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

Abstract

PM 2000 offers an excellent oxidation resistance that relies on the capability to develop during service an α-alumina scale, fine, but tightly adhered to the substrate. Advantages of the scale protection at low temperatures are obtained by performing a pre-oxidation treatment prior to operation. Due to the high temperatures involved in the preoxidation treatment microstructural stability of the alloy could be affected. The aim of this study is to characterise the microstructural changes during oxidation (1100ºC /120 h) of the alloy in a fine grained state that is more suitable for room temperature applications. Special emphasis is devoted to the analysis of zones beneath the scale where tensile and fatigue fractures are usually initiated. It is interesting to remark that beneath the oxide scale grain growth occurs forming a shell of about 10 microns of coarse grains. Microtexture analysis performed at these zones reveals that population of grains with a {111} texture decreases towards the centre of the sample. The results presented in this work clearly confirms that recrystallization is not related to oxidation-induced effects but to the heterogeneous deformation produced during machining, that stimulate recrystallization to occur at much lower temperatures.