Effect of electric current on recrystallization kinetics in interstitial free steel and AZ31 magnesium alloy

Park, Ju-Won,Jeong, Hye-Jin,Jin, Sung-Woo,Kim, Moon-Jo,Lee, Kyooyoung,Kim, Jae Joong,Hong, Sung-Tae,Han, Heung Nam Elsevier 2017 Materials characterization Vol.133 No.-

<P><B>Abstract</B></P> <P>The effect of electric current on recrystallization kinetics in interstitial free (IF) steel and AZ31 magnesium alloy was investigated based on Vickers hardness measurements and microstructural observation. Electropulsing treatment (EPT) and furnace heat treatment (HT) for these work-hardened metals were carried out under various temperature and time conditions. The Vickers hardness value after EPT was clearly lower than that after HT at the same annealing condition in both IF steel and AZ31 alloy. The microstructural observation confirms that the reduced hardness value was caused by recrystallization. This implies that the recrystallization kinetics was accelerated by the athermal effect distinct from Joule heating. To identify the athermal effect of the electric current on the recrystallization kinetics, the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation was adopted. In both IF steel and AZ31 alloy, the value of the Avrami exponent, which depends on the nucleation rate in recrystallization, was higher in EPT than in HT. In addition, it was observed that the activation energy for recrystallization was reduced in EPT.</P> <P><B>Highlights</B></P> <P> <UL> <LI> For IF steel and AZ31 magnesium alloy, electropulsing treatment (EPT) and heat treatment (HT) were carried out. </LI> <LI> The effect of EPT on recrystallization was investigated by Vickers hardness measurement and microstructural analysis. </LI> <LI> The recrystallization kinetics was accelerated by EPT with a distinct effect from Joule heating. </LI> <LI> The applying electric current could reduce the recrystallization activation energy and accelerate nucleation rate. </LI> </UL> </P>

Nb과 Sn 첨가에 따른 Zr 합금의 재결정 및 TEP 거동

정흥식,오영민,정용환,김선진,Jeong, Heung-Sik,O, Yeong-Min,Jeong, Yong-Hwan,Kim, Seon-Jin 한국재료학회 2001 한국재료학회지 Vol.11 No.2

Zr-Sn-Nb 합금의 재결정에 미치는 Nb과 Sn의 첨가영향을 연구하기 위해 냉간압연한 시편을 $300^{\circ}C$~75$0^{\circ}C$의 온도구간에서 열처리한 후에 미소경도와 TEP (Thermoelectric Power)를 측정하여 재결정 거동을 조사하였으며 광학현미경, 주사전자 현미경 (SEM), 투과전자현미경 (TEM)으로 미세조직을 관찰하였다 미소경도 및 미세조직의 분석 결과에 따르면, Nb과 Sn의 첨가에 의해 재결정 활성화 에너지가 증가하여 재결정이 지연되었으며, 재결정 완료 이후의 결정립 성장도 억제되었음을 관찰하였다. Zr내의 고용도가 매우 낮은 Nb의 첨가는 석출물을 쉽게 형성하는 반면에 고용도가 비교적 큰 Sn은 기지상 내에 대부분 고용되어 석출물의 양이 매우 작았으나, Sn 첨가에 의한 재결정의 지연 효과가 더욱 컸다. Nb보다 Sn의 첨가가 Zr 합금의 재결정 거동을 효과적으로 지연시킨 것은 고용도가 높은 SR에 의한 치환형 고용체 형성과정에서 발생된 응력장이 전위의 이동을 효과적으로 억제했기 때문으로 생각된다. 한편, 회복과 재결정이 진행됨에 따라 전자 산란인자의 감소로 TEP는 증가하였으며, 재결정이 완료되면 TEP의 포화가 발생하였다. 석출물의 형성은 석출물 주변의 용질농도 감소로 인한 전자 산란인자의 감소에 기인하여 TEP의 증가를 가져왔다 To investigate the effects of the addition of Nb and Sn on the recrystallization of Zr- Sn-Nb alloys, both Vickers micro-hardness test and TEP measurement were carried out on cold-worked specimens annealed at various temperatures from $300^{\circ}C$ to 75$0^{\circ}C$. The microstructures of heat treated specimens were analyzed by optical microscope, SEM, and TEM. The study of microhardness and microstructures showed that both recrystallization process and grain growth were retarded as the activation energy was increased by the addition of Nb and Sn. Especially, the addition of Sn was more effective on retarding recrystallization. Precipitates were formed more easily when Nb was added because the solubility of Nb into Zr is lower than that of Sn. However, the recrystallization process was affected more by Sn than Nb because the strain field formed by substitutional Sn repressed the dislocation movement. TEP was increased due to the decrease of electron scattering as recovery and recrystallization were proceeded and saturated when the recrystallization completed. However, when precipitates formed, TEP was increased because the decrease of solute concentration near the precipitates caused the decrease of electron scattering.

