An in situ study of hardening and softening of iron by carbon interstitials

Transmission electron microscopy in situ straining experiments were carried out in Fe–110 at.ppm C, between 95 and 375 K, in order to determine the origin of the softening/hardening effects of carbon in an iron matrix. The local stresses in microsamples are in fair agreement with the corresponding macroscopic yield stresses as a function of temperature, and the softening/hardening effects of carbon are well reproduced at the microscopic scale. Dislocations multiply at sources and glide in {1 1 0} planes, as in pure Fe. The motion of straight screw dislocations is much steadier than in pure Fe, however, showing that the transition between the two mechanisms observed in pure Fe was shifted to a lower temperature. This allows one to interpret straightforwardly the softening effect of carbon at intermediate temperatures. The hardening effects at lower and higher temperatures are also discussed.