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1.
The effect of grain size in the range of 18 to 350 μm on the ambient-temperature creep behavior of a metastable β Ti-14.8 pct V alloy was investigated at a stress level of 95 pct yield stress. The main modes of deformation were found to
be stress-induced plate (SIP) formation and slip. In the 350-μm grain-size material, the plates were found to nucleate and grow with time, thereby contributing to the ambient-temperature
creep strain. The extent of SIP formation and growth were found to decrease with decreasing grain size, i.e., creep strain was found to decrease with decreasing grain size. The SIPs observed during creep were found to be similar to
those observed in an earlier investigation during tensile testing, which were reported to be {332}〈113〉 twins. 相似文献
2.
Yoshihiro Terada Rie Sota Naoya Ishimatsu Tatsuo Sato Koichi Ohori 《Metallurgical and Materials Transactions A》2004,35(9):3029-3032
The effect of calcium addition on the microstructure and creep strength of the die-cast AM50 magnesium alloy was investigated.
The α-Mg grains with the diameter of 4.9 μm are surrounded by the eutectic phases for the AM50-1.72 mass pct Ca alloy, while the β(Mg17Al12) particles are located mainly on the grain boundaries of the α grains for the AM50 alloy. The minimum creep rates of the AM50-1.72 mass pct Ca alloy are three orders of magnitude lower
than those of the AM50 alloy at 423 K typically below 120 MPa. The thousandfold creep strengthening by the Ca addition is
ascribed to the thermally stable eutectic phases appearing in the AM50-1.72 mass pct Ca alloy, which is expected to yield
effective grain boundary strengthening or to resist the plastic flow of the α-Mg grains. 相似文献
3.
J. A. Jiménez G. González-Doncel P. Acosta O. A. Ruano 《Metallurgical and Materials Transactions A》1994,25(6):1241-1248
The high-temperature deformation behavior of two ultrahigh boron steels containing 2.2 pct and 4.9 pct B was investigated.
Both alloys were processedvia powder metallurgy involving gas atomization and hot isostatic pressing (hipping) at various temperatures. After hipping at
700 °C, the Fe-2.2 pct B alloy showed a fine microstructure consisting of l-μm grains and small elongated borides (less than 1μm) . At 1100 °C, a coarser microstructure with rounded borides was formed. This alloy was superplastic at 850 °C with stress
exponents of about two and tensile elongations as high as 435 pct. The microstructure of the Fe-4.9 pct B alloy was similar
to that of the Fe-2.2 pct B alloy showing, in addition, coarse borides. This alloy also showed low stress exponent values
but lacked high tensile elongation (less than 65 pct), which was attributed to the presence of stress accumulation at the
interface between the matrix and the large borides. A change in the activation energy value at theα-γ transformation temperature was seen in the Fe-2.2 pct B alloy. The plastic flow data were in agreement with grain boundary
sliding and slip creep models.
J.A. JIMéNEZ, Postdoctoral Fellow, formerly with Centro Nacional de Investigaciones Metalurgicas, C.S.I.C. 相似文献
4.
Creep tests were performed on normal grade Ti-5 Al-2.5 Sn at 194, 299, 339, and 422 K. Three processing histories were studied:
bar stock and forgings made in theα andβ ranges. Creep stresses ranged from 40 to 90 pct of the 299 K tensile yield stress, σy. Not all combinations of stress and
temperature gave reliable creep data. At 60 and 80 pct σy, the forged materials were more creep resistant than the bar stock,
while at 90 pct σy all three materials were alike. The apparent activation energy for creep, about 37 kJ/mol, was about one-fourth
the energy for selfdiffusion. Activation areas were about 10⇃2; thus the rate-controlling process at the stresses used was probably the overcoming of interstitial obstacles. Observations
on thin foils showed that the bar stock and α-forged material had equiaxed grain structures, while theβ-forged material consisted of massive martensite(α’) plates. Films of retainedβ appeared to be present in manyα’ boundaries; this restricted slip to individualα’ plates. A microstructural rationale was constructed, suggesting that each material contained dislocation sources which differed
in ease of operation. Theβ-forged material was fitted into this rationale by the observation that dislocations inα’ boundaries could apparently act as sources. It was concluded that the observed transient creep had arisen from exhaustion
of the easiest sources at each stress level. 相似文献
5.
6.
