Hardening and softening of FeAl during milling and annealing

Changes in hardness of Fe–40Al powders during milling and during subsequent annealing have been examined and related to the many structural changes occurring. During milling, the material becomes significantly disordered with a small domain size and many vacancies, and refines to a nano-scale grain size, while on subsequent annealing at progressively higher temperatures these structural defects are lost as the material re-orders, loses point defects and as the grains grow to large sizes. The increase in hardness during milling can be explained by the combined contributions of vacancy hardening, ordered domain/particle hardening, and by disorder hardening within the ordered regions. Softening during annealing occurs as the domain/particle hardening, disorder hardening and vacancy hardening are successively lost. Dislocation-induced work hardening and grain size hardening are believed to play only minor roles in affecting the material hardness.