挤压AZ31B镁合金板材中温轧制变形中的组织变化

在220℃的温度条件下,对AZ31B挤压板材进行单道次轧制变形,通过光学显微镜(OM)和扫描电镜(SEM)观察了中温变形AZ31B微观组织演变。结果表明,在中温变形的初始阶段,孪生为主要的变形机制,随变形量的增加,变形畸变能的积聚以及孪晶间的相互作用,导致了动态再结晶的形核与长大,在大的变形量条件下,孪生、形变带和动态再结晶共同作用,使变形得以进行。     

Nucleation mechanisms of dynamic recrystallization in Inconel 625 superalloy deformed with different strain rates

The effects of strain rates on the hot working characteristics and nucleation mechanisms of dynamic recrystallization (DRX) were studied by optical microscopy and electron backscatter diffraction (EBSD) technique. Hot compression tests were conducted using a Gleeble-1500 simulator at a true strain of 0.7 in the temperature range of 1000 to 1150 °C and strain rate range of 0.01 to 10.00 s-1. It is found that the size and volume fraction of the DRX grains in hot-deformed Inconel 625 superalloy firstly decrease and then increase with increasing strain rate. Meanwhile, the nucleation mechanism of DRX is closely related to the deformation strain rate due to the deformation thermal effect. The discontinuous DRX (DDRX) with bulging of original grain boundaries is the primary nucleation mechanism of DRX, while the continuous DRX (CDRX) with progressive subgrain rotation acts as a secondary nucleation mechanism. The twinning formation can activate the nucleation of DRX. The effects of bulging of original grain boundaries and twinning formation are firstly gradually weakened and then strengthened with the increasing strain rate due to the deformation thermal effect. On the contrary, the effect of subgrain rotation is firstly gradually strengthened and then weakened with the increasing strain rate.     

Twinning, dynamic recovery and recrystallization in the hot rolling process of twin-roll cast AZ31B alloy

Hot-rolling experiments with a reduction from 10% to 60% in single pass were conducted on AZ31B twin-roll cast sheets. Optical microscope (OM), electron backscattered diffraction (EBSD) tests and transmission electron microscope (TEM) were used to investigate twinning and DRV/DRX behaviors at different stage in the hot-rolling process. Two types of twinning occurred in the initial stage of hot-rolling process. DRV and discontinuous recrystallization dominated at moderate strain while continuous DRX took place homogeneously throughout original grains at the largest strains.     

Formation mechanism of texture during dynamic recrystallization in γ-TiAl,nickel and copper examined by microstructure observation and grain boundary analysis based on local orientation measurements

Texture formation in γ-TiAl, nickel and copper during dynamic recrystallization (DRX) in compression deformation is studied. The formation process is discussed in relation to the mechanism of new grain formation during DRX. It is found that the behavior of texture formation varies depending on Z (the value of the Zener-Hollomon parameter) and the kind of material. During deformation in low Z conditions, sharp fiber texture develops in γ-TiAl, suggesting that strain-induced grain boundary migration is predominant in the new grain formation. No sharp texture is seen in nickel and copper after the dynamic recrystallization in low Z conditions. However, microstructure observation suggests that new grains are also generated by strain-induced grain boundary migration in these two pure metals. It is found that the difference of texture formation between γ-TiAl and the other two pure metals is ascribed to the difference in the frequency of twinning during grain boundary migration; texture develops only when the frequency is low, which is the case of γ-TiAl.     

AZ31B镁合金热压缩变形的组织变化与变形机制

对铸态AZ31B镁合金在温度280℃～440℃、应变速率0.001s-1～0.1s-1条件下进行热压缩实验,分析变形程度、应变速率和加热温度对其微观组织变化的影响,探讨合金的热压变形机制。实验结果表明,该合金热变形时发生了动态再结晶。变形温度越高、变形速率越小和变形量越大时,动态再结晶进行的越充分;变形温度越低、变形速率越大和变形量越大时,动态再结晶晶粒越细小。该合金的热变形机制是滑移孪晶联合机制。     

