Study On Cooling Rate-controlled Microstructure And Performance Modeling In Nickel-based Superalloys

Nickel-based superalloys have good corrosion resistance,oxidation resistance,high strength and long fatigue life,mainly used in the working environment of higher than 550℃,in the engine under high temperature and high stress environment turbine disc,turbine blade and combustion chamber and other key components are made of superalloy.In recent years,with the use of superalloy materials more widely,people’s demand for high temperature strength of alloy has gradually increased.By designing the heat treatment process and regulating the microstructure parameters of the precipitates in the alloy,it has become the first key scientific research problem to further explore the nature of the strengthening effect in the superalloy,and to prepare the alloy with better high-temperature mechanical properties.In order to further explore nickel-based superalloys with excellent properties,the interaction between precipitates and dislocations at the microscale was studied in this paper,and a strength model coupled with mathematical theory was established to solve the problem of insufficient accuracy of the existing models.In addition,this paper reveals the competitive and synergistic relationship between solid solution strengthening and precipitation strengthening in nickel-based superalloys,establishes the correlation between processing technology,microstructure evolution,and mechanical properties,and obtains the optimal performance of nickel-based superalloys.optimum cooling rate range.The main innovations of this paper include:(1)Considering the cooperative and competitive relationship between the solid solution strengthening mechanism induced by element diffusion and the precipitation strengthening mechanism during the processing of nickel-based superalloys,a more realistic strength model of nickel-based superalloys is established.The evolution law of the solid solution strengthening contribution and the precipitation strengthening contribution dependent on the processing cooling rate is analyzed.The research shows that the evolution of γ’ phase is dominated by the interface energy during the nucleation stage of the precipitation phase.As the precipitation phase continues to grow,elements such as aluminum,titanium,and niobium diffuse from the matrix into the precipitation phase,resulting in a very high volume fraction of the precipitation phase at low cooling rates,and a small contribution to solid solution strengthening.As the cooling rate increases,the diffusion of cobalt,chromium,molybdenum,and tungsten from the matrix is inhibited,resulting in a decrease in the volume fraction of precipitates and an raising in the contribution of solid solution strengthening.(2)Here,a unified model has been developed to establish the quantitative relationships among the aging process,microstructure,and yielding strength for a model nickel-based superalloy in a three-dimensional(3D)space,which agglomerates the three independent variables of aging temperature,cooling time,and matrix composition,and these variables dominated the size and volume fraction of precipitates,and anti-phase boundary energy.The size and volume fraction of precipitates,and the composition of matrix after the aging process can be predicted,and then integrated into the physical model to obtain the yielding strength of alloys.On average,the deviation of the yielding strength is 4%,which is far better than 15%with the existing strength model without considering the heat treatment,significantly reducing the development cycle.The size and volume fraction of precipitates decrease with the increased cooling rate,leading to that the precipitate strengthening firstly increases and then decreases.Meanwhile,this trend would result in enhancing solid solution strengthening monotonously.Especially,the critical cooling rate coordinates the relationship between the competition and cooperation owing to the obvious change of the anti-phase boundary energy together with the matrix composition,and a maximum yielding strength occurs at 166 ℃/min.The present work can provide key theoretical guidance for designing advanced alloys with excellent performance in a3 D space.