Grain Boundary Mobility in Fe-Base Oxide Dispersion Strengthened PM2000 Alloy

An alloy can be created without melting, by violently deforming mixtures of different powders. As first discovered by Benjamin, inert oxides can also be introduced into the microstructure using this technique. The resulting dispersion-strengthened alloyed powders are then consolidated using hot-isostatic pressing and extrusion, to produce a solid with a very fine grain structure on a sub-micrometer scale. Heat treatment then induces recrystallisation, either into a coarse columnar grain structure or into a fine, equiaxed set of grains. Columnar grains occur for two reasons: the oxide particles tend to become aligned along the extrusion direction, making that a favoured growth direction. Alternatively, and in the absence of particle alignment, columnar growth can be stimulated by recrystallising in a temperature gradient; the latter may be a stationary gradient or one which moves along the sample, as in zone annealing. The columnar microstructure is desirable in applications where the resistance to creep deformation is paramount. Some of the most successful commercial oxide dispersion-strengthened (ODS) alloys are based on iron or nickel. The details of these alloys have been reviewed elsewhere, but it is useful to discuss their strange isothermal recrystallisation behaviour, which was the motivation for the present work. Because of the mechanical alloying and consolidation process, the alloys have the highest stored energies reported in the context of recrystallisation, attributed to a large extent to the ultrafine grain structure prior to recrystallisation. And yet, they do not recrystallise unless the temperature is raised to a value close to the melting temperature. It could be argued that this is because of the presence of dispersoids which pin the grain boundaries. However, when recrystallisation does occur, the limiting grain size is enormous (it can be as large as the sample itself), which means that the grain size at which the pinning force becomes comparable to the driving force for recrystallisation is far greater than that of the microstructure prior to recrystallisation. Furthermore, a small amount of non-uniform deformation, or the introduction of other non-uniformities, leads to a remarkable reduction in the recrystallisation temperature and a much finer recrystallised grain size. These observations are consistent with a difficulty in nucleating recrystallisation; the cause of the difficulty is the fine and uniform grain size in the starting microstructure, so fine that the grain boundary junctions themselves are powerful pinning points which prevent the bowing of grain boundaries, a process important in the nucleation of recrystallisation. An alternative possibility is that the boundaries are made immobile by, for example, solute effects. The purpose of the present work was to see whether the mobility of moving grain boundaries in a commercial ODS mechanically alloyed ferritic steel (PM2000) could be measured directly. ISIJ International, Vol. 43 (2003), No. 5, pp. 777–783