Role of boron in enhancing creep and high temperature oxidation resistance in FeNi-based superalloy

국문 초록 (Abstract)

본 연구는 FeNi 기반 초합금에 보론을 첨가함으로써 미세구조, 기계적 특성, 크리프 저항성 및 산화 거동에 미치는 영향을 조사하였다. 이러한 초합금은 항공우주 및 에너지 산업과 같은 고온 환경에서 널리 사용되며, 본 연구는 보론을 활용하여 결정립계를 강화하고 상의 안정성을 향상시키며 산화 저항성을 증가시키기 위한 성능 개선을 목표로 하였다. 보론은 0.1 at%와 0.2 at%의 비율로 FeNi 기반 초합금에 첨가되었으며, 샘플은 923K 에서 1123K 까지의 온도 범위에서 크리프 및 산화 시험을 거쳤다. 원자 탐침 단층 촬영(APT), 투과 전자 현미경(TEM), 전자 후방산란회절(EBSD), 열중량 분석(TGA)과 같은 여러 고급 기술을 활용하여 미세구조 변화, 원소 분포 및 산화층 형성을 분석하였다. 인장 시험(최종 인장 강도, UTS 까지)과 후속 힐링 처리(healing treatment) 후, 보론이 FeNi 기반 초합금의 기계적 특성에 중요한 영향을 미친다는 것이 확인되었다. 보론 첨가는 결정립계에서 η-Ni₃Ti 석출물을 형성하여 결정립계의 응집력과 안정성을 강화했다. 원자 탐침 단층 촬영을 통해 매트릭스-η 상 계면에서 보론이 약 1.4 at% 농도로 편석되어 있어 결정립계가 강화된 것으로 나타났다. Halder-Wagner 방법을 사용한 전위 밀도 계산에 따르면, 보론이 포함된 합금에서는 회복이 덜 발생하여 전위가 유지되었고, 이는 강도 유지에 기여했다. 또한, 보론이 결정립계에서의 편석(segregation)은 연성을 증가시켰으며, B0.2 합금(보론 0.2%)은 항복 강도가 약간 감소(1269 MPa 에서 1115 MPa)했지만, 연신율은 크게 증가(1.9%에서 4.5%)하였다. 크리프 저항성 시험에서는 보론 첨가가 부분 동적 재결정화(DDRX)를 효과적으로 억제하여 결정립계 약화를 지연시키고 조기 균열 형성을 방지한다는 것이 확인되었다. B0.1 합금은 1023K 에서 320 MPa 의 조건에서 167.64 시간 동안 파단되지 않았으며, 보론이 포함되지 않은 B0 합금(72.78 시간)보다 훨씬 긴 수명을 기록했다. 이 결과는 보론이 재결정화에 필요한 에너지 역치를 높이고 결정립계 응집력을 강화하여 크리프 저항성을 향상시킨다는 것을 나타냈다. 산화 저항성 시험에서는 보론이 보호 산화층의 열적 안정성을 향상시키는 것으로 나타났다. 1023K 에서 100 시간 동안 산화 후, B0.1 합금은 약 500nm 두께의 안정적인 산화층을 유지한 반면, B0 합금은 산화층 아래에 다공성의 불균일한 메트릭스를 형성하였다. 이는 보론이 고온 환경에서 산화막의 안정성을 유지하는 데 기여함을 보여준다. 결론적으로, 본 연구는 FeNi 기반 초합금의 기계적 특성, 크리프 저항성 및 산화 안정성을 향상시키는 데 보론이 중요한 역할을 한다는 것을 강조한다. 보론의 결정립계 및 매트릭스-석출물 계면에서의 편석은 합금의 강도와 연성을 크게 향상시켜, 고온 환경에서 이 합금의 성능과 내구성을 강화한다.

