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2014 MSE Abstracts

The Effect Of Extrusion Temperature On The Microstructure, Mechanical Properties And Deformation Behavior Of Mg-1Mn-1Nd (wt%)

Authors: Ajith Chakkedath; Jan Bohlen; Sangbong Yi; Dietmar Letzig; Zhe Chen; Carl Boehlert; Maria Teresa Pérez-Prado; Javier Llorca

Abstract: An in-situ characterization technique combining mechanical testing inside a scanning electron microscope (SEM) with electron backscatter diffraction (EBSD) analysis was employed to study the tensile deformation behavior of Mg-1Mn-1Nd (wt%) extruded at two different temperatures (300°C and 275°C) over the temperature range of 50-250°C. Rare-earth additions to Mg alloys tend to reduce the strong basal texture exhibited by conventional wrought Mg alloys and this work was intended to study the effect of extrusion temperature on the deformation behavior and microstructure. EBSD was performed both before and after the deformation. A slip/twin trace analysis technique was used to identify the distribution of the deformation systems as a function of strain. Both materials showed superior high temperature strength retention compared to conventional alloys. Basal slip, prismatic slip, pyramidal <c+a> slip, and extension twinning were active at all temperatures, except for the material extruded at 300°C, in which case extension twinning was not observed at 250°C. The extent of twinning decreased with increasing temperature and basal slip was the major deformation mode at 150°C and 250°C. Basal slip was associated with high Schmid factors in all cases. Extension twinning was distributed over the entire Schmid factor range suggesting that extension twinning does not follow Schmid law. Based on the distribution of identified deformation modes and the texture, the estimated critical resolved shear stress (CRSS) ratio of extension twinning with respect to basal slip was less than 1, suggesting that the addition of Nd results in an increase in the CRSS of basal slip.

 

Fiber Diameter And Porosity Studies On Carbon Nanofiber Mats For Bioelectrodes

Authors: Duyen Do; Scott Calabrese Barton

Abstract: Carbon nanofiber mats (CNFM) having fiber diameters ranging from 2 nm to 8 μm have been investigated as electrode materials for enzymatic bioelectrodes. The morphology and porosity of CNFM were characterized by SEM and porosimetry, respectively. CNFM of desired fiber diameter were also produced by electrospinning from Polyacrylonitrile (PAN) precursor followed by thermal treatments (stabilization at 250oC and carbonization at 1000oC). Subsequently, glucose oxidase (GOx) and redox polymer mediator are immobilized on CNFM electrode by cross-linker to measure the electrochemical properties. By relating current density to CNFM thickness, fiber diameter and porosity, the optimum CNFM properties will be identified that boost the performance of bioelectrodes.

 

Simulation Of Bi-Crystal Grain Boundary Deformation In Commercially Pure Tantalum

Authors: Bret Dunlap; Philip Eisenlohr; Claudio Zambaldi; David Mercier; Yang Su; Thomas Bieler; Martin Crimp

Abstract: Nanoindentation was carried out near grain boundaries to investigate how strain transfer at the boundaries influences heterogeneous deformation in commercially pure polycrystalline tantalum. Misorientation of several grain boundaries was determined by electron backscattered diffraction. Inclinations were determined using backscattered electron contrast on focused ion beam milled cross-sections. Pile-up topographies that formed during indentation at or away from boundaries were measured by atomic force microscopy or confocal microscopy. The influence of boundaries on slip transfer was characterized through differences between topographies inside grains and close to boundaries. The parameters of a phenomenological constitutive description were determined in an iterative approach that improves the match of crystal plasticity finite element simulation of single crystal indents against corresponding experimentally measured topographies for different grain orientations. A graphical user interface that was developed allows automating the simulation of indentation at grain boundaries based on experimental parameters such as grain orientations, indent position, or boundary inclination. The ability of the simulations to accurately reproduce the effect of the grain boundaries has been assessed by carrying out a subtraction procedure with the simulated and experimentally measured pile-ups and comparing with correlated subtraction using experimental measurements. This work is supported by a grant from Sandia National Lab and by a Materials World Network grant (NSF DMR-0710570 and DFG EI681/2-1).

This work was supported in part by a grant from Sandia National Lab and by a Materials World Network grant (NSF DMR-0710570 and DFG EI681/2-1).


