Recrystallization in a directionally solidified cobalt-base superalloy

The recrystallization behavior in a range of annealing temperature from 1020 to 1280 °C in a directionally solidified cobalt-base superalloy was studied. Local recrystallization first appeared at 1040 °C. The recrystallized volume increased rapidly as increasing the annealing temperature. Pinning effect of all carbides (M₂₃C₆, M₇C₃ and MC) was observed and large amount of twin formed at low annealing temperature. The size of the recrystallized grains increased significantly at high annealing temperatures accompanied with the sharp decrease of twin. The effect of annealing temperature and the role of carbide and twin on the development of the recrystallization were discussed.     

Annealing effects on microstructure and coercive field of ferritic-martensitic ODS Eurofer steel

Oxide dispersion strengthened reduced-activation ferritic-martensitic steels are promising candidates for applications in future fusion power plants. Samples of a reduced activation ferritic-martensitic 9 wt.%Cr-oxide dispersion strengthened Eurofer steel were cold rolled to 80% reduction in thickness and annealed in vacuum for 1 h from 200 to 1350 °C to evaluate its thermal stability. Vickers microhardness testing and electron backscatter diffraction (EBSD) were used to characterize the microstructure. The microstructural changes were also followed by magnetic measurements, in particular the corresponding variation of the coercive field (H_c), as a function of the annealing treatment. Results show that magnetic measurements were sensitive to detect the changes, in particular the martensitic transformation, in samples annealed above 850 °C (austenitic regime).     

Recrystallization of the hot isostatic pressed nickel-base superalloy FGH4096: I. Microstructure and mechanism

Hot isostatic pressed (HIPed) nickel-based superalloy FGH4096 behaves a unique flow behavior in hot compression process due to the formation of necklace microstructure and leads to its characteristic dynamic recrystallization (DRX). In this process, dislocations activate the occurrence of DRX at prior particle boundaries (PPB) and the PPB is entirely covered with DRX grains, which forms the first layer in necklace structure. In this paper, two nucleation mechanisms, viz., bulge corrugation (BC) nucleation and dislocation induce phase (DIP) nucleation, are proposed. Based on the proposed mechanisms, the recrystallization firstly occurs in PPB via BC mechanism. The DIP nucleation occurs when the hot plastic deformation is carried out at the temperature below the γ′ phase solution temperature. To verify the proposed mechanisms, hot compression experiments were conducted. The models are then verified based on the experiments. Furthermore, the recrystallization activation energy of 922 kJ/mol is determined, which includes the growth energy and the two nucleation energies, viz., BC and DIP nucleation energies.     

Analysis of Micro-texture during Secondary Recrystallization in a Hi-B Electrical Steel

The understanding of Goss texture in Hi-B electrical steels possesses significant industrial and academic value, thus attracts worldwide attention. The prevailing models for sharp Goss texture formation during secondary recrystallization are CSL (coincident site lattice) boundary theory and HE (high energy) boundary theory. These theories stress the key factor of preferred growth and the difference between them only lies in the specific selection manner. This work examined the texture gradient in primarily recrystallized sheet and demonstrated its possible influence on the formation of secondary grains, and then determined the micro- texture during different stages of secondary recrystallization using EBSD (electron back-scattered diffraction) technique, finally analyzed a special type of secondary grains with near Brass orientation, which were detected in the later stage of secondary recrystallization, and discussed its origin and effect in terms of surface energy effect. The results indicate that texture gradient in primarily recrystallized sheet will lead to a multi-stage formation of Goss texture, namely, early stage of secondary grains with various orientations in subsurface region, intermediate stage of preferred growth of Goss grains into center layer and re-grow back to the surface and the final stage of Goss grain growth by swallowing slowly the island grains with the help of H2 atmosphere.     

Characterisation of surface recrystallisation in a grit-blasted single-crystal superalloy

In this work, detailed experimental insights regarding the microstructural features on surface recrystallisation of a grit-blasted single crystal superalloy DD6 have been obtained by transmission electron microcopy (TEM), scanning transmission electron microcopy (STEM), scanning electron microscopy and electron backscattered diffraction (EBSD) techniques. The broadened diffraction ring arcs typical for micro-textures revealed the fragmented finer grains after surface grit-blasting. Energy dispersive spectroscopy analysis and STEM mapping confirmed the characteristic elements for differentiation of γ′ and γ phase, and the precipitated secondary or tertiary γ′ in the recrystallised region was characterised by dark field TEM. Moreover, the recrystallisation depth was feasibly revealed by ion-etching and further used for EBSD imaging without polishing. The resulted EBSD pole figures and misorientation profiles confirmed that the recrystallised region exhibited a rather random orientation, destroying the original single crystal type.     

