On the Nonbasal Slip in Magnesium Crystals

Abstract

In order to determine the characteristics and controlling mechanisms of the nonbasal slip which is the secondary slip system of several hcp metals such as magnesium, tensile tests were performed on single crystals of high purity magnesium having a special orientation whose tensile axis is parallel to the (0001) basal plane, and thereby the temperature and strain rate dependence of flow stress of the nonbasal slip was investigated.
Following are remarkable characteristics of the nonbasal slip in magnesium: (1) The basal slip has a wide easy glide region more than 100% in shear strain, while the nonbasal slip has no plastic region corresponding to this easy glide and rapid work hardening occurs at the early stage of deformation. Below 100°C, this rapid hardening continues up to fracture and the elongation to fracture is limited to only several per cent. At above 200°C, however, the work hardening rate becomes practically zero during further deformation beyond a certain strain. With rising temperature, this critical strain decreases and the ductility is markedly developed. (2) The strain rate dependence of flow stress is very remarkable and is about 10² times as large as that of the basal slip observed at low temperatures. (3) The temperature dependence of the flow stress is also very remarkable. Below about 350°C, the flow stress increases rapidly with lowering temperature, while above this temperature the flow stress does not so much depend on the test temperature. (4) The slip bands observed are short in length and occurs in association with frequent cross slips. These results, indicate that the superjogs formed by easy cross gliding of the screw dislocations from the nonbasal plane to the primary slip (0001) plane, would play an important role in the nonbasal slip. It is further considered that the thermally activated process which controls the nonbasal slip is varied with temperature as follows; when the strain rate is 2.3×10⁻³ sec⁻¹, the interaction of glide dislocations with trail dislocation loops left behind moving screw dislocations from room temperature to about 250°C, the climbing motion of the superjogs on screw dislocations from about 250°C to 350°C, and the process of bowing out on the nonbasal plane of the constricted and recombined screw dislocations initially extending on the basal plane about 350°C.