Investigation On The Microstructure And Properties Of Mg-2Bi-x(Zn,Al) Alloys

Magnesium is widely used in the field of lightweight structural materials because of its small density and high specific strength.Magnesium has a dense hexagonal structure and can move at the same time,resulting in its low strength and poor plasticity,which significantly limits its application.On the basis of low alloying of Mg-Bi base alloy,the extrusion,heat treatment,cooling rate and the trace addition of Zn and Al elements on the microstructure and mechanical properties of Mg-Bi base alloy were studied systematically.The effect of the extrusion temperature on the microstructure and properties of Mg-2Bi alloy was firstly investigated.It is found that the addition of Bi not only can refine the magnesium matrix,but also will produce the Mg3Bi2 hard phase,which has a reinforcing effect on the matrix.Mg-2Bi alloy was started simultaneously with the base plane<a>,cylinder<a>and cone plane<c+a>sliding during after extrusion at 300℃.With increase of extrusion temperature,the grain size and recrystallization of Mg-2Bi alloy increase,but there is no obvious relationship between the texture and extrusion temperature after extrusion at 100℃,the Mg-2Bi alloy shows excellent mechanical properties with yield strength of 190.3 MPa and elongation of 26.83%.After annealing at 200℃ for 120 min and extrusion at 100℃,the yield strength,tensile strength and elongation of Mg-2Bi alloy are 188.2 MPa,194.8 MPa and 24.1%.After annealing at 200℃ for 90 min,the elongation of Mg-2Bi alloy exceeds 105.8%,and the yield strength reaches 168.36 MPa.On the basis of low-temperature extrusion,the effect of pouring cooling rate on the microstructure and mechanical properties of Mg-2Bi alloy was investigated.The results show that the cooling rate significantly affects the grain size of the alloy and the precipitation of the second phase,and the slow cooling rate is easy to produce the second phase.After solution strength treatment with different cooling,rates,the yield strength is 168.6 MPa and 172.5 MPa,but the elongation of Mg-2Bi alloy prepared with high cooling rate is up to 47.3%,which is much higher than that of Mg-2Bi alloy prepared with low cooling rate.After adding 1 wt.%Zn and 1 wt.%Al,the as-cast microstructure of Mg-2Bi alloy can be refined.The addition of Zn increases the hardness,while the addition of Al improves the plasticity of Mg-2Bi alloy.The yield strength of the two alloys are 218.9 MPa and 259.6 MPa,respectively.After the pre-deformation and annealing of the two alloys,there are less difference for the mechanical properties for the two alloys,and their yield strength are 275.9 MPa and 278.5 MPa,respectively,indicating that the pre-deformation and annealing of the extrusion alloy can significantly enhance the mechanical properties of the material.