Effect of low-temperature phase transition on mechanical behavior of Fe-Cu alloys

Mechanical tests of thermally aged Fe-Cu alloys were performed in the temperature range between 97 K and 297 K in order to investigate their low-temperature mechanical behavior. Tests performed below 122 K have shown that the material breaks in a random fashion already in the elastic region, while above it a clearly pronounced yield point is observed. This sudden change of the mechanical behavior has been rationalized on the basis of atomistic simulations, addressing the interaction of dislocations with Cu precipitates. The latter study has revealed the presence of bcc to fcc transition induced by dislocations which is a temperature dependent process. It is suppressed with increasing temperature and enhanced with increasing a precipitate size. This transition, efficient at low temperature, leads to the transformation of Cu precipitates into non-coherent particles, which act as stronger obstacles and cause the experimentally observed premature failure. The presence of small non-coherent Cu-precipitates, expected to form according to atomistic predictions, and not observed prior to deformation, was confirmed by means of transmission electron microscopy.