The Structure And Properties Of Fe-based Amorphous Alloys And Composites Prepared By HVOF Thermal Spraying

Fe-based amorphous coatings are promising for industrial applications due to the combination of high strength/hardness,excellent wear/corrosion resistance and relatively low raw material cost.However,the monolithic amorphous coatings still suffer from poor fracture resistance and toughness due to the natural brittleness of amorphous phase.To solve the problem of poor toughness,an effective method is to add second phases to form amorphous matrix composite coatings.Moreover,based on the researches of amorphous coatings,3D printing of large amorphous alloys by the HVOF thermal spray system was attempted.Therefore,the fabrication of amorphous matrix composite coatings with addition of various second phases and 3D printing of large dimension of Fe-based amorphous alloys and composite are the two main topics in the dissertation.Based on a systematic study on preparation,structure and properties of the fabricated Fe-based amorphous composite coatings and the 3D printed components,it is anticipated that Fe-based amorphous alloys can come to the reality of applications.Based on the Fe48Cr15Mo14C15B6Y2 amorphous alloy system,both amorphous coatings with thickness of about 400 um and superlarge size amorphous sample up to several centimeters were prepared by using HVOF thermal spray technique.The microstructure,mechanical properties and corrosion behavior of the prepared components were systematically investigated by means of X-ray diffraction(XRD),optical microscopy(OM),differential scanning calorimetry(DSC),scanning electron microscopy(SEM),transmission electron microscopy(TEM),3D X-ray tomography(XRT),micro-hardness measurement,friction/wear tester,electrochemical workstation,mechanical testing system,drop-ball impact tester and fatigue tester.The fracture toughness of 3D Fe-based BMG and BMG composites was measured by three-point single edge notched bending tests.The main results obtained are as follows:(1)To solve the unavoidable oxidation problems in preparation of Fe-based amorphous coating by HVOF technique,the Fe-based feedstock powders were modified by electroless plating of a thin layer of Ni-(W)-P amorphous phase.The resultant modified coatings prepared by the plated powders exhibited lower oxygen content than the one prepared with the unmodified amorphous powders.By suppressing the formation of brittle oxides at intersplats,the Ni-W-P phases could improve the fracture toughness and bonding strength of coatings.Moreover,the Ni-W-P modified coating showed much lower friction coefficient(?=0.28)and lower wear rate(~ 5.2 x 10-6 mm3 N-1m-1 than the coating prepared with the original powders(?=0.42 and 12.4 x 10-6 mm3 N-1m-1).The anti-oxidation Ni-rich phase in the intersplat regions plays a positive role in the enhancement of wear resistance of the modified coating as the Ni-rich phase serves as a barrier to oxygen diffusion during friction.(2)A Fe-based amorphous composite coating reinforced with amorphous carbon phase was developed via in-situ carbonization of nylon 11 powders during HVOF thermal spraying.The amorphous composite coating exhibited higher bending strength,enhanced impact resistance and fatigue properties over the monolithic amorphous coating without reducing the corrosion resistance.Finite element simulation revealed that the ductile amorphous carbon phase could effectively alleviate stress concentration during impact process.Further,theoretical analysis suggested that the amorphous carbon phase could significantly reduce the driving force for crack propagation due to the shielding effect of the soft amorphous carbon phase,which slows down the crack propagation rate.The combination of these two effects contributes the enhancement of impact toughness and fatigue properties of the amorphous composite coating.(3)In spite of making Fe-based amorphous coating,HOVF thermal spray was also used to produce three-dimensional large-size Fe48Mo14Cr15Y2C15B6 bulk metallic glass(named as HVOF-BMG).The obtained BMG contains over 95% amorphous phase and exhibits almost fully dense structure(0.37% porosity),high hardness(1025 Hv)and high compressive strength(2.14 GPa perpendicular to the direction of deposition,1.36 GPa parallel to the direction of deposition).The fracture behavior of HVOF-BMG is fragmentation fracture which is similar to the as-cast BMG.Even though the obtained HVOF-BMG is brittle in nature,it is much tougher than the Fe-based BMG prepared by casting with the same composition,for example,the fracture toughness of the HVOF-BMG is about 12.5 MPa m1/2,2.5 times higher than that of as-cast BMG.(4)To further enhance toughness of HVOF-BMG,HVOF-BMG composites with the addition of various fractions of soft stainless steel(20-50 wt% stainless steel)were fabricated using the same HVOF thermal spray technique.The HVOF-BMG composites also have large sizes over 2 centimeters.It was found that the obtained HVOF-BMG composites exhibited more densified structure with a porosity of 0.35% and excellent mechanical properties with compressive strength of over 1.8 GPa and plastic strain of 0.7-to 2% depending on increased fraction of stainless steel.More interesting is that the HVOF-BMG composites showed pretty good fracture toughness.For examples,HVOF-BMG composites reinforced with 50 wt% stainless steel exhibited fracture toughness of 20.5 MPa m1/2,4 times high than as-cast BMG and 2 times higher than HVOF-BMG.It was revealed that the enhancement of fracture toughness in HVOF BMG composite is attributed to the strong crack deflection caused by the 316 L SS phase from reducing the driving force for crack propagation.