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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 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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Z.H. Nie Y.D. Wang G.Y. Wang J.W. Richardson G. Wang Y.D. Liu P.K. Liaw L. Zuo 《Metallurgical and Materials Transactions A》2008,39(13):3113-3119
The phase transition and influence of the applied stress on the texture evolution in the as-cast Ni-Mn-Ga ferromagnetic shape-memory
alloys were studied by the time-of-flight (TOF) neutron diffraction technique. The neutron diffraction experiments were performed
on the General Purpose Powder Diffractometer (Argonne National Laboratory). Inverse pole figures were determined from the
neutron data for characterizing the orientation distributions and variant selections of polycrystalline Ni-Mn-Ga alloys subjected
to different uniaxial compression deformations. Texture analyses reveal that the initial texture for the parent phase in the
as-cast specimen was composed of
, , , and , which was weakened after the compression deformation. Moreover, a strong preferred selection of martensitic-twin variants
(and ) was observed in the transformed martensite after a compression stress applied on the parent phase along the cyclindrical
axis of the specimens. The preferred selection of variants can be well explained by considering the grain/variant-orientation-dependent
Bain-distortion energy.
This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,”
which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science
Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee. 相似文献
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Ternary interdiffusion coefficients were measured in the Ni solid solution γ (fcc) phase of the Ni-Cr-Al system at 1100 and
1200 °C. Extensive use was made of both γ/γ and γ/γ + β (β-NiAl structure) diffusion couples. Two analysis techniques were
employed to calculate the interdiffusion coefficients. When the Matano planes for Al and Cr were not coincident, numerous
integral calculations were made to determine an average diffusion coefficient and to assess the effect of the noncoincidence
of the Matano planes. The results of the diffusivity measurements showed that
is approximately four times greater than
, while
and
are of the same magnitude. For all concentrations,
is two to three times greater than
. Both
and
increase with increasing Al concentration, whereas
and
show little concentration dependence on Cr alone. A ternary, finite-difference interdiffusion model was employed to predict
concentration profiles for the γ/γ couples utilizing the concentration dependence of the measured diffusivities. Good agreement
was observed between the predicted and measured concentration profiles for both 1100 and 1200 °C. 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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Electron-backscattered diffraction (EBSD) has been used to investigate the texture evolution during tensile deformation at
temperatures between 673 and 773 K of a coarse-grained commercial AZ31 magnesium alloy. A weak (0001) fiber texture was initially
present in the hot-rolled magnesium alloy plate. The [0001] directions of the grains spread 0 to 45 deg around the normal
direction (ND) of the magnesium alloy plate. This pre-existing weak texture evolved during tensile deformation into a strong
texture close to the {0001} 〈1
00〉. The [0001] directions of the grains rotated toward the orientations perpendicular to the tension axis of the samples,
indicating that the 〈11
0〉 slip system appeared to be the most active slip system, especially in the early stages of deformation. The EBSD Schmid-factor
analysis revealed that, however, with an increase in strain and the rotation of the (0001) slip plane, the {11
2} 〈11
〉 slip system appeared to be more favorable. The {1
00} 〈11
0〉 and {1
01} 〈11
0〉 slip systems remained favored throughout the strains investigated, indicating that {1
00} and {1
01} are two important slip planes for cross slip using the 〈11
0〉 slip vector. It is found that the misorientation across one coarse grain (as high as 38.2 deg) is accommodated by low-angle
grain boundaries (LAGBs). The formation of these LAGBs may be an intermediate stage of the coarse grain refinement that occurred
during deformation.
This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,”
which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural
Materials Committee. 相似文献
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S. Hata T. Nakano N. Kuwano M. Itakura S. Matsumura Y. Umakoshi 《Metallurgical and Materials Transactions A》2008,39(7):1610-1617
The ordering mechanism of long-period superstructures (LPSs) in Al-rich TiAl alloys has been investigated by high-resolution
transmission electron microscopy (HRTEM). The LPSs are classified in terms of arrangements of base clusters with different
shapes and compositions formed in Ti-rich (002) layers of L10-TiAl matrix: square Ti4Al, fat rhombus Ti3Al, and lean rhombus Ti2Al type clusters. The HRTEM observations revealed that antiphase boundaries of long-range-ordered LPS domains and short-range-ordered
microdomains are constructed by various space-filling arrangements of the base clusters. Such a microscopic property characterized
by the base clusters and their arrangements is markedly analogous to that of the
* special-point ordering alloys such as Ni-Mo.
This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part
of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.
相似文献
S. Hata (Associate Professor)Email: |
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K. Kakehi 《Metallurgical and Materials Transactions A》1999,30(5):1249-1259
The effect of crystallographic orientation and aging heat treatment at 850 °C on the creep rupture strength of single crystals
of a nickel-based superalloy was examined at 700 °C in detail. Initial tensile orientations were selected over a wide range
on the standard stereographic triangle. The {111}〈112〉-type slip systems were found to be operative during the creep deformation.
The creep behavior was found to be greatly influenced by the additional aging at 850 °C for 20 hours. It was found that the
effect of the aging at 850 °C was quite different between orientations favored for the
slip system and those favored for the (111)
slip system and that the creep deformation mechanisms of these two slip systems were different. In the orientations favored
for
slip systems, in the single-aged specimens, a small mean surface-to-surface spacing due to hyperfine γ′ precipitates in the matrix channel promoted the
slip and the primary creep. As a result of the additional aging at 850 °C, the hyperfine γ′ precipitates were dissolved into the matrix, and the resultant large mean surface-to-surface spacing between the cuboidal
precipitates inhibited extensive shearing of the γ-γ′ structure by the
slip system. As a result, the creep strengths of these orientations were increased in double-aged specimens; however, the
low ductility associated with the difficulty of secondary noncoplanar slip did not enlarge rupture lifetime in the double-aged
[001] specimen. In the orientations favored for the (111)
slip system, creep deformation occurred by twinning shear through γ and γ′ precipitates, and a distinct effect of the aging at 850 °C was not observed. In the multiple orientation of the {111}
-type slip systems, i.e., the
and
orientations, hyperfine precipitates improved creep strength because they prevented dislocations from gliding in the matrix
channel in the single-aged specimens. 相似文献