Differences in the microstructure of iron mechanically processed powder alloyed with interstitial and substitutional elements

Mechanically processing iron powder with interstitial and substitutional elements resulted in different microstructures (grain size, rms-strains, and phases) depending upon the alloying composition. Alloying iron powder with substitutional elements (approximately 4 at% Al, Cr, Nb, and Ti) resulted in a microstructure similar to mechanically processed iron powder: grain size approximate to 7nm, local rms strain approximate to 0.5% and bcc-Fe nanograins. Small reductions in grain size and increase in rms strain occurred with increasing radius of the substitute alloy atoms. Substitutional atoms were not uniformly distributed throughout the bcc-Fe matrix, but were distributed in clusters within the nanosgrains and/or as a thin coating on the grain boundaries. Alloying iron powder with interstitial elements (4-8 at% C and N) resulted in significantly different microstructures: decrease in grain size and increase in rms-strain occurred with increasing the interstitial concentration and a bct-Fe nanograin phase developed. Interstitial atoms were also predominately distributed in clusters and/or in a thin region along or in the grain boundaries of the iron nanograins. (C) 1997 Acta Metallurgica Inc.