铜双晶体循环变形的晶界强化效应

用组合双晶体（ＣＢ）及实际双晶体（ＲＢ）研究平行晶界在铜双晶体循环变形过程中的强化作用，结果表明随双晶体变小，其循环应力升高，晶界作用增强，扫描电镜观察发现晶界附近出现以次滑移为主的晶界影响区，结合双晶体的循环应力－应变响应和晶界影响区，讨论了双晶体的晶界强化机制。     

FATIGUE CRACKING POSSIBILITY ALONG GRAIN BOUNDARIES AND PERSISTENT SLIP BANDS IN COPPER BICRYSTALS

In this paper, the fatigue cracking possibility in different kinds of copper bicrystals with large-angle grain boundaries (GBs) and copper multicrystals containing some low-angle GBs are compared. The results showed that the fatigue cracks, in the copper bicrystals, always nucleated firstly along GBs no matter whether the GBs are perpendicular or parallel to the stress axis. Whereas, for the copper multicrystals containing low-angle GBs, the persistent slip bands (PSBs) are always the preferential sites to initiate fatigue cracks no matter whether low-angle GBs are perpendicular or parallel to the stress axis.
Additionally, the fatigue lives of the GBs, and the [1¯23] and [3¯35] grains in the [1¯23] ⊥ [3¯35] and [5¯913] ⊥ [5¯79] bicrystals were measured at different cyclic stresses and strain amplitudes. The results show that intergranular fracture always occurred prior to transgranular fracture in those bicrystals. The fatigue lives increased in the order of the GB, the [1¯23] and the [3¯35] grains in the [1¯23] ⊥ [3¯35] bicrystal under cyclic tension–tension loading. On the other hand, the fatigue life of the GB in the [5¯913] ⊥ [5¯79] bicrystal is about two to three times higher than that in the [1¯23] ⊥ [3¯35] bicrystal. Based on these experimental results from the copper bicrystals and multicrystals, it is indicated that the possibility of fatigue cracking increased in the order of low-angle GBs, PSBs and large-angle GBs. It is suggested that both the PSB–GB mechanism and the step mechanism required for GB fatigue cracking were questionable, and the interaction modes of PSBs with GBs may be more important for intergranular fatigue cracking.     

Cyclic Deformation and Fatigue Damage Behavior of Cu Monocrystals, Bicrystals and Tricrystals

1.IntroductionMostofmechanisticstudiesoncyclicdeformationhavebeencarriedoutonCucrystalsinsingle-slipori-entations.ThereareseveralexcellentreviewarticleswhichemphasizeddifferentaspectsofthesubjectI1~6].Whenawell-annea1edCucrystalsuitablyorientedforsingleslipissubjectedtocyclicdeformationbetweenconstantplasticstrainamplitudelimits7platroomtemperatureinair,itwillhardenundertheactionofcyclicstraining.Thepeakstressincreasesrapidlyatfirstandthenmoreslowly,approachingavalue,thesaturationstressTs.Ap…     

Grain boundary effects on cyclic deformation and fatigue damage

The cyclic deformation behaviors of single- and polycrystals were well documented in the past three decades. It has been recognized that there existed great difference in the fatigue damage mechanisms between single- and polycrystals, which can be mainly attributed to the effects of grain boundaries (GBs) and the crystallographic orientations. In the present work, a series of research work mainly on copper bicrystals with various GBs and different component crystals have been systematically investigated, including the macroscopic cyclic stress-strain responses and fatigue damage mechanisms in micro-scale. Firstly, direct evidence is offered to show the obvious strengthening effect caused by the large-angle GBs during cyclic deformation. The data of cyclic stress-strain responses will be presented to show the effects of the GBs and the crystallographic orientations in the macro-scale. Next, the influence of various GBs on fatigue cracking behavior will be considered for the crystalline materials. Clear evidences are shown that the interactions of persistent slip bands (PSBs) with various GBs play a decisive role in the intergranular fatigue cracking during cyclic deformation. It is suggested that the intergranular fatigue cracking strongly depends on the interactions of PSBs with GBs in fatigued crystals, rather than the GB structure itself. The underlying fatigue damage mechanisms were further discussed in terms of the different interactions between PSBs and three types of GBs.     

