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1.
2.
《Acta Materialia》2007,55(16):5333-5347
The influence of a high magnetic field on the growth of MnBi, α-Al and Al3Ni dendrites in directionally solidified Bi–Mn, Al–Cu and Al–Ni alloys have been investigated. Results indicate that the magnetic field changes the dendrite growth significantly. Indeed, the magnetic field aligns the primary dendrite arm and the effect is different for different dendrites. For the MnBi dendrite, an axial high magnetic field enhanced the growth of the primary dendrite arm along the solidification direction; however, for the α-Al and Al3Ni dendrites, the magnetic field caused the primary dendrite arm to deviate from the solidification direction. At a lower growth speed, a high magnetic field is capable of causing the occurrence of the columnar-to-equiaxed transition (CET). Moreover, it has also been observed that a high magnetic field affects the growth of the high-order (i.e., secondary and tertiary) dendrite arms of the α-Al dendrite at a higher growth speed; as a consequence, the field enhances the branching of the dendrite and the formation of the (1 1 1)-twin planes. The above results may be attributed to the alignment of the primary dendrite arm under a high magnetic field and the effect of a high magnetic field on crystalline anisotropy during directional solidification.  相似文献   

3.
4.
Binary aluminum alloys with 0.03–0.06 at.% RE (RE = Yb or Er) were aged to produce coherent, nanosize Al3RE precipitates in an α-Al matrix. The temporal evolution of precipitate radii and matrix concentrations at 300 °C were measured by transmission electron microscopy and local-electrode atom-probe tomography, respectively. The temporal dependence of the matrix concentration of each RE was utilized to determine its solubility in Al. The solubility and the coarsening rate constants were used to determine the diffusivity of each RE in α-Al and the α-Al/Al3RE interfacial free energies at 300 °C. When compared to Sc, both Yb and Er exhibited smaller solubilities but larger diffusivities in α-Al and larger α-Al/Al3RE interfacial energies.  相似文献   

5.
《Acta Materialia》2007,55(4):1377-1386
The influence of an axial high magnetic field (up to 10 T) on the liquid–solid interface morphology and microstructure of the solid has been investigated experimentally during Bridgman growth of Al–Cu hypoeutectic alloys. It is found that the field causes the interface to become destabilized and irregular, and promotes planar–cellular and cellular–dendritic transformation. The field has a great influence on the cellular and dendrite array morphology. Indeed, the field causes severe distortion in the cellular array and enhances cell branching. The field makes the morphology of the dendrite array more complex and, with the increase of the magnetic field intensities and decrease of the growth velocities, the dendrites become broken and orientate with the 〈1 1 1〉-direction along the solidification direction instead of the 〈1 0 0〉-direction. Furthermore, the field also enlarges the primary dendrite spacing and promotes the branching of the dendrites to form higher-order arms. The above phenomena may be attributed to the change of the equilibrium partition coefficient k and the liquidus slope mL caused by the field, the magnetic anisotropy of the α-Al crystal and the flow created by the thermoelectromagnetic convection.  相似文献   

6.
A preliminary investigation has been carried to evaluate the influence of Fe on Sr-modified and unmodified eutectic Al–Si alloys in as-cast and heat treatment conditions. The castings were produced in zircon-coated steel permanent mold and were solutionized at 500 °C for 8 h and followed by artificial aging at 155 °C for 5 h, i.e., T6-temper. The microstructure changes in the β-Al5FeSi particle morphology were analyzed. The results indicate that dendrite arm spacing is strongly related to the cooling rate rather than the chemical composition, increasing the iron content leads to increase porosity and hardness either in the as-cast condition or after T6-temper. The Sr-modified alloys have higher hardness than unmodified at all Fe-added values. The precipitated long branched β-platelets result in the formation of large shrinkage cavities due to the inability of liquid metal to feed the space between them during solidification.  相似文献   