Research on sub-surface recrystallization of single crystal nickel-based superalloy in micro-milling

Qi Gao,Po Jin,Ming Cai 대한기계학회 2019 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.33 No.7

The micro-structure/parts of single crystal nickel-based superalloy materials are extensively used in high temperature environments. The recrystallization mechanism of the machined workpiece sub-surface is explained through energy storage model. A micro-milling experiment of single crystal nickel-based superalloy DD98 was performed by simulating the working environment at high temperature and setting different heat treatment temperature. By scanning electron microscopy (SEM), the influence of sub-surface structural elements on the properties of micro-milling workpiece was analyzed. Experimental results indicate that the recrystallization layer thickness of single crystal nickel-based superalloy DD98 increases with the increase of heat treatment temperature. When this temperature is between 950 °C and 1150 °C, the recrystallization thickness is 5-20 µm. The recrystallization of sub-surface layer takes the form of equiaxed recrystallization. Those provide basic data for restraining recrystallization of single crystal materials and have good scientific significance.

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.

Zr-2.5%Nb 압력관 재료에서 α-Zr 영역에서의 소둔에 의한 β-상 분해가 재결정에 미치는 영향

김성수,임상엽,이경근 대한금속·재료학회 2020 대한금속·재료학회지 Vol.58 No.10

The effects of β phase decomposition on recrystallization and texture variation in Zr-2.5% Nb alloy pressure tube material were investigated. Isothermal annealing was conducted at 350 to 550 oC for 240 hours, and isothermal annealing was performed at 500 oC for 240 to 3,000 hours. The recrystallization and texture variation were analyzed by inverse pole figure variation using the XRD and EBSD methods. Annealing in α-Zr region at below 610 oC induced recrystallization and texture variation in the α-Zr. These results differ from those from a previous annealing study of the α+β region at 750-830 oC. Annealing above 400 oC for 240 hours caused β-Zr decomposition into β-Nb. The decomposition of the β-phase by annealing above 475 oC caused a contraction of 7.5% in the d(110) spacing in the β-phase, and a reduction in the volume fraction of the β phase by about 80%. It seems that the stress internally formed by the lattice contraction of the β-phase provides the driving force for recrystallization. In addition, it suggests that the newly formed α-Zr produced by β phase decomposition provides new nucleation sites for recrystallization and causes texture variation in the α-Zr. The reason why the recrystallization and the texture variation occurs only in the α-Zr stable region at below 610 oC is discussed.

Dynamic Recrystallization Behavior and Constitutive Modeling of As-Cast 30Cr2Ni4MoV Steel Based on Flow Curves

Peng Zhou,Qingxian Ma 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.2

The compression deformation of 30Cr2Ni4MoV steel at different temperatures and strain rates is carried out onGleeble 1500 thermal mechanical simulation tester. Based on the experimental flow curves, the strainhardening rate curves (θ = dσ/dε versus σ) are derived, from which the characteristic stresses and strainsare identified. Meanwhile, the dependences of the characteristic stresses and strains on Zener-Hollomonparameter are determined and the results show that the value of the critical stress of dynamic recrystallization isclose to the value of the steady stress. With the aid of the experimental flow curves, the Avrami equation isemployed to describe the kinetics of dynamic recrystallization. The time exponent (n) is expressed as a powerlaw function of Zener-Hollomon parameter and the Avrami constant (k) is determined as a function of halfof the time for the complete dynamic recrystallization (t50). Furthermore, a constitutive model is presentedbased on the rule of mixtures when the dynamic recrystallization occurs. Validation of the constitutive model isimplemented and the simulated results agree well with the experimental results.

