Interactions of Phase Transformations and Plasticity: Session 3
Program Organizers: Valery Levitas, Iowa State University

Wednesday 8:00 AM
July 12, 2017
Room: Water Tower
Location: Hyatt Regency Chicago

Session Chair: Martin Wagner, Technische Universitaet Chemnitz

8:00 AM  Invited
On Variants, Blocks and Packets of Martensite and Their Interaction with Crystal Plasticity: Thomas Antretter1; Manuel Petersmann1; Georges Cailletaud2; Ulrich Ehlenbröker3; Annika Vieweg4; 1Montanuniversitaet Leoben; 2Mines ParisTech; 3Paderborn University; 4Materials Center Leoben
    The mechanical behavior of martensitically transforming steels is a direct result of the evolution and arrangement of martensite variants inside a grain interacting with the mechanisms of crystal plasticity. This work elucidates the details of variant arrangements in blocks and packets based on crystallographic considerations. On that level, i.e. the grain level, slip systems can also be resolved. A micromechanical model is presented that includes variables such as the fractions of individual variants as well as the shear rate of individual slip systems. Open parameters of the model are identified by means of EBSD measurements carried out on specimens having experienced different preceding loading histories.

8:40 AM  
Phase-field Simulation of Size and Morphology of Lath Martensite in Steels: Yuhki Tsukada1; Emi Harata1; Toshiyuki Koyama1; Yoshinori Murata1; 1Nagoya University
    Effects of temperature and yield stress on the size and morphology of martensite in Fe-0.1mass%C alloy at 600~700 K are simulated by the phase-filed method. The fcc-bct transformation strain (Bain distortion) and slip deformation in both austenite and martensite phases are considered. The slip deformation is calculated in the region where the von Mises yield criterion is exceeded; the value of yield stress is varied in the range of 0~500 MPa. The plastic strain caused by the slip deformation in the austenite phase is assumed to be inherited by the martensite phase when the transformation progresses. Simulation results show that the Bain variant size decreases with increasing the magnitude of yield stress at 600 and 650 K. However, the Bain variant size is not influenced by the magnitude of yield stress at 700 K. The (111) habit plane clearly appears when the yield stress is high at 600 K.

9:00 AM  
On the Interaction of Dislocations and Twin Boundaries during Stress Relaxation in NiTi Martensites: Martin Wagner1; Cagatay Elibol1; 1Technische Universitaet Chemnitz
    Despite its practical importance for actuator applications, relaxation in martensitic NiTi shape memory alloys is not well understood. In this contribution, we investigate the relaxation behavior of martensitic NiTi wires under uniaxial tension. Our experimental results show that the amount of relaxation depends on the total strain as well as, surprisingly, on the strain rate during prior loading. We demonstrate that two different deformation mechanisms act prior to, and during, relaxation: stress-induced motion of twin boundaries and thermally-activated dislocation motion, respectively. Our experimental observations, including the effect of prior strain rate, can be rationalized in a simple microstructural scenario that considers the interaction of dislocations and twin boundaries. This interpretation is confirmed by an analysis of Haasen plots, clearly showing that thermally activated processes dominate the material behavior during relaxation, and that detwinning during prior loading leads to continuous microstructural changes that directly affect the mean free path of dislocations.

9:40 AM  
Ruders-like Elongation Associated with Martensitic Transformation in Steels: MASAAKI SUGIYAMA1; Teppei Uesugi1; Kengo Hata2; Tomoyuki Kakeshita1; 1Osaka University; 2Nippon Steel & Sumitomo Metal Corporation
    The Ruders-like deformation behavior appearing in the stress-strain curves has been reported in several kinds of materials such as high-Mn alloys, 9Cr-8Ni based steels, and others. The characteristic behavior is corresponding to the stress induced martensitic transformation at the yielding stress point. In order to clarify the non-homogeneous propagation behavior of band contrast regions, the microstructure change in Fe-0.1C-2Mn-9Cr-8Ni based steels has been studied using in-situ SEM observation and corresponding EBSD and ECCI techniques, and TEM analysis. A forward austenite comes stabilized by the stress induced martensite with lath microstructure under elongation, and successive martensitic transformation occurs repeatedly in front, results in the formation of superimposed band contrasts in the previous deformation regions. The whole feature of the Ruders-like deformation and following work hardening behaior has been discussed with the stress induced martensitic transformation.

10:00 AM Break