Theory and Methods for Martensite Design: Session 3
Program Organizers: Greg Olson, Northwestern University; Ricardo Komai, QuesTek Innovations LLC

Wednesday 8:00 AM
July 12, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Greg Olson, Northwestern University

8:00 AM  Invited
An Information-driven Approach to the Design of Low Hysteresis Shape Memory Alloys: Turab Lookman1; 1Los Alamos National Laboratory
    There has been considerable interest over the last few years in accelerating the process of materials design and discovery. Finding new materials with targeted properties has largely been guided by intuition and trial and error, and with increasing complexity (e.g. chemical), the number of possibilities becomes exceedingly large for an Edisonian approach to be practical. The Materials Genome Initiative (MGI) has spurred considerable activity and brought new researchers into the nascent field of materials informatics. The activity has also highlighted some of the open questions in this emerging area, including identifying key features, guiding the next experiment to aid the learning process, and incorporating domain knowledge to make better predictions. I will show how an adaptive design strategy, tightly coupled to experiments, can guide the discovery of new compositions of NiTi-based alloys with very small dissipation.

8:40 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.

9:00 AM  
Dynamic Theory of Martensitic Transformations: Basic Ideas and Applications : Mikhail Kashchenko1; 1Ural Federal University, Ural State Forestry University
    A brief overview of the results of the dynamic theory of reconstructive martensitic transformations obtained over forty years of research is given. It is shown that a high degree of completeness of the description of features of formation of martensite crystals is achieved after the introduction of the notion of the initial excited state (IES). IES arises in the elastic field of dislocation nucleation centers (DNC). IES produces a control wave process which provides the cooperative nature of the transformation. The attention is focused on fresh results. On the theoretical side this is particularly true for the formation of a real transformation twins with fragmented structures. On the experimental side this is especially true for the design of experiments to initiate the formation of lamellar or rod-like martensite crystals by two or three sources of elastic waves, respectively.

9:20 AM  
Martensitic Transformation Terminated in "Gas-liquid"-type Critical Point: Volodymyr Chernenko1; Saurabh Kabra2; Elena Villa3; Victor Lvov4; Jose Manuel Barandiaran5; Hideki Hosoda6; 1University of Basque Country (UPV/EHU)&Ikerbasque; 2Rutherford Appleton Laboratory, ISIS; 3IENI-CNR; 4Institute of Magnetism, Kyiv; 5BCMaterials & University of the Basque Country (UPV/EHU); 6Tokyo Institute of Technology
    We found that the line of martensitic transformations (MTs) of Ni-Fe(Co)Ga is terminated in a "gas-liquid"-type critical point, CP(318K, 109MPa) of the stress–temperature phase diagram. Below CP a tensile superelastic deformation of 14% and pronounced hysteresis were obtained whereas above it an anhysteretic nonlinear deformation of similar value was observed. In-situ neutron diffraction of aforementioned single crystals during compression tests at different temperatures indicates a stress-induced MT with two-phase coexistence below CP, and continuous change of the lattice parameter from austenite to a post-critical state above CP. The Landau-type theory describes the existence of an end-point on the stress-temperature phase diagram of the first-order MT. It also predicts that CP can be experimentally observed for the alloys with low shear modulus ( a few GPa). The Fe-Pd alloy was found recently to be another example of this behavior, showing a CP at 280K and 40 MPa.

9:40 AM  
A Computational Approach to Design Martensitic Microstructure in Carbon Steels: Shengyen Li1; Steve Mates1; Mark Stoudt1; Carelyn Campbell1; Greta Lindwall1; Sindhura Gangireddy1; 1National Institute of Standards and Technology
    Martensite is a critical microstructural component in many materials, including many steels. Being able to tailor the desired volume fraction and stability of martensite for specific applications has been a long time goal. A materials design infrastructure integrating phase-based, mechanistic, and constitutive models and experimental data has been developed using a python-based framework. This generic materials design toolkit (GMDT) has been implemented to provide new insights into high speed machining applications where the workpiece experiences rapid heating and deformation, which produces unexpected microstructures and material behavior. This work uses the GMDT framework to combine dilatometry, scanning electron microscopy, microhardness, X-ray diffraction, and high-strain rate Kolsky bar compression test data with existing models (e.g. Olson-Cohen, constitutive plastic deformantion) and neural networks to predict the phase transformation behavior and properties. An example relating predicited phase transformations and stress-strain relationships to the experimental observations in rapidly heated and deformed 1045 steel is presented.

10:00 AM Break