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1.
The microstructure and aging behavior of a nanocrystalline Al-Cu-Mg-Fe-Ni-Sc alloy was studied. The nanocrystalline powders
were produced by milling at liquid nitrogen temperature and then consolidated using spark plasma sintering (SPS). The microstructure
after SPS consisted of a bimodal aluminum grain structure (coarse-grained and fine-grained regions), along with Al 9FeNi and Al 2CuMg particles dispersed throughout. The microstructure observed in the as-consolidated sample is rationalized on the basis
of high current densities that are generated during sintering. Solution treatment and aging of the SPS Al-Cu-Mg-Fe-Ni-Sc sample
resulted in softening instead of hardening. This observation can be explained by the reduced amount of Cu, Mg, and Si in solid
solution available to form S′ Al 2CuMg due to the precipitation of Al 7FeCu 2 and Si-rich particles, and by the fact that rodlike S′ Al 2CuMg particles could only precipitate out in the coarse-grained regions, greatly decreasing their influence on the hardness.
This lack of precipitation in the fine-grained region is argued to represent a new physical observation and is rationalized
on the basis of physical and thermodynamic effects. The nanocrystalline SPS Al-Cu-Mg-Fe-Ni-Sc sample was also extremely thermally
stable, retaining a fine-grained structure even after solution treatment at 530°C for 5 h. The observed thermal stability
is rationalized on the basis of solute drag and Zener pinning caused by the impurities introduced during the cryomilling process. 相似文献
2.
The effects of microadditions of V and Be on age-hardening, microstructure, and mechanical behavior of Al-Li-Cu-Mg alloy 8090
are investigated. Small additions of V and Be significantly increase the peak hardness and mechanical properties of the alloy
aged at 190 °C. A transmission electron microscopy (TEM), scanning electron microscopy/energy dispersive spectroscopy (SEM/
EDS), optical, and X-ray diffraction investigation shows a higher S′ (Al 2CuMg) precipitate density for alloys containing V and Be, indicating an enhanced nucleation rate for S’ precipitation. The V and Be additions are more effective than prior-aging deformation for increasing the yield strength of
age-hardened 8090 alloy. The V addition increases ductility significantly, which is attributed to the homogeneous distribution
of slip lines and extensive cross slip that occurs during deformation of the V-containing alloy. 相似文献
3.
Microalloying additions of Si are known to increase significantly the response to age hardening of 2xxx series Al-Cu-Mg alloys,
and commercial alloys such as 2618 are based on this effect. Previous work has attributed this effect to a refined dispersion
of S′ or S phase (Al 2CuMg) precipitates. This work reports the results of a detailed microstructural characterization, employing transmission electron
microscopy-based techniques, on the effects of Si additions to a base Al-2.5Cu-1.5Mg (wt pct) alloy. It was found that the
peak hardness microstructure contains a fine and uniform dispersion of Si-modified Guinier-Preston-Bagaratsky (GPB) zones.
These zones are lath shaped, possessing {100}
α
facets, elongated along 〈100〉
α
directions and contain Si. The S phase was also observed at peak hardness, although it is concluded that these precipitates
do not contribute significantly to hardening due to their coarse dispersion, which arises from their heterogeneous nucleation
on the quenched-in defect structure. Overaging was associated with the replacement of the zones by the S phase through a process
involving dissolution and reprecipitation together with heterogeneous nucleation of S at the zones. The precipitation of ϑ′ (Al 2Cu) and σ(Al 5Cu 6Mg 2) phase was also observed in alloys containing ≥0.5 wt pct Si. It is demonstrated that the total solute content of the alloy
has a major influence on the precipitation reactions during aging. 相似文献
4.
