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1.
The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature
toughness for potential use at temperatures above ∼1000 °C has remained a persistent challenge in materials science. In this
work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions
in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods.
Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys,
which consisted of ∼21 to 38 vol. pct α-Mo phase in an intermetallic matrix of Mo 3Si and Mo 5SiB 2 (T 2), were characterized at temperatures between 25 °C and 1300 °C. The boron additions were found to confer improved “pest”
resistance (at 400 °C to 900 °C) as compared to unmodified molybdenum silicides, such as Mo 5Si 3. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those
of MoSi 2, for the I/M processed microstructures with coarse distributions of the α-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa √m at ambient temperatures to almost
12 MPa √m at 1300 °C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi 2, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa √m at 25 °C to ∼8 MPa √m at 1300 °C. These results, in particular, that the toughness and cyclic
crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of
toughening in Mo-Si-B alloys and the specific role of microstructure. 相似文献
2.
Phase relations in the Mo-Si-C system were evaluated at 1200 °C and 1600 °C within the composition range delimited by the
phases Mo 5Si 3, MoSi 2, Mo ≤5Si 3C ≤1, and SiC. The evaluation included estimation of possible equilibria from known thermodynamic data of the binary phases as
well as experimental work. For the experimental evaluation, high-purity powders were hot-pressed, heat-treated, and characterized
by X-ray diffraction and electron microprobe analysis. It is shown that MoSi 2 is in equilibrium with Mo ≤5Si 3C ≤1 at 1600 °C, as previously established by Nowotny et al, ( Monatsh. Chem., 1954, vol. 85, pp. 255-72), and at 1200 °C, in contrast to the Mo 5Si 3-SiC equilibrium reported by van Loo et al. ( High Temp.- High Press., 1982, vol. 14, pp. 25-31). The thermodynamic estimation suggests that these phase relations should extend to lower temperatures
in the range of compositions investigated. Thus, the third phase in silicon-lean MoSi 2-SiC composites should be the Nowotny phase (Mo ≤5Si 3C ≤1) instead of Mo 5Si 3. The Gibbs free energy of formation at 298 K of the idealized compound Mo 5Si 3C is estimated as -40.2 kJ/mol. 相似文献
3.
A study has been made of the role of ductile-phase toughening on the ambient temperature fracture toughness and fatigue-crack
propagation behavior of a molybdenum disilicide intermetallicmatrix composite reinforced with 20 vol pct niobium spheres.
Using disk-shaped compact DC(T) samples, only moderate improvements (∼24 pct) in fracture toughness K
lcvalues were found for the composite compared to the unreinforced MoSi 2 matrix material. Moreover, (cyclic) fatigue- crack propagation was seen at stress intensities as low as 75 to 90 pct of K
Ic, with growth rates displaying a high dependency (∼14) on the applied stress-intensity range. The lack of significant toughening
due to the incorporation of ductile Nb particles is associated with an absence of crack/particle interactions. This is attributed
to the formation of a weak reaction-layer interface and elastic mismatch stresses at the crack tip between the Nb and MoSi 2, both factors which favor interfacial debonding; moreover, the spherical morphology of Nb phase stabilizes cracking around
the particle. Results suggest that increasing the aspect ratio of the distributed Nb rein- forcement phase with attendant
interfacial debonding and eliminating possible Nb-phase em- brittlement due to interstitial impurity contamination are critical
factors for the successful development of tougher Nb/MoSi 2 structural composites.
Formerly with McDonnell
Formerly with McDonnell 相似文献
4.
The solidification behavior of two composites based on Al-Si alloy has been investigated as a function of cooling rate. Thermal
analysis techniques have been used to establish the relationship between solidification history and the microstructure developed.
The results of thermal analysis show that the characteristic parameters are influenced by the cooling rate. A marked difference
in these parameters is observed between the reinforced and the unreinforced materials at all cooling rates studied. The cooling
rates used in the present study range from 0.3 to 20 K/s. Increasing the cooling rate is shown to affect the undercooling
parameters both in the liquidus and eutectic solidification region. The eutectic growth temperature of the composites is observed
to be higher than that of the base alloy at all cooling rates. The depression in eutectic temperature ΔT is found to decrease
by 27 K for the unreinforced alloy (A356) and by 17 K for the composites (A356 + 10, 20 vol pct SiC) at a higher cooling rate
of ≃16 K/s. The presence of SiC reinforcement is observed to suppress the Mg 2Si precipitate formation and decrease the amount of heat liberated during both primary and eutectic phase formation. Dendrite
arm spacing (DAS) is correlated to the cooling rate by a relationship of the form DAS = AT
-n, where n is found to be of the order of 0.33. 相似文献
5.
