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1.
Cast Al-Ti-C grain refiners were synthesized by reacting up to 2 pct graphite particles of 20 micron average size with stirred
Al-(5 to 10) pct Ti alloy melts, which generated submicron-sized TiC particles within the melts, and their solidified structures
showed preferential segregation of the carbide phase in the grain or cell boundary regions and occasional presence of free
carbon whose amount exceeded equilibrium values. At the usual melt temperatures of below 1273 K, though, TiC formed first,
but was subsequently found to react with the melt forming a sheathing of A1 4C 3 and Ti 3AlC which resulted into poisoning of the TiC particles. However, it was possible to reverse these reactions in order to regain
the virgin TiC particles by superheating the melts in the temperature region where TiC particles are thermodynamically stable.
Grain refining tests using the TiC master alloys produced fine equiaxed grains of cast aluminum whose sizes were comparable
to that obtainable with the standard TiB 2 commercial grain refiner. TiC particles introduced via the master alloys were found to occur in the grain centers, thereby confirming that they nucleated aluminum crystals.
On leave from Regional Research Laboratory (CSIR), Bhopal, is Research Associate. 相似文献
2.
This study examined the microstructural evolution and castability of Al–Mg–Si ternary alloys with varying Si contents. Al–6Mg–xSi alloys (where x = 0, 1, 3, 5, and 7; all compositions in mass pct) were examined, with Al–6 mass pct Mg as a base alloy. The results showed that in the ternary alloys with Si ≤ 3 pct, the solidification process ended with the formation of eutectic α-Al–Mg2Si phases generated by a univariant reaction. However, in the case of ternary alloys with Si > 3 pct, solidification was completed with the formation of α-Al–Mg2Si–Si ternary eutectic phases generated by a three-phase invariant reaction. In addition to the eutectic Mg2Si phases, the primary Mg2Si phases formed in each of the ternary alloys, and the size of both sets of phases increased with increasing Si content. The two-phase eutectic α-Al–Mg2Si nucleated from the primary Mg2Si phases. The inoculated Al–6Mg–1Si alloy had the smallest grain size. Moreover, the grain-refining efficacy of the Al–5Ti–B master alloy in the ternary alloys decreased with increasing Si content in the alloys. Despite the poisoning effect of Si on the potency of TiB2 compounds in the inoculated Al–6Mg–1Si alloy, the grain size of the alloy was slightly smaller than that of the Al–6Mg binary alloy. This resulted from the increasing growth restriction factor (induced by Si addition) of the Al–6Mg–1Si alloy. In terms of the castability, the examined alloys showed different levels of susceptibility to hot tearing. Among the alloys, the ternary Al–6Mg–5Si alloy exhibited the highest susceptibility to hot tearing, whereas the Al–6Mg–7Si exhibited the lowest. The severity of hot tearing initiated by the unraveling of the bifilm was determined by the freezing range, grain size, and the amount of eutectic phases at the end of the solidification process. 相似文献
3.
The “co-poisoning” effect between Zr and Ti (derived from Al-Zr and Al-Ti-B master alloy additions) on the grain refinement
of cast aluminum alloys is studied from a crystallographic atom matching viewpoint. The edge-to-edge matching (E2EM) model
has been used to investigate the possible “poisoning” phase containing Zr/Ti, Al, and Fe in commercial grade aluminum alloys.
The results show that Al 3Ti is the most likely constituent to be poisoned due to the formation of an Al 8Fe 4Zr coating on its surface, since the Al 8Fe 4Zr phase has good crystallographic atom matching with Al 3Ti, but not with the aluminum matrix. Meanwhile, the partial dissolution of Al 3Zr nucleant particles to compensate for the loss of solute Zr aggravates the poisoning phenomenon. This proposed mechanism
is consistent with most previous experimental observations and with existing practical solutions employed in the foundry. 相似文献
4.
