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  
A Coupled Modeling and Experimental Study of the Interaction Between Phase Transformation and Slip in Shape Memory Alloys: Peter Anderson1; Harshad Paranjape2; Sivom Manchiraju1; 1The Ohio State University; 2Colorado School of Mines
    Numerous experimental studies show that plastic deformation accompanies the austenite-martensite transformation in shape memory alloys (SMAs). This contributes to functional fatigue and structural fatigue, thus reducing material performance. This interaction between phase transformation and plasticity is explored using a unique phase field-crystal plasticity (PF-CP) modeling approach that predicts the evolution of martensite microstructure at the correspondence variant scale and slip activity on the austenite slip systems. The implementation utilizing finite element approach captures the large rotation and deformation of material elements and allows complex loading and boundary conditions to be imposed. We present three outcomes from a coupled study utilizing the PF-CP approach and experiments of NiTi micropillar compression. Distinct martensite microstructure forms in various compression cycles. Slip activity is localized in bands spatially corresponding with the martensite-martensite interfaces formed under stress. Different austenite slip systems dominate in distinct locations near the austenite-martensite interface.

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