GOALI: Micromechanics Experiments and Modeling of Shape Memory Response in Ni-Ti Based Alloys

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TECHNICAL SUMMARY:Shape memory alloys are materials with remarkable properties that stem from a martensitic transformation. One of the fundamental aspects of shape memory and pseudoelastic behavior that is not well understood is how the matrix accommodates the large strain associated with the transformation. Theoretically, accommodation may be achieved either by matrix plasticity or by inducing other transformation variants. This three-year GOALI proposal will develop new, fundamental insight into the pseudoelastic shape memory behavior of several Ni-Ti based alloys, including a ternary Ni-Cu-Ti, in collaboration with GM Research. The focus will be on alloys that undergo different martensite transformations and exhibit disparate functional fatigue properties. The approach is to meld innovative micron-scale mechanical tests and advanced microstructural characterization with analytic and novel microstructure-based modeling. Uniaxial deformation of focused-ion-beam-machined microcrystals will be used to probe the static and cyclic response as a function of matrix crystal orientation, and in order to directly measure the mechanical response (stress, strain, work output) for individual variants. Post-mortem characterization using transmission electron microscopy of the remnant substructure will be coupled with analytic modeling of the possible transformations. This approach will also yield directly the relative conditions for martensite transformation versus matrix plasticity over a range of component sizes. A new microstructure-based finite element approach will be developed that explicitly tracks local, discrete phase transformations coupled with rate-dependent crystal plasticity. For the first time, this will enable the treatment of size effects and the generation of local plasticity associated with transformations?a crucial step to understand and enhance pseudoelastic and shape memory response. The proposed effort integrates advanced characterization at The Ohio State University and the GM Research center with new experimental techniques at two DOE labs ? high temperature nanoindentation/pillar testing at Oak Ridge National Laboratory and in situ pillar testing at the National Center for Electron Microscopy.NON-TECHNICAL SUMMARY:Shape memory alloys (SMAs) are materials with remarkable properties that include the ability to bend and stretch to large extent under an applied load, then spring back to their original shape when the load is removed. In addition, SMAs can change shape when heated, after which they may either maintain their newly acquired shape or return to their original shape after cooling back down to room temperature. The automotive industry has recognized the phenomenal potential of SMAs since they are remarkably simple actuation devices compared with conventional motorized actuators. For instance, they could be used as small, ?solid-state? motors that could be used to reconfigure a wide range of components, greatly reducing the complexity of such systems. However, commercially available SMA materials are not presently operated at their full potential due to degradation in their shape-changing capabilities after experiencing many temperature or stress cycles (?functional fatigue?). This program is designed to develop a fundamental understanding of the materials science aspects associated with this degradation process, as well as the development of modeling capabilities to predict and improve the functional fatigue performance for automotive, medical and other applications. A vigorous program of interchange between OSU and GM will stimulate efficient transfer of knowledge and will provide ample mechanisms for experiencing both academic and industrial environments. In an exciting outreach effort, we will develop a high school inquiry-based teaching module about shape memory alloys and their applications. Several mechanisms insuring insertion of this module into local high schools have also been defined.