Experimental Study On O-DECAP Of Pure Tungsten And Its Microstructure And Properties

As a refractory rare metal,tungsten has excellent physical and chemical properties,and has broad application prospects in aerospace,electronics,chemical industry,nuclear industry and other extreme environmental fields.However,tungsten has low temperature brittleness and recrystallization brittleness,which limits its application.Therefore,optimizing tungsten microstructure and improving its mechanical properties are the key to promote the development of tungsten materials and expand its application field.One of the ways to improve the properties of tungsten is to reduce the grain size,improve the homogeneity of the structure and increase the density under the condition of certain composition.The process method based on large plastic deformation theory has strong grain refinement effect,and the preparation process does not introduce impurities,which can significantly improve the properties of materials.Occlusive Double Equal Channel Angular Pressing（O-DECAP）is one of the large plastic deformation methods.Through multi-pass extrusion,the material can obtain a large cumulative strain,achieve fine structure,obtain ultrafine grain or even nano-crystal material,and improve the comprehensive properties of the material.In this paper,the O-DECAP deformation is simulated and experimented under the condition that the extrusion path is Bc path and the temperature is 900℃.The multi-pass deformation behavior of pure tungsten is analyzed by finite element software.The deformation can be divided into three stages:initial deformation stage,shear deformation stage and stable deformation stage.After single-pass deformation,the average strain of the specimen is 1.15.After multi-pass deformation,the strain of the specimen is worth increasing,the strain accumulation is realized,and the strain uniformity of the specimen is also improved to varying degrees.After three passes,the minimum uniformity coefficient is 0.28,and the strain uniformity of the specimen is improved greatly compared with that of the first and second passes.The process of tissue evolution in different parts of the tissues with multiple passages was discussed briefly.Microstructure and mechanical properties of deformed pure tungsten were analyzed and discussed by OM,XRD,TEM,EBSD and microhardness test.It can be seen that the average grain size of pure tungsten in initial state is 80～100μm,and the average grain size after multi-pass deformation is about 2μm.With the increase of passes,dislocations and other defects in grain and grain boundary have changed greatly,and dislocations mainly concentrate near grain boundary and transfer into grain.After deformation,grain orientation changes from{001}to{112}facets.The samples before and after deformation are mainlyα-W crystalline,and there isγ-W in the deformed samples.With the increase of passes,the microstructures change.After three passes,the microstrain is 0.07784%,and the dislocation density is 2.837×10¹⁴m^-2.After deformation,the grain boundary angle and proportion of the structure changed greatly.With the increase of passes,the proportion of large angle grain boundary decreased.After three passes of deformation,the average hardness of the specimens reaches 521±4HV,and the uniformity coefficient is 3.45%.After pure tungsten is deformed by DCEAP,XRD and TEM tests verify that there are transformations in the process of deformation,i.e.gamma-W and beta-W.The path of phase transition is simply analyzed.The shear stress leads to the slip of the maximum close-packed surface of bcc-W,which leads to the slip of atoms.When the slip of atoms is a certain displacement,they enter a relatively stable form.