A study on phase transformation and deformation behaviors considering transformation plasticity in steels

김동완 서울대학교 대학원 2016 국내박사

The recent trends in the automotive industry have mainly focused on increasing the crashworthiness properties of automobiles and at the same time decreasing the fuel consumption and gas emissions. In this point of view, Advanced High Strength Steel (AHSS) offers an opportunity for the development of cost effective and light weight parts with improved safety and optimized environmental performance for automotive applications. Starter of AHSS, for example dual phase (DP) and transformation induced plasticity (TRIP) steels, has a higher strength than previous conventional steel. Next stage, Ultra - Advanced High Strength Steel (U-AHSS) is developed. The representative of the U-AHSS is the twinning induced plasticity (TWIP) steels, which has a much strength and elongation. However this steel are yet to be commercialized in use, mainly because of the high level of Mn, which leads to associated processing problems. Thus, X-AHSS steels, with properties of both AHSS and U-AHSS, are now consider as a new option for commercialized steels. Phase transformation behaviors play an important role to decide material properties of the AHSS. Therefore it is necessary to predict accurate phase transformation behaviors using appropriate model. In this paper, there are various attempts in order to predict phase transformation behaviors of AHSS which included complex transformation behaviors. Firstly, method of dilatometric analysis to analyze the phase transformation behaviors of AHHS with retained austenite was developed. Developed dilatometric analysis method is based on lattice parameter and atomic volume of each phase and additionally consider transformation plasticity and enrich of alloying element. Using the thermodynamic data and equations embedded in the method, it can distinguish the type of BCC structure. Analysis of two specimens which has different Mn, the strong austenite stabilizer, we can verify the accuracy of developed method and find various parameters concerning transformation plasticity and fraction of retained austenite. This method is preferred to obtain the fraction of final fraction of retained austenite very simply and phase transformation kinetics of retained austenite steel. Secondly, the dissolution and precipitation of Nb, which has been known as strong carbide-forming element, play a key role in controlling phase transformation kinetics of microalloyed steels. In this study, we analyzed both numerically and experimentally the precipitation behavior of Nb-microalloyed steel and its effect on the austenite decomposition during cooling. Nb precipitation in austenite matrix could be predicted by the thermo-kinetic software MatCalc, in which interfacial energy between precipitate and matrix is calculated. The simulated precipitation kinetics were fairly well agreement with the experimental observations by TEM. Austenite decomposition, which is strongly affected by Nb precipitaion during cooling, was measured by dilatometry and was modeled on the basis of a Johnson–Mehl–Avrami–Kolmorgorov (JMAK) equation. It was confirmed that the dissolved Nb delays the austenite decomposition, whereas, the precipitated Nb performs as nucleation sites of phase transformation during the austenite decomposition.. Lastly, a finite element model was developed to predict the deformation, temperature history, carbon diffusion, phase fraction, and hardness during the carburizing heat treatment of automotive annulus gear ring, initially made of a medium carbon steel. Carburizing gas with a constant carbon potential for entire surfaces of the gear was assumed. The temperature and pressure driven carbon diffusion was solved by the finite element simulation based on Fick’s law. Both the diffusional and displacive phase transformations during the heat treatment were modeled incorporating the carbon concentration inside the gear. The constitutive equation of the transformation plasticity was incorporated into the finite element model. Strains due to the phase transformation, transformation plasticity, and thermal expansion/contraction were calculated by the finite element model. The prediction accuracy for the phase evolution, hardness distribution, and dimensional change of the gear ring was verified with the measurement data. From this study, phase transformation model in X-AHSS, which has not been clear up to now, is described well. The developed model and suggested analysis method lead to a clearer understanding about phase transformation behaviors and related phenomenon of ferrous alloys. Furthermore, using developed model coupled with finite element simulation or material property prediction model, we can predict the changes of microstructural characteristics and thermal-mechanical behaviors during complex phase transformation.

Numerical studies on the phase transformation and crack generation of hypo peritectic steel during continuous casting

