Authors: E. Héripré, D. Caldemaison, A. Roos, J. Crépin
Abstract: The aeronautic and automotive industries have shown a renewed interest in TiAl based
alloys. The main reasons for such an interest are their low density (~3,8g/cm3), a good stiffness and
a high strength for temperatures up to 750°C. However, these alloys exhibit, in their polycrystalline
form, a poor ductility at room temperature with widely scattered values. The aim of this study is
therefore to characterise their mechanical behaviour with a multiscale methodology, coupling
microstructure analysis and strain field measurements. This methodology employs orientation
imaging microscopy as well as digital imaging correlation techniques with an intragranular step size
of a few micrometers.
Two chemical compositions (47 at. % Al and 48 at. % Al) and two processing routes (casting and
powder metallurgy) are studied. Thus, four different types of final microstructures are considered,
from fully lamellar Ti3Al (a2) + TiAl (g) microstructure to bimodal ones composed of two-phase
(a2+g) lamellar grains and monolithic g grains. Firstly, the microstructure is characterised
crystallographically and morphologically. This allows the identification of a representative volume
element (RVE) inside the analysed volume. Then, uniaxial mechanical tests are performed for each
microstructure, and the strain fields are analysed with a multiscale approach, which determines the
spatial distribution of the strain field heterogeneity with respect to the different microstructures.
1330
Authors: Paul van Houtte, Anand Krishna Kanjarla, Laurent Delannay
Abstract: A CPFE model was used for an assessment of the assumptions used by the ALAMEL model concerning grain interactions. A finite element mesh was constructed for a multicrystal consisting of four grains. There were 17496 integration points per grain. The main goal was to capture the complex nature of the plastic fields in the vicinity of the grain boundaries. The distribution of strain rates, both along and perpendicular to the grain boundaries, confirms that the basic assumptions of the ALAMEL model are qualitatively correct, except at triple junctions. Splitting of one of the grains was occasionally observed, which has also been observed experimentally.
190
Authors: Ikuko Yamada, Shoichi Kume, Hiromi Nakano, Koji Watari
Abstract: Thermal properties of SiC at the micrometer-scale were measured quantitatively with a thermal microscope using thermo-reflectance and periodic heating techniques. In this study, SiC single crystal and polycrystal were investigated. The small values of standard deviation suggest that the SiC single crystal had constant thermal conductivities. For the single crystal, the average value of the thermal conductivity at the micrometer-scale was in good agreement with the macro-scale thermal conductivity value obtained by the laser flash technique. On the other hand, thermal conductivity of the polycrystal was heterogeneous at the micrometer-scale. An average thermal conductivity value of 257 Wm-1K-1 was obtained within an area of 50 m ×100 µm. The highest and lowest values of the thermal conductivity from the polycrystal were 300 and 220 Wm-1K-1, respectively.
179
Authors: Ai Qin Wang, Jing Pei Xie, Zhong Xia Liu, Ji Wen Li, Wen Yan Wang, Shu Qing Yan
Abstract: In the present work, rapidly solidified alloys strips with Al-0.24Ti and Al-21Si-0.24Ti(in
wt.%) were prepared by single roller melt-spinning method. The microstructures, phase and
morphology characteristics of the resultant strips were characterized by means of scanning electron
microscopy (SEM),transmission electric microscopy (TEM) and XRD technique. The results show
that the grains have been refined after rapid solidification processing, and the micro-nanocrystalline
grain are formed. The morphology characteristics can be changed. The microstructures of Al-0.24Ti
alloys strip are micro-nanostructure α-Al solid solutions which are similar with granular or nodular,
the corresponding SAD pattern is rings, it presents characteristic of polycrystal; Compared with
equilibrium solidification, the microstructures of hypereutectic Al-Si alloy are changed obviously.
They are composed of primary micro-nanostructure α-Al supersaturated solid solution and
nanocrystal granular (α+Si) eutectic which set in the supersaturated solid solution. The nucleation and
growth of primary silicon are suppressed and primary silicon can not precipitate, meanwhile, α-Al
phase is nucleated which prior to eutectic, therefore the microstructures become into the metastable
state. The mechanism of the formation for microstructures of melt-spinning alloys has also been
discussed.
27
Authors: Shu Yan Zhang, Jordan Schlipf, Alexander M. Korsunsky
Abstract: A traditional approach to increasing fatigue resistance of many assemblies involves the
creation of regions of compressive residual stress. For example, riveting holes used in modern
passenger aircraft are currently subjected to cold expansion using split mandrel tools. The method is
relatively expensive and not entirely problem-free. In the present study we consider a method of
creating residual stresses around drilled holes referred to as ‘dimpling’, that itself is a variation of a
novel technique known as the StressWaveTM process. An experimental procedure is described for
the creation of localised regions of significant plastic deformation and residual stress by ‘dimpling’,
allowing the production of cold-worked and residually-stressed specimens. The overall aims of this
study were to determine thickness-average residual stresses by two different techniques, namely,
one destructive technique (Sachs boring) and one non-destructive (high energy X-ray diffraction);
and to compare the results. In Sachs boring experiments the variation of strain gauge readings with
increasing diameter of the central hole was recorded. A classical elastic-ideally plastic axisymmetric
model for plane stress conditions was used in the analysis. Energy dispersive synchrotron X-ray
diffraction experiments were performed for non-destructive assessment of residual elastic strains.
