Phase Transformations and Microstructural Evolution: Poster Session
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Phase Transformations Committee
Program Organizers: Gregory Thompson, University of Alabama; Rajarshi Banerjee, University of North Texas; Sudarsanam Babu, The University of Tennessee, Knoxville; Deep Choudhuri, University of North Texas; Raju Ramanujan, Nanyang Technological University; Monica Kapoor, National Energy Technology Lab

Monday 6:00 PM
February 27, 2017
Room: Hall B1
Location: San Diego Convention Ctr

F-65: Effects of Microstructural Features on CTOD in Coarse-grained and Inter-critically-heated HAZs of Mn- and Ni-added HSLA Steels: Seok Gyu Lee1; Dong Ho Lee1; Seok Su Sohn1; Woo Gyeom Kim2; Kyung-Keun Um2; Sunghak Lee1; 1POSTECH; 2POSCO
    Reliable microscopic analysis methods were achieved for identification and quantitation of microstructures existed in heat affected zones(HAZs) of high-strength low alloy(HSLA) steels. Effects of microstructural features on critical crack tip opening displacement(CTOD) values were investigated in coarse-grained HAZ(CGHAZ) and inter-critically heated HAZ(ICHAZ). According to quantitative analysis, critical CTOD values were appropriate at about 0.2mm in Mn-added steel. In Ni-added steel, Ni promoted the formation of acicular ferrite in the CGHAZ, while it prevented the formation of granular bainite, and thus the addition of Ni resulted in high critical CTOD value of 0.30mm. However, it reduced the critical CTOD to 0.22 mm in the ICHAZ because Ni promoted the formation of martensite-austenite constituent(MA). In view of critical CTOD, the Ni-added steel was better overall than the Mn-added steel, which indicated beneficial effects of Ni addition, but the careful control of Ni addition was needed for restraining the formation of MA.

F-66: Relationship between Reverse Ferrite Transformation and Recrystallization in Low-carbon Al-containing Steels: Shih-Che Chen1; Yuan-Tsung Wang2; Chun-Te Wu1; Hung-Wei Yen1; 1National Taiwan University; 2China Steel Corporation
    In this work, the microstructural evolution after dynamic ferrite transformation at high temperature was investigated by using dynamic dilatometer and electron backscattering diffraction in Al-containing steels. It was found that the occurrence of reverse ferrite transformation is followed by recrystallization of austenite in the studied alloy. It is proposed that reverse ferrite transformation leads strain-free austenite, which serves as initiation site for austenite recrystallization. The current work elucidates the competition between transformation and recrystallization in strained austenite at high temperature.

F-67: Solidification Microstructures in Ag3Sn-Cu3Sn Pseudo-Binary Alloys: Haibo Yu1; Yu Sun1; S. Pamir Alpay1; Mark Aindow1; 1University of Connecticut
    Both the intermetallic phases θ-Ag3Sn and ε1-Cu3Sn play an important role in microelectronic applications utilizing lead-free solders, where they can be present as minority strengthening phases in the solder alloys and can form as reaction products at solder/substrate interfaces. Studying the properties of these phases is challenging because the phase equilibria make the production of good quality single-phase samples very difficult. Here we present a study on the solidification microstructures in three alloys with Ag3Sn-Cu3Sn pseudo-binary compositions to evaluate the extent to which the complex phase equilibria can be avoided. The solidification microstructures have been studied using a combination of X-ray diffraction, electron microscopy, differential scanning calorimetry, and quenching experiments. These data indicate that transformations for these alloys upon heating are consistent with the published ternary phase diagram, however, the solidification microstructures are different from what one would expect on this basis. The consequences of these observations will be discussed.

F-68: Morphology of Order-disorder Structures in Rapidly Solidified L12 Intermetallics: Nafisul Haque1; 1University of Leeds
    Utilization of intermetallics in high temperature applications is limited due to their poor room temperature ductility. One route to overcoming this is disorder trapping (and subsequent anti-phase domain formation) during rapid solidification, motivating the study of disorder trapping in intermetallics. The single-phase, L12 intermetallic β-Ni3Ge has been rapidly solidified via drop-tube processing. At low cooling rates (850 – 300 μm diameter particles, 700 – 2800 K/ s) the dominant solidification morphology, revealed after etching, is that of isolated spherulites in an otherwise featureless matrix. Selected area diffraction analysis in the TEM reveals the spherulites to be partially disordered β-Ni3Ge, whilst the featureless matrix is the fully ordered variant of the same compound. Dark-field TEM imaging has confirmed that the spherulites grow as radially emanating fingers of the ordered phase, with disordered material in the space between the fingers.

