The Effect of Stress State and Precipitation on Stress-Induced Martensitic Transformations in Polycrystalline and Single Crystal Shape Memory Alloys: Experiments and Micro-Mechanical Modeling
by
Kenneth Allen Gall
Abstract
Shape memory alloys such as CuZnAl and NiTi have been a strong focus of current research efforts primarily in the fields of basic materials science and applied mechanics. Although research efforts in both fields have uncovered many of the microscopic and macroscopic features of these alloys, there still exists some unresolved issues related to the mechanical behavior of shape memory alloys. With this, the purpose of the present work is to answer several critical questions related to the mechanical behavior of shape memory alloys: 1. Why do polycrystalline NiTi and CuZnAl shape memory alloys sometimes demonstrate tension/compression asymmetry? 2. Why do peak aged NiTi single crystals demonstrate a decrease in the orientation dependence of critical transformation stress levels? 3. What is the effect of difference aging treatments on σcr and martensite start temperatures, Ms, in polycrystalline and single crystal NiTi?
To answer the first question a micro-mechanical model is used which incorportes single crystal constitutive relationships and individual grain orientations into the self-consistent formulation. With this model it is shown that tension/compression asymmetry in CuZnAl alloys is linked to the unidirectional nature of the martensitic transformation. In NiTi alloys it is shown that tension/compression asymmetry is caused by a strong crystallographic texture of the {110}<111> type coupled with the unidirectional nature of the transformation. As experimentally observed, the model successfully captures the distinctly different martensite flow behavior and transformation stress-strain response in tension versus compression.
To properly understand stress-induced transformations in aged NiTi single crystals, a micro-mechanical model is used to quantify the stress fields outside perfectly coherent Ti11Ni14 precipitates in NiTi. Using the model, the decrease in the orientation dependence of σcr is linked to the unique orientation relationship that exists between the coherent precipitates in NiTi and the martensite correspondence variant pairs. As experimentally observed the model predicts that peak aged NiTi single crystals loaded under tension along the [100] orientation will show a dramatic decrease in σcr while those loaded along [111] will show less of a decrease in σcr. The effect of different aging treatments is properly explained by extending the local stress field model to the case of semi-coherent precipitates. In addition, the depletion of Ni in the matrix surrounding the precipitates is found to contribute to changes in σcr and Ms for aged NiTi alloys.
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