A new high-temperature-strength, high-conductivity Cu-Cr-Nb alloy with a Cr:Nb ratio of 2:1 was developed to achieve improved
performance and durability. The Cu-8 Cr-4 Nb alloy studied has demonstrated remarkable thermal and microstructural stability
after long exposures at temperatures up to 0.98 T m. This stability was mainly attributed to the slow coarsening kinetics of the Cr 2Nb precipitates present in the alloy. At all temperatures, the microstructure consists of a bimodal and sometimes trimodal
distribution of strengthening Cr 2Nb precipitates, depending on precipitation condition, i.e., from liquid or solid solution, and cooling rates. These precipitates
remain in the same size range, i.e., large precipitates of approximately 1 μm and small precipitates less than 300 nm, and
effectively pin the grain boundaries, thus retaining a fine grain size of 2.7 μm after 100 hours at 1323 K. This grain-boundary
pinning and sluggish coarsening of Cr 2Nb particles explain the retention of good mechanical properties after prolonged holding at very high temperatures, e.g.,
twothirds of the original yield strength after aging for 100 hours at 1273 K. The main sources of strengthening are the Hall-Petch
and Orowan mechanisms due mostly to small particles. The coarsening kinetics of the large precipitates are most likely governed
by grain-boundary diffusion and, to a lesser extent, volume diffusion mechanisms. 相似文献
5.
The grain boundary microstructure of Al-Cu-Li alloy AA2050 was investigated for different isothermal aging times to rationalize intergranular corrosion (IGC) characteristics. In the underaged condition, the dominant grain boundary precipitates are fine T 1 (Al 2CuLi). Extended aging revealed that grain boundaries were decorated by large T 1 precipitates and S′ phase (Al 2CuMg), with S′ growth not dimensionally constrained. Such a transition in the precipitate type at grain boundaries is a unique feature of the Al-Cu-Li system. 相似文献
6.
An 8090 Al-Li-Cu-Mg-Zr alloy in the peak-aged (T8) temper was subjected to retrogression treatment at temperatures above and
below the δ′ (Al 3Li) solvus line and immediately reaged to various tempers. Retrogression and reaging (RRA) behavior is characterized by hardness
testing, tensile testing, transmission electron microscopy (TEM), X-ray diffraction (XRD), differential scanning calorimetry
(DSC), and electrochemical polarization studies. Retrogression of the T8 temper alloy causes dissolution primarily of δ′ (Al 3Li) precipitates into solid solution that results in a decrease of hardness and tensile strength and an increase of ductility
of the alloy. Reaging of the retrogressed state causes reprecipitation of the δ′ precipitates in the matrix resulting in the restoration of strength and ductility properties. Retrogression and reaging to
the peak-aged temper, designated at T77 temper, has been found to retain the strength of the conventional T8 temper, but with
the gross aging time in the RRA temper almost twice that of the conventional T8 temper, the microstructure of the RRA temper
approaches that of the overaged (T7) temper. Thus, RRA treatment contributes to an improvement of stress corrosion cracking
(SCC) resistance over the conventional T8 temper while retaining the mechanical properties of T8 temper. 相似文献
7.
Although Al-Li-Cu alloys showed initial promise as lightweight structural materials, implementation into primary aerospace
applications has been hindered due in part to their characteristic anisotropic mechanical and fracture behaviors. The Air
Force recently developed two isotropic Al-Li-Cu-X alloys with 2.1 wt pct Li and 1.8 wt pct Li designated AF/C-489 and AF/C-458,
respectively. The elongation at peak strength was less than the required 5 pct for the 2.1 wt pct Li variant but greater than
10 pct for the 1.8 wt pct Li alloy. The objectives of our investigations were to first identify the mechanisms for the large
difference in ductility between the AF/C-489 and AF/C-458 alloys and then to develop an aging schedule to optimize the microstructure
for high ductility and strength levels. Duplex and triple aging practices were designed to minimize grain boundary precipitation
while encouraging matrix precipitation of the T 1 (Al 2CuLi) strengthening phase. Certain duplex aged conditions for the AF/C-489 alloy showed significant increases in ductility
by as much as 85 pct with a small decrease of only 6.5 and 2.5 pct in yield and ultimate tensile strength, respectively. However,
no significant variations were found through either duplex or triple aging practices for the AF/C-458 alloys, thus, indicating
a very large processing window. Grain size and δ′ (Al 3Li) volume fraction were determined to be the major cause for the differences in the mechanical properties of the two alloys. 相似文献
8.