7.
This paper presents the thermodynamic evaluation of A390 hypereutectic Al–Si alloy (Al–17% Si–4.5% Cu–0.5% Mg) and alloys up to 10% Mg, using the Factsage® software. Two critical compositions were detected at 4.2% and 7.2% Mg where the temperatures of the liquidus, the start of the binary and of the ternary eutectic reaction are changed. These critical compositions show differences in the formation of Mg2Si intermetallic particles during the solidification interval. For compositions up to 4.2% Mg, the Mg2Si intermetallic phase first appears in the ternary eutectic zone. With Mg contents between 4.2% and 7.2%, Mg2Si particle appears in both the binary and ternary eutectic reactions. Above 7.2% Mg, it solidifies as a primary phase and also during the binary and ternary reactions. The calculated liquid fraction vs. temperature curves also showed a decrease of the eutectic formation temperature (knee point temperature) with the addition of Mg content up to 4.2% Mg. This temperature becomes almost constant up to 10% Mg. The calculation of eutectic formation temperature shows a good agreement with differential scanning calorimetry (DSC) tests.  相似文献   

8.
The effects of adding 0.02 or 0.06 at.% Si to Al–0.06Sc–0.06Zr (at.%) are studied to determine the impact of Si on accelerating Al3(Sc,Zr) precipitation kinetics in dilute Al–Sc-based alloys. Precipitation in the 0.06 at.% Si alloy, measured by microhardness and atom-probe tomography (APT), is accelerated for aging times <4 h at 275 and 300 °C, compared with the 0.02 at.% Si alloy. Experimental partial radial distribution functions of the α-Al matrix of the high-Si alloy reveal considerable Si–Sc clustering, which is attributed to attractive Si–Sc binding energies at the first and second nearest-neighbor distances, as confirmed by first-principles calculations. Calculations also indicate that Si–Sc binding decreases both the vacancy formation energy near Sc and the Sc migration energy in Al. APT further demonstrates that Si partitions preferentially to the Sc-enriched core rather than the Zr-enriched shell in the core/shell Al3(Sc,Zr) (L12) precipitates in the high-Si alloy subjected to double aging (8 h/300 °C for Sc precipitation and 32 days/400 °C for Zr precipitation). Calculations of the driving force for Si partitioning confirm that: (i) Si partitions preferentially to the Al3(Sc,Zr) (L12) precipitates, occupying the Al sublattice site; (ii) Si increases the driving force for the precipitation of Al3Sc; and (iii) Si partitions preferentially to Al3Sc (L12) rather than Al3Zr (L12).  相似文献   

9.
《Intermetallics》2007,15(9):1217-1227
The isothermal oxidation kinetics of molybdenum silicide based alloys with composition (in at.%) as 76Mo–14Si–10B (MSB), 77Mo–12Si–8B–3Al (MSB3AL), and 73.4Mo–11.2Si–8.1B–7.3Al (MSB7.3AL) processed by reaction hot pressing of elemental powders, have been investigated in the temperature range of 700–1300 °C in dry air for 24 h. The microstructures of all the alloys have shown the presence of α-Mo, Mo3Si, Mo5SiB2 and SiO2 or α-Al2O3 phases. The oxidation kinetics and the resulting scale characteristics depend on the alloy composition and temperature of exposure. While all the three alloys show unabated loss of mass causing pest disintegration at 700 °C, the MSB3AL and MSB7.3AL alloys undergo large mass loss in the range of 800–900 °C as well. The loss in mass has been attributed primarily to volatilization of MoO3 as well as spallation. The oxide scales formed in the range of 700–800 °C show SiO2 and MoO3, while those formed at 900 °C or above contain Mo, MoO2 and SiO2. In addition, α-Al2O3 or mullite has been found in the oxide scales of MSB3AL and MSB7.3AL alloys. The oxidation resistance of the Mo–Si–B alloys can be enhanced in the range of 700–800 °C by pre-oxidation treatment at 1150 °C to form a protective scale of B2O3–SiO2.  相似文献   

