The effect of heat treatment on the microstructure evolution and mechanical properties of Ti-5Al-5V-5Mo-3Cr, and its potential applications in landing gears

The properties and microstructure of Ti-5Al-5V-5Mo-3Cr were characterized under various stress states after the following heat treatments: 1) annealing above the beta transus, followed by cooling at various rates and ageing for different times; 2) solution heat treatment in the alpha-beta range, fan-cooling and ageing for various temperatures and times.;Controlled cooling from above the beta transus to the ageing temperature at slower rates produces a coarser alpha+beta microstructure. Acicular alpha laths are produced with cooling rates of 1°C/min, while lamellar alpha develops at cooling rates of 3.4°C/min. The beta annealed and fan-cooled condition is characterized by relatively low strength (∼850MPa) and low ductility (∼6% elong.).The fracture mode is by intensely localized slip and the creation of transgranular cracks. Localization of slip is attributed to shearing of the nano-scale o precipitates by dislocations. A linear relationship between the grain size, d-1/2, and the yield and fracture stresses was established, as described by the Hall-Petch relation. With controlled cooling, the strength and ductility improve by precipitation of lamellar alpha within the beta matrix. Improvements in ductility and strength are achieved by reducing the slip length.;Solution heat treatment below the beta transus and fan-cooling results in complete dissolution of the as-forged acicular alpha phase. Solutionizing at 50°C below the beta transus yields a volume fraction of 16.5% primary a in a matrix of retained beta. Low angle grain boundaries and globular primary alpha, each measuring 2-4mum average in diameter, are uniformly distributed throughout the retained beta matrix. 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 alpha within the retained beta 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 the fact that the fracture mechanism, which controls the fracture stress, is the same for all these conditions. The fracture mechanism for all the solution treated conditions begins with shear decohesion of the primary alpha/beta interfaces.;Heat treatment above the beta transus temperature causes complete recrystallization of the as-forged microstructure. The as-cooled microstructure consists of equiaxed beta grains with an average grain size of 200mum. Water quenching from above the beta transus results in precipitation of a dispersion of nano-sized o phase; while the fan-cooled microstructure contains nano-sized o and alpha precipitates. Ageing of the fan-cooled microstructure at 790°C or 600°C precipitates sub micron acicular alpha throughout the beta grains. The tensile properties of this condition could not be determined using standard tensile specimens due to brittle failure at the grips.;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 beta annealed and fan cooled and for the alpha-beta 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.;The fracture toughness and stress corrosion cracking behaviour for the STA condition were evaluated and compared against other beta titanium alloys.