Structure and properties of a beta-solution treated, quenched, and aged Si-bearing near-alpha titanium-alloy

The microstructure, tensile properties, and fractographic features of a near-a titanium alloy, TM! 829(Ti-6.1 wt pct A1-3.2 wt pct Zr-3.3 wt pct Sn-1 wt pct Nb-0.5 wt pct Mo-0.32 wt pct Si) have been studied after aging over a tem-perature range of 550 °C to 950 °G for 24 hours following solution treatment in the /3 phase field at 1050 °C and water quenching. Transmission electron microscopy studies revealed that aging at 625 °C and above produced discrete silicides at a' interplatelet boundaries. However, aging at 900 °C and above has also resulted in the precipitation of /3 phase along the lath boundaries of. martensite. The silicides have been found to have a hexagonal structure
with c = 0.36 nm and a = 0.70 nm (designated as S2 by earlier workers). There is a significant improvement in yield and ultimate tensile strengths after aging at 625 °C, but there is less improvement at higher aging temperatures. The tensile ductility is found to be dras-tically reduced. While the fracture surface of the unaged specimen shows elongated dimples, the aged samples show a mixed mode of fracture, consisting of facets, featureless parallel bands, and extremely fine dimples.