Canned ECAP Processing And Deformation Mechanism Of Pure Mg At Room-temperatrue

As the lightest structure material nowadays, Mg alloys have been widelyused in automobile, transportation, electronics, aerospace, aviation and so on.However, the application of magnesium and its alloys has been greatly limitedby their poor ductility due to its hcp crystalline structure, where only limited slipsystems can be activated at room temperature. Grain refinement is an effectiveway to improve the comprehensive properties of materials, not only theplasticity and ductility could be greatly improved, but also the strength of thematerial can be significantly enhanced. Equal channel angular pressing (ECAP)is an advanced technology that can fine microstructure of materials and henceimprove the comprehensive properties and processability of materials. In orderto avoid cracking during the extrusion process, in the most case, ECAPprocessing magnesium alloys have to be performed above200℃. But graingrowth always concurrently occurs with grain refinement and the grain refiningis greatly weakened. In order to make full use of grain refinement effect of ECAP technique andavoid the grain growth at high temperature, an novel equal channel angularextrusion technique with bulk pure magnesium cladded in pure Fe sheath wasdeveloped and the large pieces of pure magnesium and magnesium alloys can beprocessed at room temperature for multi-passes which make it possible tofurther refine their grains. Further, the sheath configuration was modified andthe room-temperature multi-pass ECAP processing up to8passes of puremagnesium was accomplished and the deformation mechanism of pure Mgduring the multi-pass ECAP processing was investigated.Bulk pure magnesium with dimensions of867mm after conventionalextrusion and annealing was cladded in pure Fe sheath before ECAP processingin order to increase three-dimensional stress conditions during the deformationwhich protects pure magnesium from cracking. Cladded pure magnesium wassubjected to8-pass ECAP processing in route BCand with the extrusion rate of4mm/min at the room temperature, and the resultant grains of pure magnesiumwere refined significantly.The microstructure, texture, mechanical property and the deformationmechanism of pure magnesium specimens before and after conventionalextrusion, post-annealing and ECAP processing were analyzed by electronback-scattered diffraction(EBSD) using a field emission gun-scanning electronmicroscope and the hardness was tested by the Micro Vickers Hardness Tester.The cladded pure magnesium was ECAP processed at250℃for one pass, its microstructure and mechanical property and the deformation mechanism wereanalyzed and compared with those after one pass ECAP at room-temperature.Pure magnesium grains were refined after conventional extrusion and basaltexture with an inclination of10°~15°accompanied by some diffusive non-basaltexture was formed. The hardness of as-extruded pure magnesium increasedobviously. There is no obvious change in texture after annealing puremagnesium, but the hardness was reduced slightly.Pure magnesium subjected to ECAP processing for8passes at the roomtemperature was achieved by modifying the pure Fe sheath and the integratedpure magnesium sample without obvious crack on the surface was obtained.Pure magnesium after the room-temperature ECAP processing was significantrefined. With the increasing of ECAP passes, the grain size was reducedgradually and the microstructure was becoming homogenized. After eight passesECAP processing, the grains of pure magnesium were more uniformand thegrain size is up to several hundred nanometer, the hardness of pure magnesiumincreased with increasing the processing passes.Grain size of pure magnesium after one pass ECAP processing at250℃was larger than that after one pass ECAP at room-temperature. However, thedeformation mechanism and the hardness of pure magnesium after one passECAP at250℃is similar to that at room-temperature.