was about 0.2 mm in dia. Creep tests under constant tensile stresses were carried out in argon atmosphere.
The temperature dependence of steady-state creep rates under various stresses is shown in Fig. 1 from which three different stages can be clearly recognized; one in the high temperature region above Curie temperature, 770ยฐC, the second in the intermediate region located in the ferromagnetic temperature region, and the third is observed at lower temperatures under high stresses.
The change in temperature dependence of steadystate creep rates in ferro-and paramagnetic temperature regions is quite similar to that of diffusion rates which is shown on the lower part of the same figure.
The change in creep rates, therefore, may be mainly due to the magnetic effect on diffusion rate. In paramagnetic temperature region, the activation energy obtained for steady-state creep, Q,, at the stresses of 0.5 and 1.4 kg/mm2 is 72 f 1 kcal/mole, and is nearly equal to that for self-diffusion, Qa, in iron (57-67 kcal/mole(1*2*s)).
In ferromagnetic temperature region, the value of Q, (81 f 1 kcal/mole for the stresses of 1.4 and 2.5 kg/mm2) is somewhat larger than that of Qd reported by Buffington et d( 2) (60 kcal/mole). At present the discrepancy cannot be clearly explained because of a shortage of diffusion data. Fairly high values (78-100 kcal/mole) have so far been reported as the activation energy for steady-state creep in iron.cO*lO) They may correspond to the value obtained from ferromagnetic temperature region of the present investigation.
The temperature dependence at lower temperatures is stress-dependent ; for example, the values of Q, are 64 f 2 and 51 f 0.3 kcal/mole for the stresses of 2.5 and 4.0k g/mm2, respectively. Onemay, therefore, consider that creep deformation in this region is controlled by other mechanisms than the one discussed above.
A similar change in creep rates near Curie temperature has been also observed with iron-molybdenum, iron-silicon, and iron-cobalt alloys. In these alloys creep rates in ferromagnetic temperature region are decreased markedly with an increase in the amount of the alloying element. The behaviour of the decreasing varies with the alloying elements. Further investigation, however, should be done in order to discuss this point more fully.