Plastic Deformation And Fracture Behavior Of Commercial Pure Ultra Fine Grained Tantalum & Tungsten

Tantalum and tungsten are typical representations of bcc refractory metal.These matals play irreplaceable roles in various fields including military,chemistry and nuclear industry due to their excellent properties such as high atomic number,high density and good high temperature strength.However,the grain sizes of tantalum and tungsten produced in extremely high temperature are usually coarse,affecting their mechanical properties,which limits the further applications.So far,bulk full density tantalum and tungsten with ultra-fine grain(UFG)can be processed via severe plastic deformation(SPD)such as ECAP et al.By refining the grain,strength can be effectively improved and the toughness of tungsten can be remarkably enhanced as well.Compared with fcc metals,research work in bcc refractory metal,especially in UFG scale,is relatively less.research on basic problems still need further study,as the influence of external causes such as deformation mechanism or fracture mechanism and internal causes like deformation conditions on the deformation and fracture behavior.In this dissertation,taking tantalum as a example we studied the effect of grain size and temperature on the strain rate sensitivity and the deformation mechanism by measuring thermal activation parameters in bcc metals by regarding thermal activation theory as the breakthrough,controlling the microstructure via SPD and heat treatment and designing mechanic experiments;taking tungsten as a example studied the influence of the factors as grain size,dislocation density,sintering pore,plastic strain and fracture mode on the fracture behavior in bcc metals.The main research contents and conclusions are as following:(1)In order to simplify relevant calculation and derivation,dislocations in slipping were regarded as rigid line segments approximatively in classical thermal activation theroy.Investigators have pointed out the deviation induced by the approximation in forest mechanism of fcc metals but correlation research in bcc metals is still scarce.In this dissertation,based on the framework of classical thermal activation theory,according to different characteristics of dislocation slipping in fcc and bcc,we proposed a physical model on the basis of kink movement in dislocation slipping,which can be simultaneously applied to bcc and fcc metals.Based on the new model,a new strain rate equation dominanted by dislocation slipping was deduced and more accurate and quantitative relationship between strain rate sensitivity and activation volume was proposed.The strain rate sensitivity and activation volume of UFG tantalum were tested by strain rate jump tests and repeated stress relaxtion respectively and the experimental results matched well with theoretical values.(2)Work hardening of UFG materials processed by SPD have reached saturation state and futher capacity of dislocation propagation has been limited.Plastic instability with necking stage will be arised soon after yielding during tensile tests,leading to the difficulty in measuring strain rate sensitivity(SRS)by strain rate jump tests on uniform deformation stage.In this dissertation,by analytic derivation,it was verified that SRS can be detennined using engineering stress and engineering strain data by strain rate jump tests on plastic instability stage,which is equivalent to that measured on uniform deformation stage.(3)It is verified that kink pair nucleation mechanism of screw dislocation is the rate controlling mechanism of bcc metals during plastic deformation at low temperature.The activation volume does not change with grain size and dislocation structure.Result shows that the athermal part of flow stress is the main factor which effect the strain rate sensitivity of bcc metals in plastic deformation,inducing the decrease of strian rate sensitivity with the increasing of dislocation density and the refinement of grain size.(4)Transformation in plastic deformation mechanism leads to the change in mechanical properties.Research shows that in intermediate temperature range of Tc<T<Ta,sensitivity of temperature on the effective stress decreased significantly.There are controversies on the rate controlling mechanism in this temperature range.One side of the controversies proposed that the rate controlling mechanism transformed from one kink pair to multiple kink pairs in a screw dislocation wheras others claimed that the mobility of screw dislocation will increase with the temperature leading to the rate controlling mechanism transformed to forest mechanism.By measuring the activation volume,Helmholtz activation energy,effective stress and their temperature sensitivity,clarified that dislocation slipping was still dominant in plastic deformation in the intermediate temperature range but rate controlling mechanism of dislocation slipping changed from kink pair nucleation mechanism to forest mechanism.(5)it is generally acknowledged that the smaller of grain size and the higher of dislocation density,the higher of the tougness of commercial pure tungsten.In this dissertation,the experimental results which were opposite to the general regulation in literatures were found by modifing the microstructure,indicating the deformation histroical denpendence of toughness.It is revealed that plastic strain is the key factor of improving the toughness of tungsten in SPD process.The atomic distributions of deformed and annealed stated tungsten were characterized by the new technique APT clarifying the toughening mechanism of plastic strain.