Microstructure and Hall-Petch behavior of Fe-Co-based Hiperco (c) alloys

The microstructure and hardness of Fe-Co-based Hiperco(C) alloys were inves tigated. Scanning electron microscopy revealed elongated grains in the as-r eceived (cast and cold-rolled) alloys, and samples containing Nb had second phase precipitates. Annealing of alloys for 1 to 3 h at temperatures in th e range 700 to 800 degrees C resulted in grains becoming equiaxed, In the N b-containing alloys, the original precipitates dissolved and new precipitat es appeared. The rate of grain growth decreased with increasing Nb content, suggesting that Nb or Nb-containing precipitates were responsible for a re duction in the grain boundary mobility. The hardness as a function of grain size in the annealed samples of all of the alloys could be plotted on the same Hall-Petch curve, indicating that the yield strength of these alloys i s governed by the grain size, independent of the alloy composition and volu me fraction of precipitates.