Stress Aging Behavior And Strengthening Mechanism Of Mg-7Sn Alloy

The application of magnesium(Mg)alloys have been restricted in aerospace,automotive and industrial applications due to low high-temperature property and poor ductility at room temperature.The precipitation strengthening is the main way of magnesium alloy strengthening.Mg-Sn alloy is a potential heat-resistant magnesium alloy due to the high solid solubility of Sn in magnesium matrix at eutectic temperature and its significant decrease with the decrease of temperature,and the high thermal stability of the precipitated Mg₂Sn phase.However,the development and application of Mg-Sn alloy are restricted owe to the weak aging behavior.The size,morphology,quantity,distribution and precipitation orientation of the precipitated phase of the alloy can are regulated by alloying and mechanical heat treatment,which can effectively improve the aging hardening behavior of the alloy and shorten the peak aging time.Therefore,Mg-7Sn alloy was prepared in this paper.The microstructure evolution and mechanical properties of Mg-7Sn alloy were studied by using Zn,Y alloying and stress aging treatment respectively,and the phase precipitation behavior and strengthening mechanism of the alloy during aging were discussed.The microstructure of aging peak Mg-7Sn alloy is mainly composed of?-Mg and Mg₂Sn phase.The addition of Zn increased the mass precipitation and uniform dispersion of Mg₂Sn phase,refined the size of Mg₂Sn phase,promoted the precipitation of Mg₂Sn phase on non-basal plane,and thus enhanced the aging hardening capacity of the alloy.The aging peak hardness of the alloy increased from 64.6 HV to 69.7 HV,and the peak aging time decreased from 166 h to 142 h.The addition of Zn and Y elements formed the needlelike MgSnY phase which reduced the concentration of Sn in the magnesium matrix.Although the aging peak time of the alloy was effectively shortened(166 h to 120 h),the peak hardness of the alloy was slightly reduced with a value of 62.9 HV.The tensile test results show that Mg-7Sn-1Zn alloy has the best mechanical properties and the maximum tensile strength(?_b),yield strength(??_0.2)and fracture elongation(?_f)of the alloy are 81.2 MPa,194.5 MPa and 20.1%,respectively.The high mechanical properties of the alloy are mainly due to the precipitation strengthening of small Mg₂Sn phase with high volume fraction.Compared with the isothermal aging(160?)alloy,the grain size of the aging peak alloy is almost unchanged under the stress of 40 MPa and 80 MPa but the number of twin and Mg₂Sn phases increases with the increase of stress.Most Mg₂Sn phase lies on basal plane whereas a small amount of phase on non-basal plane in the 80 MPa stress ageing peak alloy.The morphology of Mg₂Sn phase have plate and rod and the average length of plate and rod are 30 nm and 50 nm,respectively.Two shapes of Mg₂Sn phase have an orientation relationship[1(?)1]_?//[11(?)0]_?,[101]_?//[0001]_?with?-Mg.The nucleation and volume fraction of Mg₂Sn phase are increased whereas the size decreased under stress aging.In addition,the stress aging promoted Mg₂Sn phase on non-basal plane formed.The peak hardness of the alloy increased by 5.9%from 64.6 HV to 68.4 HV,and the peak time was shortened from 166 h to 96 h.The tensile experimental results show that the alloy appears highest mechanical properties under the 80 MPa,the??_b,??_0.2 and ?_f are 217.2 MPa,163.6 MPa and 8.5%,respectively.The mechanical properties of the alloy increase are mainly attributed to the precipitation strengthening of Mg₂Sn phase and the twin boundary strengthening that contribution to the??_0.2 of alloy were 112.7 MPa and 22.5 MPa.Compared with the low temperzture(160?)stress aging alloy under 40 MPa,the size and volume fraction of plate and rod Mg₂Sn phase in the high temperzture(220?)stress aging alloy slightly decrease.It is worth noting that a nanoscale Mg₂Sn phase with an average size of 20?40 nm was precipitated at the interface of these two morphometric phases.The orientation relationship between nanoscale Mg₂Sn phase and?-Mg was[(?)12]_?//[11(?)0]_?,[(?)1(?)]_?//[0001]_?.Also,a small amount of Mg₂Sn phase on the non-basal plane is precipitated.The high temperature stress aging for a long time(>12 h)led to the growth of the Mg₂Sn phase and discontinuous distribution at the grain boundary.The peak hardness of the alloy was 72.3 HV at 12 h,and the hardness of the alloy remained almost unchanged from 12 h to 96 h.The main reason for maintaining peak hardness of the alloy for a long time(12 h?96 h)is the precipitation strengthening of a large number of fine Mg₂Sn phase with high thermal stability.The??_b,??_0.2 and ?_f of the alloy under high temperature stress age are234.9 MPa,170.4 MPa and 12.7%respectively.The contribution of precipitation strengthening to the??_0.2 of the alloy was 118.8 MPa.