Structure and properties of a near-alpha titanium-alloy after beta-solution treatment and aging at 625-degrees-c

Abstract

The microstructure, tensile properties, and fractographic features of a near alpha titanium alloy, IMI 829 (Ti-6.1A1-3.2Zr-3. 3Sn-lNb-0.5Mo-0.32Si) have been studied after aging at 625 °C for 24 hours following solution treatment in the beta phase field at 1050 °C and cooling at various rates (furnace, air, oil, or water). Transmission electron microscopy studies revealed that aging at 625 °C for 24 hours of this alloy had led to the precipitation of only one kind of silicide with a hexagonal structure (c = 0.36 nm and a = 0.70 nm). The precipitates are ellipsoidal in shape and their size depends on the cooling rate given to the alloy after beta solution treatment. Extremely fine precipitation of Ti3A1 phase has also been observed in the furnace cooled and aged condition. There is a significant improvement in both yield and ultimate tensile strengths after aging of both the oilquenched and water-quenched specimens whereas it is minimal in the case of furnace-cooled and air-cooled specimens. Aging the alloy has also resulted in a drastic reduction in both percentage elon- gation and percentage reduction in area for all the condi-tions investigated. While the fracture surfaces of the unaged specimens showed characteristic dimples, a large number of facets appeared in the fractographs of the aged specimens.