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1.
In order to elucidate the nature of the heterogeneous nucleation, a differential scanning calorimetry (DSC) thermal analysis
of pure Fe and Fe-Ni alloys (Ni content: 1.0 to 29.3 mass pct) containing TiN, Al 2O 3, and Ti 2O 3 was conducted. Then, special attention was paid to the difference in the phase of the primary crystal nucleated by the triggering
effect of a catalyst (nucleating agent). The solidification and transformation mode appearing during cooling in these alloys
is classified into three cases: F mode, FA mode, and A mode. The change of modes and the critical undercooling (Δ T) depend on the kind of catalyst used as well as the chemical composition (Ni content). In addition, in spite of the kind
of primary crystal, the value of Δ T is always small in the order of TiN, Al 2O 3, and Ti 2O 3. As a matter of fact, only TiN has a practical effect as a catalyst on the triggered nucleation of the primary crystal of
the δ phase. None of them has a practical effect on the nucleation of the primary crystal of the γ phase.
This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties
& Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002. 相似文献
2.
The Pt-Al system has high potential to act as alloy base for so-called refractory superalloys. Although the envisaged strengthening
phase Pt 3Al(r) has favorable L1 2 crystal structure only at high temperatures, even small amounts of Sc stabilized L1 2 crystal structure at low temperature. Pt-Al-Sc alloys were arc melted, heat treated, and examined by means of scanning electron
microscopy and X-ray diffraction (XRD). Pt 3Al 1−x
Sc
x
(r) forms a continuous phase field from the Al-rich side to the Sc-rich side of the Pt-Al-Sc ternary system. The absolute
value of the lattice misfit between cubic Pt 3Al 1−x
Sc
x
(r) and the matrix decreases with increasing Sc content. 相似文献
3.
A quasi-equilibrium temperature vs carbon-concentration phase diagram of rapidly solidified pseudo-binary (Fe 0.65Mn 0.35) 0.83Al 0.17-xC alloys was determined after heat treatment in the 823 to 1323 K range. Lattice parameter relationships of rapidly solidified
(Fe 0.65Mn 0.35) 0.83Al 0.17-xC and Fe 3Al-xC alloys in the ferrite, austenite, and perovskite carbide phases were established as a function of the carbon concentration.
This study shows that when a high concentration of carbon is present in the alloys a perovskite L′l 2 carbide is formed directly from the rapid solidification process. It is established also in this study that the carbon atom
contribution to the lattice parameter increase in the fcc-based cubic crystal is greater in the disordered γ-phase than in the ordered (L′l 2 structure) κ-phase. 相似文献
4.
The binary system Ti-Al has been reinvestigated in the composition range of 50 to 76 at. pct Al by X-ray diffraction, metallography,
electron probe microanalysis (EPMA), and differential thermal analysis (DTA). Heat-treated alloys (600°C to 1300°C) as well
as the as-cast alloys were investigated. Seven stable intermetallic phases were observed: TiAl, Ti 1−x
Al 1+x
, Ti 3Al 5, TiAl 2, Ti 5Al 11, TiAl 3 (h), and TiAl 3 (1); two metastable phases, TiAl 2 (m) and TiAl 3 (m), were also found. For each of these phases, the homogeneity range and the crystal chemical parameters were determined.
The temperatures of the solid-state phase reactions were re-established. On the basis of the experimental results, an improved
version of the equilibrium phase diagram has been drawn and critically compared with earlier versions presented in the literature. 相似文献
5.
The Er-Mg system was examined using differential thermal analysis (DTA), X-ray examination, metallography, and microprobe
analysis. Four intermediate phases are found to exist, and their crystal structures have been confirmed or determined as the
following: β phase (≈Er 2Mg) (cubic, cI2-W type, peritectic formation 1255 °C); ErMg (cubic, cP2-CsCl type, peritectic formation 830 °C); ErMg 2 (hexagonal, hP12-MgZn 2 type, peritectic formation 670 °C); and Er 5Mg 24 (cubic, cI58-α-Mn type, peritectic formation 600 °C). The β phase undergoes a eutectoidal decomposition at 680 °C and 30.5 at. pct Mg. A eutectic reaction was observed to occur at 570
°C and 89.5 at. pct Mg. Comparisons of the general properties between the ErMg phases and with those of the other R-Mg compounds
(R = rare earth) are briefly discussed. Properties and structures of the R-Mg, R-rich alloys are specially considered and
compared with those of a few groups of rare-earth alloys. The alloying behavior of R-rich R-Me alloys (R = Ho, Er, Tm, Lu;
Me = Mg, Cd, In, Tl) is systematically presented and/or predicted. 相似文献
6.
