Macrodeformation Twins in Single-Crystal Aluminum

Abstract

Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond nanotwins. Here, we present the first experimental demonstration of macro deformation twins in single-crystal aluminum formed under ultrahigh strain-rate (∼10 6 s −1 ), large shear strain (200%) via dynamic equal channel angular pressing. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates.When we talk about crystal deformation, what do we actually talk about? Crystal defects [1]. Crystal defects such as dislocations (line defects) and twins (planar defects) play a critical role in plastic deformation and ultimately govern the multifarious mechanical behaviors of many crystalline materials [2,3]. While both dislocation slip and deformation twinning are dependent on an intrinsic material property -stacking fault energy [4,5] (SFE), their sensitivities to SFE differ considerably. A notable example is pure aluminum, a typical face-centered cubic (fcc) metal with high SFE (104-142 mJ m −2 ) [6], in which deformation twinning rarely occurs even deformed at low temperatures and/or at high strain rates [7,8]. This rareness of deformation twinning in such materials is normally attributed to the following two reasons: (i) a large number of slip systems in fcc metals render dislocation slip a very efficient deformation mode [9,10], and (ii) the nucleation of twinning partial dislocations require much higher shear stresses than trailing partial dislocations due to the high unstable twin fault energy [11]. Searching for macro deformation twins in pure aluminum and revealing the underlying mechanisms have been of sustained interest in the past decade.Molecular dynamics (MD) simulations first predicted that nanoscale deformation twins can nucleate under high tensile stress (2.5 GPa) and high strain rate (10 7 s −1 ) in nanograined aluminum [6,12], and subsequent experiments confirmed this prediction in nanograined aluminum films under different kinds of severe plastic deformation (SPD) [13][14][15]. One explanation was proposed based on classical dislocation theory [16]: when grain size decreases to tens of nanometers, normal dislocation activities are greatly suppressed by the high fraction of grain boundaries (GBs); as a result, deformation twinning takes over as the dominant deformation mechanism [13]. Besides nanograin size effect, many simulation and experimental studies suggest that deformation twinning prefers to occur at high strain rates in fcc metals [17,18]. This rate-dependent twinning mechanism has been corrobarated by a very recent experiment on pure aluminum with comparatively large nanograins (50-100 nm) [19]. However, there has been no solid evidence for deformation twinning in single-crystal or coarse-grained pure aluminum. It is natural ...

Twinning in polycrystalline aluminium deformed by dynamic channel angular pressing

“…By contrast, the twins described in the present study as well as the "macrodeformation twins" observed by Zhao et al in Al single crystals [22] were up to tens of micrometers in thickness. To understand this controversy, a study of Miyamoto et al [28] may be of interest.…”

“…Its average value is about 4° for segment 1, while it is somewhat higher for segments 2 and 3. Noteworthy, the morphology of these twins is similar to that of the deformation twins discovered in Al single crystals by Zhao et al [22]. Small deviations from the ideal twin rotation, also similar to those observed in [22], may be induced by plastic deformation proceeded after the twin appearance [24 -26].…”

“…As for twinning in aluminium, nanoscopic twins were observed earlier in some special cases at extreme conditions of high stress [16], e.g., in nanograins [17 -19] and at crack tips [20 -21]. Recently, it has been shown that a large-scale twinning could be produced in Al single crystals using a method similar to the DCAP: the twins of about 40 μm in thickness nucleated from the surface and propagated toward the sample interior [22]. At the same time, until now there has been no observations of deformation twinning in ordinary polycrystalline aluminium.…”

Texture Development and Material Flow Behavior During Refill Friction Stir Spot Welding of AlMgSc

“…A recent study demonstrated for the first time that macrodeformation twins in high stacking fault energy materials, such as single-crystal Al, are realized at ultrahigh strain rates (~10 6 s À1 ) and large shear strains (2). [35] In the current work, however, the selected region is close to the center, which would most likely not be expected to experience such a high strain rate with a large strain. Thus, some factors other than stain rate are responsible in this case.…”

Stable Stacking Faults Bounded by Frank Partial Dislocations in Al7075 Formed through Precipitate and Dislocation Interactions

“…Producing SFs in high-SFE metals, which could potentially improve the ductility of the metals, has raised considerable interest among scholars [1][2][3][4][5][6][7]. For example, SFs and twins were found in nanocrystal aluminum powders produced by ball milling [5].…”