Research On Microstructure And Mechanical Properties Of Ultra-fine Grain Mg-Al-Ca-Mn Alloy Deformed By High Pressure Torsion

In this paper,the extruded Mg-1Al-1Ca-0.4Mn(wt.%)alloy is selected as the research object.The alloy is subjected to high-pressure torsion deformation with different turns at room temperature,and the change of the microstructure and microhardness that occurs with the increase of the turns of torsional deformations is studied.The initial structure of the alloy is changed by changing the extrusion temperature and the extrusion rate.The effect of the different initial structure of the alloy on the microstructure and microhardness during high-pressure torsion deformation is analyzed.The alloy after high-pressure torsion deformation is artificially aged at different temperatures to further improve the microhardness of the Mg-Al-Ca-Mn alloy.The average grain size of the alloy extruded at 200°C is reduced to 280nm after 20 turns of high-pressure torsion deformation.During the high-pressure torsion deformation,the coarse eutectic Mg₂Ca phase is deformed,its particle and the volume fraction decreases.At the same time,the original distribution along the extrusion direction is transformed into a uniform distribution on the entire sample,and the high strain introduced by the high-pressure torsion deformation causes some fine nano second phase to decompose and re-dissolve.The alloy extruded at 300°C also experienced a reduction in the size and content of the coarse second phase during the high-pressure torsion deformation,as well as the decomposition and re-dissolution of some nano-precipitated phases.However,after 20 turns of high-pressure torsion deformation of the alloy,The average grain size is refined to 160nm,which is smaller than the average grain size of the alloy extruded at 200°C after the same high-pressure torsion deformation process.This is due to the higher concentration of solute atoms in the alloy matrix extruded at 300°C.In that case,it has a stronger hindering and pinning effect on dislocations,thus,the density of dislocations increases and the degree of grain refinement is higher.The microhardness of the alloy extruded at 200°C becomes saturated when the equivalent strain becomes 3.0,which is 90HV,an increase of 10HV compared to the initial extruded alloy.The strengthening mechanism of the alloy is mainly grain boundary strengthening.A large number of dislocations introduced during the deformation process and dispersed second phase particles also contribute to the microhardness value of the alloy.The alloy extruded at 300°C also reaches the saturated microhardness at an equivalent strain of3.0,which is 88HV,15HV higher than the initial extruded alloy,which is attributed to the smaller grain size and higher dislocation density.That means that the different initial microstructure states have an effect on the refinement of the microstructure and the increase of microhardness of the alloy during the deformation process,but have little effect on the saturation microhardness value of the alloy after high-pressure torsion deformation.The main source of strengthening of this alloy is also the ultra-fine grain refined to sub-micron level,and the proportion of grain boundary strengthening has increased.The alloy extruded at 200°C undergoes subsequent heat treatment after 20 turns of high-pressure torsion deformation.The alloy is artificially aged at different temperatures.It is found that this alloy is different from the initial extruded alloy which has basically no age hardening phenomenon.Obvious age hardening response can be produced at low temperature,which is caused by the decomposition and re-dissolution of some nano second phase in the high-pressure torsion deformation process,which causes the solute atom content in the alloy matrix to increase.Increase the aging temperature,the microhardness continues to increase,and the time to reach peak aging is greatly shortened.After aging at a lower temperature,the alloy undergoes incomplete recovery and recrystallization.As the temperature rises,the degree of recovery and recrystallization increases.