Superplasticity Of ZK40 Magnesium Alloy Processed By Equal Channel Angular Pressing

Severe plastic deformation (SPD) which has been used to obtain ultrafine grains to improve mechanical properties of materials has drawn much attention in recent years. Equal channel angular pressing (ECAP), being one of the severe plastic deformation methods that can produce materials with bulk ultrafine grain sizes, has been attached much importance. The advantage of this processing technique has been manifested by the improved properties of materials, including mechanical properties, physical properties, and so on. In this paper, adopting ZK40 magnesium alloy as experimental materials, superplastic deformation characteristics and the laws of microstructure evolution of ZK40 processed by ECAP for different passes were investigated.At first, through tensile tests under the conditions of different temperatures and different strain rates, the influences of temperature and strain rate on superplastic deformation behaviors of ZK40 were researched, and the best temperature and the best strain rate in which ZK40 shows the best superplasticity were obtained. Furthermore, the tensile test proved that ZK40 processed by ECAP for 4 passes showed low temperature superplasticity (LTS) and high strain rate superplasticity (HSRS), but showed no superplasticity under the conditions of low temperature and high strain rate simultaneously. Subsequently, tensile specimen of ZK40 processed by ECAP for different passes were tested at the best temperature and the best strain rate. The tensile tests showed that with the increasing passes, the elongation-to-failure was improved dramatically. The elongation-to-failure of ZK40 processed by ECAP for 4 passes tested at 250℃ and 1x10-4s-1 was increased to 660% which showed that ZK40 processed by ECAP for 4 passes has fine superplasticity.TEM observations of ZK40 processed by different passes showed that the homogeneity and aspect ratio were improved with increasing number of pass. As a result of as-ECAPed materials with the non-equilibrium grain boundaries, many excessive dislocations accumulate near to the grain boundary which will activate other deformation mechanisms such as grain boundary sliding and grain turning to improve the plasticity of materials in the course of tensile test. Many kinds of grain refinement mechanisms function in the course of grain refinement ofZK40 processed by ECAP, and grain refinement begins from the deformation trap firstly to the inner grain gradually with the increasing strain.