Effect of M-C (M=Mo, Mn, and Cr) atomic pairs on creep properties of Fe-M-C ternary alloys

Effects of solute elements, Mo, Mn, and Cr, on the creep strength were examined by Fe-M-C (M=Mo, Mn, and Cr) and Fe-C alloys in view of M-C atomic pairs. The largest strengthening effect was obtained by the addition of Mo to the Fe-C alloy, and Mn, and Cr showed almost the same strengthening effects in creep tests at 773K. Measurements of instantaneous elongation and contraction as a function of stress change revealed the existence of instantaneous plastic strain during steady state creep in all alloys. This indicates that the steady state creep is controlled by recovery as in pure metals. For all alloys, the values of about 230GPa were obtained as the coefficient of strain hardening by the method proposed by Ishida and McLean. The recovery rate, r, the coefficient of strain hardening, h, and the steady state creep strain rate, Ε, satisfied the Bailey-Orowan relationship in all alloys. These results indicate that the alloying element affects mainly the recovery rate. From analysis by the model proposed by Sandstrom, it is concluded that M-C pair reduces the climb velocity of dislocations due to large interaction energies with dislocations. The magnitude of the reduction can be estimated from the binding energy between M and C atoms and the diffusion coefficient of M atom. The co-segregation of M and C atoms on edge dislocations is predicted by thermodynamic analysis, and it is pointed out that this may result in the apparent reduction of the dislocation climb velocity even in alloys containing very limited amount of alloying elements.