Lecturers & Abstracts

Magnesium alloys are very attractive materials for use in the automotive, railway, aerospace, and medical equipment industries. However, their low mechanical strength has been a barrier to their practical application. The mechanical strength of magnesium alloys has been improved by utilizing various material strengthening methods. The expansion of the applications of magnesium alloys requires the development of innovative materials. By focusing on both alloy design and process design, Kumamoto University has led the way in many innovations. In particular, the LPSO-type magnesium alloys developed in 2001 are composed of α-Mg and LPSO phases, and exhibits unique properties that combine high strength, heat resistance, and flame resistance. The LPSO structure is a structure in which hard L12 cluster arranged layers (CAL) and soft Mg layers are stacked alternately at intervals of less than 1 nm, and high strength is achieved through a new material strengthening method called kink strengthening. Recently, a Mg-Zn-Y alloy with a mille-feuille structure, in which hard and soft layers are stacked at intervals of more than 10 times that of LPSO structures, has also been kink-strengthened, and a strength of 417 MPa has been achieved. The MF and LPSO structures have a hard-soft nanolayered structure composed of a hard cluster arranged layer (CAL) and a soft Mg layer. Kink strengthening occurs even when the average CAL spacing is 320 times wider than that of the 18R-type LPSO structure. Furthermore, the kink strengthening amount in the MF structure increases as the average CAL spacing decreases, showing continuity with the LPSO structure. In this lecture, I will report the critical CAL interval for kink strengthening, the correlation between the CAL interval and the kink strengthening, and research challenges regarding MF structures.

2. Prof. Jer-Ren Yang

Distinguished Professor

Research Topics:

a. Phase transformation

b. Microstructural characterization

c. Transmission Electron Microscopy

d. Physical metallurgy

Abstract

The nano-structural characterization of Ω and S phases in an Al-Cu-Mg aluminum alloy with a minor addition of Ag

Jer-Ren Yang

Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan

For advanced applications requiring higher strength and elevated temperature resistance, Al–Cu–Mg–Ag aluminum alloys have been developed. In this work, the microstructural evolutions and mechanical properties of two experimental AA2024 alloys with the same base composition of Al-5.1Cu-1.0Mg (wt%) but different concentrations of Ag, 0 and 0.4 wt %, respectively denoted as Alloy 0A and Alloy 4A, have been investigated. Both alloys reach the peak ageing condition after T6 ageing treatment at 185 °C for 11 h; the UTS of the 4A-T6 specimen (Alloy 4A treated by T6), 530 MPa, was significantly enhanced to around 16% higher than that of the 0A-T6 specimen (Alloy 0A treated by T6), 456 MPa. In the over ageing condition of T7, 185 °C for 18 h, the strengths of both alloys decreased; however, as compared with the drop in the UTS of the 0A-T7 specimen (~ − 10%), the decrease in the UTS of the 4A-T7 specimen was only ~ − 4%. The smaller decrement of UTS in the 4A-T7 specimen was presumed to be attributable to the slightly lower volume percent of the Ω phase (3.82 vol %) and the same size of the S phase particles in comparison with the 4A-T6 specimen. In the 4A-T6 specimens, the minor Ag addition significantly promoted the formation of the Ω phase, consuming more Cu and Mg solute atoms, which led to the depletion of the Cu–Mg in the aluminum matrix, thereby preventing the coarsening of the S phase during the longer holding time from 4A-T6 to 4A-T7. After prolonged ageing at 185 °C for 100 h, the S phase in both alloys coalesced, so the better maintenance of UTS in Alloy 4A (~ − 15%) than in Alloy 0A (~ − 29%) can be confidently attributed to the thermally stable Ω phase which maintained its high volume percent with thickness of around 3 nm.