Role of grain boundaries in determining strength and plastic deformation of yttria-stabilized tetragonal zirconia bicrystals

Mechanical properties of yttria-stabilized tetragonal zirconia (YSTZ) bicrystals under compressive loading are investigated by atomistic simulations. Previous studies on deformation of single-crystal YSTZ showed that some specific orientations promote dislocation emission, tetragonal to monoclinic phase transformation, or both. In this work, nanograins with different orientations are selectively combined to generate bicrystals with various grain boundaries (GBs). Simulation results show that regardless of orientation of nanograins, the strength of YSTZ bicrystals is higher when the GB plane is parallel to the loading direction, and in the case of [011]/\( \left[ {01\bar{1}} \right] \)-oriented YSTZ bicrystal, the strength even exceeds that of the single crystal. Independent plastic deformation of individual grains and their interactions at the GB plane are believed to be responsible for the observed increase in strength. GB plane inhibits the volume expansion of transformed monoclinic phase and therefore serves as a source of strengthening. In contrast, YSTZ bicrystal displays softer behavior when GB plane is perpendicular to the loading direction. GB plane acts as the source of softening by initiating local amorphous phase formation.     

铜三晶体的循环形变行为及位错组态

采用逐级增幅方法研究了「０１１」同轴铜三晶体及一种非同轴取向平行三晶交线铜三晶体的循环形变行为,并观察了三晶交点和晶界附近的位错组态。为了对比,也研究了两种取向三晶体的组元晶粒双晶体和单晶体试样的循环形变行为。对于「００１」同轴取向平行三晶交线三晶体及其组元双晶体和单晶体,其循环硬化曲线几乎重合,其循环饱和应力应变曲线（ＣＳＳＣ）也相差不大,由于各滑移系之间的位错反应生成Ｌｏｍｅｒ－Ｃｏｔｔｅｌ锁,阻碍位错运动,所以三晶交线和晶界对轴向饱和应力几乎没有强化作用。对于「００１」取向晶体,在（００１）观察面上,可以看到很多较短的一段一段的位错墙结构,各段之间互不连通,这是各滑移系之间的错反应强烈,生成不动的位错锁的结果。这也是三晶交线和晶界无明显强化作用的原因。对于非同轴取向铜三体及组元晶粒双晶体、单晶体的循环形变     

Influence of orientation,temperature, and strain-rate on the yield strength of Fe2B single crystals

The deformation behaviour of Fe₂B single crystals with different orientations has been studied by the compression test over the temperature range 800 to 1100° C. Three slip systems (1 1 0) [0 0 1], (1 1 0) [1¯1 1], and (hkl) [1¯11] have been found to be active in the present work. The operation of (h k l) [1 ¯1 1] slip system appears to require about ten times higher stress than those of (1 1 0) [0 0 1] and (1 1 0) [1 ¯1 1] slip systems. The temperature and strain-rate dependence of the yield stress suggests that the Peierls mechanism controls the plastic deformation of Fe₂B crystals. The activation energy for the thermally activated deformation of Fe₂B varies between 6 and 10 eV and the activation volume between 10b ³ and 160b ³. The remarkably high activation energies have been discussed in connection with the large Burgers vectors of Fe₂B.     

Cyclic deformation response and dislocation structure of Cu single crystals oriented for double slip

Cyclic deformation behavior of double-slip oriented Cu single crystals with a stress axis in the [034] direction was investigated under plastic strain control mode for a shear strain amplitude range of 1 × 10⁻⁴ to 5 × 10⁻³. Dislocation structures in the tested samples were observed using a transmission electronic microscope. It has been found that the effect of the operation of critical slip in these [034] crystals on cyclic responses and dislocation structures is nearly the same as that of increase in strain amplitude. The nucleation stress and number of cycles for PSB formation at each specific strain amplitude in the double-slip oriented crystals were found to be both considerably lower than those observed in single-slip oriented crystals. This observation is in a good agreement with the Kuhlmann-Wilsdorf and Laird analysis, in that the formation of PSBs is associated with glide behavior on the secondary slip system. A dislocation “cord” structure has also been observed and is believed to be caused by the operation of the cross-slip system during cyclic deformation. Labyrinth wall structures were found to form with increase in strain amplitude by the operation of critical slip and cross-slip systems. However, the formation of labyrinth structure was suppressed by the coplanar slip at high strain amplitudes.     