EBPVD制备ODS Ni基高温合金抗氧化性能研究

采用电子束物理气相沉积技术制备了微晶氧化物弥散增强Ni基高温合金,并研究了该合金的高温氧化行为.通过最小二乘拟合得到的合金氧化动力学模型近似遵循4次方规律,并在高温氧化过程中,合金能够生成单一的Al2O3氧化膜,而且氧化膜的粘附性好,不易脱落.稀土氧化物的添加可明显地改善合金的抗高温氧化性能,提高氧化膜的粘附性.     

镍基单晶合金CMSX-6的再结晶行为及热裂倾向

在不同热处理温度和载荷下,研究了CMSX-6单晶的再结晶现象。结果表明:在低于1 150℃×4 h热处理后,没发现再结晶现象,但在载荷的作用下,单晶退火时γ'相形成元素自发偏析,造成大量的大颗粒γ'相在共晶周围析出;随着热处理温度升高,载荷影响区再结晶经历由压痕周围胞状再结晶、枝晶干再结晶、多个等轴再结晶三个阶段;在高于1 150℃×4 h热处理时,单晶随着载荷的增加,再结晶的范围和深度明显增大;以及随冷却速率的增加,再结晶晶粒之间的热裂倾向增大,致使大量热裂沿晶界分布。     

X-ray microdiffraction and EBSD study of FSP induced structural/phase transitions in a Ni-based superalloy

Severe plastic deformation during Friction Stir Processing (FSP) of an IN738 Ni-based superalloy was studied by means of X-ray polychromatic microdiffraction, EBSD, scanning electron and optical microscopies. Modeling of the physical properties and phase composition was also performed. Several distinct zones are formed during FSP including a stir zone (SZ), a thermal-mechanical affected zone (TMAZ) and a heat affected zone (HAZ). Each zone has distinct microstructure after FSP. The initial dendrite structure is preserved in the HAZ, while strengthening γ′-phase particles partially dissolve and coagulate. Plastic deformation of the base material dendrites takes place in the TMAZ and a large number of geometrically necessary dislocations are formed. The extent of deformation increases toward the SZ and the dendrite structure is completely destroyed in the SZ and replaced by a fine submicrocrystallinne microstructure.     

On anisotropy of ferritic ODS alloys

Abstract

Ferritic oxide dispersion strengthened (ODS) alloys are candidate materials to be used as cladding for long term fast reactors, due to their high strength at high temperature and good swelling and irradiation resistances. The fabrication of cladding tubes is usually made by a succession of cold deformation steps where a deformation induced anisotropic microstructure could take place, which would affect the mechanical behaviour of the tube. The characterisation of this microstructural anisotropy is one of the key issues in the development of cladding ODS tubes. In this paper, the microstructural anisotropy of a Fe–14Cr–ODS extruded bar and a Fe–12Cr–ODS plate is characterised and its effect on the mechanical properties is analysed by tensile, impact and small punch testing. In both materials, a reduction of the ductility is observed in the transverse specimens. In addition, the fracture behaviour seems to be strongly dependent on the location of the crack plane regarding the elongated grained microstructure.     

基于不同原始组织预设变形第四代单晶高温合金的再结晶行为

第四代单晶高温合金标准热处理试样和铸态试样压痕后分别在1100,1150,1200,1250,1300℃和1340℃退火处理,采用光学显微镜、扫描电镜、电子背散射仪研究不同条件的再结晶组织。结果表明:1100,1150,1200℃退火处理后,标准热处理试样和铸态试样都出现胞状再结晶。1250℃退火处理后,标准热处理试样和铸态试样都为混合再结晶。1300℃退火处理后,标准热处理试样再结晶组织全部为等轴再结晶,而铸态试样仍为混合再结晶。1340℃退火处理后,标准热处理试样和铸态试样都形成了等轴再结晶。随着退火温度升高,标准热处理试样和铸态试样的再结晶层深度明显增加,标准热处理试样再结晶深度明显大于铸态试样,相同条件下标准热处理试样的再结晶晶粒更容易长大。再结晶与基体的界面为小角度晶界、大角度晶界,而再结晶晶粒之间为小角度晶界、大角度晶界和孪晶界。孪晶在单晶高温合金再结晶的过程中发挥了重要作用。     