3. Prof. Zenji Horita

Specially Appointed Professor

Research Topics:

a. Severe plastic deformation

b. Atom diffusion

c. Microstructural characterization of KUMADAI Mg alloys

d. Superplasticity

Improving strength and ductility of Al-Mg-Si alloys containing excess Fe and Si by severe plastic deformation under high pressure

Zenji HORITA1,2,3 and Takahiro MASUDA2

Presenting author: Zenji HORITA

Email: horita.zenji.688@m.kyushu-u.ac.jp

1 Magnesium Research Center, Kumamoto University, Kumamoto 860-8555, Japan

2 School of Engineering, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan

3 Synchrotron Light Application Center, Saga University, Saga 840-8502, Japan

Keywords: Al-Mg-Si alloys, Al recycle, severe plastic deformation, grain refinement, Cu segregation, TEM, APT, strain rate sensitivity, strain rate change tests

Energy consumption is extremely large when Al ingots are produced from bauxite in comparison with the production of ingots from Al-alloys scraps. However, recycling processes is accompanied by accumulation of Fe and Si from the scraps, which then leads to significant degradation of the mechanical properties of Al alloys. This study was thus initiated to demonstrate that such a problem is solved by the application of severe plastic deformation (SPD) under high pressures such as high-pressure torsion (HPT). In this investigation, Al-Mg-Si alloys based on A6022 were fabricated so as to include excess Fe and Si with different additions of Cu as model alloys for recycling purpose. The tensile strength well exceeded 500 MPa with the total elongation more than 15% after HPT processing for 1 turn (eq=~12) in all the model alloys. The strength further increases to more than 700 MPa with increasing addition of Cu while maintaining reasonable ductility (~8%). Transmission electron microscopy (TEM) confirmed that the grain size was refined to 180 nm and further down to 160 nm with increasing Cu addition. Microstructural analyses using scanning transmission electron microscopy (STEM) and atom probe tomography (APT) revealed that Cu was segregated at grain boundaries, contributing to the increase in the tensile strength. Three dimensional image analyses were also carried out using high-energy X-rays in SPring 8 of JASRI for pores and intermetallic particles. Strain rate change tests were further performed for the evaluation of strain rate sensitivity (m). The tensile strength was increased due to the significant grain refinement, and the Cu segregations at the grain boiundaries when Cu was included. Higher ductilities were maintained due to the reduction of pore density through the HPT process and the increase in the m values by the grain refinement. Upgrade recycling of Al scraps can be well anticipated by the use of the HPT process even though the scraps contain excess Fe, Si and Cu. Details are referred to two papers in Materials Transactions (2026), doi:10.2320/matertrans.MT-L2025015 and doi:10.2320/matertrans.MT-M2025174.

4. Prof. Chun Chiu

Associate Professor

Research Topics:

a. Materials for energy storage

b. Intermetallics

c. Mechanical alloying

d. Structure-Process-Property relationship in materials

Hydrogen storage performance of ZK60 alloy subjected to severe plastic deformation and rapid solidification

Chun CHIU1, Jun-Ke HSIAO1, Chen-Kai LEE1, Zenji HORITA2, Shin-Ich INOUE2, Hsin-Chih LIN3

Presenting author: Chun CHIU

Email: cchiu@mail.ntust.edu.tw

1Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan

2Magnesium Research Center, Kumamoto University, Japan

3Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan

Keywords: ZK60 magnesium alloys, hydrogen storage, high-pressure sliding, melt spinning, mechanical milling

