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1.
Cyclic deformation behavior and dislocation substructure in the single crystals of Ti-Al alloy containing 2.0 pct Al (by atom)
with double prism slip at different cyclic strain amplitudes were examined. The testing results showed that Ti-2Al displayed
an initial slight cyclic hardening followed by a striking softening period, and a saturation stage was reached finally. Secondary
cyclic hardening was observed for most specimens before failure, specifically at the high cyclic strain amplitudes. The cyclic
resolved shear stress-strain curve (CSSC) was observed to contain a plateau in the testing strain range with a saturation
plateau stress of about 55 MPa. Trace analysis on the surface of specimens with an optical microscope shows that the (10
0) and (1
00) double prism slips can be distinguished from the traces on the (0001) surface of the fatigued specimens. The dislocation
substructure in the fatigued specimens was examined using a transmission electron microscope (TEM). The typical dislocation
configuration is the saturated dislocation bundles, which are tangled on the (10
0) and (1
00) slip planes and arranged parallel to the [0001] direction. 相似文献
2.
The boundary structure of {10
1}, {10
2}, {11
1} and, {11
2} 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 {11
1} and {11
2} twin boundaries is described here and compared with the results for {10
1} and {10
2} 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
3.
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
4.
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
5.
Eli S. Puchi-Cabrera 《Metallurgical and Materials Transactions A》2003,34(12):2837-2846
The constitutive behavior of an aluminum 0.1 wt pct Mg alloy deformed in the temperature range of 573 to 823 K at strain rates
between 0.001 and 100 s−1 is analyzed on the basis of the concept of the mechanical threshold stress (MTS),
, taking into consideration the contributions from the different strengthening mechanisms that could be present in this alloy,
, which include one component that arises from the interaction between dislocations and solute atoms,
, and another contribution from the interaction between mobile and forest dislocations,
. The evolution of
is described in terms of a generalized form of an exponential-saturation equation, whereas the characterization of the ratio,
s
i
(
, T), between the flow stress at any strain rate and temperature, and the two components
is carried out by means of the phenomenological model advanced by Kocks and co-workers. It is shown that the experimental
values of the flow stress as well as the work-hardening rate can be accurately described following this approach and that
the maximum difference between the experimental and calculated values of such a parameter is less than ±4 MPa. The analysis
conducted from continuous stress-strain curves determined at constant temperature and strain rate indicates that the relaxation
strain in the generalized form of the Sah et al. relationship displays a significant strain rate dependence. The inclusion of such a dependence into the analysis by means
of a simple parametric relationship leads to an improvement in the accuracy of the prediction of the model. 相似文献
6.
Sungsoo Kim 《Metallurgical and Materials Transactions A》2006,37(1):59-68
Recently, it was proven that delayed hydride cracking (DHC) is accompanied by deformation twinning through the texture analysis
of a fractured surface. Thus, in order to understand the operation of deformation twinning, the texture variations by rolling
were investigated using Zr-2.5 pct Nb alloy with {11
0} 〈10
0〉 texture. It was observed that deformation twinning was operated predominantly in the range of a 5 to 15 pct strain. The
basal poles were rotated in the normal direction of a rolling plane with the strain, and the (0002) texture was fully reversed
after 15 pct strain. This finding was established to be due to the operation of the {10
2} and {11
1} twinning systems through the analysis of the inverse pole figure. It appeared that the degree and easiness of the twinning
operation was affected by changing the direction of compression during rolling with respect to the initial {11
0} 〈10
0〉 texture. The contribution of deformation twinning to strain was quantitatively calculated using the change in the basal
pole components.
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
7.
Growth morphology and number
per unit volume have been determined vs withdrawal velocity V over the range 0.1 to 4 mm/s for primary Mg2Si in a Bridgman-solidified hypereutectic Al-Mg-Si alloy. Primary Mg2Si shows a transition from irregular or regular polyhedral to dendritic with increasing V, and
increases with solidification cooling rate
according to the relationship
. These results are compared with corresponding ones for Al-Mg-Si alloy wedge castings and for primary silicon in hypereutectic
Al-Si alloys. 相似文献
8.
9.
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
10.
The development of characteristic dislocation structures in pure zirconium and zircaloy-4 fatigued under pull-push strain
control as the testing temperature and the cyclic strain range varied was examined using a thin-foil transmission electron
microscopy (TEM) technique. The slip planes and the twinning planes were determined by a standard stereographic trace analysis
technique. The first-order prismatic slip {10
0} is the primary deformation mode in zirconium and zircaloy-4 fatigued from room temperature (RT) to 873 K. The pyramidal
sli {
2
} is activated at 673 K and at high cyclic strain ranges, whereas the basal slip {0001} only appears in those specimens fatigued
at 873 K. The {10
2}, {11
1}, and {11
2} types of twins were detected in specimens fatigued at RT. Twinning becomes less frequent as the testing temperature increases.
