| Molybdenum and it’s alloys with good high temperature mechanical properties, arewidely used in metallurgy, mechanical manufacturing, aerospace and nuclear powerindustries and other fields. Traditional molybdenum alloys due to less hard phase havebeen increasingly unable to meet applications in cases of high temperature friction.Carbide reinforced molybdenum alloys were researched widely at home and abroad inrecent years. Having higher melting point and hardness, carbides as additives are able toimprove the wear properties of molybdenum alloy.This project has mainly studied on the effects of carbides TiC and NbC, as well asCr on the microstructure and properties of Molybdenum alloys.The researches can bedivided into two parts. First we studied the synthetic process and microstructures ofTi-Nb-Cr-C/Mo alloys. Secondly TiC/Mo, NbC/Mo, Cr/Mo, TiC-Cr/Mo andNbC-Cr/Mo alloys were produced by directly adding carbides into molybdenum. Thenthe microstructures and wear properties of these five alloys were investigated. Andsome conclusions as following can be drew.Nb and Cr in the Ti-Nb-Cr-C/Mo alloys can dissolve in the Mo matrix, and thesolid solubility increases with the adding amount. Nb soluted could make the Mo’slattice distorted with expansion, while Cr soluted make Mo’s lattice distorted withcontraction.There were a large number of second phase particles on the matrix of theTi-Nb-Cr-C/Mo alloys which consist of oxide CrNbO4, TiO2and NbO2. There should becarbides by thermodynamics analysis, but they were not detected by XRD. So thecarbides were rare if ever generated.When TiC was added alone, molybdenum grain size decreased with TiC content.TiC has a good refining effect on molybdenum grain, and in the sintering process theycan prevent grain growth by pinning effect, and could make molybdenum grain to beunder10μm. And the micro hardness of molybdenum substrate can be improved up to319HV by the fine-grain reinforcing. The molybdenum grain size of NbC/Mo alloy and Cr/Mo alloy both grew up tomore than50μm, but decreasing with NbC content and increasing with Cr content.Grain growth is due to the solid solution formation of Cr and Nb in the molybdenum,and it could reduced the atomic diffusion activation energy, so making primary graineasy to grow up. NbC was just partly dissolved to formate solid solution, and theundissolved NbC could pinning on the grain boundary to prevent the grain growth,therefore molybdenum grain decreased with the increase of NbC content. When addingCr alone, the micro hardness of Mo alloy could have a greater improvement because ofthe solid solution strengthening effect, and micro hardness improved up to379HV, Butmicro hardness didn’t improved with the content change of NbC.The grain size of TiC-Cr/Mo alloy decreased gradually with the TiC contentincreasing, and the grain size of NbC-Cr/Mo alloy increased gradually with NbCcontent increasing. The micro hardness of composite reinforced molybdenum alloy ishigher than that of the first three single strengthening molybdenum alloys. The highestmicro hardness of TiC-Cr/Mo alloy and NbC-Cr/Mo alloy can reach509HV and393HVrespectively.At room temperature, adding TiC, NbC, Cr individually can not significantlyimprove the wear-resistant performance of molybdenum alloy. Under the oxidationcondition of500℃, when TiC content is lower than3%, the wear performanceimprovement is inapparent; TiC content is5%and7%, the wear performanceimprovement is greater. When NbC content is1%, the wear performance improvementis inapparent, NbC content is3%,5%,7%, the wear-resisting performance of the alloyhas certain improvement, but the effects are almost similar. The wear resistance of thealloy increased linearly with the increase of Cr content. The wear-resisting performanceof composite reinforced TiC-C/Mo alloy and NbC-Cr/Mo alloy were superior to singlestrengthening molybdenum alloy both under high temperature and room temperature.The wear-resisting performance of TiC-Cr/Mo alloy was superior to that of NbC-Cr/Moalloy. |
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