Influence Of Molybdenum On Microstructure And Mechanical Properties Of ZrO_2p/Ti Composites Layer Prepared By Laser Melt Injection

ZrO₂/Ti composite thermal barrier coating prepared by laser melt injection?LMI?,which realizes the integration of"bearing+heat prevention",exhibits great advantage in practical application.However,the ZrO₂/Ti metal matrix composites?MMCs?had poor plasticity,which resulted in the high crack tendency during the preparation process and hindered its further development.Optimizing the process parameters and controlling the particle distribution can not resolve the poor plasticity of the ZrO₂/Ti MMCs.In this study,the plasticity of the ZrO₂/Ti MMCs was enhanced by introducing Mo element.Based on the study of introducing Mo element during LMI,microstructure of the matrix in ZrO₂/Ti MMCs after Mo addition and the solidification process of the laser melt pool were studied.The change of the plasticity was investigated.The mechanisms of Mo addition improving the plasticity of ZrO₂/Ti MMCs were revealed.The addition of Mo element was achieved by laser melt injecting ZrO₂ particle into the surface of prefabricated Ti-Mo alloy layer,simultaneously the ZrO₂/Ti-Mo MMCs was fabricated.The cracking level of ZrO₂/Ti-Mo MMCs with different Mo addition amount was studied.The results show that with 50 wt.%Mo addition,the crack sensitivity of the composite layer is the lowest.The microstructure and solidification process of ZrO₂/Ti-Mo composite layer were analyzed by using X-ray diffraction?XRD?,scanning electron microscope?SEM?,electron micro probe analyzer?EMPA?and electron back-scattered diffraction?EBSD?.The results show that the composite layer matrix consists of Mo?Ti?solid solution,?-Ti?Mo?,eutectic structure and other phases.The crystallization process of the molten pool is as follows:Mo?Ti?solid solution with high melting point firstly nucleated from the liquid and grown into the initial Mo?Ti?dendrites;the?-Ti?Mo?was formed by the peritectic reaction between the liquid and the initial Mo?Ti?dendrites;the eutectic reaction occurred in the residual liquid among?-Ti?Mo?grains.The?-Ti?Mo?retained in the form of metastable phase and hadn't transform into?-Ti,which was resulted from the stabilizing effect of Mo element.The plasticity of the composite layers with different Mo addition amount was studied by micro-hardness and nano-indentation test.As the amount of Mo addition increased,the plasticity of the composite layer was gradually improved,and the matrix hardness decreased from 927 HV to 621 HV.Compared with the ZrO₂/Ti composite layer,the plastic deformation work of the ZrO₂/Ti-Mo composite layer increased from 286.2×10^-12 J to398.4×10^-12 J,and the proportion of plastic deformation work in the total deformation work increased from 55.2%to 66.5%.The mechanisms of Mo element enhancing plasticity of the ZrO₂/Ti MMCs were revealed by elemental analysis,observation of slip bands and micro-cracks in the fracture.The content of O in the matrix of ZrO₂/Ti MMCs,which damages the plasticity of Ti alloy,was decreased with Mo addition.The crystal structure of the ZrO₂/Ti MMCs matrix was transformed from hexagonal close-packed to body-centered cubic with Mo addition,which resulted in the transition of the grain slip mode from single slip to multi-slip and cross-slip,and coordinated deformation among grains.The generation and propagation of cracks were inhibited,which was attributed to the grain refinement resulting from Mo addition.