The high thermodynamic stability of alloys composed of platinum group metals and group IVB and VB metals has been explained
by an electronic interaction analogous to the Lewis acid-base concept for nontransition elements. The analogy is further demonstrated
by the titration of palladium by addition of niobium. The activity of niobium in solid palladium was measured as a function
of concentration by solid-state galvanic cells and study of the ternary oxide phase diagram. The galvanic cells were of the
type Pt/NbO 2,Nb 2O 4.8/YDTJNbO y,Nb pd/Pt where the solid electrolyte is yttria-doped thoria (YDT). Ternary phase diagrams for the Pd-Nb-0 and Rh-Nb-0 systems were
obtained by characterizing samples equilibrated at 1000 °C. The phase relationships found in the ternary diagrams were also
used to derive thermochemical data for the alloys. Thermochemical quantities for other acid-base stabilized alloys such as
Nb-Rh, Ti-Pd, and Ti-Rh were also measured. The excess partial molar ΔG xs/R of niobium at infinite dilution was determined to be -31 kilo-Kelvin at 1000 °C, and the AG°JR of formation of a mole of NbPd 3.55 is —21 kilo-Kelvin. These results and those for the other systems are used to assess the importance of valence electron configuration,
nuclear charge, and crystal field effects in the context of generalized Lewis acid-base theory. It is concluded that both
the nuclear charge of the atom and crystal field splitting of the valence orbitals significantly affect the basicity of the
platinum group metals. 相似文献
7.
A comparative study has been made of the room- and elevated-temperature properties, room-temperature fracture toughness, fatigue-crack
propagation rates, and 650 °C creep properties of Ti-24Al-14Nb-3V-0.5Mo with and without 0.9 at. pct Si. Both alloys have
microstructures consisting of the α
2, B2, and the orthorhombic O phase, with different proportions of the α
2 phase relative to the (O + B2) mixtures, depending on solution-treatment temperature. The alloy with a Si addition contains
additional primary ζ-Ti 5Si 3 particles distributed in the (O + B2) matrix. Tests of mechanical properties showed that the incorporation of a small fraction
(about 0.03 by volume) of the Ti 5Si 3 phase leads to greater room-temperature and elevated-temperature strengths, but lower room-temperature elongations and fracture
toughness as compared with the base alloy. Alloys containing greater volume fractions of the α
2 phase exhibited better tensile ductility, and this was attributed to the concurrent stabilization of the B2 phase. Examination
of tensile-tested and fatigued specimens indicates that the primary failure mode of the alloys, regardless of Si addition,
was due to the brittleness of the α
2 phase; the silicide particles that debonded from the matrix also contribute to cracking in the monotonic loading mode. Up
to a 20 pct improvement in creep-rupture life was observed in the Si-containing alloys, and this was interpreted in terms
of the solute-strengthening effect of Si. While the incorporated Ti 5Si 3 phase has an unfavorable effect on ductility and room-temperature fracture toughness, the difference in fatigue-crack propagation
rates between the alloys with and without Si is minimal. It is concluded that the controlling factor for the fatigue failure
in orthorhombic alloys is related to the ( α
2 + O + B2) microstructure, instead of the Ti 5Si 3 particles. 相似文献
8.
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
9.
The effect of Ni content on microstructure and mechanical properties of the CrMnFeCoNi high entropy alloy (HEA) has been studied. The Ni content varied from 0 to 20 at% in the composition (CrMnFeMn)100?xNix, where x?=?0, 2.5, 5, 10, 15, and 20 at%. The alloys were synthesized by vacuum arc melting and the microstructure as well as hardness of the as-cast alloys were studied. Alloys with low Ni content (x?≤?2.5%) consists of a two-phase microstructure of dendritic and inter-dendritic regions with fcc (matrix) and tetragonal (sigma) crystal structure, respectively. When the Ni content is 5 at%, two-phase structure with fcc (matrix) and bcc (secondary phase) is observed, with the addition of Mn-rich inclusions that are present in the entire matrix. Alloys with higher Ni content (x?≥?10, at%) exhibit a single phase of fcc structure. Hardness of the HEAs decreases from 320 to 120 Hv with increase in Ni content, and the high hardness of these alloys with low Ni content is due to the mixture of both fcc and hard tetragonal (sigma) phases. 相似文献
10.
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
11.