Magnesium alloys have attracted extensive attention as promising solid-state hydrogen storage materials. However, the drawback with their sluggish hydrogen reaction kinetics needs to be solved before practical industrial application. In this presentation, preliminary experimental results on the hydrogen storage properties of ZK60 alloy prepared by severe plastic deformation and rapid solidification will be reported. Two different processing routes, including high-pressure sliding (HPS) and melt spinning (MS), were applied to ZK60 and ZK60Mm alloys (Mm = Ce-based mischmetal). The severely deformed ZK60 alloy was processed by high-pressure sliding (HPS), while the rapidly solidified ZK60Mm alloy was prepared by melt-spinning (MS). Both the HPS and MS processes were effective in producing small grain size of less than 3 μm. XRD analysis results showed that ZK60-HPS consisted of α-Mg, MgZn2, and Zn2Zr phases, while ZK60Mm-MS consisted of α-Mg phase, indicating that most of the Zn and Mm dissolved in Mg. Before hydrogenation, the ZK60-HPS sample was subjected to manual filing, while the ZK60-MS ribbon was mechanically milled. The ZK60-HPS prepared at 1 GPa absorbed 4 wt% of hydrogen at 300 oC. Further increasing the HPS pressure to 2 GPa did not improve the hydrogen storage performance. The HPS pressure in this range was less critical for enhancing the kinetics and capacity. 2h-milled ZK60Mm-MS absorbed 5 wt% of hydrogen after 20 cycles at 300 oC; however, the storage capacity decreased to 2.5 wt% for the 5h-milled sample, implying welding of the powder during the prolonged milling process. With the addition of activated carbon as a milling control agent, the hydrogen storage capacity of ZK60Mm-MS increased to 6 wt%. The findings highlight the critical role of processing techniques and additives in optimizing the hydrogen storage properties of the ZK60-based Mg alloys.

5. Prof. Hung-Wei Yen

Professor

Research Topics:

a. Mechanical behavior of metals

b. Machine learning

c. Electron microscopy and atom probe tomography

d. Materials Microstructures & Defects

Double aging: novel and green aging strategy of aluminum alloys for automobile applications

Hung-Wei Yen

Presenting author: Hung-Wei Yen

This study demonstrates a novel aging strategy-double-aging-which enables superior thermal stability against softening during automotive paint baking in AA7075 alloy subjected to Hot Form Quench process. This new strategy involves pre-aging at for 2 hours followed by high-temperature aging at for 20 minutes, significantly reducing processing time compared to the typical T6 tempering. Integrated characterization via transmission electron microscopy and atom probe tomography reveals the nanostructural origins of this enhanced stability. The DA process promotes Cu segregation toward GPI zones or η' nanoprecipitates at elevated temperatures, leading to a higher density of finer precipitates after paint baking. The outcome achieves a high yield strength of 548 MPa, avoiding the property degradation seen in T6 condition. This novel approach offers an efficient metallurgical strategy for high-volume manufacturing and property optimization for aluminum alloys.

6. Prof. Takao Tsumuraya

Associate Professor

Research Topics:

a. Development of light metal materials by controlling alloy composition

b. Alloy design

c. Microstructural characterization

d. Thermal processing

First-Principles Study on the Stability of Laves Phases

in Mg–Al–Ca Alloys and Al₈(Mn, Fe)₅ Precipitates

Takao Tsumuraya

In recent years, the development of lightweight and heat-resistant magnesium (Mg) alloys has attracted considerable attention. Among them, Mg–Al–Ca alloys are promising materials because they exhibit both high thermal conductivity and excellent high-temperature mechanical properties. These properties are closely related to the formation of AB₂-type Laves phases—C15, C14, and C36—in which Ca occupies the A site while Al or Mg occupies the B sites. The stability of these Laves phases is known to depend strongly on the atomic size ratio between the constituent elements and on the Mg substitution fraction at the B sites. However, due to the large number of possible atomic configurations, it is difficult to systematically evaluate phase stability over a wide composition range using experiments or a limited number of first-principles calculations alone.

In this study, we investigate the stability of Laves phases in the Ca(Al₁₋ₓMgₓ)₂ pseudo-binary system by combining density functional theory (DFT) calculations with the cluster expansion (CE) method. A CE model was constructed using the icet Python library, enabling efficient evaluation of formation enthalpies for a large number of atomic configurations.

The calculated formation enthalpies reveal that the C15 phase is stable only in a narrow Al-rich region, indicating that it behaves as a line compound. In contrast, the hexagonal C36 and C14 phases remain stable across a wide composition range and compete energetically. The C36 phase is favored at Al-rich compositions, whereas the C14 phase becomes slightly more stable with increasing Mg content. Analysis of the electronic density of states indicates that the stability of the Al-rich C36 phase is associated with the formation of a deep valley in the DOS near the Fermi level. The presentation will also discuss the influence of electronic structure on phase stability in these Laves phases. In addition, we examine the possible substitution of Mn by Fe in Al₈Mn₅ precipitates using a combined CE and DFT approach.

