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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 pet 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 worm 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 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
3.
A new process for producing rapidly solidified bulk alloys was developed based on the hammer- and-anvil concept. In the process,
an A1-12 wt pct Si alloy slab was built up layer by layer and then hot worked to get a solid and integral sheet. The oxygen
content of the layer-deposited alloy is less than the typical value of powder metallurgy (PM) alloys by one order, and the
cooling rate can reach 104 K/s, which is higher than that of the spray deposition process. In comparison with the ingot-processed Al-12 wt pct Si alloy,
layer-deposited alloy exhibits su- perior mechanical properties. This is attributable to the fine and uniform silicon-particle
distri- bution which not only brings on dispersion hardening effect but also raises the elongation and fracture strain. The
mechanisms responsible for this enhancement were discussed in terms of particle size and effective volume fraction. 相似文献
4.
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
5.
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
6.
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
7.
Studies of the simultaneous creep and oxidation of an Fe-lwt pct Al alloy at constant stresses of 16 to 26 MN/m2 in the temperature range 973 to 1073 K have shown that the steady state creep obeys a power law. The stress exponentn was found to be 6.9 for creep in an argon atmosphere (Po2 = 10?3 mbar). Values of the apparent activation energy for creep were in the range 225 to 351 kJ/mol and appeared to be dependent on the P0 2 of the test environment. Oxidationstrengthening (?e decreasing) occurred at 998 K but only at P0 2 = 10?3 mbar and was primarily due to intergranular oxidation and the mechanical constraint of a strongly adherent scale. Oxidationweakening (?e increasing), however, occurred at 973 to 1073 K in environments of the lowest P0 2 (10?9 and 10?5 mbar) and in the highest P0 2 (162 to 1013 mbar). The factors contributing to weakening are believed to be oxidation-induced vacancies, weakly adherent scales and the loss of solute strengthening aluminum through selective oxidation. 相似文献
8.
Joonyeon Chang Inge Moon Chongsool Choi 《Metallurgical and Materials Transactions A》1998,29(7):1873-1882
Analytical transmission electron microscopy and thermal analysis of as-extruded Al-4.7 pct Zn-2.5 pct Mg-0.2 pct Zr-X wt pct Mn alloys, with Mn contents ranging from 0.5 to 2.5 wt pct, were carried out to elucidate the microstructural change
and accompanying mechanical properties during subsequent heat treatments. The as-extruded alloy was fabricated from rapidly
solidified powder and consisted of a fine, metastable manganese dispersoid and the ternary eutectic T phase (Al2Mg3Zn3). Solution heat treatment resulted in the formation of the stable Al6Mn phase and complete dissolution of the T phase. Formation of stable Al6Mn was made by two routes: by phase transition from metastable Mn dispersoids which already existed, and from the supersaturated
solid solution by homogeneous nucleation. The density of the Al6Mn phase increased with the addition of manganese, while the shape and average size remained unchanged. A significant increase
in the hardness was observed to coincide with the formation of the Al6Mn phase. Similarly, the tensile strength increased further after the aging treatment, and the increment was constant over
the content of Mn in the alloy, which was explained by the contribution from the same amount of precipitates, MgZn2. Results of thermal analysis indicated that the dissolution of the T phase started near 180 °C and that formation of Al6Mn occurred at about 400 °C, suggesting that further enhancement of strength is possible with the modification of the heat-treatment
schedule. 相似文献
9.
《Acta Metallurgica》1976,24(6):517-522
Electron microscope observations show that aging a supersaturated solid solution of Al-0.212 wt.% Au alloy produces a distribution of plate precipitates of the equilibrium phase η (AuAl2) on the matrix cube planes without the formation of any metastable phases (contrary to previous report). Dislocations in the matrix act as heterogeneous sites for the nucleation of these precipitates. The large coherency strain field due to a mismatch of 4.8% parallel to their broad faces gives rise to a large age-hardening effect. Misfit dislocations nucleated at longer ageing times to accommodate the mismatch reduce this effect. 相似文献
10.
