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 a-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 degrees 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 mu m 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 mu m for the remnant substrate. Microtexture analysis performed at these zones reveals that grains are oriented with the < 111 > and < 110 > direction parallel to the bar axis, which contrast with the weak < 110 > 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. (C) 2007 Elsevier B.V.. All rights reserved.