In-situ observations of intrinsic grain boundary structure during thermal cycling

The advent of transmission electron microscopy has led to tremendous advances in the understanding of the equilibrium structure of interfaces over the past two decades. However, a detailed understanding of interface and interfacial structure behavior under dynamic conditions such as rapidly changing temperature and stress states is in the incipient stage. Using in-situ rapid thermal cycling of electron-transparent A1 (99.999) foils, the effects of time-dependent, nonhomogeneous stress states and temperature fields on the grain boundary structure and grain boundary position have been directly observed using TEM. Stable grain structures were produced by first heating the foil in-situ at 673 K until all observable motion of grain boundary dislocations ceased. The temperature was then repeatedly cycled between 373 and 673 K with a maximum heating rate of {reversed tilde equals} 100 K/s and a maximum cooling rate of {reversed tilde equals} 20 K/s. The rapid motion of intrinsic grain boundary dislocations was clearly observed and led to noticeable changes in grain boundary position. The behavior was general for low-angle boundaries and for predominantly tilt, twist and mixed boundaries. The stresses generated in the foil discs were due to both thermal gradients and differences in thermal expansion and are found to be both radial and tangential in character. The behavior of two low-angle grain boundaries under these thermal cycling conditions are presented in detail. © 1989