Double-slip Orientations Research Articles

Unique damage process in micro-sized copper single crystal with double-slip orientation in response to near-[112] tension-compression fatigue

In order to clarify the fatigue process for a micro-sized copper (Cu) single crystal with a double-slip orientation, a tension-compression cyclic loading test was carried out with in situ FE-SEM observation. Although the primary slip system B4 finally brought about a crack from eminent intrusions, the process was complex due to characteristic interactions among multiple slip systems. The process was approximately divided into three stages; early slipping, cave-in and damaging. In the early stage, two slip systems with high Schmid factor (SF), B4 and C1, worked at first, however both immediately stopped working. Extrusions/intrusions were caused by the survived slip system B5 (third-highest SF). In the second stage, a triangular depression was brought about by the cross-slip from B5 to C5 and localized wavy undulation was formed by the continuous cross slips. In the final stage, though both of B4 and B5 worked, the slip system with the highest SF, B4, dominated to form the eminent extrusions/intrusions, resulting in the fatigue cracking. The applied resolved shear stress amplitude was a few megapascals, indicating significantly lower fatigue strength than that of the bulk counterpart while it was in same range for that of micro-sized Cu with a single slip orientation. The complex fatigue damage morphology of the specimen was explained by accounting for the interaction of dislocations between the slip systems.

Unique Damage Process in a Microscale Copper Single Crystal with Double-Slip Orientation Near [112] in Response to Tension-Compression Fatigue

Unique Damage Process in a Microscale Copper Single Crystal with Double-Slip Orientation Near [112] in Response to Tension-Compression Fatigue

Low cycle fatigue in aluminum single and bi-crystals: On the influence of crystal orientation

Aluminum single crystals with three different double-slip orientations ([2̅32], [3̅3̅2], [130]) and two aluminum bi-crystals – one with a high-angle grain boundary and one with a low-angle grain boundary - were cyclically deformed up to 100 cycles under constant displacement control. The distribution of the local strain and the local strain amplitudes was captured by in-situ digital image correlation (DIC). Dislocation structure analysis was performed by electron channeling contrast imaging (ECCI) and the evolution of local misorientations was recorded by high resolution electron backscatter diffraction (EBSD). The DIC results show a homogeneous strain amplitude distribution in the single crystals while the measured strain amplitude in the low-angle grain boundary bi-crystal sample differs significantly. ECCI observations reveal the presence of dislocation cells elongated along the trace of the primary {111} slip plane in all investigated crystals and the formation of deformation bands parallel to the trace of {110} planes. Deformation bands (DB) were observed in all samples but their frequency and misorientation with respect to the matrix was found to sensitively depend on the crystal orientation and the local strain amplitude. Our results on the bi-crystals show that the grain orientation mainly determines the local stresses and therefore also the formation of the associated dislocation structures rather than the grain boundary character.

Strain hardening in bent copper foils

A series of systematic tensile and microbend tests were conducted on copper foil specimens with different thicknesses. The specimens were made of a copper foil having almost unidirectional crystal orientations that was considered to be nearly single-crystal. In order to investigate the effects of slip system interactions, two different crystal orientations relative to the tensile direction were considered in the tests: one is close to coplanar double-slip orientation, and the other is close to the ideal cube orientation (the tensile direction nearly coincides to [0 0 1]) that yields multi-planar multi-slip deformation. We extended the microbend test method to include the reversal of bending, and we attempted to divide the total amount of strain-hardening into isotropic and kinematic hardening components. In the tensile tests, no systematic tendency of size dependence was observed. In the microbend tests, size-dependent kinematic hardening behavior was observed for both the crystal orientations, while size dependence of isotropic hardening was observed only for the multi-planar multi-slip case. We introduce an extended crystal plasticity model that accounts for the effects of the geometrically necessary dislocations (GNDs), which correspond to the spatial gradients of crystallographic slips. Through numerical simulations performed using the model, the origin of the size-dependent behavior observed in the microbend tests is discussed.

Cyclic deformation and fatigue damage behaviors of oriented Ag single crystal

The cyclic deformation and fatigue damage behaviors of oriented Ag single crystal were investigated in terms of cyclic stress–strain (CSS) curves, surface slip morphologies and dislocation configurations. A clear plateau region appears in the CSS curve of the Ag single crystal over a strain range of with average saturation shear stress amplitude of ∼26.6 MPa. Corresponding ladder-like persistent slip bands (PSBs) also appear in the Ag single crystal. Compared with other Cu and Ag single crystals with single or double-slip orientations, the saturation resolved shear stresses of the critical double-slip-oriented single crystals, no matter whether for Cu or Ag, are significantly higher than that of the single-slip-oriented ones. Combining the two-phase model with the hindering effect of sessile jogs on screw dislocation movement, a three-phase model is proposed to rationalize the increase of saturation stress. In addition, the surface slip morphologies of the Ag single crystal were characterized by the interactions between the primary and secondary SBs. According to the difference in their slippage, their interactions can be divided into three types, which could simultaneously appear in different local areas of the same crystal. The current experimental results extend the fundamental understanding of the intrinsic fatigue damage behaviors of fcc metal crystals.