Concurrent Recrystallization and Precipitation for Combination of Superior Precipitation and Grain Boundary Hardening in Co37Cr20Ni37Ti3Al3 High-Entropy Alloy

Linfei Xia,Qingfeng Wu,Kexuan Zhou,Bin Han,Feng He,Zhijun Wang 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.12

According to the precipitation and recrystallization behavior, the precipitation and recrystallization can be concurrently tunedtoward the excellent mechanical property. In this study, a synergistically strengthened Co37Cr20Ni37Ti3Al3 high-entropyalloy was developed through concurrent recrystallization and precipitation (CRP) approach. After the treatment, the highdensityγ' precipitates were introduced in the matrix, while the fully-recrystallized ultrafine grains and annealing twins wereobtained from the recrystallization. Compared with conventional treatment, the CRP-HEA exhibits a superior combinationof ~ 1.62GPa tensile strength and ~ 28% elongation, which is attributed to the synergistic strengthening of precipitation andgrain boundary. In addition, the deformation mechanisms were characterized by TEM. The cutting-through mechanismdominates the deformation behaviors. Moreover, interesting stacking faults in different slip planes prevail during plasticdeformation. The concurrent recrystallization and precipitation for ultra-fine grain structure with nano-precipitates wouldprovide an effective strategy to achieve outstanding combination of strength and ductility.

일방향 응고 CM247LC 초내열합금 크리프 시편 게이지에서 고온 산화가 표면 피트 및 재결정 형성에 미치는 영향

최병학,최광수,한성희,김대현,안종기,강동수,서성문 대한금속·재료학회 2023 대한금속·재료학회지 Vol.61 No.12

This study analyzed a recrystallization phenomenon that occurred simultaneously with high temperature oxidation on the surface of a directionally solidified CM247LC creep specimen, using optical and scanning electron microscopy. After heat treatment, the surface of the specimen subjected to the creep test at 982oC was oxidized by exposure to high temperature and underwent microstructural changes due to high temperature stress. The outermost layer of the oxidized surface pits was found to consist of an oxide of the Cr/Co component, and the lower layer contained an oxide of the Al component. The area adjacent to the surface oxide layer is a precipitated free zone (PFZ) depleted of the α precipitated phase. The PFZ is caused by the diffusion of the Al component from this area to the surface oxide layer, resulting in the depletion of α-Ni3Al as the main Al component. The area adjacent to the PFZ is a α coarsening layer, which is the result of increasing α phase fraction and coarsening as the Cr/Co component of this region diffuses into the PFZ of the α phase composition. This α coarsening and α rafting occurs in the direction perpendicular to the creep stress. In the EBSD analysis, the PFZ and α coarsened layer were observed to be recrystallized regions, with the recrystallization composed of a single grain including the PFZ and α coarsened layer. It is presumed that this recrystallization is caused by the residual stress of one-way solidification or the residual stress of the specimen surface processing. Accordingly, high-temperature oxidation in DS CM247LC creep gauge caused surface pits associated with recrystallization.

Purification and crystal growth of NPB via imidazolium based ionic liquids

Oh, Yong-Taeg,Shin, Dong-Chan Elsevier 2018 Journal of crystal growth Vol.487 No.-

<P><B>Abstract</B></P> <P>Here we report the production of high purity and crystallinity organic electronic material of NPB (<I>N,N</I>′-Di-[(1-naphthyl)-<I>N,N</I>′-diphenyl]-1,1′-biphenyl-4,4′-diamine (C<SUB>44</SUB>H<SUB>32</SUB>N<SUB>2</SUB>) through solution recrystallization within imidazolium based ionic liquids. When low purity NPB was recrystallized at 170 °C within C<SUB>8</SUB>MIM[TFSI], its purity was drastically improved from 82% to 99.92%. These recrystallized NPB crystals showed 0.040° FWHM (Full Width Half Maximum) of X-ray (1 1 1) diffraction peak. Such small FWHM angle indicates single-crystal like crystallinity. Initial NPB powder was dissolved at 100 °C and recrystallized at temperature above 110 °C. At higher temperature of 170 °C, a small number of bigger crystals were formed compared to those at 110 °C. This can be well explained by the classical nucleation and growth theory. Therefore, solution recrystallization process using ionic liquid might be promising for mass production of organic electronic materials by replacing the widely-used sublimation purification method.</P> <P><B>Highlights</B></P> <P> <UL> <LI> High quality NPB can be obtained through solution recrystallization. </LI> <LI> The purity of NPB was improved from 82% to 99.92% within C<SUB>8</SUB>MIM[TFSI]. </LI> <LI> The recrystallized NPB crystal has single-crystal like crystallinity. </LI> </UL> </P>

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.