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
11.
The mechanical properties of an Al-6 wt pct Zn-1.2 wt pct Mg alloy with various width of precipitate-free zones have been investigated. The width of the precipitate-free zone (PFZ) has been changed by the quench interruption technique without any appreciable change in the size and distribution of precipitates. An important relationship has been observed between the width of the PFZ and the quench-interruption period;i.e., the width of the PFZ increases in proportion to the square root of the holding time at 200°C. From the analysis of stress-strain curves as well as the observation of dislocation arrangements in slightly deformed specimens, the plastic deformation has been found to occur preferentially in the PFZ. The initial stage of deformation is much affected by the change in the width of the PFZ, but in the later stage, the work-hardening rate seems to be almost independent of the PFZ width. Tensile tests show that the ultimate tensile strength and the 0.2 pct proof stress decrease very little with increasing width of the PFZ, while the uniform elongation is practically constant regardless of the reduction in the nonuniform elongation. The work-hardening rate at the initial stage of deformation is found to decrease in proportion to the reciprocal of the PFZ width. This relationship can be explained from the dislocation model for work hardening. 相似文献
12.
13.
14.
L. Murgás Z. Homonnay S. Nagy A. Vertes 《Metallurgical and Materials Transactions A》1988,19(2):259-264
Phase transformation of the metastable Al6Fe intermetallic phase to Al3Fe in an Al-0.58 wt Pct Fe alloy was investigated by Mössbauer spectroscopy. The kinetics of the process were estimated by the Avrami equation:A=A 0e -(kt). The concentration of Al6Fe,A, was followed in time,t, at different temperatures between 650 and 580 °C. It was found that the kinetics of the phase transformation at 650 ? differed from those found in the 635 to 580 °C temperature range. In the latter case the activation energy of the overall process was 3.33 ± 0.47 eV as determined from the rate constants (K). The optimal value obtained for the parametern was 0.71. A simple model suggested for the mechanism of the transformation proved that this process was interface controlled between 635 and 580 °C. 相似文献
15.
E. Maire D. S. Wilkinson J. D. Embury H. Henein 《Metallurgical and Materials Transactions A》1998,29(10):2613-2620
Metal powders always contain a surface oxide layer, which is particularly tenacious in aluminum alloys. After hot pressing,
this oxide coats the particle boundaries and reduces the ductility. In this article, a study of the Al-17 wt pct Cu alloy
densified from rapidly solidified powder is presented. Different thermomechanical treatments were investigated to improve
the ductility of this material. Channel-die (CD) forging was performed at two temperatures (430 °C and 500 °C). Eight compression
runs were applied to the samples in each CD treatment. At 430 °C, three strain values per run were investigated (35, 50 and
70 pct). A bar was also extruded with a 40:1 ratio. Because of the small size of the samples, the ductility was assessed by
means of the ring expansion test and analyzed by post mortem (fracture surface and cross section) observations. No ductility was measured after CD compression at 430 °C, although it
appears from the fracture surface observations that increasing the strain per run has a beneficial effect. The CD compression
at 500 °C and extrusion were both successful at promoting ductility, extrusion being more effective. 相似文献
16.
