Effect Of Co,Ti Addition On Microstructure And Properties Of Ni-Cr-W-Based Superalloy By Powder Metallurgy

Ni-Cr-W-based superalloy is a typical solid solution strengthened superalloy.It is widely used in key hot end components of ramjets due to excellent comprehensive properties such as high temperature strength and plasticity.However,due to the high content of refractory metal elements such as W,Mo in the alloy,the traditional cast superalloys have serious metallurgical defects such as composition segregation and pores,which seriously affect the comprehensive properties of the alloy and restrict the further development.So,it is necessary to further improve the performance of alloys from the aspects of composition design and preparation process.In this paper,the composition design of a new Ni-Cr-W-based superalloy is carried out through the calculation of thermodynamic phase diagram and the theoretical analysis of high temperature oxidation resistance.In order to improve the oxidation and mechanical properties and increase the Al content in the new Ni-Cr-W based superalloy The composition range of the new alloy is designed,and then the powder metallurgy process is used to prepare it to eliminate the segregation and improve the hot working performance,to achieve a substantial improvement in the comprehensive mechanical properties of the alloy.After that,in this paper,the effects of Co and Ti additions on the microstructure,phase composition,mechanical properties and oxidation resistance of the new Ni-Cr-W-based powder superalloy were studied.The main innovative achievements are as follows:（1）The effects of Co and Ti additions on the phase composition of powder metallurgy Ni-Cr-W-based superalloys were calculated and analyzed by thermodynamic phase diagram.The research results show that the addition of Co element promotes the transformation of p-phase toμ-phase and increases the solid solution temperature of M₂₃C₆ carbides.The addition of Ti element increases the content ofγ’phase and p phase,and promotes the formation of carbides in the alloy.（2）It is found that the addition of Co significantly reduces the stacking fault energy（SFE）of the alloy by thermodynamic calculation.Besides,the addition of Co promotes dynamic recrystallization to refine the grain size during the process of high temperature deformation.What’s more,Co addition promotes the transformation of deformation from dislocation slip to dislocation slip and twinning combined leading to the tensile strength of alloy with 30 wt.%Co addition increased by 10.0%compared with those without Co addition,and the elongation increased by 29.6%at room temperature.And,the tensile strength of alloy with 30 wt.%Co is increased by 31.4%,and the elongation is increased by 74.5%at 850℃.（3）Ti addition further improves the strength of Ni-Cr-W-based superalloy.The tensile strength of Ni-Cr-W-based power superalloy increases with the Ti content gradually increase by adding 0-1Ti wt.%.However,Ti addition will promote the oxidation at grain boundaries and aggravate the intermediate temperature brittleness during tensile test at high temperature.What’s more,Ti addition promotes the content and size of the grain boundary carbides,which lead to stress concentration at the grain boundaries.The cracks caused by the stress concentration will lead to a decrease in the ductility and strength of Ni-Cr-W-based superalloy.（4）Ti addition has a significant effect on the oxidation resistance of Ni-Cr-W-Co based superalloys.During the oxidation process,it was found that Ti O₂ was doped in the outer Cr₂O₃ oxide film,and Ti⁴⁺in Ti O₂occupied the position of Cr³⁺in the lattice of Cr₂O₃to promote the formation of Cr vacancies to balance the charge,aggravate the outward diffusion of Cr³⁺and react with oxygen to form more Cr₂O₃,which improved the thickness of outer oxide film and the oxidation rate.In addition,adding Ti element inhibits the generation of Ni-rich oxides such as NiO and NiCr₂O₄ in the outer layer and increases the inner oxidation depth.During the oxidation process,a large number of brittle Ti N particles are found in the matrix of Ni-Cr-W-based superalloy with 1 wt.%Ti addition,which reduces the stability of the oxide film and has serious damage to the mechanical properties of the alloy.（5）The oxidation behavior of Ni-Cr-W-Co-Ti power superalloy with excellent mechanical properties co-added with Co and Ti at 800-1100℃was systematically analyzed.The calculated oxidation activation energy of the alloy is 258.8 k J/mol,which is consistent with the diffusion activation energy of Cr.During the high temperature oxidation process at 1100℃,the outer layer of Cr₂O₃ undergoes obvious volatilization reaction,which causes part of the oxide film to crack and peel off.However,the oxide film is still continuous and dense,the alloy still shows excellent oxidation resistance at 1100℃.