7. Prof. Yueh-Lien Lee

Professor

Research Topics:

a. Micro-arc oxidation

b. Mg–Al–Ca magnesium alloys

c. Corrosion resistance

d. Transmission electron microscopy

Second-Phase-Controlled Surface Engineering Approaches for Protecting Mg–8Al–4Ca KUMADAI Magnesium Alloy Against Corrosion

Chi-Hua Chiu1, Shih-Yen Huang1, Yu-Ren Chu1, Yoshihito Kawamura2, Yueh-Lien Lee1*

Presenting author: Yueh-Lien Lee

Email: yuehlien@ntu.edu.tw

1. Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei, Taiwan

2. Magnesium Research Centre, Kumamoto University, 2–39-1 Kurokami, Chuo-ku, Kumamoto 860–8555, Japan

Keywords: Micro-arc oxidation, Mg–Al–Ca magnesium alloys, β–Al–Ca phase, Corrosion resistance, Transmission electron microscopy, EIS

This study investigates the micro-arc oxidation (MAO) behavior of AZ31 and Mg–8Al–4Ca (AC84) magnesium alloys, with particular emphasis on the role of the β–Al–Ca phase. Constant-voltage MAO treatments were conducted, and the resulting coatings were characterized using SEM and TEM. At a low anodizing voltage (150 V), the relatively high conductivity of the β phase in AC84 promoted localized discharge events, producing coatings that were less uniform and thinner than those formed on AZ31. When the voltage increased (≥200 V), AC84 facilitated the formation of Mg–Ca-rich silicate/oxide phases, which improved the corrosion resistance of the coatings. However, the reduced discharge intensity observed on AC84 at higher voltages also restricted coating growth, leading to thinner coatings under identical processing conditions. Pre-removal of surface β phases prior to MAO improved coating uniformity and thickness, yielding coating characteristics more comparable to those of AZ31. These results highlight the critical role of β-phase distribution and anodizing voltage in governing MAO discharge behavior and coating evolution.

8. Prof. Takahisa Shiraishi

Associate Professor

Research Topics:

a. Development of light metal materials by controlling alloy composition

b. Alloy design

c. Microstructural characterization

d. Thermal processing

Development of MAX phase/TiAl composites for high temperature structural application

T Shiraishi

Presenting author: Takahisa Shiraishi

Email: tshiraishi@kumamoto-u.ac.jp

1Magnesium Research Center, Kumamoto University, Kumamoto, Japan

Keywords: TiAl alloy, MAX phase ceramics, Composite materials, Compositional modification

TiAl-based alloys have been widely studied as high-temperature structural materials. Dispersing ceramics such as TiC, TiN, and TiB is one effective approach to improving mechanical properties. In recent years, the MAX phase ceramics, Mn+1AXn, have attracted attention as a new reinforcement phase due to their compositional diversity and excellent mechanical properties. However, there are few reports on the compositional modification of Mn+1AXn/TiAl composites. In this study, we investigated the effect of doping with Zr, Sn, or N elements, which can dissolve in both matrix and Ti2AlC (MAX phase), on the microstructure and mechanical properties. All alloys were fabricated by arc-melting method. Zr, Sn, and N are expected to occupy the Ti, Al, and C sites, respectively. The crustal structure and microstructure of as-melted alloys were investigated by using XRD, SEM and S/TEM. The mechanical properties were mesureds at room temperature by micro-Vickers hardness, nanoindentation test and compression test. It was succeeded in the fabrication of (Ti,Zr)2AlC/(Ti,Zr)Al, Ti2(Al,Sn)C/Ti(Al,Sn), and Ti2Al(C,N)/TiAl composites. SEM observations revealed that the addition of Zr and Sn promoted the formation of Zr- and Sn-rich TiAl phases. In addition, the size and morphology of MAX phase changed from rod shape to granular shape by increasing N content. The hardness of composites increased by elemental addition, and the maximum compressive strength was highly dependent on the added elements and their concentrations. Especially, (Ti,Zr)2AlC/(Ti,Zr)Al composites showed the good strength and ductility.

