Theory and Methods for Martensite Design: Poster Session
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Monday 5:30 PM
July 10, 2017
Room: Crystal Ballroom A
Location: Hyatt Regency Chicago

P1-47: Aging-induced Multistage R-phase Transformation in Ti50-XNi50+X(X=1-1.3) Shape Memory Alloys: BO-YI LI1; SHYI-KAAN WU1; 1National Taiwan University
    This study investigated the effects on the transformation behavior of aged Ti50-XNi50+X(X=1-1.3) shape memory alloys. It was found that aging in the temperature range of 523-773K induced multistage R-phase transformation, which was analyzed by means of differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). We have proposed experimentally a general rule that explains the effects of Ni concentration and the aging temperature and time on the microstructural changes and thus the multistage R-phase transformation sequences.

P1-48: An Informatics Approach to Design New Shape Memory Alloys: Dezhen Xue1; Xiangdong Ding1; Turab Lookman2; 1State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University; 2Los Alamos National Laboratory
    The martensitic transformation serves as the basis for applications of shape memory alloys (SMAs). The ability to make rapid and accurate predictions of the transformation temperature of SMAs is therefore of much practical importance. In this study, we demonstrate that a statistical learning approach using three features or material descriptors related to the chemical bonding and atomic radii of the elements in the alloys, provides a means to predict transformation temperatures. Together with an adaptive design framework, we show that iteratively learning and improving the statistical model can accelerate the search for SMAs with targeted transformation temperatures. The possible mechanisms underlying the dependence of the transformation temperature on these features is discussed based on a Landau-type phenomenological model.

P1-49: Designing for the Magnetocaloric Effect: A Computational Design Framework: Brian Blankenau1; Elif Ertekin1; Huseyin Sehitoglu1; 1University of Illinois
    The magnetocaloric effect (MCE) exhibited by certain Heusler alloys, due to the coupling of martensitic and magnetic phase transformations, provides a promising alternative to conventional refrigeration technologies. We propose a computational framework for designing Heusler alloys, optimized to exhibit the MCE. The framework, consisting of ab-inito calculations, Monte Carlo (MC) simulations, and property prediction stages, uses density functional theory to determine an alloy’s ground state structure, energy and magnetic configuration. Repeating the calculations for austenite and martensite phases across the alloy’s composition space provides sufficient information to parameterize a composition, phase and magnetic dependent Hamiltonian. We then use MC simulations to examine the alloy’s phase transformation behavior across its composition space. Finally, we use data from the simulation to predict the degree to which various alloy compositions exhibit MCE. Currently this framework is being used to analyze Ni2MnIn and Ni2FeGa however, we hope to extend it to additional materials.

P1-50: Different Cooling Rates and Their Effect on Morphology and Transformation Kinetics of Martensite: Annika Vieweg1; Gerald Ressel1; Marina Gruber1; Petri Prevedel1; Stefan Marsoner1; Reinhold Ebner1; 1Materials Center Leoben Forschung GmbH
     The behavior of martensitic transformation is strongly depending on the cooling rate applied to the material. For a quenched and tempered steel the martensitic transformation is occurring below 500°C, but in industry, cooling rates are defined between 800-500°C. The effects of different cooling rates in the lower temperature regime were not intensively investigated in the past.To this end, a 50CrMo4 steel is quenched within a dilatometer using different cooling rates below 500°C. Martensite start temperatures are evaluated and the martensitic structure is analyzed in regard to lath width and block sizes, using TEM and EBSD, respectively, as these features are essential for its properties and subsequent martensite design. Special interest is given to the position of carbon in the as-quenched structure, which is analyzed through 3D atom probe tomography. Conclusions are drawn to the mechanical properties and tempering kinetics of the martensitic structures in dependence on different cooling rates.

