Precipitation Behaviors Of (Zr_0.7-xTi_xW_0.3)C_1-? Solid Solutions

Transition metal carbides(TMCs)have a series of advantages such as ultra-high melting point,high modulus,and corrosion resistance.Therefore,they have broad application prospects in defense and aerospace fields such as supersonic vehicles and rocket nozzles.However,the intrinsic brittleness imposes resitrictions on their application.Some scholars adopt the aging idea of solid solution to improve their toughness.This paper mainly studies the precipitation behavior of(Zr0.7-x Tix W0.3)C1-? solid solution in order to provide guidance for the design of the aging process in the future.The research ideas are as follows.The first step is to prepare low-oxygen,no-residue carbon and high-density(Zr0.7-x Tix W0.3)C1-? solid solution,and study the laws and physical mechanisms behind(Zr,Ti,W)C1-? solid solution;Then,a(Zr,Ti,W)C1-? solution model is established.And the kinetic mechanism of the phase transition of the solid solution under low temperature aging is predicted,mainly based on the proposed solution model.Finally,the phase transformation behaviors of(Zr0.7-x Tix W0.3)C1-? solid solution are studied by experimental methods The main research results are as follows:By calculating the thermodynamic process during the preparation of(Zr0.7-xTix W0.3)C1-? solid solution,it can be seen that CO2 generated during the carbon thermal reduction of WO3 and carbon vacancies formed during the sintering of(Zr0.7-x Tix W0.3)C1-? solid solution must be taken into accout.The lattice constant,grain size and Vickers hardness of(Zr0.7-x Tix W0.3)C1-?solid solution are obviously related to the content of Ti C.The lattice constant decreases with the increase of Ti C content,which happens because(Zr0.7-x Tix W0.3)C1-? solid solution lattice constant product meets Vegard's law.The grain size of(Zr0.7-x Tix W0.3)C1-?solid solution depending on the Ti C content is can be analysed from the perspective of the mixed entropy and lattice distortion.(3)The solution model of(Zr0.7-x Tix W0.3)C1-? based on the regular solution model predicts that the phase transition kinetic mechanism of(Zr0.7-x Tix W0.3)C1-? ternary solid solution at low temperature is spinodal decomposition.The results of calculations used Factsage 7.1 show that the concentration of carbon vacancies has a significant effect on the stability of the solid solution,and it also predicts that(Zr0.4Ti0.3W0.3)C1-? and(Zr0.5Ti0.2W0.3)C1-? solid solutions are at the thermodynamically non-equilibrium state at higher temperature compared to other(Zr0.7-x Tix W0.3)C1-? solid solution with different chemical composition,so they tend to decompose easier.(4)(Zr0.7-x Tix W0.3)C1-? solid solutions with five different compositions have various phase transition characteristics due to the difference of Ti C content.When x is 0,the kinetic mechanism of the phase transition of the solid solution is nucleation-growth type,and WC grains are produced during the aging process.When x is increased to 0.1,its stability is enhanced.Thus,there is only a small amount of precipitation along the grain boundary at low temperature.And it is impossible to precipitate at high temperature;as predicted by the thermodynamic calculations.When x is 0.3 or 0.5 respectively,the solid solution is prone to undergo phase transformations,and the mechanism of their phase transition is spinodal decomposition.When x reaches 0.7,the lattice distortion of the solid solution under this composition is extremely small.Therefore,compared to the other four solutions,the stability of the solid solution is the best,and there is no precipitation behavior observed during the aging process.At the same time,the solid solutions with obvious phase transition also have in common: the rate of precipitation is very slow as a result of the high diffusion activation energy of transition metal atoms.The other similarity is that the phase transition process is closely related to the defects of polycrystallines.