Solid-solution hardening and softening in binary iron alloys

Six dilute (0.2, 0.5 and 1 at %) binary iron-base alloys with Co, Cr, Al, Si, Mn and Ni were prepared after scavenging inherent carbon with Ti. From tensile and stress relaxation tests in the temperature range of 77 to 450 K, stress-strain behaviours and thermal activation parameters were analysed as functions of solute content and temperature. In the four alloys containing Ni, Mn, Al and Si, solid-solution softening occurs below 250 K while solid-solution hardening occurs above 250 K. In the alloys containing Co or Cr, neither softening nor hardening due to solute additions occurs at any temperature. Detailed analysis of thermal activation parameters leads one to conclude that the solid-solution softening in the above mentioned four alloys is due to a decrease in kink energy with increasing solute content, while in the latter two alloys no change in kink energy occurs. On the other hand, there exists a strong solute concentration dependence of the athermal component, suggesting that the solid-solution hardening is due to the interaction of dislocations with groups of substitutional solute atoms that create lattice and modulus misfits.