Papers

Authors: Xiao Tao, Yepeng Yang, Jiahui Qi, Biao Cai, W Mark Rainforth, Xiaoying Li, Hanshan Dong 

Published: 2023/07/01 

https://doi.org/10.1016/j.apsusc.2023.157137

Abstract

Nitrogen-expanded austenite (γN), also known as S phase, is a metastable, interstitial supersaturated phase, that has been extensively studied on AISI 304/316 stainless steel (SS) following N diffusion treatments at low temperatures. Substantial Cr segregation can occur in γN-304/316 under CrN formation at elevated treatment temperatures (typically at above 450 °C), resulting in degradation in corrosion performance. In contrast to 316 SS (mainly based on the Fe-Cr-Ni system), a FeCrNiSi0.5 medium entropy alloy (MEA) was investigated after active screen plasma nitriding (ASPN) at 430–480 °C in this study. Attributable to the equimolar Cr content in the substrate, the ASPN-treated MEA surface showed “excessive” N absorption, which was accompanied by severe surface micro-cracking and a topmost nanocrystalline layer. Nanoprecipitates (∼3-8 nm) can be seen in γN-MEA at treatment temperature as low as 430 °C. However, STEM-EDX analysis of those nanoprecipitates (∼5-30 nm) at 480 °C showed significant Si segregation without observable Cr redistribution, resulting in a uniform “greyish” etched treatment layer. The sluggish Cr segregation in the N-modified MEA surface could originate from both the minor Si addition (via Si-induced nanoprecipitation) and the high equimolar Ni/Cr content in the host matrix. 

Authors: Mohamed A Ali, Wessel MW Winters, Moushira A Mohamed, Dezhi Tan, Guojun Zheng, Rasmus SK Madsen, Oxana V Magdysyuk, Maria Diaz‐Lopez, Biao Cai, Nan Gong, Yijue Xu, Ivan Hung, Zhehong Gan, Sabyasachi Sen, Hong‐Tao Sun, Thomas D Bennett, Xiaofeng Liu, Yuanzheng Yue, Jianrong Qiu 

Published: 2023/03/27

https://doi.org/10.1002/ange.202218094

Abstract

Metal coordination compound (MCC) glasses [e.g., metal-organic framework (MOF) glass, coordination polymer glass, and metal inorganic-organic complex (MIOC) glass] are emerging members of the hybrid glass family. So far, a limited number of crystalline MCCs can be converted into glasses by melt-quenching. Here, we report a universal wet-chemistry method, by which the super-sized supramolecular MIOC glasses can be synthesized from non-meltable MOFs. Alcohol and acid were used as agents to inhibit crystallization. The MIOC glasses demonstrate unique features including high transparency, shaping capability, and anisotropic network. Directional photoluminescence with a large polarization ratio (≈47 %) was observed from samples doped with organic dyes. This crystallization-suppressing approach enables fabrication of super-sized MCC glasses, which cannot be achieved by conventional vitrification methods, and thus allows for exploring new MCC glasses possessing photonic functionalities. 

Authors: Zihan Song, Elodie Boller, Alexander Rack, Peter D Lee, Biao Cai

Published: 2023/3/25

https://doi.org/10.1016/j.jallcom.2022.168691 

Abstract

Magnetic fields have been widely used to control solidification processes. Here, high-speed synchrotron X-ray tomography was used to study the effect of magnetic fields on solidification. We investigated vertically upward directional solidification of an Al-Si-Cu based W319 alloy without and with a transverse magnetic field of 0.5 T while the sample was rotating. The results revealed the strong effect of a magnetic field on both the primary α-Al phase and secondary β-Al5FeSi intermetallic compounds (IMCs). Without the magnetic field, coarse primary α-Al dendrites were observed with a large macro-segregation zone. When a magnetic field is imposed, much finer dendrites with smaller primary arm spacing were obtained, while macro-segregation was almost eliminated. Segregated solutes were pushed out of the fine dendrites and piled up slightly above the solid/liquid interface, leading to a gradient distribution of the secondary β-IMCs. This work demonstrates that rotating the sample under a transversal magnetic field is a simple yet effective method to homogenise the temperature and composition distributions, which can be used to control the primary phase and the distribution of iron-rich intermetallics during solidification.

Authors: Daixiu Wei, Wu Gong, Tomohito Tsuru, Takuro Kawasaki, Stefanus Harjo, Biao Cai, Peter K Liaw, Hidemi Kato

Published: 2022/11/01

https://doi.org/10.1016/j.ijplas.2022.103417 

Abstract

The equiatomic CoCrFeMnNi Cantor alloy, a face-centered-cubic (FCC) single-phase high-entropy alloy (HEA), has attracted considerable attention owing to its high strength and good ductility over a wide temperature range. The mechanical performance of this alloy was improved by reducing the stacking fault energy (SFE) through composition modification, and thus, a series of near- or non-equiatomic HEAs that are stronger and more ductile than their predecessor have been developed. However, the plastic-deformation behavior and strengthening mechanisms have not yet been fully discovered. In this study, we investigated the yielding and hardening behaviors of the Cantor alloy and FCC-phase Co-rich HEAs with different SFEs by in situ neutron diffraction combined with the first-principles method and electron-microscopy characterizations. The Co-rich HEAs exhibited a higher intrinsic yield strength than the Cantor alloy, mainly because of the larger shear modulus or modulus misfit, and grain refinement being more effective in improving the yield strength of low-SFE HEAs. Furthermore, higher flow stresses and better ductility of the Co-rich HEAs are attributed to the greater dislocation density and a larger number of stacking faults, which enhanced the strain-hardening rate during tensile deformation. The low SFE promoted mechanical twinning, and martensitic transformation contributed to higher strain-hardening rates. The present study provides deep insight into the yielding and hardening of FCC-phase HEAs, the understanding of which is a prerequisite for developing high-performance materials.