To better understand the role of constituent particles in pitting corrosion, analytical electron microscopic studies were
performed on the constituent particles in commercial 7075-T6 and 2024-T3 alloys. Five phases, namely, Al 23CuFe 4 and amorphous SiO 2 in 7075-T6 and Al 2CuMg, Al 2Cu, and (Fe,Mn)
x
Si(Al,Cu)
y
in 2024-T3, were identified. The crystal structure and chemistry of the Al 23CuFe 4, Al 2CuMg, and Al 2Cu phases in these alloys are in good agreement with the published data. Small deviations from their stoichiometric compositions
were observed and are attributed to the influence of alloy composition on the phase chemistry. For the (Fe,Mn)
x
Si(Al,Cu)
y
(approximately, x=3 and y=11) phase, a rhombohedral structure, with lattice parameter a= b= c=1.598 nm and α= β= γ=75 deg, was identified and is believed to be a modified form of either Al 8Fe 2Si or Al 10Mn 3Si. Information from this study provided technical support for studying the electrochemical interactions between the individual
particles (or phases) and the matrix. The corrosion results are reported in a companion article. 相似文献
12.
A novel hexagonal phase (designated H (2)) has been detected as a major constituent of both Al 62Ti 10V 28 and Al 55Ti 10V 35 alloys, following chill casting and after homogenization at 1523 K. The phase H 2 has an ordered hexagonal crystal structure (space group P6 3/ mmc, α=0.558±0.001 and c=0.450±0.001 nm), similar to that of α
2-Ti 3Al, and an atomic composition of 54±1 Al−11±1 Ti−35±1 V in the chill-cast Al 55Ti 10V 35 alloy. A fine-scale, duplex lamellar structure, developed within the ordered H 2 phase in the solid state, was composed of parallel-sided multivariants of ξ-Ti 5Al 11 phase, formed parallel to (0001) H
2. The orientation relationship between constituent phases was of the form Following homogenization (1 hour at 1523 K) and isothermal heat treatment at temperatures in the range from 1073 to 1273
K, the Al 62Ti 10V 28 and Al 55Ti 10V 35 alloys were observed to develop two-phase microstructures comprising β-(Ti, V)/ δ-Al 3(Ti,V) and β/ξ-Ti 5Al 11, respectively. In both alloys, the β phase could be retained as a stable phase to room temperature, regardless of cooling rate, whereas the H 2 phase was unstable at temperatures below ∼1273 K and was consumed by the formation of ξ phase at lower temperatures. 相似文献
13.
The effects of supercooling on the microstructure of Cu-Co alloys containing 10 to 65 wt pct Co were investigated. Supercooling
of the alloys below a characteristic temperature, t
SEP, resulted in a metastable phase separation into two liquids: one Co rich (L1) and the other Cu rich (L2). The microstructure
of the phase-separated alloys consisted of spherulites of one phase embedded in a matrix of the other. The spherulites in
alloys containing less than 40 wt pct Co were solidified from the L1 melt and from L2 in alloys containing more than 55 wt
pct Co. Supercooling of copper alloys containing around 50 wt pct Co resulted in a duplex structure of fine and coarse dendrites.
Microstructural evidence was presented for the formation of a ε-Cu metastable phase in alloys containing less than 30 wt pct Co. 相似文献
14.
Amorphous magnesium-rich alloys Mg
y
X 1-y
(X=Ni or Cu and 0.82< y<0.89) have been produced by melt spinning. The crystallization kinetics of these alloys have been determined by in situ X-ray diffraction (XRD) and isothermal and isochronal differential scanning calorimetry (DSC) combined with ex situ XRD. Microstructure analysis has been performed by means of transmission electron microscopy (TEM) and electron energy loss
spectroscopy (EELS). Crystallization of the Mg-Cu alloys at high temperature takes place in two steps: primary crystallization
of Mg, followed by simultaneous crystallization of the remaining amorphous phase to Mg and Mg 2Cu. Crystallization of the Mg-Cu alloys at low temperatures takes place in one step: eutectic crystallization of Mg and Mg 2Cu. Crystallization of the Mg-Ni alloys for a Mg content, y>0.85, takes place in two steps: primary crystallization of Mg and of a metastable phase (Mg ∼5.5Ni, with Mg content y=0.85), followed by the decomposition of Mg ∼5.5Ni. Crystallization of the Mg-Ni alloys for a Mg content y<0.85 predominantly takes place in one step: eutectic crystallization of Mg and Mg 2Ni. Within the experimental window applied ( i.e., 356 K< T<520 K and 0.82< y<0.89), composition dependence of the crystallization sequence in the Mg-Cu alloys and temperature dependence of the crystallization
sequence in the Mg-Ni alloys has not been observed. 相似文献
15.
Ductile shape memory (SM) alloys of the Cu-AI-Mn system have been developed by controlling the degree of order in the β phase.
