Theory and Methods for Martensite Design: Session 4
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Wednesday 10:20 AM
July 12, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Dana Frankel, QuesTek Innovations LLC

10:20 AM  
On the Strong Dependence of Transformation Temperatures on Alloy Composition in Ni-Ti and Ni-Ti-Hf Shape Memory Alloys : Jan Frenzel1; Ingo Opahle1; André Wieczorek1; Burkhard Maass1; Ralf Drautz1; Gunther Eggeler1; 1Ruhr University Bochum
    It is well known that the martensite start temperature in NiTi-based shape memory alloys (SMAs) strongly depends on alloy composition. In the present study we take a new look on this phenomenon. Using differential scanning calorimetry and density functional theory simulations we show that the heat of transformation ΔH decreases as the Ni-concentration increases from 50.0 to 51.2 at.% for binary Ni-Ti. This causes a shift in the Gibbs free enthalpy difference of austenite Ga(T) and martensite Gm(T), which in turn results in a lower martensite start temperature. In the present work we provide precise reference data sets on the dependence of phase transformation temperatures for stoichiometric NiTi and Ni-rich Ni-Ti-Hf high temperature SMAs. We also discuss possible mechanisms how alloy chemistry governs phase transformation behavior in Ni-Ti-Hf high temperature SMAs.

10:40 AM  
Structural Stability of Ni2MnGa Modulated Martensites from First Principles: Martin Zeleny1; Ladislav Straka2; Alexei Sozinov3; Oleg Heczko2; 1NETME Centre, Faculty of Mechanical Engineering, Brno University of Technology; 2Institute of Physics, Academy of Sciences of the Czech Republic; 3Material Physics Laboratory, Lappeenranta University of Technology
    First-principles electronic structure calculations were used to explain structural changes in modulated martensites, which decrease their total energy below that of nonmodulated martensite (NM). Additionally to the 10M and 14M structures described by the nano-twinning concept as (3 -2)2 and (5 -2)2 structures, this study includes also the (2 -2)2 structure denoted as 4O. We show that the 4O exhibits the lowest total energy among all above-mentioned martensitic structures. Although it has not yet been observed experimentally in Ni-Mn-Ga, there are several reports on its existence in Ni-Mn-Sn alloys. All modulated structures are stabilized by the alternating shift of Mn and Ga atoms in the pairs of nanotwin boundaries separated by a single atomic plane. Consequently such nanostrucutres are the key and most favored building elements of all modulated Ni-Mn-Ga martensites. Moreover, they play significant role in shear-based transformation paths between different martensitic structures, which will be also briefly discussed.

11:00 AM  
The Influence of Order/Disorder Transitions on the Magneto-Thermo-Mechanical Coupling in Heusler-based FSMAs: Raymundo Arroyave1; Navdeep Singh2; Thien Duong1; Anjana Talapatra1; Ibrahim Karaman1; Peter Entel3; 1Texas A & M University; 2University of Houston; 3University of Duisburg-Essen
     In Heusler-based Ferromagnetic Shape Memory Alloys (FSMAs), the strong interactions between thermal, structural and magnetic DOF lead to correspondingly strong couplings between temperature/entropy, magnetic field/magnetization, and/or stress/strain thermodynamic conjugate pairs. Moreover, the degree of configurational order––e.g. L21/B2 order/disorder transition––can play a fundamental role in controlling the onset of the phase transformations responsible for the multifunctional behavior in these systems. Recently, for example, some of the current co-authors showed that the degree of order in Ni(Co)MnIn FSMAs can lead to dramatic changes in the phase transformation behavior of the system, leading even to the onset of glassy ferroic phase transitions.In this work, we present some examples in which DFT calculations are used to provide a better understanding of the nature of the couplings between thermally-excited degrees of freedom, including configurational (and compositional) effects.

11:20 AM  
Predicting Martensitic Start Temperatures of Ti-Ta High-temperature Shape Memory Alloys: A First-principles Investigation: Tanmoy Chakraborty1; Jutta Rogal1; Ralf Drautz1; 1Interdisciplinary Centre for Advanced Materials Simulation (ICAMS)
    In Ti-Ta-based high-temperature shape memory alloys (HTSMAs) the martensite start temperature and the stability of the shape memory effect are strongly composition dependent. For the design of new HTSMAs a detailed understanding of the composition dependence of the alloys properties is required. Here we use density functional theory in combination with the quasi-harmonic Debye model to evaluate free energy difference between austenite and martensite and estimate the transition temperature as a function of composition. We find that the relative phase stability at T = 0K and the difference in Debye temperature of martensite and austenite are the critical parameters that determine the composition dependence of Ms. From the analysis of our first-principles calculations we identify a one-dimensional descriptor for high-throughput screening of Ms for the design of new HTSMAs.

11:35 AM  
Effect of Internal Stresses on Elastic Deformation in B19’ NiTi Martensite Twins: Steffen Pfeiffer1; Martin Wagner1; 1Technische Universität Chemnitz
    B19’ NiTi martensites are heavily twinned. The anisotropic elastic properties of individ-ual variants determine the elastic deformation on several length scales. External loading generates additional internal stresses at twin boundaries because of the invariant plane condition. Based on recent ab initio results on the elastic constants of NiTi, we have developed a mechanical model on the micro-scale that fully accounts for the complex stress states in twins. Here, we study uniaxial loading of a type II twin, and we demonstrate that the additional internal stresses can be of the same order of magnitude as the external stresses, changing the direction-dependent macroscopic elastic modulus values by up to 26 %. We also discuss how internal stresses at twin boundaries affect detwinning during further uniaxial loading. Our approach allows for an in-depth analysis of microstructural deformation mechanisms and macroscopic properties of twinned NiTi martensites.

11:50 AM  
A Tool of Microstructure Analysis for Shape Memory Alloys: Nien-Ti Tsou1; Chih-Hsuan Chen2; 1National Chiao Tung University; 2National Taiwan University
    A powerful post-processing algorithm that can automatically identify the patterns, the present crystal variants, and the estimated stress distribution in the observation region is developed. The data needed is the microscopy image and the orientations of the crystallographic axes of the region, which can be obtained by scanning electron microscopy (SEM) and electron backscattering diffraction (EBSD). The tool consists: (1) The image-processing module, which can automatically identify the orientation and the distribution of the interfaces in the image data. (2) The database of the patterns of microstructure in SMAs, which stores all the possible compatible laminate patterns that may form in SMAs, generated by the compatibility theory. (3) The molecular structural mechanics model, which simulates the identified pattern with the equivalent truss. The stress distribution of the microstructure in the viewing plane can be instantly revealed.