Microstructure Evolution Of WEA331-xSi Alloys

In this dissertation,a new magnesium alloy system Mg-3Y-1.73Nd-0.75Gd-0.38Zr-1.0Al-x Si?x=0.5,1.0,1.5,2.0??named as WEA331-x Si?was achieved by adding Si and Al elements on the basis of WE43.Samples were melted in electric resistance furnace and poured into graphite mould,and the solid solution and aging treatment of the alloys were carried out.The influence of heat treatment on the phase composition,distribution and grain growth of the alloys were studied,and the effects of phases and microstructure evolution on the mechanical properties of the alloy under different heat treatment processes were analyzed.It is shown that in the microstructure of as cast alloy,the precipitated phases of the alloy without Si are mainly composed of the reticular eutectic at the grain boundary and the bulk phase in the grain.The grain of the alloy is granular crystal,and the grain size is coarser.When a small amount of Si is added to the alloy,the reticular eutectic phase at the grain boundaries and block phase in matrix are gradually reduced.There are two new phases in granular forms and dispersing on matrix,i.e.Gd5Si3 and YSi2,whose melting points are 1650 ? and 1520 ?respectively.In addition,there is a small amount of fine granular Mg₂ Si phase in the matrix.The grains of matrix separate in short rods.When the amount of Si is added more than 1.0%,the eutectic Mg₂ Si with Chinese script and the primary Mg₂ Si with large bulk appear in matrix,the grains are transformed into granular grains again.And the grain size increases gradually.When the Si content is 2.0%,the grain size of the alloy is almost equal to that of the alloy without Si.In the solid solution microstructure,the network eutectic structure at the grain boundary of WEA331 alloy basically disappeared.There are still bulk phases in the matrix,and the quantity and size of the bulk phases are increased.When Si is less than 1.0%,the microstructure of the reticular eutectic at the grain boundary is essentially disappeared.The black bulk phase is still partially retained in the matrix and spheroidized.When adding 1.0%Si to the alloy,the bulk phase further reduced,and a small amount of granular Mg₂ Si phase uniformly distributed in the matrix.When the Si mass fraction exceeds 1.0%,the Chinese script Mg₂ Si is broken into bar or granular form,bulky block Mg₂ Si phase has a tendency of spherical transformation,but a little size of block Mg₂ Si changes.After solid solution treatment,a spherical or short rod like phase separated from the grain boundaries of WEA331-1.0Si alloy.Compared with the solid solution structure,the quantity of the black mass is increased and spheroidized.The fine granular Mg₂ Si of uniform distribution and the block phase of aggregate distribution have a tendency to grow.The grain size of the alloy increased to about 36.4mm.With the increase of aging time,the numbers of precipitates at grain boundaries and Mg₂ Si in matrix gradually increased.By hardness testing,WEA331-1.0Si alloy reached the peak aging at 225? holding 12 h.The first principle are used to calculate the phase stability of compound in the WEA331-1.0Si alloy,Mg₂ Si and Mg₂₄Y₅ are brittle phases,while the rare-earth phase YSi2 is a ductile phase.From the perspective of thermal stability,the stability of Mg₂ Si is the best,the stability of YSi2 is the second,and the stability of Mg₂₄Y₅ is the worst.