고강도 냉간압조용 중탄소 Cr-Mo 합금강의 고온변형특성 및 동적재결정 거동 평가

조용덕,이희주,남성실,강현우,장병록 대한금속·재료학회 2022 대한금속·재료학회지 Vol.60 No.6

The hot deformation behavior of medium-carbon Cr-Mo alloy, which has been developed for high strength cold-heading quality wire rod, was investigated to evaluate its hot workability. A flow curve was derived using the hot torsion test, under conditions with temperatures of 1173-1273 K and strain rates of 0.1- 1.0 s-1. At lower deformation temperature and higher strain rate, the overall stress of the flow curve increased, and the flow curve showed a three-stage variation related to the offset of dynamic recrystallization and dynamic recovery. First, as the strain increased, the stress also increased due to work hardening, and reached peak stress. After that, the stress decreased due to softening of the dynamic recrystallization. And when the effect of the dynamic recrystallization and the dynamic recovery reached equilibrium, the stress became steady state. In this paper, the constitutive equation of the peak stress was established using a form of a hyperbolic sine function, and here the thermal activation energy for deformation of the specimen was 244.90 kJ/mol. The peak stresses calculated from the constitutive equation were in good agreement with the experimental results. The dynamic-recrystallized grains were observed using electron backscatter diffraction (EBSD). It showed that the volume fraction of dynamic recrystallization increased as the strain increased under hot deformation. Based on the Avrami kinetic equation, a dynamic recrystallization kinetic model was established. The volume fraction of dynamic recrystallization was predicted from the kinetic model, and can be applied at arbitrary deformation temperatures and strain rates.

Dynamic Recrystallization Mechanism and Precipitation Behavior of Mg-6Gd-3Y-3Sm-0.5Zr Alloy During Hot Compression

Hao Pang,Quanan Li,Xiaoya Chen,Peijun Chen,Xiangyu Li,Jinfeng Tan 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.2

The effects of temperature and strain on dynamic recrystallization (DRX) and dynamic precipitation of Mg-6Gd-3Y-3Sm-0.5Zr alloy were studied through uniaxial compression. The main conclusions were as follows: the increase of strain ortemperature can promote DRX. The texture strength decreases with the increase of DRX fraction. The pyramidal <a> slipand pyramidal <c + a> slip were activated to coordinate strain when deformed at high-temperature. Under the deformationcondition of T = 400 °C, ̇ = 0.002 s−1 and ε = 0.35, the mechanism of DRX was mainly discontinuous dynamic recrystallization. The rotation continuous dynamic recrystallization gradually becomes the main mechanism with the increase ofstrain and temperature. At the same time, the particle stimulated nucleation (PSN) caused by a single large-sized Mg5(Gd,Y, Sm) or the aggregation of multiple Mg5(Gd, Y, Sm) phases was also found. The dynamic precipitation can stimulate thenucleation of DRX, and the boundary of DRXed grains can become an effective nucleation site for dynamic precipitation. DRX and dynamic precipitation can promote nucleation mutually.

Bimodal Phenomenon of the Stress–Strain Curve During Hot Compression of LA43M Mg–Li Alloy

Yi Li,Yanjin Guan,Hu Chen,Jiqiang Zhai,Jun Lin,Liang Chen 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.10

In this study, the samples of LA43M Mg–Li alloy were compressed to the true strains of 0.12, 0.16, 0.36 and 0.60 under300 °C and the strain rate of 0.1 s−1. Under this condition, the stress–strain curves present a special bimodal phenomenon atthe early deformation stage, which is caused by twinning and dynamic recrystallization. In the process of hot compression,extension twins generated first. Twinning accommodated strain and the glide of twinning dislocations dissipated local strainenergy, resulting in the first local stress drop. Then extension twins coarsened and devoured all the matrix, leading to thechange of grain orientation. Subsequently, dislocations accumulated at the grain boundaries and resulted in the increase instress. With further strain, dynamic recrystallization occurred, discontinuous dynamic recrystallization is the main dynamicrecrystallization mechanism. The generation of dynamic recrystallization resulted in the strain softening and leaded tothe second local stress drop. Twinning retards the occurrence dynamic recrystallization and has a crucial influence on themicrostructure development during the hot compression of LA43M.

