Cyclic deformation behavior of non-isoaxial copper tricrystals and bicrystals

The cyclic hardening and saturation behaviors of copper tricrystals and bicrystals were investigated in strain-controlled multiple step tests. The results show that, for the inclined grain boundary (GB) bicrystal with single slip components, the cyclic stress strain (CSS) curve exhibits a plateau or quasi-plateau region, while the CSS curve of tricrystals shows no plateau. Observations of surface morphologies indicated that owing to the strain incompatibility of three grains, at lower strain amplitude the triple junction (TJ) retards obviously the primary slip in grains and makes deformation near it smaller than that near the bicrystal GB, while at higher strain amplitude slip can be distributed near the TJ homogeneously. The probability of crack initiation at the same TJ is closely related to the loading direction. The saturation dislocation structures of tricrystal specimens under the strain amplitude of the last step were explored by the electron channeling contrast technique in SEM (ECC-SEM). Loop patches with persistent slip band (PSB) ladders embedded were found even in the TJ vicinity for all grains. Dislocation-free zones (DFZ) occurred in the vicinity of TJ and GB, and the difference in shapes between them is due to the difference in internal stress field. Misoriented cell structure and dislocation wall structure were found near the crack tip, and the formation of them is associated with the cooperative action of crack tip, GB, grain orientation and the applied strain amplitude.     

疲劳裂纹尖端的位错结构

在双相钢物理短裂纹门槛区，观察到稳定的位错胞和墙结构；长裂纹门槛区，在铁素体／马氏体相界堆垛位错密度大，有形成位错胞的趋势．长裂纹扩展第二阶段，铁素体晶粒内具有单向滑移线（Ｒ＝０，－１）和正交网状（Ｒ＝－１）的位错结构，长裂纹扩展第三阶段，位错稀少，但单滑移、双交滑移位错线明显拉长，说明裂纹尖端位错组态是应变历史的产物．疲劳裂纹扩展门槛区形成的位错胞和墙是一亚稳态结构，与门槛循环应力应变处于动态平衡，也是一微观结构参数．     

Fracture toughness properties of high-strength martensitic steel within a wide hardness range

Fracture toughness tests were carried out on six grades of high-strength martensitic steel within the hardness range from 270 to 475 HB. Four types of tests were performed: (a) Charpy V-notch (CVN) impact over the temperature range −120 to 60 °C, (b) plane strain fracture toughness, K _IC , near the onset of crack growth, (c) fracture toughness, J _IC , near the initiation of slow crack growth, and (d) fracture toughness, J _iC , and crack tip opening displacement (CTOD _iC ) at the onset of slow crack growth using direct current potential drop (DCPD) technique. Further, true plane strain fracture toughness, K _o , at the onset of crack initiation was determined. Fracture toughness behavior including the measured and determined values of CVN, K _IC , K _o , J _IC , K _iC , and CTOD _iC have been interrelated over the entire hardness range using the various analytical and empirical correlations reported in the literature. The results indicate that the steel acquires the optimum fracture toughness properties at a hardness of 305 HB, corresponding to a tempering temperature of 630 °C. Further, the steel exhibits a slight 300 °C temper embrittlement phenomenon.     

IN-SITU OBSERVATION OF CRACK TIP DEFORMATION IN A DUPLEX STAINLESS STEEL

The development of dislocation structures in the plastic zone ahead of a crack tip has been in-vestigated in a duplex stainless steel during in-situ deformation experiments in a scanningtransmission electron microscope.It was found that the dislocation distribution wassignificantly different in the ferrite and in the austenite.In the ferrite grains,the dislocationsemitted by the crack tip may cross-slip out of the original slip planes and form a broad plasticzone.However,in the austenite,the dislocation free zone is small and the dislocations emittedby the crack pile up in its slip plane.The selection of slip systems at the crack tip depends onthe crack tip Schmid factors in both phases.But after large deformation,the selection of thesecond slip systems at the craek tip in austenite does not depend on the Schmid factors.     