9. Prof. Peng-Wei Chu

Associate Professor

Research Topics:

a. Electrochemistry

b. Microstructural characterization

c. Corrosion

d. Atom probe tomography

Microstructure of the Surface Corrosion Film on LPSO Phase-Containing Mg-Y-Zn-Yb-Al Alloys with Different Forming Processes

Pei-Hsuan LIU1, Chih-Wei SUNG1, Shi-Yuan PAN1, Wei-Lun HSIAO1,

Shin-ichi INOUE2, Peng-Wei CHU1,3, Yoshihito KAWAMURA2

Presenting author: Peng-Wei CHU

Email: pengweichu@mx.nthu.edu.tw

1Department of Engineering and System Science, National Tsing Hua University, Hsinchu, Taiwan

2Magnesium Research Center, Kumamoto University, Kumamoto, Japan

3Center for Nanotechnology, Materials Science, and Microsystems, National Tsing Hua University, Hsinchu, Taiwan

Keywords: magnesium alloys, long-period stacking-ordered (LPSO) phase, forming processes, surface corrosion film, scanning electron microscope/focused ion beam (SEM/FIB), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS)

Owing to their low density and high specific strength, magnesium (Mg) alloys are promising lightweight materials for various applications, particularly in the automotive and aerospace industries. The Magnesium Research Center (MRC) at Kumamoto University has developed a series of Mg alloys containing the long-period stacking-ordered (LPSO) phase, significantly enhancing their mechanical properties. In this study, we focused on Mg-2.05 at%Y-0.85 at%Zn-0.1 at%Yb-0.15 at%Al alloys containing LPSO phase fabricated by three different forming processes (cast, ingot metallurgy (IM) followed by extrusion, and rapidly solidified (RS) powder metallurgy followed by extrusion) to investigate the microstructure of the surface corrosion film formed on the alloy surface after immersion in 3.5 wt% NaCl solution. Cross-sectional scanning electron microscope/focused ion beam (SEM/FIB) observations show that the LPSO phases in these alloys act as local cathodes during immersion tests, and the surface corrosion film formed on the LPSO phase is thinner than that on the α-Mg matrix. By cross-sectional transmission electron microscope (TEM) analysis, surface corrosion films formed on both the LPSO phase and the α-Mg matrix are mainly composed of magnesium hydroxide (Mg(OH)2) with Y incorporation and Zn enrichment at the surface corrosion film/alloy interface. Both Y incorporation and Zn interfacial enrichment are more significant in the surface corrosion film on the Y- and Zn-rich LPSO phase. X-ray photoelectron spectroscopy (XPS) revealed the depth distribution of Y and Zn, in which Y is mainly in the form of yttrium hydroxide (Y(OH)3) and yttrium oxide (Y2O3). The relationships between the surface corrosion microstructure, the alloy corrosion behavior, and the underlying LPSO phase size and distribution will also be briefly discussed.

10. Prof. Te-Cheng Su

Associate Professor

Research Topics:

a. Solidification of light metals

b. Calculation of phase diagrams

c. Semi-solid rheology

d. Microstructural characterizations

Solidification microstructure analysis and heat-treatment evolution of injection-molded AZ91 and ultralight LAZ-Ca magnesium alloys and their SiC composites

Te-Cheng SU1, Ming-Hung WU1, Si-Yuan HU1, I-An CHEN1, Hao-Chuan HUANG1,

Kai-Yu LIANG1, Hsiu-Chien CHAN2, Yen-Fang SONG2, and Gung-Chian YIN2

Presenting author: Te-Cheng SU

Email: tcterrysu@ntu.edu.tw

1Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan

2National Synchrotron Radiation Research Center, Hsinchu, Taiwan

Keywords: ultralight magnesium alloys, heat treatment, nanoscale precipitates, CALPHAD, synchrotron X-ray tomography

