Change in the fractal dimension of the grain boundaries in pure Zn polycrystals during creep

The effects of creep deformation on the shape of grain boundaries were investigated on pure Zn polycrystals at 373 K. The fractal dimension of the grain boundaries D(1D2) was estimated by the box-counting method. There was then discussion on the relationship between the value of D, the microstructures, and the creep or plastic strain in the deformed specimens of metallic materials.The fractal dimension of the grain boundaries (D) increased with increasing the creep strain in pure Zn polycrystals, but the increase in the value of D levelled off when the creep strain exceeded about 0.30. The value of D decreased as the creep stress decreased. The increase in the value of D with the creep strain was correlated with the increase in the density of slip lines in the grains that formed the ledges and steps on grain boundaries. The value of D on the plane in parallel with the tensile axis was slightly larger than that on the plane transverse to the tensile axis. The mean shear strain on grain boundaries estimated from the value of D was correlated with the creep or plastic strain in the deformed specimens.     

Fatigue damage analysis in a duplex stainless steel by digital image correlation technique

Strain field measurements by digital image correlation today offer new possibilities for analysing the mechanical behaviour of materials in situ during mechanical tests. The originality of the present study is to use this technique on the micro-structural scale, in order to understand and to obtain quantitative values of the fatigue surface damage in a two-phased alloy. In this paper, low-cycle fatigue damage micromechanisms in an austenitic-ferritic stainless steel are studied. Surface damage is observed in real time, with an in situ microscopic device, during a low-cycle fatigue test performed at room temperature. Surface displacement and strain fields are calculated using digital image correlation from images taken during cycling. A detailed analysis of optical images and strain fields measured enables us to follow precisely the evolution of surface strain fields and the damage micromechanisms. Firstly, strain heterogeneities are observed in austenitic grains. Initially, the austenitic phase accommodates the cyclic plastic strain and is then followed by the ferritic phase. Microcrack initiation takes place at the ferrite/ferrite grain boundaries. Microcracks propagate to the neighbouring austenitic grains following the slip markings. Displacement and strain gradients indicate probable microcrack initiation sites.     

The influence of temperature and grain size on substructure evolution in stainless steel 316L

The flow stress of polycrystals is controlled by the processes occurring in the grain interior as well as in the mantle, i.e. at the grain boundary and its immediate vicinity. The early stages of evolution of dislocation substructure in these two regions with strain in 316L stainless steel polycrystals have been studied at 293 K, 673 K and 1123 K representing the low temperature thermal, the intermediate temperature athermal and the high temperature thermal regimes respectively. Specimens with grain sizes of 4 and 12 m were employed to determine the effect of grain size.Transmission electron microscopy studies on deformed specimens show the different roles of grain boundary and grain interior in different temperature regimes. In the low temperature regime grain boundaries act as obstacles to moving dislocations and as such high density of dislocation is found in the grain boundary vicinity. In the intermediate temperature regime the dislocations which are easily spread into the grain interior rearrange to form cell walls. In the high temperature regime grain boundaries transform to the equilibrium state and do not contain any grain boundary dislocations, and the distribution of dislocations within grains is homogeneous at all strains. Significantly higher values of dislocation densities in the vicinity as well as in the grain interior were found in the finer grain size material in the whole strain region employed.     

Microstructural evolution during formation of ultrafine grain structures by severe deformation

Abstract

High resolution electron backscattered diffraction analysis has been used to compare the evolution of the deformed state during severe deformation of two model materials: an Al–0.13 wt-%Mg alloy and an interstitial free steel. The alloys were deformed by equal channel angular extrusion up to a total effective strain of 10, at 20 and 500°C, respectively. At strains of <2, new high angle boundaries were formed from primary deformation bands. At strains of 2–5 (corresponding to high conventional strains) the new high angle grain boundaries, associated with the deformation bands, rotated towards the billet axis creating a lamella structure. Narrow bands of fine grains were also formed in unstable crystal orientations. With increasing strain the separation of the lamella high angle boundaries reduced until at very high strains their spacing was equal to the subgrain width and the long ribbon grains broke up into shorter segments. After an effective strain of 10 the microstructures for both materials consisted of submicron grains with an aspect ratio of ～3. In general it was observed that the interstitial free steel refined more rapidly with strain than the aluminium alloy.     

Mg-0.4Zn镁合金挤压板拉伸变形组织的演变

用电子背散射衍射(EBSD)技术结合原位拉伸,研究了在0％～20％应变条件下,Mg-0.4％Zn二元镁合金晶界、织构和裂纹的变化.结果 表明,在拉伸应变从0％增加到20％的过程中,随着应变量的增大材料微观组织中的孪晶逐渐增加.孪晶的类型以{10-12}拉伸孪晶为主;这种孪生使材料的组织织构类型发生了显著的变化,随着应变...     

