ATOM PROBE STUDIES OF SPINODAL PROCESSES IN DUPLEX STAINLESS-STEELS AND SINGLE-PHASE AND DUAL-PHASE FE-CR-NI ALLOYS

Duplex stainless steel alloys with chromium contents in the ferrite phase greater than 13% are subject to embrittlement due to hardening of the ferrite phase. The hardening is caused by a chromium rich phase, alpha', produced by either nucleation and growth or spinodal decomposition, and secondary mechanisms including the precipitation of carbides and G-phase. The activation energy of the embrittlement process in CF3 castings has previously been measured by mechanical testing and has been found to be close to the activation energy governing chromium diffusion in the ferrite, although one series of alloys has consistently given much lower activation energies for embrittlement. The reason for this is not known but precipitation of carbides and G-phase has been proposed to interfere with the kinetics of the spinodal in some alloys. In an attempt to simplify the situation by minimising the precipitation of other phases, a series of high purity FeCrNi alloys modelling the ferrite phase of CF3 castings were examined in the field ion microscope and atom probe. No carbide or G-phase precipitation is to be expected in such alloys so the only process occurring on ageing should be the spinodal. Single phase (ferrite) and dual phase (austenite and ferrite) alloys were produced by employing two different solution treatment temperatures. Ageing between 300-degrees and 450-degrees-C has been studied for times up to 10 000 hours. The final amplitude of the spinodal fluctuations was found to decrease with increasing Ni content although the rate of evolution of the spinodal increased with increasing Ni content. In the case of single phase alloys, the rate of increase of the spinodal amplitude was much slower than in the case of CF3 castings, although the activation energy for hardening was very similar. However, those alloys heat treated to produce a small amount of austenite were found to have an accelerated rate of amplitude increase of the spinodal, producing behaviour similar to that in CF3 castings.