Shoji Fujiwara Takatoshi Ogawa Yoshifumi Ohmura Imao Tamura 《Metallurgical and Materials Transactions A》1983,14(6):1067-1077
In Fe-4 pct Mo-0.2 pct C martensite which is a typical secondary hardening steel, premature failure o°Curred in tensile test
at 600 °C to 700°C where solute atoms could diffuse easily. To clarify this phenomenon, the quenched specimens were tempered under applied stress
and tensile-tested at room temperature. The following results were obtained: (1) Typical intergranular fracture was observed
in specimens tempered in a temperature range of 600 °C to 650 °C with tempering times of five minutes to 10 minutes and applied
stress (70 MPa to 140 MPa). (2) Based on Auger analysis, this phenomenon was considered to be caused by segregation of P,
S, and Mo on prior austenite grain boundaries due to applied stress. (3) The direction of applied stress was found to be very
significant. Namely, when the tensile direction was parallel to the applied stress during tempering, the specimen was more
brittle, and when tensile direction was normal to the applied stress, the specimen was not so brittle. (4) To reduce this
embrittlement, solution treatment temperature was adjusted, and it was found that the embrittlement was considerably reduced
both in specimens with fine prior austenite grains and with some ferrite phase on prior austenite grain boundaries.
TAKATOSHI OGAWA, formerly with Kyoto University.
YOSHIFUMI OHMURA, formerly with Kyoto University.
This paper is based on a presentation made at the “pcter G. Winchell Symposium on Tempering of Steel” held at the Louisville
Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy
and Heat Treatment Committees. 相似文献
17.
Ari Varschavsky Maria I. Perez Thomas Löbel 《Metallurgical and Materials Transactions A》1975,6(3):577-581
An electron microscopic investigation of Cu-9 wt pct Al alloy low thermal treated at 250°C for 30 min was performed in the deformed and non-deformed condition. For this composition and ordering treatment the alloy exhibits the higher increase in strength. It was found that the images formed in the non-deformed alloy, reveal the presence of ordered domains of an average size of 80Å, being the amount of order dependent of whether or not quenched-in vacancies are present. It is probable that the domains nucleate preferentially at stacking faults in the deformed condition. A periodic antiphase structure was determined from computations and comparison with electron diffraction data. The superlattice cell is based on the LI2 type, tetragonal face centered with a period approximately three times the lattice parameter of the matrix, having three variants of orientation within an ordered region. 相似文献
18.
Uniaxial tensile deformation of uranium 6 wt pct niobium: A neutron diffraction study of deformation twinning 总被引:2,自引:0,他引:2
D. W. Brown M. A. M. Bourke M. G. Stout P. S. Dunn R. D. Field D. J. Thoma 《Metallurgical and Materials Transactions A》2001,32(9):2219-2228
The deformation of polycrystalline uranium 6 wt pct niobium (U6Nb) was studied in situ during uniaxial tensile loading by time-of-flight neutron diffraction. Diffraction patterns were recorded at incremental
stresses to a maximum of 450 MPa (∼4 pct macroscopic strain). Consistent with reorientation of the martensite variants by
twinning, significant changes in the diffraction peak intensities, which were proportional to the plastic contribution of
the macroscopic strain, were observed. Both the lattice parameters (a, b, c, and γ) and interplanar spacings (d
hkl
) were determined as a function of applied stress. Phenomenologically, the highly anisotropic stress response of the lattice
parameters as well as the individual lattice spacings can be related to deformation twinning. Preliminary transmission electron
microscopy (TEM) studies identified the (
30) and (
72) as active deformation twinning systems of U6Nb in tension. 相似文献
19.
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
20.
Ari Varschavsky Maria I. Perez Thomas L?bel 《Metallurgical and Materials Transactions A》1975,6(2):577-581
An electron microscopic investigation of Cu-9 wt pct Al alloy low thermal treated at 250°C for 30 min was performed in the
deformed and non-deformed condition. For this composition and ordering treatment the alloy exhibits the higher increase in
strength. It was found that the images formed in the non-deformed alloy, reveal the presence of ordered domains of an average
size of 80?, being the amount of order dependent of whether or not quenched-in vacancies are present. It is probable that
the domains nucleate preferentially at stacking faults in the deformed condition. A periodic antiphase structure was determined
from computations and comparison with electron diffraction data. The superlattice cell is based on the LI2 type, tetragonal face centered with a period approximately three times the lattice parameter of the matrix, having three
variants of orientation within an ordered region. 相似文献