Size Effects in Martensitic Transformations: Session 3
Program Organizers: Peter Anderson, The Ohio State University

Friday 10:20 AM
July 14, 2017
Room: Wrigley
Location: Hyatt Regency Chicago

Session Chair: Maria N, Universidad del Pais Vasco

10:20 AM  
Evidence of a “Discrete” Mechanism of Stress-Induced Transformation in Nanostructured Ti-Ni SMA with Extremely Low Grain Size: Sergey Prokoshkin1; Sergey Dubinskiy1; Andrey Korotitskiy1; Vladimir Brailovski2; Alexander Glezer3; Anton Konopatsky1; Vadim Sheremetyev1; 1NUST "MISIS"; 2Ecole de Technologie Superieure; 3Kurdyumov Institute of Metal Science and Metal Physics
    The TEM and in situ X-ray studies were carried out with an objective to clarify the stress-induced transformation mechanism in Ti-50.61at.%Ni alloy with B2-austenite grain size approaching the lowest bound of nanosize range. A mixed nanosubgrained/nanograined austenite structure was obtained by annealing at 300 and 350 oC after cold rolling (CR) with a true strain of e=0.55. A pure nanograined structure was obtained by annealing at 350 and 400 oC after CR with e=1.7. It is unambiguously evidenced that a hypothesis about the transition from the “discrete” (martensitic) to “continuous” mechanism of stress-induced transformation when the grain size decreases under a 50 nm threshold is not confirmed. That means that the martensitic transformation preserves its “discrete” nature down to the smallest observable grain size of 10 nm. The shifts of the austenite and martensite X-ray lines during straining are correctly described by purely elastic deformation of the austenite and martensite.

10:40 AM  
Effects of Internal Length Scales on Behavior of SMA: Qingping Sun1; Mingpeng LI2; 1Hong Kong Univ. of Sci. & Tech.; 2Wuhan University
    We investigate the roles of internal microstructure length scales (grain size and grain boundary thickness) on the stress-induced phase transition behaviors of nanostructured shape memory alloys by using a generalized mechanism-based “amorphous-crystallite composite” model. A non-dimensional length scale is identified as the governing parameter that represents the energy competition between the crystallite and the amorphous phase. By length scale reduction, the energy of the elastic non-transformable amorphous phase will become dominant in the PT process, and eventually bring fundamental changes of stress-induced PT behaviors: breakdown of two-phase coexistence and vanishing of hysteresis. Furthermore, analytical predictions are presented and agree well with both MD simulation and experimental data of nanostructured NiTi SMAs.

11:00 AM  
Assessment of the Total Canning Compression Method for Grain Refinement of Brittle Shape Memory Alloys at the Example of Ni-Mn-Cu-Ga: Feng Chen1; Yunxiang Tong1; Bing Tian1; Li Li1; Peter Mllner2; 1Harbin Engineering University; 2Boise State University
    Grain refinement is reported to greatly improve the ductility of Ni-Mn-Ga alloy. For this purpose, we introduced the total canning compression (TCC), a processing method proved to be effective in ductile NiTi. In present work, Ni56Mn19Cu6Ga19 (at.%) alloy was deformed severely by TCC. The effects of compression deformation and post-annealing on microstructure, martensitic transformation and magnetic properties were investigated. The results showed that, after 70% deformation plus annealing for 30 min at 700 C, the grain size was greatly reduced from several hundred micrometers to 0.8-3.5 μm. Grains typically contained a single variant of twinned martensite. Martensitic transformation is suppressed by severe plastic deformation but it could be recovered by the post-annealing above 700 C, so did the magnetic properties. Annealing at 700 C decreased the transformation temperatures greatly while slightly enhanced the saturation magnetization. TCC can effectively refine the grains in brittle shape memory alloys.

11:20 AM  
Grain Size Effects on Fatigue Crack Growth in Nanocrystalline NiTi: William LePage1; Aslan Ahadi2; Qingping Sun3; John Shaw1; Samantha Daly4; 1University of Michigan; 2Japan National Institute for Materials Science; 3Hong Kong University of Science and Technology; 4University of California Santa Barbara
    This work investigates grain size effects on the fatigue crack growth of nanocrystalline NiTi with average grain sizes from 10 to 1500 nm, through the use of multiscale experimental techniques. Crack initiation and growth rates were characterized with optical digital image correlation (DIC) and infrared thermography, and then fatigue properties were explored on the microscale with in-situ, distortion-corrected SEM-DIC. High-magnification SEM micrographs were captured during crack growth in compact tension samples patterned for SEM-DIC with self-assembled nanoparticle deposition for sub-micron displacement resolution. Crack opening and closing displacements in the crack wake were computed to quantify the effect of martensitic phase transformation as an energy-dissipating mechanism during crack propagation. With the sub-micron displacement resolution of SEM-DIC, this work also enables a new length scale of assessment for models of phase transformation near crack tips. The enhanced resolution of SEM-DIC overcomes challenges previously encountered for crack-tip and crack-closure measurements in nanocrystalline alloys.

11:35 AM  
Molecular Dynamics Simulations of Grain-Size Dependent Martensitic Transformation in a NiTi Shape Memory Alloy Polycrystal: Prashanth Srinivasan1; Lucia Nicola1; Angelo Simone1; 1Delft University of Technology
     Nickel-Titanium (NiTi) alloys exist in different phases depending on temperature and stress level leading to properties such as pseudo-elasticity and shape memory effect. The austenite phase transforms into martensite either by cooling or under the application of an external load. Experimental evidence shows that the martensitic transformation is size-dependent in samples with average grain size less than 60 nm. Here molecular dynamics simulations of a NiTi polycrystal are performed with a modified embedded atom method interatomic potential. The grain size in the polycrystal ranges between 5 and 45 nm. Both temperature-induced and stress-induced martensitic transformation simulations are carried out.Preliminary results on a bicrystal suggest that decreasing the grain size inhibits the formation of a multi-variant martensite. The grain boundary acts as a nucleation site for stress-induced martensitic transformation. This analysis is extended to polycrystals. The simulations describe the structural changes during martensitic transformation in a polycrystal with atomistic resolution.

11:50 AM  
Grain Size Effect in Ti-Ta Shape Memory Alloy Thin Films: XH Z1; W Cai1; 1Harbin Institute of Technology
    It has been reported in other thin films that there is critical grain size of martensitic transformation, when the grain size is smaller than the certain critical size, martensitic transformation disappears. However, does it exist a grain size that shape memory effect disappears when it still has martensitic transformation? The critical grain size of martensitic transformation and shape memory effect of Ti-Ta thin films were investigated. Ti-Ta thin films with different grain sizes were prepared by magnetron sputtering followed by annealing. The microstructure and shape memory properties were analysised by X-ray diffraction, transmission electron microscopy, nanoindentation and atomic force microscopy. The shape recovery rate decrease with the increase of grain size. It shows the max shape recovery rate with the grain size of 10nm. When the grain size is 8nm, XRD and TEM results show the thin films are still martensite but the shape memory effect disappears.

12:10 PM Break