Both Ni-36 wt pct Sb and Ni-52.8 wt pct Sb eutectic alloys were highly undercooled and rapidly solidified with the glass-fluxing
method and drop-tube technique. Bulk samples of Ni-36 pct Sb and Ni-52.8 pct Sb eutectic alloys were undercooled by up to
225 K (0.16 T
E
) and 218 K (0.16 T
E
), respectively, with the glass-fluxing method. A transition from lamellar eutectic to anomalous eutectic was revealed beyond
a critical undercooling Δ T
1*, which was complete at an undercooling of Δ T
2*. For Ni-36 pct Sb, Δ T
1*≈60 K and Δ T
2*≈218 K; for Ni-52.8 pct Sb, Δ T
1*≈40 K and Δ T
2*≈139 K. Under a drop-tube containerless solidification condition, the eutectic microstructures of these two eutectic alloys
also exhibit such a “lamellar eutectic-anomalous eutectic” morphology transition. Meanwhile, a kind of spherical anomalous
eutectic grain was found in a Ni-36 pct Sb eutectic alloy processed by the drop-tube technique, which was ascribed to the
good spatial symmetry of the temperature field and concentration field caused by a reduced gravity condition during free fall.
During the rapid solidification of a Ni-52.8 pct Sb eutectic alloy, surface nucleation dominates the nucleation event, even
when the undercooling is relatively large. Theoretical calculations on the basis of the current eutectic growth and dendritic
growth models reveal that γ-Ni 5Sb 2 dendritic growth displaces eutectic growth at large undercoolings in these two eutectic alloys. The tendency of independent
nucleation of the two eutectic phases and their cooperative dendrite growth are responsible for the lamellar eutectic-anomalous
eutectic microstructural transition. 相似文献
6.
The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C)
and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties,
and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10 p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic
Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution
temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed
on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and
temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300
MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323
MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight
improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the
SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged
condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC
particles. A detailed discussion of the fracture mechanism is given. 相似文献
7.
The effects of directional solidification processing on the microstructural, compositional, and magnetic properties of near
eutectic Co-Sm alloys (∼9 at. pct Sm) have been studied. Because these sytems have high melting temperatures ( T
m > 1000 °C) and are quite reactive to oxidizing environments, special containment techniques during solidification were developed.
Initial investigations have been performed at modest thermal gradients in the liquid, G
L
≤ 60 °C/cm (1 °C/cm = 10 −2 K/m), and over a range of furnace (solidification) velocities, 0.8 ≤ V ≤ 45.4 cm/h (1 cm/h = 2.8 × 10 −6m/s). Since the range of G
L/V values, a measure of the degree of interfacial morphological stability, was rather low, aligned dendritic morphologies,
macrosegregation, and transition to rod eutectic growth were encountered. The primary dendrite spacing for near eutectic Sm 2Co 17/Co scaled with V
−1/2 and varied from ∼50 μm for V ≥20 cm/h to hundreds of microns for V < 10 cm/h while the rod eutectic diameter and interred spacing were an order of magnitude smaller. For both dendritic and
cooperative growth, the associated permanent magnet properties were rather poor, e.g., remanence less than 4 kG (1 gauss = 10 −4 Tesla) and coercive force less than 1 kOe (1 Oe = 79.577 A/m) for the smallest dendrite and rod diameter dimensions encountered,
although the magnetic hardness for the rod eutectic was larger than for the dendritic microstructure. Magnetization as a function
of sample orientation indicated that the easy axis of magnetization was primarily along the direction of solidification for
both ferromagnetic phases. 相似文献
8.