On intergranular mechanical interactions and the theory of deformation crystallography of metals

The equilibrium of intergranular stress and strain can be realized simultaneously, whereas five independent slip systems of the Taylor principle and the criterion of minimal internal work are unnecessary. In fact, the Taylor principle applied in current theories is incorrect both in practice and theory, in which the activation mechanism of plastic deformation systems must violate the Schmid’s law and deviate from the elastic–plastic characteristics of deformed matrix. The intergranular reaction stress (RS) during deformation can be calculated according to Hooke’s law and elastic limit without additional subjective presupposition, therefore the RS theory is established intuitively. Under the combination of the RS (the intergranular elastic effect) and the external loading the slips penetrating grains are activated and produce deformation texture, but certain non-penetrating slips near grain boundaries will become active (the intergranular plastic effect) and produce some random texture when a RS reaches the yield strength of grains. The RS theory is simple, intuitive and reasonable, based on which the texture simulation can well reproduce the texture formation of various metals under different external loadings and under different crystallographic mechanisms.     

Grain refinement in as-cast AZ80 Mg alloy under large strain deformation

Grain refinement in as-cast AZ80 magnesium alloy under large strain deformation was studied by hot multiple forging (MF). The results show that during the deformation there exists a critical strain controlling the degree of the homogeneity, which is in the range of 2–2.4. A homogeneous microstructure with fine dynamic recrystallized grains can be attained when the applied strain exceeds the critical strain and after that, it is difficult to get more grain refinement further. A main characteristic of microstructure evolution is directly associated with grain splitting due to the formation of microbands that develop in various directions. Such microbands intersect each other during hot MF, resulting in continuous subdivision of coarse grains into misoriented fine domains. Further deformation leads to increase in the number and misorientation of these boundaries and finally almost full development of fine equiaxed grains in high strain. New grains are concluded to be evolved by a kind of continuous reaction, that is essentially similar to continuous dynamic recrystallization.     

Grain orientation, deformation microstructure and flow stress

Dislocation structures in deformed metals have been analyzed quantitatively by transmission electron microscopy, high-resolution electron microscopy and Kikuchi line analysis. A general pattern for the microstructural evolution with increasing strain has been established and structural parameters have been defined and quantified. It has been found that two dislocation patterns co-exist in all grains, however, with very different characteristics dependent on grain orientation. This correlation with the grain orientation has been applied in modeling of the tensile flow stress and the flow stress anisotropy of fcc polycrystals. In conclusion some future research areas are briefly outlined.     

Analysis of activated slip systems in fatigue nickel polycrystals using the EBSD-technique in the scanning electron microscope

By comparison of the observed trace angles of active slip planes with the expected traces in plastically deformed metal polycrystals, conclusions for the local stress state within the grains of polycrystalline aggregates can be drawn. The expected slip systems can be calculated when the local stress tensor and the orientation of the crystallites in the specimen space are known. In fatigued nickel polycrystals, the crystal orientation was determined by the EBSD (electron backscattering diffraction) method in the scanning electron microscope. It was shown that at the relatively small plastic strains under fatigue conditions the crystalline interactions do not essentially influence the local stress state in the grains, but the external uniaxial stress tensor remains valid in good approximation.     

The particle image tracking technique: An accurate optical method for measuring individual kinematics of rigid particles

We present a new approach to assess the two-dimensional motion of rigid bodies in granular materials. Although it was adapted from digital image correlation technique, the heart of the presented technique relies on specific treatments related to the discrete nature of grain-displacement fields. The code called Tracker has been developed to process the digital images and measure the in-plane displacement and rotation of each individual grain from one image to another. A remarkable feature is the use of a specific strategy that allows tracking all particles, without losing any of them (which is a typical problem when tracking assemblies of discrete particles over many images). This is achieved by a two-step procedure, where, in case of problematic tracking of a grain, the size of the search zone is increased in an adaptive manner, that is, taking into account the results of tracking in the neighbourhood of the particle. The accuracy of the measured displacements and rotations was tested on both perfect synthetic images and digital photographs of a sheared assembly of grains. An automatic procedure that corrects the lens distortion further improves the quality of the measurements. The accurate assessment of the grain kinematics opens very interesting perspectives, especially in the study of displacement fluctuations in granular media.     

Dynamic recrystallization of Ni-base alloys—Experimental results and comparisons with simulations

The dynamic recrystallization of the Ni-base alloy Böhler L306 VMR (Alloy 80A) in a transient state was investigated both by light microscopy and electron backscatter diffraction (EBSD), and the experimental results were compared with those from simulations. Subgrain structures of the size of the recrystallized grains were observed close to the grain boundaries of the original grains. With increasing strain a texture developed in the deformed fraction. Strong twinning was found in the recrystallized fraction, with area fractions of the twinned grains of around 80% for higher strains. Thus the measured grain sizes strongly depend on the handling of the twins. A pronounced increase in the average grain size of the recrystallized fraction with increasing strain (time) was only observed after twin removal. There was generally good agreement between the measured and the simulated results.     