Authors: Tay Sparks, Duc Nguyen-Manh, Pengfei Zheng, Jan S Wróbel, Damian Sobieraj, Michael Gorley, Thomas Connolley, Christina Reinhard, Yiqiang Wang, Biao Cai

Published: 2022/10/01

https://doi.org/10.1016/j.jnucmat.2022.153911 

Abstract

Vanadium base alloys represent potentially promising candidate structural materials for use in nuclear fusion reactors due to vanadium's low activity, high thermal strength, and good swelling resistance. In this work, the mechanical properties of the current frontrunner vanadium base alloy, V-4Cr-4Ti, have been interrogated using in-situ high energy X-ray diffraction (XRD) tensile testing at varying temperatures. The single crystal elastic constants of the samples were determined from the in-situ XRD data and used to evaluate results from density functional theory calculations. Polycrystalline elastic properties and Zener anisotropy were calculated from the single crystal elastic constants produced, revealing the effect of elevated temperature on the alloy's elastic properties. These results characterise important thermomechanical properties, valuable in mechanical modelling, that will allow further and improved analysis of the structural suitability of V-4Cr-4Ti ahead of alloy adoption in the mainstream.

Authors: Lei Tang, Oxana V Magdysyuk, Fuqing Jiang, Yiqiang Wang, Alexander Evans, Saurabh Kabra, Biao Cai

Published: 2022/9/01

https://doi.org/10.1016/j.scriptamat.2022.114806

Abstract

Manufacturing austenitic stainless steels (ASSs) using additive manufacturing is of great interest for cryogenic applications. Here, the mechanical and microstructural responses of a 316L ASS built by laser powder bed fusion were revealed by performing in situ neutron diffraction tensile tests at the low-temperature range (from 373 to 10 K). The stacking fault energy almost linearly decreased from 29.2 ± 3.1 mJm−2 at 373 K to 7.5 ± 1.7 mJm−2 at 10 K, with a slope of 0.06 mJm−2K−1, leading to the transition of the dominant deformation mechanism from strain-induced twinning to martensite formation. As a result, excellent combinations of strength and ductility were achieved at the low-temperature range.

Authors:  Lei Tang, FQ Jiang, JS Wróbel, B Liu, S Kabra, RX Duan, JH Luan, ZB Jiao, MM Attallah, D Nguyen-Manh, Biao Cai

Published: 2022/7/20

https://doi.org/10.1016/j.jmst.2021.10.034 

Abstract

We investigated the mechanical and microstructural responses of a high-strength equal-molar medium entropy FeCrNi alloy at 293 and 15 K by in situ neutron diffraction testing. At 293 K, the alloy had a very high yield strength of 651 ± 12 MPa, with a total elongation of 48% ± 5%. At 15 K, the yield strength increased to 1092 ± 22 MPa, but the total elongation dropped to 18% ± 1%. Via analyzing the neutron diffraction data, we determined the lattice strain evolution, single-crystal elastic constants, stacking fault probability, and estimated stacking fault energy of the alloy at both temperatures, which are the critical parameters to feed into and compare against our first-principles calculations and dislocation-based slip system modeling. The density functional theory calculations show that the alloy tends to form short-range order at room temperatures. However, atom probe tomography and atomic-resolution transmission electron microscopy did not clearly identify the short-range order. Additionally, at 293 K, experimental measured single-crystal elastic constants did not agree with those determined by first-principles calculations with short-range order but agreed well with the values from the calculation with the disordered configuration at 2000 K. This suggests that the alloy is at a metastable state resulted from the fabrication methods. In view of the high yield strength of the alloy, we calculated the strengthening contribution to the yield strength from grain boundaries, dislocations, and lattice distortion. The lattice distortion contribution was based on the Varenne-Luque-Curtine strengthening theory for multi-component alloys, which was found to be 316 MPa at 293 K and increased to 629 MPa at 15 K, making a significant contribution to the high yield strength. Regarding plastic deformation, dislocation movement and multiplication were found to be the dominant hardening mechanism at both temperatures, whereas twinning and phase transformation were not prevalent. This is mainly due to the high stacking fault energy of the alloy as estimated to be 63 mJ m−2 at 293 K and 47 mJ m−2 at 15 K. This work highlights the significance of lattice distortion and dislocations played in this alloy, providing insights into the design of new multi-component alloys with superb mechanical performance for cryogenic applications.

Authors: Lei Tang, Fuqing Jiang, Huibin Liu, Saurabh Kabra, Biao Cai

Published: 2022/06/15

https://doi.org/10.1016/j.msea.2022.143211 

Abstract

High manganese steels are emerging as promising structural materials for cryogenic applications due to their low production cost and great potential in achieving excellent strength-ductility combinations. Micro-alloying serves as a desirable method in tailoring stacking fault energy (SFE) of the steels and thus tailoring the mechanical performance. In this study, we investigated the dedicate role of Cu addition played on the mechanical and microstructural responses of high manganese steels at the low-temperature range (293, 173, and 77 K) via in situ neutron diffraction and microscope characterizations. The addition of 1 wt%Cu to the steel not only effectively improved the yield strength (YS) and elongation but also increased the SFE thus postponing the martensite formation. For both high Mn steels, as deformation temperature decreased, the tensile strength was increased linearly, the formation of stacking faults and dislocation was promoted, and the SFE almost linearly decreased with a slope of about 0.06 mJm−2·K−1. The contributions to YS and flow stress from lattice friction, grain boundary, dislocation, deformation twins, and phase transformation were determined based on neutron diffraction results and previously validated models. The work revealed the critical roles of Cu micro-alloying in tailoring the SFE of TWIP steels and the resulting deformation mechanisms, paving the way in adapting new high manganese steels for cryogenic applications.

