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1.
Toughness-strength relations in the overaged 7449 al-based alloy 总被引:6,自引:0,他引:6
This article examines the relationship between plane strain fracture toughness, K
Ic
, the tensile properties, and the microstructure of the overaged 7449 aluminum-plate alloy, and compares them to the 7150
alloy. The 7449 alloy has a higher content of η′/η precipitates; and, the 7150 alloy contains a greater amount of coarse intermetallic particles, as it contains an appreciable
amount of coarse S phase (Al2CuMg), which is largely absent in the 7449 alloy. The toughness of the alloys shows an increase on overaging, and the 7449
alloy shows a reasonably linear toughness—yield strength relation on extended overaging. Several mechanisms of failure occur:
coarse voiding at intermetallics and a combined intergranular/transgranular shear fracture mode, with the former becoming
more important as overaging progresses. Drawbacks of existing models for toughness are discussed, and a new model for plane
strain fracture toughness, based on the microstructurally dependent work-hardening factor, K
A
, introduced in Ashby’s theory of work hardening, is developed. This model predicts a linear relation between K
Ic
and K
A
/0.85
/σ
ys
/0.35
, where σ
ys
is the yield strength, which is consistent with the experimental data. 相似文献
2.
The strength,fracture toughness,and low cycle fatigue behavior of 17-4 PH stainless steel 总被引:1,自引:0,他引:1
The influence of microstructure on the strength, fracture toughness and low cycle fatigue behavior of 17-4 PH stainless steel
has been examined. Aging hardening involves initial formation of coherent copper-rich clusters which transform to incoherent
fee ∈-copper precipitates upon further aging. The changes in strength level and strain hardening rates observed during aging
are consistent with previously suggested models for precipitation hardening based on differing elastic moduli. The fracture
toughness and fatigue crack growth rates were shown to be a function of microstructure and environment. At equivalent strength
levels overaging resulted in a higher fracture toughness than did underaging. The fatigue crack growth rates increased with
increasing strength level and humidity but were not a function of toughness level. Attempts to correlate the fatigue crack
growth rates with monotonie tensile properties were unsuccessful. However when final failure obeyed a critical strain criteria,
the fracture toughness behavior could be reasonably described and related to preferential void nucleation and growth at δ-ferrite-matrix
interfaces. 相似文献
3.
Evaluation of toughness in AISI 4340 alloy steel austenitized at low and high temperatures 总被引:2,自引:0,他引:2
Robert O. Ritchie Benjamin Francis William L. Server 《Metallurgical and Materials Transactions A》1976,7(5):831-838
It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of
conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,K
Ic, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data
in terms of the difference betweenK
Ic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but
can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence
of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and
other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment
procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should
not be based solely onK
Ic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required.
formerly with Effects Technology, Inc., Santa Barbara, CA 相似文献
4.
Michael J. Haynes Richard P. Gangloff 《Metallurgical and Materials Transactions A》1997,28(9):1815-1829
The plane-strain initiation fracture toughness (K
JICi
) and plane-stress crack growth resistance of two Al-Cu-Mg-Ag alloy sheets are characterized as a function of temperature
by a J-integral method. For AA2519 +Mg+Ag, K
JICi
decreases from 32.5 MPa√m at 25 °C to 28.5 MPa√m at 175 °C, while K
JICi
for a lower Cu variant increases from 34.2 MPa√m at 25 °C to 36.0 MPa√m at 150 °C. Crack-tip damage in AA2519+Mg+Ag evolves
by nucleation and growth of voids from large undissolved Al2Cu particles, but fracture resistance is controlled by void sheeting coalescence associated with dispersoids. Quantitative
fractography, three-dimensional (3-D) reconstruction of fracture surfaces, and metallographic crack profiles indicate that
void sheeting is retarded as temperature increases from 25 °C to 150°C, consistent with a rising fracture resistance. Primary
microvoids nucleate from smaller constituent particles in the low Cu alloy, and fracture strain increases. A strain-controlled
micromechanical model accurately predicts K
JICi
as a function of temperature, but includes a critical distance parameter (l*) that is not definable a priori. Nearly constant initiation toughness for AA2519+Mg+Ag is due to rising fracture strain with temperature, which balances
the effects of decreasing flow strength, work hardening, and elastic modulus on the crack-tip strain distribution. Ambient
temperature toughnesses of the low Cu variant are comparable to those of AA2519+Mg+Ag, despite increased fracture strain,
because of reduced constituent spacing and l*. 相似文献
5.
