Formation of grain boundary α in β Ti alloys: Its role in deformation and fracture behavior of these alloys

Beta-Ti alloys contain sufficient concentrations of β stabilizing alloy additions to permit retention of the metastable β phase after cooling to room temperature. Decomposition of the metastable β phase results in the formation of several possible phases, at least two of which are metastable. Concurrently, equilibrium α phase often forms first by heterogeneous nucleation at the α grain boundaries with an accompanying precipitate free zone observed adjacent to the grain boundary α. The grain boundary regions are softer than the precipitation hardened matrix. As a consequence, fracture follows the prior β grain boundaries, especially in high-strength conditions. This fracture mode results in low tensile ductility and/or fracture toughness. This article will describe methods of minimizing or eliminating grain boundary α formation by using metastable transition precipitates to nucleate α more rapidly. The effects on fracture behavior also will be described.