Investigation On Pure Tungsten By Severe Plastic Deformation

For refractory metals such as W,Mo they are usually produced by powder metallurgy in industry, which has introduced mess impurities and vices, and that producing process has resulted in a low ductility at room temperrature , which has greatly limited its application.As an effective way for producing significant grain refinement in bulk crystalline solids, severe plastic deformation (SPD)has obtained much interest. Research showes that the SPD can refine the coarse microstructure to nano-microstruture. The two SPD processing techniques of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) are currently receiving considerable attention, as both procedures are effective in refing the grains and make high-angle grain boundaries(GB).The present work is to obtain ultra-fined grain size in commercial tungsten via ECAP and HPT,studying the change of microstructure,mechanical property and thermal stability,through which we analyze the improved ductility of ultra-fined W produced by SPD. The results reveals that:The grain size of tungsten has been refined from 76.4μm to 52.5μm after 2 passes ECAP deformation at 1300℃, route Bc,without obvious refinement,but vices become smaller and the rate of vices decreases. HPT on CG W at 450℃results in homogeneous,equiaxed microstructure with an average grain size -150nm surrounded by GB with over 92% of high-angle GB;HPT processing on CG W at 490℃results in inhomogeneous microstructures with subgrains size -500nm surrounded by large fraction of low-angle GB(LAGB),and the GB are mainly made of tangled dislocations.The microhardness test reveals that micro hardness has been increased from 429HV to 500HV after HPT490℃deformation without obvious improvement. And it is nearly 2 times as that of as received one after HPT450℃deformation with the value of 900HV,which is consistent to the result of microstucture analysis. Annealing experiments and DSC analysis reveals a better thermal stability of nano-structure processed by HPT at 450℃. than the. micro-structure processed by HPT490℃.The deformation tempertature of HPT make a influence to the microstructure stability and michenical property by forming different microstructure ,including different grain size and different fraction of LAGB.