Authors: Zihan Song, Oxana V Magdysyuk, Tay Sparks, Yu-Lung Chiu, Biao Cai

Published: 2022/06/1

https://doi.org/10.1016/j.actamat.2022.117903 

Abstract

This study used high-speed synchrotron X-ray tomography to image the growth of Al2Cu intermetallic compounds in 4D (3D plus time) during solidification of Al-45wt%Cu alloy. Two categories of growth patterns (basic units and dendrites) are identified. Basic units are elongated rods whose cross-section are L, U or hollow-rectangular shapes. The transition from L pattern to U and finally to hollow-rectangular shaped morphology is observed. Faceted dendritic patterns include equiaxed prism and columnar dendrites. Self-repeated layer-by-layer stacking of the basic units (such as L shaped particles) is proposed as a governing mechanism for the growth of Al2Cu faceted dendrites. The growth orientation and morphologies of these patterns are strongly influenced by solidification conditions (temperature gradients, cooling rates and external magnetic fields). Another finding is that when rotating Al-45wt%Cu during upwards directional solidification, under a transverse magnetic field of 0.5T, highly refined and well aligned Al2Cu intermetallic compounds are obtained, much finer than those without the imposition of the magnetic field. This is attributed to a rotational stirring flow that modulates and regulates the temperature and solute distribution. The developed experimental findings provide a physical understanding of the formation of faceted intermetallic compounds during solidification.

Authors: Shunichi Tachibana, Biao Cai, Alison J Davenport, Shinichi Miura, Hongchang Wang, Igor P Dolbnya

Published: 2022/6/1

https://doi.org/10.1016/j.mtcomm.2022.103219 

Abstract

Microdefects in the rust layer of conventional steel and weathering steel were investigated by synchrotron X-ray micro tomography to understand the effect of defects on corrosion resistance. The rust layer of the weathering steel contained fewer and smaller defects than that of the conventional steel. A good correlation existed between the volume of defects and ion permeation of the rust layer obtained by EIS. In comparison with the conventional steel, the tomography results indicated that a protective layer formed on the weathering steel.

Authors: Muhammad Ayub Ansari, Andrew Crampton, Rebecca Garrard, Biao Cai, Moataz Attallah

Published: 2022/06

https://doi.org/10.1007/s00170-022-08995-7 

Abstract

This study aims to detect seeded porosity during metal additive manufacturing by employing convolutional neural networks (CNN). The study demonstrates the application of machine learning (ML) in in-process monitoring. Laser powder bed fusion (LPBF) is a selective laser melting technique used to build complex 3D parts. The current monitoring system in LPBF is inadequate to produce safety-critical parts due to the lack of automated processing of collected data. To assess the efficacy of applying ML to defect detection in LPBF by in-process images, a range of synthetic defects have been designed into cylindrical artefacts to mimic porosity occurring in different locations, shapes, and sizes. Empirical analysis has revealed the importance of accurate labelling strategies required for data-driven solutions. We formulated two labelling strategies based on the computer-aided design (CAD) file and X-ray computed tomography (XCT) scan data. A novel CNN was trained from scratch and optimised by selecting the best values of an extensive range of hyper-parameters by employing a Hyperband tuner. The model’s accuracy was 90% when trained using CAD-assisted labelling and 97% when using XCT-assisted labelling. The model successfully spotted pores as small as 0.2mm. Experiments revealed that balancing the data set improved the model’s precision from 89% to 97% and recall from 85% to 97% compared to training on an imbalanced data set. We firmly believe that the proposed model would significantly reduce post-processing costs and provide a better base model network for transfer learning of future ML models aimed at LPBF micro-defects detection.

Authors: Anastasia Vrettou, Hiroto Kitaguchi, Biao Cai, Thomas Connolley, David M Collins

Published: 2022/05/23

https://doi.org/10.1016/j.msea.2022.143091 

Abstract

The effect of Strain Path Changes (SPCs) on the mechanical properties and crystal-level features of deformation for a single phase, ferritic steel has been investigated. SPCs were applied via a two-step deformation process, which included pre-straining via cold rolling, followed by uniaxial tension. The pre-strain magnitude and direction, as well as the tensile direction, varied between the specimens. The role of texture and micromechanics were examined in-situ, via Synchrotron X-ray Diffraction (SXRD), and ex-situ, via Electron Backscatter Diffraction (EBSD). Abrupt strain paths (i.e. strain paths where the pre-strain and the subsequent loading directions differ; here they are orthogonal) result in a significant ductility reduction, becoming more prevalent for high pre-strain magnitudes. The macroscopic response, as well as the texture configuration were greatly dependent on the pre-strain direction but were insensitive to the direction of uniaxial tension. Increasing pre-strain magnitudes resulted in a stagnation of lattice strain hardening rates in all macroscopic directions and in a significant increase in the Geometrically Necessary Dislocation (GND) densities. This was vastly increased for specimens rolled perpendicular to the as-received prior rolling direction. No correlation was found between the GND density and the grain orientation, eliminating this as a controlling ductility factor for BCC ferrite. Instead, the initial texture and the texture developed in a subsequent pre-strain influences the density of dislocations accumulated in all grains, and ultimately determines ductility.

