Computational framework for simulating the deformation and fracture response of oligocrystalline shape memory alloys

Abstract

Shape memory alloys (SMAs) are a class of metallic materials that can recover their original shapes when heated above a certain temperature. Unique features including the superelasticity and shape memory effect have made SMAs attractive materials for a variety of fields ranging from bioengineering to aerospace engineering. In polycrystalline forms, the desirable properties of SMAs have been significantly limited by severe premature intergranular fractures at grain boundaries. Chen et al. (2009) showed that the intergranular fracture in Cu-based SMAs can be mitigated in fine wire forms with bamboo-shaped oligocrystalline microstructure. Tensile tests conducted in the oligocrystalline systems show that large ductility limits approaching those of a single crystal can be achieved while avoiding the issues involved in single crystal processing. It is, thus, of great importance to investigate how the microstructure and grain boundary characteristics affect phase transformation of oSMAs and how to delay transformation-induced fractures. In this thesis, an anisotropic single-crystal constitutive model is developed to study the underlying mechanism of transformation-induced fracture in oSMAs from a numerical perspective at the microstructural level. The model is based on the micromechanical constitutive framework by Thamburaja and Anand (2001) and a robust explicit integration scheme is developed to update the constitutive law. In order to investigate the effects the grain boundary characteristics have on the martensitic phase transformation and transformation-induced fracture, finite element simulations are performed for modeling the stress-strain response and martensite-austenite phase transformation under uniaxial tension loading condition for oSMA wires with triple junction structures. A quantitative analysis of the simulation results is conducted at the microstructural level in each transformation system to interpret the initiation of transformation-induced fracture. The simulations provide insights on the mechanical response, energy absorption of oSMA wires, as well as shed light on the microstructural design objectives of oSMA to avoid or delay the intergranular fracture.