The development of lightweight structural components has driven significant interest in advanced magnesium alloys and their composites. This presentation explores the microstructural evolution of injection-molded AZ91 and novel ultralight LAZ-Ca (Mg-Li-Al-Zn-Ca) magnesium alloys, along with their SiC particle (SiCp) reinforced composites, during solidification and subsequent heat treatments. By integrating CALPHAD (Calculation of Phase Diagrams) thermodynamics with multiscale characterization (SEM, Cs-corrected STEM, and XRD), we systematically analyze phase stability and the evolution of key precipitates under tailored heat-treatment schedules. A major focus is placed on the identification and strengthening mechanisms of nanoscale Al-Li-bearing precipitates, specifically MgLi2Al and AlLi, within the LAZ-Ca system. Our experimental results confirm that these nanoscale precipitates act as the primary strengthening phases during artificial aging. Furthermore, they work synergistically with thermally stable (Mg,Al)2Ca at grain boundaries, which effectively resists abnormal grain growth and maintains a refined microstructure. Additionally, a key highlight of our methodology is the application of three-dimensional (3D) synchrotron X-ray tomography. We successfully utilize this non-destructive technique to visualize, segment, and quantify the spatial distribution of internal porosity and Al-Mn intermetallic compounds (IMCs) within the alloy matrix, while also addressing the ongoing experimental challenges of enhancing phase contrast for other microstructural constituents. The fundamental insights gained from this solidification and heat-treatment analysis provide a robust baseline for alloy design. Most importantly, this combined computational and advanced imaging methodology establishes a solid foundation for our upcoming 2026 ILM joint research project, where we will extend these heat-treatment design principles to optimize thermomechanical processing and precipitation-tuned strengthening in advanced wrought Mg alloys.

11. Prof. Shin-ichi Inoue

Associate Professor

Research Topics:

a. Characteristics and analysis of non-flammable Mg alloys

b. High-temperature oxidation behavior

c. Rapid solidification

d. Biomedical magnesium alloys

Effect of strain aging on mechanical properties of

MF type Mg–Zn–Y alloys

Shin-ichi INOUE1 and Yoshihito KAWAMURA1

Presenting author: Shin-ichi INOUE

Email: shinoue7@kumamoto-u.ac.jp

1Magnesium Research Center, Kumamoto University, Kumamoto, Japan

Keywords: magnesium alloys, strain aging, kink, mechanical properties

Mg alloys are the lightest practical metals and are expected to be used as structural materials in aircraft, bullet trains, and automotive applications. However, magnesium alloys have the problem of low yield strength. In recent years, extruded Mg–Zn–Y alloys with sparsely dispersed cluster arranged layer (CAL) and cluster arranged nanoplate (CANaP) have attracted attention due to their high strength. This hard/soft layered structure is called a mille-feuille (MF) structure. Kink deformation occurs in Mg–Zn–Y alloys dispersed CAL and CANaP through plastic working, resulting in high strength. The Mg–0.4Zn–1Y alloy, which has an MF structure consisting of a-Mg/CANAP produced by the rapid solidification process, exhibited a high yield strength of 360–414 MPa, despite the small amount of additional elements. Additionally, an MF type Mg–0.4Zn–1Y alloy was attempted to be produced using the casting method. The extruded MF type Mg–0.4Zn–1Y alloy exhibited a good tensile yield strength of 310–320 MPa. However, the Mg–0.4Zn–1Y alloy with MF structure, made using the cast process, has a lower CANaP dispersion, compared with the MF type Mg–0.4Zn–1Y alloy produced using rapid solidification. In addition, extruded Mg–0.4Zn–1Y alloy with MF structure has a lower density of kink boundaries and exhibits lower mechanical properties. To improve the mechanical properties of MF-type Mg–Zn–Y alloy, it is important to increase the CANaP dispersion. CANaP is a concentration of L12 clusters on stacking faults. Therefore, we considered that it would be possible to increase the amount of CANaP precipitation by introducing strain into the pre-ageing structure to increase dislocations and stacking faults. In this study, to enhance the mechanical properties of Mg–Zn–Y cast alloys with an MF structure, we investigated the effects of pre-compression on the formation of an MF structure and its mechanical properties.

