Effect of long term aging on the microstructural stability and mechanical properties of Ti–6Al–2Cr–2Mo–2Sn–2Zr alloy

Microstructural development after long term aging and its effect on the mechanical properties have been investigated in Ti–6Al–2Cr–2Mo–2Sn–2Zr alloy. Four types of precipitates have been identified in the alloy, they are α2-Ti3Al, ω, silicides and α-TiCr2 Laves phase. α2-Ti3Al and ω are observed in the as-received materials and all aged samples. Silicides, (Ti, Zr)xSi3 (5≤x≤6), are found in the specimens aged at 550–650 °C over 500 h. The silicides have a hexagonal structure with a lattice parameter: a=7.01±0.03, c=3.63±0.06 Å. The orientation relation between the silicides and the parent β-phase has been determined as: [111]β//[0001]silicide, (101)β//(1100)silicide. Laves phase α-(Ti, Zr)Cr2 is observed in the samples aged at 450–550 °C over 500 h. It has a cubic structure (a=7.12±0.03 Å) with an orientation relationship with the α- and β-Ti matrix: [110]α-TiCr2//[0001]α-Ti//[110]β-Ti, (1̄13)α-TiCr2//(1̄100)α-Ti//(1̄10)β-Ti. Long term aging in the temperature range of 450–550 °C results in significant decrease of fracture toughness and increase of intergranular failure, due to precipitation of α2-Ti3Al and α-(Ti, Zr)Cr2 Laves phase. No considerable change in fracture toughness for the samples aged at 650 °C up to 1000 h is found in spite of precipitation of the α2-Ti3Al and the silicides. The retention of fracture toughness after aging at these temperatures has been attributed to coarsening of secondary α-Ti platelets during the aging treatment.