Phase transformation and fracture of a copper-aluminum-nickel single crystal

The goal of this research is to investigate the interaction of fracture and phase transformation in a single crystal of a CuAlNi shape memory alloy. Notched specimens of the CuAlNi crystal were tested in tension until final fracture. Because the mechanical response of shape memory alloys depends strongly on crystallographic orientation, eight different orientations of the notch were considered. The initial work focuses on the microstructure that forms at the notch tip. This microstructure takes the form of twinned martensite plates that grow away from the notch tip. Theoretical and numerical calculations are performed to predict the initial stress induced austenite-martensite microstructure at the notch tip. The linear elastic stress field at a crack tip in an anisotropic linear elasticity body is computed, and the geometry of all possible austenite-martensite interfaces is computed using the Crystallographic Theory of Martensite. A selection criterion is introduced based on computing the work available from the stress field to transform to each austenite-martensite interface. This criterion correctly gives the directions of the martensite plates observed experimentally for each orientation. The work on crack tip microstructure is extended to investigate the fracture behavior of the CuAlNi specimens. The interaction of microstructure formation with crack growth is studied, and the influence of microstructure on fracture behavior is investigated. Experimental results show that the austenite-martensite microstructure strongly affects the rate of crack growth. Moreover, the geometry of the microstructure correlates to the final fracture direction, as the crack always grows roughly 80° from the direction of the martensite plates. Some analytical models for fracture in shape memory alloys are explored to explain this behavior.