The Origin, Growth and Annihilation of Annealing Twins in Austenite

Abstract

The formation of annealing twins in austenite. during grain growth has been studied by thermionic emission microscopy. This method permits continuous, direct observation of the process as it occurs. Results of studies conducted in the temperature range of 900 to 1200°C show that annealing twins form in the corner s of grains as the boundary junction migrates. The observations support the growth-accident theory of annealing-twin nucleation and indicate that prior twin nuclei are not necessary. The exact ‘triggering’ mechanism for the twinning accident, however, remains unresolved. Annealing twins grow both by broadening and by lengthening, but only when the twin boundary coincides with a migrating grain boundary. Twin bands may coalesce by the mechanism suggested by Burke. Annealing twins are annihilated during grain growth by either grain-boundary or twin-boundary migration. However, a twin can be annihilated by twin-boundary migration only by motion of a noncoherent boundary in a direction parallel to the coherent boundary. When a noncoherent boundary migrates, steps on the. boundary are always generated. While twin bands in austenite are very stable, those which terminate within a grain (a partial twin band) are thermally unstable and anneal out easily. Thus, a mechanism exists which can account for the disappearance of certain annealing twins after grain growth ceases.