Dislocation creep | EPFL Graph Search

Dislocation creep is a deformation mechanism in crystalline materials. Dislocation creep involves the movement of dislocations through the crystal lattice of the material, in contrast to diffusion creep, in which diffusion (of vacancies) is the dominant creep mechanism. It causes plastic deformation of the individual crystals, and thus the material itself. Dislocation creep is highly sensitive to the differential stress on the material. At low temperatures, it is the dominant deformation mechanism in most crystalline materials. Some of the mechanisms described below are speculative, and either cannot be or have not been verified by experimental microstructural observation. Principles Dislocations in crystals Dislocation creep takes place due to the movement of dislocations through a crystal lattice. Each time a dislocation moves through a crystal, part of the crystal shifts by one lattice point along a plane, relative to the rest of the crystal. The plane that se

In-situ Laue Diffraction During Compression of Directionally Solidified Mo Micropillars

Julien René André Zimmermann

With recent developments in micro and nano-technologies, mechanical components such as those used in medicine or electronics tend to be miniaturized, requiring a new set of testing techniques to study their reliability and performance. One of the new methods is micro-pillar compression testing, which allows the study of mechanical properties of confined volumes precisely shaped by focused ion beam (FIB) milling from sometimes complex microstructures. This technique has garnered a great deal of interest in recent years; on one hand because of its apparent simplicity, on the other hand because it revealed an unexpected size dependence in the flow stress of single crystals when the pillar diameter was reduced below a few tenths of a micrometre. The flow stress of FIB milled face centred cubic (fcc) micro-pillars appears to follow a power law as a function of their size. In contrast, non-FIB milled, body centred cubic (bcc) eutectic Molybdenum (Mo) alloy pillars prepared by directional growth exhibit, in their pristine state, whisker-like behaviour without a size effect. Furthermore, after FIB milling or moderate pre-deformation, the whisker character is lost and the typical scatter of the flow stress usually observed for FIB milled pillars is reproduced. After large pre-deformation the scatter vanishes and the pillars exhibit no size effect. These findings show that 1) the deterministic parameter in the size effect can be correlated with the initial microstructure of the pillar, and 2) FIB milling influences this microstructure. This study aims to link the deformation modes occurring in small-scale pillars with their initial microstructure by studying directionally solidified eutectic Mo-alloy pillars with a diameter of 1 micrometre. In-situ Laue diffraction experiments during the compression of such defect-free pillars show that with readily-available dislocation sources from a concave pillar top, the {112}[111] slip system with the highest Schmid factor is initially activated and not the allegedly assumed {110}[111] slip system that is typical for bcc metals. Post-mortem slip trace analysis and TEM observations confirm this finding. Moreover, 3D atom probe measurements and TEM images show that this behaviour can be linked to the presence of both alloying elements and fcc nano-precipitates found in the bcc Mo-alloy. Both can lead to a decrease of the critical temperature and the observation of an fcc-like deformation. Additionally, it is shown that both FIB milling and pre-deformation introduce defects in the initially dislocation-free pillars. In addition to this, pillars prepared under similar conditions yield quite different microstructures. The angle between the focused ion beam and the crystal orientation is found to play an important role in the creation of defects. It can also be shown that pillars with high initial dislocation density deform bulk-like and exhibit strain hardening, while pillars with low initial dislocation content demonstrate a large scatter in the flow and yield stress and exhibit large strain burst at later deformation stages. Furthermore several slip systems are activated before the flow stress is reached for pillars containing initially low dislocation content. Interestingly the first activated slip system can be linked to the initial streaking direction; slip is observed on the {112} plane that contains excess dislocations. For both the as-prepared pillars and the pillars with low initial dislocation content, the yield seems to be source limited, which is not the case for pillars with high initial dislocation content.

EPFL2011

Creep Properties and Related Microstructural Changes of a Lamellar Titanium Aluminide Alloy before and after Helium-Implantation

Per Magnusson

The creep properties of a cast gamma-alpha-2 lamellar titanium aluminide of type Ti-46Al-2W-0.5Si (at. %) containing beta-particles was investigated using both helium-implanted and non-implanted material. Creep tests were performed in vacuum under constant stress using small miniaturized dog-bone shaped samples with gauge dimensions of 10 × 2 × 0.2 mm3. Creep temperatures ranged from 700 °C to 1000 °C and applied stresses ranged from 75 MPa to 400 MPa. Helium implantation was carried out with 0-24 MeV helium ions homogeneously implanting the miniaturized samples from 6.3 to 1333 appm He, at temperatures of 150 °C and 630 to 1000 °C. Non-implanted samples showed very good creep resistance up to temperatures of roughly 850 °C; at temperatures above 900 °C a decrease in creep resistance was observed. For temperatures up to 900 °C the activation energy of creep was determined to 405 kJ/mol and the stress exponent of 7.2. Size and geometry effects of miniaturized samples were observed in form of large scatter in creep strain rates and lower strains at fracture. Electron microscopy showed that the initial microstructure was unstable and that the alloy had a tendency to decompose the alpha-2 phase and form beta-gamma structures, loss of initial microstructure was accelerated with increasing temperature. The creep deformation mechanism was determined to be a combination of stress-induced phase change and dislocation creep. A gradual change in fracture behavior was observed, at lower temperatures fracture was brittle with little necking, at the highest temperature the fracture was completely ductile. Material crept and implanted at 700 and 800 °C exhibited embrittlement at helium contents above 10 appm, characterized by a strong loss of ductility and creep lifetime. Samples crept and implanted at 900 °C experienced embrittlement for all tested helium contents. Investigations with transmission electron microscopy revealed a substantial clustering and growth of helium bubbles at interfaces in the alpha-2 phase. An average bubble size for embrittlement between 5.3 and 6.7 nm was determined. The behavior and evolution of helium bubbles were studied with post implantation annealing experiments. Samples were implanted at 150, 630, 800 and 1000 °C and subsequently annealed at temperatures from 600 to 900 °C. Material implanted at 150 °C showed no bubbles before annealing, only after post-implantation annealing was widespread clustering observed. Material implanted at temperatures of 630 °C or higher, exhibited bubbles before post-implantation annealing. Depending on the implantation temperature, two different branches were observed in the size and distribution of helium bubbles even after post-implantation annealing. Samples implanted at 150 or 630 °C and then annealed at higher temperatures had high bubble density with small bubbles found in the entire microstructure. Samples implanted at 800 or 1000 °C had low bubble densities and the bubbles were only observed at alpha-2/beta and alpha-2/gamma interfaces, with sizes larger than the average bubble radius for embrittlement. Based on the post-implantation annealing experiments, a temperature limit for the onset of helium embrittlement between 630 and 700 °C is predicted. The helium embrittlement, the loss of initial microstructure and decreased creep resistance at temperatures above 900 °C suggests a limited applicability of lamellar TiAl in advanced nuclear reactor environments.

EPFL2012