Intergranular sustained-load cracking of Al-Zn-Mg-Cu (AA7xxx series) aluminum alloys exposed to moist air or distilled water
at temperatures in the range 283 K to 353 K (10 °C to 80 °C) has been reviewed in detail, paying particular attention to local
processes occurring in the crack-tip region during crack propagation. Distinct crack-arrest markings formed on intergranular
fracture faces generated under fixed-displacement loading conditions are not generated under monotonic rising-load conditions,
but can form under cyclic-loading conditions if loading frequencies are sufficiently low. The observed crack-arrest markings
are insensitive to applied stress intensity factor, alloy copper content and temper, but are temperature sensitive, increasing
from ~150 nm at room temperature to ~400 nm at 313 K (40 °C). A re-evaluation of published data reveals the apparent activation
energy, E
a
for crack propagation in Al-Zn-Mg(-Cu) alloys is consistently ~35 kJ/mol for temperatures above ~313 K (40 °C), independent
of copper content or the applied stress intensity factor, unless the alloy contains a significant volume fraction of S-phase, Al 2CuMg where E
a
is ~80 kJ/mol. For temperatures below ~313 K (40 °C) E
a
is independent of copper content for stress intensity factors below ~14 MNm −3/2, with a value ~80 kJ/mol but is sensitive to copper content for stress intensity factors above ~14 MNm −3/2, with E
a
, ranging from ~35 kJ/mol for copper-free alloys to ~80 kJ/mol for alloys containing 1.5 pct Cu. The apparent activation energy
for intergranular sustained-load crack initiation is consistently ~110 kJ/mol for both notched and un-notched samples. Mechanistic
implications are discussed and processes controlling crack growth, as a function of temperature, alloy copper content, and
loading conditions are proposed that are consistent with the calculated apparent activation energies and known characteristics
of intergranular sustained-load cracking. It is suggested, depending on the circumstances, that intergranular crack propagation
in humid air and distilled water can be enhanced by the generation of aluminum hydride, AlH 3, ahead of a propagating crack and/or its decomposition after formation within the confines of the nanoscale volumes available
after increments of crack growth, defined by the crack arrest markings on intergranular fracture surfaces. 相似文献
5.
An Al-1.1 Sc-1.1 Zr (wt %) master alloy with a uniform distribution of micron and submicron particles of aluminide phase Al 3(Sc 1 − x
Zr
x
) has been obtained by exposing of equal amounts of commercial Al-Sc and Al-Zr master alloys to short-time actions of low-frequency
vibrations transferred to the alloy via an irradiating plunger. Zirconium substitutes up to 50% Sc in aluminides and retains
its L1 2 lattice. The modifying capacity of the experimental master alloy is tested on cast alloy (wt %) Al-8Zn-2.4Cu-3Mg. Intense
grain refinement of this alloy is achieved by its modification with a certain amount of the master alloy. At a certain Sc
+ Zr content, a grain dendrite structure completely disappears in the alloy. 相似文献
8.
Although the grain-refinement practice is well established for wrought Al alloys, in the case of foundry alloys such as near
eutectic Al-Si alloys, the underlying mechanisms and the use of grain refiners need better understanding. Conventional grain
refiners such as Al-5Ti-1B are not effective in grain refining the Al-Si alloys due to the poisoning effect of Si. In this
work, we report the results of a newly developed grain refiner, which can effectively grain refine as well as modify eutectic
and primary Si in near eutectic Al-Si alloys. Among the material choices, the grain refining response with Al-1Ti-3B master
alloy is found to be superior compared to the conventional Al-5Ti-1B master alloy. It was also found that magnesium additions
of 0.2 wt pct along with the Al-1Ti-3B master alloy further enhance the near eutectic Al-Si alloy’s grain refining efficiency,
thus leading to improved bulk mechanical properties. We have found that magnesium essentially scavenges the oxygen present
on the surface of nucleant particles, improves wettability, and reduces the agglomeration tendency of boride particles, thereby
enhancing grain refining efficiency. It allows the nucleant particles to act as potent and active nucleation sites even at
levels as low as 0.2 pct in the Al-1Ti-3B master alloy. 相似文献
9.
