The constitutive behavior of nickel-based superalloy during the solution cooling process

【Objective】 This study aims to investigate the constitutive behavior of solution-treated FGH95 nickel-based superalloy during the continuous cooling process, with a focus on the precipitation of the secondary γ' phase and its influence on mechanical properties. The objective is to clarify the relationship between thermal history, γ' precipitation evolution, and mechanical response, and to establish a constitutive model capable of describing the material behavior during non-isothermal cooling.
【Method】 Solution-treated FGH95 alloy specimens were prepared by holding at 1 180 ℃, cooling uniaxial tensile tests were conducted under two different thermal paths, namely direct heating from room temperature and cooling interruption from the solution temperature, at temperatures ranging from room temperature to 1 000 ℃. Tension-compression cyclic tests were also performed to investigate the hardening behavior. Microstructural characterization was carried out using scanning electron microscopy under different cooling rates. Thermodynamic calculations and precipitation simulations were performed using JMatPro and MatCalc to analyze phase evolution. Based on experimental observations, a constitutive model incorporating dislocation slip resistance and precipitation strengthening was established, in which the evolution of the secondary γ' phase volume fraction was described by the JMA equation. Model parameters were calibrated by fitting experimental stress-strain curves.
【Result】 The results show that γ' phase precipitation during continuous cooling at 20 ℃/min is inevitable and mainly occurs within a high-temperature range. Compared with the direct heating condition, the specimens subjected to cooling exhibit higher yield strength and strain hardening rate at the same temperature. This difference is caused by the distinct thermal histories prior to deformation, which result in different stages of secondary γ' precipitation. During cooling from 1 180 ℃, a significant amount of secondary γ' phase has already precipitated before deformation, whereas only limited precipitation occurs during heating from room temperature. In addition, the precipitation of γ' phase continues during deformation at elevated temperatures, further enhancing strain hardening. Microstructural observations confirm that lower cooling rates promote the formation of coarser γ' precipitates, while higher cooling rates lead to finer and more densely distributed secondary γ' particles. The proposed constitutive model accurately captures the evolution of flow stress under both heating and cooling conditions and reflects the contribution of precipitation strengthening.
【Conclusion】 The constitutive behavior of solution-treated FGH95 superalloy during continuous cooling is strongly dependent on the evolution of the secondary γ' phase. The difference in mechanical properties between heating and cooling conditions originates from the distinct precipitation states induced by thermal history. The developed constitutive model, which incorporates precipitation evolution based on the JMA framework, can effectively predict the mechanical response during continuous cooling and provides a useful tool for analyzing microstructure-property relationships and optimizing heat treatment processes.