On vacancy condensation and the origin of dislocations in growth from the melt

Abstract

The formation of dislocations from condensed vacancies has boon re-examined. Conditions for transformation of vacancy clusters into dislocation loops are analysed, taking into account an entropy contribution of atoms in the dislocation coro. New evidence in the literature for a vacancy concentration beyond that of equilibrium at the advancing solid-liquid interface is used to calculate an excess vacancy concentration trapped in the solid as a function of freezing velocity. This excess leads to an estimation of the nuoleation frequency of loops and supports the condensation mechanism as the source for dislocations. The critical stage for dislocation formation in growing crystals is also shown to depend upon the temperatura distribution in the solid. .Results of this analysis are applied to the formation of dislocations in a general f.c.c. metal, and specifically in lead, aluminium and copper, for which experim ental inform ation is available. Striation formation is discussed for these metals; it is concluded that the original mechanism proposed b y Teghtsoonian and Chalmers (1951) and extended by Frank (1956) cannot be discarded.