时效制度对7B04高强铝合金力学及腐蚀性能的影响 总被引:4,自引:1,他引:3
采用常规力学性能、标准紧凑拉伸、电导率、慢应变速率拉伸(SSRT)及剥落腐蚀测试等手段,研究了不同热处理状态下7B04铝合金预拉伸板的力学及腐蚀性能。结果表明,合金的强度、韧性和腐蚀性能与时效制度密切相关。单级峰时效(T6)状态下合金的强度最高,但是其抗应力腐蚀(SCC)性能及断裂韧性最低;双级过时效(T74和T73)状态下材料的断裂韧性和抗SCC性能明显提高,但是其强度牺牲较多;与T6相比,RRA时效处理可明显提高合金的抗SCC性能,且强度牺牲较少,仅下降2%左右,同时断裂韧性也有一定提高。 相似文献
6.
The influence of microstructure on the strength and fracture toughness of 18 Ni (350) maraging steel was examined. Changes
in microstructure were followed by X-ray and neutron diffraction and by optical and electron microscopy. These observations
have been correlated with the fracture morphology established by scanning electron microscopy. Air cooling this alloy from
the austenitizing temperature results in a dislocated martensite. During the initial stage of age hardening, molybdenum atoms
tend to cluster (forming preprecipitates) and the cobalt assumes short range ordered positions. Subsequent aging results in
Ni3Mo and σ-FeTi with overaging being associated with the formation of equilibrium reverted austenite and Fe2Mo. The fracture behavior is examined in terms of elementary dislocation precipitate interactions. It is suggested that the
development of coplanar slip in the underaged conditions leads to its increased stress corrosion susceptibility and decreased
fracture toughness. The optimum aged condition is then associated with cross-slip deformation. The fracture behavior of the
overaged condition is a dynamic balance between a brittle matrix and the ductile (crack blunting) reverted austenite. 相似文献
7.
The effect of heat treatment on microstructure and cryogenic fracture properties in 5Ni and 9Ni steel 总被引:2,自引:0,他引:2
Heat treatments were utilized in 5Ni and 9Ni steel which resulted in the development of tempered microstructures which contained
either no measurable retained austenite (<0.5 pct) or approximately 4 to 5 pct retained austenite as determined by X-ray diffraction.
Microstructural observations coupled with the results of tensile testing indicated that the formation of retained austenite
correlated with a decrease in carbon content of the matrix. Relative values ofK
IC
at 77 K were estimated from slow bend precracked Charpy data using both the COD and equivalent energy measurements. In addition,
Charpy impact properties at 77 K were determined. In the 9Ni alloy, optimum fracture toughness was achieved in specimens which
contained retained austenite. This was attributed to changes in yield and work hardening behavior which accompanied the microstructural
changes. In the 5Ni alloy, fracture toughness equivalent to that observed in the 9Ni alloy was developed in grain refined
and tempered microstructures containing <0.5 pct retained austenite. A decrease in fracture toughness was observed in grain
refined 5Ni specimens containing 3.8 pct retained austenite due to the premature onset of unstable cracking. This was attributed
to the transformation of retained austenite to brittle martensite during deformation. It was concluded that the formation
of thermally stable retained austenite is beneficial to the fracture toughness of Ni steels at 77 K as a result of austenite
gettering carbon from the matrix during tempering. However, it was also concluded that the mechanical stability of the retained
austenite is critical in achieving a favorable enhancement of cryogenic fracture toughness properties.
Formerly with Union Carbide Corporation, Tarrytown, NY 相似文献
8.
A study has been made of the influence of austenitizing temperature on the ambient temperature toughness of commercial AISI
4340 ultrahigh strength steel in the as-quenched (untempered) and quenched and tempered at 200°C conditions. As suggested
in previous work, a systematic trend ofincreasing plane strain fracture toughness(K)
Ic
anddecreasing Charpy V-notch energy is observed as the austenitizing temperature is raised while the yield strength remains unaffected.