10.
The ultrasonic treatment temperatures were varied from about 100 °C above the liquidus temperature down to the Al–Si eutectic temperature, for different treatment times (0–15 s). The results showed that the ultrasonic melt treatment was very effective to convert the long plate-like Fe-intermetallic phases (up to 200 μm length) to a highly compacted fine polyhedral/globular form (<15 μm size). The critical ultrasonic treatment temperature to affect the morphology of Fe intermetallics was found to be in the range of 596–582 °C. The eutectic Si was mostly not affected by ultrasonic treatments carried out in this study (in the temperature range of 670–581 °C and for up to 10 s). It was also observed that the nucleation undercooling, which is a measure of nucleation efficiency, at the start of solidification was lowered from ~2.9 to ~0.4 °C by ultrasonic treatment. The variation of horn temperature within 20 °C above pouring temperature to 10 °C below it had no noticeable effect. The ultrasonically treated samples showed better tensile properties than the untreated samples, due to the change in morphology of the Fe-intermetallic particles.  相似文献   

11.
《Intermetallics》2007,15(9):1202-1207
The thermoelectric properties of Sb-doped Mg2Si (Mg2Si:Sb = 1:x(0.001  x  0.02)) fabricated by spark plasma sintering have been characterized by Hall effect measurements at 300 K and by measurements of electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) between 300 and 900 K. Sb-doped Mg2Si samples are n-type in the measured temperature range. The electron concentration of Sb-doped Mg2Si at 300 K ranges from 2.2 × 1019 for the Sb concentration, where x = 0.001, to 1.5 × 1020 cm−3 for x = 0.02. First-principles calculation revealed that Sb atoms are expected to be primarily located at the Si sites in Mg2Si. The electrical resistivity, Seebeck coefficient, and thermal conductivity are strongly affected by the Sb concentration. The sample x = 0.02 shows a maximum value of the figure of merit ZT, which is 0.56 at 862 K.  相似文献   

12.
《Intermetallics》2007,15(3):382-395
The effects of alloying on the microstructures, solidification path, phase stability and oxidation kinetics of Nbss/Nb5Si3 base in situ composites of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system have been investigated in this study. All the studied alloys are classified as hyper-eutectic Nb silicide base in situ composites and have lower densities compared to nickel-based superalloys. The Nb3Si silicide formed in the Hf-free alloys and transformed to Nbss and αNb5Si3 during heat treatment at 1500 °C. This transformation was enhanced by the addition of Ti. The Nbss and Nb5Si3 were the equilibrium phases in the microstructures of the Hf-free alloys. In the presence of Ti, the βNb5Si3 only partially transformed to αNb5Si3, suggesting that Ti stabilises the βNb5Si3 to lower temperatures (at least to 1300 °C). Furthermore, alloying with Hf stabilised the hexagonal γNb5Si3 (Mn5Si3-type) silicide in the Hf-containing alloys. The addition of Sn promoted the formation of the Si-rich C14 Laves phase and stabilised it at 1300 °C. This is attributed to the Sn addition decreasing the solubility of Cr in the Nbss of the Nb–Ti–Si–Al–Cr–Mo–Hf–Sn system whilst increasing the Si solubility. The Si solubility in the C14 Laves phase was in the range ∼6.6 to 10.5 at%. The lattice parameter of the Nbss in each alloy increased after heat treatment signifying the redistribution of solutes between the Nbss and the intermetallic phases. The oxidation resistance of the alloys at 800 °C and 1200 °C increased significantly by alloying with Ti and Sn. Pest oxidation behaviour was exhibited by the Nb–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–5Mo (as cast), Nb–24Ti–18Si–5Al–5Cr–2Mo (heat treated) and Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf (heat treated) alloys at 800 °C. Pesting was eliminated in the alloy Nb–24Ti–18Si–5Al–5Cr–2Mo–5Hf–5Sn at 800 °C, indicating that the addition of Sn plays an important role in controlling the pest oxidation behaviour at intermediate temperatures. The oxidation behaviour of all the alloys at 800 °C and 1200 °C was controlled by the oxidation of the Nbss and was sensitive to the area fraction of Nbss in the alloy.  相似文献   

