| Landing gear is one of the most important components in aircraft and it is closely related to the security.According to the statistical result,more than 70%aerospace accidents were resulted from the failures of landing gear.Thus,it is essential to improve the comprehensive properties of landing gear materials.Considering the rigorous service environment,the new generation landing gear material should be with high strength(ultimate tensile strength>1.9 GPa),high toughness(fracture toughness>70 MPa.m1/2)and excellent corrosion resistance(in comparable to 15-5 PH).Up to now,the most widely used landing gear material is high strength low alloy steels,such as 300M.For 300M,the strength and toughness has reached the target,but its corrosion resistance is still poor and can not meet the requirement for new generation landing gear material.Thus,surface coating is necessary to improve its corrosion resistance.In comparison to high strength low alloy steel,maraging stainless steel has become the most promising alternative material because of its superior properties including high strength,high toughness,excellent corrosion resistance and good weldability.In this study,the effect of Co on the microstructure and properties of Fe-Cr-Ni-Co-Mo-Ti maraging stainless steels were systematically studied.At atomic scale,the interaction mechanisms between Co and other alloying elements were investigated.After compositional optimum design,it was successful to develop a new maraging stainless steel with high strength,high toughness and excellent corrosion resistance.The newly developed maraging stainless steel is promising to be the new generation landing gear material.The effect of Co on corrosion resistance of maraging stainless steel was studied.It was found that Co promoted the spinodal decomposition of Cr in the aging process and deteriorated the corrosion resistance of the steel.In the studied maraging stainless steel,as spinodal decomposition amplitude of Cr increased,the sinusoidal wavelength of passive film formed on the steel surface decreased gradually.As a result,the stability of passive film decreased simultaneously,which deteriorated the corrosion resistance of the steel.According to the first principles calculation,Co would enhance the ferromagnetic interaction between Fe atoms and decrease the critical concentration to form Cr-rich cluster.Thus,Co could enhance the stability of Cr-rich cluster.The precipitation mechanism of precipitates in the aging process was studied.Two precipitates including Ni3Ti phase and R phase were observed in the maraging stainless steel.At early aging stage,Ni3Ti phase nucleated earliest and R phase nucleated at the interface between Ni3Ti phase and matrix subsequently.After R phase nucleated,as aging time prolongs,most nuclei grew up slightly to form final flake-like Mo-rich phase adjacent to Ni3Ti phase.Thus,the flake-like Mo-rich phase and Ni3Ti phase formed a core-shell structure together.In different sites,portion of nucleus grew up drastically to form large size sphere-like Mo-rich phase and even built a crosslink to each other.The effect of Co on the aging hardening behavior of the maraging stainless steel was studied.Co could both bring the peak aging time forward and increase the strength of the steel under peak aging condition.At initial aging stage,Co could increase the nucleation rate of Ni3Ti phase.As a result,at peak aging,Co increased the number density and decreased the size of Ni3Ti phase,thus enhanced the precipitation hardening effect.What's more,Co increased the nucleation rate by promoting the clusterization of Ni and Ti atoms.According to the first principles calculation,Co could increase the thermodynamic driving force of Ni-Ti clusterization.Co could also decrease the activation energy of diffusion of Ni and Ti atoms,thus enhance the dynamic driving force of Ni-Ti clusterization.A new maraging stainless steel with high strength(ultimate tensile strength=1.915 GPa),high toughness(fracture toughness=77.1 MPa.m1/2)and superior corrosion resistance(comparable to 15-5 PH)was successfully developed and was melted at 200 kg scale by double vacuum melting techniques.Its properties have reached the target of new generation landing gear material.In comparison to other commercial maraging stainless steels,the new steel shows obvious advantages on comprehensive properties. |
PDF Full Text Request