Preparation And Microstructure Of MAX Phase Composite Coating By Laser Cladding On Zirconium Alloy Surfac

Zirconium alloys have small thermal neutron absorption cross section,high thermal conductivity,moderate mechanical properties,easy processing,and good compatibility with UO₂.Therefore,they are widely used as structural materials for nuclear reactors,such as pressure tubes,vessel tubes,duct tubes,and guide tubes.However,the corrosion environment of cooling water for a long time under normal conditions,the fretting wear between the grid and the material change process,and the high temperature steam oxidation under abnormal conditions are easy to cause the failure of zirconium alloy.MAX phase has the characteristics of both ceramics and metals due to its unique nano-layered structure,which makes it have excellent radiation resistance,oxidation resistance,corrosion resistance and wear resistance,and can be used as the surface protection coating of zirconium alloy.In this paper,R60702 zirconium alloy was used as the matrix,TiAl,Ti C and Al powder were used as the pre-coating powder materials,and Ti₂AlC MAX multiphase structure was synthesized by in-situ reaction of laser melting.The composite coating with dense microstructure,good metallurgical bonding and excellent oxidation resistance was prepared.The effect of laser melting process parameters on the phase structure of the composite coating was studied.The synthesis mechanism of MAX phase in the in-situ reaction process was deeply explored,and the optimal process parameters suitable for performance experiments were determined.The fretting wear properties and high temperature oxidation properties of the composite coating were studied,and the high temperature oxidation mechanism of the composite coating was deeply explored.The oxidation experiment of exposed interface was carried out to study the effect of dilution rate on interface oxidation,and the mechanism of interface oxidation was deeply explored.According to the experimental results,the main conclusions are as follows:（1）The coating prepared by laser melting technology has good metallurgical bonding with the substrate without obvious defects such as cracks and pores.The coating is mainly composed of TiAl intermetallic compound,Ti₂AlC MAX phase,Ti C or Zr C reinforcement phase,and a small amount of Ti₃AlC MAX phase transition products.The maximum hardness of the coating is as high as 848.8 HV,which is five times that of the 170 HV zirconium alloy substrate.（2）Two synthesis mechanisms of Ti₂AlC MAX phase were obtained by observing the microstructure of the fusion coating,combining with the solidification theory of the metal and the thermophysical properties of the material.One is Ti₂AlC MAX phase with core-shell structure formed by diffusion of Al into Ti C,and the other is needle-like Ti₂AlC MAX phase precipitated by direct nucleation reaction.（3）Fretting wear results show that the average friction coefficient of the composite coating with different displacement amplitudes increases first and then decreases,which is related to the discharge of debris and friction heat.The average friction coefficient of different loads shows an increasing trend,which is due to the increase of load pressure,the friction force also increases,which leads to the corresponding increase of friction coefficient.The wear mechanism of composite coating is mainly adhesive wear caused by oxidation wear.（4）The results of DTG oxidation kinetics curve show that the oxidation weight gain of coating and zirconium alloy is 34%and 27.64%,and their theoretical complete oxidation weight gain is 70%and 35%.The oxidation degree of composite coating is only about half,while that of zirconium alloy substrate is nearly four fifths.The weight gain of the composite coating was mainly concentrated in the range of1000°C to 1200°C,indicating that during this period,the composite coating rapidly formed a dense oxide film,which prevented the subsequent oxidation process.The composite coating had excellent high temperature oxidation resistance.（5）The results of microstructure observation show that the edge position of oxide particles grows fastest,followed by the center area of oxide particles,and finally the matrix phase outside the oxide particles.This is related to the microstructure of the composite coating before oxidation,and the oxidation mechanism is further explored.（6）The electrochemical test results show that the open circuit potential（-100m V）of the composite coating is higher than that of the zirconium alloy（-400m V）.The self-corrosion current of the composite coating in the polarization curve is one order of magnitude lower than that of the zirconium alloy substrate.The composite coating has excellent corrosion resistance.The oxidation experiment of exposed interface shows that the composite coating and zirconium alloy have a kind of bimetallic corrosion effect due to their different corrosion resistance,and the mechanism of this effect is analyzed from the perspective of quantum mechanics.