Some Aspects of the Recystallisation of Dispersion-Strengthened Iron-Alumina Alloys.

Isochronal and isothermal annealing experiments have been carried out on a series of Iron-Alumina alloys containing dispersions of spherical alumina particles so that the influence of the particle size and inter particle spacing upon the recrystallisation behaviour of iron could be studied. The alloys were fabricated by powder metallurgical techniques followed by hot extrusion. This produced alloys containing dispersions of particles with average radii of between 0.15 and 0.35mum and interparticle spacings in the range 0.75-0.95mum. The progress of recrystallisation was followed by hardness measurements and by optical metallography. In all alloys, the initial grain size was standardised prior to cold rolling and annealing. At deformations of 60% reduction, the results indicated that recrystallisation was generally accelerated (compared to pure iron) in those alloys containing particles above a critical size (~0.5mum dia. ) due to the enhancement of nucleation at particle/matrix interfaces. The degree of acceleration increased with increasing particle size and decreasing interparticle spacing down to a critical spacing of about 0. 81um when the trend reversed. The recrystallised grain size was refined. Kinetic analysis indicated a reduction in the average dimensionality of growth of the nuclei compared to pure iron, where nucleation occurred predominantly at grain boundaries. A retardation of recrystallisation was observed in those alloys containing particles of a subcritical size due mainly to a reduced nucleation rate. Reductions in the deformation prior to annealing resulted in a retardation of recrystallisation in all alloys, the extent depending on their dispersion characteristics, due mainly to its influence on nucleation at the various sites. Generally, an increase in the initial grain size resulted in a retardation of recrystallisation, the extent depending on the dispersion characteristics. In one alloy containing large particles there was a retardation at small initial grain sizes. The present results have been shown to he consistent with the Mould-Cotterill hypothesis for recrystallisation in dispersed phase alloys. The importance of the selection of the correct interparticle spacing parameter in the interpretation of results has been critically discussed in the appendix.