13.
The nanocomposite of MoSi2-Mo5Si3 powder was synthesized by mechanical alloying from Mo and Si powder mixture at room temperature. The phase evaluation of powder after various milling durations and heat treatments were assessed via X-ray diffraction (XRD) and a differential thermal analysis (DTA). Morphology and microstructure of powder particles were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results revealed that nanocomposite of MoSi2-Mo5Si3 powder was synthesized by combustion reaction of Mo and Si powder using ball milling. In the early stages of ball milling β-MoSi2 was produced. However with continued milling for 48 h α-MoSi2 and Mo5Si3 phases were formed. DTA results of 24 h and 48 h as milled mechanical alloyed specimens showed a well-defined peak at 852 °C and 920 °C relating to the formation of α-MoSi2. The activation energy for 24 h and 48 h milled specimens were –128.6 KJ/mol and –121.4 KJ/mol respectively. Annealing the milled specimens at 1000 °C for 2 h revealed the phase transformation of β-MoSi2 to α-MoSi2 and the formation of Mo5Si3. The crystallite size of α-MoSi2 and Mo5Si3 were about 9 nm and 12 nm after 48 h mechanical alloying. These values increased slightly to 18 nm and 14 nm after annealing at 1000 °C.  相似文献   

14.
《Acta Materialia》2004,52(1):199-208
The microstructure of low pressure plasma spray deposited Al–12%Si has been investigated as a function of temperature during spray deposition, concentrating on deposit homogeneity, porosity, microstructure and microhardness. The deposit microstructure was determined by the temperature during spray deposition. Low temperatures promoted inhomogeneous banded microstructures, incorporating partially solidified and unmelted particles, with high levels of intersplat porosity, extended Si solubility in the α-Al matrix, amorphous and nano-sized Si precipitates and relatively high microhardness. High temperatures promoted homogeneous equiaxed microstructures, with low porosity levels, low Si solubility in the α-Al matrix, micro-sized Si precipitates, and relatively low microhardness. The measured critical transition temperature was in the range 190–345 °C, in reasonable agreement with Cantor et al.’s intermixing model of microstructure formation during spray deposition, which predicted a critical transition temperature of 328 °C. The corresponding critical intersplat time was estimated to be 1.4 × 10−4 s, indicating intermediate cooling conditions for splatting droplets arriving at the deposit surface.  相似文献   

15.
《Acta Materialia》2008,56(6):1182-1195
The transformation of Al3Zr (L12) and Al3(Zr1−xTix) (L12) precipitates to their respective equilibrium D023 structures is investigated in conventionally solidified Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys aged isothermally at 500 °C or aged isochronally in the range 300–600 °C. Titanium additions delay neither coarsening of the metastable L12 precipitates nor their transformation to the D023 structure. Both alloys overage at the same rate at or above 500 °C, during which spheroidal L12 precipitates transform to disk-shaped D023 precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the α-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D023 phase with α-Al. The transformation is very sluggish and even at 575 °C coherent L12 precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates.  相似文献   

16.
The application of ultrasonic vibration treatment (UVT) produced a nearly non-dendritic and refined semi-solid microstructure of Mg2Si/AM60 composite. The effects of UVT temperature and time on microstructure of the semi-solid slurry were studied. A good semi-solid slurry with average grain size of 75 μm and shape coefficient of 0.53 could be obtained by applying UVT at 620 °C for 60 s, which were decreased by a factor of 17/20 and increased by a factor of 3 respectively as compared to the sample without treatment. The Mg2Si and Mg17Al12 intermetallics are mainly located along the grain boundaries or dispersed uniformly in the metallic liquid matrix with network morphology. Mechanisms involved in the development of microstructure are discussed.  相似文献   

