Study On Microstructure And Mechanical Behavior Of Nanocrystalline Co By Magnetron Sputtering

The study of nanocrystalline materials has been for decades,and it is generally accepted that nanocrystalline materials have superior mechanical properties such as high strength.The strength of the coarse grain material is increasing as the grain size decreases,and the result of dislocation accumulation is considered.The law of the nanometer scale is still applicable,but when a certain critical value is reached,the intensity decreases as the size decreases,which is thought to be caused by grain boundary slip.What is the effect of the difference on the above mechanical behavior?Based on the above questions,in this paper,three kinds of nanocrystalline cobalt with different phase structure were prepared by magnetron sputtering method.The microstructures were characterized by XRD,SEM and TEM.The mechanical properties were studied by nanoindentation.And the deformation behavior of nanocrystalline materials is explained by dislocation bow model.(1)Three kinds of nanocrystalline cobalt with different structures were prepared by controlling the process parameters in the process of magnetron sputtering,and the duty ratio of programmable DC power was changed.We successfully prepared hexagonal structure,mixed phase structure,face-centered cubic structure,and then the microstructure of the three nanocrystalline materials was characterized by X-ray,scanning electron microscopy and transmission electron microscopy.Analysis of nanocrystalline materials with three different structures shows that the larger duty cycle contributes to the formation of face-centered cubic phase.The grain size of the hexagonal nanocrystalline cobalt is 37 nm,the grain size of the mixed phase nanocrystalline cobalt is 38 nm,and the grain size of the face-centered cubic nanocrystalline cobalt is 25 nm.At the same time,because the cobalt with low stacking fault energy can be relatively low,three kinds of nanocrystalline materials allpossess twin(2)V(d)(or L(d),m(d,T)and ?y(d)functions for FCC metals are developed based on the dislocation bow-out model.Assuming the size-dependent dislocation length and thus size-dependent activation volume,the present modeling framework extends the dislocation-based mechanistic description of the size-dependent properties and responses of FCC metals down into the nano-meter regime.The model also successfully predicts the rate-dependent mechanical behaviors,as well as the H-P breakdown and associated critical grain size,of FCC NC metals.The model is shown to be in fully consistent with numerous experimental studies of UFG and NC materials.Two limitations of the model however are T ? RT and a quasi-static strain rate range,which guarantee the dominating deformation mechanism of dislocation movement within grains.At once either of the limitations were got crossed,the GB-mediated deformation gradually takes a role and thus deviations from the model would occur.This unified model indicates that all size-dependent mechanical functions have intrinsic relationship from the dislocation point of view,which is expected to have further applications in constitutive law development.(3)The experimental results of nanoindentation combined with Ma et al.Show that the grain size of nanocrystalline cobalt decreases from 25 nm to 18 nm,the inverse Hall-Petch relationship occurs,the critical grain size is about 25 nm.The three kinds of phase structure of nanocrystalline cobalt showed considerable strain rate sensitivity.The strain rate sensitivity of nanocrystalline cobalt is obviously higher than that of nanocrystalline nickel,and the cobalt with low stacking fault energy,which is easy to produce stacking fault and twinning.The dislocation activity is not only in grain boundary,there are also some of the interface,which may be the reason for the high m value of the nanocrystalline cobalt.There are a large number of phase interfaces in the biphasic phase,and the phase interface has dislocation activity similar to that of the twin interface,which leads to a significant increase in the m value of the biphasic.The nanoindentation of nanocrystalline cobalt with three kinds of phase structure shows that they have obvious deformation and creep deformation.The relaxation of the unstable dislocations generated during the loading process,andthe creep deformation caused by the dislocation of the grain boundary and the twin boundary.The phase interface of the two-phase nanocrystalline cobalt is similar to that of the twin interface,resulting in a higher creep amount at the low load.