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1.
The interfaces of aluminum alloy composites (5083) reinforced by SiC particles (as-received, oxidized 3.04 wt pct and 14.06
wt pct) were studied. The composites were fabricated by compocasting and certain samples were also remelted at 800 °C for
30 minutes. The reaction mechanisms between SiC
p
and liquid Al and between the SiO 2 layer and Al(Mg) are discussed. The crystal boundaries of the MgO (or MgAl 2O 4) reaction products are believed to be the diffusion paths (or channels) during the interfacial reactions. A SiO 2 layer, formed by oxidation of the SiC particles prior to their incorporation into the melt, plays an important role in preventing
the SiC
p
from being attacked by the matrix. The interfacial reaction products are affected by both the alloy composition and the thickness
of the initial SiO 2 layer. 相似文献
2.
The redox behavior of titanium in CaO-SiO 2-TiO
x
melts was investigated using a slag-gas equilibrium technique. Titanium partitioning between Ti 3+ and Ti 4+ valency states and the ratio of activity coefficients of TiO 1.5 and TiO 2 were determined as functions of oxygen partial pressure, temperature, and slag composition. The equilibrium experiments were
carried out at temperatures between 1783 and 1903 K under CO-CO 2-Ar gas atmosphere with oxygen partial pressure ranging from 10 −12 to 10 −7 atm (1.01×10 −10 kPa to 1.01×10 −5 kPa). The slags had CaO/SiO 2 ratios between 0.55 and 1.35 and total titanium oxide concentrations from 7 to 50 mass pct. Experimental results showed that
the Ti 3+/Ti 4+ ratio in CaO-SiO 2-TiO
x
slags, containing up to 50 mass pct TiO
x
, increased with decreasing oxygen partial pressure and decreased with increasing CaO/SiO 2 ratio and decreasing temperature. Measured variation of the redox ratio Ti 3+/Ti 4+ with oxygen partial pressure closely followed the ideal behavior. Increasing the CaO/SiO 2 ratio increased the ratio of activity coefficients of TiO 1.5 and TiO 2. The effect of total titania content on this ratio was more complex and in accord with Raman spectroscopy data. 相似文献
3.
This work is focused on the possibilities of preparing Ni-Ti46 wt pct alloy by powder metallurgy methods. The self-propagating high-temperature synthesis (SHS) and combination of SHS reaction, milling, and spark plasma sintering consolidation (SPS) are explored. The aim of this work is the development of preparation method with the lowest amount of undesirable phases (mainly Ti 2Ni phase). The SHS with high heating rate (approx. 200 and 300 K min ?1) was applied. Because the SHS product is very porous, it was milled in vibratory disk milling and consolidated by SPS technique at temperatures of 1173 K, 1273 K, and 1373 K (900 °C, 1000 °C, and 1100 °C). The microstructures of samples prepared by SHS reaction and combination of SHS reaction, milling, and SPS consolidation are compared. The changes in microstructure with increasing temperature of SPS consolidation are observed. Mechanical properties are tested by hardness measurement. The way to reduce the amount of Ti 2Ni phase in structure is leaching of powder in 35 pct hydrochloric acid before SPS consolidation. 相似文献
4.
The oxidation resistance of Ti-Al intermetallics is superior to many of their counterparts at high temperatures. High-temperature
stability of these intermetallics appears to improve with a ternary alloying addition such as Mo, Nb, etc. A detailed analysis of oxidation of the Ti 3Al intermetallics, Ti 3Al-2.9 at. pct Mo and Ti 3Al-1.1 at. pct Nb, in pure oxygen using crystallographic and microscopic techniques is presented here. The alloy containing
1.1 at. pct Nb did not show an improvement in oxidation resistance over the base alloy but that containing 2.9 at. pct Mo
showed marked resistance to oxidation. Activation energies of oxidation for both the alloys were comparable with those reported
in the literature for similar compositions. TiO 2 and Al 2O 3 were the major phases present in the oxide scales of the oxidized alloys. The crystal orientations from the X-ray diffraction
(XRD) patterns and the morphologies from scanning electron microscopy (SEM) were in good agreement and were helpful in further
analysis of the oxidation process. The effective diffusion of oxygen in the oxide layers was calculated using the mole fraction
of individual oxide and the diffusivity of oxygen in pure oxide. The activation energy for the effective diffusion of oxygen
through the oxide layers was determined to be ~24 kJ/mol. The activation energy for the oxidation process was higher than
that of the diffusion of oxygen. Hence, it can be concluded that the oxidation process in both the alloys studied is not diffusion
controlled. 相似文献
5.
