Local deformation heterogeneity in cyclically deformed interstitial free steel

Heterogeneity of cyclic plastic deformation in interstitial‐free steel has been studied through a measurement of local average misorientation using electron back‐scattering diffraction technique. It is found that with increase of strain amplitude, the local average misorientation increases linearly within ±0.15 to ±0.40% and then decreases with further increase of strain amplitude. It is proposed that due to more homogeneous deformation vis‐à‐vis less generation of geometrically necessary dislocations, misorientation development per cycle becomes almost saturated in the predominantly low cycle fatigue regime. It is also found that the local average misorientation depends upon grain size in such a manner that in small grains, the misorientation is less as compared with large grains.     

The role of grain size in static and cyclic deformation behaviour of a laser reversion annealed metastable austenitic steel

Different grain sizes were created in a metastable 17Cr‐7Mn‐7Ni steel by martensite‐to‐austenite reversion at different temperatures using a laser beam. Two fully reverted material states obtained at 990°C and 780°C exhibited average grain sizes of 7.7 and 2.7 μm, respectively. The third microstructure (610°C) consisted of grains at different stages of recrystallization and deformed austenite. A hot‐pressed, coarse‐grained counterpart was studied for reference. The yield and tensile strengths increased with refined grain size, maintaining reasonable elongation except for the heterogeneous microstructure. Total strain‐controlled fatigue tests revealed increasing initial stress amplitudes but decreasing cyclic hardening and fatigue‐induced α′‐martensite formation with decreasing grain size. Fatigue life was slightly improved for the 2.7‐μm grain size. Contrary, the heterogeneous microstructure yielded an inferior lifetime, especially at high strain amplitudes. Examinations of the cyclically deformed microstructure showed that the characteristic deformation band structure was less pronounced in refined grains.     

Analysis on Microstructure and Misorientation of Ultrathin Hot Strip of Low Carbon Steel Produced by Compact Strip Production

The microstructure and misorientation of ultrathin hot strip were analyzed by CSP technology using electron back scattered diffraction(EBSD)method and Autoforge finite element program.The experimental results showed that the finishing hot rolling microstructures were the mixture of recrystallized and deformed austenite.After phase transformation,ferrite grains embody substructures and dislocations,leading to the high strength and relatively low elongation rate of the ultrathin hot strip.The FEM simulation of strain mode and distribution in deformation area has been fulfilled.The simulation results are in good agreement with the theoretical analysis and experimental results.     

A mechanism of ferrite softening in a duplex stainless steel deformed in hot torsion

A mechanism of dynamic softening of ferrite was studied in a 21Cr-10Ni-3Mo austenite/ferrite duplex stainless steel subjected to torsion at a strain rate of 0.7 s⁻¹ at 1200°C. Transmission electron microscopy together with convergent beam electron diffraction were used with major emphasis on the study of misorientations across ferrite/ferrite boundaries. No evidence of discontinuous dynamic recrystallisation involving nucleation and growth of new grains was found within ferrite contrary to some suggestions made in the literature for similar experimental conditions. The softening mechanism has been classified as extended dynamic recovery characterised by a gradual increase in misorientations between neighbouring subgrains that were created by dynamic recovery processes at the earlier stages of deformation. The resulting dislocation substructure was a complex network of subgrain boundaries composed of a mix of higher- and lower-angle walls characterised by misorientation angles not exceeding 20° at a maximum obtained strain of 1.3.     

Cyclic hardening behavior for interstitial-free steel

Strain-controlled fatigue experiments were employed to evaluate automotive-grade interstitial-free ferrite steels under R = 0. Hundreds of grains were examined by scanning electron microscope (SEM) under electron channeling contrast image technique of backscattered electron image mode (BEI/ECCI) for comprehensive comparison of micrographs with those taken under transmission electron microscope (TEM). It is clearly revealed that cyclic hardening was virtually unobvious and dislocation cell structures were very rare when Δε/2 was controlled to within 0.1%. When Δε/2 was increased to 0.2%, the general dislocation structure exhibited a predominately dislocation wall structure prior to the secondary cyclic hardening, after which the formation of dislocation cells were observed. At Δε/2 = 1.0%, following an initial rapid-hardening stage, the dislocation cell structure of low-angle misorientation formed in the early stage was gradually converted into high-angle misorientation as the cyclic strain continued to be imposed.     