The two different stress evaluation techniques used in this project led to consistent results. Good
correlation was found between the stresses obtained from X-ray diffraction results and those
deduced from Sachs boring experiments.
295
Authors: Xu Song, Shu Yan Zhang, Daniele Dini, Alexander M. Korsunsky
Abstract: Most models based on continuum mechanics do not account for inhomogeneities at the
micro-scale. This can be achieved by considering a representative volume of material and using
(poly)crystal elasto-plastic deformation theory to model the effects of grain morphology and
crystallographic orientation. In this way, the relationship between the macroscopic stress state and
the stress state at the grain level can be investigated in detail. In addition, this approach enables the
determination of the inhomogeneous fields of plastic strain, the identification of regions of localised
plasticity (persistent slip bands), grain level shakedown, and the prediction of fatigue crack
initiation using energy dissipation at the micro-scale. Elastic anisotropy is known to promote earlier
onset of yielding, and to increase the magnitude of intergranular residual stresses. The effect of
hardening behaviour of different slip systems on intergranular residual stresses is more subtle, as
discussed in the text. The present study focuses on the analysis average intergranular residual
strains and stresses that arise within the polycrystal aggregate following the application of single or
cyclic external loading. These residual strains can also be evaluated experimentally using diffraction
of penetrating radiation, e.g. neutrons or high energy X-rays, allowing comparisons with the model
predictions to be made.
271
Authors: Alexander M. Korsunsky
Abstract: The sin2ψ technique for near-surface and bulk stress evaluation is frequently considered
to be the method of reference, largely due to the historical reason of being established early on in the
development of experimental study of residual stress, and due to the widespread availability of
laboratory X-ray facilities equipped with goniometers allowing ψ-tilting to be carried out. In recent
years other diffraction-based techniques of residual strain and residual stress evaluation have been
developed, some of them based at large facilities such as synchrotrons, neutron reactors or spallation
sources, and others becoming available in the laboratory setting. It is therefore perhaps relevant and
timely to review the strengths and shortcomings of the sin2ψ technique in today’s context. In the
present study this task is addressed through the use of polycrystal elasto-plastic modelling that
allows the determination of equivalent average elastic lattice strains following complex deformation
history, and by post-processing of the model results in order to extract the parameters measurable in
diffraction experiments. In particular, it is possible to extract the simulated strain values that would
be measured at different tilt angles, and to build a family of sin2ψ plots for different reflections. It
then becomes possible to assess the degree to which the hypotheses underpinning the principle of
this method are enforced or violated; to select the most suitable reflections; and to discuss how the
method could be improved or varied to provide more reliable residual stress measurement
procedures.
219
Authors: In Soo Kim, Baig Gyu Choi, Seong Moon Seo, Doo Hyun Kim, Chang Yong Jo
Abstract: The effect of heat treatment in superalloy CM247LC on tensile properties at various
temperatures has been investigated. In the case of equiaxed poly-crystal specimen, the aged condition
(HTA condition) has higher strength than the solution treated and aged condition (HTSA condition)
at low temperature. The HTSA condition exhibits higher strength than HTA condition in
directionally solidified specimen in all temperature range. Shearing of γ´ particle is a principal
deformation process at the low temperature, and cutting of fine secondary γ´ particle plays important
role in the early stage of deformation in the HTA poly-crystal specimen. Tangles of short misfit
dislocations form at the γ/γ´ interface during the high temperature deformation due to the partial loss
of coherency at the interface. The rafting of γ´ increases tensile strength of the HTSA specimen at and
above 871°C.
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Authors: Naoshi Yamaki, Yoshiteru Aoyagi, Kazuyuki Shizawa
Abstract: A multiscale model on dislocation patterning of cell structure and subgrain for
polycrystal is newly developed on the basis of reaction-diffusion theory. A FD simulation for
dislocation patterning and a FE one for crystal deformation are simultaneously carried out for a
FCC polycrystal at large strain. Reflecting stress value on stress-effect coefficients, it is numerically
predicted that the evolution of dislocation pattern in a polycrystal is different in response to the
stress condition of each grain.
205
Authors: Paulo Rangel Rios, Martin E. Glicksman
Abstract: One common point amongst extant theories of abnormal grain growth (AGG) is that they
treat this phenomenon in terms of the relative grain size, or grain radius, of the abnormal grains.
Topological and metrical quantities of abnormal grains, such as the number of their faces, or their
grain boundary curvature, are taken into account only indirectly through the grain size itself. This
paper, by contrast, treats AGG in terms of concepts, that include both the boundary curvature and
the number of faces of the abnormal grain. Two cases are examined: 1) AGG, in which the matrix
grains are fully pinned, so normal grain growth cannot occur; 2) AGG in which the matrix grains
are free to evolve, so that normal grain growth ensues simultaneously in the matrix.
2401