조준현 서울대학교 대학원 2021 국내박사

In the continuous casting process, hypo peritectic steel has a complex phase change and a high cracking ratio. So, first, we develop a model of phase transformation that can simulate the phase change behaviors of the hypo peritectic steel. This new model is suggested to predict the behaviors of phase transformation during continuous cooling by considering the thermodynamics, empirical formulas, and carbon diffusion. Particularly, massive transformation from δ phase to γ phase and undercooling from the peritectic temperature to the formation of γ phase (dTp) are included in this model. As a result, it is showed that the phase change behaviors of the hypo peritectic steel have two paths. When the solidification is completed without the peritectic transformation to all δ phases before the temperature reaches Tps (=Tp (peritectic temperature)-dTp), the solidified δ phase is transformed to the γ phase by the massive transformation. On the other hand, when the peritectic transformation at the L/δ interface starts at Tps, the growth of the γ phase by the peritectic transformation is generated by the carbon diffusion. Using the results of the phase change model of hypo peritectic steel, the mechanisms of crack generation in the continuous casting process were investigated. So, new models are developed, such as strain rates in solid, volume contraction rates with liquid, and pore formation susceptibilities. In addition, stress model is developed for calculating the stress distribution in the solidified shell. As a result, it can be suggested that the massive transformation in solid and peritectic transformation during solidification are the main mechanisms of crack generation. In addition, it is showed that these two crack mechanisms are divided based on the linear relation between carbon contents and dTp, and that the probability of crack generation is high near the transition boundary between the two mechanisms. The crack generation ratios are analyzed by using the results of models for predicting crack generation. In order to apply the results of the models to alloying steel, an equation of effective carbon composition is suggested. As a result, it is possible to analyze the behaviors of the crack generation ratio according to the effective carbon contents at various experiments by using the temperature at which massive transformation starts and the pore formation susceptibilities at a specific dTp. Furthermore, the effects of silicon, manganese and casting speed on the behaviors of crack generation are analyzed. Casting speed, silicon concentration, and manganese concentration shifted the effective carbon composition with the maximum crack ratio. These behaviors of crack ratios according to casting speed, silicon, and manganese can be understood by the difference of δ/γ interfacial energy as the energy to overcome to generate γ phase. As a results, because the casting speed, silicon contents, and manganese contents can change the dTp by affecting the nucleation of the γ phase, it can be suggested that they can change the carbon contents with the maximum crack ratio. 연속 주조 공정에서 아포정강은 복잡한 상변화와 높은 크랙 발생 비율을 갖는다. 이를 이해하기 위해, 우선 아포정강의 상변화 거동을 모사할 수 있는 상변화 모델을 개발하였다. 이 새로운 아포정강의 상변화 모델은 열역학, 실험식, 탄소의 확산을 고려하였으며, 연속 냉각 중의 상변화 거동을 예측하고자 하였다. 특히 δ상에서 γ상으로의 매시브 변태와 포정 온도에서부터 γ 상 생성까지의 과 냉(dTp)이 상변화 모델에 포함되었다. 그 결과, 아포정강의 상변화 거동은 크게 두 가지 경로를 가짐을 확인하였다. 온도가 Tps(=Tp(포정 온도)-dTp)에 도달하기 전에 모두 δ상으로 포정 변태 없이 응고가 완료되는 경우, 응고된 δ상은 매시브 변태에 의해 γ상으로 상변화가 이루어진다. 이와는 달리, 온도가 Tps에 도달하기까지 응고가 완료되지 않았을 때, L/δ계면에서 포정 변태가 시작하는 경우, 포정 변태로 인한 γ상의 성장은 주로 탄소의 확산에 의해 이루어진다. 아포정강의 상변화 모델 결과를 이용하여, 연속 주조 공정에서의 크랙 발생 메커니즘을 규명하고자 하였다. 이를 위해 고상에서의 변형률 속도, 응고 도중 부피 수축 속도 및 공공 생성 가능성을 모델링 하였다. 추가적으로 응력 해석 모델을 개발함으로서, 응고된 쉘 내의 응력 분포를 계산하였다. 그 결과 고상에서의 매시브 변태와, 응고 도중 발생하는 포정 변태가 주된 크랙 발생 메커니즘으로 판단되었다. 또한 이 두 크랙 발생 메커니즘은 탄소 조성과 dTp 사이의 선형적인 관계를 기준으로 천이됨을 파악하였으며, 두 메커니즘이 천이되는 경계 근처에서 크랙 발생 가능성이 가장 크다는 것을 보여 주었다. 크랙 발생 예측 모델 결과를 이용하여 연속 주조 공정에서의 면세로 크랙 발생 비율 거동을 분석하였다. 이때, 앞서 진행 한 모델링 연구들을 합금강에 적용하기 위해서 유효 탄소 조성 식을 도출하여 사용하였다. 그 결과, 다양한 실험들의 유효 탄소 조성에 따른 크랙 발생 비율 거동을 특정 dTp에서의 매시브 변태 시작 시 온도와 공공 발생 가능성을 이용하여 분석이 가능하였다. 나아가 크랙 발생 거동에 대한 실리콘, 망간, 주조 속도의 영향을 분석하였다. 실리콘 농도와 주조 속도의 증가는 최대 크랙 발생 비율을 갖는 유효 탄소 조성을 높이며, 망간 농도의 증가는 최대 크랙 발생 비율을 갖는 유효 탄소 조성을 낮추었다. 우리는 이를 L/δ계면에서 γ상이 생성되기 위해 극복해야 하는 δ/γ 계면 에너지를 계산하여 분석해 보았다. 그 결과 주조 속도, 실리콘 농도, 망간 농도는 γ상의 핵생성에 영향을 주어 dTp를 변화시킬 수 있다고 판단할 수 있었다. 그리고 이러한 dTp의 변화는 최대 크랙 발생 비율을 갖는 탄소 조성을 변화 시킬 수 있다고 판단되었다.

Effects of Cr/Ni equivalent ratio and cooling rates on phase transformations and residual stresses in 17-4 PH alloy fabricated by LPBF

