Fretting Wear Of Zirconium Alloy In High Temperature And High Pressure Water Environment

During the operation of pressurized water nuclear reactor(PWR),fretting wear occurs in the contact area between fuel rod and grid due to mechanical vibration and high-speed water flow-induced vibration.which causes the damage of contact surface and the initiation and development of subsurface cracks,and then leads to the failure of the nuclear fuel assembly.The fretting wear between zirconium alloy cladding and support grid restricts the service life of fuel assembly.Therefore,it is very important to study the fretting wear behavior and mechanism of zirconium alloy cladding for improving the fretting wear resistance of nuclear fuel cladding and guaranteeing the life of nuclear power plant.The studies were conducted on the fretting wear behavior of Zr-4,Zr-1.0Nb and Zr-Nb-Fe-Cu tube against Zr-4 plate.In the high-temperature and high-pressure water environment,the fretting wear experiments of three groups of friction pairs at the same displacement amplitude of 150 ?m and Zr-1.0Nb / Zr-4 friction pairs at different displacement amplitudes were carried out.The tribological properties and microstructure evolution of zirconium alloy tubes were analyzed by means of the optical microscopy,the white light interferometer,Raman spectroscopy,scanning electron microscopy,focused ion beam and transmission electron microscope.The main results are as follows:(1)Under the displacement amplitude of 150 ?m,the surfaces of the three groups of friction pairs were severely worn,forming uniform and continuous wear scars.The main damage mechanism of Zr-4 surface is the plough and grooves caused by oxidation wear and abrasive wear,while the surface damage mechanism of Zr-1.0Nb and Zr-NbFe-Cu tube is mainly the combination effect of oxidation wear and delamination due to a large number of pits and delamination structures on the surface.In addition,the wear volume and average wear coefficient of the three kinds of tubes follow the order: Zr-4< Zr-Nb-Fe-Cu < Zr-1.0Nb tube,and the wear degree of the tube is more serious than that of the plate.(2)Under the displacement amplitude of 150 ?m,the subsurface microstructure of Zr-4,Zr-1.0Nb and Zr-Nb-Fe-Cu tubes presents are distributed in a stratified structure,that is,the oxide layer,the deformation layer and the matrix.The deformation of the plastic deformation layer mainly depends on the dislocation movement.The second phase is less in Zr-4 tube,which has a low inhibiting effect on the dislocation,resulting in serious grain deformation.Under the dual action of wear and corrosion,the growth rate of Zr-4 oxide film is fast,which plays a protective role on the surface.While a large number of second phase particles in Zr-1.0Nb tube hinder the movement of dislocations,and the low oxygen zone at the O/M interface alleviates the stress concentration,so the oxide film grows slowly.The thin oxide film is quickly removed in reciprocating motion,and then quickly regenerated in the oxygen-rich environment,which forms delamination wear on the worn surface and increasing the wear volume.Zr-Nb-Fe-Cu tube is between the two.Therefore,the wear volume follows: Zr-1.0Nb > Zr-Nb-FeCu > Zr-4 tube.(3)With the increase of displacement amplitude,the wear volume of Zr-1.0Nb tube increases and the average wear coefficient decreases,and the fretting state of the contact interface changes from partial slip to complete slip.The main damage mechanism of Zr-1.0Nb changed from oxidation wear,fatigue wear and adhesive wear at low displacement amplitude(35 ?m,50 ?m)to the combination effect of oxidation and delamination wear at high displacement amplitude(100 ?m,150 ?m).(4)The worn subsurface of Zr-1.0Nb is a multi-layer structure of oxidation layer,deformation layer/mixing layer and matrix layer.Between the interface of the loose equiaxed grains in the outer oxide film and the compact columnar grains in the inner oxide film,cracks are easy to initiate and propagate,resulting in a large number of transverse cracks.Severe plastic deformation occurred in the deformed layer,but the uneven distribution of stress and local severe wear lead to the discontinuous distribution of the deformation layer,and the mixed layer of low oxygen zone and deformation zone is formed in the local area.