It has been known experimentally that TiAl 3 acts as a powerful nucleant for the solidification of aluminum from the melt; however, a full microscopic understanding is still lacking. To improve microscopic understanding, hot rolling technique has been performed to the Al–5Ti–1B alloy and the effect of shape and size of the particles on grain refinement has been studied. The effect of hot rolling of Al–5Ti–1B master alloy on its grain refining performance and hot tearing have been studied by OM, XRD, and SEM. Hot rolling improves the grain refining performance of this master alloy, which is required to reduce hot tearing in Al–7Si–3Cu alloy. The improvement in grain refining performance of Al–5Ti–1B master alloy on rolling is due to the fracture of larger TiAl 3 particles into fine particles during rolling. The presented results illustrate that the morphology of TiAl 3 particles alter from the plate-like structure in the as-cast condition Al–5Ti–1B master alloy to the blocky type after rolling due to the fragmentation of plate-like structures. The grain refining response and effect on hot tearing of Al–7Si–3Cu alloy have been studied with as-cast and rolled Al–5Ti–1B master alloys. The results display hot-rolled master alloys revealing enhanced grain refining performance and minimizing hot tear tendency of the alloy at much lower addition level as compared to as-cast master alloys. 相似文献
10.
Dendrite coherency is important to the formation of the solidification structure and castability of alloys. The effects of
grain refinement on the dendrite coherency in A319 aluminum alloy have been studied using the two-thermocouple thermal analysis
technique in the solidifying sample. The fraction solid at the dendrite coherency point (
fs\textDCP f_{s}^{\text{DCP}} ) in the A319 alloy increases with increasing Al-5Ti-1B grain refiner, and varies from 16 to 21 pct when the amount of Al-5Ti-1B
in the alloy is in the range of 0 to 4.6 wt pct. The results also indicate that the grain refinement increases the temperature
interval of coherency ( T
N
– T
DCP) and coherency time ( t
DCP), and it can postpone dendrite coherency. These changes were interpreted based on the dendrite growth rate, the growth restriction
factor, and the microstructure. 相似文献
11.
The present work aims to report and discuss the development of a novel grain refiner (Al–Y–B master alloys) focusing on the characterization of the phenomena that exist during their production. Al–Y–B master alloy is produced by the combined employment of yttrium and boron, instead of yttrium or boron individually. It is discovered as a highly effective grain refiner for inoculating the grain size of Al–Si alloys. The crystallized microstructure can be refined though the effect of Y-based intermetallic on heterogeneity nucleus. The Y-based intermetallic is formed in the melts (Al–Y–B master alloy) by the addition of yttrium and KBF 4 powers. A approach to produce Al–Y–B master alloys as well as its characterization by means of optical micrographs and SEM is presented. The study is assessed by testing the grain refining potency of the produced Al–Y–B master alloys in binary Al–20Si alloy. It is revealed that the approach employed to produce the Al–Y–B master alloys is suitable because the size of the primary phases is significantly reduced in each of the case investigated. 相似文献
12.
Quick preheating treatment of the Al-Ti-C pellets and high-intensity ultrasonic vibration are introduced in the fabrication
of in-situ TiC
p
/Mg composites. Al-Ti-C pellets are preheated for about 130 seconds in the furnace at 1023 K (750 °C), in which magnesium
is melted as well. In this process, plenty of heat can be accumulated due to the reactive diffusion between liquid aluminum
and solid titanium in Al-Ti-C, and a small amount of Al 3Ti phase is formed as well. After adding the preheated Al-Ti-C into the molten magnesium, thermal explosion takes place in
a few seconds. In the meantime, high-intensity ultrasonic vibration is applied into the melt to disperse in-situ formed TiC particles into the matrix and degas the melt as well. Microstructural characterization indicates that in-situ formed TiC particles are spherical in morphology and smaller than 2 μm in size. Furthermore, a homogeneous microstructure with low porosity of the magnesium composite is obtained due to the effect
of ultrasonic vibration. A novel approach using the quick preheating treatment technique and high-intensity ultrasonic vibration
to synthesize in-situ TiC
p
/Mg composites is proposed in our research. 相似文献
13.
X2020 aluminum alloys were produced with variations in the Li/Cu ratio by the ultrasonic gas atomization process. In alloy
68 (Al-4.9Cu-l.2Li) and 69 (Al-4.4Cu-l.55Li) alloys, the Θ′ and T 1, phases are dominant with evidence of the T B phase. In the 70 (Al-3.5Cu-2.8Li) alloy, the δ′ phase is dominant with a trace of T 1. It was found that Θ′ and T
1 are effective strengtheners whereas δ′ provides excellent fatigue crack initiation resistance. Overall results indicate that
the fracture behavior of three RS-PM X2020 alloys is closely related to alloy production route as well as to the phases present
in the alloys.
