Dislocation Mean Free Paths and Strain Hardening of Crystals

Devincre, Benoît; Hoc, Thierry; Kubin, L.P.

doi:10.1126/science.1156101

Cited by 414 publications

(266 citation statements)

References 14 publications

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“…The key quantities are the applied resolved shear stress τ α acting on the dislocations and the zero-temperature strength or critical flow stress τ α crit required to overcome the energy barrier without thermal activation. τ α crit is due to the forest obstacles, and so is related to the dislocation densities in all slip systems α including α itself as 30,31 …”

Section: Origin Of Low Ductility and High Strain Hardeningmentioning

confidence: 99%

The origins of high hardening and low ductility in magnesium

Curtin

2015

Nature

568

170

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Magnesium is the ultimate lightweight structural metal but exhibits low ductility connected to unusual, mechanistically-unexplained, dislocation/plasticity phenomena, which make it difficult to form and use in energy-saving lightweight structures. Here, long-time molecular dynamics simulations using a DFT-validated interatomic potential reveal the fundamental origins of the previouslyunexplained phenomena and show a clear path toward design of new ductile magnesium alloys. The key c + a dislocation is metastable on easy-glide pyramidal II planes and undergoes a thermallyactivated, stress-dependent transition to one of three lower-energy, basal-dissociated immobile dislocation structures, which (i) cannot contribute to plastic straining and (ii) serve as strong obstacles to the motion of all other dislocations. The transition is intrinsic to magnesium, driven by reduction in dislocation energy and predicted to occur at very high frequency at room temperature, thus eliminating all major dislocation slip systems able to contribute to c-axis strain and leading to its low ductility.Enhancing ductility can thus be achieved by delaying, in time and temperature, the transition from the easy-glide metastable dislocation to the immobile basal-dissociated structures, and our results provide the underlying insights needed to guide the design of ductile magnesium alloys. * Corresponding author: william.curtin@epfl.ch 1 This is a post-print of the following article: Wu, Z. & Curtin, W. A. The origins of high hardening and low ductility in magnesium. Nature 526, 62-67 (2015). The final publication is available at http://dx.doi.org/10.1038/nature15364Developing lightweight structural metal is a crucial step on the path toward reduced energy consumption in many industries, especially automotive 1,2 and aerospace 3 . Magnesium (Mg) is the ultimate lightweight metal, with a density 23% that of steel and 66% that of aluminum, and so has tremendous potential to achieve energy efficiency 4 . In spite of this tantalizing property, Mg generally exhibits low ductility, insufficient for the forming and performance of structural components. The low ductility is associated with the inability of hexagonally-close-packed (hcp) Mg to deform plastically in the crystallographic c direction, which is accomplished primarily by dislocation glide on the pyramidal II plane with the c + a Burgers vector 5 (see Fig. 1).Experiments reveal a range of unusual, confounding, conflicting, and mechanistically-unexplained phenomena connected to c+a dislocations that coincide with the inability of Mg to achieve high plastic strains 6 . Uncovering and controlling the fundamental behavior of c + a dislocations is thus the key issue in Mg, and any solution would catapult Mg science, technology, and applications forward. Success would enable, for instance, lightweight automobiles that would consume less energy, independent of the energy source, and thus act as a multiplier for many other energyreduction strategies.Due to its critical importance and pro...

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Section: Origin Of Low Ductility and High Strain Hardeningmentioning

confidence: 99%

The origins of high hardening and low ductility in magnesium

Curtin

2015

Nature

568

170

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“…M etal nanoparticles with tailored crystalline phases and internal microstructures (for example, twins and stacking faults) represent a class of promising building blocks in achieving ultrahigh-strength materials [1][2][3][4][5][6][7][8] and efficient catalysts 9 . Controlled synthesis of metal nanoparticles with well-defined compositions, sizes and morphologies has been extensively studied in the last decade [10][11][12][13][14] .…”

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Ambient-stable tetragonal phase in silver nanostructures

et al. 2012

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Crystallization of noble metal atoms usually leads to the highly symmetric face-centred cubic phase that represents the thermodynamically stable structure. Introducing defective microstructures into a metal crystal lattice may induce distortions to form non-face-centered cubic phases when the lateral dimensions of objects decrease down to nanometre scale. However, stable non-face-centered cubic phases have not been reported in noble metal nanoparticles. Here we report that a stable body-centred tetragonal phase is observed in silver nanoparticles with fivefold twinning even at ambient conditions. The body-centered tetragonal phase originates from the distortion of cubic silver lattices due to internal strains in the twinned nanoparticles. The lattice distortion in the centre of such a nanoparticle is larger than that in the surfaces, indicating that the nanoparticle is composed of a highly strained core encapsulated in a less-strained sheath that helps stabilize the strained core.

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“…Subsequent transition to deformation twinning leads to a velocity burst, and deformation twins propagate at a transonic speed. Classical dislocation theory predicts that, dislocation velocity cannot exceed the shear wave velocity [16], and with increasing dislocation velocity, dislocation self-pinning and tangling ensue, leading to stress accumulation until deformation twins nucleate [25,26]. Twin propagation velocity exceeds the shear wave velocity.…”

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Macrodeformation Twins in Single-Crystal Aluminum

et al. 2016

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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 ...

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Dislocation Mean Free Paths and Strain Hardening of Crystals

Cited by 414 publications

References 14 publications

The origins of high hardening and low ductility in magnesium

The origins of high hardening and low ductility in magnesium

Ambient-stable tetragonal phase in silver nanostructures

Macrodeformation Twins in Single-Crystal Aluminum

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