Authors: Sheng Li, Biao Cai, Ranxi Duan, Lei Tang, Zihan Song, Dominic White, Oxana V Magdysyuk, Moataz M Attallah

Published: 2022/01/10

https://doi.org/10.1007/s40195-021-01317-y 

Abstract

Isotropy in microstructure and mechanical properties remains a challenge for laser powder bed fusion (LPBF) processed materials due to the epitaxial growth and rapid cooling in LPBF. In this study, a high-strength TiB2/Al-Cu composite with random texture was successfully fabricated by laser powder bed fusion (LPBF) using pre-doped TiB2/Al-Cu composite powder. A series of advanced characterisation techniques, including synchrotron X-ray tomography, correlative focussed ion beam–scanning electron microscopy (FIB-SEM), scanning transmission electron microscopy (STEM), and synchrotron in situ X-ray diffraction, were applied to investigate the defects and microstructure of the as-fabricated TiB2/Al-Cu composite across multiple length scales. The study showed ultra-fine grains with an average grain size of about 0.86 μm, and a random texture was formed in the as-fabricated condition due to rapid solidification and the TiB2 particles promoting heterogeneous nucleation. The yield strength and total elongation of the as-fabricated composite were 317 MPa and 10%, respectively. The contributions of fine grains, solid solutions, dislocations, particles, and Guinier–Preston (GP) zones were calculated. Failure was found to be initiated from the largest lack-of-fusion pore, as revealed by in situ synchrotron tomography during tensile loading. In situ synchrotron diffraction was used to characterise the lattice strain evolution during tensile loading, providing important data for the development of crystal-plasticity models.

Authors: Ranxi Duan, Sheng Li, Biao Cai, Zhi Tao, Weiwei Zhu, Fuzeng Ren, Moataz M Attallah

Published: 2021/10/01

https://doi.org/10.1016/j.compositesb.2021.109059 

Abstract

β-titanium (Ti) alloys combined with high strength and good ductility have attracted extensive research interest for use in many advanced industrial applications. In this study, laser powder bed fusion (LPBF) parameters were firstly optimised to fabricate highly dense metastable β Ti–12wt% Mo alloy with largely homogeneous structure from low-cost elemental powders. When the laser area energy density (AED) increased to 4 J/mm2 using the simple scan mode, the refractory Mo powder melted and dissolved into the Ti matrix due to the increased melting pool temperature and increased cycles of remelting. However, when the same high AED was used in the chess scanning mode, keyhole-induced defects emerged along the island boundaries. The laser beam delay (LBD) and island spacing (IS) settings were then optimised to eliminate keyhole defects. Additionally, it is found that using low and high AED (e.g. 1.6 and 4 J/mm2 respectively), the builds show significantly different microstructure and mechanical properties. In view of this, a Ti–Mo functional gradient composite (FGC) was fabricated via alternating AEDs of 1.6 and 4 J/mm2 layer-wise. A gradient distribution of α” phase with varied size and quantities across the designed layer boundaries was produced. The Ti–Mo FGC possessed a high compressive yield strength of 1173 (±15) MPa and improved strain hardening capacity. The developed approach demonstrated the potential for the fabrication of FGCs using an in situ alloying based LPBF.

Authors: Nolwenn Le Gall, Fabio Arzilli, Giuseppe La Spina, Margherita Polacci, Biao Cai, Margaret E Hartley, Nghia T Vo, Robert C Atwood, Danilo Di Genova, Sara Nonni, Edward W Llewellin, Mike R Burton, Peter D Lee

Published: 2021/08/15

https://doi.org/10.1016/j.epsl.2021.117016 

Abstract

Crystallisation is a complex process that significantly affects the rheology of magma, and thus the flow dynamics during a volcanic eruption. For example, the evolution of crystal fraction, size and shape has a strong impact on the surface crust formation of a lava flow, and accessing such information is essential for accurate modelling of lava flow dynamics. To investigate the role of crystallisation kinetics on lava flow behaviour, we performed real-time, in situ synchrotron X-ray microtomography, studying the influence of temperature-time paths on the nucleation and growth of clinopyroxene and plagioclase in an oxidised, nominally anhydrous basaltic magma. Crystallisation experiments were performed at atmospheric pressure in air and temperatures from 1250 °C to 1100 °C, using a bespoke high-temperature resistance furnace. Depending on the cooling regime (single step versus continuous), two different crystal phases (either clinopyroxene or plagioclase) were produced, and we quantified their growth from both global and individual 3D texture analyses. The textural evolution of charges suggests that suppression of crystal nucleation is due to changes in the melt composition with increasing undercooling and time. Using existing viscosity models, we inferred the effect of crystals on the viscosity evolution of our crystal-bearing samples to trace changes in rheological behaviour during lava emplacement. We observe that under continuous cooling, both the onsets of the pāhoehoe-‘a‘ā transition and of non-Newtonian behaviour occur within a shorter time frame. With varying both temperature and time, we also either reproduced or approached the clinopyroxene and plagioclase phenocryst abundances and compositions of the Etna lava used as starting material, demonstrating that real-time synchrotron X-ray tomography is an ideal approach to unravel the final solidification history of basaltic lavas. This imaging technology has indeed the potential to provide input into lava flow models and hence our ability to forecast volcanic hazards.

Authors: Xiaowei Zhang, Jeremy H Rao, Mingzong Wang, Biao Cai, Hongxi Liu

Published: 2021/01/01

https://doi.org/10.1016/j.addma.2020.101704 

Abstract

This study demonstrates a novel design of a three-dimensional structural hood to remove the vapor and spatter generated during the Direct energy deposition (DED) process. The hood is composed of three planes, including an inlet block, an outlet block and a shell, with three splitter structures in each block. Computational fluid dynamics (CFD) simulation was implemented to match the inlet block with the outlet block, installing a horizontal splitter and a vertical splitter, respectively. With the proposed new designs, the gas flow state of vapor and spatter removal hood structures of different splitters is distinct. Through numerical simulation, one of the structures beneficial to vapor and spatter removal during DED process is proposed. The objects of different structural hoods were built by selective laser melting (SLM) additive manufacturing and the reliability of the simulation results is verified through experiments.

