| In this paper,pulse electrodeposition was used to prepare fine and uniform nanocrystalline Ni-Co alloy and ultrafine-grained Ni-Co alloy with twin structure by controlling the parameters of electrodeposition and additives.The composition,microstructure of nanocrystalline Ni-Co alloy and ultrafine-grained Ni-Co alloy were analyzed by means of X-ray diffractmeter(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),energy dispersive scanning electron microscopy(EDS)and electron-probe microanalyzer(EPMA).Meanwhile,the mechanical properties of nanocrystalline Ni-Co alloy and thermal stability of ultrafine-grained Ni-Co alloy were also studied.The main results are shown as follows:(1)Nanocrystalline and ultrafine-grained Ni-Co alloy were successfully prepared by pulse electrodeposition.According to the composition analysis,the results show that the content of cobalt in alloy is much higher than that in bath.With the addition of cobalt element,the surface morphology of the alloy changed from the initial conical shape to the spherical shape and then to the dendritic morphology,which was due to the refinement of the coating grain.When it comes to grain refinement,the effect of saccharin on grain refinement is much greater than that of cobalt.When the cobalt content is less than 50.04%,the alloy is a single face-centered cubic structure(FCC).When its content exceeds 70.03%,the alloy has a mixed phase structure with face-centered cubic structure(FCC)and dense hexagonal structure(HCP).With the increase of cobalt content,the(200)texture phenomenon is weakened.TEM and HRTEM results show that a large number of twin structures are found in four kinds of ultrafine-grained Ni-Co alloys.(2)The nano-indentation results of three kinds of ultrafine-grained Ni-Co alloys with different cobalt content show that there is no obvious dependence between the elastic modulus and the strain rate,which is an insensitive mechanical property index.It is mainly determined by the internal structure of the material itself.The hardness is dependent on the strain rate,and the higher the strain rate is,the higher the hardness is.When the cobalt content in the alloy is 80.12%,the Pop-in phenomenon is caused by the martensitic transformation,and the hardness decreases obviously.At the stage of holding,the creep of the three alloys is obvious.The creep analysis of different periods shows that,in the early stage of loading,the creep rate of the three alloys is very large,and then it becomes very slow at the later stage of holding.This is mainly due to the transformation of creep mechanism from the early deformation mechanism dominated by rapid absorption of dislocation to the deformation mechanism dominated by grain boundary slip or diffusion coordination.(3)The thermal stability of four nanocrystalline Ni-Co alloys with different cobalt contents was studied.The EPMA results show that the content of trace impurities sulfur increased gradually with the increase of cobalt content in the coating.DTA shows that the peak temperature(T_p)increased with the increase of heating rate(b).According to the improved Kissinger equation,the activation energies of the four samples were calculated,which is 1.25 eV,1.44 eV,1.94 eV and 2.10 eV,respectively.It is proved that the addition of cobalt can improve the thermal stability of Ni-Co alloy.The average grain size of the Ni-Co alloy is estimated by Scherrer formula.The statistical results show that the tempering time only accounts for 25%of the total tempering time,but the change of average grain size accounts for more than80%of the total change.The comparing results show that the thermal stability of Ni-9.94Co alloy is the worst,and that of Ni-70.03Co alloy is the best.By analyzing some diffraction peaks of XRD spectra,there is a change in the structure from dual-phase to single-phase at tempering temperature 400℃and tempering time 15min.By analyzing the influencing factors of thermal stability,it is found that the thermal stability of alloy can be improved by the addition of cobalt and trace impurities sulfur. |
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