In situ diffraction investigations of mechanisms that lead to path dependent mechanical behaviours of nickel-titanium shape memory alloys
Development and characterization of Shape Memory alloys (SMAs) has taken big steps over half a century in understanding the role of alloying components, processing of alloys and microstructural optimization to real life engineering application. This has aided in the growth of application of SMAs to multi-billion-dollar industry but the component design with SMAs still follows that traditional route of prototyping and laboratory testing. Implementation of numerical simulations would help in making component’s design-time more efficient and design-cycle cost effective, but few bottle necks are present to fully implement the numerical models. Out of them, lack of multiaxial experimental data to deconvolute and interpret the role of multiple deformation mechanism has been on the top priority. Historically, lack of experimental capabilities to study in situ deformation mechanism like phase transformation and reorientation of variants was the main hindrance. But with development of modern diffraction technique, it has been possible to study deformation and correlate them with microstructure evolution. In this current work, we aim to provide some insights quantify evolution of internal state variables (ISVs) like volume fraction under biaxial loading conditions.The first part of the thesis examines the role of loading history on reorientation mechanics of martensitic NiTi. Deformation under mechanical loads of two sets of specimens with different initial microstructure were compared. Neutron diffraction data were collected at different interval for the texture evolution information. First set of samples with “Preferred Variants” deformed by deformation twins along with transformation twins whereas “Self-Accommodated” samples showed reoriented by transformation twins only. Although, the samples have same processing route, alterations in microstructure caused variation in stress strain response highlighting the influence of loading history. In the second part, austenitic NiTi are subjected to three loading paths under biaxial loading conditions and diffraction data are collected from neutron diffraction and HEDM experiments at various loads. Three loading paths included both proportional and non-proportional mechanical loads on a cruciform sample and evolution of volume fraction can be studied independently for different paths. Different volume fraction of austenite was obtained upon loading to same strain value as loading path influenced the transformation kinetics. The contribution from transformation kinetics to total strain was higher than the reorientation of martensitic phase at the current loading levels. The results show volume fraction evolution are path dependent in nature. Change of loading path change the favorability of austenite grains and new set of grains transformed that gave continuation to transformation kinetics. Thus, from the current analysis, we provide a method to quantify volume fraction analysis during biaxial loading that is first data set of the its kind. The other important outcome of the current project is the ability to deconvolute active mechanism like transformation and reorientation under multiaxial projects. This has huge potential in studying mechanism like plasticity and transformation in TRIP steels under multiaxial loading. Furthermore, the characterization of complex deformation mechanism in SMAs can aid in calibrating and validating material models for implementation in various numerical framework.
multiaxial loading, NiTiNOL, shape memory alloys, neutron diffraction, high energy x-ray diffraction, path dependence
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Colorado School of Mines. Arthur Lakes Library