Study On Deformation Microstructure And Properties Of Nanocrystalline Nickel And Nickel-iron Alloy

X-ray diffraction line profile analysis is an impotant method to investigatenanocrystalline (nc) materials, which is widely used to quantitatively characterize themicrostructural parameters. Although this method only gives semi-quantitative results,there are still several advantages in most research field due to its convenience. In thispaper, the microstructural evolution during plastic deformation of nanocrystalline Ni,Ni-20wt%Fe (Ni-20Fe) and Ni-30wt%Fe (Ni-30Fe) with a mean grain size of around20nm is quantitatively evaluated by X-ray diffraction line profile analysis. On this basis,the effects of temperature, deformation modes and stacking fault energy (SFE) on themicrostructural evolution are analyzed. Moreover, the mechanical properties of suchthree nanocrysatalline metals are investiged by the hardness measurement. Finally, thecorrelation between microstructure and mechanical property is explored.The microstructural evolution of nc metals show that:(1) The grain shapes of ncmetals remain equiaxed after plastic deformation. With increasing equivalent strain,the mean grain size increases. A (200)preferred orientation is observed in all threenc metals and can be further strengthed by deformation. The decrease in the(111)/(200)diffraction intensity ration with increasing equivalent strain indicates thatgrain rotation occurs during deformation. Such neighboring grains with similarorientation can lead to the grain coalesce/growth occurs.(2) A correlated trend in theevolution of dislocation density and stacking fault probability is found in the three ncmetals. In the early stage of deformation, as a result of high-density dialocations,which can provide a high probability for the formation of stacking faults, both stackingfault probability and dislocation density increase. In the late stage of deformation, dueto apparent grain growth, dislocation-dislocation and dislocation-twin interaction,stacking fault probability and dislocation density decrease to some extent. Furthermore,the normalizd densities of dislocation density and stacking fault probability in nc Niare compared and suggest that the normalized density of stacking fault deviatesdownward significantly from that of dislocation density when the grain size increasesabove35nm. This is accounted for the grain size dependence of dislocation activity.For nc metals, there exist a crossover from partial dislocation-mediated process toperfect dislocation-mediated process when the grain reach a critical size. The criticalgrain size for nc Ni, Ni-20Fe and Ni-30Fe is around20nm,25nm and30nm, respectively.(3) Temperature has an important effect on the microstructure of ncmetals. An obvious grain growth under elevated temperature in the as-received nc Ni.Meanwhile, the original (200)preferrd orientation disappears in the annealed samplebut has a random grain orientation distribution. In contrast, the pre-deformed nc niceksamples are subjected to recovery and recrystallization, resulting in abnormal graingrowth and strong (200)recrystallization texture. It is also found that massivecrystalline defects produced by pre-deformation plays an important role in theannealing process and final grain orientation of nc metals. Based on the fitting resultfrom the DSC curves of recrystallization, it is concluded that a small plastic strain canenhance the thermal stability of nc grains by postsponing recrystallisation to a hightemperature.(4) The mean grain size of rolling side is smaller than that of non-rollingside after the pack-rolling deformation. Compared to conventional cold-rolling, thegrain growth rate is still slower. The grains in non-rolling side tend to form (200)preferred orientation by the applied shear stress, whereas the grains in rolling side tendto form (220)preferred orientation under plane strain compression. Quantitativetexture analysis shows that the volume fractions of {001}100and {001}110components decrease with increasing equivalent strain, whereas those of{110}001,{110}112,{112}110and {123}634componentsincrease for both the roll-bonding side and non-roll-bonding side. Especillay forroll-bonding side, the {001}100and {001}110components nearlydisappear, instead the volume fractions of other texture components is higher than thatof non-rolling side. Detailed microstructure analysis suggests such a remnant of theoriginal cube component can be primarily attributed to partial dislocation slip andgrain coarsening.(5) The SFE of nc Ni-Fe alloys decreases with increasing Fe content.Due to different SFEs, there are some discrepancy in cross-slip and emission ofdislocations from the grain boundary. The critical equivalent strain corresponding tothe saturation of dislocation density in nc Ni, Ni-20Fe and Ni-30Fe is around0.31,0.12and0.06, respectively, whereas the saturation of stacking fault probability occursat0.28,0.11and0.07. Moreover, since Ni has a relatively high SFE, the dislocationdensity and stacking fault probability of nc Ni are smaller than those of nc Ni-20Feand Ni-30Fe alloys.The mechanical property of nc metals show that:(1) For nc Ni, Ni-20Fe andNi-30Fe, the hardness increases at first and then slows down. The critical equivalentstrain corresponding to the transition between strain hardening and strain softening is about0.30,0.10and0.05, respectively, which is related to Fe content. In addition, thecritical equivalent strain corresponding to the microstructure evolution is in goodagreement with the critical equivalent strain corresponding to the transition betweenstrain hardening and strain softening, indicating such mechanical transition is stilldominated by microstructure. At early stage of deformation, the increase in hardness is apositive contribution of the high-density crystal defects, exhibiting strain hardening;while at late stage of deformation, with crystal defects being saturated and even tendingto decrease, the hardness starts to decrease due to the negative contribution of grain size,exhibiting strain softening. Based on the contributions of crystal defect and grain size tostrain hardening and strain softening, the critical grain size corresponding to mechanicaltransition for nc Ni, Ni-20Fe and Ni-30Fe is32nm,26nm and24nm, respectively.(2)The nucleation and storage of crystal defect is influenced by SFE to some extent. Thelower the SFE, the easier the accumulation and saturation of grain boundarydislocations. With the faster increase in flow stress which depend on crystal defect, thenc metal with low SFE enter into strain hardening more quickly. Besides, due to thefaster grain growth in low SFE metals, there will be a lower flow stress which dependon grain size, and thus strain softening occurs at relatively small equivalent stain.