Stress-state dependence of slip in Titanium-6Al-4V and other H.C.P. metals

The plastic deformation in hcp metals is complex, with the associated dislocation core structures and properties not well understood on many slip planes in most hcp metals. A first step in establishing the dislocation properties is to examine the stable stacking fault energy and its structure on relevant slip planes. However, this has been perplexing in the hcp structure due to additional in-plane displacements on both side of the slip plane. Here, density functional theory guided by crystal symmetry analysis is used to study all relevant stable stacking faults in 6 hcp metals (Mg, Ti, Zr, Re, Zn, Cd). Specially, the stable stacking fault energy, position, and structure on the Basal, Prism I and II, Pyramidal I and II planes are determined using all-periodic supercells with full atomic relaxation. All metals show similar stacking fault position and structure as dictated by crystal symmetry, but the associated stacking fault energy, being governed by the atomic bonding, differs significantly among them. Stacking faults on all the slip planes except the Basal plane show substantial out-of-plane displacements while stacking faults on the Prism II, Pyramidal I and II planes show additional in-plane displacements, all extending to multiple atom layers. The in-plane displacements are not captured in the standard computational approach for stacking faults, and significant differences are shown in the energies of such stacking faults between the standard approach and fully-relaxed case. The existence of well-defined stable stacking fault on the Pyramidal planes suggests zonal dislocations are unlikely. Calculations on the equilibrium partial separation further suggests c + a dissociation into three * Corresponding author Email address: zhaoxuan.wu@epfl.ch (Zhaoxuan Wu) Preprint submitted to Acta Materialia October 9, 2016This is a pre-print of the following article: Yin, Binglun; Wu, Zhaoxuan; Curtin, W.A. Acta Materialia 2017, 123, 223--234.. The formal publication is available at http://dx.doi.org/10.1016/j.actamat.2016.10.042 partials on the Pyramidal I plane is unlikely and c dissociation on Prism planes is unlikely to be stable against climb-dissociation onto the Basal planes in these metals.

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“…The Prism I plane has two layers of atoms and thus two slip planes of different structure and interplanar spacing [22,17], as shown in Fig. 2b.…”

Section: Stacking Faults Based On Symmetry In Hcp Crystalsmentioning

confidence: 99%

Comprehensive first-principles study of stable stacking faults in hcp metals

2017

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“…For GND calculation of α-Ti the following dislocation types were used: 3 <a> screw and 6 <c+a> screw; 3 < a > edge on basal plane; 3 < a > edge on first order prismatic plane; 6 < a> edge on first order pyramidal plane; 12 <c+ a > edge on first order pyramidal plane [44]. The GND density for each type of dislocation is found using a standard linear programming algorithm by minimizing the possible total GND line energy [45].…”

Section: Materials and Experimental Proceduresmentioning

confidence: 99%

Slip band–grain boundary interactions in commercial-purity titanium

2014

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“…Frequent cross-slip of hc + ai dislocations can occur in early stage, room temperature deformation (10-15). However, the propensity and nature of hc + ai cross-slip differ among different materials and can also depend on the applied stress and temperature in the same material (12,(16)(17)(18)(19)(20)(21)(22) (Fig. 2 and refs.…”

mentioning

confidence: 99%

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

Curtin

2016

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

110

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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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Stress-state dependence of slip in Titanium-6Al-4V and other H.C.P. metals

Cited by 290 publications

References 51 publications

Comprehensive first-principles study of stable stacking faults in hcp metals

Comprehensive first-principles study of stable stacking faults in hcp metals

Slip band–grain boundary interactions in commercial-purity titanium

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

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