Transmission and scanning electron microscope study on the secondary cyclic hardening behavior of interstitial-free steel

Strain controlled fatigue experiment was employed to evaluate automotive grade interstitial-free ferrite steel. Hundreds of grains were examined by scanning electron microscope under electron channeling contrast image technique of backscattered electron image mode for comprehensive comparison of micrographs with those taken under transmission electron microscope. The cyclic stress responses clearly revealed that rapid hardening occurs at the early stage of cycling as a result of multiplication of dislocations to develop loop patches, dipolar walls and dislocation cells at various total strain amplitudes. After primary rapid hardening, stress responses varied from being saturated to further hardening according to dislocation structure evolution at various strain amplitudes. The fatigue failure was always accompanied with further hardening including secondary hardening. The corresponding dislocation structures with the three types of hardening behaviors are discussed. Once the secondary hardening starts, dislocation cells began to develop along grain boundaries in the low strain region and then extended into grain interiors as strain amplitudes increased and cycling went on. The secondary hardening rates were found to be directly proportional to their strain amplitudes.     

Dislocation boundaries in drawn single crystal copper wires produced by Ohno continuous casting

Dislocation boundaries in drawn single-crystal copper wires produced by Ohno continuous casting have been studied via electron backscattering diffraction and transmission electron microscopy. In the undeformed wires, there are subgrains with misorientation angle lower than 4.2°. For the cold-drawn wires, we measured misorientation angle and spatial distribution of dislocation boundaries, analysing the formation mechanism of dislocation boundaries parallel to drawing direction. Regarding spatial distribution of high-angle dislocation boundaries, at the strains more than 2.77, the boundaries spread from the centre to the surface regions with increasing strain. Regarding the angular distribution of dislocation boundary misorientation, at the strain lower than 2.77, there is one peak lower than 5°. Increasing the strain to 4.12, a bimodal distribution of misorientation angles is observed. One is lower than 5°, and the other is between 45 and 50°. For dislocation boundaries parallel to drawing direction, although at low strains there are different deformation bands with different microstructures, at high strain the microstructures are characterized as dislocation boundaries parallel to drawing direction formed by two approaches: the interaction between two kinds of boundaries and the increase in misorientation angle of boundaries shared by some dislocation cells.     

Microstructure characterization based on the type of deformed grains in cold-rolled,Cu-added,bake-hardenable steel

The electron backscatter diffraction technique has been used to characterize the microstructure of deformed grains in cold-rolled, Cu-added, bake-hardenable steel. A new scheme based on the kind and number of average orientations, as determined from a unique grain map of the deformed grains, was developed in order to classify deformed grains by type. The α-fiber components, γ-fiber components and random orientations, those which could not be assigned to either γ-fiber or α-fiber components, were used to define the average orientation of unique grains within individual deformed grains. The microstructures of deformed grains in as-rolled specimens were analyzed based on the Taylor factor, stored energy, and misorientation. The relative levels and distributions of the Taylor factor, the stored energy and the misorientation were examined in terms of the types of deformed grains.     

Dislocation structures in cyclically deformed nickel polycrystals

The effect of the grain orientation and the plastic strain amplitude _pa on the saturated dislocation structure was studied on individual grains of cyclically deformed nickel polycrystals by means of scanning electron microscopy using the electron back scattering pattern technique and the channelling contrast of back scattered electrons. The main features of the dislocation configuration in a grain were found to be essentially determined by the crystallographic axial orientation of the grain. A labyrinth-like dislocation pattern is typical for grains with axial orientations near [001], a patch pattern exists in grains with a loading axis (LA) near [011] and fragmented dislocation walls are dominant in grains with LA near [ 11]. Grains with axial orientations in the central part of the stereographic standard triangle contain a bundle arrangement of dislocation structures. All four types of dislocation structures, but mostly the bundle type, can occur together with the ladder structure of persistent slip bands. Cell patterns were found to be a result of a modification of the bundle and patch configuration at high deformation amplitudes. The mesoscopic dimensions of the dislocation patterns turned out to depend on _pa in the same way for all grain orientations: while the thickness of regions with high dislocation density is reduced with increasing _pa, the width of regions with low dislocation density remains roughly constant.     

Analysis of strengthening mechanisms in a severely-plastically-deformed Al–Mg–Si alloy with submicron grain size

Methods of severe plastic deformation of ductile metals and alloys offer the possibility of processing engineering materials to very high strength with good ductility. After typical amounts of processing strain, a submicrocrystalline material is obtained, with boundaries of rather low misorientation angles and grains containing a high density of dislocations. In the present study, an Al–Mg–Si alloy was severely plastically deformed by equal channel angular pressing (ECAP) to produce such a material. The material was subsequently annealed for dislocation recovery and grain growth. The strength of materials in various deformed and annealed states is examined and the respective contributions of loosely-arranged dislocations, many grain boundaries, as well as dispersed particles are deduced. It is shown that dislocation strengthening is significant in as-deformed, as well as lightly annealed materials, with grain boundary strengthening providing the major contribution thereafter.     