The schematic map of the cyclic deformation modes as a function of the plastic strain range and the test temperature is described.
The dislocation configurations in fatigued pure zirconium specimens evolve from a planar arrangement to a cell structure as
the test temperature and the strain range increase. For zircaloy-4, the fatigued dislocation structure is parallel dislocation
lines at RT, cells at 673 K, and two sets of approximate mutually perpendicular dislocation bands at 873 K, respectively.
Finally, the fatigued dislocation-structure evolution map with the cyclic strain range and the test temperature are qualitatively
established for zirconium and zircaloy-4, respectively. The effect factors on the fatigue mechanism and the thermodynamic
and dynamic criteria of the dislocation-pattern evolution are discussed. 相似文献
11.
Evolution of microstructure and texture in Mg-Al-Zn alloys during electron-beam and gas tungsten arc welding 总被引:3,自引:0,他引:3
The evolution of microstructure and texture in the AZ-series Mg alloys subjected to electron-beam welding and gas tungsten
arc welding are examined. Electron-beam welding is demonstrated to be a promising means of welding delicate Mg plates, bars,
or tubes with a thickness of up to 50 mm; gas tungsten arc welding is limited to lower-end thin Mg sheets. The grains in the
fusion zone (FZ) are nearly equiaxed in shape and ∼8 μm or less in size, due to the rapid cooling rate. The as-welded FZ microhardness and tensile strength are higher than the
base metals due to the smaller grain size. The weld efficiency, defined as the postweld microhardness or tensile strength
at the mid-FZ region divided by that of the unwelded base metal, is around 110 to 125 pct for electron-beam welding and 97
to 110 pct for gas tungsten arc welding. There are three main texture components present in the electron-beam-welded (EBW)
FZ, i.e.,
(with TD//
),
(with ND∧
∼15 deg), and
(with WD∧
∼30 deg), where TD, ND, and WD are the transverse, normal, and welding directions, respectively. The crystal growth tends
to align toward the most closed-packed direction,
. The texture in gas tungsten arc welded (GTAW) specimens is more diverse and complicated than the EBW counterparts, due to
the limited and shallow FZ and the lower cooling rate. The cooling rates calculated by the three-dimensional (3-D) and two-dimensional
(2-D) heat-transfer models are considered to be the lower and upper bounds. The cooling rate increases with decreasing Al
content, increasing weld speed, and increasing distance from the weld top surface. The influences of the FZ location, welding
speed, and alloy content on the resulting texture components are rationalized and discussed. 相似文献
12.
Creep crack growth rate (
) is usually characterized in terms of macroscopic load parameters, such as C*, C
t
and C(t), through the constant load test. However, load parameters are continuously changing during the test, and so is
. Here, by conducting constant C
t
and constant
tests, quasi-steady-state crack growth was obtained where
remained almost constant. Results indicate the
∼[C
t
]0.76 correlation, which differ from the
∼[C
t
]0.96 correlation of the constant load test. Discrepancies can be ascribed to the inclusion of the stage II data, which showed
no correlation between
and C
p
in the constant load analysis. Finally, the crack growth rate was well predicted using the Monkmam-Grant analysis in creep
crack growth. 相似文献
13.
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
14.
Crystallographic details of precipitates in Fe-22Cr-21Ni-6Mo-(N) superaustenitic stainless steels aged at 900 °C 总被引:1,自引:0,他引:1
Tae-Ho Lee Sung-Joon Kim Yun-Chul Jung 《Metallurgical and Materials Transactions A》2000,31(7):1713-1723
Crystallographic features of second phases and the effect of nitrogen addition on the microstructural evolution in superaustenitic
Fe-22Cr-21Ni-6Mo-(N) (all in wt pct) stainless steels during isothermal aging at 900 °C were investigated using scanning electron
microscopy (SEM) and transmission electron microscopy (TEM). Both alloys with and without nitrogen contained sigma phase and
M23C6 carbides in the solution treated condition. While four phases (sigma, M23C6, M6C, and chi) of intermetallics and carbides appeared sequentially as a function of aging time in the nitrogen-free alloy, two
nitrides (Cr2N and AlN) were additionally observed after long time aging of the nitrogen-containing alloy. The addition of nitrogen into
Fe-22Cr-21Ni-6Mo steel promoted a finer and more uniform distribution of precipitates during isothermal aging. The exact identification
and crystallography of various second phases were confirmed from the analyses of selected area diffraction patterns from various
orientations, stereographic projection, and energy dispersive spectroscopy. The orientation relationships between the precipitates
and austenite matrix can be summarized as follows: (1) two carbides (M23C6 and M6C): cube-on-cube orientation relationship; (2) chi phase: Kurdjumov-Sachs (K-S) orientation relationship; (3) two nitrides
(Cr2N and (AlN): (11
0)nitrides //(211)
γ
and [0001]nitrides //[
11]
γ
; and (4) sigma phase: (1) (
11)
γ
//(00
)
σ
and [
0]
γ
//[
0]
σ
or (2) (
10)
γ
//(
10)
σ
and [
2]
γ
//[113]
σ
. For the sigma phase, the former orientation relationship was predominant throughout aging, and the latter orientation relationship
was occasionally observed under limited aging conditions. 相似文献
15.
