Preparation And Mechanical Properties Of In-situ Cr₃C₂-Ni₃（Al,Cr）/Ni Composites

Mechanical properties of Ni-based superalloys strongly depend onγ’-Ni₃Al precipitates and hard particles.However,the traditional Ni-based composites have reached the serviceability limit as to satisfy the requirement of higher speed and thrust to weight ratio for advanced aerospace engines.Intermetallic compounds（Ni₃Al,etc.）and ceramic particles reinforced Ni matrix composites have attracted broad interest and significant prospects.In this paper,pure Cr₂AlC ceramic powders were firstly prepared by pressure-free sintering.Then,we successfully fabricated a novel in-situ Cr₃C₂particulate combined withγ’-Ni₃（Al,Cr）precipitate synergistically reinforced pure Ni matrix composites by reactive hot press techniques.The composites with different volume fraction of Cr₂AlC were characterized by X-ray diffraction（XRD）,scanning electron microscopy（SEM）and energy dispersive analysis（EDS）.The phase evolution,microstructure and mechanical properties of 10%～40%Cr₂AlC/Ni composites were studied,in addition the contributions of different strengthening mechanisms were calculated.Moreover,changes of microstructure and mechanical properties of30%Cr₂AlC/Ni and 40%Cr₂AlC/Ni composites after heat treatment were analysis.The results of reserch shows as flowing:（1）According to the molar ratio of Cr:Al:C=2:1.2:1,the mass of Cr,Al and graphite powders were calculated and mixed with agate balls.Thereupon,the mixture was annealed up to 1400-1450℃with an average heating rate of 10℃/min and isothermally held for 10 min to obtain Cr₂AlC ceramic.（2）Cr₂AlC/Ni composites with ceramic content of 10%～40%were prepared by hot pressing sintering（1200℃）.Cr₂AlC was completely decomposed at high temperature,which transformed Cr₃C₂ hard particles.Moreover,depleted Al atoms and partial Cr atoms spread into Ni matrix to formγ-Ni.As the concentration of Al and Cr atoms increased in the matrix,γ’-Ni₃（Al,Cr）precipitates would be generated,giving rise to the formation of hybrid Cr₃C₂-γ’/Ni composites.（3）The microstructure of the composites is relatively compact,in which there isγ’phase in the composites of 30%Cr₂AlC/Ni and 40%Cr₂AlC/Ni,and the phase evolution process:γ-γ+γ’-γ’.The in-situ Cr₃C₂ particles andγ’-Ni₃（Al,Cr）were uniformly distributed in theγ-Ni matrix.The Ni matrix with higher ductility is filled between the hard Cr₃C₂ particles andγ’phase,and the Cr₃C₂ particles form a strong bonding interface with the matrix.（4）Cr₃C₂-Ni₃（Al,Cr）/Ni composites prepared by hot pressing sintering have excellent mechanical properties.The composites with 30%and 40%Cr₂AlC/Ni exhibit substantial reinforcement in flexural strength,which is 1790±73 MPa and 1752±25MPa respectively,and substantial reinforcement in tensile strength which is 947±19MPa and 846±15.9 MPa,respectively.The material not only obtained high strength but also a high fracture toughness.The mechanical properties of the composite in this works are at the forefront of the pure Ni matrix composites.（5）The 30%Cr₂AlC/Ni and 40%Cr₂AlC/Ni composites obtained by hot pressing were further heat-treated,and the phase structure of the composites unchanged.The nanoscaleγ’phase was uniformly distributed in the matrix.The tensile strength and yield strength of the composites were significantly increased,and the yield strength was 798±17.5MPa and 967±16MPa,respectively.