China Foundry (Sep 2015)
Effects of Y content on microstructures and mechanical properties of as-cast Mg-Zn-Nd
Abstract
The effects of Y addition amount on the microstructures and mechanical properties of as-cast Mg-Zn-Nd alloy have been investigated by using an optical microscope, a scanning electron microscope, back-scattered electronic imaging technique, an X-ray diffractometer, a differential thermal analyzer and a universal testing machine. There are three kinds of ternary phases in the Mg-Zn-Y system alloys, such as I phase (Mg3Zn6Y), W phase (Mg3Zn3Y2) and Z or X phase (Mg12ZnY). The experimental results in the present study indicate that the Mg-Zn-RE (RE includes Y and Nd) ternary phases change from the I + W phases in turn to unique W, W + Z and unique Z as the Y content increases from 0% to 3%. Simultaneously, their distribution gradually changes from small particle-like form to continuous network form. The grain size first decreases as the Y content increases from 0% to 1% Y, then increases when the Y content exceeds 1% and finally decreases again when the content exceeds 3% due to the variation of growth restriction factor caused by the increased Y element and the change of the ternary phases. The hardness continuously increases because of the increased ternary phase amount. The ultimate tensile strength and elongation first increase within the range of 0-1% Y, also due to the increased ternary phase amount and grain refinement, and then decreases because of the grain coarsening, porosity formation and continuous network distribution of the ternary phases. The grain bonding strength of the W phase-containing alloys is quite strong and the W phase is an ideal strengthening phase if a given amount of it distributes in discontinuous and small-sized form. The alloy with 1% Y only has one ternary phase of W, but has the best combination of mechanical properties. The fracture regimes of these alloys always present a transgranular mode.