Oxidation experiments have been conducted on Ti 3Al in the temperature range 1023 to 1273 K in a one-atmosphere pure oxygen environment. The oxidation products were analyzed
using the scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques
and found to be predominantly TiO 2 (rutile). The oxidation rate was observed to obey the parabolic rate law. Diffusivity data were obtained using the parabolic
rate constant for interdiffusion of Ti and O in the oxide layer. Parabolic rate constants were calculated from oxidation rate
data, and Valenci equations for flat sheets were used to calculate diffusion coefficients. The activation energy, Q, was found to be 295.43±5.90 kJ/mol, and the frequency factor, D
0, was calculated to be 0.68±0.01 m 2/s for oxygen in the TiO 2. The activation energy obtained in this study matches closely with that of oxygen diffusion in TiO 2 reported in the literature. 相似文献
6.
The selective oxidation of Al‐free and Al‐added TWIP steel after full austenitic annealing at 800°C in a N 2 + 10%H 2 gas atmosphere with a dew point of ?17°C was investigated by means of transmission electron microscopy. A thick MnO layer was formed at the surface of Al‐free TWIP steel after the recrystallization annealing. Small crystalline c‐ xMnO · SiO 2 ( x > 2) particles and amorphous a‐ xMnO · SiO 2 ( x < 0.9) particles were found at the MnO/steel interface. In the subsurface, the Mn depletion resulted in the formation of a narrow ferrite layer. The annealing of the Al‐added TWIP steel also resulted in the formation of a thick MnO surface layer. At the MnO/steel interface, Kirkendall voids were formed between the amorphous a‐ xMnO · SiO 2 ( x < 0.9) oxide and crystalline c‐ xMnO · Al 2O 3 oxide in the case of Al‐added TWIP steel. In the subsurface, a thin layer was depleted of Mn and the original austenite had transformed into ferrite. Internal oxidation of Al to Al 2O 3 and the formation of crystalline c‐ xMnO · Al 2O 3 ( x > 1) compound oxide particles were found to occur at the grain boundaries of the Mn‐depleted ferritic zone. The present contribution highlights the implications of the selective oxidation of TWIP steels for their processing in continuous annealing and continuous hot dip galvanizing lines. 相似文献
7.
Ti/Si/TiC powders with molar ratios of 1:1:2 (M1) and 2:2:3 (M2) were prepared for the synthesis of a ternary carbide Ti 3SiC 2 by using the mixture method for 24 hours in an Ar atmosphere. The synthesis process was conducted at 1200 °C to 1400 °C under
a pressure of 50 MPa, using the pulse-discharge sintering (PDS) technique. After sintering, the phase constituents and microstructures
of the samples were analyzed by X-ray diffraction (XRD) technique and observed by optical microscopy and scanning electron
microscopy. The results showed that the phases in all the samples consisted of Ti 3SiC 2 and small amounts of TiC, and the optimum sintering temperature was found to be in the relatively low range of 1250 °C to
1300 °C. By the standard additive method, the relative content of Ti 3SiC 2 was calculated. For the M1 samples, the lowest TiC content can be only decreased to about 3 to 4 wt pct, whereas the content
of Ti 3SiC 2 in the M2 samples is always lower than that in the M1 samples. When the M2 powder was sintered at 1300 °C for 8 to 240 minutes,
the TiC peaks were found to show a very low intensity, and the corresponding content of Ti 3SiC 2 was calculated to be higher than 99 wt pct. The grain size of Ti 3SiC 2 increased from 5 to 10 μm to 80 to 100 μm in the entire applied sintering temperature range. The relative density of the
M2 samples was measured to be higher than 99 pct at sintering temperatures above 1275 °C. It indicates that the PDS technique
can rapidly synthesize high-content Ti 3SiC 2 from the Ti/Si/TiC powders in a relatively low temperature range. 相似文献
8.
This work aims to describe the effect of magnesium on the microstructure, phase composition, amount of undesirable Ti2Ni phase, martensitic transformation, mechanical properties, and corrosion resistance of NiTi alloy. To minimize the quantity of Ti2Ni phase, we use the magnesium as an element with high affinity to oxygen, because this phase is stabilized by oxygen. Various quantities of magnesium (1, 3, and 5 wt pct) were tested. Self-propagating high-temperature synthesis (SHS) was used as a production method of the alloys. The samples prepared by SHS were pulverized by a vibrating mill, and the obtained powders were used for consolidation by means of spark plasma sintering. Results showed a significant reduction of the content of undesirable Ti2Ni phase by the addition of magnesium. Further, magnesium increased corrosion resistance and yield strength. 相似文献
9.
