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1.
The effect of powder particle size on the microstructure, mechanical properties, and fracture behavior of Al-20 wt pct Si
alloy powders was studied in both the gas-atomized and extruded conditions. The microstructure of the as-atomized powders
consisted of fine Si particles and that of the extruded bars showed a homogeneous distribution of fine eutectic Si and primary
Si particles embedded in the Al matrix. The grain size of fcc-Al varied from 150 to 600 nm and the size of the eutectic Si
and primary Si was about 100 to 200 nm in the extruded bars. The room-temperature tensile strength of the alloy with a powder
size <26 μm was 322 MPa, while for the coarser powder (45 to 106 μm), it was 230 MPa. The tensile strength of the extruded bar from the fine powder (<26 μm) was also higher than that of the Al-20 wt pct Si-3 wt pct Fe (powder size: 60 to 120 μm) alloys. With decreasing powder size from 45 to 106 μm to <26 μm, the specific wear of all the alloys decreased significantly at all sliding speeds due to the higher strength achieved by
ultrafine-grained constituent phases. The thickness of the deformed layer of the alloy from the coarse powder (10 μm at 3.5 m/s) was larger on the worn surface in comparison to the bars from the fine powders (5 μm at 3.5 m/s), attributed to the lower strength of the bars with coarse powders. 相似文献
2.
The elevated temperature deformation characteristics of a rapidly solidified Al-8.4 wt pct Fe-3.6 wt pct Ce alloy have been
investigated. Constant true strain rate compression tests were performed between 523 and 823 K at strain rates ranging from
10−6 to 10−3 s−1. At temperatures below approximately 723 K, the alloy is significantly stronger than oxide dispersion strengthened (ODS)
aluminum. However, at higher temperatures, the strength of the Al-Fe-Ce alloy falls rapidly with increasing temperature while
ODS aluminum exhibits an apparent threshold stress. It is shown that particle coarsening cannot fully account for the reduction
in strength of the Al-Fe-Ce alloy at elevated temperatures. The true activation energy for deformation of the Al-Fe-Ce alloy
at temperatures between 723 and 773 K is significantly greater than that for self-diffusion in the matrix. This is unlike
the behavior of ODS alloys, which contain nondeformable particles and exhibit true activation energies close to that for self-diffusion
in the matrix. Since abnormally high true activation energies for deformation are also exhibited by materials containing deformable
particles, such as γ′ strengthened superalloys, it is concluded that elevated temperature deformation in ythe Al-Fe-Ce alloy involves deformation
of both the matrix and the precipitates. The loss of strength of the Al-Fe-Ce alloy appears to be related to a reduction in
strength of at least some of the second phase particles at temperatures above 723 K.
Formerly Research Assistant, Department of Materials Science and Engineering, Stanford University. 相似文献
3.
The squeeze casting of an Al-7 wt pct Si alloy was carried out in order to investigate the conditions for the formation and
the prevention of macrosegregation. The effects of process parameters such as applied pressure, die temperature, pouring temperature,
delay time, degassing, and inoculation on the formation of macrosegregation were investigated, in correlation with the evolution
of macrostructure and shrinkage defects. Three critical applied pressures were defined, based on the experimental results
for the squeeze-cast Al-7 wt pct Si. The first is the critical applied pressure under which shrinkage defects form (P
SC). The second is the critical applied pressure above which macrosegregates form (P
MS). The third is the critical applied pressure above which and under which minor segregation forms. (P
m and P
MS, respectively). With the concept of these three critical pressures, an experimental diagram describing the optimum process
conditions was proposed for obtaining sound squeeze castings. It was concluded that sound castings without macrosegregation
and shrinkage defects can only be obtained when the applied pressure is in the range where P
SC < P<P
m (<P
MS). Both degassing and inoculation treatments greatly enhanced the soundness of the castings. It was also found that the pouring
temperature and the delay time should not exceed T
D-critical and t
D-critical, respectively, in order to achieve sound castings. 相似文献
4.