안소희 Graduate School, Yonsei University 2025 국내박사

본 연구는 17-4 PH 합금의 LPBF 공정 및 후열처리 동안의 상변태에 대해 연구하여 LPBF로 출력된 17-4 PH 합금의 결함을 개선하고자 하였다. LPBF 출력 17-4 PH 합금의 미세구조를 관찰하고, 응고 모드가 합금 원소 편석과 출력 중 석출물 형성에 미치는 영향을 파악하였다. 또한 마르텐사이트 변태를 통해 인장 잔류 응력장을 제어하는 방법을 제안하였으며, 고전적 핵생성 및 성장 이론에 기반하여 17-4 PH 합금의 LPBF 출력 중 상변태를 예측하는 모델링을 수행하여 최종 미세조직을 예측하는 모델을 제시하였다. 기존의 17-4 PH 스테인리스강과 달리, LPBF로 출력된 17-4 PH 합금에서는 급속 냉각으로 인한 Cu 편석과 반복적인 가열로 인한 ε-Cu 석출물이 관찰된다. Cu 편석과 ε-Cu 석출물이 시효 처리 중 석출 경화에 미치는 부정적인 영향을 개선하기 위한 용체화 처리 최적화를 수행하였다. ε-Cu 석출물은 고온, 장시간의 용체화 처리 후에도 기지 내로 용해되지 않았으나, 용체화 처리 온도와 시간이 증가할수록 석출물의 평균 직경이 감소하는 경향이 나타났다. 이에따라 1200 ℃-8시간 용체화 처리가 기존의 1050 ℃ 용체화 처리보다 시편의 경도를 향상시켰으며, 적층 제조 시편에서는 적어도 1100 ℃ 이상의 용체화 처리가 효과적임을 확인하였다. 17-4 PH 합금은 Creq/Nieq 및 냉각속도에 따라 다양한 미세조직으로 출력될 수 있다. 본 연구는 Creq/Nieq 비율과 VED를 변화시켜 미세조직을 제어하고 이를 통해 인장 잔류 응력을 제어하는 방법을 제안하였으며, 17-4 PH 합금의 LPBF 출력 미세 조직을 예측하는 다이어그램을 제시하였다. 마르텐사이트 변태 시 발생하는 인장잔류 응력 상쇄의 메커니즘을 규명하였으며, 마르텐사이트 변태는 Creq/Nieq 비율이 낮고 냉각속도가 느릴수록 더욱 용이하게 발생하여 LPBF 공정 중 인장 잔류 응력을 효과적으로 상쇄함을 확인하였다. 빠른 냉각속도로 인해 LPBF 출력 17-4 PH 합금의 최종 미세조직은 델타 페라이트, 마르텐사이트, 잔류 오스테나이트로 구성된다. 최종 미세조직을 결정하는 것은 델타 페라이트로 부터 오스테나이트가 형성되는 과정으로, 최종 미세조직을 예측하기 위해서는 이 상변태를 예측하는 것이 중요하다. 따라서 고전 핵생성 및 성장 이론을 기반으로 한 상변태 모델을 시뮬레이션된 온도프로파일과 결합하여 최종 미세조직 예 측을 수행하였다. 시뮬레이션 결과를 실제 상분율과 비교하여 모델이 높은 정합성 (R2 =0.9405)을 달성한 것을 확인하였다. 본 연구는 LPBF에서 최종 미세조직을 예측하기 위해 위치별 온도 프로파일을 고려하는 것이 중요하다는 것을 보여준다. This study investigated the phase transformations and changes in residual stress fields during the laser powder bed fusion (LPBF) process and post heat treatment of 17-4 PH alloys. Aimed at advancing both theoretical understanding and practical applications, this research bridges academic insights with industrial relevance. By providing a comprehensive analysis of microstructure control, phase transformation modeling, and residual stress management, the study establishes a scientific foundation for obtaining reliability of LPBF-fabricated 17-4 PH alloy. The microstructures of LPBF-fabricated 17-4 PH alloys were observed, and the influence of solidification modes on solute segregation and early precipitation was investigated. Various conditions of solution treatments were conducted to assess their effectiveness in homogenizing the matrix. Additionally, effects of chemical compositions and volumetric energy densities (VED) on the printed microstructures were studied and a microstructure prediction diagram was proposed. In addition, a method for controlling residual stress fields through martensitic transformation was also proposed based on the established relationship between microstructures and residual stress fields. Recognizing the critical importance of microstructure prediction, numerical modeling for phase transformation of the 17-4 PH alloy was conducted based on the classical nucleation and growth theory. 1. Homogenization through solution treatment and its effects on the precipitation-hardening 17-4 PH stainless steel (SS) exhibits high strength and good corrosion resistance via Cu-precipitation hardening. Unlike conventional wrought 17-4PH SS, Cu segregation and ε-Cu precipitates are observed in additively manufactured (AM) 17-4PH SS owing to the repeated rapid cooling after heating, which characterizes the AM process. In this study, solution treatment was conducted under various temperatures (1,000, 1,050, 1,100, and 1,200 °C) and durations (1, 2, 4, and 8 h) to minimize the negative effects of Cu segregation and ε-Cu precipitates on precipitation hardening. Because the Cu segregation and precipitates formed during the LPBF process are challenging to fully homogenize even with the most intensive solution treatment, it is important to minimize Cu segregation during the printing. The mechanical properties and microstructures of each condition for the Cu precipitation behavior were examined. Although the ε-Cu precipitates did not disappear after solution treatment, the average diameter of the ε-Cu precipitates tended to decrease with increasing solution treatment temperature and duration. Therefore, solution treatment at a temperature of 1,200 °C for 8 h was the best, resulting in increase of hardness about 128 HV xi compared to the conventional solution treatment at 1,050 °C. Solution treatment on at least 1,100 °C is effective in AM. 2. Understanding the relationship between martensitic transformation and residual stress fields The 17–4 PH alloy can be printed into diverse microstructures. Existing reports on the alloy have focused on the effect of Creq/Nieq ratios and the processing parameters of LPBF on the final microstructures of the δ-ferrite and α’-martensite phases in the alloy. However, the effect of microstructural variations on the residual stress fields of LPBF-fabricated 17–4 PH SS has yet to be fully understood. This study bridges the gap between what is known about the microstructure and residual stresses by proposing an alloy-equivalent phase prediction diagram for the AM of the 17–4 PH alloy by varying the Creq/Nieq ratio and VED to control residual stresses via microstructure manipulation. Dilatometry and Satoh tests were performed to elucidate the mechanism of the offset effect in martensite transformation. To control the microstructures, samples were fabricated with various Creq/Nieq ratios (2.05, 2.15, 2.30, and 2.49) and laser powers (130, 150, 170, and 190 W). The 17–4 PH alloy exhibited a low Ms temperature and approximately net zero residual stresses in the Satoh test. The study confirmed that martensitic transformation, which occurs more readily at lower Creq/Nieq ratios and higher laser powers, effectively offsets tensile residual stresses during the LPBF process, resulting in almost zero tensile residual stresses on fully martensitic specimens. 3. Kinetic modeling of the δ-ferrite to γ-austenite phase transformation 17-4 PH SS fabricated by LPBF retains δ-ferrite due to rapid cooling rates of up to 107 K/s. γ-austenite, transformed from δ-ferrite, further transforms into α’-martensite under sufficient cooling rates, resulting in a final microstructure composed of a mixture of retained δ-ferrite, α’-martensite, and retained austenite. The cooling rate, controlled by the VED, and the alloy composition (Creq/Nieq ratio) can influence the microstructure in LPBF-fabricated 17-4 PH SS. These microstructures influence not only mechanical properties but also tensile residual stress fields, highlighting the importance of predicting the final microstructure. This study focuses on the kinetic modeling of δ-to-γ phase transformation under various thermal profiles, induced by different VEDs (Laser powers: 130 W, 150 W, 170 W, and 190 W), in alloys with distinct Creq/Nieq ratios (2.05, 2.15, 2.30, and 2.49). Numerical modeling, based on nucleation and growth theory, was combined with simulated thermal profiles. The model’s predictions were validated by comparing the simulated microstructures with experimentally observed ones, achieving a high accuracy (R² = 0.9405). This study emphasizes the importance of considering site-specific thermal profiles to predict final microstructures in LPBF, offering new insights for microstructure prediction in additive manufacturing.

