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Heterogeneous nucleation of pb particles embedded in a Zn matrix 总被引:1,自引:0,他引:1
R. Goswami K. Chattopadhyay W. T. Kim B. Cantor 《Metallurgical and Materials Transactions A》1992,23(12):3207-3218
Zinc-10 and 20 wt pct Pb alloys have been rapidly solidified by melt spinning to obtain a very fine scale dispersion of nanometer-sized
Pb particles embedded in Zn matrix. The microstructure and crystallography of the Pb particles have been studied using transmission
electron microscopy (TEM). Each embedded Pb particle is a single crystal, with a truncated hexagonal biprism shape with the
6/mmm Zn matrix point group symmetry surrounded by and
facets. The Pb particles solidify with a well-defined orientation relationship with the Zn matrix of
. The melting and solidification behavior of the Pb particle have been studied using differential scanning calorimetry (DSC).
The Pb particles solidify with an undercooling of approximately 30 K, by heterogeneous nucleation on the {0001} facets of
the surrounding Zn matrix, with an apparent contact angle of 23 deg. 相似文献
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The deformation characteristics contributing to the superior cavitation erosion properties of HCP cobalt single crystals have
been determined. Results indicate that its erosion response is highly orientation sensitive. A homogeneous distribution of
and
glide occurs in {0110} crystals, whereas slip in the (0001) crystals is much more heterogeneous and consists mainly of
dislocations. Continued exposure to cavitation nucleates a large number of twins, predominantly on the
and
planes in the
and (0001) crystals respectively. The former twins are finer and more needle-like than the latter. The
crystals are also significantly more erosion resistant than the (0001) crystals. The twin density increases continuously
with cavitation exposure until a dense network of twins spans the entire exposed area. This fine-scale twinning is considered
responsible for the superior erosion resistance of the metal. 相似文献
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Joo Hyun Park Sang-Beom Lee Henri R. Gaye 《Metallurgical and Materials Transactions B》2008,39(6):853-861
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The crystallographic direction of growth in directionally solidified magnesium alloy AZ91 has been studied by TEM and EBSP
techniques in SEM. The main direction of growth is found to be
. The dendrites have sixfold symmetry around the main direction, with secondary arms lying along the traces of the (0001),
, and
-planes, respectively. The secondary arms lying in the basal plane are crystallographically of the same type as the main direction:
and
. 相似文献
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The dynamics of dislocations in both steady state and transient creep in alpha iron of about 99.5 pct purity was investigated
in the temperature interval 773 to 923 K, and the applied stress range 24.5 to 220.5 MN m−2. The applied stress sensitivity parameter of the steady state creep rate m∲ = (∂ In ε/∂ In σ)
T
increased linearly with increasing applied stress σ from about 5 at σ = 24.5 MN m−2 to about 12 at σ = 196 MN m−2. The apparent activation energy of steady state creep rate
was found to decrease linearly with stress from 89 K cal mol−1 at σ = 98 MN m−2 to 81 K cal mol−1 at a = 147 MN m−2. Measurements of the mean effective stress σ* by the strain transient dip test technique led to a nonlinear relation between σ* and σ, indicating a dependence of the ratio σ*/σ on the applied stress. The effective stress sensitivity parameter
was lower than m′.However, the apparent activation energy
was equal toQ. Using the stress sensitivity technique, the relation between transient creep rate and effective stress at various constant
internal stresses and temperatures was obtained. The effective stress sensitivity of transient creep rate
was found to be lessthan that of steady creep rate. 相似文献
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Twins as barriers to basal slip in hexagonal-close-packed metals 总被引:2,自引:0,他引:2
The boundary structure of
,
,
, and
twins in hexagonal-close-packed (hcp) metals and the interaction of crystal dislocations with the first two twin types have
been studied previously using atomic-scale computer simulation. The interaction of crystal dislocations with
and
twin boundaries is described here and compared with the results for
and
twins. These four twins are found to create barriers to the motion of crystal dislocations gliding on the basal plane, and
the strength of the barrier depends in a relatively complex manner on crystallographic parameters and details of the atomic
structures of the interfaces. In some circumstances, crystal dislocations can be transmitted through the twin boundary, thereby
creating twinning dislocations.
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献
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Interactions between deformation twin and 120 deg-rotational domain boundary were studied by transmission electron microscopy
in a two-phase TiAl-based alloy with fully lamellar structure deformed at room temperature. Three types of the interaction
were observed, depending on the interaction geometry and crystallography faced by the incident twinning Shockleys. It was
found that the incident twinning shear could be accommodated into the barrier domain by a reaction involving emission of 1/2
{111}
B
slip in all the three types of interactions presumably since the slip required a small critical resolved shear stress (CRSS)
and was always favored by the pile-up stress. Several reaction schemes involving 1/2
{111}
B
slip for each type of the interactions were proposed by considering whether the reaction resulted in a reduced elastic energy
and if the dissociated dislocations were able to glide away to minimize the total elastic energy associated with a long-range
stress field of a pileup of the incident twinning partials. It is suggested that whether a reaction scheme is feasible would
depend on behavior of other product dislocation except 1/2
{111}
B
. 相似文献
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The competition of slip among
, and
slip modes of hcp metals has been analyzed geometrically in terms of a critical resolved shear stress, CRSS, criterion. Under
the action of an applied stress slip systems of one or more modes may be activated depending on the value of the CRSS and
on the orientation of the slip systems with respect to the applied stress. If the CRSS of a given slip mode should exceed
a limiting value relative to the CRSS of the other modes, however, the given mode becomes inoperative even under the most
favorably stressed conditions. It is found by an examination of the yield loci that basal slip is inoperative if α2 < cos θ; prism slip is inoperative if α2 < α1 sin θ; and pyramidal slip is inoperative if α2 > cos θ + α1 sin θ where
and
are, respectively, the ratios of CRSS for prism and pyramidal slips relative to basal slip, and ϕ is the angle between the
(0001) and
normals. Since the value of ϕ is a function ofc/a, the limiting values of α1 and α1 depend on thec/a ratio of the crystal structure. 相似文献