971 research outputs found
Approach and avoidance movements are unaffected by cognitive conflict: A comparison of the Simon effect and stimulus-response compatibility
Participants in this study reached from central fixation to a lateral position that either contained or was opposite to the stimulus. Cognitive conflict was induced when the stimulus and response directions did not correspond. In the Simon task, the response direction was cued by the color of the lateral stimulus, and corresponding and noncorresponding trials varied randomly in the same block of trials, resulting in high uncertainty and long reaction times (RTs). In the stimulus-response compatibility (SRC) task, participants reached toward or away from the stimulus in separate blocks of trials, resulting in low uncertainty and short RTs. In the SRC task, cognitive conflict in noncorresponding trials slowed down RTs but hardly affected reach trajectories. In the Simon task, both RTs and reach trajectories were strongly influenced by stimulus-response correspondence. Despite the overall longer RTs in the Simon task, reaches were less direct and deviated toward the stimulus in noncorresponding trials. Thus, cognitive conflict was resolved before movement initiation in the SRC task, whereas it leaked into movement execution in the Simon task. Current theories of the Simon effect, such as the gating of response activation or response code decay, are inconsistent with our results. We propose that the SRC task was decomposed as approaching and avoiding the stimulus, which is sustained by stereotyped visuomotor routines. With complex stimulus-response relationships (Simon task), responses had to be coded as leftward and rightward, with more uncertainty about how to execute the action. This uncertainty permitted cognitive conflict to leak into the movement executio
On Extracting Mechanical Properties from Nanoindentation at Temperatures up to 1000C
Alloyed MCrAlY bond coats, where M is usually cobalt and/or nickel, are
essential parts of modern turbine blades, imparting environmental resistance
while mediating thermal expansivity differences. Nanoindentation allows the
determination of their properties without the complexities of traditional
mechanical tests, but was not previously possible near turbine operating
temperatures.
Here, we determine the hardness and modulus of CMSX-4 and an Amdry-386 bond
coat by nanoindentation up to 1000C. Both materials exhibit a
constant hardness until 400C followed by considerable softening,
which in CMSX-4 is attributed to the multiple slip systems operating underneath
a Berkovich indenter.
The creep behaviour has been investigated via the nanoindentation hold
segments. Above 700C, the observed creep exponents match the
temperature-dependence of literature values in CMSX-4. In Amdry-386,
nanoindentation produces creep exponents very close to literature data,
implying high-temperature nanoindentation may be powerful in characterising
these coatings and providing inputs for material, model and process
optimisations
Softening non-metallic crystals by inhomogeneous elasticity
High temperature structural materials must be resistant to cracking and oxidation. However, most oxidation resistant materials are brittle and a significant reduction in their yield stress is required if they are to be resistant to cracking. It is shown, using density functional theory, that if a crystal's unit cell elastically deforms in an inhomogeneous manner, the yield stress is greatly reduced, consistent with observations in layered compounds, such as TiâSiCâ, NbâCoâ, WâBâ
, TaâC and TaâCâ. The mechanism by which elastic inhomogeneity reduces the yield stress is explained and the effect demonstrated in a complex metallic alloy, even though the electronegativity differences within the unit cell are less than in the layered compounds. Substantial changes appear possible, suggesting this is a first step in developing a simple way of controlling plastic flow in non-metallic crystals, enabling materials with a greater oxidation resistance and hence a higher temperature capability to be used.The work was supported by the EPSRC/Rolls-Royce Strategic Partnership (EP/M005607/1)
Origins of limited non-basal plasticity in {\mu}-phase at room temperature
We unveil a new non-basal slip mechanism in the {\mu}-phase at room
temperature using nanomechanical testing, transmission electron microscopy and
atomistic simulations. The (1-105) planar faults with a displacement vector of
0.07[-5502] can be formed by dislocation glide. They do not disrupt the
Frank-Kasper packing and therefore enable the accommodation of plastic strain
at low temperatures without requiring atomic diffusion. The intersections
between the (1-105) planar faults and basal slip result in stress concentration
and crack nucleation during loading
Microcompression experiments on glasses â strain rate sensitive cracking behavior
Figure 11 â microcompression experiments on glasses showing stable crack growth (a) and reversible deformation (b)
It is well known that the mechanical properties of glasses are closely related to their atomic structure. The exact structure-property-relationship, however, is only poorly understood even for fundamental mechanisms like shear and densification. Nanomechanical test methods like micropillar compression and nano indentation can help fill this gap. In this study a sodium-boro-silicate glass is quenched from different temperatures to induce changes in the atomic structure. Micropillar compression was used to introduce plastic deformation into these glasses at room temperature under a uniaxial stress state. By changing the strain rate it is shown that deformation shifts from completely reversible deformation, to stable crack growth, and finally brittle failure. It is shown that by changing the glass structure, the strain rates corresponding to these deformation regimes are shifted. Finally, the occurrence of shear and densification is discussed. These findings are analysed against the background of the glass structure.
