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1.
Differential scanning calorimetry and electron diffraction investigation on low-temperature aging in Al-Zn-Mg alloys 总被引:3,自引:0,他引:3
X. J. Jiang J. Tafto B. Noble B. Holme G. Waterloo 《Metallurgical and Materials Transactions A》2000,31(2):339-348
Differential scanning calorimetry (DSC) has been combined with transmission electron microscopy (TEM) to investigate the low-temperature
decomposition processes taking place in an Al-5 wt pct Zn-1 wt pct Mg alloy. It was confirmed that two types of GP zones,
i.e., GP(I) (solute-rich clusters) and GP(II) (vacancy-rich clusters), formed independently during decomposition of the supersaturated
solid solution. The GP(I) zones form at a relatively low aging temperature and dissolve when the aging temperature is increased.
The GP(II) zones are stable over a wider range of temperatures. To investigate the nature of the zones in the Al-Zn-Mg alloy,
differential scanning calorimetry and transmission electron microscopy have also been carried out on binary Al-Zn alloys containing
5 wt pct and 10 wt pct Zn. In these Al-Zn alloys, GP zones formed rapidly during quenching, and they gave rise to characteristic
electron diffraction patterns identical to those from GP(II) in the Al-Zn-Mg alloy system, implying that GP(II) zones in Al-Zn-Mg
alloys are very similar to the zones formed in binary Al-Zn alloys. Thus, it is likely that GP(II) zones in Al-Zn-Mg alloys
are zinc-rich clusters. In the Al-5 wt pct Zn-1 wt pct Mg alloy, both GP(I) and GP(II) were found to transform to η′ and/or η particles during heating in the differential scanning calorimeter. The η′ was also observed to form after prolonged isothermal aging of the Al-Zn-Mg alloy at 75 °C or after short aging times at
125 °C. 相似文献
2.
The growth of spherical precipitates in an Al-5 wt pct Zn-2 wt pct Mg alloy was monitored by hardness measurements and transmission electron microscopy. The growth of precipitates from 15 to 80Å in diameter was followed for various aging times at temperatures of 30, 70, 100, and 150°C. For short times at low aging temperatures precipitate growth followed at 1/9 law, whereas growth at longer times, or higher temperatures, followed at 1’3 law. Various contrast experiments lead to the conclusion that the precipitates produced at all but the shortest aging times at the lowest aging temperatures have a hexagonal structure and are spherical precursors of the η′ phase. The advantages and disadvantages associated with the various imaging techniques for small precipitates utilized in this study are discussed. 相似文献
3.
The growth of spherical precipitates in an Al-5 wt pct Zn-2 wt pct Mg alloy was monitored by hardness measurements and transmission
electron microscopy. The growth of precipitates from 15 to 80Å in diameter was followed for various aging times at temperatures
of 30, 70, 100, and 150°C. For short times at low aging temperatures precipitate growth followed at
1/9 law, whereas growth at longer times, or higher temperatures, followed at
1’3 law. Various contrast experiments lead to the conclusion that the precipitates produced at all but the shortest aging times
at the lowest aging temperatures have a hexagonal structure and are spherical precursors of the η′ phase. The advantages and
disadvantages associated with the various imaging techniques for small precipitates utilized in this study are discussed.
Formerly Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,
MA. 02139 相似文献
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Cumulative zirconium and zirconium alloy creep data over a broad range of stresses (0.1 to 115 MPa) and temperatures (300
°C to 850 °C) were analyzed based on an extensive literature review and experiments. Zirconium obeys traditional power-law
creep with a stress exponent of approximately 6.4 over stain rates and temperatures usually associated with the conventional
“five-power-law” regime. The measured activation energies for creep correlated with the activation energies for zirconium
self-diffusion. Thus, dislocation climb, rather than the often assumed glide mechanism, appears to be rate controlling. The
common zirconium alloys (i. e., Zircaloys) have higher creep strength than zirconium. The stress exponents of the creep data in the five-power-law regime
were determined to be 4.8 and 5.0 for Zircaloy-2 and Zircaloy-4, respectively. The creep strength of irradiated Zircaloy appears
to increase relative to unirradiated material. It was found that the creep behavior of zirconium was not sensitive to oxygen
in the environment over the temperature range examined. 相似文献
6.