F-69: Phase Transformation Kinetics of Fe16N2 Based Rare-earth-free Permanent Magnets: Md Mehedi1; Yanfeng Jiang1; Jian-Ping Wang1; 1University of Minnesota
    Rare-earths are expensive, the extraction process is energy intensive and hazardous for the environment and human health. Fe16N2 is regarded as one of the potential candidates of rare-earth-free, permanent magnets for its high magnetocrytalline anisotropy and high saturation magnetization.We are reporting the solid state phase transformation kinetics of Fe16N2 to understand the thermodynamic behavior of the permanent magnetic material. The iron raw material is prepared in foil shape and then nitrided at 250-700°C for 5-48 hours to obtain the iron nitride phases. Later the sample is quenched from 400-1000°C to obtain the martensitic crystal structure of Fe16N2. Finally we heated the sample from 100-400°C for 3-24 hours to study the decomposition of the Fe16N2 phase. The transformation kinetics are fitted using the Avrami equation of solid state phase transformation, and the activation energy is calculated based on the fitted result.

F-70: The Role of Grain Size Distribution in Nanocrystalline Shape Memory Alloys: Jakub Mikula1; Jerry Quek Siu Sin1; Shailendra P. Joshi2; David T. Wu1; Rajeev Ahluwalia1; 1A*Star; 2NUS
    We have carried out a detailed study on unimodal and bimodal grain size distributions in shape memory alloys (SMA) and their role on the mechanical properties (shape memory effect and pseudo-elastic behaviour). We show that the microstructure and mechanical properties of SMA can be controlled by the grain size distribution. Our focus is on SMA undergoing cubic to tetragonal transformations. The dynamics of the phase transformation was investigated using the phase field modelling within the framework of Ginzburg-Landau theory. For a bimodal distribution, while the typical twin structure is observed for large grains, smaller austenite grains tend to transform into a single variant of martensite or the transformation within them may become completely suppressed. This significantly alters the stress-strain curve.

F-71: W, Nb, and Cr Effects on High-temperature Tensile Properties in Heat-resistant Austenitic Cast Steels: Yong Hee Jo1; SeungMun Jung1; Seok Su Sohn1; Won-Mi Choi1; Byeong-Joo Lee1; Yong-Jun Oh2; Gi-Yong Kim3; Seongsik Jang3; Sunghak Lee1; 1Pohang University of Science and Technology; 2Hanbat National University; 3Key Yang Precision
    Heat-resistant austenitic cast steels have been actively developed because excellent high-temperature properties are required in automotive turbo-chargers for retaining their structures at extremely high exhaust gas temperatures. Three heat-resistant austenitic cast steels were fabricated by varying contents of W, Nb, and Cr, and their high-temperature tensile properties were interpreted by deformation and fracture mechanisms related with carbides and austenite matrix. According to high-temperature tensile test results, the Cr-added steel containing many M7C3 carbides showed the higher strength than the Nb-added steel containing many hard MC carbides. This was because the strength could be enhanced by distributing carbides having slightly lower hardness such as M7C3 carbides in the strengthened matrix as the strengthened matrix could hold hard carbides in it. Hard MC carbides reduced the hardness of austenite matrix, and the hardness gab between carbide and matrix promoted the microcracking at MC carbides and deterioration of high-temperature strength.

F-72: Controlling of Mechanical Properties on SUS310S Substrate Used at Superconducting Wire: Seung-gyu Kim1; Najung Kim1; Sung-gi Choi1; Oh-min Kwon1; Dongilk Kwon1; 1Seoul National University
    Most companies related to Superconductor ask for making cheaper products to make competitiveness in world market. So, the stainless steel get interests from researchers. But, Yield strength and tensile stress of superconductor with stainless substrate decrease sharply after processing. The mechanical properties of superconductor are mostly influenced by the substrate. For understanding this variation, we follow and check each processing steps of substrate using nano indentation testing and EBSD for micro-structure. We evaluate microstructures inside of substrates with nano-indentation from pre-process one to end-process one for comparison. As a result, we find that twins in the substrate after rolling go through recrystallization and recovery in high temperature process. Based on mechanical and microscopic analysis, we propose a method for controlling the major factor in lowering the mechanical properties of the substrate. Controlling the development of a hardened structures such as a twin in substrates can reduce the degree of re-crystallization.

