Pack aluminisation kinetics of nickel rods and foams

The kinetics of pack aluminisation has been described through a multiphase, finite difference model considering diffusion of aluminium into nickel under conditions of concentration dependent diffusivity and finite specimen dimensions, allowing complete conversion of the specimen to the equilibrium intermetallic phase. For a cylindrical geometry, the model accurately predicts the growth of an outer Ni₂Al₃ shell, the increase in diameter, and the average composition as measured for 0.5 and 1 mm diameter nickel rods aluminised at 1000°C for up to 4 h. For nickel foams with hollow struts, which are modelled as thin sheets, good agreement is found with the average aluminium concentration measured on two foams with strut wall thicknesses of 41 and 84 μm. The measured aluminisation kinetics for a foam with strut thickness of 6 μm, however, is much slower than predicted owing to the high surface area of this foam, which prevents the centre of the specimen from reaching the equilibrium aluminium surface concentration.