Formation mechanism of large grains inside annealed microstructure of GH4169 superalloy by cellular automation method

In authors’ previous work [Mater. Charact. 141 (2018) 212–222], it was found that the heterogeneous deformed microstructures can be replaced by the relatively homogeneous recrystallized grains through an annealing treatment. However, there are still some relatively large recrystallized grains. To find the reasons for the formation of large grains, some new annealing treatment tests were done, and the cellular automation (CA) simulations were carried out in the present work. The experimental results showed that the microstructural evolution during annealing treatment is significantly affected by the content of δ phase. So, the effects of δ phase on the nucleation and growth of grains are carefully considered in the CA model to accurately simulate the microstructural evolution behavior. By the CA simulation, it is found that the dislocation density rapidly decreases due to the nucleation of static recrystallization (SRX) and the growth of dynamc recrystallization (DRX) nuclei at the early stage of annealing. The high initial dislocation density can provide the high velocity for the growth of DRX nuclei, which is responsible for the formation of coarse grains. However, the growth rate of SRX nuclei is relatively small due to the low dislocation density and pinning effects of δ phase.     

ODS技改项目的实施与环境保护

实施 ODS技术改造项目 ,是履行《蒙特利尔议定书》的重要举措。本文论述了如何利用国际 ML F多边基金组织的赠款做好工商制冷设备的制冷剂替代和设备改造的技术工作 ,阐明了 ODS技改项目的实施对环境保护和企业发展的深远影响     

In situ SEM studies of the low cycle fatigue behavior of DZ4 superalloy at elevated temperature: Effect of partial recrystallization

The influence of partial recrystallization (RX) on the low cycle fatigue (LCF) behavior of directionally solidified DZ4 superalloy was studied using in situ scanning electron microscopy. Three typical recrystallization microstructures were prepared on the fatigue samples, via shot peening and subsequent heat treatment. The effect of RX on LCF life is evidently related to the specific RX microstructure. The RX layer does not necessarily reduce the LCF life. The fatigue crack initiation and short crack propagation behaviors for both raw DZ4 and recrystallized samples were in situ examined, which rationalized the observed effect of recrystallization on the LCF life variation.     

Treatment of misorientation data to determine the fraction of recrystallized grains in a partially recrystallized metal

A method has been developed to differentiate recrystallized from unrecrystallized grains in partially annealed samples based on the point-to-point misorientation within the grains. Recrystallized grains exhibit a low average point-to-point misorientation whereas the contrary is the case for deformed grains. The analysis for recrystallized fraction was calibrated based on this distinguishing feature, and it was found that the average point-to-point misorientation within a grain in a fully recrystallized sample was less than 0.7°. Based on this calibration, partially recrystallized samples were analysed for their degree of recrystallization, and the technique was validated using microhardness measurements. The analysis of the misorientation data was in excellent agreement with the hardness data. There are three factors which distinguish the current method in comparison to the earlier work: a fixed limit is maintained on the minimum number of pixels which may constitute a grain; pattern quality is not considered; and the recrystallization criterion is calibrated to the experimental data. Nevertheless, in the early stages of annealing, where recovery is likely to have the most influence on the drop in hardness, the analysis of electron backscattered diffraction (EBSD) data underpredicts the recrystallized fraction. The analysis was also conducted on a cold-rolled, nonannealed sample, and from this, the error of the technique is estimated to be a maximum of 0.06 recrystallized volume fraction.     

On the room-temperature annealing of cryogenically rolled copper

The electron-backscatter-diffraction (EBSD) technique was applied to investigate room-temperature annealing processes in cryogenically rolled copper during long-term (∼1.5 years) storage at ambient temperature. Static recrystallization appeared to be nucleated as result of both grain-boundary bulging and recovery. A bimodal recrystallized grain size distribution appeared to be a result of these two competing mechanisms. The ultra-fine grain copper produced via cryogenic deformation was deduced to be prone to abnormal grain growth after long static storage at room-temperature.     