A study on the thermo/mechanical behavior in friction stir welding of steel

조훈휘 서울대학교 대학원 2014 국내박사

마찰교반용접(FSW)은 고상에서 재료를 접합할 수 있는 매우 획기적인 기술로서, 1991년 영국의 TWI(The Welding Institute)에서 처음으로 개발되었다. 이 용접 방법은 액상에서 고상으로 가는 과정이 공정 중에 일어나지 않기 때문에 일반적인 용접 방법에 비해 잔류 응력과 뒤틀림, 결함 등을 최소화할 수 있다. 또한, 이 공정은 보통 접합부에 미세한 결정립을 발달시킴으로써 기계적 강도를 향상시키는 장점을 가지고 있기 때문에 다른 일반적인 용접 방법의 대안으로 떠오르고 있다. 마찰교반용접은 초기에는 녹는점이 낮은 재료, 즉 알루미늄 합금같은 재료에 주로 적용되어 왔다. 반면에, 강도가 높은 재료, 즉 철강 같은 녹는점이 높은 재료에는 그 적용이 제한되어 있는 실정이었다. 그러나, 툴의 발달과 공정의 최적화에 관한 연구가 차츰 축적됨으로써, 강도가 높은 철강 계열에도 최근에는 이 공정이 적용되고 있다. 본 고에서는 실험과 전산모사 방법을 결합함으로써 철강의 마찰교반용접에서의 열적/기계적 거동을 기술하였다. 이에 더하여 계산된 결과들은 실험 결과들을 직접적으로 예측하기 위해 비교되었고, 검증되었다. 먼저, 마찰교반용접 중에 상변태가 일어나지 않는 강의 미세조직 변화가 분석되었다. 409 페리틱 스테인리스 강을 마찰교반용접을 통해 성공적으로 접합시켰으며, 우수한 강도의 접합부를 도출하였다. 현저하게 미세한 결정립이 접합부에서 관찰되었으며, 이는 연속동적 재결정에 의한 현상이라 판단되었다. 또한, 소각 입계가 접합부에서 기본 재료에 비해 상당히 증가하였다. 툴 삽입 깊이가 증가함에 따라 소각 입계와 경도가 증가하였고, 반면에 결정립 크기는 감소하였다. 다음으로, 마찰교반용접에 대한 열-기계적 모델을 Eulerian 유한체적법을 이용하여 개발하였으며, 정상상태하에서 FSW 공정 중의 열적/기계적 거동 변화를 예측하였다. 계산된 결과들은 측정된 온도 변화 이력과 미세조직 변화와 비교되었고, 검증되었다. 점소성 self-consistent 접근법을 이용하여 접합부의 집합조직이 예측되었고, 툴 근처에서 재료의 유동이 계산되었다. 계산된 결과를 통해, 열은 주로 재료와 툴의 경계에서 발생하는 것을 확인하였고, 점성도는 교반부와 열-기계적 영향부에서 급격하게 변화하는 것을 확인하였다. 재료의 유동은 주로 툴의 회전방향과 진행방향이 반대인 지점에서 발생하는 것을 예측하였다. 또한, 계산을 통해 접합부에서는 주로 전단 집합조직이 발달하는 것을 확인하였다. 측정된 온도 변화 이력과 미세조직적 특성은 계산된 결과와 잘 일치하였다. 다음으로, 상변태가 공정 중에 일어나는 강의 미세조직 변화가 분석되었다. 다양한 미세조직이 접합부 지역에서 발달하게 되는데, 발달된 미세조직들은 소각 입계의 재배열, 연속동적 재결정, 상변태에 의해 설명될 수 있다. 교반부의 대부분에서는 바늘모양의 베이나이틱 페라이트가 상변태에 의해 생성되었다. 연속동적 재결정이 일어나는 열-기계적 영향부에서는 미세한 결정립이 분포되어 있는 미세조직을 관찰할 수 있었다. 또한, 교반부에서는 상변태로 인해 전단 집합조직이 관찰되지 않았으며, 교반부의 경도는 상변태로 인해 발달된 상으로 인해 다른 접합부보다 높게 관찰되었다. 마지막으로, 개발된 열-기계적 모델에 상변태 모델을 결합하여 공정 중에 강의 상변태가 어떻게 진행되는지를 예측하였다. 마찰열과 과도한 소성 변형은 공정 중에 있는 강의 상변태에 큰 영향을 미친다. 따라서 열간 압연 중에서 흔히 사용되는 오스테나이트 결정립 크기 모델을 적용함으로써 적당한 상변태 모델을 만들고 이를 기존의 열-기계적 모델에 결합하였다. 측정된 온도와 상분율이 계산된 결과와 잘 일치하였다. 본 연구를 통해, 아직까지 명확하지 밝혀지지 않았던 철강의 마찰교반용접 중의 열적/기계적 거동을 분명하게 이해할 수 있었다. 개발된 모델과 제안된 방법은 철강의 마찰교반용접의 열적/기계적 거동에 대한 근본적인 이해를 가능하게 하였으며, 더욱이 개발된 모델은 계산만으로도 마찰교반용접으로 인해 나타나는 미세조직적 특징 변화들을 직접적으로 예측할 수 있는 충분한 가능성을 증명해주었다. Friction stir welding (FSW) is a solid state joining process invented by The Welding Institute (TWI, UK) in 1991. The process leads to lower residual stress and distortion in comparison with the fusion-based welding processes, since no melting of the material occurs during the process. Also, the process reduces manufacturing costs on account of the elimination of defects, shielding gas, and costly weld preparation. In addition, the process produces high-quality joints with a finer homogeneous microstructure and superior mechanical properties compared with the fusion-based welding processes. Initially, the FSW process was used only for non-ferrous alloys with a low melting temperature, such as aluminum alloys. In contrast, application of FSW to ferrous alloys including high strength steel with high melting temperatures has been limited due to high temperatures and severe wear conditions induced by the welding tool during the process. However, continued research into this process has brought some success in the joining of ferrous alloys, and this practical success requires a clearer understanding of the FSW joints of ferrous alloys. In this thesis, the thermo/mechanical behavior in FSW of steel is investigated based on the microstructural and numerical approaches. Also, the simulated results are directly compared with the experimental results for validation of the developed model. Firstly, the microstructural change in the FSWed region of steel, where phase transformation does not occur, is analyzed. High-quality, defect-free welds are successfully produced in 409 ferritic stainless steel by FSW. A remarkably fine-grained microstructure was observed in the stir zone, and the fraction of low angle grain boundary in the stir zone (SZ) significantly increased as compared to that in the base material. An increase in plunging depth led to an increase of the fraction of low angle grain boundary, a decrease in grain size, and an increase in hardness in the stir zone. Secondly, a three-dimensional thermo-mechanical simulation of FSW processes is carried out for ferritic stainless steel by utilizing an Eulerian finite volume (FV) method under the steady state condition, and the simulation result is compared directly with both the measured temperature histories during FSW and the microstructural changes after FSW. Based on a visco-plastic self-consistent (VPSC) approach for polycrystal, the texture development in the FSWed material is determined from the velocity gradients along the streamlines in the material flow field. The simulation results show that the heat is generated mainly near the interface between the tool and the workpiece, and that the viscosity changes drastically in the vicinity of the boundary between the SZ and the TMAZ. From the predicted streamlines, it can be indicated that the strong material flow mainly develops on the retreating side of the tool. Also, the simulation results show that the shear deformation texture is significantly developed in the FSWed region. The measured temperatures and microstructural characteristics agree fairly well with the predicted data. Thirdly, microstructural evolution during FSW of a high-strength linepipe steel with phase transformation is studied. The various grain structures developed through a complex process including the rearrangement of low-angle boundaries, continuous dynamic recrystallization and phase transformation. In most parts of the SZ, acicular-shaped bainitic ferrites were formed by the phase transformation during the FSW process. A fine-grained microstructure developed mainly in the TMAZ, where continuous dynamic recrystallization occurs. The shear texture in the SZ became considerably weak due to the phase transformation during the FSW process. The hardness of the SZ was significantly higher than that of the other FSWed regions due to the bainitic ferrites. Lastly, the developed thermo-mechanical model is coupled with the phase transformation model. Frictional heat and severe plastic deformation would affect phase transformation behavior of steel during FSW. Thus, the appropriate model is developed using the austenite grain size evolution model. The measured temperatures and phase fraction agree fairly well with the predicted data. From this study, thermo/mechanical behavior in FSW of steel, which has not been clear up to now, is described well. The developed model and suggested method lead to a clearer understanding about FSW of ferrous alloys. Furthermore, the rigorous numerical model coupled with experimental results demonstrates sufficient possibilities, which can predict directly the changes of microstructural characteristics during the FSW process.

