Book Description

The gliding of one metal crystal with respect o another parallel to their mutual grain boundary has been studied in pure aluminum bicrystals during isothermal creep at temperatures ranging from 200 to 650 degrees C under static stresses of 10 to 1,600 psi. The motion is spasmodic and begins with an induction period. Its direction is determined by that of the maximum resolved shearing stress in the plane of the grain boundary, but its rate depends as well upon the angle through which the active slip systems are bent as they cross the grain boundary. The overall gliding rate is a linear function of the cube root of time and bears and Arrhenius relationship to the temperature, and its logarithm is proportional to the stress, over small ranges of stress. Shearing occurs with a zone of finite thickness, which becomes thicker with time, increasing temperature, and increasing stress. The metal within this zone is composed of subgrains that rotate back and forth about an axis which is sometimes the octahedral axis of the parent crystal. Rupture, which is never intergranular, is preceded by a sudden increase in the rate of uniform extension in one of the crystals. The mechanism of grain-boundary gliding is described as a coordinated alternation of slip and recovery in a chain of subgrains along the grain boundary. This concept is employed to account both for the rate of primary creep and for the transition from primary to steady-state creep.