In situ TEM study of twin boundary migration in sub-micron Be fibers

Abstract

Deformation twinning in hexagonal crystals is often considered as a way to palliate the lack of independent slip systems. This mechanism might be either exacerbated or shut down in small-scale crystals whose mechanical behavior can significantly deviate from bulk materials. Here, we show that sub-micron beryllium fibers initially free of dislocation and tensile tested in-situ in a transmission electron microscope (TEM) deform by a {1012} 1011 twin thickening. The propagation speed of the twin boundary seems to be entirely controlled by the nucleation of twinning dislocations directly from the surface. The shear produced is in agreement with the repeated lateral motion of twinning dislocations. We demonstrate that the activation volume (V) associated with the twin boundary propagation can be retrieved from the measure of the twin boundary speed as the stress decreases as in a classical relaxation mechanical test. The value of V ≈ 8.3 ± 3.3 × 10 −29 m 3 is comparable to the value expected from surface nucleation. This text is a revised version of the article published in Acta Materialia, 96 (2015) 57-65

A Continuum Multi-Disconnection-Mode model for grain boundary migration

“…Zhang and Xiang (2018) developed a continuum model for shear coupling in low angle GBs in terms of the motion and reaction of the constituent dislocations that constitute the GB structure. Recent experiments and simulations demonstrated that the shear-coupling of highangle GBs (Rajabzadeh et al, 2013a,b;Mompiou et al, 2015) is associated with the motion of line defects known as disconnections (Bollmann, 1970;Ashby, 1972;Hirth and Pond, 1996).…”

Deformation mechanisms in submicron Be wires

“…Indeed individual wires tested in SEM show a remarkable high yield stress around 2 GPa. 18 This fragile-like behavior is reinforced by the hci axis orientation of the wire that is unfavorable for the easy prismatic and basal dislocation glide. On the contrary, the twinned area can plastically deform by prismatic slip because its hci axis is almost perpendicular to the wire axis.…”

“…The wires were deformed by applying a uniaxial displacement on the sample glued on a copper grid. 18 In situ observations were performed on a JEOL2010 TEM (JEOL Ltd., Tokyo, Japan) operated at 200 kV. Video sequences were recorded on a hard drive/DVD writer using a MEGAVIEW III camera (Olympus Soft Imaging Solutions GmbH, Münster, Germany) at 22-25 fps.…”

“…In a previous work, we have shown that a twin boundary can propagate under stress by the nucleation of twinning dislocations presumably from the surface. 18 In the present article, our aim is to gain a deep understanding of the elementary mechanisms of plastic deformation at play in Be wires with the knowledge of the initial microstructure. Thus, we report in situ and postmortem transmission electron microscope (TEM) observations of submicron Be wires deformed in tension and fractured.…”

In Situ TEM Micropillar Compression Testing in Irradiated Oxide Dispersion Strengthened Alloys

“…Micro-tensile testing also encounters size effects in pure metals, such as copper [33], and requires understanding of both the obstacle spacing, microstructural constraints, and the dimensional constraints as discussed in Section 2. [42] and Cu [71], observations of twin boundaries in Be [73], and tensile strengths and modulus of Co nanowires conducted in situ SEM [72]. Depending on the length scale and initial sample structure, sample preparation time varied from large amounts for FIB-prepared samples in [42] to quick placement of nanowires in [72].…”