P1-51: In-situ SAXS Study on the Evolution of Nanodomains in 250ºC Early-aged Ti48.7Ni51.3 Strain Glass: Yung Chien Huang1; Cheng Si Tsao2; Shyi Kaan Wu3; 1Dept. of Materials Science and Engineering, National Taiwan University,; 2Nuclear Fuel and Materials Division, Institute of Nuclear Energy Research; 3Dept. of Materials Science and Engineering, National Taiwan University
    The evolution of nanostructure in Ti48.7Ni51.3 strain glass from the initial defect clusters to nanoprecipitates is investigated by in-situ small-angle X-ray scattering (SAXS) technique at 250ºC aging for 20 hrs. The in-situ SAXS study provides the details of phase transition, including temporary evolutions of nanodomains’ morphology and volume-fraction. The evolution of hardness vs. the aging time is also studied. According to the consistently concurrent evolutions of morphology, volume-fraction and hardness, the mechanism of nanodomains evolution during the isothermal aging can be divided into two regimes: (1) the nanodomains with the thickness slight change at the first 5 hrs aging, and (2) the nanodomains with the thickness/volume-fraction significant increase after 5 hrs aging. The study of frequency-dependent Tg temperature indicates the characteristic of strain glass vanishes gradually with increasing the aging time, in which the phase transition changes from the strain glass transition to the martensitic transformation.

P1-52: Martensitic Transformation Entropy Change in Metamagnetic Shape Memory Alloys: Eduard Cesari1; Joan Torrens-Serra1; Concepció Seguí1; Vicente Recarte2; José Ignacio Pérez-Landazábal2; Victor A. L'vov3; 1Universitat de les Illes Balears; 2Univ. Pública de Navarra, Institute for Advanced Materials; 3Taras Shevchenko National University
    Different aspects of the entropy change, ΔS, in the martensitic transformation (MT) of several Ni-Mn-based metamagnetic shape memory alloys are analyzed. Considering the main contributions to ΔS, namely lattice or vibrational, ΔSvib, and magnetic contributions, ΔSmag , it has been fully clarified that the main factor for the decrease of ΔS with (Tc,a - Tm ) is the temperature dependence of austenite magnetization. A phenomenological model has been proposed to describe the observed behavior in terms of the evolution of ΔSmag. Very recently the concept of magnetically driven MT has been introduced. Within this framework, the observed decrease of ΔS as the MT temperature is lowered as well as the arrest of the forward MT under strong magnetic fields is properly reproduced by the Landau formalism applied to metamagnetic SMAs. The comparison with the results obtained in ferromagnetic SMA like Ni-Mn-Ga and Ni-Fe-Ga will be considered.

P1-53: On the Asymmetry of the Forward and Reverse Martensitic Transformations in Shape Memory Alloys: Dezso Beke1; Melinda Bolgar1; Laszlo Toth1; Lajos Daroczi1; 1Department of Solid State Physics, University of Debrecen
    It is illustrated that the asymmetry of the DSC peaks and exponents of the probability densities of amplitude, α, or energy, ε, of acoustic noises during heating and cooling can be grouped into two groups: i) the relative changes of the exponents during cooling and heating (γε=(εhc)c as well as γα=(αhc)/αc)) are either positive or ii) negative. For positive γ values the number of hits and the total energy of acoustic emission, AE, is larger for cooling than heating, and the situation is just the reverse for negative asymmetry. Our interpretation is based on the different ways the relaxation of the elastic energy during cooling and heating. It is illustrated that if the relaxed fraction of the total elastic energy (which would be stored/relaxed without relaxations during heating as well as cooling) during heating is larger than the corresponding relaxed fraction during cooling than the asymmetry is positive.

P1-54: Phase Field Simulation of B2-R Transformation in a Porous NiTi Alloy: Changbo Ke1; Shanshan Cao1; Xin-Ping Zhang1; 1South China University of Technology
    B2-R transformation in a porous NiTi alloy is studied by means of phase field modeling which considers inhomogeneous elasticity and defines a smooth transition with elastic constants. Compressive stress inherited from sintering process is assumed to surround the pores which are treated with zero modulus. Effects of porosity (pore volume fraction) and in particular pore size on the transformation behavior are investigated. Simulation results show that the featured herring-bone microstructure with four R-phase variants meeting at <010>B2 can be reproduced, the twinning planes are determined as {1 0 1}B2 and {0 0 1}B2. The nucleation behavior and accommodation patterns of R-phase strongly depend on the pore size. Intensive study indicates that increasing the porosity with pore size smaller than 10 nm, the poly-twinned R-phase microstructure with long range spatial correlations can be broken to nano-sized domains with random spatial alignment.

