Yield Point of Sapphire

Yielding in sapphire undergoing basal slip appears to occur by a dislocation‐multiplication mechanism and thus depends on the initial density of dislocation sources. In verneuil and Czochralski sapphire, these sources appear to be associated with surface damage, and yield drops can be enhanced by mechanical or thermal treatments that improve the surface perfection. Annealing and flame polishing also increased the lower yield stress. This hardening appears to be caused by formation of gaseous or solid precipitates. In addition, a yield drop can be induced in sapphire by strain aging; this phenomenon does not appear to be associated with impurities.