This effect is seen under both static <slowbend> and dynamic (impact) loading conditions, and is rationalized in terms of
a differing response of the microstructure, produced by each austenitizing treatment, to the influence of notch root radius
on toughness. Since failure in all microstructures was observed to proceed primarily by a ductile rupture (microvoid coalescence)
mechanism, an analysis is presented to explain these results, similar to that reported previously for stress-controlled fracture,
based on the assumption that ductile rupture can be considered to be strain-controlled. Under such conditions, the decrease
in V-notch Charpy energy is associated with a reduction in critical fracture strain at increasing austenitizing temperatures,
consistent with an observed decrease in uniaxial and plane strain ductility. The increase in sharp-crack fracture toughness,
on the other hand, is associated with an increase in “characteristic distance” for ductile fracture, resulting from dissolution
of void-initiating particles at high austenitizing temperatures. The microstructural factors which affect this behavior are
discussed, and in particular the specific role of retained austenite is examined. No evidence was found that the enhancement
of fracture toughness at high austenitizing temperatures was due to the presence of films of retained austenite. The significance
of this work on commonly-used Charpy/KIc empirical correlations is briefly discussed.
formerly with Lawrence Berkeley Laboratory, Berkeley, CA 相似文献
9.
The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without
cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at
673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties
were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence
of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of
interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of
transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness
of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which
had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raisedM
S
but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at
673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably
Mo2C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated
the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions. 相似文献
10.
Two microscopic ductile fracture processes have been established in a fracture tough superalloy, Inconel 718, aged to five
strength levels. At yield strengths less than 800 MPa, the mechanism is a slow tearing process within large pockets of inhomogeneous
carbides and nitrides, giving rise to plane strain fracture toughness (K
IC)values greater than 120 MPa-m1/2. At yield strengths greater than 900 MPa, the mechanism involves fracture initiation at carbides and nitrides followed by
off crack plane void sheet growth nucleated at the Laves (σ) phases. Here, the fracture toughness drops to about 80 MPa-m1/2. A Mode I normal strain growth model for low yield strength conditions and a shear strain void sheet model for high yield
strength ones are shown to model KIC data obtained from a J-integral evaluation of compact tension results. 相似文献
11.
Robert O. Ritchie Benjamin Francis William L. Server 《Metallurgical and Materials Transactions A》1976,7(6):831-838
It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,K Ic, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenK Ic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onK Ic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required. 相似文献
12.
Z. M. Sun T. Kobayashi H. Fukumasu I. Yamamoto K. Shibue 《Metallurgical and Materials Transactions A》1998,29(1):263-277
A γ-base TiAl alloy with duplex microstructure of lamellar colonies and equiaxed γ grains was prepared with a reactive sintering method. Tensile tests and fracture toughness tests at loading velocities up
to 12 m/s (strain rate for tensile tests up to 3.2×102/s) were carried out. The micro-structure of the alloy before and after tensile deformation was carefully examined with a
scanning electron microscope (SEM) and a transmission electron microscope (TEM). The fractography of the tensile specimens
and fracture toughness specimens was studied. The experimental results demonstrated that the ultimate tensile strength (UTS)
and yield strength (YS) increase with increasing strain rate up to 10/s and subsequently level off. The UTS and YS exhibited
similar strain rate sensitivity. The strain rate sensitivity exponent at strain rates lower than 10/s is about 1.5×10−2 and at higher strain rates is almost zero. In this study, fracture toughness was found to be less sensitive to the loading
velocity, having values of around 25 MPa √m, which is believed to be attributed to the high strain rate experienced at the
crack tip. The predominant deformation mechanism for the strain rates used in this study was found to be twinning. However,
in the low strain rate range, the dislocation motion mechanism was operative at the initial deformation stage and twinning
dominated the later stage of the deformation process. In the high strain rate range, the entire deformation process was dominated
by twinning. The interaction between deformation twinning and grain boundaries resulted in intergranular fracture in the γ grains and delamination of α
2/γ interfaces in the lamellar colonies. 相似文献
13.
