Effects of Pre-Strain on the Evolution of Microstructure and Strain Hardening of Extruded Az31 Magnesium Alloy

© 2017 Materials Research. 2017; 20(4): 1003-1009 DOI: http://dx.doi.org/10.1590/1980-5373-MR-2016-0498 Effects of Pre-Strain on the Evolution of Microstructure and Strain Hardening of Extruded Az31 Magnesium Alloy Lifei Wang a,b,c,*, Miao Caoa, Shuming Yangd, Hua Zhanga,b,c, Dongya Wangc, Xiaoqing Caoa a Shanxi ley laboratory of advanced magnesium-based materials, Taiyuan 030024, China Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China c College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, China d College of basic education, National University of defense technology, Changsha 410072, China b Received: July 02, 2016; Accepted: May 02, 2017 Pre-compression 3% and pre-stretch 3% subsequent annealing at 200ºC for 2h are conducted on AZ31 magnesium alloys, then inverse tensile and compressive deformation are carried out at room temperature, respectively. During inverse tension 3% deformation on 1st pre-compression samples, detwinning behavior happens; after 2nd pre-compression 3%, the volume fraction of {10-12} extension twins decreases comparing with 1st pre-compression. Due to the interaction of dislocation and induced twinning lamellas, strain hardening rate (θ) increases on 1st and 2nd pre-compression samples. {10-12} tensile twinning is restrained during inverse compressive deformation by pre-stretch process. Owing to the decreasing amount of twins, the texture strengthening in compressive deformation weakens. So the slop of stage III in strain hardening rate sustaining reduces after 1st pre-stretch 3% and 2nd prestretch 3% deformation during inverse compression deformation. Keywords: AZ31 magnesium alloy, microstructure, pre-strain, strain hardening 1. Introduction Due to advantages of high specific strength, low density and so on, magnesium alloys have been attracted by a large number of industrial sectors1,2. However, the basal plane only provides two independent slipping systems owing to its hexagonal close packed (HCP) structure which cannot fit Von-Misses criterion with five slipping systems3,4. At this time, twinning plays an important role to coordinate the deformation. Twinning is a polar deformation mechanism which is active only when the load is on specific direction of c-axis of Mg grains (compression perpendicular to c-axis or tension parallel to c-axis)5,6. Currently, the effects of pre-twinning induced by pre-strain on the properties of magnesium alloys are widely investigated. Song et al.7 cold pre-rolled on AZ31 magnesium plates along transverse direction for a small thickness reduction, the strength improved and yield asymmetry was weakened. Xu et al.8 conducted multidirectional pre-compression along three directions of magnesium blocks, the tension-compression asymmetry reduced. Lou et al.9 carried out dynamic plastic pre-deformation along the rolling direction at a smaller degree, the ductility improved largely. Ozaki et al.10 indicated that pre-compression along extrusion direction to induce {1012} extension twins, fatigue life of magnesium alloys got improved due to twinning-detwinning behavior during * e-mail: cycling deformation. Xin et al.11 reported that maximum thickness reduction per pass of Mg alloys sheet during rolling at 300 °C increased after pre-extension twinning. While, Zhang et al.12 pre-stretched on Mg alloy sheet and subsequent annealing, the formability enhanced greatly after pre-strain process. Besides, pre-strain plays also an important role on the strain hardening of magnesium alloys. Sarker et al.13 indicated the strain hardening rate of AM30 magnesium alloys increased with a positive slope after pre-compression along extrusion direction to induce {10-12} extension twinning during stage B. Wu et al.14 introduced the excess twinned grains through pre-compression along the rolling direction, the rapid strain hardening reduced after the pre-twinning process. Thus, pre-strain can be regarded as an effective way to modify the microstructures and properties (strength7, yield asymmetry8, ductility9, fatigue life10, and formability11,12 et al.) of Mg alloys. Besides, it has an important effect on the strain hardening during deformation13,14. About the strain hardening, pre-compression process is mainly focused; however, the effects of pre-stretch have rarely investigated. While it is much more significant to make clear how the pre-strain affects the strain hardening behavior, no matter pre-compression but also pre-stretch, especially during cycling deformation. Therefore, the effects of pre-compression and pre-stretch on the evolution of microstructure and strain hardening of Mg alloys have been systemic studied in this paper. 1004 Wang et al. 2. Experimental Procedure As-extruded AZ31 magnesium bars with a diameter of 16mm are used as the initial materials. And a diameter and length with 12mm×100mm billets are cut from the bars for pre-strain process. Firstly, the billets are 1st pre-compression by 3% along extrusion direction (ED). While subsequent annealing process is carried out at 200ºC for 2h to remove the dislocations. After that, reversed tension 3% is conducted. Then 2nd pre-compression is taken again for 3% degree. At last, the tensile samples are wire-cutting from as-received, 1st pre-compression and 2nd pre-compression billets to tensile fracture. The dimensions of tensile specimens are with nominal gage of 6mm×36mm. In order to avoid bending-buckling of Mg billet during pre-compression, a special die is used to hold the samples, as shown in section view Figure 1. Two separate holders are used to provide side stress during so as to prevent lateral strain. Figure 1. The section view of pre-compression die on Mg alloy billets The same dimensions with 12mm×100mm billets are used for pre-stretch process. Firstly, the billets are 1st pre-stretch 3% along ED. After annealing at 200ºC for 2h, pre-compression 3% is taken out. Then 2nd pre-stretch 3% is Materials Research conducted. At the end, the compressed specimens are taken from as-received, 1st pre-stretched and 2nd pre-stretched billets. The compressive specimens are with the height and diameter with 15mm×10mm. Tensile and compressive tests are carried out on a CMT6305-300KN electronic universal testing machine at room temperature. The strain rate was set at 10-3s-1. The microstructure is observed by optical microscopy and electron backscatter diffraction (EBSD). The EBSD test is conducted on ED-TD plane of the Mg specimens by a Zeiss EVO 50 SEM and the data were processed by INCA OXFORD crystal software. 3. Results and Discussions Microstructure and (0002) pole figure of as-extruded AZ31 Mg alloys are shown in Figure 2. It can be seen that equiaxial grains distribute and there are no twins emerging in the microstructure. The grain size is about 15.5μm. Besides, (0002) pole figure of Mg billet expresses a typical (...truncated)