A comprehensive study on the effect of heat treatment on the fracture behaviors and structural properties of Mg-Li alloys using RSM

In the present work, the effects of annealing temperature on fracture toughness and anisotropy properties of Mg-Li alloys have been studied for the first time, comprehensively. To do this, two different annealing temperatures, including 200 degrees C and 350 degrees C, have been exerted on the produced sheets of LZ71 and LZ91 alloys. Also, microstructure observations, microhardness measurements, and phase analysis of the samples have been accomplished using optical microscopy (OM), vickers microhardness test, and x-ray Diffraction (XRD) analysis, respectively. Moreover, uniaxial tensile tests were accomplished to study the mechanical and anisotropy properties of the produced plates. Furthermore, to evaluate the fracture properties and observe the fracture behavior of the samples, the first mode fracture test and fractography have been carried out. From the OM photographs and XRD results, present Mg-Li alloys contain dual-phase microstructures including a partitioned lattice Hexagonal Closest Packed (HCP) structures of alpha-Mg phase spread in a Body Centered Cubic (BCC) matrix of ss-Mg-Li. Calculated anisotropy coefficients demonstrated that r-values of as-rolled and annealed specimens of LZ71 were more than that of LZ91 in all directions. The results of performed fracture tests showed that the fracture toughness values of all samples of LZ71 were considerably more than that of LZ91 while the trends of the changes in fracture toughness of the alloys were not the same. In addition, in the present study, Response Surface Method (RSM) has been applied to fit a model between variables and responses.