 

Effects Of Mg-Vacancy And Sb-Compensation On The Thermoelectric Properties Of Mg2.08-xSi0.364Sn0.6Sb0.036+x Materials

Authors: Peng Gao; Tim Hogan

Abstract: Mg2(Si,Sn) solid solution are promising thermoelectric materials due to the excellent thermoelectric properties. The constituent elements of the materials are abundant in earth and they are non-toxic. It is also of very low density which is favored in device fabrication. High ZT of ~1.5 has been found in Mg2.08Si0.364Sn0.6Sb0.036 compounds in our previous study. But composition analysis on the specimen showed that Mg was deficient on most samples, which indicating the formation of the Mg-vacancy. Compared with other reported high ZT results, more Sb dopant was needed in our work. It has been reported that the Mg-vacancy and Sb-compensation could lead to band structure engineering of the Mg2Si1-xSbx materials. Thus, it is worthwhile investigating the Mg-vacancy and Sb-compensation effects in the Mg2.08-xSi0.364Sn0.6Sb0.036+x materials, to better understand the origin of the high ZT in this material system.

This work was supported in part by DOE-EFRC

 

In-Situ Scanning Electron Microscopy Observations Of Tensile And Tensile-Creep Deformation Of A Ti-8Al-1Mo-1V (wt.%) Alloy.

Authors: Indraroop GhoshDastidar; Thomas Bieler; Martin Crimp; Adam Pilchak; Carl Boehlert

Abstract: The deformation behavior of titanium can be complex due the relatively low crystal symmetry of the HCP phase (compared with cubic metals). Here the deformation behavior of a Ti-8Al-1Mo-1V(wt.%) alloy was investigated during in-situ deformation inside a SEM. Tensile experiments were performed at room temperature (RT), 260oC and 455oC, while tensile-creep experiments were performed at 370oC and 455oC. EBSD was performed both before and after the deformation and slip trace analysis was used to identify the activation of the different slip modes as a function of temperature and the associated global stress state Schmid factors. The material exhibited a very weak fiber texture with the c-axis 30o off from the normal sheet direction. During the tensile tests extensive slip occurred. Prismatic slip made up the majority of the observed slip systems during the RT tensile test, while basal and prismatic slip were nearly equally active during the 455oC tensile test. Grain boundary sliding (GBS) and dislocation slip appeared to be the dominant deformation mechanisms during the creep deformation. Cracking occurred both during the tensile tests as well as during the tensile-creep tests, the source of cracking being the triple point junctions and the grain boundaries. The effect of alloy elements on tensile and tensile-creep deformation mechanisms will be discussed with respect to related experiments on other alloys.

This work was supported in part by US DOE/BES grant No. DE-FG02-09ER46637

   

Study Of Slip In High Purity Single Crystal Nb For Accelerator Cavities

Authors: Di Kang; Derek Baars; Thomas Bieler; Chris Compton

Abstract: Nb has been the material used for building accelerator cavities over the past couple decades, as it has the highest superconducting transition temperature in elemental metals. Crystal orientation dependent slip system activities affect the shape change of ingot slices during deep drawing, and form a dislocation substructure that affects subsequent recrystallization and ultimately, cavity performance. Two groups of single crystal tensile specimens with different orientations were extracted from a large grain ingot slice. The first group was deformed monotonically to 40% engineering strain. Analyses suggest that slip on {112} planes controlled the work hardening behavior. The second group was heat treated at 800 ºC for two hours, and then deformed incrementally to 40% engineering strain. The results indicate that the heat treated group had lower yield strengths, and in most cases, the rotations of crystals differed from corresponding specimens in the first group. Trace analyses reveal that {110} slip was favored for the heat treated group over {112} slip, which could be a result of the lower initial dislocation content. Slip traces were not observed at early deformation stages for some specimens, which could be due to homogeneous slip on {110} planes. Despite the small differences in the initial orientations and the potentially different operating slip systems, the stress-strain curves for corresponding specimens are quite similar. This variation in initial specimen states poses a challenge to interpreting and predicting the deformation behavior of Nb, in that initial dislocation density needs to be installed in a useful material model.