Deformation mechanism in αCu-Al single crystals and bicrystals under scratching with pyramidal indentor

In order to elucidate the deformation mechanism of materials in abrasive wear process, scratching tests were carried out on the (111) face of Cu-14.7 at % Al single crystals and 13 b bicrystals with pyramidal indentor. In the scratching on the single crystals three kinds of scratching directions, [11 ¯2], [¯1¯12] and [¯110], were chosen. In the case of bicrystals, the [1 ¯21] and [1¯10] directions were adopted. After scratching the dislocation structure, slip trace patterns and surface profiles across the scratched track were examined. In addition, the dislocation distributions inside the crystals were revealed by successively removing thin layers and developing etch pits on the exposed surface.The slip traces on either side of the scratched track are produced more extensively than those around the indented point. These slip traces are observed only in the surface of about 100 m deep, and are not observed in the deeper area. It is found that the microscopic deformations produced due to the scratching consist of three kinds of deformation: formed by indentation, formed by both normal and frictional stresses in the surface layer and formed by stress which is caused in certain limited depth.The swells of the material were produced in the front of the indentor due to the slips on the {111} crystal faces which are arranged so as to be diverging into the inside. The azimuths of formation of the swells are [11¯2] and [¯211] in the [11¯2] and [¯110] scratching respectively and [1¯21], [¯211] in the [¯1¯12] scratching.In the scratching of the bicrystal, the propagation of dislocation in the surface layer of the one side crystal is obstructed by the grain boundary. The microscopic deformation range on the dislocation order is affected by the distribution density of the grain boundaries.     

An investigation into the influence of grain boundary misorientation on the tensile strength of SiC bicrystals

In this work, molecular dynamics (MD) and Car-Parrinello molecular dynamics (CPMD) simulation-based analyses are performed to understand the influence of grain boundary (GB) misorientation on the tensile strength of SiC bicrystals. The tensile strength is governed by the changes in electron density and bond strength of atoms in GBs. An investigation of dislocation activity during mechanical deformation shows that the extent of the propagation of dislocations across the bicrystal grains is directly proportional to the extent of GB misorientation. An analytical relation that predicts the tensile strength as a function of GB misorientation is developed.     

一个特殊单滑移取向铜单晶疲劳位错结构研究

为了更细致地揭示面心立方金属单晶体的循环变形机制,利用扫描电镜电子通道衬度(SEM-ECC)技术观察研究了Schmid因子为0.5的[41841]单滑移取向铜单晶体的循环饱和位错结构.实验表明,在单滑移铜单晶体中,胞结构除了在高应变幅下的循环变形中出现外,还可能出现在循环应力-应变(CSS)曲线平台区的较低塑性应变幅下.驻留滑移带(PSBs)会随应变幅的增大而在试样表面聚集成内部含有位错胞的粗滑移带,带内的位错胞结构被认为是由于带内滑移阻力增大引起的应变集中所致形成的.此外,CSS曲线高应变幅区起始部分对应的循环饱和位错结构观察揭示出迷宫结构和胞结构是由PSBs逐渐演变而成的.     

Some aspects of the static and dynamic compressive behaviour of β-brass single crystals

The compressive behaviour of β-brass single crystals has been investigated in uninterrupted static and dynamic tests and in interrupted tests using static-static, static-dynamic, dynamic-dynamic and dynamic-static loading sequences. Static and dynamic strain-rates were 1.5×10^?4 and 3.2×10³ sec^?1 respectively. Slip traces on the statically deformed crystals were wavy and deformation occurred by single slip on either (ī01) [111] or (¯211) [111] or by a transition mode involving both (ī01) [111] and (¯211 [111]. Except for anomalous behaviour in the dynamic reload following static preload the dynamic slip traces were straight with deformation occurring by multiple slip on four {110} planes involving two 〈111〉 directions. It is shown that there is no direct causal relationship between the lower work-hardening rate and level of flow stress and the crystallography of slip in dynamic deformation. The work-hardening rate and flow stress in static and dynamic loading are rather determined by the dynamics of the deformation. The differences in the substructural features as observed by transmission electron microscopy arise principally from the differences in the slip modes and cannot be interpreted as controlling the stress-strain behaviour. The low work-hardening rate and flow stress in dynamic deformation is believed to be due to the production of short-lived disorder. The absence of a/2〈111〉 dislocations in thin foils is explained in terms of the fast reordering reaction in β-brass.     

