Investigation On The Formation Mechanism And Mechanical Properties Of(TiNb)C Surface Reinforced Layer In Titanium-niobium Alloy

Titanium-niobium(TiNb)alloy has excellent high temperature strength and lower density,which has a wide range of applications in aviation,aerospace and nuclear industries.However,duo to the comparative deficiency in wear resistance,its application in wear condition is limited.Therefore,improving the wear resistance of TiNb alloy has very important practical significance.In this paper,the in-situ solid-state carbonization reaction is adopted to produce the(TiNb)C surface reinforced layer(SRL).Adjusting the process parameters,controlling compactness of surface reinforcement layer and fraction of(TiNb)C,realizing the controllable preparation of TiNb based(TiNb)C surface reinforced composite.The effects of preparation parameters on the phase,microstructure of the(TiNb)C SRL were researched;microstructure evolution,formation mechanism,growth mechanism,mechanical properties,friction and wear properties of the SRL were investigated.The results of research show as following:(1)Proceeding the in-situ solid-state carbonization reaction at 1050℃-1150℃,the surface of TiNb alloy form the Ti-Nb-C system and happen the solid phase transformation.The stable phases(TiNb)C and(TiNb)2C can be formed.When HT300 gray cast iron is adopted as carbon source,the(TiNb)C SRL with high density,good bonding interface and layered structure can be prepared,and the growth relationship between the thickness(d)and time(t)of the SRL prepared by isothermal treatment at 1150℃ satisfies d2=2.84t.(2)The change of diffusion flux and concentration of carbon atoms leads to happen two different reaction mechanisms during the formation of SRL,namely,one-step reaction:TiNb+C→(TiNb)CⅠ and two-step reaction:TiNb+C→(TiNb)2C+C→(TiNb)CⅡ+Nb.The transformation of reaction mechanism lead to the mutation of microstructure and grain size,and along the diffusion direction formed Layer-I quasi nano sized spherical particles,Layer-II micro sized rice particles and Layer-Ⅲ submicro sized equiaxed particles,and more,the orientation relationship of Layer-Ⅱ is<011>(TiNb)CI//<111>TiNb,and the(TiNb)CⅠ particles have Core-Rim(C-R)structure.The thermodynamic difference of heterogeneous elements in Ti-Nb-C system lead to the formation of solubility gap islands,which will lead to the intermediate phase(Ti0.7Nb0.3)C formed in one-step reaction happen the thermodynamic spinodal decomposition,and transform into(TiNb)CⅠ with C-R structure by diffusion-segregation process.(3)The research results of grain morphology and growth behavior show that Layer-I have the highest nucleation rate,the particles nucleate and grow with spherical new phase mechanism,the limited growth space makes the particles exhibit nearly spherical.(TiNb)CⅠ in Layer-II grows with lamellar new phase and directional attachment mixing mechanism,along<011>to form lamellared(TiNb)CⅠ particles.The particles in layer-III take(TiNb)2C as the parent phase,nucleate and grow with spherical new phase mechanism,exhibit with submicron cubic shape.(4)The results of nano indentation show that the average hardness of the SRL is about 31.22 GPa,which is about 7.2 times than the value of TiNb substrate.Meanwhile,the average elastic modulus of Layer-II and Layer-III are 358.37 GPa and 417.86 GPa,respectively,and Layer-II has higher H/E and H3/E*2 values,which indicates that layer Ⅱ has higher ability to resist failure and crack propagation.The micro-indentation results show that the fracture toughness KC of Layer-II and Layer-III is 4.58 MPa-m1/2 and 2.94 MPa-m1/2,respectively.Then,the interface between SRL and TiNb substrate has good bonding interface,and the fracture toughness KC at the interface is 2.57-3.44 MPa-m1/2.The results of three-point bending test show that the TiNb strengthened by SRL have higher deformation resistance,and the maximum bending strength is about 1.5 times than that of the TiNb substrate.(5)The results of dry ball-disk sliding friction show that the wear morphology of layer Ⅱexhibit with flake peeling and pitted spalling under 10 N and 20 N loads,respectively,both of which are belong to surface fatigue wear.Under 20 N loads,the wear surface of layer Ⅲ is shallow and smooth,and main wear mechanism is micro cutting wear.In terms of friction performance,the friction coefficient of the SRL/YG6 friction pair is about 0.084.Low friction coefficient means good surface friction performance,which benefit to reduce the friction resistance.At the same time,excellent anti-wear performance and low wear rate ensure the durability of surface accuracy and extend the service life of materials.