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Various sizes, fractions and morphologies of interdendritic σ phase in the as cast N08028 alloy due to different solidification conditions are characterised. Dissolution behaviours of the secondary phase in both the as cast and the forged alloys are investigated during heat treatment at 1100, 1150 and 1200°C. A recently developed analytical model, subjected to necessary modification, is applied to describe the dissolution process of σ phase in the alloys with and without prior hot deformation. A recipe derived from transformed fraction is applied to evaluate the effective activation energy for dissolution. It has been found that, the prior hot deformation accelerates the dissolution process, which can be attributed to the decreased particle size and the reduced effective activation energy.

Vacuum hot roll bonding of titanium alloy and stainless steel using a nickel interlayer was investigated. No obvious reaction or diffusion layer occurs at the interface between stainless steel and nickel. The interface between titanium alloy and nickel consists of an occludent layer and diffusion layers, and there are the intermetallic compounds (TiNi3, TiNi, Ti2Ni and their mixtures) in the layers. The total thickness of intermetallic layers at the interface between titanium alloy and nickel increases with the bonding temperature, and the tensile strength of roll bonded joints decreases with the bonding temperature. The maximum tensile strength of 440·1 MPa was obtained at the bonding temperature of 760°C, the reduction of 20% and the rolling speed of 38 mm s–1.