Influence of gas bubbles on creep of copper under small shear stresses

Abstract

It is known that gas bubbles on grain boundaries may grow under small applied tensile stresses and an important factor, particularly at low stress levels, is the contribution that this growth makes to the creep strain. Bubble growth also requires accommodation by volume expansion and bubble coarsening phenomena may add to further dilation. The purpose of the present experiments was to eliminate these further variables by using specimens in the form of helical coils so that the creep rate of copper could be determined under small shear stresses for material of a constant grain size with and without intergranular bubbles. Under the conditions described, the creep rate with gas bubbles is three times greater than that without gas bubbles. This can be interpreted in terms of the creep strain arising from grain boundary diffusion controlled bubble growth with consequent atom plating on grain boundaries nearly perpendicular to the tensile stress component, with the converse situation of bubble shrinkage on boundaries nearly perpendicular to the compressive stress component. It is concluded that under low stresses the existence of bubbles can substantially increase the creep rate, and the influences of bubble radius and centre–to–centre spacing and of grain size are also considered. © 1986 The Institute of Metals.