Investigation On Improvement Of The Ductility Of Electrodeposited Nanocrystaliine Ni And Alloys

In this paper, microcrystalline nickel (mc-Ni)， ultrafine grainednickel (UFG-Ni), nanocrystalline (nc) nickel with broad/narrow grain sizedistributions (BGSD/NGSD nc-Ni) and nc Ni-2.4%Co and Ni-15.4%Cowere fabricated by electrodeposition by changing the type of substrates,additives and bath composition. The microstructure, tensile properties o fall deposits was investigated, and the way to improve the ductility of ncNi and alloys was analyzed. The experimental results are shown asfollows:Compared with the low ductility of UFG-Ni processed using SPDtechniques, electrodeposited UFG-Ni exhibits a high tensile ductility of10.44%, which is comparable to that of mc-Ni. The rationale is that UFGNi exhibits pronounced strain hardening until necking due to its highstorage capacity of dislocations.For BGSD nc-Ni, the existence of some large grains allowsappropriate dislocation accumulation, resulting in a higher level of strainhardening capacity, and consequently, BGSD nc-Ni has a markedlyenhanced ductility of8.4%. The intrinsic tensile curve for BGSD nc-Nishows a constant stress deformation stage, and fracture by shear; Wh ile aincomplete tensile curve shows premature plastic instability before theappearance of the flat plastic region, mainly because that during thedeposition of nickel, flaws like vacancies and porosity may for m in thedeposit due to the cathode hydrogen evolution, resulting in theenhancement of crack formation at the grain boundary when deformed,and finally fracture in vertical.After annealing at150℃, the grain size changes slightly. Abnormalgrain growth was observed for annealing at200℃. Further annealing at 300℃, extensive grain growth took place, most grains grew to microscale.Thermal annealing can lead to the relaxation of grain boundary and theincreased order of grain boundaries, which can make the grain boundaryactivities such as the dislocation emission and grain boundary slidingmore difficult, resulting in the increase in strength and the loss of tensileelongation after annealing below200℃.For nc Ni-2.4%Co and Ni-15.4%Co, the grain sizes and theirdistributions are comparable. In addition, twins were observed in bothalloys. The Ni-15.4%Co alloy has a higher work hardening rate, yield andultimate tensile strengths and ductility than the Ni-2.4%Co alloy. Theseresults indicate that decreasing the stacking fault energy increases th estrain hardening rate of the nc fcc metals, and consequently increase boththe tensile strengt h and the ductility.