Formerly Research Assistant, Massachusetts Institute of Technology. 相似文献
14.
A356 with scandium (Sc) addition provides interesting results beyond costs. For the practical use of Sc, the effects of Sc on castability must be considered. Fluidity and hot cracking are important factors defining the castability of aluminum casting alloys. In the present work, the influence of Sc addition on the castability of A356 hypoeutectic Al–Si alloy was investigated, which was evaluated through fluidity and hot cracking susceptibility. The fluidity of the alloys was studied by measuring the total volume of solidified aluminum in a multi-channel mold. The hot cracking susceptibility of the alloys was evaluated by using a constrained-rod casting mold test. The results of the fluidity and hot cracking susceptibility test were supported by microstructural analysis. The results indicate that 0.2 wt% Sc addition significantly increases the fluidity of A356 alloy, due to the grain refinement and eutectic Si modification by changing the solidification mode. However, the fluidity slightly decreases when the Sc content increases to 0.4 wt% due to the formation of primary Al 2Si 2Sc intermetallic phase. The hot cracking of A356 alloy was completely diminished when Sc was added to the alloy. 相似文献
15.
Nanocrystalline equiatomic high-entropy alloys (HEAs) have been synthesized by mechanical alloying in the Cu-Ni-Co-Zn-Al-Ti
system from the binary CuNi alloy to the hexanary CuNiCoZnAlTi alloy. An attempt also has been made to find the influence
of nonequiatomic compositions on the HEA formation by varying the Cu content up to 50 at. pct (Cu
x
NiCoZnAlTi; x = 0, 8.33, 33.33, 49.98 at. pct). The phase formation and stability of mechanically alloyed powder at an elevated temperature
(1073 K [800 °C] for 1 hour) were studied. The nanocrystalline equiatomic Cu-Ni-Co-Zn-Al-Ti alloys have a face-centered cubic
(fcc) structure up to quinary compositions and have a body-centered cubic (bcc) structure in a hexanary alloy. In nonequiatomic
alloys, bcc is the dominating phase in the alloys containing 0 and 8.33 at. pct of Cu, and the fcc phase was observed in alloys
with 33.33 and 49.98 at. pct of Cu. The Vicker’s bulk hardness and compressive strength of the equiatomic nanocrystalline
hexanary CuNiCoZnAlTi HEA after hot isostatic pressing is 8.79 GPa, and the compressive strength is 2.76 GPa. The hardness
of these HEAs is higher than most commercial hard facing alloys ( e.g., Stellite, which is 4.94 GPa). 相似文献
16.
The electrochemical codeposition of Mg-Li-Zn-Mn alloys on a molybdenum electrode in LiCl-KCl-MgCl 2-ZnCl 2-MnCl 2 melts at 943 K (670 °C) was investigated. Preparation of the alloys by electrolysis was proven feasible in LiCl-KCl-MgCl 2-ZnCl 2-MnCl 2 melts from cyclic voltammograms and chronopotentiometry measurements. X–ray diffraction (XRD) indicated that Mg-Li-Zn-Mn
alloys with different phases were prepared via galvanostatic electrolysis. The microstructure of typical α + Mg 7Zn 3 phase of Mg-Li-Zn-Mn alloys was characterized by an optical microscope and scanning electronic microscopy). The analysis
by energy dispersive spectrometry showed that the addition of ZnCl 2 leads to the formation of intermetallic Mg 7Zn 3 distributed in grain boundaries, whereas Mn mainly existed on polygon particles. The results of inductively coupled plasma
analysis showed that the chemical compositions of alloys were consistent with the phase structures of XRD patterns. 相似文献
17.
