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1.
An investigation has been made into the solidification behavior and microstructural evolution of AM50, AM70, and AM90 alloys
during rheo-diecasting, their processibility, and the resulting mechanical properties. It was found that solidification of
AM series alloys under intensive melt shearing in the unique twin-screw slurry maker during rheo-diecasting gave rise to numerous
spheroidal primary magnesium (Mg) particles that were uniformly present in the microstructure. As a result, the network of
the β-Mg17Al12 phase was consistently interrupted by these spheroidal and ductile particles. Such a microstructure reduced the obstacle
of deformation and the harmfulness of the β-Mg17Al12 network on ductility, and therefore improved the ductility of rheo-diecast AM alloys. It was shown that, even with 9 wt pct
Al, the elongation of rheo-diecast AM90 still achieved (9 ± 1.2) pct. Rheodiecasting thus provides an attractive processing
route for upgrading the alloy specification of AM series alloys by increasing the aluminum (Al) content while ensuring ductility.
Assessment of the processibility of AM series alloys for semisolid processing showed that high Al content AM series alloys
are more suitable for rheo-diecasting than low Al content alloys, because of the lower sensitivity of solid fraction to temperature,
the lower liquidus temperature, and the smaller interval between the semisolid processing temperature and the complete solidification
temperature. 相似文献
2.
The influence of microstructure on the fracture toughness of Ti-23A1-9Nb-2Mo-1Zr-1.2Si (at. pct) and Ti-23A1-11Nb-0.9Si (at.
pct) Ti3Al-based alloys has been investigated. Basket-weave microstructures comprising different volume fractions of α
2 and retained β phases were produced by systematic heat treatments. Besides the volume fraction of the retained β phase, the average size of the β laths has also been used to characterize these microstructures. The toughness of both alloys was examined at room temperature,
and the brittle transgranular fracture modes were found to be controlled by microstructure. However, the toughness is not
determined solely by the volume fraction of the retained β phase, and a linear relationship has been obtained between the fracture toughness and the average size of the retained β laths. It appears therefore that the toughness of Ti3Al-based alloys at room temperature is controlled primarily by the width of retained β laths rather than by the retained β volume fraction. 相似文献
3.
A. K. Mukhopadhyay 《Metallurgical and Materials Transactions A》2001,32(8):1949-1958
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-Al2Cu) 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
4.
Noam Bernstein Ramasis Goswami Ronald L. Holtz 《Metallurgical and Materials Transactions A》2012,43(6):2166-2176
Alloying Al with Mg can improve its structural properties but also can lead to the formation of grain-boundary precipitates
of β-Mg2Al3 that lead to failure by intergranular fracture and corrosion. Simulating the properties of the β phase is difficult because
it has a complex structure with more than 1000 atoms per unit cell. We approximate the experimental β structure by the β′
structure, which has about 300 atoms per unit cell, and we compute the fracture behavior of the material from density functional
theory calculations of relevant surface and interface energies. We report also on experimental measurements of the orientation
and fracture properties of the α-Al(Mg)–β-Mg2Al3 interface and compare them with the atomistic simulations. We have computed the surface energy of face-centered cubic α-Al
with up to 10 at. pct Mg, as well as the decohesion energy of β′-Mg2Al3 and the interfacial decohesion energy between β′-Mg2Al3 and pure α-Al with geometry similar to that observed experimentally. We find that the β′-Mg2Al3 decohesion energy is nearly isotropic and is lower than the pure Al surface energy and the α-Al–β′-Mg2Al3 interface decohesion energy. This result is consistent with the experimental observations of fracture within the β phase
rather than at the α-Al(Mg)–β-Mg2Al3 interface or within the α-Al(Mg) phase. 相似文献
5.
Sarah Brennan Katrina Bermudez Nagraj S. Kulkarni Yongho Sohn 《Metallurgical and Materials Transactions A》2012,43(11):4043-4052
Solid-to-solid diffusion couples were assembled and annealed to examine the diffusion between pure Mg (99.96?pct) and Al (99.999?pct). Diffusion anneals were carried out at 573?K, 623?K and 673?K (300?°C, 350?°C and 400?°C) for 720, 360, and 240?hours, respectively. Optical and scanning electron microscopes were used to identify the formation of the intermetallic phases, ??-Mg17Al12, and ??-Mg2Al3, as well as the absence of the ??-Mg23Al30 in the diffusion couples. The thicknesses of the ??-Mg17Al12 and ??-Mg2Al3 phases were measured and the parabolic growth constants were calculated to determine the activation energies for growth. Concentration profiles were determined with electron microprobe analysis using pure elemental standards. Composition-dependent interdiffusion coefficients in Mg-solid solution, ??-Mg17Al12, ??-Mg2Al3, and Al-solid solutions were calculated based on the Boltzmann-Matano analysis. Integrated and average effective interdiffusion coefficients for each phase were also calculated, and the magnitude was the highest for the ??-Mg2Al3 phase, followed by ??-Mg17Al12, Al-solid solution, and Mg-solid solution. Intrinsic diffusion coefficients based on Huemann??s analysis (e.g., marker plane) were determined for the ~ Mg-62 at. pct Al in the ??-Mg2Al3 phase. Activation energies and the pre-exponential factors for the interdiffusion and intrinsic diffusion coefficients were calculated for the temperature range examined. The ??-Mg2Al3 phase was found to have the lowest activation energies for growth and interdiffusion among all four phases studied. At the marker location in the ??-Mg2Al3 phase, the intrinsic diffusion of Al was found to be faster than that of Mg. Extrapolations of the impurity diffusion coefficients in the terminal solid solutions were made and compared with the available self-diffusion and impurity diffusion data from the literature. Thermodynamic factor, tracer diffusion coefficients, and atomic mobilities at the marker plane composition were approximated using the available literature values of Mg activity in the ??-Mg2Al3 phase. 相似文献
6.