Effects of thickness and precracking on the fracture toughness of particle-reinforced al-alloy composites 总被引:4,自引:0,他引:4
A. B. Pandey B. S. Majumdar D. B. Miracle 《Metallurgical and Materials Transactions A》1998,29(4):1237-1243
The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the underaged condition increased
significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM-E 813 for
a valid J
Ic
test. The influence of thickness was considerably lower in the peak-aged (PA) condition. This relative insensitivity is believed
to be due to the low strain to failure associated with severe flow localization in the PA condition. The effect of precracking
on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness
on specimen thickness and precracking is explained in terms of the hydrostatic stress, which has a strong influence on the
plastic straining capability of the DRA material. The fracture toughness was modeled using a critical strain formulation,
with the void growth strain dependent on hydrostatic stress through the Rice and Tracey model. The predicted toughnesses for
the thick and thin specimens were in good agreement with the experimental data. 相似文献
14.
N. U. Deshpande A. M. Gokhale D. K. Denzer John Liu 《Metallurgical and Materials Transactions A》1998,29(4):1191-1201
The fracture toughness of Al-Zn-Mg-Cu-based 7XXX aluminum alloys decreases with an increase in the extent of recrystallization.
In this contribution, the fracture path of plane-strain fracture-toughness specimens of 7050 alloy (a typical alloy of the
7XXX series) is quantitatively characterized as a function of degree of recrystallization, specimen orientation, and aging
condition. The fracture path is quantitatively correlated to fracture toughness, and the bulk microstructural attributes estimated
via stereological analysis. In the companion article, these quantitative data are used to develop and verify a multiple-fracture
micromechanism-based model that relates the fracture toughness to a number of microstructural parameters of the partially
recrystallized alloy plate. 相似文献
15.
K. T. Venkateswara Rao J. C. McNulty R. O. Ritchie 《Metallurgical and Materials Transactions A》1993,24(10):2233-2245
Aluminum-lithium alloys are currently being considered for applications at moderately elevated temperatures; accordingly,
a study has been made on the effects of prolonged (100 and 1000 hours overaging) thermal exposure at 149 °C and 260 °C on
the mechanical properties of a peakaged Al-Li-Cu-Mg-Zr alloy 8090-T8771. In the as-received T8771 temper, the alloy exhibits
an excellent combination of strength (˜500 MPa) and toughness (35 MPa√m) with moderate tensile elongation (4 pct). Overaging
at 149 °C results in a ˜50 pct reduction in ductility and toughness, primarily associated with the growth of equilibrium phases
along grain/subgrain boundaries, resulting in formation of solute-depleted precipitate-free zones and coarsening of matrix8' andS precipitates; strength levels and fatigue-crack growth rates, however, remain largely unchanged. Thermal exposures at 260
°C, conversely, lead to dramatic reductions in strength (by ˜50 to 80 pct), toughness (by ˜30 pct) and fatigue-crack propagation
resistance; crack-growth rates at all ΔK levels above ~5 MPa√m are 2 to 3 orders of magnitude faster. Microstructurally, this was associated with complete dissolution
of δ′, severe coarsening ofS andT
2
precipitates in the matrix, and formation of equilibrium Cu- and Mg-rich intermetallic phases in the matrix and along grain
boundaries. The resulting lack of planar-slip deformation and low yield strength of 8090 following overaging exposures at
260 °C increase the cumulative crack-tip damage per cycle and reduce the tendency for crack-path deflection, thereby accelerating
fatigue-crack growth rates. Despite this degradation in properties, the 8090-T8771 alloy has better strength retention and
generally superior fatigue-crack growth properties compared to similarly overaged Al-Li-Cu-Zr 2090 and Al-Cu-Zn-Mg 7150 alloys.
formerly with the University of California,
formerly with the University of California, 相似文献
16.
A. Bhattacharjee P. Ghosal T. K. Nandy S. V. Kamat A. K. Gogia S. Bhargava 《Transactions of the Indian Institute of Metals》2008,61(5):399-405
The tensile and fracture toughness behaviour of Ti-10V-4.5Fe-3Al alloy in the beta solution treated condition was investigated
as a function of β grain size and the behaviour was compared with that of Ti-10V-2Fe-3Al alloy. It was found that this alloy,
unlike the Ti-10V-2Fe-3Al alloy, did not exhibit any stress induced martensitic transformation during tensile testing. This
was attributed to the higher stability of the β phase in this alloy as compared to that in Ti-10V-2Fe-3Al alloy because of
the higher iron content. The yield strength, ultimate tensile strength, total elongation and fracture toughness in this alloy
were found to decrease with increase in grain size. The yield strength of this alloy was found to obey the Hall-Petch relationship
with grain size, similar to the observation in Ti-10V-2Fe-3Al alloy. However, the σi value was higher and k value lower than that observed in Ti-10V-2Fe-3Al alloy. The fracture toughness in Ti-10V-4.5Fe-3Al
alloy was also found to increase linearly with inverse square root of grain size; however, the slope was significantly less
than that observed in Ti-10V-2Fe-3Al alloy. 相似文献
17.