This work was supported in part by U.S. Department of Energy, Office of High Energy Physics, Grant No. DE-FG02-09ER41638

 

Modeling Lithium Ion Diffusion In The Fast-Ion Conducting Garnet Type Solid Electrolyte LLT Using Molecular Dynamics

Authors: Matthew Klenk; Yuxing Wang; Wei Lai

Abstract: Recent developments in the field of solid state secondary lithium ion batteries has yielded new compounds that show high room temperature ionic conductivity and good stability to lithium metal. One class of compounds of particular interest is the garnet series Li7-xLa3Zr2-xTaxO12. The preliminary results presented here are of molecular dynamic simulations for the X=2 (LLT) compound using the DL_POLY software. Employing randomized initial structures from energy minimization studies previously performed by the group, NPT and NVE simulations were carried out, mapping the lithium diffusion pathways in the crystal represented by isosurfaces. This work explores the understanding how the local environment of lithium plays a role in the diffusion of the ion. The results presented are to demonstrate the feasibility of our model with the expectation to apply it to other compounds, to better understand the effects of composition on the local environment of lithium. These simulations are to help supplement the impedance spectroscopy, X-ray diffraction, and neutron diffraction experiments that have been carried out in the literature to give an atomistic perspective on solid electrolyte conduction. The mechanism of a single lithium diffusion step has not yet been rigorously studied in the literature, and this study looks to investigate the jumping of a single lithium between neighboring 24d and 48g lattice positions.

This work was supported in part by the Ceramics Program of National Science Foundation

  

Engineering Ytterbium-Based Alloys For Space Based Cooling Applications

Authors: Gloria J. Lehr; Donald T. Morelli

Abstract: The United States Air Force uses infrared sensors on satellites to detect and track unique infrared signatures that occur around the globe originating from missile or rocket launches. Infrared signals are detected by sensors and in order to increase the signal to noise ratio, these sensors must be cooled to an optimal temperature, typically in the cryogenic range (< 123K, or -150 degrees C). Currently the Air Force utilizes mechanical devices to achieve the necessary cooling; however, these mechanical cryocoolers have several drawbacks including: limited lifetime due to moving parts, difficulty of production and integration, poor scalability, large size and mass, narrow range of temperatures, and finally the introduction of vibrations into the system. Many if not all of these disadvantages could be overcome by using a solid state cooling device such as a thermoelectric cooler. However, the current thermoelectric cooling technology is limited by the poor efficiency of the basic materials, and in particular, there are no know good thermoelectrics for cooling below 150 K. This work investigates the cooling potential of unique ytterbium-based materials and methods for increasing their efficiency for the Air Force’s space-based cooling applications.

This work was supported in part by the Air Force Office of Scientific Research under Multi-University Research Initiative (MURI) “Cryogenic Peltier Cooling” under contract number FA9950-10-10533.

  

Microscopic Phase-Field Simulation Of Precipitation And Atomic Site Preference In A Ni3(Al1-xFex) Alloy

Authors: Minjie Liang; Carl J. Boehlert

Abstract: The intermetallic compound Ni3Al is the main strengthening phase in Ni-based superalloys, which has attracted great interest in view of the increased yield stress that occurs with increased temperatures. However, the Ni3Al alloy is brittle. It has been found that the addition of ternary alloying elements can greatly improve the mechanical properties of Ni3Al. These properties can be affected by the presence of the precipitated phase. Therefore, it is important to investigate precipitation in the alloy. It is difficult to obtain this information with experimental methods, but computer simulations provide valuable information. In this research, a microscopic phase-field model is used to investigate the early precipitation events in a Ni3(Al1-xFex) alloy by simulating the atomic morphology, calculating long-range order parameters and concentration, and characterizing the site occupation of Ni, Al, Fe atoms in the gamma-prime ordered phase. The temporal evolution of the atomic site occupation and the atomic anti-sites behavior was revealed in detail with Fe content change. It was shown that such research could predict volume fraction of phase, precipitation mechanism, the size of phase, coarsening and atomic site preference. The above research results could provide information valuable for controlling and optimizing the heat treatment process, thereby improving the alloy making it more suitable for industrial production.