Cyclic deformation of pure aluminum single crystals with double-slip orientations

Abstract Fully reversed tension–compression fatigue tests were performed on pure aluminum single crystals with the [1 1 2], [1 1 4] and [1 1 2 0] double-slip orientations under constant plastic strain amplitudes in the range of 5 × 10−4 and 1 × 10−2. No stress saturation was obtained at any strain level in the cyclic hardening curves (CHCs). The CHCs were characterized as: initial hardening, intermediate softening and secondary hardening. The degree of the softening was found to be strongly dependent on the stress-axis: the softening stages were significant for the [1 1 2 0] and [1 1 4] specimens, and conversely, indefinable at any plastic strain amplitude for the [1 1 2] specimens. The changes in surface topography were also found to be dependent on the crystal orientation. Characteristic macrobands in the order of several hundred micrometers were formed in the [1 1 2 0] specimens. The appearance of cyclic softening during fatigue deformation in aluminum single crystals is explained in terms of both the activity of cross-slip and the strain localization in the macrobands.

Mechanical testing device for in situ experiments on reversibility of dislocation motion in silicon

We present a general-purpose mechanical testing device built and tested for in situ experiments using synchrotron radiation. A new setup was used in order to investigate the reversibility of dislocation motion under stress reversal. The observations were done by in situ transmission X-ray topography, at the ID 19 beamline of the ESRF. Specimens of FZ silicon single crystals, with a [ 1 1 ̄ 4 ] axis (double slip orientation), were submitted to load cycles in tension and compression ( τ=10–20 MPa) at high temperature (650–750 °C). Dislocations were created at surfaces by Vickers indents. They were observed using a monochromatic beam ( λ=0.035 nm) and a 220 diffraction vector. The images were recorded both on X-ray films and using the FRELON X-ray camera. The first observations show that dislocations may behave in different ways within the same specimen, and that their motion can be partially reversible.

Low-cycle fatigue and microstructure of Cu–Fe single crystals with a double-slip orientation

Low-cycle fatigue behavior of [1 1 3] double-slip oriented Cu–Fe alloy single crystals with and without precipitated f.c.c. γ-Fe particles were examined under plastic-strain-controlled fatigue tests and compared with that of single-slip oriented crystals such as [4 1 9] . Although the γ-Fe particles are coherent and shearable, no cyclic softening was observed in all the particle bearing specimens. The cyclic stress–strain curves of the [1 1 3] crystals show a plateau region similar to those in the [4 1 9] crystals. On the other hand, shear stress amplitude increased monotonically with increase in shear strain amplitude on solution treated specimens. Dislocation microstructures characteristic of cyclic deformation were more clearly developed in the [1 1 3] crystals. Magnetic measurement of fatigued specimens revealed that more particles had transformed martensitically into b.c.c. α-Fe in the [1 1 3] crystals than in the [4 1 9] crystals. The above results can be understood reasonably if orientation-dependent deformation and transformation characteristics are taken into account.

Cyclic deformation behavior of double-slip-oriented copper single crystals IV: change of hysteresis loop shape and formation of persistent slip bands

Abstract The changes of hysteresis loop shape parameter V H with number of cycles N in copper single crystals oriented for conjugate, critical and coplanar double slip were detected systematically during cyclic deformation. The results of multiple slip-oriented copper single crystals [001], [111] and [011] were also included for summary. It was shown that, unlike the result obtained for single slip crystals, the crystallographic orientation has a strong effect on the trend of hysteresis loop shape change during cycling for double slip-oriented crystals. The persistent slip bands (PSBs) may occur at certain strain amplitudes within or even below the plateau region in the cyclic stress–strain (CSS) curve for [112] crystals, while it is very hard for PSBs to form in [233] and [017] crystals. In brief, whether V H could be adopted to determine the PSB nucleation stress is closely related to the existence or non-existence of a plateau or quasi-plateau region in the CSS curves of the crystals with various orientations. The PSB nucleation stresses determined by the parameter V H for copper single crystals with different types of double slip orientations were compared and discussed.