In the aged Mg–0.4Zn–1Y alloy with a pre-compression ratio of 0%, the CANaP dispersion was 26 mm-1. The CANaP dispersion of the aged Mg–0.4Zn–1Y with 10% pre-compression was approximately 46 mm-1, indicating that CANaP was densely formed. This suggests that applying pre-compression before aging treatment is effective in improving the CANaP dispersion. The yield strength of extruded Mg–0.4Zn–1Y alloy with 0% pre-compression was 332 MPa. On the other hand, the yield strength of the extruded alloy with 10% pre-compression was 358 MPa. This suggests that the improvement in CANaP dispersion due to pre-compression resulted in an enhancement of the kink dispersion in the extruded alloy.

12. Prof. Shao-Pu Tsai

Assistant Professor

Research Topics:

a. Metal additive manufacturing

b. 3D EBSD

c. Microstructural Characterizations

d. Titanium alloys

High-temperature mechanical properties of sustainable 8xxx-series aluminum alloys made by laser-powder bed fusion

Shao-Pu TSAI1, Ji-Yun WANG1, 2

Presenting author: Shao-Pu TSAI

Email: shaoputsai@ntu.edu.tw

1Department of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan

2Max Planck Institute for sustainable materials, Düsseldorf, Germany

Keywords: sustainable metallurgy; 8xxx-series aluminum alloys; laser-powder bed fusion; mechanical properties

Sustainable 8xxx-series aluminum alloys were fabricated by laser-powder bed fusion (LPBF), and the alloys exhibited an iron tolerance of 4.58 wt.%, above which hot cracking occurred extensively. Uniaxial tensile tests were performed at room temperature (RT), 300 °C, and 400 °C, applying a strain rate of 10-4/s, after a 30 min soaking at each temperature to ensure temperature stabilization and possible microstructural variations. Mechanical results show that the best combination of strength and elongation takes place at 300 °C. Reducing the test temperature increased strength but decreased ductility; increasing deformation temperature decreased both strength and ductility at the same time. To rationalize this non-monotonic mechanical behaviour, detailed microstructural characterization was carried out. Electron backscatter diffraction (EBSD) data revealed that dynamic recrystallizataion (DRX) took place at both 300 °C, and 400 °C, with more extensive DRX at 400 °C. Since DRX typically enhances ductility, EBSD alone could not fully explain the inferior ductility at 400 °C. To gain insights from higher resolution, transmission electron microscopy (TEM) was employed to probe dislocatioin configuration, quantify dislocation density calculation, and assess possible precipitation states. On average, the as-built sample has dislocations quite randomly distributed, and the dislocation density is 2.1 × 1013/m2. Straining at RT, 300 °C, and 400 °C led to increased dislocation densities of 7.8 × 1013/m2, 1.1 × 1014/m2, 9.4 × 1013/m2, respectively. The peak at 300 °C followed by a decrease at 400 °C was conjectured to result from severe DRX at 400 °C. Moreover, deformation led to heterogeneous dislocation distribution, with locally dislocation-rich and dislocation-lean cellular regions. The precipitation behaviour were in principle similar to those observed in our previous in-situ heating TEM experiments: At 300 °C, only cellular structures with a distribution of cellular sizes were observed. 400 °C gave rise to decomposed cells and formation of two types of precipitates, i.e., Al12(Fe, Mo)3Si and D023 Al3Zr. The Al3Zr precipitates were frequently located at grain boundaries and caused precipitation-free zones (PFZs). The PFZs, together with Zr-enriched precipitation (~ 100 nm) are proposed to promote boundary decohesion and thereby facilitates fracture at 400 °C, accounting for degradation in tensile performance at the highest tensile testing temperature, i.e., 400 °C.

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