Abstract
It is well known that alloys containing fine dispersed particles are superior in high temperature creep strength. The mechanism of the creep, however, has not been made clear.
In the present work, the mechanisms of the creep deformation of this kind of alloy are investigated, using the Nimonic type alloy, Ni-20% Cr-4% Al and Ni-20% Cr-35% C0-4% Al alloys containing fine rf particles. The results obtained are as follows:
(1) The steady state creep rate \dotε of the Ni-Cr-Al alloy is not linearly proportional to the applied stresses on the bilogarithmic scale. This is due to the presence of two different creep mechanisms corresponding to low and high applied stresses. The creep rate at low stress is controlled by the rate of climbing of edge dislocation and non-conservative motion of jog in screw dislocation, and at high stress cross slip of dislocation is a rate controlling process of the steady state creep. In the case of the former, the activation energy is nearly equal to that of the vacancy diffusion, while in the case of the latter the energy is much larger than the former. The thermally activated process of this cross slipping is considered to be due to the contraction of extended dislocation. In the highest stress region mechanical twins are observed.
(2) The cross slipping of dislocation is not observed in Ni-20 Cr-35 C0-4 Al alloy, with a low stacking fault energy, when crept under a high stress, and the activation energy is nearly equal to that of the vacancy diffusion. Therefore, the rate controlling process of creep deformation of the alloy is thought to be not cross slipping but climbing of dislocation.
