Assessment of factors influencing surface recrystallisation during high temperature exposure of fine-grained PM 2000 alloy

PM 2000 offers an excellent oxidation resistance that relies on its capability to develop in service a fine α-alumina scale, tightly adhered to the substrate. The advantages of the scale protection at low temperatures are obtained by performing a pre-oxidation treatment prior to operation. The aim of this study is to characterise the microstructural changes during oxidation (1100 °C/120 h) of the alloy in a fine-grained state, which make it more suitable for room temperature applications. With regards to the as-received material, oxidation treatment does not produce macroscopic changes in terms of grain size and texture. Beneath the scale, however, a shell about 10 μm thick containing coarse grains was formed. The grain size in this zone is in the order of the shell thickness, which is considerably higher than the grain size of about 2 μm for the remnant substrate. Microtexture analysis performed at these zones reveals that grains are oriented with the 〈1 1 1〉 and 〈1 1 0〉 direction parallel to the bar axis, which contrast with the weak 〈1 1 0〉 fibber texture of the bulk. The results presented in this work clearly confirm that surface recrystallisation is not related to oxidation-induced effects but to the heterogeneous deformation produced during grinding, that stimulate recrystallisation to occur at much lower temperatures.