A Study On The Tensile Deformation And Fracture Behaviors Of As-extruded Magnesium Alloys

As the lightest structural metallic materials, magnesium alloys have high specific rigidity, high specific strength, excellent electromagnetic shielding ability, damping and heat dissipating properties. Therefore, they have wide application prospects in the fields of aeronautic and astronautic instruments, automobile panels and 3C products. Now, the dosage of cast magnesium alloy products is bigger than that of wrought magnesium alloy products. Wrought magnesium alloy products mainly obtained by extrusion, forging, rolling have finer integrated mechanical properties compared with cast magnesium alloy products and are able to meet increased demands for diversified structural application. Nevertheless, up to now, there has been little study on microstructures, mechanical properties and fracture behavior of wrought magnesium alloys, which have held up the development of wrought magnesium alloys. The microstructure evolution, tensile deformation and fracture behaviors of the as-extruded AZ91, ZK60 and AZ31 magnesium alloys under different conditions have been studied systematically by using optical microscope(OM), X-ray diffraction (XRD), and scanning electron microscope(SEM). The main research items and results are as follows:The effects of heat treatment on the tensile deformation and fracture behavior of as-extruded AZ91 and ZK60 magnesium alloys were examined. The results show that the ductility is significantly enhanced by solution to the as-extruded AZ91 alloy, in contrast, the ductility and tensile strength are reduced by solution to the as-extruded ZK60 alloy, and the hardness values and tensile strength of the alloys are significantly enhanced by the special combination of solution and artificial aging treatment. The grain sizes of the as-extruded alloys are coarsened and precipitated fine particles during solution and artificial aging treatment. The fracture surfaces are featured as a typical ductile fracture, evidenced by the existence of dimples on the fracture surfaces as a result of growth and coalescence of microvoids for the as-extruded and solution treated alloys. However, the fracture mechanisms changed from typical ductile tearing to mixture of ductile and brittle for aged AZ91 alloy. And the fracture surfaces of aged ZK60 alloy assume the features of cleavage fracture. The surfaces are characterized by cleavage steps and cleavage rivers.The application of magnesium alloys are mainly limited by their poor plasticity at ambient temperature, so it is very important for magnesium alloys to acquire properties of large elongation and superplasticity by grain refinement. The results indicate that continuous extrusion forming (CONFORM) can remarkably refines the grains structure and modities texture of AZ31 magnesium alloys and significantly enhances the ductility. The fracture mechanisms changed from mixture of ductile and brittle to dimpled rupture compared with the conventional direct extrusion.Additional, the tensile deformation and fracture behavior of spray deposited FVS0812 heat-resistant aluminum alloy sheets were studied by uniaxial tension tests at temperatures ranging from 250℃to 450℃and strain rates from 0.001 to 0.1s-1.The associated fracture surfaces were examined by SEM. The results show that the total elongations to fracture increase not only with increasing temperature but also with increasing strain rate, which is in marked contrast with the normal inverse dependence of elongation on the strain rate exhibited by the conventional aluminum alloy sheets. The SEM fracture analysis indicates that the dependence of elongation on the strain rate may be due to higher dynamic recovery in the fine grained materials produced by spray deposition.