Design of Superelastic Secondary-Phase-Toughened Alloys

Abstract

As the global demand for metal increases, the resulting production is creating a growing strain on the environment. One strategy to combat this sustainability challenge is to increase material lifetime through alloy design aimed at improving mechanical performance under difficult conditions like fatigue. This thesis presents an alloy design strategy to benefit from transformation toughening without retention of the transformation product by designing a multi-phase alloy combining a superelastic phase with a stable elasto-plastic phase. In particular, nano- and micro-scale precipitates, lamellar structures, and bulk matrix morphologies of TiNi are investigated in combination with stable (V,Nb)-Ti matrix phases. These structures allow the study of phase stability as a function of morphology, size, and composition. The phase stability of each form of the superelastic phase is not only investigated in isolation, but also in its role in the various phase mixtures within the alloy. The effects of the complex microstructure on the phase stability as well as the effects of the transformation on the deformation micro-mechanics of the microstructure have been probed through a variety of in-situ micro-mechanical tests, including in-situ SEM tensile experiments combined with 𝜇- DIC, in-situ synchrotron tensile experiments, in-situ TEM micro-pillar compression tests, and in-situ SEM crack propagation experiments. These results provide a statistical view of the importance of the properties of the incorporated phases on phase stability, co-deformation, and ultimately crack propagation behavior.