A simple process was developed to fabricate ceramic-reinforced copper and silver matrix composites by electroless plating
and hot pressing at 873 K and 300 MPa, in air. Composites were produced containing 10 to 40 vol pct ceramic reinforcements
of different sizes and shapes including silicon carbide whiskers (SiC
w
), alumina particles (Al 2O 3p
), carbon short fibers (Carbon
sf
), and Saffil short fibers (Saffil
sf
) (3.8 pct SiO 2-96.2 pct Al 2O 3) uniformly distributed within the matrix. The hardness and bending strength of the composites were much higher than those
of the pure matrices. The electrical conductivity, measured by a four-point probe method, was similar to that of traditional
CdO/Ag electrical contact materials. The surface morphologies and cross-sectional microstructures of the arc-eroded Al 2O 3p
/Ag composites were similar to those of conventional CdO/Ag and SnO 2/Ag and exhibited a good arc-erosion resistance. These composites combine the high strength and elevated-temperature stability
of the ceramic reinforcements with the good electrical and thermal conductivity of the two metallic matrices. 相似文献
10.
The rate of nitrogen dissolution in CaO-Al 2O 3-SiO 2 and CaO-Al 2O 3-TiO
x
melts was measured by 14N– 15N isotope exchange reaction. The rate constant for the CaO-Al 2O 3-SiO 2 melts at the ratio of mass pct CaO/mass pct Al 2O 3 = 1 increases as SiO 2 content increases, whereas the rate constant for the same melts at the ratio of mass pct CaO/mass pct SiO 2 = 1 increases as Al 2O 3 content increases. The rate constant for the CaO-Al 2O 3-TiO
x
melts at the ratio of mass pct CaO/mass pct Al 2O 3 = 1 decreases as the TiO
x
content increases. The activation energies of nitrogen dissolution in CaO-Al 2O 3-SiO 2 melts are about 1.5 to 3 times larger than that of molten pure iron. Moreover, the rate constant of nitrogen dissolution
is independent of the ratio of Ti 3+/Ti 4+. 相似文献
11.
Isothermal oxidation behavior of Ti-48.6 at. pct Al alloy was studied in pure dry oxygen over the temperature range 850 °C
to 1000 °C. The oxidation was essentially parabolic at all temperatures with significant increase in the rate at 1000 °C.
Effective activation energy of 404 kJ/mol was deduced. The oxidation products were a mixture of TiO 2 (rutile) and α-Al 2O 3 at all temperatures. An external protective layer of alumina was not observed on this alloy at any of the temperatures studied.
A layered structure of oxides was formed on the alloy at 1000 °C. 相似文献
12.
Plasma processing offers improved thermodynamics and kinetics over conventional, thermal processing. In the current work,
the reduction of TiO 2 was investigated in a moderate-pressure ( p<46 torr) nonequilibrium hydrogen plasma at temperatures below 1273 K; the effect of plasma power, plasma pressure, time,
and applied voltage on the extent of the reduction was examined. Reduction of powdered TiO 2 at the surface of a packed bed has produced up to 60 pct conversion of TiO 2 to Ti 2O 3 in only 5 minutes of plasma-specimen contact. While the plasma-assisted reduction occurs at the surface, the reduction of
TiO 2 to Ti 50O 99 within the interior of the bed by diatomic hydrogen establishes a value of
that leads to the reoxidation of the Ti 2O 3. The continued reduction of the surface material by monatomic hydrogen from the plasma balances this oxidation process, and
a steady-state condition is established. When the interior of the bed is completely reduced to Ti 50O 99, the partial pressure of water vapor declines, and further reduction of Ti 2O 3 at the surface can proceed. It is hypothesized that the reduction process involves the formation of a Magneli-like oxide
between Ti 2O 3 and TiO. 相似文献
13.
Equilibrium studies between CaO-SiO 2-10 pct MgO-Al 2O 3-TiO 1.5-TiO 2 slags, carbon-saturated iron, and a carbon monoxide atmosphere were performed at 1773 K to determine the activities of TiO 1.5 and TiO 2 in the slag. These thermodynamic parameters are required to predict the formation of titanium carbonitride in the blast furnace.
In order to calculate the activity of titanium oxide, the activity coefficient of titanium in carbon-saturated iron-carbon-titanium
alloys was determined by measuring the solubility of titanium in carbon-saturated iron in equilibrium with titanium carbide.
