The Fine-grained Rolling Process And High Temperature Superplastic Deformation Behaviors Of Mg-AI-Zn System Magnesium Alloys

Magnesium and its alloys are promising materials in military,automotive,medical,computer and other industries due to their low density and high specific strength.However,the commercial applications of magnesium alloys has been limited mainly because of their low formability due to their hexagonal close-packed crystal structure having a limited number of operative slip systems at room temperature.Therefore,how to improve the formability of magnesium alloys,especially those have high aluminum content,has confused the researchers a long time.Recently,Severe plastic deformation(SPD)techniques,like friction stir process(FSP),high pressure torsion(HPT)and equal-channel angular processing(ECAP)have been utilized to produce ultrar-fine grained AZ91 alloys.SPD usually leads to a prominent grain refinement and texture weakening,which therefore,improves the plastic deformation and strength of the materials.Note that investigation of a convenient fine-grained rolling process for Mg-Al-Zn alloys is necessary to expand the application of magnesium alloy for.The main research contents of this thesis are the fine-grained rolling process and high temperature superplastic deformation behaviors of Mg-Al-Zn system magnesium alloys,and the conclusions are as follows:(1)Developed a convenient rolling process for achieving fine-grained superplastic Mg-Al-Zn sheets,resolve the formability of magnesium alloys.The rolled AZ91 sheets after annealing has an average grain size of 3.1μm and the exhibits excellent properties at room temperature and elevated temperatures.At a strain rate of 1.0×10-3 s-1,at 25 Κand 300 ℃,elongation-to-failure could reach 563%and 735%,are observed.(2)It revealed that the optimum initial state for AZ91 alloy is as-extruded state for achieving fine-grained superplastic sheets compared with the as-cast state and cast-rolling state.At 300 ℃,the as-extruded&rolled alloy has a much greater superplastic elongation(697.0%)than the as-cast&rolled alloy(302.4%)and the cast-rolling&rolled alloy(403.0%).(3)The optimum pretreatment process is 430 ℃&3 h for the as-extruded AZ91 alloys.At 300 ℃,the three rolled alloys with different pretreatment had obtained superplastic deformation.Note that the as-extruded&rolled AZ91 alloy by homogenization at 430℃ for 3 h exhibited a higher superplastic elongation(694.3%)compared to rolled AZ91 alloy without pretreatment(510.9%)and rolled AZ91 alloy by solid solution at 415 ℃for 24 h(448.3%).(4)The influence of Al-content(3%,6%,9%)on the superplasticity of Mg-Al-Zn alloys was investigated.The elongations-to-failure for the AZ31,AZ61 and AZ91 alloys were 108.8%,326.8%,478.0%respectively at 250 0C,and 182.6%,346.2%,694.3%at 300℃.With the Al-content increasing,the Mg17Al12 particles were increased,which resulted in significant enhancements in the superplasticity.(5)It is revealed that the liquid phase auxiliary GBS was considered to be the main superplastic deformation mechanism.Since the particle clustering,growth and changing in morphology are always correlated to the GBS,a convenient strategy for the GBS observation was provided in this study by examining the morphology and size evolutions of second-phase Mg17Al12 particles.