 

A Dynamic Hardening Rule For Crystal Plasticity With A Generalization To The Classical Hardening Rule For Single Crystal Nb

Authors: Aboozar Mapar; Thomas R. Bieler; Farhang Pourboghrat; Christopher C. Compton

Abstract: The classical empirical crystal plasticity hardening rule assumes that the increase in critical resolved shear stress on a slip system during deformation is directly proportional to the increase in shear strain on each of the other slip systems. Although this assumption works well for polycrystals, it cannot accurately predict the deformation of a single crystal, due to the ability of dislocations to escape from a free surface.

To address this issue, a dynamic hardening model is proposed which can increase the accuracy and numerical stability of crystal plasticity models. The classical hardening model is a special case of this model. A crystal plasticity model based on this dynamic hardening rule was calibrated for single crystal Niobium (Nb) and used to simulate the deformation of many tensile samples with different crystal orientations. Comparison to the experiments showed that the dynamic hardening rule considerably increases the accuracy of the crystal plasticity model.

This is; however, only one side of the story. To accurately predict the deformation of a crystal, one needs to consider the evolution of the texture. Comparison of texture evolution from experiments and simulation showed that the model can accurately predict the texture evolution for some crystal orientations.

This work was supported in part by the U.S. Department of Energy, Office of High Energy Physics, through Grant No. DE-S0004222.

 

Creep And Fatigue Analysis Of Friction Stir Welded Al 2139-T8 Alloy

Authors: Uchechi Okeke; Tomoko Sano; Jian Yu; Chian-Fong Yen; Carl Boehlert

Abstract: Aluminum alloys are commonly used for structural applications due to their high strength and low weight. Welding techniques are often applied to join two or more aluminum alloy plates together. The welding process introduces heat, plastic deformation, and chemical variation into the weld joints and modifies the microstructure, strength, and elongation-to-failure of the welded region. The Al 2xxx alloy series is difficult to weld using conventional methods, therefore friction stir welding is being studied. Samples studied were extracted from two plates of Al 2139-T8 alloys friction stir welded together. Fatigue and creep tests were performed on samples from the unwelded, or base metal (BM), region and the friction stir welded region (FSW). The results of the room temperature fatigue testsat 100, 150, 200, and 250 MPa indicated no significant differences in performance between the BM and the FSW regions. The creep test results at 250C and 300C at 25MPa and 50MPa reveal that the FSW region has significantly poorer creepresistance than the BM samples. Backscattered electron (BSE) images were taken of the microstructures of failed samples of both tests to try to understand the different fracture behaviors of the materials.

This work was supported in part by NSF, Army Research Laboratory

  

In-Situ Characterization Of Deformation Twinning In Pure Titanium

Authors: H. Phukan; L. Wang; C. Zhang; T.R. Bieler; A.J. Beaudoin; J.S. Park

Abstract: Deformation behavior in polycrystalline materials is heterogeneous in nature due to inherent anisotropy and interaction among the constituent grains. In hexagonal metals like titanium the local stress-strain response is strongly dependent upon the texture and c to a ratio. For ‘soft’ orientations, where the c-axis is almost normal to the direction of the applied load, first order prismatic slip is favored. On the other hand, for ‘hard’ orientations, where the c-axis is almost parallel to the loading direction, deformation twinning is often observed. The conditions that lead to the nucleation and evolution of such twins are not clearly understood. In the present work, a strongly textured specimen of commercial purity titanium is subjected to a few percent strain in a tensile test. 3D X-ray diffraction is used for in-situ characterization of type 1 (T1) extension twinning events. A total of eleven layers along the gage length of the specimen were evaluated. The local stress and center of mass positions of the grains are evaluated for each layer at every stress state. The slip transfer parameter is used to ascertain whether the T1 twins observed were nucleated as a result of slip transfer across a grain boundary (S+T twins) or twin induced shear transfer (T+T). This study is expected to facilitate the development of reliable predictive models for meso scale deformation behavior that include mechanical twinning.

