STUDY ON ELEVATED TEMPERA-TURE MECHANICAL PROPERTIES OF AM50HP WITH ND

Effects of Nd on the mechanical properties of die casting magnesium alloysAM50 are investigated. It is found that the ultimate tensile strength, yield strength and elongationof the AM50 alloy at elevated temperatures up to 200 deg C increase with an addition of Nd.Generally, the most significant improvement in the tensile properties is attained at 100 deg C. Thepresence of Mg12Nd is proved by X-ray diffraction. Microstructural observations reveal that theaddition of Nd can lead to the dispersion-strengthening. Due to the presence of Mg12Nd, the slidingof grain boundaries is restrained and the strength of the alloy is enhanced. It can be seen from thetensile fractographs that the fracture surfaces exhibit the mixed fracture characteristics, andless brittle fracture is observed with increasing the temperature. Quasi-cleavage fracture isthought as the main fracture mode. TEM observations expose the formation of dislocation network,slip lines and dislocation pile-ups.

Microstructure and elevated-temperature tensile properties of differential pressure sand cast Mg-4Y-3Nd-0.5Zr alloy

The microstructures of an Mg-4Y-3Nd-0.5Zr alloy by differential pressure casting were investigated using scanning electron microscopy （SEM） and transmission electron microscopy （TEM）, and its tensile deformation behavior was measured using a Gleeble1500D themo-simulation machine in the temperature range of 200 to 400 ℃ at initial strain rates of 5×10^-4 to 10^-1 s^-1. Results show that the as-cast microstructure consists of primary α-Mg phase and bone-shaped Mg5RE eutectic phase distributed along the grain boundary. The eutectic phase is dissolved into the matrix after solution treatment and subsequently precipitates during peak aging. Tensile deformation tests show that the strain rate has little effect on stress under 300 ℃. Tensile stress decreases with an increase in temperature and the higher strain rate leads to an increase in stress above 300 ℃. The fracture mechanism exhibits a mixed quasi-cleavage fracture at 200 ℃, while the fracture above 300 ℃ is a ductile fracture. The dimples are melted at 400 ℃ with the lowest strain rate of 10^-4 s^-1.

Effect of heat treatment on mechanical properties and microstructure evolution of Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy

Regarding the as-cast Mg-9.5Gd-4Y-2.2Zn-0.5Zr alloy,the effect of heat treatment on its properties at room temperature(RT),as well as the mechanical properties and microstructure evolution of various peak-aging samples at different tensile temperatures were discussed in this article.The results indicated that the optimal heat treatment process of the alloy was:520℃×24 h+200℃×112 h.Under this condition,the yield strength(YS),ultimate tensile strength(UTS)and elongation(EL)at RT were:238 MPa,327 MPa and 2.5%,respectively.As the tensile temperature increases,the strength increases firstly and then decreases,but the ductility increases monotonously.The microstructures evolution of 200℃ peak-aging(200PA)and 250℃ peak-aging(250PA)samples were different with the increasing tensile tenperature.When tensile test processed at 150℃,the denseβ’phase and rod-shaped basalγ’phase will be formed in the 200PA sample.However,at 300℃,theβ’phases disappeared.Theβ’and LPSO phases in the 250PA sample coarsened gradually as the tensile temperature increased,and 14H-LPSO phases were formed during tensile at 300℃.The 200PA sample reached the highest strength when tensile at 150℃,which was attributed to the hindrance of the basal dislocation and non-basal dislocation slip by the prismaticβ’phases and the newly formed basalγ’precipitates.