This study of Al-Zn-Mg alloys was undertaken to establish the specific combination of mechanical and thermal conditions which
is necessary for stress-corrosion susceptibility to occur in these alloys. The work included a study by electron transmission
microscopy of the microstructure of this alloy system. A new mechanism is proposed to account for the observations made. It
suggests that the high corrosion potential at grain boundaries in stress-corrosion susceptibile Al-Zn-Mg alloys is due to
segregation of zinc and magnesium. By taking precautions to reduce this segregation, the stress-corrosion life can be increased. 相似文献
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L. L. Rokhlin N. R. Bochvar N. P. Leonova I. E. Tarytina 《Russian Metallurgy (Metally)》2011,(1):55-59
Metallography and electrical resistivity and hardness measurements are used to study the aging and recrystallization of Al-rich
Al-Sc-Cr-Zr alloys with 0.1–0.4% Sc, 1.0% Cr, and 0.2% Zr. During aging at 350°C, the hardness of the alloys is found to increase
substantially even after 0.5-h aging as compared to that of the deformed state. Variations of the hardness and electrical
resistivity of all alloys exhibit similar behavior; however, the higher the scandium content in the alloys, the higher the
strengthening effect reached after aging for 2–4 h. Combined small scandium, chromium, and zirconium additions to aluminum
are shown to increase its recrystallization temperature; therewith, the effect of scandium on the increase in the temperatures
of the onset and finish of recrystallization is more substantial. 相似文献
9.
Al-Zn-Mg系铝合金应力腐蚀性能 总被引:4,自引:0,他引:4
研究了热处理制度和时效工艺的改变对Al-Zn-Mg系铝合金的组织结构、力学性能和应力腐 蚀性能的影响。研究结果表明:高温预析出可以改变Al-Zn-Mg系铝合金晶界的析出相大小和分 布,从而改善其抗应力腐蚀性能;在T6和T612种人工时效条件下,预析出的合金的抗应力腐蚀 性能均好于无预析出的合金。在自然时效状态下,引入超声波,对无预析出合金的应力腐蚀性能 进行了初步探索,发现超声波可以提高合金的抗应力腐蚀性能,而对合金的硬度无影响。 相似文献
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Corrosion fatigue crack propagation tests were performed on commercial 7075 alloys. Testing was done in a 3.5 pct sodium chloride
solution under constant impressed potential and under reversed anodic-cathodic current conditions. Results indicated that
a cathodic potential of -1.400 V vs SCE was sufficient to reduce corrosion fatigue crack growth rates to the level observed
in dry argon. By alternately impressing anodic and cathodic currents, it was shown that anodic potentials enhance the crystallographic
dependence of the fracture mode, resulting in brittle striations, while cathodic potentials result in ductile striations formed
by shear. Modification of the alloy chemistry and lower impurity content resulted in a two-fold reduction in crack growth
rates. Thermomechanical treatment of these alloys to refine the grain size proved detrimental. Adding an inhibitor to the
sodium chloride solution was found to be the most effective means for reducing corrosion fatigue crack growth rates. A model
for the environment-surface interaction is suggested. 相似文献
13.
Nejdet Kayali Raşit Zengin Osman Adiguzel 《Metallurgical and Materials Transactions A》2000,31(2):349-354
Shape memory alloys exhibit superelasticity when they are deformed in a temperature range where the thermoelastic martensite
forms on application of a strain. The martensite persists upon removal of the applied strain, and the alloy recovers the original
shape on heating over the reverse-transformation temperature after removing the strain. The β-phase CuZnAl alloys have β-type superlattice in the parent case, and M9R or M18R martensites occur on quenching the alloys from the homogenization temperature.
The basal plane of martensite is exposed to hexagonal distortion with martensitic transformation as well as the monoclinic
distortion in the crystal structure, and splittings are observed in some selected diffraction-peak pairs due to the differences
in atom sizes in lattice points. These pairs have a great importance as ordering criteria and satisfy a special relation between
Miller indices. The present text reports the variation of the differences in interplane spacings (Δd) between some selected planes upon the further aging at room temperature at which alloys are fully martensitic. The decrease
of Δd during the aging implies that the monoclinic distortion decreases. The mass increases are caused by the oxidation upon heating
the alloys at high temperatures close to the betatizing temperature at free atmosphere. 相似文献
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E. Di Russo M. Conserva F. Gatto H. Markus 《Metallurgical and Materials Transactions B》1973,4(4):1133-1144
An investigation was carried out to determine the metallurgical properties of Al-Zn-Mg and Al-Zn-Mg-Cu alloy products processed
according to newly developed Final Thermomechanical Treatments (FTMT) of T-AHA type. The results show that these cycles can
be utilized to produce wrought products of high purity Al-Zn-Mg(-Cu) alloys characterized by equivalent toughness and ductility
and much higher strength than conventionally processed commercial purity materials. Based on transmission electron microscopy
studies, it was found that such improved behavior of FTMT material is attributable to the superposition of hardening effects,
from aging precipitation and from dislocations. Preliminary stress-corrosion and fatigue tests indicate that these properties
are not substantially influenced by T-AHA thermomechanical process. Further work is needed in this area, in order to better
understand the directions to follow for developing better alloys. 相似文献
18.
