The Study Of The Microstructure Evolution And Corrosion Behaviour Of Mg-Al Alloys

Alloying is an effective way to develop high performance and low cost cast Mg alloys,and the thermoplastic deformation process is an effective means to improve both the mechanical properties and corrosion resistance of Mg alloys with high Al.In this paper,Mg-9Al-1x（x=Cu,Zn）alloys were prepared by gravity casting method by adding Cu and Zn elements to Mg-Al alloys,and alloys were subjected to hot extrusion and equal channel angular pressing（ECAP）at different temperatures（250℃,300℃,350℃ and 400℃,respectively）.The effects of the two deformation methods on the microstructure evolution and corrosion properties of the alloys were systematically investigated using characterisation techniques such as SEM,TEM,EBSD and electrochemical testing.Mg-9Al-1Cu was hot extruded at 300℃.The microstructure evolution,second phase precipitation behaviour and mechanical property changes of the alloy before and after hot extrusion were investigated.It was found that the microstructure of the as-cast alloy was dominated by coarseα-Mg dendrites and bandedβ-Mg₁₇Al₁₂phases,and the Cu elements were mainly enriched in the tightly packed hexagonal structure of Mg Al Cu rings.In the extruded alloy,the Mg Al Cu phase is fragmented and diffusely distributed in the form of rods,while theβ-Mg₁₇Al₁₂ phase is refined and distributed in the form of a spherical mesh at the grain boundaries.The alloy shows significant grain refinement and discontinuous dynamic recrystallization（DDRX）after hot extrusion.In addition,the extruded alloy exhibits a significant selective orientation,with most of the grain c-axis perpendicular to the ED（extrusion direction）;the extruded alloy ages at a constant temperature of 180℃,producing a significant age strengthening,with the 24h aging alloy showing the best age strengthening with a tensile strength of 262 MPa.The DRX mechanism and texture behaviour of the Mg-9Al-1Zn alloy were investigated by ECAP deformation at different temperatures.The results show that the DRX mechanism is dominated by twin-induced dynamic recrystallization（TDRX）for ECAP deformation at 250℃ and 300℃,and DDRX for ECAP deformation at 350℃ and 400℃.In addition,the{10（?）2}tensile twins of the ECAP deformed alloys at 250℃ and 300℃ were formed preferentially in grains with a tendency to[01（?）0]and[（?）2（?）0]orientations,respectively.The alloy texture weakened with increasing deformation temperature and the twinning decreased,with the maximum pole density decreasing from 13.76 mud to 8.62 mud,respectively.The corrosion resistance of thermally deformed alloys was investigated in 3.5wt.%NaCl solution and simulated body fluid（SBF）solution,respectively.The results show that grain refinement,second phase reticulation and detwinning during thermoplastic deformation can significantly improve the corrosion resistance of the alloy.