Yang Leng 《Metallurgical and Materials Transactions A》1995,26(2):315-328
Creep crack growth (CCG) has been investigated in an 8009 (Al-Fe-V-S) P/M alloy at 175 °, 250 °, and 316 ° and in a 2618 ingot
alloy at 150 °, 175 °, and 200 °. Under sustained load, subcritical crack growth is observed at stress intensity levels lower
thanK
ic
; for 2618, at 200 °, crack growth is observed at stress intensities more than 40 pct lower thanK
ic
. Alloys 8009 and 2618 exhibit creep brittle behavior,i.e., very limited creep deformation, during CCG. The CCG rates do not correlate with CCG parameters C* and C but correlate with
the stress intensity factor,K, and theJ integral. Generally, crack growth rates increase with increasing temperature. Micromechanisms of CCG have been studied with
regard to microstructural deg-radation, environmental attack, and creep damage. Although theoretical estimation indicates
that CCG resistance decreases with second-phase coarsening, such coarsening has not been observed at the crack tip. Also,
no evidence is found for hydrogen- or oxygen-induced crack growth in comparing test results in moist air and in vacuum. Creep
deformation and cavitation ahead of crack tip are responsible for observed time-dependent crack growth. Based on the cavitation
damage in the elastic field, a micromechanical model is proposed which semiquantitatively explains the correlations between
the creep crack growth rate and stress intensity factor,K. 相似文献
7.
J. A. del Valle M. T. Pérez-Prado O. A. Ruano 《Metallurgical and Materials Transactions A》2005,36(6):1427-1438
The microstructural evolution during tensile deformation of an AZ31 alloy with grain size ranging from 17 to 40 μm, at intermediate temperatures, has been studied using electron backscattered diffraction (EBSD) and optical microscopy (OM)
as the main characterization tools. Two deformation regimes could be distinguished. In the high-strain-rate regime, the stress
exponent was found to be about 6, and the activation energy is close to that for Mg self-diffusion. These values are indicative
of climb-controlled creep. In the lower strain rate range, elongations higher than 300 pct were measured. In this range, significant
dynamic grain growth takes place during the test, and thus, the predominant deformation mechanisms have been investigated
by means of strain-rate-change tests. It was found that the stress exponent varied during the test between 1.7 and 2.5, while the activation energy remains close to that for grain-boundary diffusion. The EBSD analysis
revealed, additionally, the appearance of low to moderately misoriented boundaries that tend to lay perpendicular to the tensile
axis. The enhanced ductility of this AZ31 alloy in this regime is attributed to the operation of a sequence of deformation
mechanisms. Initially, grain-boundary sliding governs deformation; once dynamic grain growth occurs, dislocation slip becomes
gradually more important. Dislocation interaction gives rise to the appearance of new interfaces by continuous dynamic recrystallization
(CDRX). 相似文献
8.
A eutectic alloy, Ag-32.2 at. pct Mg, has been directionally solidified at growth rates,R, ranging from 0.9 to 63.9 cm/h to produce an aligned lamellar structure. The alloy consists of AgMg, an ordered CsCl type
phase, and a solid solution of approximate composition Ag-27 at. pct Mg. The investigation consisted of studying ordering
effects in the Ag-27 pct Mg phase on creep behavior of the aligned eutectic at temperatures between 210 and 270°C. Ordering
of Ag-27 pet Mg markedly reduced creep rate and increased rupture life of the eutectic alloy. An increase in R resulted in
a finer interlamellar spacing and further reduced creep rate for both ordered and disordered material. An increase in the
apparent activation energy for creep with order is consistent with an increase in the activation energy for vacancy motion
in the Ag3Mg phase. Creep tests on a single-phase Ag3Mg alloy confirmed this conclusion. Both the ordered and disordered eutectic alloy exhibited a high sensitivity of steady-state
creep rate, ε, to stress. The stress dependence was analyzed in terms of a proposed mechanism of creep in this alloy. Slip
processes have been observed by optical microscopy, and fractographic techniques have been employed to explain the influence
of long range order on rupture. Implications of this work to design of creep-resistant eutectic composites are discussed. 相似文献
9.