고온 압축변형된 니켈기 초내열 718 합금 결정립 조직의 EBSD 분석

나영상,이만형,염종택,박노광 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.3

In order to characterize the dynamic change of microstructures in commercially-available superalloy 718, a series of compression tests were conducted by varying test temperature and strain rate. Orientation relationship and the misorientations between recrystallized grains as well as unrecrystallized grains were thoroughly investigated with EBSD technique. The dynamically-recrystallized grain size was similar regardless of the compression test condition except for the sample tested at 1066℃-0.005/sec condition, where the dynamically-recrystallized grain size was as large as 20 μm in average diameter. In case of the sample tested at 1066℃, flow stress drops caused by the strain softening were almost same at the strain rates of 0.5/sec and 0.005/sec in spite of the different fraction of dynamic recrystallization. It was concluded from the observation of the misorientation angle distribution that it was due to the active dynamic recovery in the unrecrystallized large grains at high temperature. It was also suggested from the variations of CSL boundary distribution that the characteristic redistribution of the dynamically-recrystallized grain boundary might be taken place during the compression at high temperature and low strain rate.

마그네슘 합금의 온간 동적재결정 구성방정식 최적화

조윤희(Yooney Cho),윤종헌(Jonghun Yoon) 한국산학기술학회 2017 한국산학기술학회논문지 Vol.18 No.6

상용 마그네슘 합금의 경우, 상온에서 낮은 성형성을 갖기 때문에, 온간 성형 조건 하에서 성형 공정을 수행하는 것이 일반적이다. 마그네슘 합금은 온간 성형 과정 중에 동적 재결정(dynamic recrystallization, DRX)이 발생하여, 초기 결정립 사이즈가 급격하게 작아지며, 내부 전위 밀도가 낮아지게 된다. 이에 따라, 유동 응력 곡선은 세 단계의 복잡한 변형 경화 및 연화 현상을 보이게 된다. 첫 번째 구간에서는 변형률이 증가함에 따라, 가공 경화에 의해 응력이 증가하는 경향을 보이며, 두 번째 구간에서는 동적 재결정 현상에 의한 가공 연화로 응력이 갑작스럽게 감소한다. 세 번째 구간에서는 가공 경화와 가공 연화 사이의 평형에 의해, 응력이 일정하게 나타난다. 본 연구에서는, 성형 온도 300℃, 변형률 속도는 0.001, 0.1, 1, 10/sec에서 AZ80 합금의 구성 방정식의 18개 변수들을 체계적으로 최적화하며, 유동 곡선의 정확도를 높일 수 있는 방식에 대해 제안하려고 한다. 또한 AZ80외에 AZ61도 추가적으로 최적화여 본 논문에서 제안한 최적화 방식의 성능을 증명하였다. A hot forming process is required for Mg alloys to enhance the formability and plastic workabilitydue tothe insufficient formability at room temperature. Mg alloy undergoes dynamic recrystallization (DRX) during the hotworking process, which is a restoration or softening mechanism thatreduces the dislocation density and releases theaccumulated energy to facilitate plastic deformation. The flow stress curve shows three stages of complicated strainhardening and softening phenomena. As the strain increases, the stress also increases due to work hardening, and itabruptly decreases work softening by dynamic recrystallization. It then maintains a steady-state region due to theequilibrium between the work hardening and softening. In this paper, an efficient optimization process is proposedfor the material model of the dynamic recrystallization to improve the accuracy of the flow curve. A total of 18variables of the constitutive equation of AZ80 alloy were systematically optimized at an elevated formingtemperature(300℃) with various strain rates(0.001, 0.1, 1, 10/sec). The proposed method was validated by applying it to the constitutive equation of AZ61 alloy.

Constitutive Analysis and Processing Map for Hot Working of a Ni-Cu Alloy

Gholam Reza Ebrahimi,Amir Momeni,Seyed Mehdi Abbasi,Hossein Monajatizadeh 대한금속·재료학회 2013 METALS AND MATERIALS International Vol.19 No.1

The hot deformation behavior of a Ni-Cu alloy was studied using hot compression testing in the temperature range of 950 °C-1150 °C and at strain rates of 0.001 s.1-1 s.1. Flow curves at low strain rates, up to 0.01 s.1, were typical of DRX characterized by a single peak, while at higher strain rates, the typical form of a DRX flow curve was not observed. The power-law constitutive equation was used to correlate flow stress to strain rate and temperature, and the apparent activation energy of hot deformation was determined to be about 462.4 kJ/mol. The peak strain and stress were related to the Zener-Hollomon parameter and the modeling formula was proposed. The dependence of flow stress to the Z changed at ln Z=38.5,which was considered to be a critical condition for the change in the mechanism of dynamic recrystallization. The efficiency of power dissipation was determined to be between 10-35 percent at different deformation conditions. According to the dynamic material model, stable flow was predicted for the studied temperature and strain rate ranges. Highly serrated grain boundaries at low strain rates were considered to be a reason for the occurrence of continuous dynamic recrystallization. On the contrary, at high strain rates, equiaxed grain structure was attributed to the typical discontinuous dynamic recrystallization.

