Validating a polycrystal model for the elastoplastic response of magnesium alloy AZ31 using in situ neutron diffraction

Agnew, Sean R.; Brown, Donald W.; Tomé, Carlos N.

doi:10.1016/j.actamat.2006.06.020

Cited by 408 publications

(201 citation statements)

References 25 publications

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“…I and II planes. The above conclusions are all in general agreement with experiments on Mg where deformation mechanisms are most clearly discernable and rationalize the experimental ambiguities (15,(57)(58)(59) in the competition between Pyr. II and Pyr.…”

Section: Resultssupporting

confidence: 77%

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

Curtin

2016

Proc. Natl. Acad. Sci. U.S.A.

109

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Hexagonal close-packed (hcp) metals such as Mg, Ti, and Zr are lightweight and/or durable metals with critical structural applications in the automotive (Mg), aerospace (Ti), and nuclear (Zr) industries. The hcp structure, however, brings significant complications in the mechanisms of plastic deformation, strengthening, and ductility, and these complications pose significant challenges in advancing the science and engineering of these metals. In hcp metals, generalized plasticity requires the activation of slip on pyramidal planes, but the structure, motion, and cross-slip of the associated 〈c + a〉 dislocations are not well established even though they determine ductility and influence strengthening. Here, atomistic simulations in Mg reveal the unusual mechanism of 〈c + a〉 dislocation cross-slip between pyramidal I and II planes, which occurs by cross-slip of the individual partial dislocations. The energy barrier is controlled by a fundamental step/ jog energy and the near-core energy difference between pyramidal 〈c + a〉 dislocations. The near-core energy difference can be changed by nonglide stresses, leading to tension-compression asymmetry and even a switch in absolute stability from one glide plane to the other, both features observed experimentally in Mg, Ti, and their alloys. The unique cross-slip mechanism is governed by common features of the generalized stacking fault energy surfaces of hcp pyramidal planes and is thus expected to be generic to all hcp metals. An analytical model is developed to predict the cross-slip barrier as a function of the near-core energy difference and applied stresses and quantifies the controlling features of cross-slip and pyramidal I/II stability across the family of hcp metals.P lastic deformation of crystalline materials occurs mainly by slip along low-index atomic planes. Slip regions are bounded by line defects called dislocations (1), which are characterized by the crystallographic slip increment known as the Burgers vector b and the local line direction ξ. The transition from slipped to unslipped regions is resolved at the atomic scale by local atomic rearrangements creating a dislocation "core" structure. Slip occurs by motion of this core, and the surrounding elastic fields, driven by an applied stress. Plasticity is thus controlled by the details of the core region, which differ significantly among face-centered cubic (fcc), bodycentered cubic (bcc), and hexagonal close-packed (hcp) metals (2). The motion of the dislocation core is usually confined to glide within the slip plane defined by the normal vector n = b × ξ. Screw dislocations have b × ξ = 0, however, and so do not have a unique slip plane and can therefore "cross-slip" among glide planes that share a common Burgers vector. Cross-slip is a crucial process in plasticity, acting to spread slip spatially on multiple nonparallel planes and to enable dislocation multiplication and annihilation processes; all of these processes affect ductility and ultimate strength of the material. Whereas the atomistic mecha...

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Section: Resultssupporting

confidence: 77%

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

Curtin

2016

Proc. Natl. Acad. Sci. U.S.A.

109

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“…This means that in axial tension or compression the resolved shear stress for basal slip, which is the easiest deformation mode in magnesium, is very low and other deformation modes control yield despite their higher CRSS. [13][14][15] When tested in tension, it has been demonstrated that the activation of prismatic slip controls the yield stress. [13][14][15] In compression, the grains are favorably oriented for activation of f10 " 12g twinning, and the CRSS for this mode is much less than prismatic slip in conventional magnesium alloys.…”

Section: Origin and Elimination Of Asymmetrymentioning

confidence: 99%

“…[13][14][15] When tested in tension, it has been demonstrated that the activation of prismatic slip controls the yield stress. [13][14][15] In compression, the grains are favorably oriented for activation of f10 " 12g twinning, and the CRSS for this mode is much less than prismatic slip in conventional magnesium alloys. [13][14][15] Thus, in compression, f10 " 12g twinning activates and produces yield at a much lower stress than that needed to activate prismatic slip, hence producing mechanical asymmetry.…”

Section: Origin and Elimination Of Asymmetrymentioning

confidence: 99%

Effect of Precipitate Shape and Habit on Mechanical Asymmetry in Magnesium Alloys

Robson

Stanford

Barnett

2012

Metall Mater Trans A

104

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Asymmetric yield behavior in tension and compression is a common and usually undesirable feature of wrought magnesium alloys. To prevent yield asymmetry, it is necessary to favor slip over twinning, as it is the unidirectional nature of twinning combined with the strong textures produced in wrought magnesium alloys that produce yield asymmetry. In this article, the potential to use precipitates to strengthen selectively against twin growth is discussed. The effect of precipitate's shape and habit on strengthening of slip and twinning is calculated using simple Orowan-based models. It is shown that basal plate precipitates, although being poor strengtheners against basal slip, are good strengtheners against twin growth. This is because they produce the maximum unrelaxed back-stress when they remain unsheared inside the twin. The predictions of the model have been validated against experiments for two alloys that form different precipitate types: AZ91 (basal plates). and Z5 (c-axis rods). Crystal plasticity modeling has been used to predict that an optimized distribution of basal plate precipitates is expected to strongly reduce yield asymmetry, even in strongly textured magnesium alloy.

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“…For example, the related polymer polyethylene is employed for neutron moderation at the LANSCE facility (for a discussion of the forms of polyethylene see [3]). Previous applications of SMARTS have been focused on metallic materials (see for example [4][5][6][7][8][9]). This is partially due to difficulties in interpreting diffraction patterns of amorphous materials [10], and partially because of the relatively few polymers that could be successfully probed in the bulk by diffraction methods.…”

Section: Introductionmentioning

confidence: 99%

In-situ Measurement of Crystalline Lattice Strains in Polytetrafluoroethylene

et al. 2007

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Strain measurements by neutron diffraction are employed as an in situ technique to obtain insight into the deformation modes of crystalline domains in a deformed semi-crystalline polymer. The SMARTS (Spectrometer for MAterials Research at Temperature and Stress) diffractometer has been used to measure the crystalline lattice displacements in polytetrafluoroethylene (PTFE) for crystalline phase IV (at room temperature) in tension and compression and for crystalline phase I (at 60°C) in compression. The chemical structure of PTFE, -(C 2 F 4 )-n , makes it ideally suited for investigation by neutron methods as it is free of hydrogen that results in limited penetration depths and poor diffraction acquisition in most polymers. Deformation parallel to the prismatic plane normals is shown to occur by inter-polymer chain compression with a modulus ∼10× bulk, while deformation parallel to the basal plane normal occurs by intra-polymer chain compression with a modulus ∼1000× bulk, corresponding with theoretical values for a PTFE chain modulus. Deformation parallel to the pyramidal plane normals is accommodated by interpolymer chain shear.

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Validating a polycrystal model for the elastoplastic response of magnesium alloy AZ31 using in situ neutron diffraction

Cited by 408 publications

References 25 publications

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

Mechanism and energetics of 〈c + a〉 dislocation cross-slip in hcp metals

Effect of Precipitate Shape and Habit on Mechanical Asymmetry in Magnesium Alloys

In-situ Measurement of Crystalline Lattice Strains in Polytetrafluoroethylene

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