A study has been made of the fracture toughness/resistance-curve (R-curve) and cyclic fatigue-crack propagation behavior in
a molybdenum disilicide composite, ductile-phase toughened with nominally 20 vol pct Nb-wire mesh reinforcements (Nb
m
/MoSi 2); results are compared with monolithic MoSi 2 and MoSi 2 reinforced with 20 vol pct spherical Nb particles (Nb
p
/MoSi 2). It is found that the high aspect ratio wire reinforcements induce significant toughening in MoSi 2, both under monotonic and cyclic fatigue loading conditions. Specifically, the Nb
m
/MoSi 2 composite exhibits R-curve behavior with a steady-state fracture toughness of ∼13 MPa
, compared to unstable fracture at K
c
values below 5 MPa
in unreinforced MoSi 2 or Nb
p
/MoSi 2. Such behavior is seen to be associated with extensive crack deflection within the reaction layer between Nb and the matrix,
which leads to crack bridging by the unbroken ductile phase. Similarly, resistance to fatigue-crack growth is found to be
far superior in the wire-reinforced composite over pure MoSi 2 and Nb
p
/MoSi 2. Although crack paths are again characterized by extensive deflection along the Nb/matrix reaction layer, the role of crack
bridging is diminished under cyclic loading due to fatigue failure of the Nb. Instead, the superior fatigue properties of
the Nb
m
/MoSi 2 composite are found to be associated with high levels of crack closure that result from highly deflected crack paths along
the (Nb,Mo) 5Si 3 reaction layer interface. 相似文献
9.
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
10.
As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to
generate a constitution diagram for INCONEL 718 * in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and
scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are
used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram
are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and
a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification
sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued
γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made
using the Scheil solidification model, and the fraction of each phase in the eutectic is calculated via the lever rule. These are compared with experimentally determined values and found to be in good agreement. 相似文献
11.
The solidification behavior of two composites based on Al-Si alloy has been investigated as a function of cooling rate. Thermal analysis techniques have been used to establish the relationship between solidification history and the microstructure developed. The results of thermal analysis show that the characteristic parameters are influenced by the cooling rate. A marked difference in these parameters is observed between the reinforced and the unreinforced materials at all cooling rates studied. The cooling rates used in the present study range from 0.3 to 20 K/s. Increasing the cooling rate is shown to affect the undercooling parameters both in the liquidus and eutectic solidification region. The eutectic growth temperature of the composites is observed to be higher than that of the base alloy at all cooling rates. The depression in eutectic temperature ΔT is found to decrease by 27 K for the unreinforced alloy (A356) and by 17 K for the com- posites (A356 + 10, 20 vol pct SiC) at a higher cooling rate of ≃16 K/s. The presence of SiC reinforcement is observed to suppress the Mg2Si precipitate formation and decrease the amount of heat liberated during both primary and eutectic phase formation. Dendrite arm spacing (DAS) is correlated to the cooling rate by a relationship of the form DAS =A(T)
-n, wheren is found to be of the order of 0.33. 相似文献
12.
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- βAl 5FeSi eutectic point. Solidification via the binary Al- βAl 5FeSi 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
14.
The hot-working behavior of two metal matrix composites (7090 + 20 vol pct SiC whiskers and 6061 + 20 vol pct SiC whiskers)
and their powder metallurgy matrix alloys (7090 and 6061) was studied by hot torsion testing. Flow stress (σ o) and strain-to-failure (ε
f
) data were generated at deformation temperatures and strain rates corresponding to the potential range for commercially hot-working
these alloys. Based on the hot torsion data, hot-working parameters were recommended where σ o was low and ε
f
was high. Strain rate sensitivities and activation energies of deformation were computed for the alloys.
Formerly with Martin Marietta Laboratories, Baltimore, MD 相似文献
15.
The mechanical and tribological properties of white cast irons are strongly dependent on whether they contain M 7C 3 or M 3C carbides (M = Fe, Cr, etc.). In an effort to improve the wear resistance of such materials, the United States Bureau of Mines has studied the effects
of adding 0.3 to 2.3 wt pct (throughout) Si to hypoeutectic irons containing approximately 8.5 pct Cr and 6.0 pct Ni. The
eutectic carbides formed were identified by electron microprobe analysis, X-ray diffraction, and scanning electron (SEM) and
optical microscopies. In addition, differential thermal analysis (DTA) was used to study the process of solidification. At
Si contents of 0.3 and 1.2 pct, the eutectic carbides exhibited a duplex structure, consisting of cores of M 7C 3 surrounded by shells of M 3C. Additionally, the microstructure contained ledeburite (M 3C + γFe (austenite)). At the higher Si content of 1.6 pct, the eutectic carbides consisted entirely of M 7C 3, and some ledeburite remained. Last, when the Si content was raised to 2.3 pct, the eutectic carbides again consisted entirely
of M 7C 3, but ledeburite was no longer formed. These observations can be explained in terms of the effects of Si and, to a lesser
extent, of Ni on the shape of the liquidus surface of the metastable Fe-Cr-C phase diagram. The addition of Si reduces the
roles played by the four-phase class II p reaction L + M 7C 3 → M 3C + γFe and the ledeburitic eutectic reaction L → M 3C + γFe in the overall process of solidification.
