| These materials in the form of bulk nanostructured structures have become important in many technologies which include aerospace and manufacture industry due to their im-proved physical, chemical and mechanical properties. Electrodeposition is a versatile and inexpensive method for fabricating dense bulk nanocrystalline metals. A great deal of re-search has been devoted to studying experimentally their mechanical behavior and plastic deformation mechanisms using various methods which include mechanical properties, ten-sile tests and compress tests. But the fatigue behavior and fracture mechanism in these ma-terials are still a controversy issue. Further exploration on this problem is clearly vitally significant from viewpoints of fundamental research as well as applications.Determine the optimum process parameters by the concentration of brightener A and B, mixing method, bath temperature, pH and current density affected the preparation and microstructure of nanocrystalline alloys. A superior quality nanocrystalline Ni-Fe alloys was prepared by this electrodeposition technique. Normal mechanical performance of these as-deposited nanocrystalline materials were analyzed by Microhardness tester and tensile testing system. Strain controlled, fatigue tests were conducted on a servovalve controlled electrohydraulic testing machine for analyzing fatigue behavior at room temperature.The EDS, XRD, SEM and TEM were used to examine composition, phase composition, micro-structure and fracture surface and to identify fatigue fracture mechanism.The main results of this paper are shown as follows:(1) A detailed discussion about the effect of the concentration of brightener A,B, mixing method, bath temperature, pH value and current density to the surface of mi-cro-morphology, microstructure and microhardness has been given. The analysis of expe-rimental result shows the optimum parameters of fabricating nanocrystalline Ni-Fe alloys: the concentrations of brightener A,B are 0.5g/L and 2g/L; mixing method is cathode inter-mittent agitation; bath temperature is 60℃±1℃; pH value is 3.8±0.1; current density is 7A/dm2.(2) It was demonstrated that the percent of Fe is 17.65wt.%, the grain sizes of this material are fine, the orientation of the crystallography is uniform. To statistic 300 size of grains, it can be obtained that the range of grain size is 5--60nm and the average grain size is about 23nm. The microhardness of nanocrystalline Ni-Fe alloys is 553.40Hv. During the strain rate increases from 10-3s-1to 100s-1, the result has showed that the yield strength, ul-timate tensile strength and the elongation are basically unchanged.(3) As the strain amplitude increases the amount of the number of cycles to failure decreases from the 0.6% strain amplitude to the 1.2% strain amplitude. When strain ampli-tude is lower than 0.6%, Specimens have no fracture until 105 times. Cyclic hardening has been observed at fatigue process. Shear zone appears on the surface of nanocrystalline Ni-Fe alloys, and as the strain amplitude increases shear zone are more and more concen-trated.(4) The failure of the nanocrystalline Ni-Fe alloys was found to be somewhat strain-dependent. When strain amplitude is greater than 0.8%, the sample fractured into two parts. Signatures of micro-void coalescence fracture and intergranular fracture can be identified during TEM and SEM investigations. |
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