Slip behaviour near a grain boundary in high-cycle fatigue of poly-crystal copper

In order to investigate the characteristic slip behaviour near a grain boundary in high‐cycle fatigue, a high‐cycle fatigue test is carried out using a copper poly‐crystal specimen, which consists of several tens of grains. Seventeen persistent slip bands (PSBs) are observed along the grain boundaries. Their location and the activated slip system are different from those expected by the Schmid factor. After the fatigue test, the crystalline orientation and the three‐dimensional shape of each grain are specified by the repetition of polishing and observation by means of an orientation‐imaging microscope (OIM). A finite‐element method (FEM) analysis is also conducted for the specimen with the same orientation and shape of grains taking into account the anisotropy. This analysis reveals that the shear stress concentrates near the grain boundaries where the PSBs are formed. The activated slip systems predicted by the maximum resolved shear stress agree well with those observed in the fatigue experiment. Thus, the characteristic slip near the grain boundary in the poly‐crystal is governed by the concentrated resolved shear stress on the specific slip system due to the deformation constraint by neighbouring crystals.     

铜单晶体在循环形变中形成的形变带

系统研究了循环形变铜单昌体中宏观形变带的产生规律和特征及相应的位错结构,并对其形成机制进行了综合探讨。结果表明,在不同取向铜单昌体的循环形变中,形变带ＤＢＩ近似沿主滑移面发展。而ＤＢⅡ的惯习面接近传统的扭折面｛１０１｝,两者成严格的正交分布。     

Half-cycle slip activity of persistent slip bands at different stages of fatigue life of polycrystalline nickel

The slip activity of persistent slip bands (PSBs) in polycrystalline nickel was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The half-cycle slip activity as well as the local shear strain amplitudes was investigated after half-cycle deformation at different numbers of cycles in the domain of stress saturation. Moreover, the fraction of grains containing cumulated PSBs and the accumulated volume fraction of PSBs was estimated depending on the number of cycles during fatigue life. The volume fraction of active PSBs during half-cycle deformation is significantly lower than the cumulated PSB volume and decreases with increasing number of cycles. Additionally, an increasing localization of cyclic plastic strain within the PSBs was observed. However, with increasing number of cycles the average local shear strain amplitude remains almost unchanged. Thus, PSBs in polycrystals are subjected to a life history which is characterized by active and inactive periods of their half-cycle slip activity during cyclic deformation at different stages of the saturation state.     

Cyclic deformation behavior of double-slip-oriented copper single crystals I: coplanar double slip orientation on 011-1̄11 side of the stereographic triangle

The cyclic deformation behaviors of [2̄33] coplanar double-slip-oriented and [4̄ 18 41] single-slip-oriented copper single crystals were investigated at constant plastic shear strain amplitude γ_pl in the range of about 10⁻⁴–10⁻² at ambient temperature in air. It was revealed that the cyclic deformation behavior of copper single crystal oriented on the 011-1̄11 side is distinctly dissimilar from that on the 001-1̄11 and 001-011 sides in the stereographic triangle. The plot of initial hardening rate θ_0.2 against γ_pl of [2̄33] crystal exhibits two regions as presented for single-slip-oriented crystals. The critical strain amplitude (≈3.5×10⁻³), corresponding to the occurrence of the secondary hardening stage in the cyclic hardening curve of the [2̄33] crystal, was found to be an intermediate value between that for single-slip-oriented single crystals and polycrystals. The result shows that the cyclic hardening behavior of the [2̄33] crystal, as compared with that of single-slip-oriented crystals, is more close to that of polycrystals. Instead of a clear plateau, the cyclic stress–strain (CSS) curves of the [2̄33] crystals shows a quasi-plateau over the range of about 3.0×10⁻⁴–2.0×10⁻³, which would be greatly attributed to the mode of dislocation interactions between slip systems operating in the crystal. The habit plane of two types of deformation bands DBI and DBII, formed in the cyclically deformed [2̄33] crystal, are perpendicular to each other strictly, and they develop with increasing applied strain amplitude.     