The microstructure and tensile behavior of an Al-3Cu-l.6Li-0.8Mg-0.2Zr alloy, produced by splatquenched powder metallurgy
processing, were studied. The alloy exhibited homogeneous deformation, both in bulk samples and during in situ TEM studies. This is in contrast to the strain localization that is frequently observed in Mg-free Al-Cu-Li-X alloys. The
difference in deformation mode is attributed to a fine distribution of Ś (Al 2CuMg) which precipitates up to the grain boundaries. A processing treatment involving 2 pct stretch prior to aging resulted
in a yield strength of 555 MPa, a reduction in area of 29 pct, and a strain to fracture of 8.8 pct. This represents an attractive
improvement in specific properties compared with 7075-T76 having a similar texture. 相似文献
9.
The microstructural evolution of Mg-7Al-2Sn (AT72) alloy processed by super vacuum die-casting and heat treated at various conditions was studied. The results showed that the dendritic microstructure in the as-cast AT72 alloy consisted of α-Mg, Mg 2Sn, and Mg 17Al 12 phases. After solution treatment at temperatures ranging from 663 K to 703 K (390 °C to 430 °C), the Mg 17Al 12 phase dissolved into the Mg matrix entirely, while the Mg 2Sn phase partially dissolved into matrix. An average grain size of about 40 μm in the alloy could be achieved after solution treatment at 683 K (410 °C) for 16 hours. A large amount of lath-shaped precipitates of Mg 2Sn and Mg 17Al 12 was observed in the aged AT72 alloy. The results of tensile property evaluation at room temperature showed that the ductility of the solution-treated alloy was dramatically improved, in comparison with the as-cast alloy. In the peak aged condition, the tensile strength of the alloy was increased, which was attributed to the deposition of fine Mg 17Al 12 and Mg 2Sn precipitates during the aging treatment. 相似文献
10.
The influence of small additions of C, Zr, and Hf, alone or in combination with B, on the microstructure and tensile behavior
of substoichiometric FeAl was investigated. Tensile prop-erties were determined from 300 to 1100 K on powder which was consolidated
by hot extrusion. All materials possessed some ductility at room temperature, although ternary additions generally reduced
ductility compared to the binary alloy. Adding B to the C- and Zr-containing alloys changed the fracture mode from intergranular
to transgranular and restored the ductility to ap-proximately 5 pct elongation. Additions of Zr and Hf increased strength
up to about 900 K, which was related to a combination of grain refinement and precipitation hardening. Fe 6Al 6Zr and Fe 6Al 6Hf precipitates, both with identical body-centered tetragonal structures, were iden-tified as the principal second phases
in these alloys. Strength decreased steadily as temperature increased above 700 K, as diffusion-assisted mechanisms, including
grain boundary sliding and cavitation, became operative. Although all alloys had similar strengths at 1100 K, Hf additions
significantly improved high-temperature ductility by suppressing cavitation. 相似文献
11.
The effects of the addition of Ca and Ce to the AT42(Mg-4Al-2Sn) alloy on the microstructural modification and deformation, as well as the fracture mechanisms of squeeze cast magnesium alloys, were investigated in this study. Microstructural analyses indicated that the AT42 alloy contained Mg 17Al 12 and Mg 2Sn particles precipitated along cell boundaries, whereas long, needle-shaped CaMgSn particles were precipitated additionally in the AT42-0.5Ca and AT42-1Ca alloys. In the AT42-1Ca-0.5Ce and AT42-1Ca-1Ce alloys containing Al 11Ce 3 particles as well as Mg 17Al 12, Mg 2Sn, and CaMgSn particles, the overall distribution of precipitates was homogeneously modified considerably as the solidification cell size was refined. According to the observation of deformation and the fracture processes of the AT42-1Ca alloys, the fracture proceeded mainly along cracked, needle-shaped CaMgSn particles at a relatively low stress-intensity factor level. However, in the AT42-1Ca-1Ce alloys, the deformation and fracture proceeded into cells rather than into cell boundaries as twins were developed actively inside cells, although few microcracks were initiated at the precipitates.?Thus, the AT42-1Ca-1Ce alloy had the highest strength, ductility, and fracture toughness simultaneously because of the increase in the volume fraction of hard precipitates and the development of many twins in the Mg matrix. 相似文献
12.
