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  
Space Group Consideration of Martensitic Transformation in Ferrous Alloy: Takuya Ohba1; 1Shimane University
    Space group consideration of martensitic transformation in ferrous alloy is discussed. Crystal structure of austenite is face centered cubic, whose space group is Fm3m (225) and that of martensite is BCT structure, whose space group is P42/n (86). Number of symmetry operations of Fm3m is 192 and that of P42/n is 8. Therefore, 24 variants appear. Group-subgroup consideration is applied; that is, (1) removing translational symmetry and two-fold axis at the first step, (2) removing three-fold axis, and (3) removing two-fold axis again.

2:40 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.

3:00 PM  
Sandwich-like Strain Glass Phase Diagram of Ti49Ni51−xPdx: Shuai Ren1; Dezhen Xue1; Kazuhiro Otsuka2; Xiaobing Ren2; 1Xi'an Jiaotong University; 2National Institute for Materials Science
    Two kinds of phase diagrams can be observed in ferroelastic materials. A glass phase diagram is characterized by a boundary separating a long-range-ordered ferroelastic phase and a short-range-ordered strain glass, while the other kind of phase diagrams exhibit a phase boundary separating two different ferroelastic phases. Here we report a phase diagram, in which a strain glass state is sandwiched between two distinct ferroelastic phases. This phase diagram bridges the one with glass-state and the one with phase boundary. We thus established a 3D phase diagram of Ti50−yNi50+y−xPdx ternary alloys. An understanding from Landau free energy landscape suggests the transforming doping end plays three roles in influencing the ferroic matrix: (1) to destabilize the ferroic matrix phase, (2) to stabilize another ferroic phase different from the matrix one, and (3) to create random local fields. The competition between these effects determines various phase diagrams in doped ferroic materials.

3:15 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.

3:30 PM Break