P1-55: Stochastic Models for the Space-time Evolution of Martensitic Avalanches: pierluigi cesana1; Ben hambly2; John Ball2; 1Kyushu University; 2Oxford University
     In this talk we present a stochastic model for the description of martensitic microstructure as an avalanche process. Our approach to the analysis of the model is based on an associated general branching randomwalk process. Comparisons are reported for numerical and analytical solutions and experimental observations.

P1-56: Structures of Thermo-induced and Strain-induced Martensite in High Carbon Fe-Ni-C Alloy: Yulin Chen1; Dehai Ping2; Xinqing Zhao1; 1Beihang University; 2Dalian Jiaotong University
    It has been well accepted that the martensite formed in high carbon steels by quenching exhibits remarkable tetragonality because of the supersaturation of carbon in -Fe, and that the tetragonality is a consequence of carbon ordering onto the preferential octahedral sites of -Fe lattice. In this paper, two kinds of martensite were produced in high carbon Fe-Ni-C steel, i.e. the martensite by quenching and the martensite by strain deformation. It was found that the strain induced martensite exhibit no tetragonality, in contrast to the thermo-induced martensite with significant tetragonality. The origin of the abnormal nontetragonality of high carbon martensite in Fe-Ni-C steel was discussed according to the order-disorder transition of carbon atoms in octahedral interstices of bcc lattice. Clarifying the origin of abnormal tetragonality in high martensite is helpful to understand the martensitic transformation mechanism in carbon steels.

P1-57: Thermal Characterization of the Multistage Martensitic Transformation in TiNi: I. Ruiz-Larrea1; A. López-Echarri1; T. Breczewski1; G.A. López1; I. Lopez-Ferreño1; M.L. Nó1; J. San Juan1; 1Facultad de Ciencia y Tecnología, Universidad del País Vasco
    A polycrystalline TiNi shape memory alloy (49.7 at% Ti - 50.3 at% Ni) has been studied by precision DSC and adiabatic calorimetrimetry. Due to the thermal treatment, the sample presents a multistage martensitic transformation (MT) which is only stabilized after 75 thermal cycles. The kinetic of the MT as well as the relationships between the Cp-peaks in the forward and the reverse MT has been established by using a careful calculation of the peaks enthalpy. For this purpose, a previous determination of the lattice contribution to the specific heat was done. The progressive nucleation of the various martensitic phases throughout the forward transformation is found dependent on both the alloy composition and its thermal history. Finally, the temperature memory effect under partial MT has been characterized.

P1-58: XRD and EBSD Studies on the Microstructural Evolution and Crystallization Behavior of Amorphous Ti50Ni25Cu25 Shape Memory Ribbon: Chih-Hsuan Chen1; Hsin-Kai Wang2; Shyi-Kaan Wu2; 1Dept. of Mechanical Engineering, National Taiwan University; 2Dept. of Materials Science and Engineering, National Taiwan University
    This study aims at understanding the microstructural evolution and crystallization behavior of Ti50Ni25Cu25 shape memory ribbon, which is amorphous under as-spun condition. XRD and EBSD experiments were carried out after crystallizing and aging the ribbon at 500℃ for different heat-treating intervals. XRD results indicate that the ribbon shows different crystallization orientations at contact surface and free surface. The contact surface shows strong (200)B2 preferred diffraction but the free surface shows (110)B2 preferred diffraction, indicating that preferred texture had been introduced and established during the melt-spinning process. EBSD results on the contact side show no preferred crystal orientation, indicating the textured microstructure cannot be detected after chemically polishing (removal of about 1 μm) and only exists at the very near surface. Furthermore, the cross-sectional crystal orientation map of 15-min-treated ribbon shows nearly even grain size, about 290 nm, at both contact and free surfaces.