Study On Microstructure And Properties Of Nb-Ti-Zr Refractory High-entropy Alloys Fabricated By Laser Cladding

Refractory high entropy alloy（RHEA）possesses outstanding potential for applications in aerospace,military equipment,et al.The protective coating of RHEA prepared by laser cladding（LC）is expected to improve the service limit of key components.However,the laser cladding process is extremely difficult and challenging due to the extremely high melting-point and room temperature brittleness of RHEA.So far,no detailed studies have been carried out regarding the process optimization and performance control of RHEA coatings.In this thesis,a new Mo-Nb-Ti-Zr RHEA coatings system was designed and prepared on the Ti6Al4V surface by laser cladding.The conflict between the dilution and microhardness,porosity of the RHEA coating is solved by multi-objective process parameter optimization method.The effect of Mo-doping on the phase constitution and microstructure of the RHEA coatings was analyzed in detail.The driving forces of micro-segregation of the RHEA coatings under non-equilibrium solidification conditions caused by rapid cooling and the strengthening mechanism and crack sensitivity of the coatings under multi-HCP phase precipitation were investigated.The effects of Mo-doping on the high-temperature phase stability and dry friction wear behavior of RHEA coatings were analyzed.The stability enhancement of phase caused by low diffusion rate Mo-doping are explained.Finally,a link between the microstructure and the tribological behavior was established.The relevant findings are as follows:Single-factor processing experiments displayed that the changes in processing parameters result in a deep-penetration phenomenon at the bottom region of the coating,which was related to the direction change of Marangoni convection caused by changes in the melt pool temperature.The microhardness of Mo Nb Ti Zr RHEA coating was controlled by the processing parameters and negatively correlated with the dilution,and the content of strengthening elements（Mo,Nb）determined the the microhardness of coating.Changes in scanning speed will not only increase the heat input to the molten pool by decreasing powder density,but also reduce the temperature of molten pool by decreasing the laser specific energy.Multi-objective processing parameter optimization indicated that the scanning speed had a more significant effect on the geometric characteristics,porosity,and microhardness of the RHEA coating.For the RHEA coating that meet the expected performance,the optimal processing parameters combination was laser power 2700 W,powder feeding rate 8 g/min and scanning speed3 mm/s.The experimental results were in good agreement with the predicted values.The Mo_0.2Nb Ti Zr coating was a simple BCC solid solution phase,and the coatings precipitated several HCP phases with the addition of Mo content.Mo-doping exacerbated the micro-segregation of the dendritic structure and promoted the thermomechanical instability of the coating,leading to the precipitation of the HCP phase.The formation of the HCP phase is the result of continuous exsoluation of the supersaturated solid solution by atomic diffusion,and the final morphology and structure of the HCP phase depends on the ratio of elemental Ti to Zr within the phase.The increase in Mo content facilitated solid-solution strengthening,second-phase strengthening,fine-grain strengthening,and nano-twinning strengthening,resulting in a maximum microhardness enhancement of 109%to 732.67 HV0.3 for the Mo_xNb Ti Zr coatings.However,the HCP phase induced a large number of twin nucleation,which increased the crack sensitivity of the coatings and caused a gradual decrease in the fracture toughness.Study revealed that the Mo element with low diffusion rate reduced the diffusion rate of the alloys system,increased the start and end temperatures of phase transition of the Mo_1.0Nb Ti Zr coating,which promoted the thermal stability of the BCC phase.After annealing at 1000°C,the Ti₄Nb phase precipitated at the grain boundaries of the Mo_0.2Nb Ti Zr coating and the BCC phase began to instability.The Mo_1.0 Nb Ti Zr coating maintained a stable BCC structure,but a large number of needle-like HCP phases precipitated at the grain boundaries.After annealing at 1300°C,the BCC phase of the Mo_0.2 Nb Ti Zr coating was completely transformed into the Ti₄Nb phase,and the Mo_1.0Nb Ti Zr coating still maintains good high-temperature stability.During the process of friction and wear at room temperature,Mo-doping promotes the generation of destructive oxides and exacerbates the brittleness,and the coating undergoed a transition from oxidation wear to three-body wear+brittle spalling,resulting in a significant reduction in the wear resistance of the coating.During the process of friction and wear at 600℃,the rise in average temperature and flash temperature promoted the oxidation of the coating and the sintering of the enamel layer,and the Mo_0.6 Nb Ti Zr coating exhibits the best wear resistance.However,the further increases in Mo content lead to severe abrasive wear,severe delamination spalling and oxidation wear,which exacerbates material loss.There are 93 figures,21 tables and 135 references in this thesis.