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.