This work focuses on the effect of the second phase on the ambient temperature creep deformation mechanisms of titanium alloys, using Ti-6.0 wt pct Mn and Ti-8.1 wt pct V with Widmanstätten microstructures as the model systems. In Part I it was observed that the presence of a second phase can affect the tensile deformation behavior. Likewise, the creep deformation mechanisms of the two-phase alloys differ from the mechanisms of single-phase alloys. These α-β deformation mechanisms include twinning in fine grains of the α phase and stress-induced hexagonal martensite in the β phase of Ti-8.1 V. This is the first time that twinning in the α phase and stress-induced martensite in the β phase are reported as creep deformation mechanisms in an α-β titanium alloy. Several factors, including elastic interaction effects, shear stress due to deformation products in adjacent phases, and the stability of the β phase, affect the creep deformation mechanisms in these alloys. Models for the time-dependent growth of martensite are suggested. In addition, the difference between tensile and creep deformation in regard to accumulation of stresses to reach the critical stresses is described. 相似文献
10.
Creep and microstructure of magnesium-aluminum-calcium based alloys 总被引:13,自引:0,他引:13
Alan A. Luo Bob R. Powell Michael P. Balogh 《Metallurgical and Materials Transactions A》2002,33(3):567-574
This article describes the creep and microstructure of Mg-Al-Ca-based magnesium alloys (designated as ACX alloys, where A
stands for aluminum; C for calcium; and X for strontium or silicon) developed for automotive powertrain applications. Important
creep parameters, i.e., secondary creep rate and creep strength, for the new alloys are reported. Creep properties of the new alloys are significantly
better than those of the AE42 (Mg-4 pct* Al-2 pct RE**) alloy, which is the benchmark creep-resistant magnesium die-casting
alloy. Creep mechanisms for different temperature/stress regimes are proposed. A ternary intermetallic phase, (Mg,Al)2Ca, was identified in the microstructure of the ACX alloys and is proposed to be responsible for the improved creep resistance
of the alloys.
All concentrations in wt. pct, unless otherwise stated.
RE stands for a combination of rare earth elements, i.e., misch metal, in this case. 相似文献
11.
12.
John D. Whittenberger 《Metallurgical and Materials Transactions A》1976,7(4):611-619
A study of the relationship between structure and mechanical properties of thin TD-NiCr sheet indicated that the elevated
temperature tensile, stress-rupture, and creep strength properties are dependent on grain aspect ratio and sheet thickness.
In general, the strength properties increase with increasing grain aspect ratio and sheet thickness. Tensile testing revealed
an absence of ductility at elevated temperatures(T ≥ 1144 K). Significant creep damage, as determined by subsequent tensile testing at room temperature, occurs after very small
amounts (< 0.1 pct) of prior creep deformation at elevated temperatures (1144 ≤T ≤ 1477 K). A threshold stress for creep appears to exist. Creep exposure below the threshold stress atT ≥ 1366 K results in almost full retention of room temperature tensile properties. 相似文献
13.
Sensitivity of the elevated temperature (above 1/2 Tm) deformation of TD-nickel to grain size and shape was examined in both tension and creep. Elevated temperature strength increased
with increasing grain diameter and increasingL/D ratio. Temperature sensitivity of the yield stress, as well as high (compared to self diffusion) apparent tensile activation
enthalp-ies were the result of the internal stress not being proportional to the shear modulus. Creep activation enthalpies
increased with increasingL/D ratio and, to a lesser extent, increasing grain diameter, reaching high values which may be apparent values. The thoria particle
dis-persion may have been altered by elevated temperature tensile and creep deformation. 相似文献
14.
Brian D. Worth J. Wayne Jones John E. Allison 《Metallurgical and Materials Transactions A》1995,26(11):2947-2959
The influence of microstructure on creep deformation was examined in the near-y TiAl alloy Ti-49A1-1V. Specifically, microstructures
with varying volume fractions of lamellar constituent were produced through thermomechanical processing. Creep studies were
conducted on these various microstructures under constant load in air at temperatures between 760 °C and 870 °C and at stresses
ranging from 50 to 200 MPa. Microstructure significantly influences the creep behavior of this alloy, with a fully lamellar
microstructure yielding the highest creep resistance of the microstructures examined. Creep resistance is dependent on the
volume fraction of lamellar constituent, with the lowest creep resistance observed at intermediate lamellar volume fractions.
Examination of the creep deformation structure revealed planar slip of dislocations in the equiaxed y microstructure, while
subboundary formation was observed in the duplex microstructure. The decrease in creep resistance of the duplex microstructure,
compared with the equiaxed y microstructure, is attributed to an increase in dislocation mobility within the equiaxedy constituent, that results from partitioning of oxygen from the γ phase to the α2 phase. Dislocation motion in the fully lamellar microstructure was confined to the individual lamellae, with no evidence
of shearing of γ/γ or γ/α2 interfaces. This suggests that the high creep resistance of the fully lamellar microstructure is a result of the fine spacing
of the lamellar structure, which results in a decreased effective slip length for dislocation motion over that found in the
duplex and equiaxed y microstructures.