Computational Study of Microstructure Evolution during Phase Transformations

Yu, Taiwu ProQuest Dissertations & Theses The Ohio State Uni 2021 해외박사(DDOD)

Phase transformation is always a critical topic in the study of materials science. Most people have been familiar with some transformations between solid and liquid, such as ice to water, or transformations between liquid and gas, such as water to vapor. Besides, the phase transformations in solids also occur everywhere. Some solid phase transformations occur due to temperature variations. Those transformations may also be affected by external stress or strain, as seen in shape memory alloys (SMAs). The solid-solid transformation is considered to be the one of the most effective ways to tailor the microstructure and properties of the alloys, moreover, it sometimes strengthens the structural materials. There are some types of solid-state phase transformations that are hard to characterize in the traditional experiments. The difficulty mainly comes from two aspects. Firstly, some of the phase transformations happen too fast, such as martensitic transformation. The speed of the martensitic transformation is close to the speed of sound traveling in solids (~1000m/s), which makes it difficult to know how it starts and evolves. Secondly, some of the phase transformation processes are too slow, such as oxidation. It could take years to form a continuous layer of oxides in microns.With the fast development of high-performance computing, the study of phase transformations through computational tools attracts more and more attention. The objective of this thesis is to apply computational tools to study the two types of phase transformations and their corresponding mechanical properties: precipitation and martensitic transformation.As one of the most important structural phase transformations discovered in metallurgy and materials science, martensitic transformation (MT) has been attracting continued attention since its discovery in the late nineteenth century till today because it relates closely to the functional properties of NiTi-based alloy such as the superelasticity and shape memory effect. Most importantly, MT can be tailored through nano-scale defects in materials. Firstly, nano-scale defects in the B2 parent phase are known to have profound impacts on the properties of NiTi-based shape memory alloys. We employed the phase field models (PFM) to study the effects of two typical nano-scale defects, nano-scale precipitates and voids, on MT. The simulation of precipitation unveiled the mechanical and chemical effects on the behavior of MT in NiTi-Hf alloys. Moreover, the simulation of MT with the coexistence of precipitates explained the mechanism of two typical patterns of martensite. The results indicates that the stress-strain response has great dependence on the concentration heterogeneity in the matrix as well as precipitate microstructures. Through the simulation we proved the feasibility to achieve linear or quasi-linear superelasticity with high recoverable strain (up to 4%) in NiTi-Hf alloys after the precipitation. In the simulation of MT under the effects of nano voids in NiTi, we observed that martensite could be confined in the interspacing area between voids. Besides, MT could be triggered at lower critical stress with larger volume fraction of voids. This simulation may shed lights on the design of the porous NiTi alloys for the biomedical application.In superalloys, the microstructure of precipitates can be altered by the formation of an oxide layer on the surface. It is observed that the γ' precipitates dissolve at the near-surface region with the formation of the oxide layer in the alloy. We employed DICTRA module in Thermo-calc Software to solve the multicomponent diffusion equations in alloy H282 with an outward flux of chromium or aluminum due to oxidation and applied PFM to simulate the dissolution of precipitates. The local variation of precipitates? volume fraction as a function of oxidation time has been quantitatively determined. The calculation of precipitates depletion depth shows good agreement with the experiments. The highly heterogeneous structure of γ' precipitates is expected to have a significant effect on the creep behavior of the alloy.

Investigation of phase transformation and mechanical properties of refractory high entropy superalloy