CRACK INITIATION AND STEADY PROPAGATION OF ELASTIC-PLASTIC MATERIALS IN PLANE STRESS CONDITION

The crack tip stress-strain fields of the elastic-plastic cracked specimens have been analyzedusing finite element calculations.The crack initiation and steady propagation behaviours havealso been investigated by means of slip line pattern etching technique and mechanical tests.The results show that there are HRR near field and distant field in the crack tip region,andthe later depends on the specimen configuration.The crack initiation behaviour is controlledby a single parameter J.In contrast,the steady crack propagation is affected by the distantstrain field and can not be described by single parameter only.     

Strain and texture evolution during mechanical loading of a crack tip in martensitic shape-memory NiTi

In situ synchrotron X-ray diffraction measurements are used to create two-dimensional maps of elastic strain and texture, averaged over a compact-tension specimen thickness, near a crack tip in a martensitic NiTi alloy. After fatigue crack propagation, the material ahead of the crack and in its wake exhibits a strong texture, which is eliminated by subsequent shape-memory heat treatment, indicating that this texture is due to detwinning, the main deformation mechanism of NiTi. Upon subsequent application of a static tensile stresses, the highly textured zone reappears and grows around the crack tip as the applied stress is increased. At the highest applied stress intensity of 35 MPa m^1/2, large tensile strains are measured ahead of the crack tip and considerable elastic anisotropy is observed. This detwinning zone is similar to the plastic zone produced by dislocation slip present around cracks in other metals. The texture in this zone is not significantly altered after mechanical unloading, despite the development of substantial triaxial compressive residual strains in this zone.     

双相不锈钢中裂纹尖端塑性变形行为的透射电镜原位观察

在透射电子显微镜下对铁素体-奥氏体双相不锈钢薄膜试样进行了动态拉伸试验,原位观察了裂纹尖端塑性变形过程和位错分布,发现在铁素体相和奥氏体相中,裂尖位错组态有明显差异。在铁素体相中,裂尖无位错区较大,且让错易发生交滑移,在裂纹前端形成较大的塑性区。在奥氏体相中,裂尖无位错区较小,发射的位错在其滑移面内逆塞积。两相中裂尖滑移系的选择都与裂尖Schmid因子有关。但当形变较大时,奥氏体相中裂尖二次滑移系的选择不再为裂尖Schmid因子所支配。     

TEM OBSERVATION OF LOCAL DEFORMATION BAND AT CRACK TIP

The thin foil specimen of a ferrite-austenite duplex stainless steel was tensiled under transmis-sion electron microscope(TEM).It was found that both in ferrite and austenite the localdeformation band at crack tip was formed near to the crack propagating direction.Its forma-tion was related with the crack tip Schmid factor,dislocation shielding,latent hardening andhardening coefficient.When the crack tip emitted dislocations to a slip system by the action ofpure mode Ⅱ stress resolute,and the decreasing rate of hardening coefficient was suitable,lo-cal deformation band was easy to form.     

Evaluating the stress intensity factor using white light photoelastic experiment

In this study, a method is proposed to evaluate the stress intensity factor using isochromatics multiplied and extracted from original isochromatic images obtained from a white light photoelastic experiment. This method utilizes R, G and B isochromatics which are captured using a color CCD camera and an image processor in dark- and light-field circular polariscopes. First, for each color, the dark-field intensities are subtracted from the light-field intensities so that each point of the resultant image has a positive, zero or negative value. The isochromatics are then obtained as a series of points whose value is zero, which are extracted using image processing. The fringe orders are assigned to the extracted isochromatics using calibration curves. The stress intensity factor is determined using the Irwin method. This method is applied to an epoxy resin plate with a crack formed at one side under 3-point bending. The result shows that the stress intensity factor is accurately evaluated using R, G and B isochromatics near the crack tip. This article based on a presentation made in the symposium “The 4th International Conference on Fracture and Strength of Solid”, held at POSTECH, Pohang, Korea, August 16–18 under the auspices of Far East and Ocean Fracture Society (FEOPS),et al.     

