Multi-objective calibration of elastic-viscoplastic models to capture the elevated-temperature creep and tensile behaviour of alloy 617

High-temperature engineering systems are expected to operate under sustained thermal and mechanical loads while also undergoing repeated thermal transients. Consequently, it is essential to develop material models capable of accurately predicting behaviour under a range of loading and temperature conditions. This study presents a multi-objective calibration framework for semi-empirical elastic-viscoplastic (EVP) models, aimed at simultaneously capturing the creep and tensile behaviour of Alloy 617 at $800°C$, $900°C$, and $1000°C$. A three-stage calibration workflow was developed using a multi-objective genetic algorithm (MOGA) to identify a unified set of material parameters for two EVP-based models: (i) one incorporating classical creep damage (EVP-CD), and (ii) another incorporating work-based damage (EVP-WD). Both models were calibrated using short-term high-temperature creep and tensile datasets while their validation was performed against longer-term creep datasets obtained at a given temperature. The results demonstrate that the calibrated material models can capture both high-temperature creep and tensile behaviour. Notably, the EVP-WD model exhibited better accuracy in reproducing the full tensile stress-strain response to failure, albeit with greater calibration difficulty. The proposed approach paves the way for the development of a single material model applicable to multiple service conditions, thereby simplifying and improving the accuracy of fitness-for-service assessments of high-temperature engineering components.