B. O. Han F. A. Mohamed Z. Lee S. R. Nutt E. J. Lavernia 《Metallurgical and Materials Transactions A》2003,34(3):603-613
In the present study, the relationships between the structure and properties of a cryomilled Al-7.5 pct Mg alloy were investigated.
The microstructure of the cryomilled Al-7.5 pct Mg alloy consisted of equiaxed grains with an approximate size of 300 nm.
Thermal treatment had only a minor effect on microstructure, as evidenced by X-ray diffraction (XRD) and transmission electron
microscopy (TEM) results. The tensile behavior was characterized by high strength, high ductility, and low-strain-hardening.
The tensile deformation was relatively uniform, with limited necking deformation, and fracture surfaces were characterized
by microdimples. The variation of strain rates from 4 · 10−4 to 4 · 10−2 s−1 had an insignificant effect on tensile behavior. Comparison of compressive and tensile behavior revealed similar moduli and
yield strengths, although the postyield behavior was markedly asymmetric. The present results indicate that grain-size effects,
solid-solution strengthening, Orowan strengthening, and dislocation strengthening contribute significantly to the properties
of a cryomilled Al-7.5 pct Mg alloy. 相似文献
5.
The effect of quenching condition on the mechanical properties of an A356 (Al-7 wt pct Si-0.4 wt pct Mg) casting alloy has
been studied using a combination of mechanical testing, differential scanning calorimetry (DSC), and transmission electron
microscopy (TEM). As the quench rate decreases from 250 °C/s to 0.5 °C/s, the ultimate tensile strength (UTS) and yield strength
decrease by approximately 27 and 33 pct, respectively. The ductility also decreases with decreasing quench rate. It appears
that with the peak-aged condition, both the UTS and yield strength are a logarithmic function of the quench rate,i.e., UTS orσ
y
=A logR +B. The termA is a measure of quench sensitivity. For both UTS and yield strength of the peak-aged A356 alloy,A is approximately 32 to 33 MPa/log (°C/s). The peak-aged A356 alloy is more quench sensitive than the aluminum alloy 6063.
For 6063,A is approximately 10 MPa/log (°C/s). The higher quench sensitivity of A356 is probably due to the high level of excess Si.
A lower quench rate results in a lower level of solute supersaturation in the α-Al matrix and a decreased amount of excess
Si in the matrix after quenching. Both of these mechanisms play important roles in causing the decrease in the strength of
the peak-aged A356 with decreasing the quench rate. 相似文献
6.
7.
Limited results on the fatigue of pseudo-elastic material indicate that, as a class, these materials should have truly outstanding
fatigue properties. To check this, the mechanisms of cyclic deformation and fracture have been studied in Cu−Al−Ni chosen
because its transformation behavior is well understood. Since this alloy is notoriously brittle, pulsating compression fatigue
tests were carried out in polycrystalline material. The details of the stress-induced martensite behavior were studied byin situ video observations. The alloy was found to undergo cyclic hardening and failure eventually occurred by multiple nucleation
of cracks at grain boundaries, by a mechanism similar in principle to that which occurs in regular metals cycled at high plastic
strains. The Coffin-Manson law was obeyed. 相似文献
8.
D. J. Field G. M. Scamans E. P. Butler 《Metallurgical and Materials Transactions A》1987,18(4):463-472
The high temperature oxidation of Al-Mg alloys is characterized by the rapid formation of thick, micro-crystalline oxide films.
The oxidation kinetics of an Al-4.2 wt pct Mg alloy under dry and moist 20 pct O2/Ar have been measured, and oxide films grown on bulk specimens complementary to the weight gain curves have been characterized
using electron optical techniques (TEM, SEM). Initial oxidation takes place by the nucleation and growth of primary crystalline
oxides at the oxide/metal interface and by the formation of secondary oxides of MgO by the reduction of the original amorphous
over-layer of γ-Al2O3 by Mg. Subsequent oxidation is dominated by the further nucleation and growth of primary oxides. The presence of water vapor
in the oxidizing environment initially reduces oxidation rates through a modification of the mechanical properties of the
amorphous overlayer but does not affect the overall oxidation mechanism. A microstructural model has been developed which
describes oxidation of Al-Mg alloys in terms of fracture of the original air-formed film by primary MgO nucleation and growth
and modification to this film by the presence of water vapor in the oxidizing environment.