Aging kinetics of aluminum alloy 7050 总被引:1,自引:0,他引:1
J. T. Staley 《Metallurgical and Materials Transactions B》1974,5(4):929-932
Precipitation hardening at room temperature (natural aging) and isothermal precipitation hardening at 422 to 455 K beyond
the point of peak strength (overaging) were studied. Natural aging kinetics of the increase in strength and resistivity were
comparable with those of alloy 7075. Alloy 7050, however, develops peak strength by aging at higher temperatures. This ability
of 7050 to develop peak strength upon aging in the temperature region used to improve the resistance to stress-corrosion cracking
and toughness of 7XXX alloys is attributed to the effect of Cu on increasing the temperature range of G.P zone stability. 相似文献
18.
Flow behavior and microstructural evolution in an Al-Cu eutectic alloy of equiaxed grains were investigated over ε ≃ 2× 10−6 to 2 × 10−2 s−1 andT = 400° to 540 °C. Depending on the test conditions, there appeared either strain hardening or strain softening predominantly
in the early part of the σ-ε curves. The microstructural observations showed evidence for grain growth, development of zig-zag
boundaries, dislocation interactions, and cavitation. The grain growth adequately accounts for the observed strain hardening
at higher temperatures and lower strain rates. However, at lower temperatures the strain hardening can be only partly accounted
for by the observed grain growth; under this condition, some dislocation interactions also contribute to the strain hardening.
The presence of cavitation causes strain softening predominantly at higher strain rates. Therefore, to develop a proper understanding
of the superplastic behavior of the Al-Cu eutectic alloy, it is necessary to take into account the influence of dislocation
interactions and cavitation along with that of grain growth. 相似文献
19.
K. S. Ravichandran D. B. Miracle M. G. Mendiratta 《Metallurgical and Materials Transactions A》1996,27(9):2583-2592
A detailed investigation of the effects of microstructural changes on the mechanical behavior of twoin situ intermetallic composites with Cr and Cr2Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic)
and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature
and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250 ‡C (fine microstructure) and
1500 ‡C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection
curve) with the coarsening of the microstructures was noted for both alloys below 1000 ‡C. The Cr-13Hf alloy retained strength
to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence
of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility
at and above 200 ‡C; ductility in primary Cr could be seen only at and above 1000 ‡C for the fine microstructure. In other
words, the temperature at which ductility was first observed decreased from about 1000 ‡C to about 200 ‡C due to high-temperature
heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with
increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures
were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to
pure Cr at 1200 ‡C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The
effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial
elements in Cr, on the observed mechanical behavior are discussed.
Formerly Research Scientist, Materials and Processes, UES, Inc. 相似文献
20.
Work-softening behavior of the ultrafine-grained Al alloy processed by high-strain-rate,dissimilar-channel angular pressing 总被引:1,自引:0,他引:1
Jae-Chul Lee Jin-Yoo Shu Jae Pyung Ahn 《Metallurgical and Materials Transactions A》2003,34(3):625-632
Commercially pure Al alloy sheets were processed using high-strain-rate, dissimilar-channel angular pressing (DCAP) for as
many as 100 passages through a channel with φ=120 deg, to investigate the work hardening caused by a low strain level (ɛ<2) and the work softening caused by an extremely high strain level (3<ɛ<58). The hardness of the alloy increased significantly by a factor of 2 at strains less than ∼2, while a gradual decrease
in the hardness was observed at strains greater then ∼2. The work-hardening and the work-softening behaviors observed from
the Al alloy were analyzed by correlating the measured properties with microstructural evolutions observed by transmission
electron microscopy (TEM). A detailed microstructural-evolution sequence occurring at successive strain stages was also investigated
based on TEM and electron backscattered diffraction (EBSD). 相似文献