The microstucture,mechanical properties and fracture behaviors of semi-continuous cast Mg-8Gd-3Y-0.5Zr(wt.%,GW83K) alloy after different heat treatments were investigated.Almost all the eutectic compounds were dissolved into the matrix and there was no evident grain growth after optimum solution treatment at 500 ℃for 4h.Further ageing at low temperatures led to significant precipitation hardening,which strengthened the alloy.Peak-aged at 200 ℃,the alloy had the highest ultimate tensile strength(UTS) and lowest elongation at 395 MPa and 2.8%,respectively.When aged at 225 ℃ for 15 h,the alloy exhibited prominent mechanical performance with UTS and elongation of 363 MPa and 5.8%,respectively.With regard to microstructure and tensile properties,the processes of 500 ℃,4h+225℃,15 h are selected as the optimal heat treatment conditions.The alloy under different conditions shows different fracture behaviors:in the as-cast alloy,a quasi-cleavage pattern is observed;after solution treatment,the alloy exhibits a trans-granular quasi-cleavage fracture;after being peak-aged at 200 ℃ and 225 ℃,the fracture mode is a mixed mode of trans-granular and intergranular fracture,in which the inter-granular mode is dominant in the alloy peak-aged at 200 ℃.
The microstructure and mechanical properties of Mg-Zn-Ho-Zr alloys have been investigated in detail. The grain size of the as-cast Mg-Zn-Ho-Zr alloy was greatly decreased by the addition of Ho, and the grain growth during solution treatment was suppressed by Mg-Zn-Ho phases formed at grain boundaries. These thermally stable Mg-Zn-Ho phases could not completely dissolve into the matrix dur- ing solution treatment, and the strengthening effect of solution-plus-ageing treatment weakened. The addition of Ho can greatly enhance the high-temperature elongation of the Mg-Zn-Ho-Zr alloy, but the increase of high-temperature tensile strength was just a little.
The sliding friction and wear behaviors of Mg-11Y-5Gd-2Zn-0.5Zr （wt%） alloy were investigated under oil lubricant condition by pin-on-disk configuration with a constant sliding distance of 1,000 m in the temperature range of 25-200℃. Results indicate that the volumetric wear rates and average friction coefficients decrease with the increase of sliding speeds, and increase with the increase of test temperature below 150℃. The hard and thermally stable Mg12（Y,Gd）Zn phase with long-period stacking order structure in the alloy presents significant wear resistance, The wear mechanism below 100℃ is abrasive wear as a result of plastic extrusion deformation. The corporate effects of severe abrasive, oxidative, and delaminating wear result in the tribological mechanism above 100℃.
Characterizations of phases in Mg-10%Y-5%Gd-2%Zn-0.5%Zr(WGZ1052) alloy during heat treatments were investigated by OM,XRD,SEM and EDS.The mechanisms of microstructure evolution were discussed.The results show that,after high temperature heat treatments,the Mg12ZnY phases still exist.During solution-treatment at 535 °C,the amount of the long-period stacking order structures decreases.At 545 °C for 20 h and 24 h,there are still remnant Mg12ZnY compounds in the Mg matrix,the shape of which does not change and the amount does not decrease obviously.
The microstructure and crystallographic texture characteristics of an extruded ZK60 Mg alloy subjected to cyclic extrusion and compression(CEC) up to 8 passes at 503 K were investigated.The local crystallographic texture,grain size and distribution,and grain boundary character distributions were analyzed using high-resolution electron backscatter diffraction(EBSD).The results indicate that the microstructure is refined significantly by the CEC processing and the distributions of grain size tend to be more uniform with increasing CEC pass number.The fraction of low angle grain boundaries(LAGBs) decreases after CEC deformation,and a high fraction of high angle grain boundaries(HAGBs) is revealed after 8 passes of CEC.Moreover,the initial fiber texture becomes random during CEC processing and develops a new texture.
