| Zirconium(Zr)and its alloys have wide structural applications in biomedical,chemical and nuclear industries due mainly to their excellent bio-compatibility,good corrosion resistance and low thermal-neutron absorption.However,low hardness and poor wear resistance make them often suffer from rapid wear failure under frictional conditions.In this thesis,a typical commercial Zr alloy(Zr702)was used as the substrate,and CrHfNbTaTiZr-based refractory medium-/high-entropy alloy coatings were successfully prepared on the surface of the substrate by pulse laser cladding(PLC)techniques.The composition,microstructure,hardness and wear characteristics have been carefully characterized and analyzed by the combined use of XRD,EBSD,ECC,EDS and second electron image techniques.The main research conclusions are as follows:(1)CrNbHfTaTiZr-based refractory medium-/high-entropy alloy coatings with good metallurgical bonding were successfully prepared on the surface of the Zr702 substrate by laser cladding.Cladding zone and heat affected zone are formed on the cross-section of each coating specimens.The thickness of the coatings were about 380-420?m and the cladding zones mainly are high angle boundaries with scattered orientations.(2)NbTiZr and HfZr Ticoatings are composed of body centered cubic solid-solution phase and hexagonal close-packed solid-solution phases;The NbTiZr is mainly composed of irregular-bulk grains and the HfTiZr is mainly composed of fine plates.The hardnesses of the NbTiZr and the HfTiZr coatings are 510±22 HV and 794±42 HV,respectively,which are significantly higher than that of the substrate(196±4 HV).Below the coatings,there exist heat-affected zones consisting of a mixture of bulk?-Zr grains and fine martensite plates.Average thicknesses of the heat-affected zones in both the specimens are 180-190?m.The main wear mechanism of the NbTiZr coating is mainly oxidative and abrasive wear,while the HfTiZr coating is abrasive and adhesive wear.The wear rate of NbTiZr(5.2×10-5mm3 N-1 m-1)is about 11%than that of the Zr702 substrate(4.58×10-4 mm3 N-1 m-1),while the HfTiZr coating(3.16×10-4 mm3 N-1 m-1)is approximately 69%of the substrate.The wear resistance of the NbTiZr and the HfTiZr coatings are significantly better than that of the Zr702 substrate.(3)NbTaTiZr is composed of body centered cubic solid-solution phase,while CrNbTiZr is mainly composed of body centered cubic solid-solution phase and Cr2Zr Laves phase.The hardnesses of NbTaTiZr and CrNbTiZr coatings are 550±26 HV and 637±46 HV,respectively.The addition of Taor Crelements can increase the hardness of the NbTiZr coating.NbTaTiZr is mainly composed of irregular-bulk grains,and CrNbTiZr is mainly composed of cellular grains and interdendritic structure.The main wear mechanism of the NbTaTiZr coating is mainly oxidative and abrasive wear,while the CrNbTiZr coating is abrasive and adhesive wear.The wear rates of NbTaTiZr and CrNbTiZr coatings are 4×10-6mm3 N-1 m-1 and 1.99×10-4 mm3 N-1 m-1,respectively.The wear resistance of the NbTaTiZr coating is remarkable higher than that of the Zr702 substrate,while the wear resistance of the CrNbTiZr coating is slightly better than that of the substrate.(4)HfNbTaTiZr is mainly composed of body centered cubic solid-solution phase and hexagonal close-packed lamellar structure,and CrHfTaTiZr is mainly composed of body centered cubic solid-solution phase,hexagonal close-packed solid solution phase and Cr2TaLaves phase.The hardness of HfNbTaTiZr and CrHfTaTiZr coatings are 668±42 HV and701±46 HV,respectively.The main wear mechanism of the HfNbTaTiZr coating is mainly abrasive and adhesive wear,while CrHfTaTiZr coating is mainly adhesive and abrasive wear.HfNbTaTiZr and CrHfTaTiZr coatings show relatively similar wear performance,with wear rates of 3.07×10-4 mm3 N-1 m-1 and 3.08×10-4 mm3 N-1 m-1,respectively,which are just a little better than that of the substrate.(5)Further analysis the phase formation of CrHfNbTaTiZr-based refractory medium-/high-entropy alloy coatings shows that as the number of elements increases,the number and types of intermetallic compounds in CrHfNbTaTiZr-based refractory medium-/high-entropy alloy coatings gradually increase.Comparative analysis of hardnesses and wear resistance shows that this excellent mechanical performance is attributable to the solid-solution strengthening and grain refinement strengthening caused by the rapid cooling of the laser cladding.The NbTaTiZr medium-entropy alloy coating with relatively few element types performs good hardness(550±26 HV)and the most excellent wear resistance(4.0×10-6mm3N-1 m-1).It can be seen that medium-entropy alloy coatings with reasonable and appropriate element types are easier to form simple phases and can exhibit better overall performance than high-entropy alloys.This study should be able to provide an important reference for preparing low-cost,high-performance refractory medium-/high-entropy alloy coatings on Zr alloys and facilitate an in-depth understanding of the performance optimization mechanism of such coatings. |
PDF Full Text Request