Additions of Mn to the binary Cu-Al alloy stabilize the β phase and widen the single-phase region to lower temperature and
lower Al contents. It is shown that Cu-Al-Mn alloys with low Al contents have either the disordered A2 structure or the ordered
L2 1 structure with a lower degree of order and that they exhibit excellent ductility. The disordered A2 phase martensitically
transforms to the disordered Al phase with a high density of twins. The martensite phase formed from the ordered L2 1 phase has the 18R structure. The SM effect accompanies both the A2 → Al and L2 1 → 18R martensitic transformations. These alloys exhibit 15 pct strain to failure, 60 to 90 pct rolling reduction without
cracking, and 80 to 90 pct recovery from bend test in the martensitic condition. Experimental results on the microstructure,
crystal structure, mechanical properties, and shape memory behavior in the ductile Cu-AI-Mn alloys are presented and discussed. 相似文献
16.
In the context of developing tungsten free cobalt alloys, the physical metallurgical properties of γ′ precipitate strengthened Co–Ti–V alloys were investigated. In this study, few alloys were cast and heat treated to study systematic effects on the properties. The addition of V to the Co–Ti system decreases γ′ solvus temperature, whereas it increases solidus temperature. The γ–γ′ lattice parameter misfit decreases with V addition. The γ′ precipitates have cuboidal with round corners morphology, and the extent of roundedness of corners increases with V addition. Density functional theory calculations were performed to understand the experimental observation of phase transformation temperatures, lattice misfit, and γ′ precipitate morphology. The calculations indicate that magnitude of the heat of formation of Co3(Ti,V) in the L12 crystal structure decreases with V addition. The γ–γ′ interfacial energy at 0 K is predicted to increase with V addition to the Co–Ti system. 相似文献
17.
We have observed the icosahedral quasicrystals in the Zn- and rare earths-containing Mg alloys. The quasicrystals in these alloys contains Mg, Zn and rare earths elements. Three crystal phases, i.e. W phase, W′ phase and MgZn 2-type Laves phase, coexist with the quasicrystals in this kind of alloys. XRD analysis suggest that the quasicrystals in experimental alloys may have the same structure but different compositions to those in the Mg-Al-Zn [5] and Ga-Mg-Zn [6] alloys. 相似文献
19.
A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use
in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture
lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the ~923 K to 1173 K (650 °C to
900 °C) temperature range due to the formation of a protective Al 2O 3 scale rather than the Cr 2O 3 scales that form on conventional stainless steel alloys. This article overviews the alloy design approaches used to obtain
high-temperature creep strength in AFA alloys via considerations of phase equilibrium from thermodynamic calculations as well as microstructure characterization. Strengthening
precipitates under evaluation include MC-type carbides or intermetallic phases such as NiAl-B2, Fe 2(Mo,Nb)-Laves, Ni 3Al-L1 2, etc. in the austenitic single-phase matrix. Creep, tensile, and oxidation properties of the AFA alloys are discussed relative
to compositional and microstructural factors. 相似文献
20.
The effects of small additions of Ag on the aging of wrought Al-Cu-Mg(-Li) alloys, involving widespread nucleation of Cu-rich
Ω (and T 1) phase precipitates, are known. This article examines the influence of small additions of Ag on the nature of Cu-rich θ-Al 2Cu, S-Al 2CuMg, and T 1-Al 2CuLi phases present in appropriate as-cast Al-Cu, Al-Cu-Mg, and Al-Cu-Li-Mg alloys. Using a combination of light microscopy,
scanning electron microscopy (SEM), and electron probe microanalysis (EPMA), it is shown that neither independent nor combined
additions of Ag and Mg to the binary Al-Cu alloy alter the composition of the θ phase; however, there are differences in the ways the θ phase is evolved in Al-Cu-Mg alloys with and without Ag. Ag additions to the Al-Cu-Mg alloy result in the formation of an
Al-Cu-Mg-based ternary phase, having a Cu content similar to that of the θ phase and containing small amounts of Mg; rapid rates of cooling cause the retention of this phase in the as-cast alloy.
Relatively large amounts of both Ag and Mg are always located in the peripheral regions of such a phase. This phase is readily
replaced by θ phase upon annealing at 450 °C. The S phase of the ternary Al-Cu-Mg system is identified in this alloy and is found to dissolve
small amounts of Ag. In the case of the Al-Cu-Li-Mg-Ag alloy, two major changes are observed: both Ag and Mg are always present
in relatively large amounts in the peripheral regions of the T 1 phase, and the S-phase particles are once again found to dissolve small amounts of Ag. These results are discussed in light
of the known compositional features of the precipitates formed in the artificially aged Al-Cu-Mg(-Li)-Ag alloys, to reveal
that examination of phases present in the as-cast microstructure is a contributory step toward determining the locations of
trace alloy additions in the phase precipitates of interest. 相似文献
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