铁素体不锈钢焊接接头的再结晶现象

考察了新型铁素体不锈钢焊接接头中的再结晶现象，分析了其形成特点及相关影响因素。     

Tailoring the secondary phases and mechanical properties of ODS steel by heat treatment

The oxide dispersion strengthened (ODS) steel with the nominal composition of Fe–14Cr–2W–0.3Ti–0.2V–0.07Ta–0.3Y₂O₃ (wt%) was fabricated by mechanical alloying and hot isostatic pressing (HIP). In order to optimize the relative volume fraction of secondary phases, the as-HIPed ODS steel was annealed at 800 °C, 1000 °C, 1200 °C for 5 h, respectively. The microstructures and different secondary phases of the as-HIPed and annealed ODS samples were identified by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The tensile properties of all the ODS steels at room temperature were also investigated. The results indicate that annealing is an effective way to control the microstructure and the integral secondary phases. The annealing process promotes the dissolution of M₂₃C₆ particles, thus promoting the precipitation of TiC. No obvious coarsening of Y₂Ti₂O₇ nanoparticles can be observed during annealing. The tensile results indicate that the annealed ODS sample with the optimized secondary phases and high density possesses the best mechanical properties.     

基于ODS FRF某城际车工作状态下模态贡献分析

为研究某城际车工作状态下模态贡献问题,首先用激振器对该城际车进行了静态台架实验,利用LMS. Test. Lab软件识别其模态参数。然后对该城际列车进行线路实验,测试其不同速度级下的加速度振动响应。根据ODS FRF概念,给出了城际车在ODS FRF包络曲线峰值处的工作变形。最后根据模态叠加原理,得出不同速度级下,该城际车不同频率下工作变形主要由车体沉浮、车体点头及车体1阶垂弯贡献的结论。同时结果显示,不同速度级下,同一频率下工作变形可能由不同固有模态贡献;不同速度级下,同一工作变形频率可能会发生偏移。     

Methods and mechanisms for uniformly refining deformed mixed and coarse grains inside a solution-treated Ni-based superalloy by two-stage heat treatment

The uniform refinement mechanisms and methods of deformed mixed and coarse grains inside a solution-treatment Ni-based superalloy during two-stage annealing treatment have been investigated.The two-stage heat treatment experiments include an aging annealing treatment(AT)and a subsequent recrystallization annealing treatment(RT).The object of AT is to precipitate some δ phases and consume part of storage energy to inhibit the grain growth during RT,while the RT is to refine mixed and coarse grains by recrystallization.It can be found that the recrystallization grains will quickly grow up to a large size when the AT time is too low or the RT temperature is too high,while the deformed coarse grains cannot be eliminated when the AT time is too long or the RT temperature is too low.In addition,the mixed microstructure composed of some abnormal coarse recrystallization grains(ACRGs)and a large number of fine grains can be observed in the annealed specimen when the AT time is 3 h and RT tem-perature is 980℃.The phenomenon attributes to the uneven distribution of δ phase resulted from the heterogeneous deformation energy when the AT time is too short.In the regions with a large number of δ phases,the recrystallization nucleation rate is promoted and the growth of grains is limited,which results in fine grains.However,in the regions with few δ phases,the recrystallization grains around grain boundaries can easily grow up,and the new recrystallization nucleus is difficult to form inside grain,which leads to ACRGs.Thus,in order to obtain uniform and fine annealed microstructure,it is a prereq-uisite to precipitate even-distributed δ phase by choosing a suitable AT time,such as 12 h.Moreover,a relative high RT temperature is also needed to promote the recrystallization nucleation around δ phase.The optimal annealing parameters range for uniformly refining mixed crystal can be summarized as:900℃×12 h+990℃×(40-60 min)and 900℃×12 h+1000℃×(10-15 min).     

Automatic determination of recrystallization parameters based on EBSD mapping

A new automatic algorithm for determining the recrystallization parameters V_V, S_V and <λ> based on EBSD mapping is presented in this paper. The algorithm is validated on aluminium deformed to high strains. The algorithm is also compared with other methods using the exact same sets of samples, and it is found that the present method gives better results for S_V.