Two Al-Mg-Ge alloys with compositions Al-0.87Mg-0.43Ge (at. pct) and Al-0.59Mg-0.71Ge (at. pct) were investigated and compared
using high-resolution transmission electron microscopy, annular dark-field scanning transmission electron microscopy, and
nano-beam electron diffraction. The alloys contained fine needle- and lath-shaped precipitates after aging at 473 K (200 °C)
for 16 hours, which produced hardnesses similar to those measured in comparable Al-Mg-Si alloys. The β″ phase was not observed. Instead, hardness was achieved by β′-like and disordered precipitates in the Mg-rich alloy, and U1-like and disordered precipitates in the Ge-rich alloy. In
all cases, the fine precipitates had structures containing an ordered near-hexagonal network of Ge atoms with a = b ≈ 0.4 nm, which could be visualized directly in annular dark-field mode. The network is very similar to the recently discovered
Si network that relates all precipitate structures in the Al-Mg-Si alloys. The orientation of the precipitate unit cells and
the Ge network relative to the Al matrix differed from what has been observed for β′ and U1 in the Al-Mg-Si system. 相似文献
18.
Hot-rolled, binary Mg-Nd alloys with compositions ≥0.095 at. pct undergo the texture weakening phenomenon that has been reported
in a number of Mg–rare earth (RE) alloys. However, alloys with compositions ≤0.01 at. pct retain a strong basal texture typical
of pure Mg and other Mg alloys. Measurements of intragranular misorientation axes obtained using electron backscatter diffraction
(EBSD) show that more dilute alloys contain predominantly basal $ < a > $ < a > dislocations, while richer alloys contain primarily prismatic $ < a > $ < a > dislocations. It is suggested that this change in dislocation content is related to a change in the dynamic recrystallization
(DRX) mechanism. Metastable second-phase Mg
x
Nd 1–x
intermetallic particles are present within the alloys, and an annealing study indicates that the alloys undergoing texture
weakening have grain sizes well predicted by classical Zener drag theory. Even though the more dilute alloys also contain
second-phase particles, they are not sufficient to induce pinning. The promotion of nonbasal slip and the reduction in grain
boundary mobility due to Zener drag are suggested as controlling mechanisms that promote the observed texture weakening phenomena. 相似文献
19.
This article presents a novel study on electrochemical codeposition of Mg-Li-Ce-La alloys on a molybdenum in LiCl-KCl-MgCl 2-KF melts containing RE 2(CO 3) 3 (rich in cerium) at a temperature range of 953 K to 1073 K (680 °C to 800 °C). The cyclic voltammetry proved the feasibility
of the production of the alloys. The factors that affect the current efficiency were investigated. The optimal electrolytic
temperature and cathodic current density was 1023 K (750 °C) and 15.9 A·cm −2, respectively. The chemical content, phases, morphology, and distribution of alloy elements were analyzed by inductively
coupled plasma mass spectrometry, X-ray diffraction, and scanning electron microscopy, respectively. The intermetallic compounds
between Mg and Ce (La) distribute in the grain boundary of the alloys, present as reticulate structures, and refine the grains. 相似文献
20.
The microstructures of as-cast and heat-treated biomedical Co-Cr-Mo (ASTM F75) alloys with four different carbon contents
were investigated. The as-cast alloys were solution treated at 1473 to 1548 K for 0 to 43.2 ks. The precipitates in the matrix
were electrolytically extracted from the as-cast and heat-treated alloys. An M 23C 6 type carbide and an intermetallic σ phase (Co(Cr,Mo)) were detected as precipitates in the as-cast Co-28Cr-6Mo-0.12C alloy;
an M 23C 6 type carbide, a σ phase, an η phase (M 6C-M 12C type carbide), and a π phase (M 2T 3X type carbide with a β-manganese structure) were detected in the as-cast Co-28Cr-6Mo-0.15C alloy; and an M 23C 6 type carbide and an η phase were detected in the as-cast Co-28Cr-6Mo-0.25C and Co-28Cr-6Mo-0.35C alloys. After solution treatment, complete precipitate
dissolution occurred in all four alloys. Under incomplete precipitate dissolution conditions, the phase and shape of precipitates
depended on the heat-treatment conditions and the carbon content in the alloys. The π phase was detected in the alloys with carbon contents of 0.15, 0.25, and 0.35 mass pct after heat treatment at high temperature
such as 1548 K for a short holding time of less than 1.8 ks. The presence of the π phase in the Co-Cr-Mo alloys has been revealed in this study for the first time. 相似文献
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