The solubility and the activity coefficient of titanium obtained were 1.3 pct and 0.023 relative to 1 wt pct titanium in liquid
iron or 0.0013 relative to pure solid titanium at 1773 K, respectively. Over the concentration range studied, the effect of
the TiO
x
content on its activity coefficient is small. In the slag system studied containing 35 to 50 pct CaO, 25 to 45 pct SiO 2, 7 to 22 pct Al 2O 3, and 10 pct MgO, the activity coefficients of TiO 1.5 and TiO 2 relative to pure solid standard states range from 2.3 to 8.8 and from 0.1 to 0.3, respectively. Using thermodynamic data
obtained, the prediction of the formation of titanium carbonitride was made. Assuming hypothetical ‘TiO 2,’ i.e., total titanium in the slag expressed as TiO 2, and using the values of the activity coefficients of TiO 1.5 and TiO 2 determined, the equilibrium distribution of titanium between blast furnace-type slags and carbon-saturated iron was computed.
The value of [pct Ti]/(pct ‘TiO 2’) ranges from 0.1 to 0.2. 相似文献
14.
The high temperature oxidation of Al-Mg alloys is characterized by the rapid formation of thick, micro-crystalline oxide films. The oxidation kinetics of an Al-4.2 wt pct Mg alloy under dry and moist 20 pct O 2/Ar have been measured, and oxide films grown on bulk specimens complementary to the weight gain curves have been characterized using electron optical techniques (TEM, SEM). Initial oxidation takes place by the nucleation and growth of primary crystalline oxides at the oxide/metal interface and by the formation of secondary oxides of MgO by the reduction of the original amorphous over-layer of γ-Al 2O 3 by Mg. Subsequent oxidation is dominated by the further nucleation and growth of primary oxides. The presence of water vapor in the oxidizing environment initially reduces oxidation rates through a modification of the mechanical properties of the amorphous overlayer but does not affect the overall oxidation mechanism. A microstructural model has been developed which describes oxidation of Al-Mg alloys in terms of fracture of the original air-formed film by primary MgO nucleation and growth and modification to this film by the presence of water vapor in the oxidizing environment. 相似文献
15.
NiTi/Ti 3SiC 2 interpenetrating composites that combine two unique material systems—a shape memory alloy (SMA) and a MAX phase—demonstrating two different pseudoelastic mechanisms, were processed using spark plasma sintering. The goal of mixing these two material systems was to enhance the damping behavior and thermo-mechanical response of the composite by combining two pseudoelastic mechanisms, i.e., reversible stress-induced martensitic transformation in SMA and reversible incipient kink band formation in MAX phase. Equal volume fractions of equiatomic NiTi and Ti 3SiC 2 were used. Microstructural characterization was conducted using scanning electron microscopy to study the distribution of NiTi, Ti 3SiC 2, and remnant porosity in the composite. Thermo-mechanical testing in the form of thermal cycles under constant stress levels was performed in order to characterize shape memory behavior and thereby introducing residual stresses in the composites. Evolution of two-way shape memory effect was studied and related to the presence of residual stresses in the composites. Damping behavior, implying the energy dissipation per loading–unloading cycle under increasing compressive stresses, of pure NiTi, pure Ti 3SiC 2, as-sintered, and thermo-mechanically cycled (TC) NiTi/Ti 3SiC 2 composites, was investigated and compared to the literature data. In this study, the highest energy dissipation was observed for the TC composite followed by the as-sintered (AS) composite, pure NiTi, and pure Ti 3SiC 2 when compared at the same applied stress levels. Both the AS and TC composites showed higher damping up to 200 MPa stress than any of the metal—MAX phase composites reported in the literature to date. The ability to enhance the performance of the composite by controlling the thermo-mechanical loading paths was further discussed. 相似文献
16.
The crystalline structures in the Ti-Al-C and Ti-Si-C systems are analyzed, and experiments are conducted with VT6 titanium alloy and eutectoidal Silumin (Al-12% Si) subjected to electroexplosive alloying and electron-beam treatment. Diffraction analysis reveals the formation of MAX phases (Ti 3SiC 2 and Ti 3AlC) in the modified layer of these alloys. 相似文献
17.
Anin situ morphological study of the oxidation of electron transparent specimens of aluminum and aluminum alloys containing zinc and magnesium has been carried out in the temperature range 400 to 520°C using the hot stage of a 1 MeV transmission electron microscope. The structure and morphology of the crystalline oxide produced in each alloy has been carefully examined by selected area electron diffraction and stereomicroscopy. In pure aluminum, oxidation takes place after a temperature dependent induction period, by the nucleation of crystalline γ-Al2O3 at the amorphous oxide/metal interface. This process is delayed by additions of zinc which modify the structure of the oxide. In alloys containing magnesium, oxidation takes place by the rapid nucleation and growth of MgAl2O4 or MgO, with a secondary form of magnesia developing from the reduction of the amorphous γ-Al2O3 surface layer. 相似文献
18.