김상준 서울대학교 대학원 2022 국내박사

Ni계 초내열 합금을 대체하기 위한 고온 소재 연구는 수십 년간 강도 높게 연구되고 있으며, 대표적인 예로 Nb계 합금, Mo계 합금 그리고 세라믹 복합재가 있다. 최근 고엔트로피 초내열 합금이라 불리는 소재가 고온에서 우수한 물성으로 Ni계 초합금을 대체할 차세대 고온 소재로써 주목 받고 있다. 고엔트로피 초내열 합금은 Ni계 초합금이 가지고 있는 고유한 미세구조를 BCC 결정구조를 가지는 초내열 금속에 적용한다는 점에서 향후 고온 소재로써 가능성이 매우 높으나, 현재까지 보고된 연구의 수가 매우 적고 고엔트로피 합금이 가지는 다성분계 특성으로 인해 상태도가 존재하지 않아 합금 개발에 있어 어려움이 있다. 이에 있어서 가장 시급한 문제는 기존의 개발된 합금들과 같이 체계적으로 미세구조를 제어할 수 있는 방법이 요원하다는 것이다. 상태도는 합금의 열역학적 거동을 예측함에 있어 가장 유용한 정보라고 할 수 있으나 고엔트로피 초내열 합금이 유래된 고엔트로피와 마찬가지로 그 상태도를 예측하기가 매우 어렵다. 또다른 도전 과제는 기계적 물성 평가에 대한 부재인데, 현재의 고엔트로피 초내열 합금의 기계적 물성은 상온과 고온에서 모두 압축 실험을 통해 평가된 것으로 보고되어 있다. 고온 소재로써 성능이 평가되는 항목인 크리프와 피로 특성은 모두 인장 변형 모드에서 이루어지기 때문에, 이에 대한 평가가 필요한 상황이다. 본 연구에서는 현재의 고엔트로피 초내열 합금이 가지는 문제점을 극복하고 새로운 고엔트로피 초내열 합금을 개발하는데 필요한 가이드라인을 만들기 위하여, 고엔트로피 초내열 합금의 상변태와 기계적 특성에 관한 체계적인 실험을 진행하였다. 우선, 원소가 가지는 역할을 고려하여 Ti-Zr-Nb-Ta-Al 5원계 합금계를 설계하였고, 상태도를 작도하기 위해 이를 (TiZr)-(Nb,Ta)-Al 3원계로 변환하였다. 조합 실험법을 바탕으로 다양한 온도 및 조성에 대한 미세구조 정보를 취합하여, 합금계 내 BCC 용해도간극을 준정량적으로 작도하였다. 작도한 준정량적 BCC 용해도간극 (miscibility gap)을 바탕으로 고엔트로피 초내열 합금의 상변태 거동을 2가지 측면에서 고찰하였다. 우선, (TiZr)90-x(NbTa)xAl10 (at. %) 합금계 내에서 (Ti, Zr)과 (Nb, Ta) 간의 비율이 상변태 메커니즘에 비치는 영향을 분석하였다. 열유량 분석과 짧은 시효 처리 이후 미세구조를 관찰하는 두가지 방법을 통해 취합된 정보를 바탕으로, 준정량적 BCC 용해도간극을 (TiZr)90-x(NbTa)xAl10 (at. %) 합금계 내에 작도하였다. (Ti, Zr)과 (Nb, Ta) 간의 비율은 부정합 핵 생성 및 성장 혹은 정합 스피노달 분해 (spinodal decomposition) 메커니즘 결정하였고, 기지와 석출물 간의 관계에도 영향을 미쳤다. (TiZr)40(NbTa)60-xAlx (at.%) 합금계 내에서는 시효처리에서 미세구조의 형성을 고찰하였으며, 800도에서 Al을 포함하지 않는 합금은 부정합 핵 생성 및 성장 상변태 메커니즘을 보였으며, Al을 포함하는 합금은 정합 스피노달 분해 메커니즘을 보였다. 정합 스피노달 분해 메커니즘의 초기 단계에는 바구니 무늬 형태 (basket-weave structure)의 미세구조가 발현 되었으나, 이러한 구조는 B2 상분율과 큰 격자 상수 차이로 인하여 열역학적으로 불안정한 거동을 보이고, 둥근 석출물 형태(round-shape precipitates structure)의 미세구조로 위상 상 역전(topological phase inversion)을 보였다. 이러한 안정상과 상변태 정보들을 모두 취합하여, Al을 x축으로 하는 (TiZr)40(NbTa)60-xAlx (x = 0 ~ 25 at. %) 합금계의 상 평형도 을 작도하였으며, 이러한 준 정량적 도식을 바탕으로 고엔트로피 초내열 합금의 상 형성을 설명할 수 있었다. 고엔트로피 초내열 합금의 기계적 물성을 평가하기 위하여, (TiZr)40(NbTa)40Al10 (at. %) 조성을 선택하여 바구니 무늬 미세구조와 둥근 석출물 형태의 미세구조의 두가지 시편을 준비하여, 동일한 조성에서 미세구조에 따른 기계적 물성 차이를 고찰하였다. 상온에서 바구니 무늬 구조의 합금은 높은 강도와 취성을 나타내었고, 둥근 석출물 구조의 합금은 낮은 강도와 높은 연신율을 보였다. 고온 압축 실험에서 두 합금은 동일한 기계적 거동을 보였는데, 이것은 하중 조건에서 바구니 무늬 구조에서 둥근 석출물 구조로의 미세구조 변환이 가속화되었기 때문이다. A2와 B2 상이 안정하게 공존할 수 있는 온도에서는 결정이 균일하게 변태하며 래프팅(rafting) 현상이 관찰되었다. 인장 실험과 파괴 인성 실험에서는 낮은 연성과 낮은 파괴 인성 값을 보였는데, 이것은 결정립계 파괴 메커니즘 때문이다. 결정립계에서 보인 석출물이 결여된 A2 상이 인장 변형에서 약점으로 작용하였다. 본 연구에서는 넓은 조성 및 온도 범위에서의 고엔트로피 초내열 합금의 상 형성을 체계적으로 이해하였으며, 고엔트로피 초내열 합금의 기계적 물성을 평가하여 최초로 보고하였다. 본 연구 결과가 초내열 합금은 상 형성 거동과 다양한 하중 환경에서의 기계적 거동에 대한 체계적은 분석을 제공함으로써, 향후 우수한 물성의 고엔트로피 합금을 개발하는데 지침서가 될 수 있을 것으로 사료된다. For a few decades, there have been extensive investments to develop alternative materials of Ni-based superalloy such as Nb-based alloys, Mo-based alloys or ceramic composites for high temperature application. Recently, a new class of alloy called refractory high entropy superalloy (RHSA) has attracted significant attention due to its superior strength at high temperatures. RHSAs were based on the intriguing idea that structure of nano-precipitate of Ni-based superalloy can be introduced to the refractory alloys with BCC crystal structure. However, very little reported work and the absence of phase diagrams due to the multi-principal characteristics of high entropy alloy makes it challenging to develop new RHSA with the properties refused for high temperature materials. The most critical challenge for RHSAs is to establish systematic methods for controlling the microstructure like other conventional alloys. Phase diagram is the most helpful tool for describing and predicting the thermodynamic behavior of alloy, but it is not well available for RHSA. In addition, the phase diagram of RHSA is complicated to establish because of the combination of alloying elements with multi-principal characteristics. Another challenge is the evaluation of mechanical behavior for RHSA. All of the currently reported mechanical properties of RHSA so far were performed through compression tests at both room temperature and high temperature. However, the mechanical properties for evaluating high-temperature materials are creep, fracture toughness and fatigue, all of which are conducted in tensile modes. To overcome the problems of the current RHSA research and provide a valuable guideline for designing novel RHSAs within the Ti-Zr-Nb-Ta-Al alloys, systematic research on phase transformation and mechanical properties was conducted in this study. At first, the Ti-Zr-Nb-Ta-Al high entropy alloy system was designed with the consideration of the thermodynamic behavior of each alloying elements. The designed Ti-Zr-Nb-Ta-Al quinary system was simplified as (Ti50Zr50)-(Nb50Ta50)-Al ternary system to obtain systematic information about phase equilibrium. A combinatorial approach using available literature data was applied to investigate the BCC miscibility gap in a wide range of compositions. And finally, the semi-quantitative miscibility gap contour was predicted. Based on the predicted semi-quantitative miscibility gap information obtained, phase transformation in RHSA was experimentally investigated in two directions. The effects of TiZr/NbTa ratio on the phase transformation mechanism were investigated in the (TiZr)90-x(NbTa)xAl10 (at. %). From the present experimental study, the semi-quantitative BCC miscibility gap was obtained in the (TiZr)90-x(NbTa)xAl10 (at. %) system. It was revealed that the ratio between A2 former transition metals (Nb and Ta) and B2 former transition metals (Ti and Zr) can affect the temperature ranges where the incoherent nucleation and growth mechanism and coherent spinodal decomposition of BCC phases (A2 and B2) appear. The effects of Al on the microstructural evolution were investigated in the (TiZr)40(NbTa)60-xAlx (at. %) system. Incoherent nucleation and growth of A2 phase were observed at the alloy without Al. At the initial stage of coherent spinodal decomposition in the alloy with Al, the basket-weave microstructure was formed. The microstructure from the coherent spinodal decomposition was thermally unstable showing topological phase inversion due to a small volume fraction of B2 and large lattice misfit between A2 and B2 phase. By collecting all the microstructure and phase transformation results, a microstructural evolution diagram in (TiZr)40(NbTa)60-xAlx (x = 0 ~ 25 at. %) isopleth was depicted. These phase diagram can be applied to control the microstructure of the RHSA by providing useful insight on the phase transformation mechanism in the RHSA system. To evaluate the mechanical properties of RHSA, the alloy (TiZr)40(NbTa)40Al10 (at. %) was selected. In particular, two separate samples with either basket-weave microstructure or a round-shape B2 precipitates microstructure were prepared to investigate the effects of microstructure on mechanical properties. The alloy with a basket-weave structure showed high compressive strength with brittle characteristics, and the alloy with round-shape precipitates structure low strength and higher elongation at room temperature. In the high temperature compression tests, both RHSAs showed similar mechanical behavior, which is because microstructure change from a basket-weave structure to a round-shape precipitates structure was accelerated under the load condition. The temperature where A2+B2 structure is stable, grains were deformed homogeneously and rafting phenomena were observed. In tensile and fracture toughness tests, the RHSA showed little ductility and fracture toughness values due to the prevalent inter-granular fracture mechanism. In the grain boundaries, there was A2 denuded zone which was a weak spot at the deformation. In this study, a phase transformation in wide ranges of composition and temperature of RHSA was investigated and microstructural evolution was systematically understood for the first time. Mechanical evaluations of RHSA were conducted at high temperatures using tensile and fracture toughness tests. This study shall be a valuable guideline for developing a novel RHSA.