This work was supported in part by Material World Network  NSF-DMR-1108211   DFG ZA 523/3-1, use of APS supported by DOE/BES

 

Multifunctional Nancomposite Foams For Space Applications

Authors: Diandra Rollins; Lawrence T. Drzal

Abstract: Materials combined with a small amount of nanofillers offer new possibilities in the synthesizing of multifunctional materials. One novel nanomaterial is graphene, which due to its hexagonal atomic structure has excellent mechanical, thermal, barrier, flammability reducing and electrical properties. A procedure developed at Michigan State University creates graphene nanoplatelets 1-5 layers thick which are a more robust form of graphene and can be produced at cost competitive prices compared to other additives and fillers. The addition of these nanoplatelets to a polymer foam offer improved mechanical, thermal and electrical properties, at an overall lower cost that allows the foam to maintain its unique cellular structure and low density. A foam material with such combination of properties has potential applications in space technology as the nanocomposite foam would have ranges of stiffness and resilience that are outside the limits of pure polymer foams, be flame resistant, demonstrate electrical and thermal conductivity and yet be both weight and cost effective space stable materials. This research is directed at understanding the physical and chemical challenges associated with embedding graphene nanoplatelets in the struts and cell walls of a polymer foam in order to achieve percolation. This relies on the creation of a good dispersion using both mechanical and chemical methods while varying concentration and the physical properties of the platelets. Investigating each of these effects will help to determine the best GnP selection and dispersion methods to create the optimal multifunctional nanocomposite foam for space applications.

This work was supported in part by NASA Space Technology Research Fellowship

  

Preparation Of Poly (Lactic Acid)/Polystyrene Bioblend Hollow Microparticles Embedded With Nanoparticles Via A One-Step Emulsion-Diffusion Method

Authors: Anna Song; Ilsoon Lee

Abstract: Poly (lactic acid) (PLA) is a highly potential drug delivery carrier because of its biodegradation and biocompatibility. However, the brittleness and high cost of PLA limit its application. PLA combined with polystyrene (PS) has been considered as a potential bioblend for biomedical applications. In this work, PLA and PS have been dissolved in ethyl acetate under heating, which is a good solvent for PLA but a non-good solvent for PS. This PLA/PS blending solution has then been mixed with a 1:1(v/v) water/glycerol system to form an oil-in-water emulsion, which is followed by the diffusion process to obtain the spherical particles. From the SEM result, hollow microparticles embedded with nanoparticles can be clearly observed. Electron energy loss spectroscopy (EELS) will be employed to define the composition of the dispersive nanoparticles and the continuous phase of the microparticles, respectively. This type of microparticles can be used to design a “two-stage” release system for drug delivery due to the two separated polymer phases and will be tested in the future work.

This work was supported in part by DOD SERDP, SPG

 

Quantifying Nanoindentation Deformation Processes Near Grain Boundaries In Alpha-Titanium Using Microscopic Characterization And Crystal Plasticity Modeling

Authors: Y. Su; C. Zambaldi; D. Mercier; P. Eisenlohr; T. R. Bieler; M.A. Crimp

Abstract: To understand the roles different grain boundaries play in plastic deformation of commercially pure Titanium, instrumented sphero-conical nanoindentations were placed at preselected grain boundaries where corresponding grain orientations were mapped by electron backscatter diffraction. The topographies of the nanoindents were measured using atomic force microscopy. The effects of grain boundary misorientation and boundary inclination (determined from focused ion beam sections) on indentation pile-ups were categorized by slip transmission parameters. These bi-crystal indentations were simulated using crystal plasticity finite element (CPFE) models to better understand the details of the mechanical response of the different slip systems. Indents in grain interiors were used to identify adjustable parameters in the material model using a single crystal optimization process to match simulated and experimental indentation topographies [Zambaldi et al. J. Mater. Res. 27, 356–367 (2012). ].

This work was supported in part by the NSF Materials World Network Grant DMR-1108211 and corresponding DFG grant ZA523/3-1.

  

Optimization Of The Seebeck Coefficient For Low Temperature Thermoelectric Use Of PtSb2 By Tellurium Doping

Authors: Spencer Waldrop; Donald Morelli

Abstract: As part of the ever changing energy demand landscape, thermoelectric materials have shown promise with their capacity to transform thermal energy into electrical energy. The properties of import to a thermoelectric material are its Seebeck coefficient, electrical conductivity, and thermal conductivity. As a generalization, the bulk of research on thermoelectric materials has been concerned with optimization at high temperatures. However, little work has been performed on low temperature thermoelectric materials where there are many venues of application such as in earth orbiting satellites.