Recrystallized cube grains in an Al–Mg–Si alloy dependent on prior cold rolling

This paper reports on the experimental investigation of an industrial Al–Mg–Si alloy, which was subjected to different cold rolling reductions and subsequently solution annealed. Based on large-scale electron backscatter detection (EBSD) measurements, it provides an analysis of the area fraction, size and number density of cube grains in the fully recrystallized microstructure. The area fraction and number density of recrystallized cube grains increase continuously with increasing strain, but the cube grain size equals the average grain size independent of prior strain. The recrystallization advantage of cube grains decreases rapidly with increasing misorientation from the ideal cube component. The technological relevance of this misorientation dependence and its possible micro-mechanical origins are discussed.     

Influence of Hydrogen on Dislocation Structure in a Duplex Stainless Steel

The dislocation structure of specimens of aduplex stainless steel with precharged hydrogen hasbeen investigated by transmission electronmicroscopy.The result showed the hydrogen acti-vated dislocation source on phase boundaries offerrite/austenite and makes certain dislocations inaustenite grains move.The dislocation appearancesin each phase in specimens uncharged withhydrogen have been given for comparison.     

Microstructural evolution over a large strain range in aluminium deformed by cyclic-extrusion–compression

Polycrystalline pure aluminium (99.99%) has been deformed at room temperature by the Cyclic-Extrusion–Compression (CEC)-method to strains in the range 0.9–60 (1–67 cycles). At different strains, the microstructure and local crystallography have been characterised in particular by transmission electron microscopy. It has been found that the microstructure develops from a cell block structure into an almost equiaxed structure of cells and subgrains, that the spacing between the boundaries subdividing the structure is almost unaffected by the strain and that the misorientation across these boundaries increases with the strain over the whole strain range. At the largest strain, the average misorientation across the deformation induced boundaries is 25°. The flow stress in compression is measured after the cyclic deformation and it is found that the flow stress increases with strain towards a saturation level which is reached at a relatively low strain. The discussion comprises the effect of deformation mode and plastic strain over a large strain range on the microstructural evolution and mechanical behaviour of aluminium.     

TEM studies of strain-induced martensitic formation in fatigued Fe-18Mn alloy

Austenitic Fe-18Mn alloy was cyclically deformed at various total strain amplitudes. The structural changes induced by cyclic straining in Fe-18Mn alloy were investigated by transmission electron microscopy. At a low strain amplitude of 0.4%, the formation ofε-martensite associated with deformation twins and stacking faults was observed in this alloy. As the applied strain amplitude was increased to 1.0%,α′-martensite embryos were induced in the alloy investigated. These embryos coalesce into a lath structure upon subsequent cyclic deformation.     

Deformed microstructure evolution in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s

Deformed microstructure in AM60B Mg alloy under hypervelocity impact at a velocity of 5 km s⁻¹ were investigated through optical microscope, scanning electron microscope and transmission electron microscope. The results show that four deformed zones around the crater can be classified based on the different deformed microstructure, including ultrafine grain zone, ultrafine grain and deformation twin zone, high and low density deformation twin zones. The dislocation slipping, deformation twins and ultrafine grains are the dominant components in the four deformed zones, and the evolution of deformed microstructure is speculated based on the deformed microstructure observed in four zones. Slipping and twinning play a critical role for the formation of the dynamic recrystallized grains, and twinning-induced rotational dynamic recrystallization mechanism is thought to be the main mechanism for the formation of ultrafine grains. The microhardness and dynamic compressive strength in different deformed zones were measured, and the high microhardness and yield strength in ultrafine grain zone should be attributed to the strain hardening and grain refining.     

AZ31B镁合金压-压循环载荷下变形行为及变形机制演化EI北大核心CSCD

为探讨AZ31B挤压态镁合金棒材沿径向取样的循环变形特征,开展了0.75%,1.0%,2.0%和4.0%应变幅下应变控制的非对称压-压循环变形实验。结果表明:在小应变幅(0.75%,1.0%)下,循环变形的滞回曲线表现出较好的对称性;在大应变幅(2.0%,4.0%)下,滞回曲线对称性差,且在滞回曲线上出现拐点;随着循环周次增加,塑性应变幅呈现下降趋势,材料均表现出循环硬化行为,在小应变幅下循环拉伸阶段对材料硬化率远大于压缩阶段的硬化率,而在大应变幅下这种区别并不明显。分析表明,沿径向取向的〈1120〉丝织构镁合金,小应变幅下位错滑移在整个寿命周期内作用更大;大应变幅下,随着塑性变形的增加,循环过程中变形机制发生演化,较低临界剪切应力(critical resolved shear stress,CRSS)的基面位错和拉伸孪生不能完全满足变形要求,较高CRSS滑移系启动及残余孪晶使得滞回曲线出现拐点;循环变形过程中不完全的孪生-去孪生过程使基体中存在大量残余孪晶,影响了循环变形过程的硬化率,同时降低了疲劳寿命。     