Phase Transformation and Low Temperature Mechanical Properties of the Weld Joint of 9% Ni Steel Welded using Newly Developed Fe-based Filler materials : 신규 개발된 Fe계 용가재를 적용해 용접된 9% Ni강 용접 조인트의 저온 기계적 특성과 상변태

최광수 경북대학교 대학원 2025 국내박사

국제 해사 기구의 해양 유해 가스 배출 규정이 강화되면서 친환경 에너지원 인 액화 천연가스에 대한 요구가 증대되고 있다. 현재 액화 천연 가스 저장 탱크의 내벽 재료로 사용되는 9% Ni강은 저온에서 안정적인 인성을 가질 수 있는 FCC 결정 구조를 기지로 하는 Ni계 초합금 용가재를 적용하여 용접 된다. 용가재로 사용되는 Ni계 초합금은 Ni, Mo, Cr, W, Nb, Ti등의 고가의 원소로 구성되어 있어, 피용접재 인 9% Ni강에 비해 가격 경쟁력이 크게 낮다. 액화천연가스를 이용하는 친환경 산업 의 지속적인 성장이 예상됨에 따라, 산업계에서는 9% Ni강 용접의 경제성을 개선하 기 위해 용접 공정 설계를 최적화하는 노력이 이어지고 있다. 그러나, 이러한 노력 과 더불어 원천적으로 가격 경쟁력이 있는 신규 용가재 합금의 설계가 필수적으로 요구된다. 본 논문은 9% Ni강의 경제적인 가스 텅스텐 아크 용접을 위해 새로운 Fe계 용가재 합금 설계와 이를 9% Ni강 용접 조인트에 적용하기 위한 연구를 수행하였다. Fe계 용가재 합금 조성은 9% Ni강에 첨가하여 오스테나이트 안정화 원소 Ni, Mn과 마르텐사이트 형성 원소인 Co,C를 첨가하였으며, CCT 곡선상에서 마르텐사이트 형성 147 온도가 Ms ≥대기 온도 (25 ℃)≥Mf으로 제어되어 용접과 같은 급속 응고 환경에서 오스테나이트와-마르텐사이트 2상 구조를 안정적으로 형성할 수 있도록 설계 되었 다. 맞대기 용접된 9% Ni강 조인트의 WM에서 오스테나이트가 둘러진 패킷 래스 마르 텐사이트 미세조직을 형성하였고, 이러한 WM의 미세조직은 급속 응고 환경에서 오스 테나의트-마르텐사이트 2상 구조를 형성하기 위해 마르텐사이트 형성 변형을 최소화 시킨 미세조직 형성 거동에 따른 것으로 고찰했다. HAZ는 용접 시 가해지는 최대 온 +도를 유한요소해석을 통해 예측하고, 상변태 거동을 예측하고 미세조직을 분석하였 다. 추가적으로, WM과 BM의 상평형도를 고찰하여 BM에는 상변태 효과 없으면서, WM 에는 템퍼링 상변태 효과를 가 할 수 있는 용접 후 열처리를 수행하였다. 열처리 효 과로 WM에 래스 형상의 시멘타이트 상을 패킷 래스 마르텐사이트 내부에 형성하였 다. 이러한 용접 후 열처리는 WM의 상변태를 통한 경도 감소와 HAZ 영역의 압축 잔 류 응력 감소 효과를 나타내고 이는 용접 조인트의 인성 개선 효과를 나타냈다. 본 연구는 9% Ni강을 용접하기 위한 새로 개발된 Fe계 용가재의 합금 설계 단계, 용접 조인트 제작, 용접 후 열처리의 효과까지 신소재 용가재 개발에 대한 전 체적인 연구 단계를 수행했다. 이러한 연구 결과들의 고찰들을 응용함으로써 경제적 인 액화 천연 가스 저장 탱크 제작 및 설계에 방향을 제시할 수 있을 것으로 사료된 다.

