Formation of X phases and origin of grain refinement effect in Cu-Al-Ni shape memory alloys added with titanium.

Abstract

Grain refinement effect in Ti-doped Cu–Al–Ni shape memory alloys has been studied using microscopic and microanalytical methods. Titanium is found to decrease as-cast grain size and reduce grain coarsening rate during betatization/homogenization heating but does not affect much on large grain refinement during hot working. A eutectic phase separation takes place during casting and Ti-rich eutectic equilibrates on subsequent heating to produce homogeneously distributed μm-order XL particles at more than 0.38 mass% Ti additions, whereas two-orders-of-magnitude smaller XS phase tends to precipitate in the β phase which has less than 0.05 mass% solid solubility of Ti.The grain refinement upon casting is considered to originate from the constitutional supercooling effect and the reduced diffusion rate by presence of Ti. However, the ultimate refinement effect depends significantly on the pinning effect of XS which suppresses the growth of β grains.