Coarsening of Laves phase and creep behaviour of a Re-containing 10% Cr-3% Co-3% W steel

Abstract

The coarsening of Laves phase in a Re-containing 10% Cr-3% Co-3% W steel during both creep and ageing at 923 K was investigated. The depletion of W solutes from the ferrite matrix is accompanied with the precipitation of Laves phase, wherein creep strain accelerates both these processes. The equilibrium W content in the ferritic matrix of 1.24 wt% and the volume fraction of Laves phase of 1.6% is reached after 500 h of creep and 1000 h of ageing. This is related to lower activation energy for W diffusion upon creep testing. The coarsening of Laves phase obeys Ostwald ripening and can be described in terms of Lifshitz-Slyozov-Wagner theory: d 4 – d 0 4 = K p (t-t 0 ), where K p = 3.70–3.80 × 10 −11 μm 4 s −1 for both creep and ageing suggesting that the coarsening of these particles should be controlled by grain boundary diffusion. No strain-induced coarsening of Laves phase is observed. Growth of the Laves phase particles located on the lath boundaries and the high-angle boundaries of the blocks, packets and prior austenite grains occurs in independent manner. The Laves phase particles located on the high-angle boundaries are susceptible to coarsening that leads to appearance of the particles with dimensions >200 nm. The Laves phase particles located on the lath boundaries dissolve with a low rate. Dense chains of Laves phase on the lath boundaries retain up to high creep time giving a significant contribution to creep behaviour.