Formerly at Imperial College, London. 相似文献
9.
The squeeze casting of an Al-4.5 wt pct Cu alloy was carried out to investigate the conditions for the formation and the prevention
of macrosegregation. The effects of the process parameters, applied pressure, die temperature, pouring temperature, delay
time, and humidity on the formation of macrosegregation were investigated in correlation with the evolution of macrostructure
and shrinkage defects. Two critical applied pressures were defined: one is the critical applied pressure, P
SC
, under which shrinkage defects form, and the other is the critical applied pressure, P
MS
, above which macrosegregates form in the squeeze castings. A quantitative diagram describing the optimum process conditions
was proposed for obtaining sound squeeze castings. It was found that the pouring temperature, the die temperature, the delay
time, and the humidity are closely related to the two critical applied pressures P
SC
and P
MS
, in different manners. It was concluded that sound castings without macrosegregation and shrinkage defects can only be obtained
when the applied pressure is in the range of P
SC
<P<P
MS
. 相似文献
10.
Jien-Wei Yeh Shi-Ying Yuan Chao-Hung Peng 《Metallurgical and Materials Transactions A》1999,30(9):2503-2512
A reciprocating extrusion (RE) process has been developed for producing A1-12 wt pct Si bulk alloys with fine and uniform
microstructures and superior properties. Two starting forms were used: disks produced by the hammer-and-anvil method and cast
billets produced by casting. Variations of micro-structure and mechanical properties with the number of extrusion passes are
investigated for these two starting forms. The results show that the porosity along the interfaces between the rapidly solidified
layers could be completely eliminated to give a sound matrix. The Si-phase particles in both cases could be refined and distributed
uniformly. The strength and ductility of all specimens are also enhanced, until the microstructure reaches an optimum state,
as the number of extrusion passes increases. The tensile properties of the rapidly solidified Al-Si alloys are found to be
superior to those of ingot-processed alloys, due to the inherent finer particles produced by rapid solidification. The mechanism
for the improvement of the microstructures and properties is also discussed. 相似文献
11.
A certain degree of cold working is advantageous in developing a fine microstructure with minute silicon crystals for eutectic
and/or hypereutectic Al-Si cast alloys. A novel process, repeated thermomechanical treatment (RTMT), was applied to an Al-12.6 wt pct Si cast alloy. The process involves multiple-pass cold working (less than a 20 pct
reduction in section area) and heat treatment at 793 K for 3.6 ks. Cold-work annealing was repeated up to about an 80 pct reduction in section from the beginning. The RTMT material showed a refined microstructure
with high ductility. Most silicon crystals were fragmented to only a few micrometers and were spheroidized. The RTMT material
showed such marked plasticity that it could be wrought up to a 99 pct reduction in section at room temperature. The Cold-worked
RTMT materials exhibited an excellent balance between tensile strength and elongation and a higher strain hardening than the
cast material. 相似文献
12.
13.
14.
15.
Dong Hyuk Shin Duck-Young Hwang Yong-Jun Oh Kyung-Tae Park 《Metallurgical and Materials Transactions A》2004,35(3):825-837
Tensile tests were carried out at temperatures of 673 to 773 K and strain rates of 1×10−3 to 1 s−1 on an ultrafine-grained (UFG) 5083 Al alloy containing 0.2 wt pct Sc fabricated by equal-channel angular pressing, in order
to examine its high-strain-rate superplastic characteristics. The mechanical data for the alloy at 723 and 773 K exhibited
a sigmoidal behavior in a double logarithmic plot of the maximum true stress vs true strain rate. The strain-rate sensitivity was 0.25 to 0.3 in the low-(
<5×10−3 s−1) and high- (
>5×10−2 s−1) strain-rate regions, and ∼0.5 in the intermediate-strain-rate region (5×10−3 s−1<
<5 × 10−2 s−1). The maximum elongation to failure of ∼740 pct was obtained at 1×10−2 s−1 and 773 K. By contrast, no sigmoidal behavior was observed at 673 K. Instead, the strain-rate sensitivity of 0.3 was measured
in both intermediate-and low-strain-rate regions, but it was about 0.25 in the high-strain-rate region. High-strain-rate superplasticity
(HSRS) in the intermediate-strain-rate region at 723 and 773 K was dominated by grain-boundary sliding (GBS) associated with
continuous recrystallization and preservation of fine recrystallized grains by second-phase particles. However, the activation
energy for HSRS of the present alloy was lower than that predicted for any standard high-temperature deformation mechanism.
