Quasimartensitic Modulations: Session 5
Program Organizers: Avadh Saxena, Los Alamos National Laboratory

Friday 2:00 PM
July 14, 2017
Room: Wrigley
Location: Hyatt Regency Chicago

Session Chair: Jason Lashley, Los Alamos National Laboratory

2:00 PM  
Flexoelectricity and The Polarity of Complex Ferroelastic Twin Patterns: Ekhard Salje1; Suzhi Li1; Massimiliano Stengel2; Peter Gumbsch3; Xiangdong Ding1; 1State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University; 2Institut de Ciencia de Materials de Barcelona (ICMAB-CSIC); 3 Institute for Applied Materials, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany
    We study the interplay of ferroelastic twin patterns and electrical polarization. Our molecular dynamics simulations reproduce polarity in straight twin walls as observed experimentally. We show, by making contact with continuum theory, that the effect is governed by linear flexoelectricity. Complex twin patterns, with very high densities of kinks and/or junctions, produce winding structures in the dipolar field, which are reminiscent of polarization vortices. By means of a "cold shearing" technique, we produce patches with high vortex densities; these unexpectedly show a net macroscopic polarization even if neither the original sample nor the applied mechanical perturbation breaks inversion symmetry by itself. These results explain experimental observations of "parasitic" polarity in the paraelectric phase of BaTiO3 and LaAlO3.

2:20 PM  
Nanoscaled Martensitic Transition and Its Abnormal Properties in Shape Memory Alloys: Dong Wang1; Yunzhi Wang2; Xiaobing Ren3; 1Xi'an Jiaotong University; 2The Ohio State University; 3National Institute for Materials Science
    By impurity doping in shape memory alloys (SMAs), we propose a mechanism that leads to nanoscaled martensitic transformation accompanying with superelasticity with slim hysteresis across a wide temperature range. Computer simulations using the Landau theory of phase transformations and Khachaturyan’s microelasticity theory predict the formation of randomly distributed nanosized, singlevariant martensitic domains and a generic ‘‘phase diagram’’ including all the strain states in NiTi SMAs, which agrees well with experimental measurements. These nanoscale martensitic domains are frustrated and cannot evolve into long-range-ordered, internally twinned structures (i.e. long-range strain ordering). Such a structural state is found to evolve gradually upon loading and unloading or heating and cooling across a wide temperature range with narrow hysteresis. This continuous transition process shows lots of unique properties.

2:40 PM  
Multi-step Martensitic Transformation in TiNiHf High Temperature Shape Memory Alloy Powder Fabricated by Plasma Rotating Electrode Process: XiaoYang Yi1; XiangLong Meng1; ZhiYong Gao1; Wei Cai1; 1Harbin Institute of Technology
    The Ti-rich TiNiHf alloy powders with various particle size are fabricated successfully by the plasma rotating electrode process (PREP). The as-received alloy powders are mainly spherical or ellipsoidal ball with approximate 10~200Ám in size and exhibited cellular morphology. In addition, it varies from uniform equiaxed grains to heterogeneous dendrite with the increasing of particle size. Apart from the typical self-accommodation martensitic morphologies with spear-like, stripe-like and mosaic-like, it also shows bending lath-like martensite other than the previous straight lath-like martensite . At the same time, the as-received powders with various particle size show distinct transformation behaviors: two-step martensitic transformation occurs for alloy powders with φ45~75Ám in size; three-step martensitic transformation appears for alloy powders with φ75~90Ám, φ90~125Ám, and φ125~150Ám in size. However, single step martensitic transformation happens for powders with various particle size after solution treated at 900℃/1h

3:00 PM  
Glass Phase Boundary and Extraordinary Behaviors in Ferroelastic Systems: Zhijian Zhou1; Jian Cui1; Xiaobing Ren1; Yuanchao Ji1; 1Xi'an Jiaotong University
    Phase transition paths play a vital role on the mechanical and functional properties of martensitic material. In this work, strain glass state separating different phase transition paths is widely found in TiNi, TiNiCu, FeNiCoAlTa and other alloys. Here, strain glass phase boundary (GPB) acts as a bridge to obtain new phase transition path and associated properties by introducing proper defects (point defects, precipitates). Furthermore, extraordinary behaviors such as isothermal martensitic transformation, large linear elastic strain or magnetostrain were discovered in the critical region. External stress field gives arise to change GPB towards morphotropic phase boundary (MPB), which enhanced many properties in ferroelectric and ferromagnetic system. The competition between suppressing certain long-range order martensite and stabilizing another martensitic transition path should be responsible for these extraordinary behavior and properties around GPB. The finding of GPB may help to discover novel strategy to design new transforming material.

3:15 PM Break