Thermal stability of nanocrystalline Fe-10 wt.% Al produced by cryogenic mechanical alloying

Nanocrystalline Fe-10 wt.% Al material is synthesized using cryogenic high energy ball milling (cryomilling). The resultant powders are compacted in a rigid die at 350 MPa and 823 K in an argon atmosphere. The grain size of the powder compacts, determined using transmission electron microscopy, is found to be 11 +/- 5 nm. Subsequent heat treatment for I hour at temperatures of 1073 K and 1223 K reveals that fine grain sizes of 13 +/- 6 nm and 16 +/- 7 nm, respectively, are maintained. Only when the heat treatment temperature is increased to 1373 K, or over 75% of the melting point of Fe, do relatively large grains form, approaching 100 nm. This level of thermal stability is shown to exceed that of pure Fe processed under identical conditions, by a significant margin. The present results indicate that the increase in thermal stability may originate from the pinning effect of fine dispersoids formed during cryomilling and subsequent heat treatment.