다국어 초록 (Multilingual Abstract)

This study investigates the effect of boron addition on the microstructure, mechanical properties, creep resistance, and oxidation behavior of FeNi-based superalloys, which are widely used in high-temperature applications such as aerospace and energy industries. The research aimed to enhance the performance of these alloys by utilizing boron to strengthen grain boundaries, improve phase stability, and increase oxidation resistance.
Boron was added in varying amounts (0.1 at% and 0.2 at%) to FeNi-based superalloys, and the samples were subjected to creep, and oxidation tests at temperatures ranging from 923K to 1123K. Several advanced techniques, including Atom Probe Tomography (APT), Transmission Electron Microscopy (TEM), Electron Backscatter Diffraction (EBSD), and Thermogravimetric Analysis (TGA), were used to investigate microstructural changes, elemental segregation, and oxidation layer formation.
After tensile testing up to ultimate tensile strength (UTS) and subsequent healing treatment, boron was shown to significantly influence the mechanical properties of FeNi-based superalloys. Boron addition resulted in the formation of η-Ni3Ti precipitates at grain boundaries, enhancing grain boundary cohesion and stability. Atom Probe Tomography (APT) revealed that boron segregation at the matrix-η phase interface reached concentrations of around 1.4 at%, strengthening the grain boundaries. Dislocation density calculations using the Halder-Wagner method showed that recovery was less pronounced in boron-containing alloys, thereby maintaining strength by retaining dislocations. In addition, segregation of boron at grain boundaries increased ductility by reinforcing the boundaries. For example, the B0.2 alloy, with 0.2% boron, exhibited only a slight reduction in yield strength (from 1269 MPa to 1115 MPa) but showed a significant increase in elongation at fracture (from 1.9% to 4.5%). Creep resistance tests demonstrated that boron addition effectively suppressed partial dynamic recrystallization (DDRX), delaying grain boundary weakening and preventing premature crack formation. The B0.1 alloy exhibited a rupture time of 167.64 hours at 1023K and 320 MPa, significantly longer than the 72.78 hours observed for the boron-free B0 alloy. This indicated that boron improved creep resistance by increasing the energy threshold necessary for recrystallization and enhancing cohesion at grain boundaries. Oxidation tests revealed that boron improves the thermal stability of the protective oxide layer. After 100 hours of oxidation at 1023 K, the B0.1 alloy maintained a stable oxide layer with a thickness of approximately 500 nm, whereas the B0 alloy formed a porous and uneven matrix beneath the oxide layer. This indicates that boron contributes to maintaining the stability of the oxide layer in high-temperature environments.
In conclusion, this study highlights the vital role of boron in improving the mechanical properties, creep resistance, and oxidation stability of FeNi-based superalloys. Boron segregation at grain boundaries and matrix-precipitate interfaces provides significant strengthening, enhancing both the strength and ductility of these alloys in high-temperature environments.

목차 (Table of Contents)

