The Effects Of Zr On High Temperature Property Of The Mg-2.8Nd-0.35Zn Magnesium Alloy

Effects of Zr on the microstructure and mechanical properties at elevated temperature were investigated by means of optical microscope and scanning electronic microscope(SEM).Process of settling down of Zr particles in melt was calculated by using second law. The experimental results show both grain size and grain morphology of Mg-Nd-Zn alloys are significantly affected by addition of Zr. Mean grain size at the centre of samples is about 1200μm when no Zr addition is made. Grain size decreases dramatically to 520μm when 0.2%Zr addition and further decrease with the increased Zr addition. Grain size is 98μm when Zr addition reaches 0.6%, then decreases slightly. Grain size decreases only 5μm when Zr addition varies from 0.6% to 1.2%. This means that there is a critical value for Zr addition until 1.2% after which the grain size changes little. Effects of elements on grain size of Mg alloys were quantitatively estimated by using Grain Restriction Factor. The results show that affect of Zr is the most important. Both tensile strength and elongation of Mg-Nd-Zn alloys at elevated temperature were increased with the increasing of Zr addition. SEM observation of fractograph for.6%Zr addition shows that plastic pits and tearing ridges are uniformly distributed. This indicates Mg-Nd-Zn alloys refined by Zr has a high ductility. Under 75 and 100MPa at 250℃, the creep-resistance is improved with the increased Zr addition. The creep rate is the lowest when 1.2%Zr was added. The main factor affecting setting of Zr particle is its size based on calculating for Zr particle setting down in melt. Setting distance has little change when the size of Zr particle is less than 10μm.Zr particle can be stable in the melt as long as setting time for Zr particle is reasonably controlled. This is beneficial to the refinement of Zr-containing Mg alloys. When the size of Zr particle is more than 100μm, the setting distance changes greatly even its size changes a little. Zr particle is susceptible to set down, resulting in detriment to refinement of Zr-containing Mg alloys.