镁及镁合金动态再结晶研究进展

综述了镁及镁合金动态再结晶方面的研究现状，介绍了镁及镁合金室温或高温塑性变形行为，包括应力一应变特征及其影响因素、应变速率方程和流变应力方程；描述了该合金在不同变形条件下发生塑性变形时的位错、孪晶、亚结构等微观组织演变以及各种动态再结晶如孪生动态再结晶、低温动态再结晶、连续动态再结晶、不连续动态再结晶和旋转动态再结晶的机理及其特点；最后讨论了动态再结晶与塑性变形之间的相互关系，并提出了镁及镁合金动态再结晶研究的发展趋势。     

Simulation of strain-softening behaviors in an AZ31 Mg alloy showing distinct twin-induced reorientation before a peak stress

To investigate strain-softening behavior during plastic deformation of an AZ31 Mg alloy, cylindrical specimens were compressed in a rolling direction at 300 °C. Experimental evidence revealed that an inhomogeneous microstructure evolved due to the softening behavior associated with deformation at elevated temperatures. The large grains that reoriented as a result of deformation twinning were free of dynamic recrystallization (DRX). Fine grains nucleated at grain boundaries of grains were deformed by a slip-dominated mechanism, which accommodated the external strain. A visco-plastic self-consistent (VPSC) polycrystal model was used to simulate softening of the flow stress curve and texture evolution during uniaxial compression. A softening scheme was implemented in the polycrystal model to predict the softening phenomenon and texture evolution after the peak stress. The original VPSC model was modified to simulate texture evolution in an AZ31 Mg alloy that exhibited twin-dominated deformation before the peak stress.     

Texture Evolution and Dynamic Recrystallization of Zr-1Sn-0.3Nb-0.3Fe-0.1Cr Alloy During Hot Rolling

In the present work,the sheets of Zr-1Sn-0.3Nb-0.3Fe-0.1Cr alloy were hot rolled with different reductions(10%,30%,50%,and 60%) at 1023 K and 1073 K.The micro structure evolutions including grain micro structure,texture,and dislocation were investigated,using electron backscattering diffraction and transmission electron microscope.The results showed that dislocation slip,twinning,and dynamic recrystallization(DRX) were the main deformation mechanisms.DRX was found to be promoted by larger reduction and higher rolling temperature.The predominant texture formed during hot rolling was basal 0001//ND,whose intensity reached peak value after 30% reduction hot rolling.While the intensity of DRX texture 10-10//ND and1-210//ND increased with increasing reduction and temperature.This study provided an effective way to tailor the texture and microstructure of the alloy,for optimizing process parameters.     

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.     

Effect of microstructural inhomogeneities on the evolution of texture during rolling and recrystallization of copper

Microstructural inhomogeneities in polycrystalline copper was intentionally varied by the deformation at the liquid nitrogen temperature and the rolling without lubrication. The effect of different rolling conditions on the evolution of microstructure and texture during rolling and subsequent recrystallization was studied by means of X-ray texture measurement and microstructural observations. Applying cold rolling at a low temperature led to the formation of deformation twins in grains at the Cu-orientation, and shear bands developed when cold rolling was performed without lubrication. The evolution of the normal rolling texture was suppressed by deformation twinning. Shear bands in deformed structure resulted in a very weak cube-recrystallization texture.     

Effect of temperature on microstructure and texture evolution of Mg-Zn-Er alloy during hot compression

The microstructure and texture evolutions in Mg-Zn-Er alloy during hot compression were investigated by using optical microscope (OM), field emission scanning electron microscope (EBSD) and transmission electron microscope (TEM). The results indicate that the temperature plays an important role in dynamic recrystallization (DRX) mechanism. The twin dynamic recrystallization (TDRX) is induced at a strain of 0.6 because of the activation of non-basal slip «a+c» dislocations at 200 °C. Meanwhile, the continuous DRX (CDRX) occurs at 350 °C, which is identified by the typical necklace-like structure around the residual initial grains. The DRX contributes to the modification of texture significantly. The tension twins are responsible for the weak texture at 200 °C. Meanwhile, the decrease in the basal texture is ascribed to the DRX sites which transfer from twin boundaries to initial grain boundaries as the temperature is increased from 200 to 350 °C.     