16.
A new experimental method is given for recovering the probability-distribution function S
v
(
|Δg). The function S
v
(
|Δg) is the grain-boundary area per unit volume as a function of grain-boundary plane orientation (
), given a lattice misorientation (Δg) between the adjoining grains. The grain-boundary normal (
) is expressed in the crystal frame in which the misorientation Δg originates. The proposed method recovers the three-dimensional S
v
(
|Δg) function using data taken from two-dimensional section planes. The method requires the measurement of many grain-boundary
trace (in-plane) angles and lengths associated with grain boundaries of lattice misorientation. All such boundary traces may
be observed from a single section plane if the crystallographic texture is sufficiently random. In heavily textured microstructures,
the method requires the researcher to observe traces from multiple oblique section planes cut through the material. A method
of quantitatively estimating whether the texture is sufficiently random is given. Simulations on both textured and nontextured
microstructures demonstrate the validity of the method. Experimentally, the new method is used to analyze boundaries of misorientation
(Σ3) observed in 304 stainless steel. Calculated grain-boundary plane-probability functions are shown to be consistent with
what is already known.
This article is based on a presentation made at the symposium “Characterization and Representation of Material Microstructures
in 3-D” held October 8–10, 2002, in Columbus, OH, under the auspices of ASM International’s Phase Transformations committee. 相似文献
17.
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
18.
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
19.
M2C carbides were observed to precipitate within the austenite matrix of an Fe-24.6Mn-6.6Al-3. IMo-1.0C alloy after quenching
from 1200 °C and aging at 750 °C. By means of transmission electron microscopy (TEM) and diffraction techniques, the orientation
relationships between M2C (p) and the austenite (γ) matrix were determined to be: (0001)p//(111)γ, (11–
0)p// (1
0)γ, (
100)p//(11
)γ. M2C carbide has been reported by many researchers to precipitate from the ferrite matrix or along austenite/ferrite boundaries
in alloy steels containing Mo. However, little information concerning the formation of M2C in the austenite matrix has been provided. This investigation presents the first evidence for the existence of M2C carbide wholly within the austenite matrix and its relationship to the austenite. The energy-dispersive spectrometry (EDS)
analyses were performed on M2C carbides, and the results indicate that the solubility of the M2C carbide for foreign atoms other than Mo is very limited. 相似文献
20.
A novel in situ process was developed to produce titanium matrix composites reinforced with TiB and TiC of different mole ratios in which
traditional ingot metallurgy plus self-propagation hightemperature synthesis (SHS) reactions between Ti and B4C, graphite powder were used. Microstructures of (TiB+TiC)/Ti in situ composites were comprehensively characterized using scanning electron microscopy (SEM), transmission electron microscopy
(TEM), and high-resolution transmission electron microscopy (HRTEM). Solidification paths were investigated using a differential
scanning calorimeter (DSC). Results show that there is an apparent difference in morphologies of reinforcements. The reinforcements
nucleate and grow from the melt in a way of dissolution precipitation. The different morphologies are related to their solidification
paths and the particular crystal structure of the reinforcement. TiB grows along the [010] direction and forms short-fiber
shape due to its B27 structure, whereas TiC with NaCl type structure grows in a dendritic, equiaxed, or near-equiaxed shape.
The DSC results and analysis of the phase diagram yield three stages for the solidification paths of in situ synthesized titanium matrix composites: (1) primary phase, (2) monovariant binary eutectic, and (3) invariant ternary eutectic.
The addition of graphite adjusts the solidification paths and forms more dendritic primary TiC. The addition of aluminum does
not change the solidification paths. However, the reinforcements grow finer and lead to equiaxed or near-equiaxed TiC morphologies.
The following consistent crystallographic relationships between TiB and titanium were observed by HRTEM, i.e., [010]TiB//[
]Ti, (100)TiB//(
)Ti, (001)TiB//(0002)Ti, (
)TiB//(
)Ti and [001]TiB//[
]Ti, (
)TiB//(
)Ti, (200)TiB//(0002)Ti. The formation of the preceding crystallographic relationships is related to the growth mechanism of TiB. It also helps to
minimize the lattice strain at the interfaces between TiB and the titanium matrix. 相似文献