Low-cycle fatigue behavior and microstructural evolution in a low-carbon carbide-free bainitic steel

This paper deals with the cyclic deformation behavior and microstructural evolution in a low-carbon carbide-free bainitic steel with two different microstructures. Low-cycle fatigue tests were performed at room temperature at various strain amplitudes under total strain control. The variations of the amount of retained austenite and the substructures versus the number of fatigue cycles were evaluated by the X-ray diffraction technique and electron microscopy. Fatigue test results demonstrate that the two microstructures exhibit very similar cyclic stress responses, i.e. initial cyclic hardening followed by cyclic softening or by cyclic saturation and softening till failure, depending on the strain amplitude applied. Parametric studies of the microstructure–property relationship indicate that the major cause for the initial cyclic hardening is neither martensitic transformation nor increased dislocation density. Based on these results and considering the initial high density of dislocations, which are pre-existent and mobile in the starting microstructure and which are entangled, rearranged or annihilated with cycling, the mechanisms responsible for the initial cyclic hardening followed by softening are analyzed.     

Effect of vanadium micro-alloying on the microstructure evolution and mechanical properties of 718H pre-hardened mold steel

The effects of different contents of vanadium(V)(0.1,0.2,and 0.3 wt%) on the microstructure evolution and mechanical properties of 718H steel were investigated.The precipitate was characterized by means of atom probe tomography(APT) and bright-field transmission electron microscopy(TEM).The increase in V content has great benefits for strength,but has an adverse effect on impact toughness.The strength increase can be attributed to the influence of V addition on dislocation density,misorientation gradient,and fine scale MC precipitates.Precipitation strengthening mainly contributes to the V-added steel by analyzing various strengthening mechanisms.However,fine scale MC precipitates can pin dislocation leading to a decrease in its mobility.A large number of immovable dislocations will increase the dislocation accumulation,internal stress and brittle cracking,resulting in a gradual decrease in impact toughness with the V addition.In addition,compared with V-free steel,the dissolved V content in austenite decreases the grain boundary energy and inhibits the diffusion of the C atoms,ultimately reducing the transformation range of pearlite(P).     

EBSD Investigation on Oriented Nucleation in IF Steels

The mechanism responsible for the formation of recrystallization texture in cold-rolled Ti bearing interstitial free （IF） steel sheets was investigated using electron back-scatter diffraction （EBSD）. In addition, the origin of nuclei with specific orientations was studied. The formation of recrystallization texture was explained by oriented nucleation. Most nuclei have a high misorientation angle of 25-55° with the surrounding deformed matrices, but no specific orientation of misorientation axis between the nucleus and the surrounding deformed matrix is observed. The stored energy of deformed grains is in the decreasing order of the {111}〈112〉,{111}〈110〉, {112}〈110〉 and {001}〈110〉 orientations. New {111}〈110〉 grains are nucleated within deformed {111}〈112〉 grains and new {111}〈112〉 grains originate in the deformed {111}〈110〉 grains.     

18.5%Cr高Mn型节镍双相不锈钢大变形热压缩的组织和再结晶行为

使用热模拟试验机在1123~1423 K/0.01~10 s^-1变形条件下对18.5%对Cr高Mn节镍型双相不锈钢进行了变形量为70%的大变形热压缩,研究其在热变形过程中两相的亚结构特征和软化机理。结果表明,在0.01~0.1 s^-1/1123~1223 K范围的热压缩软化以铁素体相的再结晶为主,而在0.1 s^-1/1323~1423 K和10 s^-1/1223 K范围的热压缩软化以奥氏体相的再结晶为主。在变形温度为1223 K、应变速率由0.01 s^-1增大到10 s^-1的条件下铁素体相内的位错缠结向胞状结构演化并出现位错线,奥氏体相内的亚结构则转变为细小的再结晶晶粒。应变速率为0.1 s^-1、变形温度由1123 K提高到1323 K时铁素体相内的位错增加,变形晶粒向胞状组织演化而奥氏体相内的位错减少,由回复组织转变为再结晶组织。根据热变形方程计算出表观应力指数n=7.13,热变形激活能Q=514.29 kJ/mol,并建立了Z参数关系本构方程。根据加工硬化率得到再结晶临界条件,并确定了Z参数与再结晶临界条件的关系。对热加工图的分析结果表明,随着变形量的增大失稳区逐渐减小,最佳加工区域为1348~1423 K/1~10 s^-1,功率耗散系数大于0.4。