Development of creep-resistant Mg–Sn-based alloys utilizing rapid phase transformation during deformation

권재호 울산과학기술원 Ulsan National Institute of Science and Te 2026 국내박사

고탄소강의 열간압연 공정의 냉각대에서의 온도 예측모델 정도 향상

나태호 포항공과대학교 철강대학원 2016 국내석사

To improve the accuracy of temperature prediction of high carbon steel in ROT of hot-rolling mill, the influencing factors in temperature calculation such as transformation kinetics and enthalpy change were investigated using heat equation. The transformation kinetics was measured with dilatation curve and compared to the calculated one by current model. Enthalpy change was attempted to measure. It was successful at a specific temperature, but not measurable at the lower temperature due to an experimental limitation. Enthalpy change data from the model was compared to the one from the recently released thermodynamic database. It was proved that inaccurate evaluation of transformation kinetics is mainly responsible for incorrect temperature prediction, besides, reliability on thermodynamic quantities such as enthalpy change associated with the phase transformation needs to be improved for better temperature prediction. This study reveals that vigorous consideration of transformation kinetics and thermodynamic quantity will provide better accuracy in temperature prediction in particular high carbon steel.

Grain Boundary Nucleation of Displacive Phase Transformations in Steels

송태진 포항공과대학교 철강대학원 2012 국내박사

Austenite Grain Boundaries (AGBs) are known to be the most preferable nucleation site for reconstructive ferrite transformation due to the presence of excess free-volume which provides a higher probability for satisfying the local atomic configuration appropriate for the nucleation by random phase fluctuations. On the other hand, it has also been reported that nucleation of displacive phase transformations, i.e. both the isothermal bainitic transformation and the athermal martensitic transformation, preferentially occurred at prior AGBs. However, no clear mechanism for the preferred nucleation of displacive phase transformations at AGBs has been successfully postulated. This is primarily due to the difficulties in experimental demonstration. For example, crystallographic features of a nucleation of bainitic ferrite at AGBs cannot be directly investigated in low-carbon, lean-alloyed steels due to the presence of martensitic transformation products which hinder the precise determination of the crystallographic orientation of the prior austenite. On the other hand, it is well known that both the bainite and martensite hold specific orientation relationships (OR) with respect to austenite, and that the transformations are confined to occur within a single austenite grain. These characteristic features of displacive transformation make it possible to determine the orientation of prior austenite grain and the structure of prior AGBs from the orientation distribution of transformed products measured by the Electron Back Scattered Diffraction (EBSD) technique. In the present study, the nucleation behavior of upper bainite in low-carbon, lean-alloyed steel was investigated by Scanning Electron Microscopy (SEM)-EBSD. It is shown that the bainitic ferrite nucleated at AGBs having the crystallographic orientation of a variant with a Kurdjumov-Sachs (K-S) OR with respect to one of the neighboring austenite grain. This variant selection rule indicates that the bainitic ferrite nucleation is enhanced by interfacial energy reduction as the initial AGBs are converted to low energy inter-phase boundaries (IPBs) in the process. As the nucleation of upper bainite is controlled by an interfacial energy reduction mechanism, the transformation behavior of upper bainite is strongly affected by B addition. It was observed that the addition of B clearly affected both the transformation kinetics of isothermal bainitic transformation and the morphology of the bainite. The addition of B increased the incubation time for the formation of a detectable amount of transformation product, and reduced the overall transformation kinetics. The bainite microstructure consisted of a bainitic ferrite matrix and the Martensite/Austenite (M/A) constituent. The M/A constituent had an elongated morphology in B-free steel, whereas the M/A constituent in B-added steel had a granular structure. In B-free steel, bainitic ferrite nucleated homogeneously within the austenite grains, and the overall transformation was controlled by the increase of transformed area within the initial austenite grain. In contrast, in B-added steel, the transformation was confined to the interior of the austenite grains, and the overall transformation kinetics was controlled by an increase in the number of the locally transformed grains. The variant of bainitic ferrite in B-added steel was selected within a group of K-S variants related to the same Bain variant. The characteristic bainite microstructure in B-added steel is therefore due to the inhibition of the bainitic ferrite nucleation at ABGs. The microstructure and crystallography of BCC α’-martensite formed in a sensitized AISI 304 stainless steel was also studied in detail by means of Transmission Electron Microscopy convergent beam Kikuchi line pattern analysis. It was suggested that GBs containing Intrinsic Grain Boundary Dislocation (IGBD) might act as preferential nucleation site according to a faulting mechanism of martensitic nucleation proposed by Olson and Cohen. Different from the nucleation behavior of an upper bainite in low carbon, lean-alloyed steels, the martensite variants do not appear to be selected to achieve interfacial energy reduction, or the easy accommodation of transformation strain by slip in the neighboring austenite grain. This implies that both the martensite and the upper bainite preferentially nucleate at prior AGBs, but the operating nucleation mechanisms are totally different. From the observations that interfacial energy reduction mechanism plays a pivotal role in the nucleation of upper bainite, specific alloy designs were made to produce ultra-high strength (UHS) martensitic grades in conventional continuous galvanizing lines. The thermal treatments related to the hot dip galvanizing and galvannealing processes needed to obtain a Zn or Zn-alloy coated martensitic steel did not degrade the mechanical properties significantly when the coating process was applied to the un-transformed austenite phase. Therefore, it is shown that martensitic steel designed according to the critical physical metallurgy principles can be successfully processed in galvanizing lines not originally intended to produce UHS grades.