Microstructure,mechanical Properties And Deformation Mechanism Of Nanostructured Pure Niobium

This paper selects pure niobium as the research object,and two different processes are used to process it.First,I homogenize the material structure through the process of combining multi-directional forging and annealing,and perform rotation accelerated shot peening(RASP)treatment on the annealed pure niobium samples to prepare gradient nanostructured pure niobium with different thicknesses.Second,I process pure niobium samples with equal channel angular extrusion(ECAP)and unidirectional rolling at room temperature(the rolling reduction is 90%)to prepare nano-lamellar pure niobium with different deformation passes.For the pure niobium samples with different nanostructures prepared under the two processes,I test the mechanical properties of the samples by using the stretching machines,hardness testers and other equipment.Meanwhile,to study the relationship between the microstructure and mechanical properties of body-centered cubic nanostructure pure niobium,I characterize their microstructures by using SEM,EBSD,XRD,TEM and other equipment.The main conclusions are as follows:(1)After RASP treatment,the surface of annealed pure Nb has severe plastic deformation,the roughness increases,and the grain size,microstrain and dislocation density are distributed in a gradient along the thickness.The surface grain is obviously refined and the grain boundary is blurred.The surface layer of the sample is nanocrystalline,and the grain refinement effect is obvious.According to XRD statistics,the average grain size is about 90 nm,and then the grain size increases gradually.The maximum depth of deformation layer is 50μm,there is no obvious boundary between fine grain layer and coarse grain layer.(2)After RASP treatment,the surface hardness and yield strength of the material are significantly improved,but the plasticity decreases compared with the annealed state.Two reasons for the decrease of uniform elongation are shown as follows.First,the cyclic impact of steel ball causes work hardening of the sample.Second,the shot peening speed is faster,resulting in more holes,cracks and other defects on the surface.The gradient structure formed by shot peening of coarse-grained samples with different thickness has similar morphology.There are arc cracks on the surface layer,and the crack depth is 20μm or so.As the thickness of the material increases,the volume ratio occupied by the core layer becomes higher,which prolongs the stage where the elastic-plastic interface expands from the core to the surface layer,the yield strength decreases,and the uniform elongation increases.Therefore,the 2 mm sample has the best performance under the rasp parameter treatment in this paper.(3)The fracture mode of gradient nanostructured pure niobium is basically ductile fracture,and the fracture presents a typical dimple state.The dimple size also showed a gradient change.Near the surface of the sample are dimples with smaller size,which are relatively flat as a whole.With the increase of depth,the dimple size gradually becomes larger and deeper,and there are holes.The large dimple reflects that the core of the material has good plasticity.(4)Nano-lamellar pure niobium can be effectively prepared by the combination of B_CECAP and room temperature unidirectional rolling.The width of nano lamellae in 8P+CR and16P+CR is about 50 nm.Due to the recovery effect caused by the increase of temperature in room temperature rolling and the micro-shearing behavior in the process,there are some relatively coarse lamellae in 8P+CR and 16P+CR.(5)The strength of 8P+CR and 16P+CR is nearly ten times higher than that of the original sample.With the increase of ECAP deformation passes,the strength and hardness are only slightly different;however,the plasticity and strength are the opposite trend,and the plasticity decreases sharply after 90%rolling.The sharp decrease in plasticity can be attributed to the dislocation density and,the decreasing lamellar spacing with rolling strain and the proportion ofΣ3 boundary increases.(6)According to EBSD analysis,the texture of 8P+CR is mainly concentrated on theγorientation line,and some of them gather on theαorientation line,while the texture of 16P+CR mainly concentrated on theα-oriented line.Therefore,it can be considered that with the increase of ECAP deformation passes,the texture of ECAP+CR gradually changes transfer fromγorientation line toαorientation line.