The evolved microstructure ahead of an arrested fatigue crack in Haynes 230

The dislocation microstructure beneath surface slip traces produced by fatigue loading of Haynes 230 was revealed to be a function of distance from a crack tip. The microstructure beneath these traces evolves from planar slip bands with increasing dislocation density and decreasing interband spacing as the crack tip approaches one of refined subgrains and lamellar bands at and in the vicinity of the crack tip. Similarly, beneath fatigue striations the microstructure evolves from nanosized subgrains to a banded structure with increasing distance from the fracture surface. These structures are significantly different to those predicted to develop under fatigue loading of a planar slip material. The evolved structures are considered in terms of the microstructure generated by severe plastic deformation.     

铝合金搭接搅拌摩擦焊接头拉伸变形的数值模拟

通过对2A12铝合金搭接搅拌摩擦焊接头的强度试验,研究了搭接搅拌摩擦焊接头的断裂行为,应用有限元方法对2A12铝合金板材搭接搅拌摩擦焊接头的拉伸力学响应进行了模拟计算,获得了搭接搅拌摩擦焊接头在外载作用下的局部应力应变分布与变形规律.结果表明,在搭接接头的搭接面上存在两个类裂纹的未焊接区域,塑性变形首先在搭接界面裂纹尖端区域发生,上板受拉侧裂纹尖端周围区域的塑性变形最大,是导致搭接搅拌摩擦焊接头上板的前进侧热影响区断裂的主要原因.     

Numerical simulations of hydrogen–dislocation interactions in fcc stainless steels.: part II: hydrogen effects on crack tip plasticity at a stress corrosion crack

The discrete simulation method for hydrogen–dislocation interactions is applied to the study of Stress Corrosion Cracking (SCC). We recall the main results of the experimental study of the fracture micro-crystallography in austenitic stainless steels, along with the successive stages of the Corrosion Enhanced Plasticity Model. Numerical simulations allow the assessment of the critical parameters affecting the model stages. Solute hydrogen promotes the formation of dense dislocation pile-ups, and a ‘zigzag’ type of fracture along alternating slip planes at the SC crack tip. We provide an analytical expression for the stress field of a dilatation line in the vicinity of a crack, from which we derive all the hydrogen–crack–dislocation elastic interactions terms. Diffusing hydrogen also has a marked pinning effect on a dislocation source at a crack tip. This effect exhibits a strong dependence on the crystal orientation. These results are discussed from the viewpoint of SCC fracture mechanisms.     

平面应力条件下弹塑性材料的起裂与裂纹稳态扩展

根据有限元计算分析的弹塑性裂纹体在平面应力条件下的裂纹尖端应力应变场,采用滑移线浸蚀以及断裂力学的试验方法,研究了该条件下的起裂和稳态扩展现象。结果表明,在裂纹尖端存在着HRR近程场以及取决于试样几何类型的远程场。起裂由单参数J控制,而稳态扩展行为则受远程应变场影响,不能由单参数描述。     

阳极电流对纯铜裂尖表面形变和总体形变影响的对比

采用微机控制激光散斑干涉技术，对多晶纯铜平板预裂纹试样在空气及在３．５％ＮａＣｌ溶液中外加阳极电流条件下的裂纹张开位移进行了原位测量，并对比了这两种条件下，裂尖表面应变场的变化，研究了阳极溶解对裂尖材料总体形变和表面形变的影响。结果表明：加阳极电流后，裂尖表面应变量和应变范围都明显增大，而反映裂尖总体形变的张开位移变化很小。阳极溶解对金属内部材料的形变没有直接的影响，只能缓解金属表面的变形硬化，促进金属的表面变形．     

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [$ \bar 1 $ \bar 1 11] and [$ \bar 1 $ \bar 1 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.     

