Fatigue fracture at copper bicrystal interfaces: fractography (original) (raw)
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Acta Materialia, 2001
This work investigated the dislocation arrangements, crystallographic characteristics and fatigue crack initiation of deformation bands (DBs) in a [5 12 20] copper single crystal cyclically deformed at high strain amplitude (g pl = 8×10 Ϫ3 ). The surface morphology of the fatigued copper crystal was observed to display the following features. (1) There is only one group of fine slip bands (SBs), which seem to carry little plastic strain. (2) Intensive DBs, with a width of 50-60 µm and spacing of 100-110 µm, are homogeneously distributed on the whole surface of the crystal and perpendicular to the SBs. The dislocation patterns within the SBs are often characterized by irregular structures with no persistent feature, indicating that these SBs are not typical persistent slip bands (PSBs). (4) The microstructure of the DBs can be classified into two types. One is the regular, 100% ladder-like PSBs in parallel and can be defined as the developing DB; the one is composed of parallel dislocation walls and is named the well-developed DB. (5) With further cyclic deformation, fatigue cracks always nucleate within the DBs rather than within the SBs or PSBs. Based on the observations above, the crystallographic characteristics and dislocation arrangements of DBs are discussed in combination with the plastic strain distribution and fatigue cracking mechanism within DBs. It is suggested that there is a transformation of deformation mode from slipping on the (111) plane in the developing DBs to shearing on the (101) plane in the well-developed DBs. Furthermore, the fatigue cracking within the DBs carrying high plastic strain can be attributed to the surface roughness caused by the shearing irreversibility of DBs.
Fatigue fracture at bicrystal interfaces: experiment and theory
Acta Materialia, 1998
ÐCopper bicrystals with isoaxial [149] twist boundaries and a (149)/(001)``random'' boundary were notched at the interface, left or right regarding the slip geometry, and intergranular cracks were propagated under strain control, to study how dislocation structure, crystallography and strain localization aect crack growth. Forward slip at the crack surface was favored regardless of growth direction or misorientation, whereas pre-existent multiple slip around the boundary reduced the dependence of crack kinetics on the growth direction. A model is proposed for intergranular crack growth based on crack tip deformation via crystallographic slip bands and simplifying assumptions about the eects of mode I stress ®elds on the strain. The experimental and theoretical results, along with published data, provide evidence that an optimal slip geometry exists for fatigue crack propagation. This idea is used to explain experimental results on directional dependence of intergranular cracking of bicrystals, suggesting that the kinematics of deformation has to be considered besides the energetics of dislocation nucleation at a crack tip to explain such results.
Dislocation structures around crack tips of fatigued polycrystalline copper
Materials Science and Engineering: A, 2005
Dislocation structures near fatigue cracks of polycrystalline copper specimens were analyzed using the electron channelling contrast imaging (ECCI) technique. Prior to the ECCI observations, optical microscopy was conducted to classify the fatigue crack morphologies into several kinds. It was found that the dislocation structures were correlated with the slip morphologies observed using the optical microscope. The cell structure almost corresponded to the severely deformed plastic zone where the individual slip bands could not be identified. The labyrinth dislocation structure was detected at the double-slip region. Ladder-like dislocation structure was detected ahead of the Stage I type fatigue crack. Hence, it can be said that the persistent slip band (PSB) was a favorable crack path. However, the microscopic route of the crack growth was not along the PSB but along the cell structure, which was developed locally in the vicinity of the crack tip.
