Researches On Preparation And Mechanical Behavior Of Diphase Nanocrystalline Ni-Co Alloys

The nano-structure is defined as the structures which scale below100nanometers atleast on one dimensionality in three-dimensional space. The materials which composed ofnano-structure have the advanced properties in optics and thermodynamics. Theproperties and nature of the nano-materials caught the attention of people, however, thereare some aspects have not been know, such as the anti-Hall-Petch phenomenon and themechanism of deformation. Some interpretation in the mechanism of mechanics propertyhas been done using molecular dynamics theoretically, but there is no support ofexperiment data. Therefore, it need a series of data to compare and ensure the moleculartheory, realize the advantages and disadvantages of materials, and then improve theproperties through ameliorate the materials. In a word, we should research the propertiesof materials in detail.In this paper, Base on the electrolytic deposition method in literature and combinethe actual situation, we obtain the Ni-Co nano-alloy with smooth surface by adjusting thecurrent density and plating solution at same time optimizing the ratio of additive. Theratio are Ni-35.3%Co, Ni-41.6%Co, Ni-69.9%Co, respectively. XRD, TEM, SEM, EDS,EBSD has been used to analysis the component, characterization of morphology, and themicrostructure. Then we do the nano indentation experiment to the samples and measurethe mechanics property. Here is the experimental results:1.The nano-alloy of Ni-35.3%Co is only face-centered cubic structure through theanalysis of XRD, SEM, EDS, EBSD, in other words, Co element enter into the Ni latticeby replacement. The average grain size is large as21nm of it and there is no cluster, wecan see the particles chequered with black and white which because the angular differenceof grain boundary between the grains, we do not find the second phase particles aroundthe grain, at the same time, We also observe no hole and the compactness is perfect. 2.The nano-alloy of Ni-41.6%Co is also only face-centered cubic structure.Compared the base peak, diffraction peak of this material shift to left, the reason is thatthe material expand when adding Co atom form solid solution. In addition, it proves thefact that the texture decrease continually as the Co content increase (the size forcomputation is16nm). There are equiaxed grain and twin crystal structure existsimultaneously in this alloy.3.There are six diffraction peak with、(111)、、(200)、(311)、(222)for Ni-69.9%Co nano alloy,which grain size is15nm. The size of the three kinds ofnanometer materials is constantly decreases, which suggested that material is constantlyrefined with the increase of cobalt content. This is because when the cobalt atoms movedinto the crystal lattice, the lattice distortion is serious with the increasing of content, sothat caused a lot of material internal defects, which can make material more refined. Inaddition, when the Co content increased to a certain value, the material would changefrom a single surface heart structure into a face-centered structure and close the six-partystructure state, this situation would also promote tiny change of material. The alloycontains a lot of twin crystal structure, we consider this is due to the orientation ofdifferent grain caused by overlapping, called moire interference fringes.4.Nanoindentation results show that the mechanical properties of the materials hasvery strong dependence on the strain rate, and with high strain rate sensitivity. Incontrast to corresponding boundary adjustment simulation theory, this study obtaineddifferences with both m and v, also with the traditional deformation dislocationmechanism of coarse grained materials. Under high strain rate, materials’ deformationmechanism is main on dislocation regulation, and when under low strain rate, grainboundary regulating mechanism may also be involved, in this way, the deformationmainly grain happened with mutual coordination of boundary and dislocation mechanism.In addition, the results show that with the increasing of Co content, the series ofstrength and plastic of materials will be enhanced. However, with the continue increase ofcobalt content, namely reach Ni-69.9%Co nano alloy, its strength and plasticity declined.First of all, the material performance enhancement is due to the addition of cobalt, makes the materials form substitutional solid solution, that is solid solution strengthening.Moreover, fault energy also plays a considerable role, in lower fault energy, the first formof fault hinders the later’s dislocations proliferation, then achieves a certain reinforcement.And small size would increase the strength of the material. While in cobalt content of69.9%namely duplex alloy, the plastic and strength of material decrease to a certainextent, this is because there is bipolar and twin crystal structure in the material, whichdeformation mechanism translates into partial dislocation activation and bipolarcoordinate deformation.