位错和位错偶沿单─滑移系从裂纹尖端的发射

采用计算机模拟了位错和位错偶沿单一滑移系从裂纹尖端的发射,考察了滑移面取向、外加载荷、晶格摩擦力以及位错发射的临界应力强度因子对所发射的位错数量、塑性区与无位错区大小以及裂关残余应力强度因子的影响研究表明,位错从裂纹尖端发射的临界应力强度因子对无位错区的存在和其大小起决定作用,而外加载荷与晶格摩擦力主要影响位错发射的数量以及塑性区大小．在Ｉ型载荷作用下,滑移面与裂纹面的夹角越大,从裂尖发射出的位错数量越多,位错对裂纹的屏蔽效应也越大当裂纹发射位错后的残余应力强度因子仍然较大时,位错偶就有可能在裂纹尖端附近产生井沿着几个滑移面发射,但发射出的位错偶对裂纹没有明显的屏蔽作用在滑移面不垂直于裂纹面时,发射出的位错或位错偶关于裂纹面呈不对称分布     

Discrete dislocation modeling of fatigue crack propagation

Analyses of the growth of a plane strain crack subject to remote mode I cyclic loading under small-scale yielding are carried out using discrete dislocation dynamics. Cracks along a metal–rigid substrate interface and in a single crystal are studied. The formulation is the same as that used to analyze crack growth under monotonic loading conditions, differing only in the remote stress intensity factor being a cyclic function of time. Plastic deformation is modeled through the motion of edge dislocations in an elastic solid with the lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles and dislocation annihilation being incorporated through a set of constitutive rules. An irreversible relation is specified between the opening traction and the displacement jump across a cohesive surface ahead of the initial crack tip in order to simulate cyclic loading in an oxidizing environment. The cyclic crack growth rate log(da/dN) versus applied stress intensity factor range log(ΔK_I) curve that emerges naturally from the solution of the boundary value problem shows distinct threshold and Paris law regimes. Paris law exponents in the range 4 to 8 are obtained for the parameters employed here. Furthermore, rather uniformly spaced slip bands corresponding to surface striations develop in the wakes of the propagating cracks.     

Stress fields and geometrically necessary dislocation density distributions near the head of a blocked slip band

We have examined the interaction of a blocked slip band and a grain boundary in deformed titanium using high-resolution electron backscatter diffraction and atomic force microscopy. From these observations, we have deduced the active dislocation types and assessed the dislocation reactions involved within a selected grain. Dislocation sources have been activated on a prism slip plane, producing a planar slip band and a pile-up of dislocations in a near screw alignment at the grain boundary. This pile-up has resulted in activation of plasticity in the neighbouring grain and left the boundary with a number of dislocations in a pile-up. Examination of the elastic stress state ahead of the pile-up reveals a characteristic “one over the square root of distance” dependence for the shear stress resolved on the active slip plane. This observation validates a dislocation mechanics model given by Eshelby, Frank and Nabarro in 1951 and not previously directly tested, despite its importance in underpinning our understanding of grain size strengthening, fracture initiation, short fatigue crack propagation, fatigue crack initiation and many more phenomena. The analysis also provides a method to measure the resistance to slip transfer of an individual grain boundary in a polycrystalline material. For the boundary and slip systems analysed here a Hall–Petch coefficient of K = 0.41 MPa m^½ was determined.     

An intrinsic effect of hydrogen on cyclic slip deformation around a {1 1 0} fatigue crack in Fe–3.2 wt.% Si alloy

The effect of gaseous hydrogen on cyclic slip behavior around a fatigue crack tip introduced along the {1 1 0} plane in a Fe–3.2 wt.% Si alloy is precisely investigated by cross-sectional transmission electron microscopy and fractography. The results clearly suggest that the fatigue crack growth rate is promoted by hydrogen, whereas the number of dislocations emitted per load cycle is reduced. In addition, dislocation distribution is localized around the crack, causing quasi-brittle crack morphology. A sustained load test confirms that no subcritical crack growth caused by cleavage or micro-void coalescence exists along the {1 1 0} plane, which indicates that the observed increase in the fatigue crack growth rate is correlated solely to the intrinsic effect of hydrogen on the cyclic slip-off process around the crack tip.