Mechanically Alloyed ODS Steel and Ni-base Superalloy: DSC Studies of Directional Recrystallisation

K. Mino, H. Harada, H.K.D.H. Bhadeshia and M. Yamazaki


A precision differential scanning calorimeter (DSC) has been used to characterise the directional0recrystallisation behaviour of mechanically alloyed, oxide dispersion strengthened steel (MA956) and nickel-base superalloy (TMO-2) with the aim of revealing details about the mechanism of recrystallisation.

It is common practice to associate the onset of recrystallisation in ODS alloys during continuous heating with a well defined “recrystallisation temperature”, a parameter implied to be a characteristic of the as-received material. Our experiments on MA956 (a mechanically alloyed and yttrium oxide dispersion strengthened steel) show that during continuous heating (5-40°C min-1), the recrystallisation process occurs over a temperature interval as large as 100°C. Furthermore, the temperature at which crystallisation begins appears to be sensitive to the heating rate. The results are not surprising when interpreted in terms of reaction rate theory, if it is assumed that very little recovery occurs during heating elevated temperatures. Evidence is presented to demonstrate that this is the case, and the results are interpreted using kinetic theory whose form predicts that recrystallisation should be retarded as the heating rate is increased.

TMO-2 is a mechanically alloyed, yttrium oxide dispersion strengthened nickel-base superalloy with the important characteristic that there is a significant temperature interval between γ' solution and the onset of directional recrystallisation. This facilitates the easier interpretation of DSC data with little interference between the endothermic γ' solution and exothermic stored energy release processes.

Materials Science Forum Vols. 88-90 (1992) 213-220

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