Classical and Non-Classical Mechanisms of Alpha Nucleation In Titanium Alloys

Classical nucleation and growth is the commonly accepted mechanism of second phase precipitation in many metallic alloy systems including titanium alloys. This report presents experimental evidence of a non-classical decomposition mechanism in metallic alloys, involving local compositional fluctuations of small amplitude within the parent phase, leading to homogeneous precipitation of the product phase, with a different crystal symmetry, also referred to as pseudo-spinodal decomposition. An extremely large density of nucleation sites coupled with far-from equilibrium compositions of both parent and product phases, at the formation stage, are characteristic features of this non-classical precipitation mechanism. Experimental observations indicate the possibility of pseudo-spinodal decomposition in case of the intra-granular precipitation of alpha-Ti within the matrix of the beta-Ti alloy, Ti-5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe), during isothermal annealing after directly quenching from the high temperature single beta phase field to certain temperatures.