Research Papers

A Plastic Deformation Mechanism by Necklace Dislocations Near Crack-like Defects in Nanotwinned Metals

[+] Author and Article Information
Haofei Zhou

Centre of Advanced Mechanics and Materials,
Applied Mechanics Laboratory,
Department of Engineering Mechanics,
Tsinghua University,
Beijing 100084, China
School of Engineering,
Brown University,
Providence RI 02912

Huajian Gao

School of Engineering,
Brown University,
Providence RI 02912
e-mail: huajian_gao@brown.edu

1Corresponding author.

Manuscript received January 26, 2015; final manuscript received April 21, 2015; published online June 3, 2015. Assoc. Editor: A. Amine Benzerga.

J. Appl. Mech 82(7), 071015 (Jul 01, 2015) (5 pages) Paper No: JAM-15-1049; doi: 10.1115/1.4030417 History: Received January 26, 2015; Revised April 21, 2015; Online June 03, 2015

Nanotwinned metals are a class of hierarchically structured materials that appear to transcend the limits of conventional material systems by exhibiting an exceptional combination of superior strength, ductility and resistance to fracture, fatigue, and wear. Recently, we reported a type of necklace dislocations in nanotwinned metals which become operative when the twin boundary (TB) spacing falls below a few nanometers. Here, we show that the presence of a cracklike defect as the dominant dislocation source could allow the same mechanism to operate at much larger twin spacings. This finding calls for further theoretical and experimental investigations of a new type of TB related dislocation mechanism which may play particularly important roles in crack-tip deformation in nanotwinned metals.

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Fig. 1

Four TB mediated dislocation mechanisms in nanotwinned metals: (a) Hall–Petch type dislocation; (b) twinning partial dislocation; (c) threading dislocation; and (d) necklace dislocation

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Fig. 2

Schematic of a necklace dislocation spreading away from a crack tip in the nanotwinned structure. The Burgers vector bf denotes the slip direction as well as the full dislocation character of the necklace dislocation.

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Fig. 3

(a) Initial configuration of a simulated nanotwinned crystal sample with system dimensions of 30 × 50 × 40 nm3 and twin thickness of λ = 10 nm. The twin planes are slightly tilted relative to the X-axis of a Cartesian coordinate system of XYZ, with an inclination angle of θ = 0 deg, 5 deg, and 10 deg in various samples. An initial crack of length 10 nm is introduced along the Y-axis of the sample. Uniaxial tensile loading is applied along the X-direction while traction-free boundary conditions are adopted in the other two directions. (b) Schematic illustration of the crystallographic orientations of the crack and available slip planes in the first twin.

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Fig. 4

Snapshots illustrating emission and spreading of necklace dislocations at a crack tip in nanotwinned samples with sample title angle of: (a) θ = 5 deg and (b) θ = 10 deg. The necklace dislocations are formed via dislocation transmission across the twin planes while propagating parallel to the twins on inclined slip planes.

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Fig. 5

Necklace dislocation structures with extended dislocation segments lying on: (a) inclined slip planes and (b) on both inclined slip planes and twin planes

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Fig. 6

Typical necklace dislocations observed in the simulations. The extended dislocation segments are indicated by capitals and unit jogs are marked with triangles. Only atoms in dislocation cores are shown.

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Fig. 7

Burgers vector analysis for the jog structure of a necklace dislocation at a twin plane with the help of a double Thompson tetrahedron




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