纯钛板材冷拉成形过程中的微观组织与织构演变(英文)

研究纯钛板材冷拉成形过程中微观组织及织构演化规律。半球形壳体件在深拉延过程中由于各部位变形模式及应变形式的不同会形成胀形区、拉深区及法兰区等3个区域。结果表明,在拉深件的3个区域中塑性应变均由位错滑移与变形孪晶共同作用。纯钛板材及其拉深件中的织构包含轧制织构与再结晶织构。由于变形孪晶与位错滑移对织构的影响规律不同,初始板材织构的强度及类型在深拉过程中不断变化。变形孪晶对初始织构具有弱化作用,特别是对于再结晶织构,这种弱化效应更为明显。由于拉深区产生的孪晶较多,再结晶织构消失。此外,大拉伸变形时位错滑移为主导机制,织构强化效应明显。     

Self-consistent polycrystal modelling of dynamic recrystallization during the shear deformation of a Ti IF steel

The texture variations associated with the occurrence of dynamic recrystallization (DRX) during the high temperature torsion of a Ti interstitial-free steel were examined. Both texture formation mechanisms of DRX were considered: i.e. oriented nucleation as well as selective growth. These processes were simulated with the aid of a self-consistent polycrystal model that takes crystallographic glide and DRX into account and also allows for the grain shape changes that take place during deformation. To handle the variations in volume fraction associated with the different orientations caused by DRX, the so-called volume transfer scheme was employed. By comparing the texture predictions with the experimental observations, it is shown that the self-consistent model reproduces the DRX textures more faithfully than previous relaxed constraint models. The results also indicate that the main texture formation mechanism during DRX is oriented nucleation followed by generalized growth. By contrast, selective growth, as modelled in terms of coincident site lattice rotations or plane matching, appears to be of little importance under the present conditions.     

Effect of deformation mode,dynamic recrystallization and twinning on rolling texture evolution of AZ31 magnesium alloys

In order to obtain quantitative relationship between (0002) texture intensity and hot rolling conditions, conventional rolling experiments on AZ31 magnesium alloys were performed with 20%–40% reductions and temperatures within the range of 300–500 °C. Shear strain and equivalent strain distributions along the thickness of the rolled sheets were calculated experimentally using embedded pin in a rolling sheet. Rolling microstructures and textures in the sheet surface and center layers of the AZ31 alloys were measured by optical microscopy (OM), X-ray diffractometry (XRD) and electron back scatter diffraction (EBSD). Effects of the rolling strain, dynamic recrystallization (DRX) and twinning on the texture evolution of the AZ31 alloys were investigated quantitatively. It is found that the highest (0002) basal texture intensities are obtained at a starting rolling temperature of 400 °C under the same strain. Strain–temperature dependency of the (0002) texture intensity of the AZ31 alloy is derived.     

AZ31镁合金挤压板材的中温变形机理及软化机制(英文)

研究AZ31镁合金挤压板材在473～523K的温度范围内。应变速率0.001～1.0s-1压缩时的流变应力行为,计算板材沿挤压方向压缩时的激活能,并结合光学显微镜和透射电子显微镜探讨合金软化机制和变形机理之间的联系。结果表明,在中温下沿挤压方向压缩时,AZ31挤压态镁合金的变形激活能为174.18kJ/mol。这说明,由热激活位错交滑移所控制的动态再结晶是合金中温变形的主要软化机制。位错滑移是中温变形的主要变形机理,而孪生的作用则不大。其主要的动态再结晶机制为持续动态再结晶,并伴随少量的孪生动态再结晶。     

AZ31B镁合金板材中晶界滑移的晶体塑性仿真

建立一种耦合滑移、动态再结晶以及晶界滑移的晶体塑性模型以仿真镁合金的高温变形行为及织构演化.首先,通过实验测量单轴拉伸、压缩后的织构以及显微组织演化,研究AZ31B镁合金在300°C的变形机制.结果发现,动态再结晶在应变小于0.2时起到细化晶粒的作用,之后晶界滑移在变形过程中起显著作用.此外,建立晶界滑移模型来评估由晶...     