The present study is concerned with γ-(Ti 52Al 48) 100−x
B
x
( x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure
of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α 2 and γ with a mean grain size of 200 nm. Besides α 2 and γ phases, binary and 0.5 pct B alloys contain Ti 2AlN and Al 2O 3 phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing
treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ
grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility,
but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength
than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed
alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature
of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine
5 pct B alloy is dominated by the grain boundary sliding mechanism.
This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented
at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the
Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees. 相似文献
13.
The low-carbon steel workpieces are deformed by equal-channel angular pressing at 293 K (20 °C) up to an equivalent strain of ~12 using route B
c, which results in the bulk ultrafine-grained (UFG) structure with high dislocation density and partial dissolution of cementite. The yield strength (YS) is enhanced from 208 (as-received) to 872 MPa and the tensile strength is increased from 362 to 996 MPa, but the material loses total elongation (TE) from 36.2 to 2.9 pct. Cold rolling of equal-channel angular pressed steel produces the refined structure of grain size 0.11 μm. The YS increases further to 924 MPa with a marginal gain in ductility due to the reappearance of the γ fiber component. Flash annealing the samples, which were equal-channel angular pressed followed by cold rolling, at 873 K (600 °C) results in 27 pct of micron-sized (9 µm) ferrite grains in submicron-sized (<1 µm) matrix with a reduced defect density and small amount of precipitation of cementite. TE increases from 2.9 to 23.3 pct. The material retains a YS of 484 MPa and tensile strength of 517 MPa, which are higher than those of the as-received material. The UFG grains are failed by cleavage, but the micron-sized grains display ductile fracture. The ductility of the flash-annealed material is recovered significantly due to bimodal grain size distribution in ferrite and the development of a good amount of γ fiber texture components. The major contribution toward recovery of ductility comes from the bimodal grain size distribution in ferrite rather the precipitation of cementite. 相似文献
14.
Aluminum alloy matrix composites containing 1 to 30 wt pct of fibrous and particulate nonmetals varying in size from 0.06
μm to 840 μm were fabricated. The composites were cast into cylindrical molds for friction and wear tests, hot extrusion and
tensile tests. The distribution of the nonmetals in the cast ingots was homogeneous. Friction and wear tests were done on
a pin (52100 bearing steel) and dish type machine without lubrication. It was found that composites containing ∼10 wt pct
or more of SiC, TiC, Si 3N 4, Al 2O 3, glass, solid waste slag, and silica sand wear less than the pure matrix alloy, but have slightly higher average coefficients
of friction. Wear in composites containing soft particles, especially MgO and boron nitride was higher than the pure matrix
alloy. The average coefficient of friction of all the composites was in the range of 0.35 to 0.58. Increasing the sliding
velocity reduced this range to ∼ 0.4 to 0.45. The longitudinal tensile properties of the extruded composites (with the exception
of loss of ductility in some cases) are comparable to that of the matrix alloys. Improvements in strength or ductility were
noted. For example, addition of 15 wt pct of 3 μm size Al 2O 3 particles raised the yield and ultimate strength of the Al-4 pct Cu-0.75 pct Mg alloy matrix from 227 to 302 MPa, and 356
to 403 MPa, respectively. The corresponding percent elongation decreased from 25.8 to 12.5. The fact that the various composites
can be readily cast and hot formed suggests a variety of engineering applications.
AKIRA SATO, formerly Visiting Scientist at Massachusetts Institute of Technology, Cambridge. 相似文献
15.