N.H. Macmillan, for-merly with the Albany Research Center. 相似文献
16.
A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum
alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy
and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied
in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain
refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance
from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis
and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture
surface analysis (phase identification), respectively. The results showed that the local solidification time (viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced
effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain
boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching.
The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al 5FeSi, α - Al 15(Fe,Mn) 3Si 2, and Al 2Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties
to DAS allows for the use of the latter as an indicator of the expected properties of the alloy. 相似文献
17.
A new method to determine directly the solid fraction using the cooling curve was proposed for solidification of undercooled
melts. Then, to construct three different baselines, a sudden function ξ
α
( x) is introduced. In terms of the ξ
α
( x) function, accordingly, the solid fractions during solidification of Ni-3.3 wt pct B, Al-7 wt pct Si, Al-14 wt pct Cu, and
Fe-4.56 wt pct Ni alloys were predicted. The predictions of the primary, the regular lamellar eutectic, the anomalous eutectic,
and the peritectic phases from cooling curves of the solidified samples coincide with the results of measurement or the available
methods. 相似文献
18.
Conclusions We examined the concentration and temperature dependences of the specific electrical resistivity in the two- (MoSi 2-SiC) and three-phase (MoSi 2-SiC-Y 2O 3) systems in the temperature range 100–1800°C. In the two-phase system, the lowest TCR in heating to 1800°C was recorded for the materials with the mass content of SiC of 20–40%. The results show that the TCR of the two-phase materials of the MoSi 2-SiC system can be reduced by adding yttrium oxide to them.Translated from Poroshkovaya Metallurgiya, No. 6(306), pp. 83–85, June, 1988. 相似文献
19.
The constitution of the Pb-Sn-Sr system from the Pb-Sn binary up to 36 at. pct Sr was determined by differential thermal analysis,
metallography, microprobe analysis, and X-ray diffraction. Pb 3Sr forms a continuous series of solid solutions with Sn 3Sr, and is referred to here as the 8 phase. Sn 4Sr was the only other intermetallic phase found and is designated here as γ. A eutectic-like trough is formed between (Pb)
and δ. It originates at 1.0 at. pct Sr and 324.5 °C (the (Pb)/Pb 3Sr eutectic) and falls monotonically to ~75 at. pct Pb, 24.5 at. pct Sn, and 0.45 at. pct Sr at 283 °C. At 283 °C, a Class
II, four-phase reaction occurs: L + δ → (Pb) + γ. A eutectic-like trough between (Pb) and γ falls from the four-phase plane at 283 °C to the ternary eutectic at
~26 at. pct Pb, ~74 at. pct Sn and <0.3 at. pct Sr at 182 °C. The ternary eutectic reaction is L → (Pb) + (Sn) + γ. 相似文献
20.
The liquidus surface of the C-Cr-Fe system has been experimentally determined in the Fe-rich region —C ≤6 wt pct, Cr ≤40 wt
pct —using a sensitive differential thermal analysis technique, along with optical and scanning electron microscopy and X-ray
diffraction. Previous liquidus surfaces for this system have differed on the extent of the (Cr,Fe) 23C 6 liquidus field, with one version reporting its existence at ∼20 wt pet Cr, and others finding that it did not occur at Cr
levels of less than ∼60 wt pct. The present investigation provides evidence in favor of the second contention, with the (Cr,Fe) 23C 6 field not being detected at Cr ≤40 wt pct. Changes are proposed to the accepted liquidus surface in respect of the compositions
of the invariant reactions— L + αδFe ⇌ γFe + (Cr,Fe) 7C 3 and L + (Cr,Fe) 7C 3 ⇌ γFe + (Fe,Cr) 3C —and of the monovariant eutectic valley— L⇌ γFe + (Cr,Fe) 7C 3. 相似文献
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