Formation of B19′ martensite in annealed Ti44.6Ni40.1Cu15.3 thin films and their shape recovery characteristic

The microstructure and shape memory behavior of Ti44.6Ni40.1Cu15.3 thin films annealed at 773, 873, 973 K for 1 h were investigated. The Ti(Ni,Cu)2 particles precipitated in the grain boundaries and grain interiors. As the annealing temperature increased, the grain boundary precipitates became prominent. With increasing annealing temperature, the size of the precipitates increased while the density of precipitates decreased. Unexpected B19′ martensite formed in the annealed thin films, resulting from the composition variation in the matrix induced by precipitation process. Both of the maximum recoverable strain and the critical stress for slip decreased with increasing annealing temperature, which was mainly attributed to the coarsening of the Ti(Ni,Cu)2 precipitates.