Authors: Zihan Song, Oxana V Magdysyuk, Lei Tang, Tay Sparks, Biao Cai

Published: 2021 

https://doi.org/10.1016/j.jallcom.2021.158604 

Abstract

High-speed synchrotron tomography was used to investigate the nucleation and growth dynamics of Al13Fe4 intermetallic during solidification of an Al-5wt%Fe alloy, providing new insights into its formation process. The majority of Al13Fe4 intermetallics nucleated near the surface oxide of the specimen and a few nucleated at Al13Fe4 phase. Al13Fe4 crystals grew into a variety of shapes, including plate-like, hexagonal tabular, stair-like and V-shaped, which can be attributed to the crystal structure of this compound and its susceptibility to twinning. Hole-like defects filled with aluminium melt were observed within the intermetallics. Oriented particle attachment mechanism was proposed to explain the formation of the Al13Fe4 intermetallic, which needs further experiments and simulation to confirm.

Authors: Ranxi Duan, Sheng Li, Biao Cai, Weiwei Zhu, Fuzeng Ren, Moataz M Attallah

Published: 2021/01/01

https://doi.org/10.1016/j.addma.2020.101708

Abstract

Biocompatible β Ti-alloys with high strength and low modulus are of interest for additive manufacturing of biomedical implants. In this study, a metastable β Ti-12Mo-6Zr-2Fe (TMZF) alloy with highly dense structure was successfully fabricated by laser powder bed fusion (LPBF) from low-cost elemental powders. The applied different scanning strategies (simple and chess scan), and post heat treatment can regulate both the texture and secondary phases. The formation of strong {100}< 001 > texture leads to the low elastic modulus of TMZF alloys, while nano-sized α" phases induce significantly strengthening effect. The as-fabricated TMZF alloy via simple scanning strategy shows considerably high strength due to the high density of α", ω phases and sessile dislocations, but it is brittle owing to the presence of ω phase. The TMZF alloy, manufactured using chess scanning strategy, possessed high yield strength of 1,026 MPa, low modulus of 85.7 GPa and good ductility of 12.7%. This results from its unique hierarchical microstructure containing α" phases, heterogeneous grains and the formation of {100}< 001 > texture. After solution heat treatment, the specimens exhibit stronger {100} < 001 > texture, hence lower modulus of 70.9 GPa. High yield strength of 943 MPa was maintained due to the formation of plate-like α" precipitates. The TMZF alloy fabricated by in-situ alloying based LPBF demonstrates comparable strength to that of Ti-6Al-4V alloy, but much lower elastic modulus, suggesting that it could be a potential candidate for some implant applications.

Authors: Lei Tang, Li Wang, Minshi Wang, Huibin Liu, Saurabh Kabra, Yulung Chiu, Biao Cai

Published: 2020/11/01

https://doi.org/10.1016/j.actamat.2020.09.075

Abstract

High manganese steels are promising candidates for applications in cryogenic environments. In this study, we investigate the mechanical and microstructural responses of a high manganese twinning induced plasticity (TWIP) steel at a low-temperature range (from 373 to 77 K) via in situ neutron diffraction qualification and correlative microscopy characterization. During plastic deformation, stacking fault probability and dislocation density increased at a faster rate at a lower temperature, hence, higher dislocation density and denser mechanical twins were observed, confirmed by microscopic observation. Stacking fault energy was estimated, dropping linearly from 34.8 mJm−2 at 373 K to 17.2 mJm−2 at 77 K. A small amount of austenite transferred to martensite when deforming at 77 K. The contributions to flow stress from solutes, grain boundary, dislocation, and twinning were determined at different temperatures, which shows that the high work strain hardening capacity of the TWIP steel originates from the synergetic strengthening effects of dislocations and twin-twin networks. These findings reveal the relationship among stacking fault energy, microstructure, and deformation mechanisms at the low-temperature range, paving a way in designing TWIP steels with the superb mechanical performance for cryogenic applications.

Authors: B Cai, A Kao, E Boller, OV Magdysyuk, RC Atwood, NT Vo, K Pericleous, PD Lee

Published: 2020/09/01

https://doi.org/10.1016/j.actamat.2020.06.041 

Abstract

A key technique for controlling solidification microstructures is magneto-hydrodynamics (MHD), resulting from imposing a magnetic field to solidifying metals and alloys. Applications range from bulk stirring to flow control and turbulence damping via the induced Lorentz force. Over the past two decades the Lorentz force caused by the interaction of thermoelectric currents and a magnetic field, a MHD phenomenon known as Thermoelectric Magnetohydrodynamics (TEMHD), was also shown to drive inter-dendritic flow altering microstructural evolution. In this contribution, high-speed synchrotron X-ray tomography and high-performance computational simulation are coupled to reveal the evolution, dynamics and mechanisms of solidification within a magnetic field, resolving the complex interplay and competing flow effects arising from Lorentz forces of different origins. The study enabled us to reveal the mechanisms disrupting the traditional columnar dendritic solidification microstructure, ranging from an Archimedes screw-like structure, to one with a highly refined dendritic primary array. We also demonstrate that alloy composition can be tailored to increase or decrease the influence of MHD depending on the Seebeck coefficient and relative density of the primary phase and interdendritic liquid. This work paves the way towards novel computational and experimental methods of exploiting and optimising the application of MHD in solidification processes, together with the calculated design of novel alloys that utilise these forces.

Authors: T Nelson, B Cai, N Warnken, PD Lee, E Boller, OV Magdysyuk, NR Green

Published: 2020/04/15

https://doi.org/10.1016/j.scriptamat.2019.12.026 

Abstract

The effect of gravity on thermo-solutal convection and its impact on solidification dynamics of an Al-15 wt%Cu alloy were studied using high speed synchrotron tomography. A method for mapping the composition of the solidifying samples was developed, enabling three-dimensional quantification of the time evolved solute concentration and dendrite morphology. Differences in solute segregation, dendrite morphology and fragmentation between upwards and downwards solidification were identified, which were attributed to buoyancy-modulated thermal-solutal convection.