Prediction of Microstructure Evolution during Hot Forging using Grain Aggregate Model for Dynamic Recrystallization

Lee, Ho Won,Kang, Seong-Hoon,Lee, Youngseon 한국정밀공학회 2014 International Journal of Precision Engineering and Vol.15 No.6

In this study, dynamic recrystallization during nonisothermal hot compression test was numerically simulated by finite element analysis using new grain aggregate model for dynamic recrystallization. This model was developed based on mean field approach by assuming grain aggregate as representative element. For each grain aggregate, changes of state variables were calculated using three sub-models for work hardening, nucleation, and nucleus growth. A conventional single parameter dislocation density model was used to calculate change of dislocation density in grains. For modeling nucleation, constant nucleation rate and nucleation criterion developed by Roberts and Ahlblom were used. It was assumed that the nucleation occurs when the dislocation density of certain grain reaches a critical nucleation criterion. Conventional rate theory was used to model nucleus growth. The developed dynamic recrystallization model was validated by comparing with isothermal hot compression of pure copper. Then, the finite element analysis was conducted to predict the local changes of microstructure and average grain size by using the grain aggregate model. The predicted results were compared with nonisothermal hot compression results. The simulation results were in reasonably good agreement with experimentally obtained microstructures and the calculation time was much shorter than cellular automata-finite element method.

Prediction of Microstructure Evolution during Hot Forging using Grain Aggregate Model for Dynamic Recrystallization

이호원,강승훈,이영선 한국정밀공학회 2014 International Journal of Precision Engineering and Vol. No.

In this study, dynamic recrystallization during nonisothermal hot compression test was numerically simulated by finite elementanalysis using new grain aggregate model for dynamic recrystallization. This model was developed based on mean field approachby assuming grain aggregate as representative element. For each grain aggregate, changes of state variables were calculated usingthree sub-models for work hardening, nucleation, and nucleus growth. A conventional single parameter dislocation density model wasused to calculate change of dislocation density in grains. For modeling nucleation, constant nucleation rate and nucleation criteriondeveloped by Roberts and Ahlblom were used. It was assumed that the nucleation occurs when the dislocation density of certain grainreaches a critical nucleation criterion. Conventional rate theory was used to model nucleus growth. The developed dynamicrecrystallization model was validated by comparing with isothermal hot compression of pure copper. Then, the finite element analysiswas conducted to predict the local changes of microstructure and average grain size by using the grain aggregate model. Thepredicted results were compared with nonisothermal hot compression results. The simulation results were in reasonably goodagreement with experimentally obtained microstructures and the calculation time was much shorter than cellular automata-finiteelement method.

Underlying mechanisms of drastic reduction in yield asymmetry of extruded Mg-Sn-Zn alloy by Al addition

Kim, Sang-Hoon,Park, Sung Hyuk Elsevier 2018 Materials science & engineering. properties, micro Vol.733 No.-

<P><B>Abstract</B></P> <P>This study demonstrates that the addition of Al to the Mg-7Sn-1Zn alloy significantly reduces the tension–compression yield asymmetry of the extruded alloy and that this reduction is strongly related to the variation in the area fraction of deformed grains that do not undergo dynamic recrystallization. The decrease in the amount of these grains by Al addition has adverse effects on the improvement in the tensile yield strength but beneficial effects on that in the compressive yield strength. The relationship between the microstructural characteristics of deformed grains and the yield strengths of the extruded alloy is discussed herein.</P>

Annealing effect on microstructure, hardness and creep properties of the foils of alloy 617

Sanjeev Kumar Sharma,Feng Xuin Li,Ki-Ju Kang 대한기계학회 2008 대한기계학회 춘추학술대회 Vol.2008 No.5

The rolling and annealing effect on the microstructure and high temperature creep properties of Ni-base alloy 617 foils were investigated. Two types of foil specimens with different thickness reduction were prepared by thermo-mechanical processing, which is a combination of cold rolling and static recrystallization. The thermo mechanical processing (TMP) is a way to considerably influence the grain size. The recrystallization and grain growth were observed readily at both lower and higher annealing temperatures. The grain sizes were strongly dependent on the thickness reduction and the annealing temperature. The grain sizes were observed bigger at higher annealing temperature than lower annealing temperature. Due to the microstructure analysis, the carburization was observed at lower annealing temperature and lower thickness reduction. The uniform coarse grain sizes increase the resistance for creep deformation and crack growth owing to the precipitation hardening. The lower thickness reduction foils were found better creep properties.