Microstructure,Strength,Plasticity,and Fracture Toughness Of Mg-Gd-Y-Zn-Zr Alloy

In this thesis,an Mg-Gd-Y-Zn-Zr alloy ingot was selected and processed by homogenization treatment,extrusion deformation and aging treatment.The influence of different heat treatment processes as well as different extrusion parameters on the microstructure and mechanical properties of the alloy was studied through microscopic observation and mechanical property testing,and the fracture toughness was further studied.As-cast Mg-Gd-Y-Zn-Zr alloy is composed of an equiaxed crystal and a large number of Mg3 Gd phases distributed at the grain boundary,as well as a small amount of lamellar LPSO.the as-quenched sample after homogenization treatment at 510? for8 h will obtain a good solid solution effect.There are a large amount of Mg3 Gd phases at the grain boundary dissolved into the matrix.And a large number of lamellar14H-LPSO are generated in which the sample cooled in furnace.After homogenization treatment,the mechanical properties of the alloy were improved.However,the plasticity of the sample cooled in furnace is not as good as that of the as-quenched ones,mainly because the LPSO do not have directionality and is likely to be the source of cracks.After the extrusion deformation of the alloys,the microstructure of the alloys could be a significant bimodal structure,a large proportion of dynamic recrystallization occurs in the alloy.And with the extrusion temperature and extrusion rate increase,the proportion of recrystallization area increases a lot.The mechanical property of the as-quenched sample is very close to the sample cooled in furnace at relatively low extrusion temperature.However,the sample cooled in furnace has lower strength and greater elongation compared with the as-quenched sample,after the temperature reaches a certain degree,this is related to a large number of LPSO which contains in the sample cooled in furnace..The alloy has a good aging effect after age treatment and can precipitate a large number of nanoscale ?' phases to strengthen the alloys.The higher the extrusion temperature is,the more effective the aging precipitation effect of the alloy will be.The fracture toughness of as-cast Mg-Gd-Y-Zn-Zr alloy is low,after homogenization treatment,the fracture toughness of alloy can be improved,it mainly due to solution strengthening,but the fracture toughness of the sample cooled in furnace instead of lower,this is mainly because the LPSO phase does not have the same orientation,then is unable to take advantage of the ability to improve plasticity.Besides,a large number of LPSO phase at grain boundaries are prone to become the source of the crack.The key of nanometer indentation method is to obtain accurate critical indentation energy?*.The nano-indentation test results are in good agreement with the three-point bending test results of magnesium alloy in as-extrusion sample and T5 sample,and the difference is between 1 and 2.but the result of as-cast alloy needs to be further improved in accuracy.Though the scratch test results are in biggish gap with the three-point bending test results,they follow the same trend,which indicating that the scratch method can reflect the influence of microstructure changes on fracture toughness of magnesium alloy to a certain extent.