Authors: Lei Tang, Kun Yan, Biao Cai, Yiqiang Wang, Bin Liu, Saurabh Kabra, Moataz M Attallah, Yong Liu

Published: 2020/03/15

https://doi.org/10.1016/j.scriptamat.2019.11.026 

Abstract

Deformation mechanisms of high entropy alloys (HEAs) at cryogenic temperatures have attracted extensive research interest. We used in situ neutron diffraction to study the tensile behavior of a face-centered-cubic HEA at 77 and 15 K and compared its stacking fault energy (SFE) at ambient and cryogenic temperatures. The SFE dropped from 28 mJm−2 at 293 K to 11 mJm−2 at 15 K, leading to the transition of deformation mechanism from deformation-induced twinning to martensite phase transformation. As a result, excellent balance of strength and ductility was achieved at both temperatures. This finding highlights the importance of SFE for cryogenic alloy design.

Authors: Martin B Østergaard, Manlin Zhang, Xiaomei Shen, Rasmus R Petersen, Jakob König, Peter D Lee, Yuanzheng Yue, Biao Cai

Published: 2020/03/1

https://doi.org/10.1016/j.actamat.2020.02.060 

Abstract

Glass foams are attractive thermal insulation materials, thus, the thermal conductivity (λ) is crucial for their insulating performance. Understanding the foaming process is critical for process optimization. Here, we applied high-speed synchrotron X-ray tomography to investigate the change in pore structure during the foaming process, quantifying the foam structures and porosity dynamically. The results can provide guidance for the manufacturing of glass foams. The 3D pore structures were also used to computationally determine λ of glass foams using image-based modelling. We then used the simulated λ to develop a new analytical model to predict the porosity dependence of λ. The λ values of the glass foams when the porosity is within 40% to 95% predicted by the new model are in excellent agreement with the experimental data collected from the literature, with an average error of only 0.7%, which performs better than previously proposed models.

Authors: YQ Wang, SJ Clark, Biao Cai, D Alba Venero, K Yan, Mike Gorley, Elizabeth Surrey, DG McCartney, S Sridhar, PD Lee

Published: 2020/01/01

https://doi.org/10.1016/j.scriptamat.2019.08.016 

Abstract

Ti-containing micro-alloyed steels are often alloyed with molybdenum (Mo) to reduce nano-precipitate coarsening, although the mechanism is still disputed. Using small angle neutron scattering we characterised the precipitate composition and coarsening of Ti-alloyed and Ti-Mo-alloyed steels. The results demonstrate ~25 at.% of Ti is substituted by Mo in the (Ti, Mo)C precipitates, increasing both the precipitate volume percent and average size. Mo alloying did not retard precipitation coarsening, but improved lattice misfit between precipitate and matrix, contributing to better ageing resistance of the Ti-Mo-alloyed steel. This new understanding opens opportunities for designing ageing-resistant micro-alloyed steels with lean alloying elements.

Authors: Yiqiang Wang, Sridhar Seetharaman, Graham McCartney, Vit Janik, Kun Yan, Biao Cai, Samuel Clark, Peter Lee, Diego Alba Venero

Published: 2019/12/06

http://doi.org/10.5286/ISIS.E.RB1620206

Abstract

Ever-more stringent environmental regulations have driven the automotive industry to continuously develop lightweight solutions in automotive body sheet materials, without compromising of passenger safety and manufacturing robustness. Steels with small amounts of alloying elements such as vanadium, molybdenum (also called micro-alloyed steels) strengthened via nano-precipitates, are potential candidate materials fitting these requirements. To be able to control the dispersion of nano-precipitates and understands its effect on mechanical properties, a detailed understanding of their formation and growth kinetics at the nanoscale is essential. This proposal is a part of an ongoing EPSRC funded (£600K from Sep. 2014-Sep. 2017) project, which aims to develop new micro-alloyed steels with enhanced strength. This project is collaboration between the University of Warwick and Tata steel.

Authors: Fabio Arzilli, Giuseppe La Spina, Mike R Burton, Margherita Polacci, Nolwenn Le Gall, Margaret E Hartley, Danilo Di Genova, Biao Cai, Nghia T Vo, Emily C Bamber, Sara Nonni, Robert Atwood, Edward W Llewellin, Richard A Brooker, Heidy M Mader, Peter D Lee

Published: 2019/10/21

https://doi.org/10.1038/s41561-019-0468-6 

Abstract

Basaltic eruptions are the most common form of volcanism on Earth and planetary bodies. The low viscosity of basaltic magmas inhibits fragmentation, which favours effusive and lava-fountaining activity, yet highly explosive, hazardous basaltic eruptions occur. The processes that promote fragmentation of basaltic magma remain unclear and are subject to debate. Here we used a numerical conduit model to show that a rapid magma ascent during explosive eruptions produces a large undercooling. In situ experiments revealed that undercooling drives exceptionally rapid (in minutes) crystallization, which induces a step change in viscosity that triggers magma fragmentation. The experimentally produced textures are consistent with basaltic Plinian eruption products. We applied a numerical model to investigate basaltic magma fragmentation over a wide parameter space and found that all basaltic volcanoes have the potential to produce highly explosive eruptions. The critical requirements are initial magma temperatures lower than 1,100 °C to reach a syn-eruptive crystal content of over 30 vol%, and thus a magma viscosity around 105 Pa s, which our results suggest is the minimum viscosity required for the fragmentation of fast ascending basaltic magmas. These temperature, crystal content and viscosity requirements reveal how typically effusive basaltic volcanoes can produce unexpected highly explosive and hazardous eruptions.

