PhD Thesis Defence: Mr. Chandan Kumar (24/04/25)
Thesis title:
A Comprehensive Evaluation of High-Temperature Deformation and Damage Behavior of IN740H, a γʹ-Lean Model Ni-Based Superalloy
Faculty advisor(s):
Prof. Praveen Kumar, Prof. Gaurav Tomar
When?
24th April, 2025 (Thursday), 03:00 PM (India Standard Time)
Mode Online
https://teams.microsoft.com/l/meetup-join/19%3ameeting_OThiNjdiZjItMmViNS00MTRhLTgyMmQtNWVlYzY1YTU5NTA1%40thread.v2/0?context=%7b%22Tid%22%3a%226f15cd97-f6a7-41e3-b2c5-ad4193976476%22%2c%22Oid%22%3a%22db32af09-2e7a-4ab0-b793-ab66876ce0e6%22%7d
Abstract
Advanced ultra-supercritical (AUSC) power plants represent a significant leap in thermal efficiency and environmental sustainability for coal-based energy production. By operating at higher temperatures (≥ 760 °C) and steam pressures (≥ 35 MPa), AUSC technology is expected to improve the thermal efficiency of powerplants to ≈50 %, thereby reducing fuel consumption and greenhouse gas emissions; however, it warrants the discovery of a new class of materials for long-term use under such harsh conditions, which also includes high-temperature corrosion. Based on the creep-rupture life, IN740H, a γʹ-lean Ni-based superalloy, is considered as a potential material to be used in high-temperature sections of AUSC powerplants; however, mechanistic insights into high-temperature deformation and damage behavior of this alloy are not available. Accordingly, in this study, we aim to establish structure-property correlations for IN740H by systematically studying its high-temperature flow behavior under constant strain rate, stress conditions, and cyclic loading conditions. At first, the tensile deformation behavior of IN740H was examined across a temperature range of 25-800 °C by applying constant nominal strain rates of 10-3 to 10-5 s-1. Distinct stress-strain curve serrations, consistent with dynamic strain aging (DSA), were predominantly observed between 400-600 °C. Notably, the alloy displayed negative strain rate sensitivity in the DSA regime. Furthermore, the alloy also showed yield strength anomaly (YSA), wherein the yield strength decreased initially with temperature, followed by an increase above a specific temperature, which was strain-rate dependent. Subsequently, creep tests were performed on the IN740H sample by applying constant nominal stress at temperatures above 700 °C. The material showed classic signatures of the presence of threshold stress, marked by observation of a high apparent creep stress exponent, n, (e.g., 10 at 750 °C), and a high apparent activation energy for creep, Qc (e.g., ~545 kJ/mol), along with a rapid increase in n at lower stresses. Accounting for the temperature-dependent threshold stress led to a decrease in the values of QC and n to 270 kJ/mol and 4, respectively, which in corroboration to the microstructural studies, confirm the dislocation climb over γʹ precipitates as the dominant creep mechanism. Furthermore, creep-fatigue experiments were conducted on IN740H, at 750 °C with a strain ratio of -1, a strain amplitude of either 0.3 or 0.5 %, and a dwell period at the maximum strain of 0, 120, and 600 s. The effect of creep and fatigue on fatigue-life as well as microstructure was systematically evaluated, and a strain energy density-based model that accounted for primary creep parameters was developed to predict creep-fatigue lifetime. The developed model was in good agreement with experimental results, with predicted life within a factor of 2 of the experimental observations. This comprehensive investigation underscores the potential of IN740H to meet the demanding operational requirements of AUSC power plants, contributing to more efficient and environmentally friendly energy production.