Quasimartensitic Modulations: Session 4
Program Organizers: Avadh Saxena, Los Alamos National Laboratory

Thursday 2:00 PM
July 13, 2017
Room: Wrigley
Location: Hyatt Regency Chicago

Session Chair: Qianglong Liang, Xi'an Jiaotong University

2:00 PM  
The Martensitic Transformation in Indium-Thallium Alloys: Trevor Finlayson1; Garry McIntyre2; Kirrily Rule2; 1University of Melbourne; 2Australian Nuclear Science and Technology Organisation
    Micrographs of indium-thallium alloys appear in text books to illustrate the microstructure typical of some martensitic transformations. The In-Tl transformation is from cubic to tetragonal, with c/a typically 1.028. The transformation was suggested to arise from [0][ ̅0] phonon softening, based on reports of the temperature dependence of the (c11 c12)/2 elastic modulus. However, on account of the large attenuation of [110][11 ̅0] ultrasonic waves, this modulus could only be estimated from the measurements of other elastic moduli. Previous measurements of the low-q [0][ ̅0] phonon frequencies, with decreasing temperature, showed an increase in frequency, not a decrease, as expected for phonon softening. In our current research on In-27.5at%Tl, both the neutron Laue technique and cold, triple-axis spectrometry are being used to study this martensitic transformation. The results from these experiments using the KOALA and SIKA instruments on the OPAL Research Reactor at Lucas Heights, NSW, will be presented.

2:20 PM  
The Role of Band Structure in Quasimartensitic Transitions: Jason Lashley1; Trevor Finlayson1; 1Los Alamos National Laboratory
    Using a variety of thermodynamic measurements made in magnetic fields, we show evidence that the modulation in many shape-memory alloys is related to significant changes in the electronic structure. We observe that the martensitic transitions in three shape-memory alloys are significantly altered in these alloys by the application of a magnetic field. Investigations of AuZn were performed using an ultrasonic pulse-echo technique in magnetic fields up to 45T. Quantum oscillations in the speed of the longitudinal sound waves propagating in the [110] direction indicated a strong acoustic de Haas–van Alphen-type effect and give information about part of the Fermi surface. The effect of the magnetic field is considered unusual as many influential investigations of martensitic transitions have emphasized that the structural transitions are primarily lattice dynamical and are driven by the entropy due to the phonons.

2:40 PM  
Effect of Driving Rate on Martensitic to Quasimartensitic Transition: Mingpeng LI1; Qingping Sun2; 1Wuhan University; 2Hong Kong Univ. of Sci. & Tech.
    Effects of driving rate on non-isothermal martensitic transition modes and spatiotemporal patterns are studied based on Ginzburg-Landau theory and heat equation. We find that the traditional nucleation-growth paradigm breaks down due to fast self-heating under high driving rate as far as the adiabatic elastic modulus becomes positive. With the increase of driving rate, the mode of phase transition gradually changes from the traditional nucleation-growth of scattered domains to emergence of periodic domain patterns and eventually to stable and homogenous deformation. Such changes is essentially governed by the ratio of external time scale of driving (heat release) and internal time scale of heat conduction. Moreover, the scaling law of the spatiotemporal patterns and the experiment verification using Digital Image Correlation method on nano-gained polycrystalline NiTi thin strips are presented.

3:00 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

3:15 PM  
Bending Induced Pseudoelasticity in Multi-twinned α-Fe Nanowires: Orientation-dependent Mechanism: Yang Yang1; Suzhi Li1; Xiangdong Ding1; Jun Sun1; Ekhard Salje1; 1State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University
    Molecular dynamics simulations on bent multi-twinned α-Fe nanowires, with the orientation of x-[100], y-[011], z-[0-10], show a novel interface driven pseudoelasticity upon loading/unloading cycles. Further studies show that the underlying mechanisms are orientation dependent. When the wire is bent in x-y plane, the pseudoelasticity is mediated by the accumulation and disappearance of a/6<111> partial dislocations in the conventional {112}/<111> twin boundaries. This kind of pseudo-elasticity shows almost no size independence, and can be extend to bulk materials. In contrast, when the wire is bent in x-z plane, the pseudo-elasticity is stemmed from formation of non-conventional {110} interfaces, which provide a large part of the driving force for shape recovery upon unloading. Such bending pseudo-elasticity is related to twin boundary density, and can be extended to a wide range of wire diameters by seeding enough conventional twin boundaries in the sample.

3:30 PM Break