• Table of Contents
• Table of Contents.....................................................................................................i
• List of Figures.........................................................................................................v
• List of Tables.........................................................................................................xii
• List of Abbreviations...........................................................................................xiv
• Abstract................................................................................................................xvi
• Chapter 1. Introduction.........................................................................................1
• 1.1. Materials for High-temperature Application..................................................................1
• References............................................................................................................................6
• Chapter 2. Literature Review................................................................................7
• 2.1. Superalloys..................................................................................................................7
• 2.1.1. Classification and development of superalloys............................................8
• 2.1.2. Processing of superalloy.............................................................................13
• 2.1.3. Research and development of FeNi-based superalloy................................14
• 2.1.4. Ductility dip cracking in superalloy............................................................21
• 2.2. Creep...........................................................................................................................24
• 2.2.1. Larson-Miller approach for evaluating creep performance........................27
• 2.2.2. Correlation between strain rate sensitivity and stress hardening exponents in creep mechanisms..................................................................................28
• 2.2.3. Steady state creep deformation of Superalloy.............................................33
• 2.3. Elements interaction and the role of interstitial atoms.........................................35
• 2.3.1. Segregation mechanisms of interstitial elements........................................41
• 2.3.2. Boron segregation effect in superalloy........................................................48
• 2.3.3. Precipitation of metal compounds containing boron..................................49
• 2.4. Recrystallization................................................................................................52
• 2.4.1. Dynamic recrystallization..........................................................................54
• 2.4.2. Discontinuous Dynamic Recrystallization.................................................56
• 2.4.3. Dynamic Recrystallization during creep of superalloy...............................59
• 2.5. Oxidation of metal....................................................................................................60
• 2.5.1. Oxidation of superalloy..............................................................................60
• 2.5.2. Oxidation Behavior in Superalloys with Boron Additions..........................62
• References..........................................................................................................................65
• Chapter 3. Motivation and Research Objective.................................................75
• References..........................................................................................................................83
• Chapter 4. Experimental Procedure...................................................................84
• 4.1. Sample Preparation..................................................................................................84
• 4.1.1. Materials....................................................................................................84
• 4.1.2. Heat treatment............................................................................................84
• 4.2. Metallurgical Analysis and Evaluation..................................................................86
• 4.2.1. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).................................................................................................86
• 4.2.2. Optical Microscopy (OM)..........................................................................86
• 4.2.3. X-ray Diffraction (XRD)............................................................................86
• 4.2.4. Scanning Electron Microscopy (SEM).......................................................86
• 4.2.5. Electron Backscattered Diffraction (EBSD)...............................................87
• 4.2.6. Transmission Electron Microscope (TEM)................................................87
• 4.2.7. Thermogravimetric Analyzers (TGA)........................................................87
• 4.2.8. Differential Scanning Calorimetry (DSC)..................................................88
• 4.2.9. Glow Discharge Optical Emission Spectrometry (GD-OES).....................88
• 4.2.10. Glow Discharge Mass Spectrometer (GD-MS)........................................88
• 4.2.11. Atom Probe Tomography (APT)...............................................................89
• 4.3. Mechanical Property Testing..................................................................................91
• 4.3.1. Tensile Test.................................................................................................91
• 4.3.2. Hardness Test.............................................................................................91
• 4.3.3. Strain Rate Change (SRC) Test...................................................................91
• 4.3.4. Creep Test...................................................................................................92
• References..........................................................................................................................93
• Chapter 5. Enhanced crack propagation resistance after healing treatment by addition of boron in FeNi-base superalloy..........................................................94
• 5.1. Background of the Research...................................................................................94
• 5.2. Experimental Procedure..........................................................................................96
• 5.3. Results.......................................................................................................................99
• 5.3.1. Mechanical properties and microstructure of aged alloys...........................99
• 5.3.2. Evolution of microstructure and mechanical properties after healing heat treatment..................................................................................................110
• 5.4. Discussion...............................................................................................................120
• 5.4.1. Formation of η phase after healing heat treatment....................................120
• 5.4.2. Role of boron in healing heat treatment....................................................121
• 5.5. Summary.................................................................................................................130
• References........................................................................................................................132
• Chapter 6. Suppression of recrystallization during creep by addition of boron in Fe-Ni based superalloy...................................................................................136
• 6.1. Background of the Research.................................................................................136
• 6.2. Experimental Procedure........................................................................................137
• 6.3. Results.....................................................................................................................142
• 6.3.1. Microstructure of aged alloys...................................................................142
• 6.3.2. Creep properties of alloys.........................................................................149
• 6.3.3. Evolution of microstructure during creep test...........................................151
• 6.4. Discussion...............................................................................................................167
• 6.4.1. Role of boron............................................................................................167
• 6.4.2. Discontinuous dynamic recrystallization (DDRX) during Creep.............172
• 6.5. Summary.................................................................................................................176
• References........................................................................................................................178
• Chapter 7. Improving oxidation resistance by addition of boron in FeNi-based superalloy............................................................................................................181
• 7.1. Background of the Research.................................................................................181
• 7.2. Experimental Procedure........................................................................................183
• 7.3. Results and Discussion..........................................................................................185
• 7.3.1. Microstructure and mechanical properties of aged alloys.........................185
• 7.3.2. Thermal oxidation behavior under isothermal conditions........................194
• 7.3.3. Analysis of the oxide layer using GD-MS and GDS for identification of the location and role of boron.........................................................................197
• 7.4. Summary.................................................................................................................200
• References........................................................................................................................201
• Chapter 8. Conclusion…....................................................................................202
• Abstract in Korean...................................................................................................204