Evolution of local atomic structure in a melt-spun Ni25Ti 50Cu25 shape memory alloy during crystallization

Abstract

The local atomic environment of a melt-spun Ni25Ti 50Cu25 amorphous alloy and bond evolution during crystallization were studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and differential scanning calorimetry. In the amorphous alloy, the interatomic distances of Ni-Ti and Cu-Ti are distinct from Ti-Ti and can be indicative of the formation of two types of dominant polyhedra or distorted polyhedral clusters centered with Ni and Cu, with the majority of shell atoms being Ti. The overall increase in the coordination numbers of Ni, Ti, and Cu by crystallization and evidence for structural relaxation suggest that the melt-spun ribbon contains a combination of ordered structures and free volume prior to the heat treatment. Copper and nickel are co-located as their absorption spectra are similar. Although crystallization occurs rapidly (within 4 min at 500 °C), the local atomic environment change persists at longer annealing durations (up to 10 min). An increase in the Ti-Ti and Cu-Cu homo-bond fractions at short and intermediate annealing times suggests that these species segregate between Cu-rich and Cu-poor phases. Crystallization continues through a short-range Ti and Cu diffusion-dominated process, as the near-neighbor structures of Ti and Cu change considerably more than for Ni during annealing. This homogenizes the microstructure followed by possible precipitation of a TiCu compound. © 2010 2011 Taylor & Francis.