Effect of the grain boundary on the evolution of deformation in a bicrystal

The role of grain boundary constraint in strain localization and concomitant constitutive response was examined by performing a series of uniaxial compression tests on a tantalum bicrystal. Tantalum single crystals were diffusion bonded to form a (011) 90^∘ twist boundary that was compressed along the common [011] direction. The plastic deformation resulted in the creation of deformation bands away from the highly constraining grain boundary, resembling those bands known from single crystal plastic deformation. Near the grain boundary, such deformation band formation could not be detected. Instead a distinctive pattern of crystal lattice rotation was observed that filled a rather large volume (several millimeters in size) around the bicrystal grain boundary. The internal deformation band structure as well as the crystal lattice rotation pattern near the bicrystal grain boundary were characterized and found to give greater rates of work hardening in the neighborhood of the grain boundary.     

Size and microstructure effects on the mechanical behavior of FCC bicrystals by quasicontinuum method

The effects of structure and size on the deformation of <110> tilt bicrystals in copper are investigated by concurrent multiscale simulations at zero temperature. In the simulation of eleven grain boundary (GB) structures, a direct relation is shown between structural units and sliding at GBs. We find that GB sliding operates by atom shuffling events localized on one particular type of structural units, which are present in the GB period. When this type of unit is absent, the GB deformation process occurs by migration, or GB-mediated nucleation of partial dislocations with no sliding, depending on the initial GB configuration. The elastic limit causing sliding is found to vary slightly at zero temperature, but no correlation was obtained with the GB energy at equilibrium. Additionally, both modulus of rigidity, and elastic limit remain constant as the bicrystal size varies from 1 nm up to 25 nm. However, differences in the stress relaxation after sliding are observed with respect to the size.     

Cyclic Saturation Dislocation Pattern Observation in the Vicinity of Grain Boundaries by Electron Channeling Contrast Technique

State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Chinese Academy of Sciences,Shenyang, 110015, China)Abstract:The cyclic saturation dislocation patterns within grains and in the vicinity of low-angle grain boundaries in fatigued copper crystal were successfully observed by electron channeling contrast technique in SEM. The results show that the dislocation patterns within grains consisted of typical two-phase structure, i.e. persistent slip bands (PSB) and veins. With increasing plastic strain amplitude (γp1 ≥1.7×10-3), large amount of PSBs and regufar dislocation walls were observed.The dislocation walls and PSBs could cross through the low-angle grain boundaries continuously except that the dislocation-free zone (DFZs) appeared at some local regions. Combining with the cyclic stress-strain response and dislocation patterns, the effect of low-angle grain boundaries on cyclic deformation behavior was discussed.     

Grain Boundary Triple Junction Effects on the Fatigue Deformation and Cracking Behaviors of Copper Tricrystal and Tetracrystals

Cyclic symmetrical tension-compression fatigue tests in an axial plastic strain range of 2.0×10-4 to 1.5×10-3 were performed on three copper tetracrystal specimens containing two grain boundary triple lines as well as one copper tricrystal specimen employing a multiple step method. Experimental results show that the strengthening effect of triple junction (TJ) on axial saturation stress increased with increasing plastic strain amplitude. The strengthening effects owe much to the strain incompatibilities at TJ. The cyclic stress-strain (CSS) curves of tetracrystals are higher than that of tricrystal. At low strain amplitude, deformation at TJ is smaller than that near grain boundary (GB), which results in that the width of TJ effect zone is smaller than that near GB. Whether GB split or not is associated with the angle between GB and loading axis, activation of slip systems beside GB and the accommodation and annihilation of residual dislocations on GB planes.