Cyclic deformation behavior of double-slip-oriented copper single crystals III: conjugate double slip orientation on 001–11 side of the stereographic triangle

The cyclic deformation behaviors of [2̄23] and [1̄12] conjugate double-slip-oriented copper single crystals were investigated at constant plastic shear strain amplitude γpl in the range of about 10−4–10−2 at ambient temperature in air. The results of [1̄17] and [001] crystals are included for comparison. Unlike the plot of initial hardening rate θ0.2 against γpl of the [2̄23] crystal which does not exhibit two obvious regions, the plot for the [1̄12] crystal exhibits two regions similar to those in single-slip-oriented crystals. The cyclic stress–strain (CSS) curve of the [1̄12] crystal shows a clear plateau over the range of about 5.0×10−4–4.0×10−3, which is narrower than that of the single slip crystals (6.0×10−5–7.5×10−3) although the saturation shear stress is nearly the same. However, no obvious saturation phenomenon was observed in the cyclically deformed [2̄23] crystals over the range of plastic strain amplitude investigated. Even though the orientations of [1̄12], [1̄17] and [2̄23] crystals are on the same side of the stereographic triangle, their plateau behaviors in the CSS curve are quite different, i.e. the plateau regions of the CSS curves for the crystals located on both sides of the [1̄12] crystal tend to die out except that an obvious plateau region occurs in the cyclically deformed [1̄12] crystals. The results above are mainly attributed to the rather different slip deformation characteristics associated with the different crystal orientations. In addition, two types of deformation bands DBI and DBII, formed in the cyclically deformed [1̄12] and [2̄23] crystals, were also discussed in terms of crystallographic geometry.

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 γpl 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 θ0.2 against γpl 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.

Deformation mechanism and dislocation structure of high‐purity molybdenum single crystals at low temperatures

AbstractHigh purity molybdenum single crystals were deformed in tension and compression along the symmetric double slip orientation [110] in the temperature range from 300 K down to 0.5 K with strain rates between 10−3 and 10−5 s−1. The activation volume was measured by stress relaxation tests. The dislocation structure of the deformed crystals (T → 1.85 K) was examined by high voltage electron microscopy. It was established that the low temperature increase of the critical shear stress exhibits three distinct temperature regimes with different temperature dependences. These non‐uniformities are discussed in terms of recently developed theories of kink‐pair formation and kink‐kink interactions on screw dislocations in bcc metals. The HVEM observations suggest that the mobility of screw dislocations at very low temperatures should be determined by the combined effects of the PEIERLS barriers and the jog dragging.

The influence of pre-strain on deformation twinning in niobium single crystals

Single crystals of niobium were grown by the electron beam floating zone technique and purified by annealing in ultra high vacua. Crystals orientated for either single or double slip were predeformed at room temperature or 158°K in order to investigate the effects of various dislocation distributions on the twinning behaviour. It was found that pre-strain always raises the twinning stress and that more twins form in prestrained specimens. Pre-strain at 158°K in both double and single slip orientations and at room temperature in double slip orientations, is effective in suppressing twinning at temperatures down to 77°K, but not at 55°K and below. The twinning stress continues to increase slightly with decreasing temperature down to 20°K and there is evidence that twinning is always accompanied by some slip. In very pure pre-strained crystals, twinning is not restored by annealing. It is concluded that a homogeneous distribution of dislocations is the main requirement for effective twin suppression and that dislocations which intersect the twinning plane are more effective than those which are parallel to it. A possible nucleation mechanism is the dissociation of screw dislocations, formed by micro-slip, at or below the macroscopic yield stress at which these dislocations move.

Plastic behaviour of Fe-3% Si alloy single crystals

Fe-3% Si alloy single crystals of single slip and double slip orientation have been tested in tension at temperatures between 113 K and 473 K. The stress-strain curves exhibit a yield drop followed by inhomogeneous yielding (yield propagation stage), a parabolic and an approximately linear parts. The slip line observations carried out in various stages of deformation show that only at the beginning of the parabolic part of stress-strain curve the specimen is completely filled out with slip bands and the deformation becomes relatively homogeneous. With both single slip and double slip orientations deformation takes place predominantly in one slip system only. At higher temperatures ageing occurs during deformation.

The effect of plastic deformation mode upon the recrystallization behaviour of copper-cobalt crystals

Two-phase copper—cobalt crystals have been prepared and plastically deformed 1. (a) in tension using crystals of initial single- and double-slip orientations, and 2. (b) by cross-rolling up to 30 per cent R.A. These were subsequently annealed above the particle solvus temperature, and the kinetics of recrystallization were determined—the structural changes being followed by transmission electron microscopy. It is shown that the ease of reorystallization is associated with appreciable non-uniform secondary (e.g. cross and conjugate) slip resulting from unequally stressed slip systems. Recrystallization only occurs after high strains associated with decreasing rates of work hardening. Only alloys deformed to lower strains than these annealed to inherently stable recovered dislocation arrays. In all cases significant dislocation rearrangement occurred prior to recrystallization. Thus in technological processing operations, particle-pinning is necessary to provide the principal source of recrystallization resistance if a high stored energy is required.