Study Of Refinement Mechanism Of Al-5Ti-1B On Mg-xLi-3Al(x=5,8) Alloys

As the lightest metallic structural materials,Mg-Li alloys have attracted considerable attention in automobile,aerospace,defense,machinery and electronics industries owing to their low density,feeblish anisotropy,high specific stiffness and strength.However,Mg-Li alloys usually suffer from connatural drawback of low strength due to micro-porosity and shrinkage porosity.These defects hinder the further application of Mg-Li alloys.Grain refinement is one of the most effective methods to simultaneous improve strength and ductility for most engineering alloys,which is of great importance for the production of Mg-Li alloys.In general,the mechanical properties are directly related to the grain size according to the Hall-Petch equation and thus it is necessary to reduce grain size of the cast alloy as much as possible to achieve superior mechanical properties.In this paper,the grain refinement efficiency of Mg-5Li-3Al??phase?and Mg-8Li-3Al??+?phases?alloys inoculated by Al-5Ti-1B master alloy and the influences of grain size on the mechanical properties were investigated.With the addition of 1.0 wt%Al-5Ti-1B master alloy,the average grain size of Mg-5Li-3Al and Mg-8Li-3Al alloys were observably reduced.The diminution of grain size leads to a significant increases in both strength and elongation.The average grain size of as-cast Mg-5Li-3Al alloy decreases markedly from¹20?m to50?m,its ultimate tensile strength?UTS?and elongation increased from 154 MPa and 7.8%to216 MPa and 12.2%,respectively.The?-Mg phase of as-cast Mg-8Li-3Al alloy is dramatically refined and many of them convert into spherical.The average grain size of as-rolled Mg-8Li-3Al alloy decreases observably from¹08?m to 48?m,its ultimate tensile strength?UTS?and elongation increased from 138 MPa and 10.2%to 167 MPa and 17.8%,respectively.Thus,fine grain strengthening is the strengthening mechanism of investigated alloys with Al-5Ti-1B master alloy.The grain refinement mechanism can be explained by heterogeneous nucleation theory.XRD and EDS analysis indicate show that Al-5Ti-1B master alloy is composed of?-Al,Al₃Ti and TiB₂.Although this master alloy was added in the Mg-5Li-3Al and Mg-8Li-3Al alloys,chemical reaction is not detectd between matrix and refiner so that there is no any new phase in the alloys.In addition,Al₃Ti is not detected in the alloys by XRD.It can be indicated that Al₃Ti has resolved during the process of alloy smelting.Crystallographic calculations based on the disregistry model of two-dimensional lattices and EDS analysis indicate that TiB₂particles are the potent heterogeneous nucleating substrates for?-Mg and?-Li grains.To begin with,TiB₂ particles are discovered in the?-Mg and?-Li grains.Furthermore,with the crystallographic orientation relationships of?0001?_TiB2//?0001?_?-Mg and?0001?_TiB2//?111?_?-Li,the smallest disregistries between TiB₂ with?-Mg and?-Li are 5.6%and 6.73%,respectively.Therefore,the grain refinement mechanism is heterogeneous nucleation.A mass of nuclei lead to a high nucleation rate in the solidification process,which refines the alloy's grains.Moreover,according to the result of calculation,the theoretical high utilization of TiB₂particles in Mg-5Li-3Al??phase?and Mg-8Li-3Al??+?phases?alloys are 47.6%and 67.3%,respectively.