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 exami ned 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 773 K. Measurements of instantaneous elongation and contract ion 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 al l alloys, the values of about 230 GPa were obtained as the coefficient of s train hardening by the method proposed by Ishida and McLean. The recovery r ate, r, the coefficient of strain hardening, it, and the steady state creep strain rate, (<epsilon>) over dot satisfied the Bailey-Orowan relationship , (<epsilon>) over dot =r/h, in all alloys. These results indicate that the alloying element affects mainly the recovery rate. From analysis by the mo del proposed by Sandstrom, it is concluded that M-C pair reduces the climb velocity of dislocations due to large interaction energies with dislocation s. The magnitude of the reduction can be estimated from the binding energy between the M and C atoms and the diffusion coefficient of the M atom. The co-segregation of the M and C atoms on edge dislocations is predicted by th ermodynamic analysis, and it is pointed out that this may result in the app arent reduction of the dislocation climb velocity even in alloys containing a very limited amount of the alloying elements.