The low activation energy was likely the result of the not-fully equilibrated microstructure due to the prior severe plastic
deformation (SPD). For 673 K, the mechanical data and the microstructural examination revealed that viscous glide was a dominant
deformation mechanism in the intermediate- and low-strain-rate regions. Deformation in the high-strain-rate region at all
testing temperatures was attributed to dislocation breakaway from solute atmospheres. 相似文献
16.
17.
The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C)
and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties,
and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic
Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution
temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed
on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and
temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300
MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323
MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight
improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the
SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged
condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC
particles. A detailed discussion of the fracture mechanism is given. 相似文献
18.
S. M. Skolianos T. Z. Kattamis O. F. Devereux 《Metallurgical and Materials Transactions A》1989,20(11):2499-2516
The microstructure and corrosion behavior of as-cast and heat-treated Al-4.5 pct Cu-2.0 pct Mn alloy specimens solidified
at various cooling rates were investigated. The equilibrium phases Al6Mn and θ-Al2Cu, which are observed in the conventionally solidified alloy in the as-cast condition, were not detected in rapidly solidified
(melt-spun) material. Instead, the ternary compound Al20Cu2Mn3 was present in addition to the α phase, which was present in all cases. The morphological and kinetic nature of corrosion
was investigated metallographically and through potentiostatic techniques in 3.5 wt pct NaCl aqueous solution. Corrosion of
the as-cast material was described by two anodic reactions: corrosion of the intermetallic phases and pitting of the α-Al
solid solution. The corrosion rate increased with cooling rate from that for the furnace-cooled alloy to that for the copper
mold-cast alloy and, subsequently, decreased in the rapidly solidified alloy. In the heat-treated material, corrosion could
be described by two anodic reactions: corrosion of Al20Cu2Mn3 precipitate particles and pitting of the α-Al matrix.
S.M. Skolianos, formerly Graduate Student, Department of Metallurgy, University of Connecticut 相似文献
19.
A transmission electron microscopy study of the structural changes which attend aging at 180°C with and without pre-aging
at 100°C was conducted on a high purity aluminum alloy containing 6.8 pct Zn and 2.3 pct Mg. The refinement in precipitate
dispersion accompanying multiple aging is caused by the operation of aging sequences which differ from those occurring in
material given the single age at 180°C only. The high nucleation rate which occurs during the low temperature pre-aging treatment
is responsible for the observed precipitate refinement. The results of this investigation appear to agree favorably with the
Pashleyet al. model of multiple aging. 相似文献
20.
The morphology, crystallography, and nature of precipitates in a quenched and aged Zr-2.5 wt pct Nb alloy has been studied
by transmission electron microscopy. The needle-shaped matrix precipitates and equiaxed twin boundary nucleated precipitates
produced by aging at 500 °C were the equilibrium Nb-rich β2 phase. On aging at 600 °C, the matrix precipitation was a mixture of β2 needles and coarse metastable Zr-rich β1 particles, while only β1 particles were found at twin boundaries. The growth direction of the needle-shaped particles, 6.6 deg to 8.2 deg from (1-100)h, and their orientation relationship can be predicted by an invariant line strain model. The β1 precipitates have the Burgers orientation relationship. The formation of metastable β1 and stable β2 particles is considered from the free energy approach of Menon, Banerjee, and Krishnan. 相似文献