Authors: Shenghang Xu, Meng Du, Jia Li, Kun Yan, Biao Cai, Quanfeng He, Qihong Fang, Oxana Magdysyuk, Bin Liu, Yong Yang, Yong Liu

Published: 2019/12/01

https://doi.org/10.1016/j.mtla.2019.100463 

Abstract

Nature materials, such as bones and nacre, achieve excellent balance of toughness and strength via a hierarchical “brick-and-mortar” microstructure, which is an attractive model for engineering materials design. Here, we produced nacre-like Ti–Ta metallic composites via a powder metallurgy process, during which mixed powders were sintered by spark plasma sintering, followed by hot and cold rolling and then annealing. The structure consists of soft Ta-enriched regions and hard Ti-enriched regions in a hierarchical and laminated fashion. The microstructural heterogeneity spans several scales due to the diffusion between Ti and Ta. This yields a novel metal–metal composite with a balanced combination of strength and ductility (1226 MPa ultimate tensile strength and 20.8% elongation), outperforming most of conventional Ti based alloys and composites. Via the complementary in situ synchrotron X-ray diffraction and electron microscopies, it is found out that multiple micromechanisms are active, including nano-particle and dislocation localized strengthening as well as phase transformation induced plasticity. The manufacturing route developed here is versatile, capable of making high performance bio-mimic metallic composites.

Authors: S Bhagavath, Biao Cai, R Atwood, M Li, B Ghaffari, PD Lee, S Karagadde

Published: 2019/10/15

https://doi.org/10.1007/s11661-019-05378-8 

Abstract

In die-casting processes, the high cooling rates and pressures affect the alloy solidification and deformation behavior, and thereby impact the final mechanical properties of cast components. In this study, isothermal semi-solid compression and subsequent cooling of aluminum die-cast alloy specimens were characterized using fast synchrotron tomography. This enabled the investigation and quantification of gas and shrinkage porosity evolution during deformation and solidification. The analysis of the 4D images (3D plus time) revealed two distinct mechanisms by which porosity formed; (i) deformation-induced growth due to the enrichment of local hydrogen content by the advective hydrogen transport, as well as a pressure drop in the dilatant shear bands, and (ii) diffusion-controlled growth during the solidification. The rates of pore growth were quantified throughout the process, and a Gaussian distribution function was found to represent the variation in the pore growth rate in both regimes. Using a one-dimensional diffusion model for hydrogen pore growth, the hydrogen flux required for driving pore growth during these regimes was estimated, providing a new insight into the role of advective transport associated with the deformation in the mushy region.

Authors: Martin B Østergaard, Biao Cai, Rasmus R Petersen, Jakob König, Peter D Lee, Yuanzheng Yue

Published: 2019/09/01

https://doi.org/10.1016/j.matlet.2019.04.106 

Abstract

The thermal conductivity (λ) of glass foams is thought to depend on pore size. We report on the impact of pore size, determined using X-ray microtomography, and percentage porosity on the λ of glass foams. Glass foams were prepared by heating powder mixtures of obsolete cathode ray tube (CRT) panel glass, Mn3O4 and carbon as foaming agents, and K3PO4 as additive, to a suitable temperature above Tg, and subsequent cooling. Here, we report for the first time a correlation between λ and pore size in the range 0.10–0.16 mm showing a decrease from 57 to 49 mW m−1 K−1 with increasing the pore size for glass foams with porosities of 87–90%. This indicates that the pore structure should be optimized in order to improve the insulating performance of glass foams.

Authors: Hongchang Wang, Robert C Atwood, Matthew James Pankhurst, Yogesh Kashyap, Biao Cai, Tunhe Zhou, Peter David Lee, Michael Drakopoulos, Kawal Sawhney

Published: 2019/06/20

https://doi.org/10.1038/s41598-019-45561-w 

Abstract

High energy X-ray phase contrast tomography is tremendously beneficial to the study of thick and dense materials with poor attenuation contrast. Recently, the X-ray speckle-based imaging technique has attracted widespread interest because multimodal contrast images can now be retrieved simultaneously using an inexpensive wavefront modulator and a less stringent experimental setup. However, it is time-consuming to perform high resolution phase tomography with the conventional step-scan mode because the accumulated time overhead severely limits the speed of data acquisition for each projection. Although phase information can be extracted from a single speckle image, the spatial resolution is deteriorated due to the use of a large correlation window to track the speckle displacement. Here we report a fast data acquisition strategy utilising a fly-scan mode for near field X-ray speckle-based phase tomography. Compared to the existing step-scan scheme, the data acquisition time can be significantly reduced by more than one order of magnitude without compromising spatial resolution. Furthermore, we have extended the proposed speckle-based fly-scan phase tomography into the previously challenging high X-ray energy region (120 keV). This development opens up opportunities for a wide range of applications where exposure time and radiation dose are critical.

Authors: Amy Nommeots-Nomm, Cosimo Ligorio, AJ Bodey, Biao Cai, JR Jones, PD Lee, Gowsihan Poologasundarampillai