D. R. Swinbourne G. G. Barbante A. Strahan 《Metallurgical and Materials Transactions A》1996,27(10):3187-3191
In the final stages of the smelting of copper anode slimes, a silver alloy, known as “doré,” is produced. Oxidation refining
is used to remove copper since this element interferes with subsequent electroparting of the small amounts of gold and platinum
group metals in the doré. The gold content of doré can be greatly increased by gold scrap additions and this may affect the
minimum achievable copper content of doré. In this work, silver-gold-copper alloys were oxidized by injecting pure oxygen
at 1100 °C in the absence of any slag cover. For the gold contents expected in practice, the equilibrium copper content of
the doré did not increase significantly as the gold content increased. However, at the other extreme of composition, the equilibrium
copper content was a very strong function of the silver content of the gold bullion. The activity coefficient of copper in
silver-gold alloys was calculated and compared to those predicted from a ternary subregular solution model of the system Ag-Au-Cu.
Satisfactory agreement was found. 相似文献
19.
A study has been made of the effect of Ti additions on the microstructure in two stage and single stage aging of an Al-5 Zn-2
Mg alloy. The influence of aging treatment was examined for alloys with and without Ti addition. The alloys were solution
heat treated at 510°C, air cooled then aged at various temperatures below the GP zone solvus for short times before aging
above the GP zone solvus at 200°C. The Ti addition alloy yields a small precipitate free zone (PFZ ∼ 0.3 μ) and a higher density
of precipitate near the edge of the PFZ than in the midgrain region. The ternary alloy without Ti addition produces a wide
range of PFZ sizes (0.4 to 4 μ) depending upon the aging temperature below the GP zone solvus. Significant changes in the
morphology of the matrix precipitate, the amount of grain boundary precipitate, and the width of the PFZ, were observed in
single stage aging at 200°C as a result of the Ti addition. These results could be explained in terms of Ti interacting with
vacancies or solute atoms, causing the changes in the vacancy and/or solute concentration profiles. A model based upon solute
and vacancy concentrations coupled with a time at temperature effect has been developed to allow the interpretation of the
observed two-stage aging results in an Al-Zn-Mg alloy.
Formerly Graduate Research Assistant, Materials Engineering, Rensselaer Polytechnic Institute, Troy, New York. 相似文献
20.
C. D. S. Tuck 《Metallurgical and Materials Transactions A》1985,16(8):1503-1514
Al4.5Zn1.5Mg and Al5Mg were reacted in water-vapor saturated air (WVSA) at 120°C and tensile tested. After an initial loss
of ductility with exposure time, probably caused by hydrogen embrittlement of the grain boundaries, between 15 hours and 25
hours exposure the mechanical properties of Al4.5Zn1.5Mg improved, this effect being due both to a reduced corrosion activity
of the grain boundaries in producing embrittling hydrogen at the external surface and to grain boundary MgZn2 precipitates acting as hydrogen traps. After 25 hours exposure water was shown to penetrate the grain boundaries, and a layered
corrosion product identified as the aluminum hydroxides boehmite and diaspore was formed. This resulted in a marked fall of
ductility. Re-solution heat treatment and reaging partially recovered the mechanical properties of Al4.5Zn1.5Mg if the exposure
time was less than 50 hours, and would not recover properties for longer exposure times. Small additions (0.1 pct) of iron
and nickel to Al4.5Zn1.5Mg lessened the grain boundaries’ sensitivity to corrosive attack whereas the addition of 0.1 pct
copper did not. Also, the former two additions did not cause the relative ductility increase during 15 to 25 hours exposure
in WVSA at 120°C shown by Al4.5Zn1.5Mg. It is proposed that these elements alter the magnesium segregation levels at the grain
boundaries which in turn affects their electrochemical attack. 相似文献