F-73: Study on the High Temperature Phase Equilibrium Relationship in CaO-SiO2-10%La2O3-Nb2O5 System: Jiyu Qiu1; Chengjun Liu1; Zhaoyun Wang1; Junjie Shi1; Lifeng Sun1; 1School of Metallurgy, Northeastern University
    Phase diagram is widely used as the most intuitive thermodynamic tool, the partition of independent region is the decisive factor to determine equilibrium relationship for sub-solid phases. The comprehensive utilization of REE-Nb-Fe deposits is severly restricted due to the lack of thermodynamic research on the phase equilibrium relationships. In this paper, the independent regions within the specific composition range of CaO-SiO2-La2O3-Nb2O5 system were distinguished and the phase equilibrium relationship for 10%La2O3 section was determined using the slow cooling experiment and high temperature equilibrium experiment. It was found the independent triangle region contain SiO2, CaO·Nb2O5 and CaO·SiO2 in CaO-SiO2-Nb2O5 ternary phase diagram changed as an independent tetrahedral region which contain SiO2, CaO·Nb2O5, CaO·SiO2 and CaO-Nb2O5-La2O3 ternary compounds with the addition of 10% La2O3. Meanwhile, the low melting temperature region became larger and the eutectic line and eutectic point moved toward higher SiO2 composition after La2O3 addition.

F-74: Improved Electrochemical Discharge Kinetics of V-based BCC Metal Hydrides via Microstructure Reduction: Nicholas Weadock1; Heng Yang1; Hongjin Tan2; Brent Fultz1; 1California Institute of Technology; 2Liox
    Vanadium based BCC alloys are a promising high capacity anode material for metal hydride type batteries. Vanadium alloys containing titanium and nickel have been shown to unmix into a microstructure consisting of a majority V-rich region and a minority TiNi region. The TiNi rich region serves as an electrocatalyst to promote hydrogen evolution, which diffuses into the V-rich region to form a metal hydride. Typical alloy preparation via arc melting produces majority V-rich regions on the order of 10 microns within a matrix of TiNi. Reducing the length scale of the unmixing will improve the kinetics of the material by reducing the hydrogen diffusion length and increasing surface area for electrocatalysis. We developed quaternary TiVNiCr alloys produced via suction casting which have improved cycling kinetics over the traditional arc melted alloy. The mechanism for improved kinetics was investigated via SEM, electrochemical methods, and in-situ and ex-situ XRD.

F-75: Structure-Property Relations in Doped Ni-Mn-Ga Heusler Alloys for Magnetocaloric Applications: Michael McLeod1; Zafer Turgut2; Bhaskar Majumdar1; 1New Mexico Tech; 2Wright Patterson AFB
    We report here on our ongoing work in non-stoichiometric Heusler based Ni2MnGa alloys for solid state refrigeration close to room temperature. These alloys have demonstrated 'giant' magnetocaloric effects (MCE) as a result of magnetic field induced martensitic structural transformations. One of the drawbacks of these alloys is that the magnetostructural transformation occurs over a narrow range that is typically tens of degrees far from the required refrigeration temperature; and in addition, the MCE value can be low. In this work, we have attempted to address both of these deficiencies by systematically doping with Al, In, and B. The rationale and results from these dopant studies will be discussed. These alloys also exhibit significant MCE enhancement through constant stress thermal cycling. The role of texture in such thermal cycling induced MCE improvements will be discussed.

F-76: In-situ High Energy XRD Study of Optimal Annealing for a Novel Nb/NiTi Nanocomposite: Fangmin Guo1; Shijie Hao1; Lishan Cui1; Yang Ren1; 1China University of Petroleum (Beijing)
     Exploiting the superior mechanical properties of embedded nanowires in composites has proven to be challenging. Until recently, it is found that the ultra-large elastic strain of Nb nanowires can be exploited in a phase transforming NiTi matrix. We herein found that the elastic strain limit of embedded nanowires strongly depents on the heat treatment temperature of the Nb nanowires-NiTi composite. In this study, the effect of heat treatment temperature on the elastic strain limit of the nanowires embedded in the transforming NiTi matrix was studied by in-situ high energy XRD. It is found that the Nb nanowires gradually transformed into nanorods with increasing treatment temperature from 350℃ to 700℃, and accordingly the elastic strain limit of nanowires decreased from 6.1% to 2.6%. It is revealed the relationship between the elastic strain limit of nanowires and the treatment temperature of composite.

F-77: Relationship of Microstructural Evolution to Magnetic Properties of Alnico Magnets: Wei Tang1; Lin Zhou1; Andriy Palasyuk1; Kevin Dennis1; Jun Cui1; Matthew Kramer1; Iver Anderson1; 1Ames Lab of DOE
    Alnico is an attractive non-rare earth permanent magnet due to its excellent thermal stability and corrosion resistance. However, the best coercivity Hcj and maximum energy product (BH)max of commercially available alnico 8 magnets are only about 2000 Oe and 6.0 MGOe. A key to improving the magnetic properties is to enhance Hcj. It is generally recognized that Hcj depends on the shape anisotropy of ferromagnetic Fe-Co precipitates (α1-phase) that is finely dispersed in a non-magnetic Ni-Al-based matrix phase (α2-phase) by spinodal decomposition (SD) through appropriate heat treatments. The relationship of composition, microstructure and processing to magnetic properties was systematically studied and revealed. A Hcj of nearly 3000 Oe was achieved, which is very promising to greatly improve the magnetic properties of the magnets. Supported by the DOE-EERE-VT-EDT program under the DREaM project at the Ames Laboratory, operated for the U.S. DOE by Iowa State University under contract no. DE-AC02-07CH11358.

F-78: Microstructure Evolution in Martensitic NiTi Using High Energy Diffraction Microscopy: Ashley Bucsek1; Harshad Paranjape1; Branden Kappes1; Darren Dale2; Peter Ko2; Margaret Koker2; Aaron Stebner1; 1Colorado School of Mines; 2Cornell High Energy Synchrotron Source
    The remarkable properties of shape memory alloys (SMA), including superelasticity and shape memory effect, originate from a martensitic phase transformation and the evolution of the martensite microstructure in response to thermo-mechanical loading. While micromechanical theories for martensitic phase transformations have been developed since the mid-20th century, experiments to validate these theories on the microstructural scale are relatively new, challenging, and often limited to surface measurements. To address this open area, we utilize High-Energy Diffraction Microscopy (HEDM), a non-destructive 3D X-ray diffraction technique, to quantify microstructural evolution in martensitic NiTi. The results include martensite correspondent variant evolution during detwinning/reorientation and estimates of misorientation spread and lattice strain in the variants. These results can be used to inform and validate microstructural models of martensite deformation and potentially develop numerical schemes to estimate elastic properties of monoclinic martensite.

F-79: Phase Equilibria in the Al-Co-Ni Alloy System: Yang Zhou1; Philip Nash1; 1Illinois Institute of Technology
    The phase equilibria in the the Ni-rich region (<50 at%Al) of the Al-Co-Ni system were studied experimentally for three isothermal sections at 1100℃, 800℃and 600℃, as well as the liquidus projection. Metallography, energy dispersive spectroscopy, differential scanning calorimetry and X-Ray Diffraction were used for characterization and determination of the phases within the ternary system. Phase boundaries in the isothermal sections and eutectic line in the liquidus projection are modified compared to previous research. Comparison is made to the isothermal sections computed using Thermo-Calc and the TCNi6 database. Curie temperature and lattice parameters of γ, γ’ and β phases were determined from DSC/TG and X-Ray diffraction results.

F-81: Effect of Composition and Thermal Processing on Transformation Characteristics and Equilibrium Phase Stability in NiTiHf High Temperature Shape Memory Alloys: Tejas Umale1; Bradley Tomes1; Ibrahim Karaman1; Anjana Talapatra1; Raymundo Arroyave1; Ruben Santamarta2; 1Texas A&M University; 2Universitat de les Illes Balears, Palma de Mallorca, Spain
    Ni-Ti-Hf high temperature shape memory alloys are extensively investigated because of their relatively high transformation temperatures and excellent mechanical properties. Chemistry and thermal processing tend to govern the transformation characteristics through the formation of so-called H-phase nano-precipitates and thus provide control on shape memory response. In the present work, we aim at understanding the role of chemistry and thermal processing on the transformation characteristics of a wide range of compositions, varying nickel from Ni=49.8% to Ni=51% and hafnium from Hf=5% to Hf=30%. In particular, we’ve revealed the role of Ni content on the transformation temperatures at different Hf contents. We’ve also studied the phase stability in the NiTiHf system at various temperatures, by constructing equilibrium ternary phase diagram isotherms with the help of diffusion multiple experiments. Knowledge of equilibrium phases at different temperatures aids in understanding the H-phase precipitate chemistry more clearly, which is important for developing predictive precipitate models.

F-82: Application of ASTAR/PED Orientation Microscopy Technique in Grain Boundary Character Distribution of Nano-size Pure Zirconium: Iman Ghamarian1; Peyman Samimi1; Gregory Rohrer1; Peter Collins1; 1Iowa State University
    Recently, research efforts have directly incorporated the characteristics of the grain boundaries, including the five orientation parameters (three for the misorientation made by the grain boundary in Euler space and two for the normal of the grain boundary plane in spherical coordinate). In order to properly characterize these parameters, it is necessary to conduct orientation microscopy. Further, when the counting statistics of grain boundary character are sufficient (i.e., >100,000 grain boundaries), it allows for additional aspects (e.g., grain boundary energy) of the material to be understood. While some work has been conducted on coarse grained materials, little work has been conducted on ultrafine grained materials. In this research, grain boundary character distributions of nano-size pure zirconium deposited by PVD on a fused silica was studied using a TEM-based orientation imaging microscopy technique, ASTAR/precession electron diffraction, which enables orientation studies with a spatial resolution of a few nanometers.