Published: 2019/06/01

https://doi.org/10.1016/j.mtadv.2019.100011 

Abstract

Bioglass® was the first material to form a stable chemical bond with human tissue. Since its discovery, a key goal was to produce three-dimensional (3D) porous scaffolds which can host and guide tissue repair, in particular, regeneration of long bone defects resulting from trauma or disease. Producing 3D scaffolds from bioactive glasses is challenging because of crystallization events that occur while the glass particles densify at high temperatures. Bioactive glasses such as the 13–93 composition can be sintered by viscous flow sintering at temperatures above the glass transition onset (Tg) and below the crystallization temperature (Tc). There is, however, very little literature on viscous flow sintering of bioactive glasses, and none of which focuses on the viscous flow sintering of glass scaffolds in four dimensions (4D) (3D + time). Here, high-resolution synchrotron-sourced X-ray computed tomography (sCT) was used to capture and quantify viscous flow sintering of an additively manufactured bioactive glass scaffold in 4D. In situ sCT allowed the simultaneous quantification of individual particle (local) structural changes and the scaffold's (global) dimensional changes during the sintering cycle. Densification, calculated as change in surface area, occurred in three distinct stages, confirming classical sintering theory. Importantly, our observations show for the first time that the local and global contributions to densification are significantly different at each of these stages: local sintering dominates stages 1 and 2, while global sintering is more prevalent in stage 3. During stage 1, small particles coalesced to larger particles because of their higher driving force for viscous flow at lower temperatures, while large angular particles became less faceted (angular regions had a local small radius of curvature). A transition in the rate of sintering was then observed in which significant viscous flow occurred, resulting in large reduction of surface area, total strut volume, and interparticle porosity because the majority of the printed particles coalesced to become continuous struts (stage 2). Transition from stage 2 to stage 3 was distinctly obvious when interparticle pores became isolated and closed, while the sintering rate significantly reduced. During stage 3, at the local scale, isolated pores either became more spherical or reduced in size and disappeared depending on their initial morphology. During stage 3, sintering of the scaffolds continued at the strut level, with interstrut porosity reducing, while globally the strut diameter increased in size, suggesting overall shrinkage of the scaffold with the flow of material via the strut contacts.

This study provides novel insights into viscous flow in a complex non-idealized construct, where, locally, particles are not spherical and are of a range of sizes, leading to a random distribution of interparticle porosity, while globally, predesigned porosity between the struts exists to allow the construct to support tissue growth. This is the first time that the three stages of densification have been captured at the local and global scales simultaneously. The insights provided here should accelerate the development of 3D bioactive glass scaffolds.

Authors: S Bhagavath, B Cai, R Atwood, PD Lee, S Karagadde

Published: 2019/05

https://dx.doi.org/10.1088/1757-899X/529/1/012053 

Abstract

The alloy casting process is one of the major manufacturing processes to produce near net shape components. The casing process is prone to a wide variety of defects, with hot tear being one of the most detrimental. The two main factors generally recognized as the primary cause for formation of hot tears are the mechanical response of the mush (which effects its permeability), and the solidification range (solidification time). The response of the mushy zone under deformation is mainly affected by the solid fraction, strain rate and grain morphology. Even though the science behind the formation of hot tear is understood, there is no general criterion to quantify the hot tear formation under varying casting conditions. The development of ultra-fast X-ray imaging has facilitated the means to quantify the effects of the critical parameters in-situ and develop better correlations for hot tear prediction. The in situ experiments will also provide insights into mush rheology, which has significant influence on hot tear formation. In this study, isothermal semi solid compression studies of Al-Si-Cu alloys were carried out using specially built thermo-mechanical rig. We studied the effects of the strain rate in the range of 2 × 10-4–0.02/s and solid fraction (∼0.6-0.9) on the mechanical response of the mushy zone. The sample were characterized before and after deformation using X-ray micro tomography. The data was subjected to an image processing routine and the amount of porosity and hot tear was quantified. The stress-strain curve of the semisolid alloys showed a characteristic strain softening behaviour for semi solid samples with ∼0.6-0.7 solid fraction, irrespective of loading rates, whereas the behaviour at higher fractions were that of constant flow stress. Additionally, in situ compression experiments were carried out, wherein the liquid channel thickness at various strain values were measured. Isolated liquid channels were formed under loading, from where the hot tears were found to nucleate. Hot tear susceptibility was found to increase with increasing strain rate and rheology of the mush, which is dependent on solid fraction.

Authors: B Cai, A Kao, PD Lee, E Boller, H Basevi, AB Phillion, A Leonardis, K Pericleous

Published: 2019/05/01

https://doi.org/10.1016/j.scriptamat.2019.02.007 

Abstract

Fe contamination is a serious composition barrier for Al recycling. In Fe-containing Al-Si-Cu alloy, a brittle and plate-shaped β phase forms, degrading the mechanical properties. Here, 4D (3D plus time) synchrotron X-ray tomography was used to observe the directional solidification of Fe-containing Al-Si-Cu alloy. The quantification of the coupled growth of the primary and β phase via machine learning and particle tracking, demonstrates that the final size of the β intermetallics were strongly influenced by the solute segregation and space available for growth whereas the β orientation was controlled by the temperature gradient direction. The work can be used to validate predictive models.

Authors: Xin Wang, Zaiwang Huang, Biao Cai, Ning Zhou, Oxana Magdysyuk, Yanfei Gao, Shesh Srivatsa, Liming Tan, Liang Jiang

Published: 2019/04/15

https://doi.org/10.1016/j.actamat.2019.02.012 

Abstract

Controlling the final grain size in a uniform manner in powder metallurgy nickel-based superalloys is important since a number of mechanical properties are closely related to it. However, it has been widely documented that powder metallurgy superalloys are prone to suffer from growth of abnormally large grains (ALGs) during supersolvus heat treatment, which is harmful to in-service mechanical performance. The underlying mechanisms behind the formation of ALGs are not yet fully understood. In this research, ALGs were intentionally created using spherical indentation applied to a polycrystalline nickel-based superalloy at room temperature, establishing a deformation gradient underneath the indentation impression, which was quantitatively determined using finite element modeling and synchrotron diffraction. Subsequent supersolvus heat treatment leads to the formation of ALGs in a narrow strain range, which also coincides with the contour of residual plastic strain in a range of about 2%–10%. The formation mechanisms can be attributed to: (1) nucleation sites available for recrystallization are limited, (2) gradient distribution of stored energy across grain boundary. The proposed mechanisms were validated by the phase-field simulation. This research provides a deeper insight in understanding the formation of ALGs in polycrystalline nickel-based superalloy components during heat treatment, when subsurface plastic deformation caused by (mis)handling occurs or small residual strain has been retained from hot/cold working before supersolvus heat treatment.