Study On Microstructure Morphological Control And Properties Of Mg-Y-Zn Alloy With LPSO Phase

Over the years, Magnesium alloy never fail to fascinate the scientific and technical worker due to its low density, excellent electromagnetic shielding and high damping performance, Magnesium alloy has a wide application in the sports equipment, aerospace and electronics. Unfortunately, the untreated microstructure of the alloy has a negative effect on the abrasive wear resistance, the corrosion resistance and strength, thus restricting its further use. Accordingly, in order to improve the properties of the Mg-Y-Zn alloy, the modification,the mechanical stirring, the isothermal heat treatment and the sub-rapid solidification method were selected to study on the microstructure and properties of Mg₉₇Zn₁Y₂ alloy in the present paper. The main conclusions are as follows:（1） After the element Si was added into the Mg₉₇Zn₁Y₂ alloy, the black particle phase was emerged in the alloy, LPSO phase was transformed from lath shap into island state, and the bulky Chinese-script shape of Mg2 Si phase was shown in Mg₉₇Zn₁Y₂-1.8wt.%Si alloy, the hardness was increased with the element Si content increasing. As the the element Si content was 1.8wt.%Si, the hardness value of the alloy was 95.5Hv, increased by 40%. The tensile strength and the elongation of 0.2wt.%Si-containing alloy were 220 Mpa and 10%, which increased by 24% and 36% comparing to cast Mg₉₇Zn₁Y₂ alloy, respectively. The tiny particle phase of Mg5Si6 or Mg2 Si was distributed evenly around the magnesium matrix, which prevented the Cl- of the corrosive liquid into the matrix, thus reducing corrosion rate of the alloy.（2）After the Mg₉₇Zn₁Y₂ alloy was brought into mechanical stirring, the LPSO phase was transformed lath shape into dissociate island, the microstructure morphology was from ice-like dendrite to the broken dendrite, then the large pieces of the broken dendrite became small and there were a few fragments at the same time, and finally became rosiness shape. As the stirring time went up to 5min, the hardness value, the tensile strength and the elongation were 90.2HV, 191 Mpa and 11%, respectively. With the stirring time prolonging, the corrosion rate fell down due to the microstructure of the alloy distributed evenly resulting in generating the additional passivation, thus impeding corrosion. The microstructure of alloy was easier to gain rosiness shape by high stirring speed shear rate, As the stirring speed reached 700r/min,the tensile strength and the elongation were 110 HV, 210 MPa and 8.3%, which increased by61%, 18.6% and 13.7%, respectively. With the stirring speed ascending, rosiness shape helpfully improve the corrosion performance, while the grain boundary became bulky after mechanical stirring, due to the corrosion mainly occurs at the grain boundary and grainboundary was susceptible to corrosion, was easier to show up, thus accelerating the corrosion rate of the alloy.（3） The Mg₉₇Zn₁Y₂ alloy was held at 540℃, 575℃ and 600℃ by isothermal temperature heat treatment at 20 min, the size order evolution of the microstructure were large-small-large,which goes through three processes, namely coarsing, microstructure separation and spheroidizing. The evolution trend of the microstructure was dendrite-irregular spherical block-round-shape at different holding time. As the holding time about 15 min, the shape of the microstructure was even, round and spherical grain.（4）The microstructure of Mg₉₇Zn₁Y₂ alloy consisted of three areas by negative pressure suction, they were small equiaxed crystal zone, columnar structure and bulky isometric zone.As the diameter of the alloy was 8mm, the primary dendrite arm spacing was 25μm, then the cooling rate was around 63.8k/s. As the diameter of the alloy was 4mm, the secondary dendrite arm spacing was 9.67μm, then the cooling rate was around 138k/s arm spacing. With the cooling rate increasing, the grain boundary segregation was reduced and the element enrichment could be effectively restrained, the refined microstructure tended to distribute evenly around the matrix.