17.
Dense (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composites with adjustable content of (ZrB2 + SiC) reinforcements (0–30 vol.%) were prepared by in situ hot-pressing. The microstructure, room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composites were investigated and compared with monolithic Zr2[Al(Si)]4C5 ceramic. ZrB2 and SiC incorporated by in situ reaction significantly improve the mechanical properties of Z2[Al(Si)]4C5 by the synergistic action of many mechanisms including particulate reinforcement, crack deflection, branching, bridging, “self-reinforced” microstructure and grain-refinement. With (ZrB2 + SiC) content increasing, the flexural strength, toughness and Vickers hardness show a nearly linear increase from 353 to 621 MPa, 3.88 to 7.85 MPa·m1/2, and 11.7 to 16.7 GPa, respectively. Especially, the 30 vol.% (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composite retains a high modulus up to 1511 °C (357 GPa, 86% of that at 25 °C) and superior strength (404 MPa) at 1300 °C in air. The composite shows higher thermal conductivity (25–1200 °C) and excellent thermal shock resistance at ΔT up to 550 °C. Superior properties render the composites a promising prospect as ultra-high-temperature ceramics.  相似文献   

18.
《Intermetallics》2005,13(7):749-755
The superplastic properties of two intermetallic Ti–46.8Al–1.2(Mo,Si) and Ti–46Al–1.5(Mo,Si) (at.%) materials produced by arc melting and processed by hot extrusion in the temperature range between 1200 and 1250 °C were studied. The materials exhibited an equiaxic near γ microstructure with γ grains finer than 1 μm and some band like region of γ grains with a size ranging from 5 to 20 μm. The finer grained zone contained a volume fraction of about 12 vol% in the 46.8Al material and about 25 vol% in the 46Al material of finely dispersed α2-Ti3Al particles. Mechanical tests performed on both materials at strain rates ranging from 4.6×10−4 to 10−2 s−1 in the temperature range of 975–1050 °C showed strain rate sensitivity exponents of about 0.5 at most strain rates. A maximum elongation to failure of about 300% was obtained for the 46.8Al material while about 900% was recorded for the 46Al material at 1050 °C at a relatively high strain rate of 8×10−3 s−1. This difference is attributed to the larger volume fraction of α2-phase particles in the 46Al material that leads to a decrease of the number and size of band like regions of coarse γ grains. The microstructure in the fine-grained areas of both materials remains essentially constant during deformation. The mechanical behavior at high temperature of these superplastic materials can be explained by considering grain boundary sliding as the dominant deformation mechanism.  相似文献   

19.
The microstructures of Si–Mn–P alloy manufactured by the technique of combining phosphorus transportation and alloy melting were investigated using electron probe micro-analyzer (EPMA). The phase compositions were determined by energy spectrum and the varieties of eutectic morphologies were discussed. It is found that there is no ternary compound but Si, MnP and MnSi1.75?x could appear when the Si–Mn–P alloy's composition is proper. Microstructure is greatly refined by rapid solidification technique and the amount of eutectic phases change with faster cooling rates. Moreover, primary Si or MnP are surrounded firstly by the binary eutectic (Si + MnP) and then the ternary eutectic (Si + MnSi1.75?x + MnP) which also exhibit binary structures due to divorced eutectic determined by the particularity of some Si–Mn–P alloys.  相似文献   

20.
A. Gali  H. Bei  E.P. George 《Acta Materialia》2009,57(13):3823-3829
The thermal stability of Cr–Cr3Si lamellar eutectic composites was investigated at temperatures up to 1400 °C. In drop-cast Cr–Cr3Si, coarsening was found to be interface controlled. The coarsening rate could be reduced by microalloying with Ce and Re, two elements which were chosen because they were expected to segregate to the Cr–Cr3Si interfaces. Similarly, directional solidification, which is also expected to lower the Cr–Cr3Si interfacial energy, was found to dramatically decrease the coarsening rate.  相似文献   

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