Microstructure And Properties Evolution And Thermal Stability Study Of Pure Molybdenum By High Pressure Torsion

With the continuous development of science and technology,refractory molybdenum materials have been more and more widely required in the fields of aerospace,national defense,military industry and nuclear physics.At present,powder metallurgy is the main method of making molybdenum,but there are some problems such as coarse grain and low density.However,due to the difficult deformation of molybdenum,the problem of a large number of defects still can not completely solved.The high pressure torsion(HPT)process is a typical sever plastic deformation method,which can effectively close the pores and refine the grain.The molybdenum materials obtained after HPT have excellent comprehensive properties.The application of ultrafine molybdenum obtained by HPT process widens its application field and enables it to serve in worse conditions.In this paper,form 0.5 to 5 revolutions of HPT tests of pure molybdenum bar after fully annealing were carried out by using semi-restricted HPT mold at room temperature(RT)and 6GPa.The samples after the deformation were analyzed by metallographic observation(OM),X-ray diffraction analysis(XRD),electron back scattering diffraction(EBSD)analysis,transmission electron microscope(TEM),nano indentation testing technology,microhardness test,heat treatment methods.The microstructure and property evolution of pure molybdenum under different strain variables and the thermal stability of the microstructure were studied at several scales Research shows that: With the increase of high pressure torsional deformation strain,the grain was refined,the grain is elongated along the shear direction,the dislocation density increases obviously,large Angle grain boundary proportion increases,the grain has a certain orientation,and obvious texture can be observed.When the number of torsion turns is 5,after the strain variable increases further,the grain is no longer elongated,basically all of them are equiaxed crystals,dislocation density decreases slightly,the ratio of large Angle grain boundary increases further,the texture is more pronounced.The performance tests of the samples in different tissue states showed that the hardness of the materials gradually increased with the increase of the number of revolution,and the hardness of the same sample gradually increased from the center to the edge,and the difference decreased with the increase of the number of revolution.The annealing experiments with different revolution numbers showed that the recrystallization behavior of the samples was significantly different at the same heating rate and time,and the microstructure and microhardness of the samples were also significantly different after annealing.In general,the recrystallization temperature of the sample increases under the same heating rate and holding time with the increase of HPT strain.After recrystallization,the microhardness of the material was lower than that of the samples without overheating treatment after HPT deformation,but higher than that before HPT deformation.The annealing test results show that HPT deformation can change the recrystallization behavior and improve the thermal stability of the material.