Temperature Dependence of Work Hardening in High Purity Iron Single Crystals

The three-stage work hardening in high purity iron single crystals was investigated between room temperature and 373 K with special attention to stages 0 and I, and mechanisms for these stages are proposed. With increasing temperature from room temperature to about 350 K, the size of stage 0 decreases and the length of stage I increases. The hardening rate in stage I decreases with increasing temperature and becomes negative (work softening) above a temperature which depends on the crystallographic orientation of the specimen axis. The secondary screw dislocations are produced before the macroscopic yielding, and act as the forest dislocations to produce jogs on the primary screws. This process is the cause of stage 0. The work softening is caused by the annihilation of these jogs. The density of secondary screws produced before the macroscopic yielding decreases with increasing temperature and hence stage 0 becomes smaller. The length of stage I is determined by the dislocation structure formed in stage 0, so that it depends on temperature. Above about 350 K, the mobility of screw dislocations approaches that of edge dislocations and little secondary screws are formed before the macroscopic yielding. The work hardening behavior is similar to that in FCC metals in this temperature range.