M. H. Mulazimoglu A. Zaluska F. Paray J. E. Gruzleski 《Metallurgical and Materials Transactions A》1997,28(6):1289-1295
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 (Al8Fe2Si) rather than β-AlFeSi (Al5FeSi) 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″-Mg2Si 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-Mg2Si phase. As a result, the strontium-treated alloy exhibits a resistance to overaging. 相似文献
7.
Yoshihiro Terada Rie Sota Naoya Ishimatsu Tatsuo Sato Koichi Ohori 《Metallurgical and Materials Transactions A》2004,35(9):3029-3032
The effect of calcium addition on the microstructure and creep strength of the die-cast AM50 magnesium alloy was investigated.
The α-Mg grains with the diameter of 4.9 μm are surrounded by the eutectic phases for the AM50-1.72 mass pct Ca alloy, while the β(Mg17Al12) particles are located mainly on the grain boundaries of the α grains for the AM50 alloy. The minimum creep rates of the AM50-1.72 mass pct Ca alloy are three orders of magnitude lower
than those of the AM50 alloy at 423 K typically below 120 MPa. The thousandfold creep strengthening by the Ca addition is
ascribed to the thermally stable eutectic phases appearing in the AM50-1.72 mass pct Ca alloy, which is expected to yield
effective grain boundary strengthening or to resist the plastic flow of the α-Mg grains. 相似文献
8.
9.
C. D. Marioara S. J. Andersen H. W. Zandbergen R. Holmestad 《Metallurgical and Materials Transactions A》2005,36(3):691-702
To study how changes in solute elements affect precipitation, six Al-Mg-Si alloys aged at 175 °C were investigated by transmission
electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears
after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with
β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg5Si6 phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions
of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate
microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the
content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior. 相似文献
10.
Matsuda Kenji Ikeno Susumu Terayama Kiyoshi Matsui Hiroaki Sato Tatsuo Uetani Yasuhiro 《Metallurgical and Materials Transactions A》2005,36(8):2007-2012
Differential scanning calorimetry (DSC) curves were obtained from Al-1.0 mass pct Mg2Si (balanced) and Al-1.0 mass pct Mg2Si −0.4 mass pct Si (excess Si) alloys, and precipitates corresponding to each peak at the DSC curve were interpreted by means
of high-resolution transmission electron microscopy (HRTEM) observation in order to understand the precipitation sequence
of metastable phases. Five peaks were obtained on the DSC curves, from which four were exothermic (A, C, D, and E) and one
endothermic (B). Upon HRTEM observation, the peaks for the excess Si alloy were explained as follows: peak A–B: Guinier-Preston
(GP) zones and random-type precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation
of the β″ phase; peak C: β″ phase and precipitation of type B; peak D: dissolution of the β″ phase; precipitation of type A and β′ phase; and peak E: dissolution of the type B, type A, and β′ precipitation of the (β+Si) phase. This result is quite different from that in the balanced alloy as follows: peak A–B: GP zones and random-type
precipitates; peak B: dissolution of the GP zones and the random-type precipitates, precipitation of the parallelogram-type
precipitate; peak C: parallelogram-type precipitate and precipitation of β′ phase; peak D: β′ phase, dissolution of parallelogram-type precipitate; and peak E: the β-(Mg2Si). 相似文献
11.
J. A. Taylor G. B. Schaffer D. H. StJohn 《Metallurgical and Materials Transactions A》1999,30(6):1651-1655
The mechanism by which iron causes casting defects in the AA309 (Al-5 pct Si-1.2 pct Cu-0.5 pct Mg) may be related to the
solidification sequence of the alloy. Superimposing calculated segregation lines on the liquidus projection of the ternary
Al-Si-Fe phase diagram suggests that porosity is minimized at a critical iron content when solidification proceeds directly
from the primary field to the ternary Al-Si-βAl5FeSi eutectic point. Solidification via the binary Al-βAl5FeSi eutectic is detrimental to casting integrity. This hypothesis was tested by comparing the critical iron content observed
in the standard AA309 alloy to that of a high-silicon (10 pct Si) variant of this alloy. 相似文献
12.
The effects of 0.2, 0.6, and 1.0 wt pct Zr additions on the microstructure and creep behavior of AZ91 Mg alloy were investigated
by impression tests carried out under constant punching stress (σ
imp) in the range 100 to 650 MPa, corresponding to the modulus-compensated stress levels of
0.007 £ s\textimp \mathord