An investigation of the material PtSb2 was performed to examine the effects of tellurium doping on the Seebeck coefficient. Stoichiometric amounts of platinum and antimony were reacted at 800 C for 4 days and subsequently sintered using Spark Plasma Sintering at 900 C and 60 MPa for 30 minutes. X-ray diffraction was performed before and after sintering to ensure a single phased sample was produced. It was seen that nominal PtSb2 has a positive Seebeck coefficient which peaks with a magnitude of 230 microV/K. The compositions PtSb2-xTex ; x = 0.0005, 0.001, 0.002, 0.005, 0.02, and 0.04 were examined. At all tellurium dopant concentrations the Seebeck coefficient was seen to be negative and readily dependent in magnitude on the concentration. The highest magnitude of Seebeck coefficient seen in these concentrations was found in PtSb2-xTex where x = 0.0005. These results inspire further investigation of this material to find at what concentration of tellurium the Seebeck coefficient will be fully maximized.

This work was supported in part by Air Force Office of Scientific Research under the Multi-University Research Initiative (MURI)

 

Impression Creep Behavior Of Cast Mg-10Gd-3Y-0.5Zr (wt.%) Alloy At Elevated Temperatures

Authors: Huan Wang; Qudong Wang; Carl J. Boehlert; Jie Yuan

Abstract: The impression creep behavior of a cast Mg-10Gd-3Y-0.5Zr (GW103) alloy was investigated by flat cylindrical indenter at temperatures ranging from 250 to 325 oC and stresses ranging from 80 to 505 MPa. The impression creep stress exponents varied from 1.36 to 5.10, which were lower at lower temperatures and stresses. The impression creep activation energies increased from 106.11 kJ/mol to 190.65 kJ/mol with increasing stress. Dislocation-controlled creep was suggested in high temperature and stress regime, while grain boundary sliding could contribute more at lower temperatures and stresses. The zone just beneath the indenter almost maintained the same during impression creep, while the zone inside cycle segments under the indenter deformed severely. The zone at the edge of the indenter underwent largest stress and strain, resulting in broken of grain boundaries and bending of most intragranular precipitates. Intergranular crack was also observed in this zone due to severe deformation. The impression creep data could be converted to be consistent with conventional tensile creep data of the same alloy in certain regions by conversion factors, indicating that impression creep testing is a valuable method to characterize creep behavior and localized deformation in Mg-RE alloy.

  

Reducing The Thermal Conductivity In Ge-Sb-Te Alloys Through The Incorporation Of Amorphous Ge2Sb2Se5 Particles

Authors: Jared Williams; Donald Morelli

Abstract: We currently live in a world where energy demands continue to escalate, and the way we currently meet those energy needs, via fossil fuels and nonrenewable resources, will very soon no longer be available. Much of today’s research in materials science, chemical engineering, and physics is focused on finding, and improving, alternative forms of energy generation. Cars, power plants, trains, boats, and even solar cells, emit heat as a waste product from their processes. Thermoelectric materials possess the unique ability to convert wasted heat from various thermodynamic processes into useful electrical current. If even a fraction of that heat could be captured and converted back into electrical current, a significant improvement in the efficiency would be achieved. The mechanisms which govern waste heat recovery are exhibited in all materials, but are especially high in a handful of semiconductors. My current research involves developing new thermoelectric materials, and engineering these materials for power generation. Ge4SbTe5 and its relatives, with equal numbers of atoms on the cation and anion sites, form stably in the cubic rocksalt structure. For thermoelectric applications a cubic compound is advantageous because there is no issue regarding anisotropy of the thermoelectric properties. This study investigated the feasibility of incorporating amorphous Ge2Sb2Se5 particles into the Ge4SbTe5 crystal matrix and their effects on the thermoelectric properties.