Materials Characterization, 2018
Low-cycle fatigue tests of copper single crystals with the [001] stress axis were performed at room temperature under constant plastic shear strain amplitudes between γ pl = 3.5 × 10 − 4 and 1.0 × 10 − 2. Characteristic dislocation structures, which are highly dependent on a given plastic strain amplitude, were observed by high voltage scanning transmission electron microscopy. A vein-like dislocation structure, having a parallelepiped shape with two longitudinal (100) and (001) sides, was periodically formed along the [010] direction at around γ pl = 1.0 × 10 − 3. Labyrinth structure with the (100) and (001) dislocation walls was then gradually developed with increasing plastic strain amplitude. 2. Experimental Procedure Single crystal plates of copper (purity: 99.9%) with the (100)
Dislocation distributions associated with fatigue cracking on cleavage planes
Materials Science and Engineering, 1986
An attempt is made to answer the following question: does the dislocation distribution control the intrinsic fatigue resistance of b.c.c. metals? To evaluate this, experiments involving hydrogen-induced fatigue cracking of several Fe-3wt. %Si single crystals oriented in the (100) direction were performed. Microminiature samples of the round compact type were loaded under constant AK with alternating blocks of four constant test frequencies o~ to produce different single-cycle crack advances. Hydrogen apparently has the unusual effect or requiring more than 1 cycle to advance the crack at high frequencies but only part of a cycle to advance the crack at low frequencies. To estimate the deformation gradients, both direct transmission electron microscopy and channeling were used. Low energy dislocation configurations mostly consisted of dislocation dipoles on {110 ~ and (121 } planes. Both the local hardening on slip bands from dipole configurations and the thermal component of the flow stress from the strain rate sensitivity of Fe-Si increased with increasing ¢o. It is proposed that these combined effects can explain the increases in da/dt with increasing ¢o in terms of a hydrogen embrittlement model.
Hardness of fatigued copper polycrystals and their relation to their dislocation structure
Materials Science and Engineering: A, 1990
ABSTRACT The Vickers hardness was determined for copper polycrystals cyclically strained at a constant plastic strain amplitude up to fracture. The dependence of the hardness on the plastic strain amplitude exhibits a minimum at a strain of 1 × 10−3 followed by a sharp increase. This is explained in terms of the observed dislocation structure produced during fatigue. Using polishing and sectioning techniques, the same value of hardness has been found on the surface and in the bulk of the material.
Metals, 2020
There have been a number of studies on dipole separations in cyclically deformed FCC single crystals in single slip while there are no such studies in multiple slip. The dipole heights provide insight into the presence of long-range internal stresses (LRIS). In this study, we investigated how LRIS compare with the single slip studies through measuring the dislocation of dipole heights. [001] oriented copper single crystals were cyclically deformed in strain-control to saturation at ambient temperature. Transmission electron microscopy (TEM) confirms a labyrinth dislocation microstructure with high dislocation density walls and low dislocation density channels. The maximum dipole heights under the saturation stress were approximately independent of location, being nearly equal in the walls and within the channels. This, by itself, supports a uniform stress across the microstructure and low long-range internal stresses. The maximum value for dipole heights suggests dipole strengths (local stresses) that are about a factor of 2.4 higher than the applied stress based on the usual athermal equations. Considering the small "extra" stress that may be provided by tripoles or small dislocation pileups , a nearly homogenous stress distribution with only small internal stresses may be present, which is consistent with the observation of uniform dipole height across the heterogeneous dislocation microstructure. This observation that the stress state appears to be homogenous and higher than the applied stress has also been reported in the case of cyclically deformed metals in single slip.
Dislocation dynamics during cyclic loading in copper single crystal
Materialia, 2019
Crystalline plasticity can take place through numerous, small, uncorrelated dislocation motions (mild plasticity) or through collaborative events: dislocation avalanches (wild plasticity). Here, we study the correlation between dislocation patterning under cyclic loading and the nature of collective dislocation dynamics. The dislocation motion of a [110] oriented pure copper single crystal was dynamically followed using Acoustic Emission (AE) for different imposed stress amplitudes. The dislocation structure between each cyclic stress step was investigated using Electron BackScattered Diffraction (EBSD) and Rotational-Electron Channeling Contrast Imaging (R-ECCI) in a Scanning Electron Microscope (SEM). At low imposed stress, when the structure consists of dislocation cells, few dislocation avalanches are observed, while for a wall structure, at higher imposed stress, the contribution of avalanches is increased during the first cycles. For a given stress amplitude, the evolution of mild plasticity is synchronous with the plastic strain-rate, and rapidly vanishes after few cycles due to work hardening. The mean free path of the dislocations in this mild plasticity regime corresponds to the characteristic size of the dislocation structure (cell size, distance between walls). From one stress level to another, brutal rearrangements of the dislocation structure occur within a few numbers of cycles. Those rearrangements take place, at least partly, through dislocation avalanches. Upon reloading at a larger stress amplitude, dislocation avalanches can travel over distances much larger than the former dislocation mean free path. As the dislocation avalanches spread within the crystal, the memory of the previous dislocation structure is lost and a new dislocation structure emerges.