Mechanical properties of pure molybdenum in micro and nano scale during high pressure torsion
Because of its high strength at room temperature and high temperature, excellent wear resistance, corrosion resistance and good thermal physical properties, refractory metal molybdenum is widely used in aerospace, electrical and electronic fields. Molybdenum products produced by traditional P / M and plastic processing methods often have problems such as insufficient performance improvement and uneven distribution caused by residual pores, coarse grains, impurity enrichment at grain boundaries and deformation texture, which limits the engineering application of molybdenum. High pressure torsion (HPT) process, as a typical representative of SPD process, is considered to have excellent grain refinement effect. It can obtain ultra-fine grain or even nanocrystalline materials, and improve the comprehensive properties of f7l materials. At present, the mechanical tests of HPT samples mainly focus on microhardness and uniaxial tension, and the results mainly reflect the influence of microstructure evolution on the macro mechanical properties. However, for the ultra-fine products obtained by large plastic deformation, it is of great theoretical significance to study the mechanical properties obtained by the coordinated deformation of several grains at micro and nano scale. Therefore, nano indentation test was used to characterize the evolution of mechanical properties of pure molybdenum during HPT. The technology can be used to analyze the elastic modulus, hardness, fracture toughness, viscoelastic or creep behavior of materials in micro nano scale . At present, the application of nano indentation is mostly limited to thin film materials, and the research on bulk materials, especially large plastic deformation specimens, is rarely reported. In addition, the stress-strain curves of different deformed materials were obtained by using finite element simulation technology. The elastic and plastic changes of materials were analyzed combined with microstructure evolution. The relationship model between yield strength and equivalent strain in HPT process was established, and the hardening behavior of materials was discussed.
The material used in HPT deformation experiment is sintered industrial pure molybdenum with purity of 99.95% and density of more than 98%. Its grain size is very coarse and uneven, with an average size of 356 + 64 um. In the experiment, rzu200hf pressure torsion special experimental machine and quasi restricted HPT mold developed by our research group were used. HPT experiments were carried out at room temperature with a pressure of 6 GPA, a torsion speed of 1 R / min, and a torsion number of 1, 2 and 5 turns, respectively. The microstructure of the sample was characterized by jsm-7001f field emission scanning electron microscope equipped with sem-ebsd system and Fei TECNAI G2 f20s twin field emission transmission electron microscope. The nano indentation test was carried out in the same area by Agilent nano indentor G200 nano indentation instrument. In order to ensure the accuracy of the test, more than five effective tests were carried out on each sample, and the average value of the test results was taken after removing the influence of error.
