Size Effects in Martensitic Transformations: Session 1
Program Organizers: Peter Anderson, The Ohio State University

Monday 2:00 PM
July 10, 2017
Room: Comiskey
Location: Hyatt Regency Chicago

Session Chair: Peter Anderson, The Ohio State University

2:00 PM  Invited
Superelasticity and Shape Memory at Nano-scale Studied by In-Situ Electron Microscopy: Maria Nó1; Jose San Juan1; 1Universidad del Pais Vasco
     Martensitic transformation in shape memory alloys (SMA) takes place usually by heterogeneous nucleation of martensite plates and further progress by motion of their interfaces. The involved microscopic mechanisms are of paramount importance to understand the kinetics of the transformation, particularly when it is taking place in confined volumes at small scale, and In-situ experiments constitute fundamental tools to get information about such mechanisms.In the present talk, several studies on nucleation and growing of martensite, in different SMA, approached by in-situ electron microscopy will be presented. Both kind of transformations, thermally (heating and cooling) and stress induced (superelastic), have been studied by using different in-situ holders at the scanning (SEM) and transmission (TEM & HRTEM) electron microscopes. Selected examples of the capabilities of these advanced techniques will be provided, and the advantages, possibilities and limitations of each technique will be overviewed.

2:40 PM  
Size Effect and Scaling-law for Superelasticity in Cu-Al-Ni Shape Memory Alloys at Nano-scale: Jose San Juan1; Jose Gómez-Cortés1; Iñaki López-Ferreño1; Andrei Chuvilin2; Sergio Molina3; Jesús Hernández-Saz3; Maria Nó1; 1Universidad del Pais Vasco; 2CIC NanoGUNE; 3Universidad de Cádiz
     New properties, as ultra-high mechanical damping and a very fast response along thousand of cycles, have been reported in Cu-Al-Ni SMA at nano-scale. However there is a lack of a quantitative characterization of the observed size effects in SMA.The goal of this work is to offer such quantitative characterization and analysis by presenting the evolution of the superelastic behaviour of a series of pillars covering a broad range of size diameters from few hundred of nanometres to some microns. Then they have been tested by nano-compression tests by using an instrumented nano-indenter. The obtained results show a clear size effect on the critical stress for the stress-induced transformation during superelastic behaviour. Finally, the scaling law for the size effect on the critical stress for superelasticity has been experimentally established, and discussed on the physical base of the nucleation mechanism.

3:00 PM  
Biased Target Ion Beam Deposition and Nanoskiving for Fabricating NiTi Alloy Nanowires with Extended Length and High Uniformity: Huilong Hou1; Reginald Hamilton2; 1University of Maryland, College Park; 2The Pennsylvania State University
    Nanoskiving is a novel nanofabrication technique to produce shape memory alloy nanowires. Our previous work was the first to successfully fabricate NiTi alloy nanowires using nanoskiving, a top-down approach leveraging thin film technology and ultramicrotomy for ultra-thin sectioning. Herein, we utilized biased target ion beam deposition technology to fabricate nanoscale NiTi alloy thin films. In contrast to our previous work, rapid thermal annealing was employed for heat treatment, and phase transformation between B2 and R-phase was confirmed using stress-temperature and diffraction measurements. Ultramicrotome was programmable and facilitated sectioning thin films to produce nanowires with thickness-to-width ratios ranging from 4:1 to 16:1. Energy dispersive x-ray spectroscopy confirmed the elemental make-up of Ni and Ti within the wires. Nanowires exhibited distinct ribbon-like curvatures, hinging on thickness-to-width ratio. The results demonstrate nanoskiving is potential to produce NiTi alloy nanowires that are continuous with an unprecedented length on the order of hundreds of micrometers.

3:15 PM  
Nanoscale Phase Field Modeling and Simulations of Martensitic Phase Transformations and Twinning at Finite Strains: Anup Basak1; Valery Levitas1; 1Iowa State University
    A thermodynamically consistent multiphase phase field theory for mutlivariant martensitic phase transformations is formulated considering interfacial stresses at finite strain. The variant-variant transformations (twinning) are described with the same accuracy and possibility of parameter calibration as the austenite-martensite transformation. That means that each of them can be effectively described by a single order parameter and the lattice instability conditions are satisfied. Finite element algorithm and procedure for solution of 3D problems under complex loading is developed. Material parameters are calibrated for martensitic transformation in NiAl alloy. Various problems of modeling of martensitic nanostructure during temperature and stress-induced transformations are solved. The effects of sample size and widths of austenite-martensite and martensite-martensite interfaces, complex loading, and interfacial stresses on the nanostructure formation have been studied.

3:30 PM Break