A transmission electron microscopy (TEM) study of a 6201 aluminum alloy to which controlled strontium additions were made
has revealed important differences compared to the same alloy free of strontium. In the as-cast state, strontium favors the
formation of α-AlFeSi (Al 8Fe 2Si) rather than β-AlFeSi (Al 5FeSi) phase, resulting in a greater quantity of excess silicon present in the strontium-treated alloy. During heat treatment,
the excess silicon allows a greater density of finer β″-Mg 2Si precipitates to form, leading to increased tensile strength values and increased electrical resistivity. Strontium also
retards the growth of the precipitates formed during heat treatment and inhibits formation of the equilibrium β-Mg 2Si phase. As a result, the strontium-treated alloy exhibits a resistance to overaging. 相似文献
16.
The solidification microstructure and mechanical properties of as-cast Mg-Al-Sn alloys have been investigated using computational
thermodynamics and experiments. The as-cast microstructure of Mg-Al-Sn alloys consists of α-Mg, Mg 17Al 12, and Mg 2Sn phases. The amount of Mg 17Al 12 and Mg 2Sn phases formed increases with increasing Al and Sn content and shows good agreement between the experimental results and
the Scheil solidification calculations. Generally, the yield strength of as-cast alloys increases with Al and Sn content,
whereas the ductility decreases. This study has confirmed an early development of Mg-7Al-2Sn alloy for structural applications
and has led to a promising new Mg-7Al-5Sn alloy with significantly improved strength and ductility comparable with commercial
AZ91 alloy. 相似文献
18.
The effects of nano particles on double shear strength and tribological properties of A356 alloy reinforced with Al 2O 3 nano particles of size 30 nm were investigated. The percentage inclusions of Al 2O 3 were varied from 0.5 to 1.5 wt%. The particles were added with stirring at 400 rpm and squeeze casting at 750 °C and pressure of 600 MPa in a squeeze casting machine. Comparison of the performance of as cast samples of A356/Al 2O 3 nano composite was conducted. The tribological properties of the samples were also investigated by pin-on-disk tests at 10, 30 and 50 N load, sliding speed 0.534 m/s and sliding distance 1100 m in dry condition. SEM images of microstructure analysis of the composite, Al 2O 3 (0.5 and 1 %) particles were well dispersed in the A356 alloy matrix. Partial agglomeration was observed in metal matrix composite with higher (1.5 %) Al 2O 3 particle contents. The nano dispersed composites containing 0.5 and 1 wt% of Al 2O 3 nano particles exhibited the highest double shear strength, lesser wear loss and coefficient of friction. 相似文献
19.
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 (Al 2CuMg), 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
20.
Combined additions of Ge and Si to Al are known to produce higher precipitation hardening than that which occurs in the constituent
binaries, when the total amounts of alloying atoms are the same for all the alloys investigated. In the resultant Al-Ge-Si
alloys, the diamond cubic precipitates contain both Ge and Si and are designated as GeSi. During artificial aging at 160 °C,
the GeSi precipitates are commonly present in three forms, i.e., equiaxed, 〈100〉 Al lath, and triangular plate. The equiaxed form is the dominant one of the three. This article examines the influence of varying
amounts ( i.e., 2 to 4 wt pct) of Cu additions on the morphology of GeSi precipitates formed in an Al-2.6 wt pct Ge-1.04 wt pct Si alloy
during artificial aging at 160 °C. It is shown that Cu additions have the remarkable effect of maximizing the nucleation frequency
of the 〈100〉 Al lath form and simultaneously suppressing the nucleation of the equiaxed and the plate forms of the GeSi precipitates. Increasing
Cu additions also increase the homogeneity and cause refinement of the 〈100〉 Al laths. These results are discussed in light of (1) the critical requirement of vacancies for the nucleation and growth of
GeSi precipitates having an atomic volume larger than Al and (2) the crystallographic nature of the negative dilation strains
that develop locally in the Cu-rich regions of the Al matrix. It is further shown that, in the alloys containing increased
levels ( i.e., exceeding about 2.5 wt pct) of Cu, the precipitation of ϑ′ (metastable ϑ-Al 2Cu) phase occurs, and that the nucleation of Cu-rich ϑ′ precipitates occurs upon the 〈100〉 Al laths of GeSi. The latter effect is discussed in terms of the attainment of both the nucleation site and the necessary solute
supersaturation at the 〈100〉 Al GeSi/ α-Al interfaces. 相似文献
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