EBM 공정에 적합한 저비용 고분율 γ＇ 초내열합금 설계 및 변형거동 고찰 = Novel Alloy Design of Cost-Reduced and Weldable High γ＇ Superalloy for AM and Its Deformation Behavior

다국어 초록 (Multilingual Abstract)

The application of Additive Manufacturing (AM) technology to high-γ′ Ni-based superalloys for next-generation gas turbine components has been severely restricted by their inherent susceptibility to hot cracking and high intrinsic material costs. This study aims to overcome these dual challenges by developing a novel cost-effective and high-strength Ni-based superalloy, optimized for the AM process. A multiscale alloy design strategy utilizing thermodynamic simulations (Thermo-Calc) and Density Functional Theory (DFT) calculations was employed to screen 36,450 alloy candidates. The design focused on minimizing the solidification temperature range and Cracking Susceptibility Coefficient (CSC) by strictly controlling grain boundary segregating elements (B, C, Zr), while simultaneously reducing high-cost refractory elements (Ta, Hf) to ensure economic feasibility (≤ 130% cost of pure Ni). The newly developed alloy fabricated via EBM under optimized process conditions exhibited excellent processability with a relative density as high as 99.98%. Notably, the crack density was significantly reduced to 0.08%, which is superior to that of the commercial MAR-M247 alloy (1.80%), demonstrating the effectiveness of the anti-cracking design strategy. The chemical composition of the as-built parts matched the target composition well, with no significant evaporation of volatile elements such as Al. Microstructural analysis revealed that the as-built developed alloy possessed a columnar grain structure with a strong <001> texture along the build direction and a high γ′ area fraction of 61.2%. To address the issue of coarse intergranular γ′ precipitates formed during the EBM process, a novel multistep solution heat treatment was designed. This tailored heat treatment successfully homogenized the microstructure by dissolving coarse precipitates and re-precipitating fine, cuboidal γ′ particles within the grain interior, leading to improved microhardness (464 Hv). Mechanical testing demonstrated that the newly developed alloy exhibits superior tensile strength compared to commercial cast alloys (CM247LC, MAR-M247) and other AM superalloys. Specifically, the ultimate tensile strength at 871 °C reached 1,185 MPa, showing remarkable high-temperature performance. Furthermore, in creep tests conducted at 815 °C/448 MPa, the alloy achieved a rupture life of 333 hours. Transmission Electron Microscopy (TEM) analysis revealed that the primary deformation mechanisms were Anti-Phase Boundary (APB) shearing and the formation of isolated Stacking Faults (SFs) rather than microtwinning. This behavior, attributed to the increased planar fault energy, along with the formation of stable interfacial dislocation networks, effectively impeded dislocation motion, thereby enhancing both tensile and creep resistance. In conclusion, this study successfully developed a novel high-γ′ superalloy that achieves both cost-competitiveness and high mechanical performance, proving its potential as a promising material for additively manufactured hot-section components in advanced gas turbines.

목차 (Table of Contents)

• Abstract ·········································································································3
• List of tables and figures ······································································6
• 제1 장. 서 론 ······························································································ 11
• 제2 장. 이론적 배경
• 제1 절. Ni기 초내열합금의 미세조직 및 물성 ··································16
• 1) Ni기 초내열합금 ·············································································16
• 2) Ni기 초내열합금의 주요 강화기구 및 석출상 ························· 23
• 3) Ni기 초내열합금의 변형기구 ·······················································26
• 제2 절. Ni기 초내열합금의 적층제조 ··················································34
• 1) Additive Manufacturing (AM) ···················································34
• 2) Ni기 초내열합금의 적층제조 및 고온균열 ······························· 44
• 제3 장. 실험 방법
• 제1 절. 적층제조용 Ni기 초내열합금 설계 ········································54
• 제2 절. 개발합금의 적층제조 ································································59
• 제3 절. 시편 준비 및 미세조직 분석··················································60
• 제4 절. 기계적 물성 평가······································································61
• 제4 장. 결과 및 고찰
• 제1 절. 적층제조용 저비용-고강도 초내열합금 설계······················62
• 1) 합금 설계 전략 제시 ···································································62
• 2) Screening을 통한 최종 합금 성분계 도출 ····························· 73
• 제2 절. EBM 공정으로 제작된 개발합금의 미세조직 분석·········· 76
• 1) 개발합금 분말 분석 ·····································································76
• 2) 개발합금의 적층제조 용이성 평가 ··········································· 80
• 3) 미세조직 분석 ···············································································86
• 4) 고온균열 기구 고찰 ·····································································89
• 5) 열처리 최적화·················································································94
• 제3 절. EBM 공정으로 제작된 개발합금의 물성 평가 ·················· 99
• 1) 개발합금 EBM 성형체의 인장 특성 및 변형기구 분석········ 99
• 2) 개발합금 EBM 성형체의 크리프 특성 및 변형기구 분석·· 105
• 제5 장. 결론 ·······························································································111
• References ·································································································114