Large Deformation Preparation Of Ag / Ni Fiber Composite Electrical Contact Material

Based on conventional plastic processes such as common extrusion, rolling and torsion, Severe Plastic Deformation (SPD) was developed during 1990s'. Materials received shear stress and produced shear strain during SPD which was different with the normal strain plastic theory during conventional plastic processes. SPD could intensively refine the grain size to submicrometer until nanometer directly, and improve properties of materials. Until now, SPD was acknowledged by international materials society to be one of the most effective methods to prepare bulk nanocrystalline and ultra fine grained materials by international academic materials.Ag/Ni belongs to the Ag alloys systems with limited dissolution during liquid state and indiscerptibility during solid state, and can only be prepared by powder metallurgy, restricting the development and application of new type Ag/Ni products. In present P.H.D thesis, Ag/Ni electrical contact materials with composite degree of 1, composite degree of 2, composite degree of 3 and composite degree of 4 were produced by SPD such as multiple clad extrusion-drawing, cluster extrusion-drawing etc. The microstructures, mechanical properties, electrical properties, the form process of contact, the direct current characters and arc erosion morphology were systematically studied by SEM, EDS and TEM etc., and the main results are as follows:The morphologies evolution of Ag/Ni20 filamentary composites were observed by SEM. With the increasing of composite degree, refinement of filament, fracture, and spheroildization or particle-like happened in the reinforce phase of Ni, and finally changed into the complex structures which were composed of short filaments and particle-like. The thickness of most filaments were less than 200 nm, and sizes of particle were 180～300nm。 TEM results showed that, dislocation band structures with low angle misorientation and band-like subgrain structure were formed in Ag/Ni20 filament composites after SPD. With the increasing of composite degree, some subgrains changed to high grain boundaries. The obvious interdiffusion was found in sample with composite degree of 4, (Ag,Ni) solid solution interface was formed between Ag and Ni grains.SPD can improve the ultimate tensile strength (UTS) of Ag/Ni20 filamentary composites to 1200 MPa, which were about 3 times more than that before SPD. The results shows that the relationship between UTS and the true strain parametersη, the volume fraction of Ni reinforce phase and the microstructure of Ni reinforce phase. UTS increased with true strain and the Ni content. The relationship between UTS and true strain or the Ni content fits well with Hall-Petch relationship, which could be depicted by dislocation strength model. UTS and component volume fraction fit well with mix rule. The strength calculating model was built by combining the Cu/Nb model and the microstructure analysis. The strength results calculated by the mathetical model fited well with the experiment results.The resistivity of Ag/Ni20 filamentary composites at different condition and composite degree were measured by the four-point method. The results show that resistivity increased with true strain. Resistivity and the thickness of Ag phase fited well with Sondheimer relationship. The increasing of resistivity could be attributed to the piling up of dislocations and interface scattering. We built the resistivity model of Ag/Ni20 filament composites with composite degree of 1, and found that the same changing tendency between calculated value and experimental value.We also built the 5 strain area model of contact by the analysis of Ag/Ni20 filamentary composite contact. The calculating results of the deformation state in individual area and deformation degree indicated that, tensile deformation was the main deformation style in areal, area 3, depress deformation is the main deformation style in area2, deflection occurred in area 4, and nearly no deformation occurred in area 5.At the condition of direct current of 17V,15A, and resistance load, Ag/Ni20 filament composite contact showed typical anode arc. Anode material transferred to cathode, and finally pit formed in the surface of anode while bulge formed in cathode surface. The arc characters could be attributed to the contact material and electrode voltage. There were several typical arc erosion morphologies such as mushy stage like solidification material, coral like structure, frame structure, hole or gas cavity, and crack.