BRIAN D. WORTH, formerly with the Department of Materials Science and Engineering, The University of Michigan 相似文献
15.
M. L. Öveçoglu O. N. Senkov F. H. Froes N. Srisukhumbowornchai 《Metallurgical and Materials Transactions A》1999,30(3):751-761
Prealloyed, gas-atomized (GA) Ti-47Al-3Cr alloy powder, containing about 70 pct of the α
2 (Ti3Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600 °C, but decomposed
at higher temperatures, with an exothermic reaction peak at 624 °C as the material transformed to a mixture of α
2 and γ and then to a fully γ structure at 722 °C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP’ing)
of the amorphous powder at 725 °C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200 °C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and
analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only
the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours,
with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning
force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The
minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains. 相似文献
16.
The residual microstructure and mechanical response of shock-loaded stainless steel (AISI-304) of four different grain sizes-23,
55, 85 and 187 Μm-was investigated. In addition to mechanical twinning and planar dislocation arrays, transformation to both
e and α martensite occurred in all shock-loaded specimens but became more extensive with decreasing grain size. In comparison
to the Hall-Petch behavior of yield and early flow stress observed for the material after 5.2 pct cold rolling, the strengthening
efficiency of shock loading decreased with increasing grain size. Shock loading enhanced the strain-induced transformation
to α martensite during subsequent tensile deformation. 相似文献
17.
Creep deformation mechanisms in high-pressure die-cast magnesium-aluminum-base alloys 总被引:2,自引:0,他引:2
W. Blum Y. J. Li X. H. Zeng P. Zhang B. von Großmann C. Haberling 《Metallurgical and Materials Transactions A》2005,36(7):1721-1728
Creep of die-cast Mg alloys is described as an integral part of their plastic deformation behavior in terms of stress-strain-rate-strain
relations. Creep tests yield information on yield stress, work hardening, maximum deformation resistance (minimum creep rate),
and work softening. Testing in compression avoids influences by fracture. Data on the alloy AJ52 (5Al, 2Sr) in the temperature
range between 135 °C and 190 °C are presented and compared to those for AZ91 and AS21. Die-cast Mg-Al alloys consist of fine
grains with a grain boundary region containing intermetallic precipitates. Transmission electron microscopic observations
indicate that basal glide is the dominant mechanism of deformation being supplemented by nonbasal glide and twinning to maintain
compatiblity between the grains. The deformation resistance can be modeled with a composite approach assuming that the grain
boundary region is relatively hard due to precipitation of intermetallic phases. The differences in long-term creep resistance
at low stress are explained in terms of different strength and stability of precipitates in the different alloys.
This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium
Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of the ASM-MSCTS
Phase Transformations Committee. 相似文献
18.
M. Dilip Bhandarkar M. Shanthidas Bhat Victor F. Zackay Earl R. Parker 《Metallurgical and Materials Transactions A》1975,6(6):1281-1289
A Laves phase, Fe2Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7
at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome
by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to
600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests
conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy
were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of
steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture
and creep strengths were considerably increased.
Formerly with Lawrence Berkeley Laboratory. 相似文献
19.
Masaaki Tabuchi Yuji Nakasone Toshio Ohba Koichi Yagi Tatsuhiko Tanabe 《Metallurgical and Materials Transactions A》1995,26(2):383-389
Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and
in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment
on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed
to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental
effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment
at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered
to prevent the grain boundary sliding in lower-carbon alloys. 相似文献
20.
Sang H. Kim Nack J. Kim H. H. Chung Sung G. Pyo S. J. Hwang 《Metallurgical and Materials Transactions A》1998,29(9):2273-2283
The present study is concerned with γ-(Ti52Al48)100−x
B
x
(x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure
of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α2 and γ with a mean grain size of 200 nm. Besides α2 and γ phases, binary and 0.5 pct B alloys contain Ti2AlN and Al2O3 phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing
treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ
grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility,
but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength
than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed
alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature
of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine
5 pct B alloy is dominated by the grain boundary sliding mechanism.
This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented
at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the
Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees. 相似文献