Mechanical Properties of Solution Treated and Aged Ti-5Al-5V-5Mo-3Cr: An Attempt to Define Critical Fracture Properties On the Basis of Microstructural Features

Solution heat treatment below the β transus and fan-cooling results in complete dissolution of the as-forged acicular α phase. The microstructure and mechanical properties were characterized after solutionizing at 50°C below the β transus. The tensile strength in this condition is relatively low, i.e. (~ 900MPa) and the ductility relatively high (~ 16% elong.). With ageing in the 500°C to 600°C temperature range, precipitation of α within the retained β begins within 5 minutes of the start of ageing. Precipitating is heterogeneously nucleated at dislocations and grain boundaries. The yield and ultimate tensile strengths reach values of roughly 1200 and 1300MPa, respectively, and remain relatively constant for up 48 hours ageing.
The fracture stresses for the solution treated condition and for material subsequently aged at 500°C and 600°C are quite similar in magnitude. This similarity is due to a common fracture mechanism which controls the fracture stress for all these conditions. The fracture mechanism for all the solution treated conditions begins with shear decohesion of the primary α/β interfaces.
For each condition, the damage mechanisms and final fracture modes were evaluated and rationalized on the basis of microstructural features. The yield and fracture stresses for the various conditions were calculated and plotted on a two-principal stress axis coordinate system, thus creating the failure envelope for Ti-5553. For the α/β solution heat treated and aged conditions the yield and fracture envelopes are two concentric ellipses in good agreement with the shear strain energy (von Mises) model for failure.