.ag / Sno <sub> 2 </ Sub> Materials During Severe Plastic Deformation, Microstructure Evolution, To Strengthen The Mechanism And Performance

It is well known that Ag/SnO₂ electric contact composites would substitute Ag/CdO composites for they are nontoxic,and they will be put to use widely in the future.But it is a bottleneck for developing this kind material that they are difficult to manufacture.Therefore,to develop advanced electric contact materials,it is very important to understand the strengthening mechanisms and deformation mechanisms of Ag/SnO₂ materials.The electric contact materials of Ag/SnO₂ are fabricated by Reactive Synthesis and Powder Metallurgy in this thesis.They are processed by the severe plastic deformation.The microstructure and physical performance of Ag/SnO₂ materials are studied in different stage.The strengthened mechanisms are studied too.By analyzing the evolution of the microstructure of Ag/SnO₂ materials,it is founded that the severe plastic deformation is beneficial to improve the microstructure of materials.Before severe plastic deformation,SnO₂ particles present as agglomerations and fiber,however,after it,the SnO₂ particles become more tiny and distribute homogenously in the Ag matrix.So that the microstructure homogenization is realized.Ag/SnO₂ materials are processed with the true strain of 3.89,7.78 and 11.7.For Powder Metallurgy,with the true strain increasing,the tensile strength,plastic and electricity are improved;further more,for the Reactive Synthesis,with the true strain increasing,the tensile strength reduced,percentage elongation and electricity increased.The tensile strength evolvement trend is studied.The strengthened mechanisms which related to microstructure are analyzed.The strength of Ag/SnO₂ materials which fabricated by Reactive Synthesis is greater than Powder Metallurgy.For Reactive Synthesis,the strengthened mechanisms of Ag/SnO₂ materials mainly include the fiber-strengthened mechanism,dispersion-strengthened mechanism, thermal -dislocation -strengthened mechanism and machining- hardening mechanism. While the true strain is small,the strengthened mechanisms of Ag/SnO₂ materials mainly include the fiber-strengthened mechanism.However,while the true strain is great,the SnO₂ particles distribute homogenously in the Ag matrix,the strengthened mechanism is dispersion-strengthened mechanism.For Powder Metallurgy,the strengthened mechanism of Ag/SnO₂ materials mainly include the dispersion -strengthened mechanism.Tensile fracture surfaces are observed.Dimple fracture is the main fracture mode of composites for both methods.The dimples are more are smaller and homogenous, while comparing the methods of Reactive Synthesis and Powder Metallurgy.With the true strain increasing,the dimples are smaller and homogenously more and more.It is related the situation that SnO₂ particles distribute in Ag matrix.