This work was supported in part by Department of Energy

 

Angle-Dependent Performance Of Thin-Film, Transparent Photovoltaics

Authors: Margaret Young; Yunhua Ding; Richard Lunt

Abstract: Understanding the angle dependent performance is an important consideration for building integrated photovoltaics (PVs), such as transparent PV windows, where illumination angles are rarely at normal incidence. While the transfer matrix model (TMM) has been widely utilized to model optical interference and quantum efficiency in thin-film PVs at normal incidence, self-consistent simulations for PVs under oblique illumination have not yet been demonstrated. We derive an updated model that is self-consistent for all angles, light polarizations, and electrical / optical configurations, and experimentally verify the predicted angular quantum efficiency response of planar heterojunction (PHJ) transparent PVs. We subsequently use this model to optimize PHJ transparent PVs for maximum short circuit photocurrent density (Jsc) and transparency as a function of the multivariable landscape under a variety of optical and electrical configurations, showing that it is possible to greatly reduce the angle-dependent roll-off in efficiency by moving in this multi-parameter space. We will provide insights into the lesson learned for designing devices that can reduce this roll-off and increase overall yearly power output.

This work was supported in part by National Science Foundation (Career Grant #: 1254662)

 

Study Of The Subsurface Slip Activity Of Polycrystalline Ti-5Al-2.5Sn Alloy With Crystal Plasticity Finite Element Method Using 3D Microstructure

Authors: Chen Zhang; Hongmei Li; Philip Eisenlohr; Thomas R. Bieler; Martin A. Crimp; Carl J. Boehlert

Abstract: The study of slip activity of polycrystalline material generally focuses on the sample surface since the subsurface slip activity is difficult to characterize using conventional experimental methods. However, the heterogeneous deformation process of polycrystalline material is three dimensional by nature, indicating that understanding the subsurface slip activity is crucial for the study of heterogeneous deformation of polycrystalline material. Computational models with a microstructure that is representative of the sample are generally used to provide insight about the slip activation and propagation during plastic deformation. In this study, a new method is used to simulate the deformation process of a Ti-5Al-2.5Sn sample deformed under uniaxial tension at room temperature using a Crystal Plasticity Finite Element (CPFE) model with realistic 3D microstructure based on Electron Backscatter Diffraction (EBSD) and Differential Aperture X-Ray Microscopy (DAXM) data. Schmid analysis and generalized m’ factor analysis were used to analyze the subsurface slip activity, using the local stress tensor and local accumulative shear from simulation. Correlation between experiment observation of the sample surface and simulation results was attempted to gain better understanding of the effect of subsurface slip activity on the deformation history visible on the surface of polycrystalline Titanium alloys. The DAXM characterization was conducted at beamline 34-ID-E, Advanced Photon Source at Argonne National Lab. Supported by DOE/BES grant DE-FG02-09ER46637.

This work was supported in part by DOE/BES grant DE-FG02-09ER46637

 

In-Situ HE-XRD Characterization Of Microstructure Evolution In SAC Solder Joints With Different Cooling And Thermal Cycling Conditions

Authors: Quan Zhou; Huili Xu; Choong-Un Kim; Thomas R. Bieler; Tae-Kyu Lee

Abstract: Lead freed solders have replaced the conventional leaded solder in the electronic packaging industry for several years, but there still exist unpredictable failures arising from the highly anisotropic properties of Sn-based solder joints. In our previous study, the effects of cooling rates on the microstructure and grain orientation evolution in lead-free solder joints in ball grid array packages was characterized using polarized light microscopy and electron backscattered diffraction (EBSD). This study shows that either higher or lower cooling rates than in common use will refine grains by promoting new grain generation or triggering mechanical twinning. The amount of microstructural refinement varied depending on the cooling rate, joint location, and the original crystal orientation(s). The observations of microstructure change depend on the general anisotropy of the joint arising from its initial crystal orientations. New grains and twins with different orientations weaken the joint anisotropy and new interfaces and orientations slow the damaging effects on electromigration. With the possibility to improve the mechanical properties by modifying the microstructures, cooling rate experiments were designed for in-situ measurements of a joint using high-energy X-Ray diffraction (HE XRD) at beamline 6-ID-D to investigate the microstructure evolution and its effect on mechanical properties. Microstructure evolution during the cool down following solidification was captured, and the related mechanical response during thermal cycling was further characterized. Structure-property relationships are discussed as well as fundamental analysis of grain refinement and mechanical twin formation. The findings will provide guidance for package design and process control in the electronic packaging industry.

This work was supported in part by NSF-GOALI Contract 1006656 and Cisco Systems Inc., San Jose, CA. Use of the Advanced Photon Source was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH1