Phase Transformations and Microstructural Evolution: Shape Memory Alloys, and General
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

Wednesday 8:30 AM
March 1, 2017
Room: 16B
Location: San Diego Convention Ctr

Session Chair: Deep Choudhuri, University of North Texas

8:30 AM  
Phase Transformations in NiTi Alloys under Biaxial Stress: Efthymios Polatidis1; Wei-Neng Hsu2; Steven Van Petegem1; Helena Van Swygenhoven2; 1Paul Scherrer Institute; 2Paul Scherrer Institute & EPFL
    The superelastic behavior, in NiTi alloys, originates from the reversible stress-induced austenite-to-martensite phase transformation. The majority of previous studies, on superelastic NiTi alloys, were conducted in either uniaxial tension/compression or torsion, however, during manufacturing processes and operational conditions these materials can undergo complex biaxial states or strain path changes. It is thus important to study the transformation behavior of these materials under biaxial loading or complex stain-path changes. In the present study, biaxial cruciform specimens were developed and deformed in-situ, employing a miniaturized biaxial tensile machine while investigated with synchrotron X-ray diffraction on the MS beamline at the Swiss Light Source, SLS. The obtained results are discussed with respect to the localization/homogenization of the phase transformation (as observed by Digital Image Correlation, DIC), the martensite variant selection and the evolution of the microstructure as a function of the applied strain and the applied load-paths.

8:50 AM  
The Effect of the Heat Treatment Temperature on the Thermodynamic Properties of the 55.89wt%Ni-Ti Shape Memory Alloy: Ben Fraj Boutheina1; Zoubeir Tourki1; 1Mechanical Laboratory of Sousse
    In this study, the relationship between the heat treatment temperature and the thermodynamic properties of the Ni rich NiTi shape memory alloy (SMA) was investigated. The characterization of the thermoelastic martensitic transformation was based on differential scanning calorimetry (DSC) experiments. It has been shown that the equilibrium temperature, enthalpy and entropy are strongly influenced by the variation of the heat treatment temperature. Moreover, the evolution of both elastic and dissipation energies as function of this parameter during the transformation process was discussed. The obtained results were largely in line with previously published studies.

9:10 AM  
Revealing Transformation and Deformation Mechanisms in Niti-based High Temperature Shape Memory Alloys through Microstructural Investigations: Lee Casalena1; Fan Yang1; Daniel Coughlin2; Glen Bigelow3; Darrell Gaydosh3; Santo Padula3; Othmane Benafan3; Ronald Noebe3; Peter Anderson1; Yunzhi Wang1; Michael Mills1; 1The Ohio State University; 2Los Alamos National Laboratory; 3NASA Glenn Research Center
    High temperature shape memory alloys such as NiTiHf and NiTiAu are of significant interest to the automotive and aerospace industries for potential light-weight solid-state actuator applications. However, a thorough understanding of the complex microstructural mechanisms occurring is needed to maximize their transformation temperatures and work output, while minimizing the detrimental cyclic instability leading to functional fatigue. NiTiHf can be tailored to achieve a favorable balance of properties, including high strength, stability, and work output at temperatures approaching 300C. Constant-force thermal cycling (CFTC) experiments performed on NiTiAu have demonstrated remarkable compositionally insensitive work output above 400C. The behavior in both systems is strongly influenced by the formation of secondary phases. Advanced scanning transmission electron microscopy (STEM) based characterization techniques are being used to explore and understand the unusual behavior seen in these alloy systems.

9:30 AM  
Microstructural Effects on Stress-Induced Martensite in NCAXB Alloys: Cheng Zhang1; Kenneth Vecchio1; 1Department of NanoEngineering and Materials Science and Engineering Program, University of California, San Diego
    To achieve superelasticity in Fe-based alloys, a certain combination of matrix strengthening, precipitation hardening, and microstructural texturing is required. CALPHAD methods are employed here to examine compositions in NCAXB-type ferrous alloys. In Fe-Ni-Co-Al-X-B (X= Ta, Cr, Nb, V, or W) superelastic alloys, the gamma prime ((Fe, Co, Ni)3(Al, X)) phase is available for precipitation strengthening, in addition to matrix solid solution strengthening; both of which are tracked in the CALPHAD modeling. The hardness of Fe-Ni-Co-Al-X-B alloys was measured after aging at different temperatures and times. Tensile tests show that the strength of the gamma matrix affects the stress-induced martensitic (SIM) transformation. XRD results shows that the value of tetragonality (c/a) also has an effect on the SIM transformation. The coherency relationship between gamma-prime particles and the gamma matrix is discussed in Fe-based superelastic alloys to elucidate its effect on SIM, and the Clausius-Clapeyron relationship in current alloys is evaluated.