The Y modification of a two-phase ( γ+ α
2) TiAl-(Mn,Mo,C) alloy was studied with an aim to improve, mainly, the oxidation resistance and the mechanical properties
in a high-temperature air environment. The experimental alloy was prepared by the elemental powder metallurgy (EPM) method.
The addition of up to 0.6 at. pct Y resulted in a significant improvement in tensile properties and compressive yield strength
and an anomalous yielding phenomenon withal. Two structural characteristics were identified: first, microstructural refinement
in terms of the grain size as well as the interlamellar spacing, and second, precipitation of fine oxides that might scavenge
harmful oxygen. Deformation was found to be mainly provided by 1/2<110] ordinary dislocations and a much lesser amount of
<011] superdislocations as compared to what has been reported in other ( γ+ α
2) TiAl alloys. The oxidation resistance of the experimental alloy was evaluated by air-exposure tests at 800 °C, from which
the oxidation kinetics and the morphological and phase characteristics of the oxide scales were analyzed. With a Y addition,
the constituents of the oxide scale changed from those of the Y-free alloy. In the case of the Y-free alloy, the oxide scale
which formed upon extended air exposure (350 hours) at 800 °C consisted of a mixture of TiO 2 and α-Al 2O 3. In the case of the alloy modified with Y (0.6 at. pct), however, the oxide scale formed in an identical environment was
considerably different: it consisted of a complex mixture of TiO 2, α-Al 2O 3, Y 2O 3, and Al 5Y 3O 12. The formation of the multiphase (Y,Al)O-rich oxide scale impedes the oxygen transport and the thermal-expansion stress in
the Al 2O 3 layer. It is also suggested that a Y addition reduces the oxygen solubility and concentration of oxygen vacancices in the
TiO 2 layer.
This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented
at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint
Committee on Mechanical Behavior of Materials. 相似文献
19.
The high temperature oxidation of Al-Mg alloys is characterized by the rapid formation of thick, micro-crystalline oxide films.
The oxidation kinetics of an Al-4.2 wt pct Mg alloy under dry and moist 20 pct O 2/Ar have been measured, and oxide films grown on bulk specimens complementary to the weight gain curves have been characterized
using electron optical techniques (TEM, SEM). Initial oxidation takes place by the nucleation and growth of primary crystalline
oxides at the oxide/metal interface and by the formation of secondary oxides of MgO by the reduction of the original amorphous
over-layer of γ-Al 2O 3 by Mg. Subsequent oxidation is dominated by the further nucleation and growth of primary oxides. The presence of water vapor
in the oxidizing environment initially reduces oxidation rates through a modification of the mechanical properties of the
amorphous overlayer but does not affect the overall oxidation mechanism. A microstructural model has been developed which
describes oxidation of Al-Mg alloys in terms of fracture of the original air-formed film by primary MgO nucleation and growth
and modification to this film by the presence of water vapor in the oxidizing environment.
Formerly at Imperial College, London. 相似文献
20.
Copper mold cast cylinders of (Ti 0.5Cu 0.25Ni 0.15Sn 0.05Zr 0.05) 100−x
Mo
x
composites are prepared. Addition of Mo in the bulk glass-forming alloy induces the formation of a dendrite/matrix composite.
For 3-mm-diameter cylinders, the matrix exhibits a homogenous ultrafine microstructure for Mo content of 2.5 at. pct, and
a fine eutectic microstructure for 5 at. pct Mo. For 5-mm-diameter cylinders, the matrix exhibits a dendritic microstructure
for 2.5 at. pct Mo, and exhibits a coarser eutectic microstructure for 5 at. pct Mo. Despite the formation of a dendrite/nanostructured
matrix composite in the cylinders, the quenched surface layer with a nanoscale grain size dominates the deformation and fracture
of the 3-mm-diameter cylinders. More than 56 vol pct quenched layer leads to a distensile fracture mode and the samples exhibit
high fracture strength and high Young’s modulus but low ductility. For 5-mm-diameter cylinders, the composite microstructure
becomes dominant due to its more than 64 vol pct volume fraction leading to a cone-shaped fracture surface. The samples exhibit
lower yield strength and lower Young’s modulus but better ductility compared to the 3-mm-diameter cylinders. The mechanical
behavior of the Mo-bearing composites strongly depends on the microstructural homogeneity and casting defects formed upon
solidification. 相似文献
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