The Fracture of Cu–Al–Ni Shape Memory Alloy

Abstract

The initiation and propagation of cracks during both quenching and deformation in polycrystalline Cu–Al–Ni alloys have been investigated under various conditions. The results obtained are summarized as follows; (1) Quenching-cracks may be avoided by choosing the temperature of quenching media close to or below the Ms temperature of the alloys, so that many deformation modes are available under given thermal stresses. (2) In the superelastic temperature region, cracks which are formed either by quenching stress or by external stress were found to be initiated and propagate along grain boundaries. This was ascribed to the stresses which accumulated at grain boundaries due to large elastic anisotropy of the present alloys. (3) In specimens in a martensitic state, the propagation paths of cracks were found to easily deviate from grain boundaries, although they are initiated at three-fold nodes of grain boundaries at the very beginning. This is because the stress concentrations at crack tips become more essential in this case, since stresses at grain boundaries are easily accommodated by many deformation modes in a martensitic state.As a whole, it was concluded that the brittleness of the present alloy and other β phase alloys are due to the large elastic anisotropy and grain sizes, and that the large ductility in Ti–Ni alloys are due to the small elastic anisotropy and grain size of the latter.