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Deformation of micrometer and mm-sized Fe2.4wt.%Si single- and bi-crystals with a high angle grain boundary at room temperature
Plasticity in body-centred cubic (BCC) metals, including dislocation
interactions at grain boundaries, is much less understood than in face-centred
cubic (FCC) metals. At low temperatures additional resistance to dislocation
motion due to the Peierls barrier becomes important, which increases the
complexity of plasticity. Iron-silicon steel is an interesting, model BCC
material since the evolution of the dislocation structure in
specifically-oriented grains and at particular grain boundaries have
far-reaching effects not only on the deformation behaviour but also on the
magnetic properties, which are important in its final application as electrical
steel. In this study, two different orientations of micropillars (1, 2, 4
microns in diameter) and macropillars (2500 microns) and their corresponding bi
crystals are analysed after compression experiments with respect to the effect
of size on strength and dislocation structures. Using different experimental
methods, such as slip trace analysis, plane tilt analysis and cross-sectional
EBSD, we show that direct slip transmission occurs, and different slip systems
are active in the bi-crystals compared to their single-crystal counterparts.
However, in spite of direct transmission and a very high transmission factor,
dislocation pile-up at the grain boundary is also observed at early stages of
deformation. Moreover, an effect of size scaling with the pillar size in single
crystals and the grain size in bi-crystals is found, which is consistent with
investigations elsewhere in FCC metals
Investigation of the electroplastic effect using nanoindentation
A promising approach to deform metallic-intermetallic composite materials is the application of electric current pulses during the deformation process to achieve a lower yield strength and enhanced elongation to fracture. This is known as the electroplastic effect. In this work, a novel setup to study the electroplastic effect during nanoindentation on individual phases and well-defined interfaces was developed. Using a eutectic Al-Al2Cu alloy as a model material, electroplastic nanoindentation results were directly compared with macroscopic electroplastic compression tests. The results of the micro- and macroscopic investigations reveal current induced displacement shifts and stress drops, respectively, with the first displacement shift/stress drop being higher than the subsequent ones. A higher current intensity, higher loading rate and larger pulsing interval all cause increased displacement shifts. This observation, in conjunction with the fact that the first displacement shift is highest, strongly indicates that de-pinning of dislocations from obstacles dominates the mechanical response, rather than solely thermal effects
Room temperature deformation mechanisms of the C14 Laves Phase in the MgâAlâCa system
In order to improve the creep resistance of magnesium alloys and thereby increase their operating temperature, hard intermetallic phases can be incorporated in the microstructure. In particular the addition of Al or Ca to Mg results in the formation of a skeleton-like intermetallic structure at the grain boundaries. This structure consists predominately of Laves phases, which reduces the minimum creep rate by a few orders of magnitude. In bulk, these Laves phases are extremely brittle at low temperatures, limiting our understanding of the underlying mechanisms of plasticity. Additionally, the small size of the microstructural features in technical alloys make bulk-scale tests unsuitable for studying these phases. Using nanomechanical testing (nanoindentation and microcompression) in individual grains, cracking can be suppressed and plastic deformation can be observed [1]. Micropillars were milled using FIB in individual grains of a polycrystalline specimen, and orientations determined by EBSD to activate and interrogate slip systems. These data have then been combined with slip line analysis around indents. Such an approach reveals the presence of pyramidal, prismatic and basal slip at ambient conditions, with pyramidal 1st order being the predominant slip plane. Critical resolved shear stresses for these slip systems have been calculated, and TEM analysis of the deformation microstructure performed. This work therefore exemplifies how nanomechanical testing in conjunction with electron microscopy can extend the current knowledge of plasticity in macroscopically brittle crystals.
[1] S. Korte, W.J. Clegg, Studying Plasticity in Hard and Soft NbâCo Intermetallics, Advanced Engineering Materials, 14, No. 11 (2012), 991-99
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