It has been possible to measure the average interfacial velocities of grain boundaries during the process of recrystallisation of an oxide dispersion-strengthened iron-base alloy manufactured using the mechanical alloying technique. The measurements could be made using optical microscopy because the recrystallised grains are orders of magnitude larger than the surrounding unrecrystallised material. Furthermore, the peculiar way in which the alloy recrystallises, made it possible to observe large, flat segments of boundaries as they advanced into the higher energy matrix. The data have been analysed bearing in mind the pinning effect of the non-random distribution of oxide particles. It is concluded that the boundaries have a high mobility. This conclusion adds to the growing evidence that the alloy has great difficulty in nucleating recrystallisation because of the ultrafine and uniform starting-microstructure in which the grain boundary junctions are so closely spaced that they prevent strain-induced grain boundary migration; in other words, the junctions themselves are powerful pinning points.
Oxide-dispersion-strengthened (ODS) alloys produced by mechanical alloying (MA) are very prone to the occurrence of porosity. The reason for this has not been fully understood so far. In this work, a model ferritic ODS alloy has been examined by optical microscopy and scanning electron microscopy in both the powder form and the consolidated condition. Submicron-sized pores have been found in the as-mechanically alloyed powder and in the consolidated alloy. Moreover, pores can be observed in loose powder particles annealed at 1000 ЊC and 1100 ЊC. Based on these results, it has been suggested that the MA process might be largely responsible for the porosity problem in ODS alloys.
In this work, the porosity in a iron-based oxide dispersion-strengthened (ODS) alloy, namely, PM2000, has been studied together with its recrystallization behavior. It has been found that pores are only found in coarse secondary recrystallized grains formed at the early stage of recrystallization. It is suggested that a lack of fast diffusion paths, particularly grain boundaries, will prevent gas trapped in the materials during mechanical alloying (MA) from diffusing away, and porosity is formed as a result. Thus, it is proposed that an extended anneal below the secondary recrystallization temperature will help to reduce the subsequent evolution of porosity in these materials, and the method has been demonstrated to work successfully in PM2000.
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