Effects of deformation parameters on microstructure and mechanical properties of magnesium alloy AZ31B

Plastic deformation and dynamic recrystallization (DRX) behaviors of magnesium alloy AZ31B during thermal compression and extrusion processes were studied.In addition, effects of deformation temperature and rates on the microstructure and mechanical properties were investigated.The results show that the DRX grains nucleate initially at the primary grain boundaries and the twin boundaries, and the twinning plays an important role in the grain refinement.The DRX grain size depends on the deformation temperature and strain rate The average grain size is only 1 μm when the strain rate is 5 s-1 and temperature is 250 ℃.It is also found that the DRX grain can grow up quickly at the elevated temperature.The microstructure of extruded rods was consisted of tiny equal-axis DRX grains and some elongated grains.The rods extruded slowly have tiny grains and exhibit good mechanical properties.     

Mg-3Y合金大应变量轧制过程中的孪生辅助动态再结晶及组织演变

采用预挤压加单道次大应变量热轧制的方法制备了Mg-3Y（质量分数,%）合金板材。并研究了大应变量轧制过程中不同孪晶类型对合金动态再结晶（DRX）及组织演变的影响。结果表明,在挤压比为8:1的预挤压过程中,合金内部发生了几乎完全的动态再结晶。而在接下来的大应变量热轧制过程中,孪生变形尤其是■压缩孪晶及■双孪晶在协调合金的塑性应变中发挥了重要作用。此外,大量动态再结晶在压缩孪晶及双孪晶内部发生,并扩展到非孪晶区域,有效缓解了轧制过程中的内应力集中。上述2个过程对提高合金在大应变量轧制中的成形性均起到了促进作用。     

The role of Zr-rich cores in strength differential effect in an extruded Mg–Zn–Zr alloy

This work examines the effects of extrusion parameters namely ratio and temperature on recrystallization behavior of a Mg–Zn–Zr alloy, and their consequent effects on anisotropy in the mechanical properties. Upon extrusion, the characteristic Zr-rich cores that do not recrystallize form the so-called “soft stringers” which are deformed bands elongated in the extrusion direction and curled around the extrusion axis. At higher extrusion ratio, there is more twinning contribution and the DRX response is improved, making the recryatallized grains finer and increasing the proportion of recrystallized Zr-rich cores. A basal texture after extrusion and the directional activation of tensile twinning cause anisotropy in the mechanical properties. In addition, the microstructural features such as large unrecrystallized regions and coarse crystallized grains also contribute in the strength differential effect. Further slip in the strain-hardened unrecrystallized grains is inhibited while twin activation under favorable orientation becomes easier in the coarse recrystallized grains. A higher proportion of large unrecrystallized and coarse crystallized gains in the case of lower extrusion ratio result in a much higher strength differential effect (100 MPa) in comparison to the one caused by the crystallographic texture only (25 MPa).     

Effect of Hot Deformation on Texture and Microstructure in Fe-Mn Austenitic Steel During Compression Loading

Hot compression tests of a new high-Mn austenitic steel were carried out at deformation temperatures of 700, 800, 900, and 1000 °C under strain rate of 0.01 s^?1. The hot deformation behavior was investigated by the analyses of flow curves, texture, and deformed microstructures. Microstructures of the deformed specimens and macrotexture were examined using electron backscatter diffraction and x-ray diffraction methods, respectively. The results showed that the flow stress depended strongly on the deformation temperature and decreased by increasing deformation temperature. The microstructural evidence indicated that the dynamic recrystallization (DRX) process of experimental steel was initiated at 800 °C with necklace structure. The volume fraction of DRX grains was considerably increased by increasing deformation temperature to 1000 °C. Texture of the DRX grains tended to become a weak texture and was associated with the formation of Goss and R-Cube components. Meanwhile, martensitic transformation was detected in the hot-deformed austenite. The martensitic transformation was the most difficult in the DRX grains because of the effect of small grain size. The tendency of transformation was decreased after compression at 1000 °C.