数字图像相关测量钢绞线弹性模量的应用研究

该文利用非接触式数字图像相关方法测量了预应力钢绞线的弹性模量。测量过程中用CCD相机记录不同载荷下钢绞线表面的数字图像,再利用数字图像相关方法对所采集的序列数字图像进行分析从中精确提取不同载荷下钢绞线表面的纵向平均正应变。根据试验机的载荷信息、钢绞线的参考截面积和由数字图像相关方法测量的平均正应变绘制出钢绞线的应力-应...     

Q235碳素钢应变强化相变中铁素体的取向特征

利用背散射电子衍射取向成像技术分析了在热模拟单向压缩条件下Q235碳素钢应变强化相变中铁素体晶粒的取向（差）变化特点。结果表明，奥氏体的状态影响应变诱导出的铁素体的取向，奥氏体的动态再结晶使应变诱导出的铁素体的取向随机分布，在铁素体的内部基本上没有小角晶界，随着形变温度的降低和应变量的增大，铁素体取向的择优性变强，铁素体内部的小角晶界增加，这是细小铁素体动态再结晶的表现，相变，形变以及铁素体的动态再结晶都影响＜111＞方向线织构的形成。     

Microstructure evolution during dynamic recrystallization of hot deformed superalloy 718

Microstructure evolution during dynamic recrystallization (DRX) of superalloy 718 was studied by optical microscope and electron backscatter diffraction (EBSD) technique. Compression tests were performed at different strains at temperatures from 950 °C to 1120 °C with a strain rate of 10⁻¹ s⁻¹. Microstructure observations show that the recrystallized grain size as well as the fraction of new grains increases with the increasing temperature. A power exponent relationship is obtained between the dynamically recrystallized grain size and the peak stress. It is found that different nucleation mechanisms for DRX are operated in hot deformed superalloy 718, which is closely related to deformation temperatures. DRX nucleation and development are discussed in consideration of subgrain rotation or twinning taking place near the original grain boundaries. Particular attention is also paid to the role of continuous dynamic recrystallization (CDRX) at both higher and lower temperatures.     

镁合金AZ31高温形变机制的织构分析

利用X射线衍射和背散射电子衍射方法测定了镁合金AZ3l高温动态再结晶和超塑形变时的宏观和微观织构,分析了晶粒内部的形变机制.结果表明,在动态再结晶和超塑形变过程中,晶粒内部的滑移机制仍起重要作用,表现为再结晶晶粒出现择优取向以及一些晶粒可充分均匀形变成长条状.宏观织构的测定表明,具有不同初始织构的两类样品高温动态再结晶时,新晶粒有不同的取向择优过程,形成相似的织构;长条形变晶粒内部开动的滑移系也有一定的差异.分析了不同温度下相同的织构对应的不同塑变机理取向成像分析表明,基面织构取向的晶粒间总伴随着较高比例的小角晶界和30°(0001)的取向关系,这是六方结构的六次对称性限制了动态再结晶时(亚)晶粒间取向差的有效增大的缘故.     

Polycrystal models for the analysis of intergranular crack growth in metallic materials

In polycrystal materials the intergranular decohesion is one important damage phenomena that leads to microcrack initiation. The paper presents a mesoscale model, which is focused on the brittle intergranular damage process in metallic polycrystals. The model reproduces the crack initiation and propagation along cohesive grain boundaries between brittle grains. An advanced Voronoi algorithm is applied to generate polycrystal material structures based on arbitrary distribution functions of grain size. Therewith, the authors are more flexible to represent realistic grain size distributions. The polycrystal model is applied to analyze the crack initiation and propagation in statically loaded samples of aluminium on the mesoscale without the necessity of initial damage definition.     

Microstructure tailoring of Al0.5CoCrFeMnNi to achieve high strength and high uniform strain using severe plastic deformation and an annealing treatment

Ultrafine-grained alloys fabricated by severe plastic deformation (SPD) have high strength but often poor uniform ductility.SPD via high-ratio differential speed rolling (HRDSR) followed by an annealing treatment was applied to Al0.5CoCrFeMnNi to design the microstructure from which both high strength and high uniform strain can be achieved.The optimized microstructure was composed of an ultrafine-grained FCC matrix (1.7-2 μm) with a high fraction of high-angle grain boundaries (61 ％-66 ％) and ultrafine BCC particles (with a size of 0.6-1 μm and a volume fraction of 8 ％-9.3 ％) distributed uniformly at the grain boundaries of the FCC matrix.In the severely plastically deformed microstructure,the nucleation kinetics of the BCC phase was accelerated.Continuous static recrystallization (CSRX) took place during the annealing process at 1273 K.Precipitation of the BCC phase particles occurring concurrently with CSRX effectively retarded the grain growth of the FCC grains.The precipitation of the hard and brittle σ phase was,however,suppressed.The annealed sample processed by HRDSR with the optimized microstructure exhibited a high tensile strength of over 1 GPa with a good uniform elongation of 14 ％-20 ％.These tensile properties are comparable to those of transformation-induced plasticity steel.Strengthening mechanisms of the severely plastically deformed alloy before and after annealing were identified,and each strengthening mechanism contribution was estimated.The calculated results matched well with the experimental results.