9:50 AM  
Role of Granular Constraint and Surface Effects on the Phase Transformation Mechanics in Shape Memory Alloys: Harshad Paranjape1; Partha Paul2; Hemant Sharma3; Jun-sang Park3; Peter Kenesei3; Catherine Brinson2; Aaron Stebner1; 1Colorado School of Mines; 2Northwestern University; 3Argonne National Laboratory
    Microstructural and structural constraint influences the superelastic performance of polycrystalline Shape Memory Alloy (SMA) applications in two ways. The interaction between austenite grain neighbors creates a complex stress state and potentially a less efficient martensite microstructure. Second the grains in the specimen interior vs. surface inherit a disparate strain state during any training/pre-deformation procedures applied. We combine in-situ measurements of deformation and microstructure using 3D X-ray diffraction (3DXRD) based experiments with microstructural, anisotropic elastic simulations to quantify the relative contributions of the two influences in a NiTi-based SMA. The simulations, informed by microstructural data from the 3DXRD experiments, provide rich information on the intragranular stress state, driving forces for martensitic transformation and theoretical transformation strains. These results are key to the development of processing paths and optimal microstructures for high performing SMAs and this approach demonstrates 3DXRD experiments as a tool to inform microstructural models of phase transformation.

10:10 AM Break

10:30 AM  Invited
Characterization of Microstructural Evolution in a High Entropy Alloy with a Complex Nanoscale Microstructure: Jacob Jensen1; John Sosa1; Dan Huber1; Gopal Viswanathan1; Robert Williams1; Hamish Fraser1; 1The Ohio State University
    High-entropy alloys (HEAs) are a new class of materials garnering a great deal of attention due to their intriguing balance of properties, including high strength, ductility, and corrosion resistance. HEAs appear to offer new pathways to lightweighting in structural applications, however, to realize this potential requires considerable alloy development that will rely on ICME and a detailed knowledge of the microstructural evolution of these compositionally complex alloys. One such HEA, AlMo0.5NbTa0.5TiZr, was selected to be the basis for this characterization study due to its strength at elevated temperature, low density, and interesting nanoscale interpenetrating microstructure. HEA samples were vacuum arc melted followed by hot isostatic pressing and homogenization at 1400 C for 24 hrs with a furnace cool of 10 C/min in an inert gas atmosphere. Samples were quenched during the furnace cool and have been characterized to reveal the underlying precipitation mechanism in this HEA.

10:50 AM  
Tailoring the Microstructure of Intermetallic Films by Seed Layer Mediated Crystallization from an Amorphous Phase: Rohit Sarkar1; Jagannathan Rajagopalan1; 1Arizona State University
    Independently controlling the size, aspect ratio and distribution of grains in nanostructured films would enable us to tune their mechanical behavior for specific applications. We developed a novel technique to synthesize intermetallic films with tailored microstructures by altering the recrystallization process of an initially amorphous film. Using sputtering, thin, crystalline layers of Ti and Al were sandwiched between amorphous layers of TiNi and TiAl to act as preferential nucleation sites (seeds). The amorphous films were subsequently annealed in vacuum to obtain a crystalline microstructure. Transmission electron microscopy and x-ray diffraction revealed that the seeds reduced the crystallization temperature while simultaneously curtailing grain growth. The final microstructure of the seeded films remained in the nanocrystalline/ultrafine-grained regime whereas films without seeds formed large microcrystalline grains. Furthermore, by varying the seed layer spacing we were able to control the aspect ratio of grains across the thickness, as revealed by cross-sectional TEM analysis.

11:10 AM  
Unraveling the Growth Process of an Irregular Eutectic: Ashwin Shahani1; Xianghui Xiao2; Peter Voorhees1; 1Northwestern University; 2Argonne National Laboratory
    Eutectic systems are ubiquitous in nature, and have been discovered in a vast array or organic, metallic, and semi-metallic alloys. One of the most important eutectic growth processes from a technological standpoint involves a faceted phase: the Si phase in the Al-Si irregular eutectic. Unfortunately, the mechanism by which this eutectic grows is widely disputed. Here, we provide the long sought answer to this age-old conundrum. We have interrogated the real-time dynamics of irregular eutectic growth using reconstructions from four-dimensional (i.e., time and space resolved) X-ray tomography. Our results show that the eutectic growth process is markedly different from previously employed theories based upon "quench and look"-type experiments. In light of our experimental findings, we present a coherent growth model of irregular eutectic solidification.

11:30 AM  
A Combinatorial Assessment of AlxCrCuFeNi2 (0<x<1.5) High Entropy Alloys: Microstructure, Microhardness, and Magnetic Properties: Tushar Borkar1; Bharat Gwalani2; Deep Choudhuri2; Calvin Mikler2; Chris Yannetta2; Xi Chen3; Raju Ramanujan3; Mark Styles4; Mark Gibson4; Rajarshi Banerjee2; 1Cleveland State University; 2University of North Texas; 3Nanyang Technological University; 4CSIRO Manufacturing
    Laser additive manufacturing is becoming increasingly important in advanced manufacturing; current research efforts are focused on optimizing the parameters for processing mature alloys from powder feedstock to achieve properties, at least equivalent to, or better than, conventionally processed counterparts. Laser additive manufacturing also opens up a new horizon in terms of processing functionally graded materials that are difficult, if not impossible, to process using conventional techniques. This talk discusses a novel combinatorial approach for assessing composition-microstructure-microhardness-magnetic property relationships, using laser deposited compositionally graded AlxCrCuFeNi2 (0 < x < 1.5) high entropy alloys (HEAs). The composition gradient has been achieved from CrCuFeNi2 to Al1.5CrCuFeNi2 over a length of ~25 mm, deposited using the laser engineered net shaping process from a blend of elemental powders. Such graded alloys are highly attractive candidates for investigating the influence of systematic compositional changes on microstructural evolution and concurrent physical and mechanical properties in HEAs.