Preparation And Properties Of Duplex TiC-Ni₃（Al,Ti） Particles Synergistically Reinforced Nickel Matrix Composites

With the increasing advancement in aerospace industries,the conventional nickel-based superalloys have reached their bottleneck as increasingly stringent service requirements.Herein,exploring a novel kind of nickel-based composite material that integrates the intriguing performances in terms of light weight,high strength,superior thermal and ablation resistance,and long-term service life can totally offset the benefits of Ni matrix composite in comparison with conventional superalloys.Heretofore,this dissertation explores for the first time that the ternary layered Ti₂Al C is used as a precursor,and the in-situ Ti C ceramic and Ni₃（Al,Ti）intermetallic compound synergistically enhanced nickel matrix composite material are successfully fabricated by using powder metallurgy combined with in-situ reaction technology.The morphology of Ti C and Ni₃（Al,Ti）enhancers and their interface orientation relationship with the Ni matrix as well as their strengthening mechanism were revealed.The mechanical properties of the prepared Ti C-Ni₃（Al,Ti）/Ni composites were improved by optimizing the heat-treatment process.The effect of the heat-treatment process on the microstructure and properties of the composite materials was elucidated.The high-temperature mechanical properties of Inconel718,a kind of Ni-based superalloy,were further improved by the filler of in-situ Ti C particulate.This thesis has achieved the following innovative results:（1）Using Ti₂Al C and Ni as starting materials,in-situ Ti C ceramic particles and nano Ni₃（Al,Ti）intermetallic compound synergistically reinforced Ni matrix composites were prepared by hot-press sintering and pressureless-pressurized densification sintering processes,respectively.（2）The chemical reaction between Ti₂Al C and Ni occurred at about 900 ℃.As the reaction progressed,Al-Ti atoms gradually dissociated from Ti₂Al C and solidly dissolved into Ni lattice.Meanwhile,Ti₂Al C spontaneously transformed into Ti C.In-situ Ti C particulate was considered as the heterogeneous nucleus of Ni matrix,which effectively refined the grain size of Ni.When the dissociated Al-Ti atom exceeded the solid solubility in the Ni matrix（7 at.%）,the nano-Ni₃（Al,Ti）would be precipitated out from the matrix.（3）The morphology and size of Ti C particulates mainly depend on the sintering temperature.At 1200 ℃ sintering,the in-situ Ti C particles are small in size,at the sub-micron scale,and partially retain the sheet-like structure of Ti₂Al C.As the sintering temperature rises to 1400 ℃,Ti C gradually grows,and the particle size increases to 2-5μm.The particle growth of Ti C follows a non-classical crystal growth mechanism.（4）The lattice mismatch between in-situ Ti C and Ni matrix is calculated to be 6%,which belongs to the semi-coherent lattice interface.The interface between nano Ni₃（Al,Ti）precipitate and the Ni matrix is determined to be the coherent lattice interface;The interface orientation relationship between Ti C-Ni₃（Al,Ti）and Ni matrix can be expressed as[001]Ti C//[011]Ni,（020）Ti C//（111）Ni and[100]γ′//[100]γ,（010）γ’//（010）γ.（5）The morphology and size of Ni₃（Al,Ti）in composites can be regulated by subsequent solid solution and aging treatment.As for the Ti C-Ni₃（Al,Ti）/Ni composites prepared by hot-press sintering 30 vol.%Ti₂Al C/Ni mixtures,nanoscale Ni₃（Al,Ti）continually precipitated out from the matrix and grew slowly after solution treatment at1200 ℃ for 1 h along with annealing at 750 ℃.As the annealing time increased from 20h to 100 h,the particle size of Ni₃（Al,Ti）grew from 43 nm to 75 nm.The coherent Ni₃（Al,Ti）is considered as the precipitation strengthening phase of Ni matrix,which effectively improves the high-temperature strength and thermal stability of the matrix.（6）Due to the synergistic reinforcement of in-situ duplex Ti C and Ni₃（Al,Ti）reinforcement,Ti C-Ni₃（Al,Ti）/Ni composites exhibit excellent mechanical properties.The yield strength,tensile strength,elongation,Vickers hardness,and fracture toughness of the composite materials optimized by heat treatment can reach 1230 MPa,1301 MPa,6.45%,5.09 GPa,and 31.7 MPa·m^1/2,respectively.The improvement of mechanical properties is mainly due to the combined effect of multiple strengthening mechanisms such as grain refinement,solid solution,and load transmission strengthening.（7）The prepared Ti C-Ni₃（Al,Ti）/Ni composites exhibited excellent high-temperature mechanical properties.At 600°C,the yield strength and compressive strength of the Ti C-Ni₃（Al,Ti）/Ni composites reached 1450 MPa and 1535 MPa,respectively.When the temperature rises to 700°C,the yield strength and compressive strength of the composite were 810 MPa and 973 MPa.However,at 800°C,the yield and compressive strengths are reduced to 405 MPa and 485 MPa.At 700-800 ℃,the performance degradation of Ti C-Ni₃（Al,Ti）/Ni composites is mainly due to the low diffusion activation of the pure Ni matrix,and the matrix undergoes recrystallization.（8）Dispersion-enhanced Inconel718 composite material with in-situ ultrafine Ti C particles（about 230 nm）was successfully prepared by high-energy ball milling combined with hot pressing sintering process.In-situ submicron Ti C（about 230 nm）particles further improve the mechanical properties of Inconel718 superalloys.The tensile strength and elongation of the 5 vol.%Ti₂Al C/Inconel718 composite reached 1404 MPa and 9.8%,respectively.At 600 ℃,700 ℃,800 ℃ and 900 ℃,the tensile strengths are determined to be 1333 MPa,1010 MPa,685 MPa and 276 MPa.It is 9.2%,14.6%,14.2%,and 55%higher than the pure Inconel718 material,respectively.Its excellent performance is mainly attributed to the comprehensive reinforcement mechanisms in terms of dislocation strengthening、Orowan diffusion load transmission strengthening.