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1.
S. Asgari 《Metallurgical and Materials Transactions A》2006,37(7):2051-2057
This article presents results of an investigation on age-hardening behavior of superalloy AEREX 350. Microhardness testing was employed to evaluate the age-hardening response of the alloy while optical, scanning, and transmission electron microscopy techniques were used to characterize the major phases formed during the aging process. No significant hardening was found in solution-treated samples aged at temperatures up to about 680 °C. Aging at 700 °C up to 950 °C, however, caused a characteristic hardening response. This hardening was concurrent with the formation of γ', an ordered phase with L12 structure, as fine precipitate distributed throughout the fcc matrix. In the temperature range of 800 °C to 1055 °C, a new phase called η with D024 structure was formed. Two morphologies of η phase were found: the discrete blocky precipitates mainly at grain boundaries and elongated plates with a Widmanstätten appearance within the grains. The latter morphology was predominant at higher aging temperatures. This was attributed to accelerated diffusion of solute to the incoherent tips of Widmanstätten plates at high temperatures. No evidence of any precipitates was found in the microstructure of samples aged at 1060 °C, implying that this temperature was above the solvus temperatures of all precipitates in the AEREX 350 alloy. Based on the results presented in this investigation, it is suggested that considerable improvement in the properties of the alloy may be achieved through modification of the commercial heat-treatment practice. 相似文献
2.
Z. Jin C. Cady G. T. Gray III Y. -W. Kim 《Metallurgical and Materials Transactions A》2000,31(3):1007-1016
The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001 to 2000 s−1 and temperatures from −196 °C to 1200 °C. The temperature dependence of the yield and flow stresses is found to depend on
the strain rate. At strain rates of 0.001 and 0.1 s−1, the yield stress decreases as the temperature increases, with a plateau between 600 °C and 800 °C. At strain rates of 35
and 2000 s−1, the yield stress exhibits a positive temperature dependence at temperatures above 600 °C; however, postyield flow stresses
exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high
temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The workhardening-rate plateau is
seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1 s−1 and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s−1, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200 °C). The anomalous (positive)
temperature dependence of the yield stress at high strain rates (>1 s−1) and high temperatures (>600 °C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s−1) and high temperatures (>900 °C) is thought to be due to a thermally activated dislocation-jog climb process in the grain
interiors and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence
of the flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating. 相似文献
3.
Yulong Zhang Li Wang Kip O. Findley John G. Speer 《Metallurgical and Materials Transactions A》2017,48(5):2140-2149
With an aim to elucidate the influence of temperature and grain size on austenite stability, a commercial cold-rolled 7Mn steel was annealed at 893 K (620 °C) for times varying between 3 minutes and 96 hours to develop different grain sizes. The austenite fraction after 3 minutes was 34.7 vol pct, and at longer times was around 40 pct. An elongated microstructure was retained after shorter annealing times while other conditions exhibited equiaxed ferrite and austenite grains. All conditions exhibit similar temperature dependence of mechanical properties. With increasing test temperature, the yield and tensile strength decrease gradually, while the uniform and total elongation increase, followed by an abrupt drop in strength and ductility at 393 K (120 °C). The Olson–Cohen model was applied to fit the transformed austenite fractions for strained tensile samples, measured by means of XRD. The fit results indicate that the parameters α and β decrease with increasing test temperature, consistent with increased austenite stability. The 7Mn steels exhibit a distinct temperature dependence of the work hardening rate. Optimized austenite stability provides continuous work hardening in the temperature range of 298 K to 353 K (25 °C to 80 °C). The yield and tensile strengths have a strong dependence on grain size, although grain size variations have less effect on uniform and total elongation. 相似文献
4.
Z. Jin C. Cady G. T. Gray Y. -W. Kim 《Metallurgical and Materials Transactions A》2000,31(13):1007-1016
The mechanical behavior of a fine-grained duplex γ-TiAl alloy was studied in compression at strain rates ranging from 0.001
to 2000 s−1 and temperatures from −196°C to 1200°C. The temperature dependence of the yield and flow stresses is found to depend on the
strain rate. At strain rates of 0.001 and 0.1 s−1, the yield stress decreases as the temperature increases, with a plateau between 600°C and 800°C. At strain rates of 35 and
2000 s−1, the yield stress exhibits a positive temperature dependence at temperatures above 600°C; however, postyield flow stresses
exhibit a reduced temperature dependency. The work-hardening rate decreases dramatically with temperature at low and high
temperatures, with a plateau occurring at intermediate temperatures for all strain rates. The work-hardening-rate plateau
is seen to extend to higher temperatures as the strain rate increases. The strain-rate sensitivity at strain rates of 0.1
s−1 and greater is lower than 0.1, although it increases slightly with temperature. At 0.001 s−1, the strain-rate sensitivity increases dramatically at high temperatures (equal to 4.5 at 1200°C). The anomalous (positive)
temperature dependence of the yield stress at high strain rates (>1 s−1) and high temperatures (>600°C) is explained via a dislocation-jog pinning mechanism. The negative temperature dependence of the yield stress at low strain rates (<1 s−1) and high temperatures (>900°C) is though to be due to a thermally activated dislocation-jog climb process in the grain interiors
and/or deformation and recovery processes at/near grain boundaries. The decreased anomalous temperature dependence of the
flow stress at high strain rates and high temperatures is ascribed to dynamic recovery promoted by adiabatic heating.
Z. JIN, formerly Technical Staff Member, Materials Science and Technology Division, Los Alamos National Laboratory 相似文献
5.
Compression testing of a Ti-44Al-llNb alloy was carried out at various temperatures and for different microstructures. Annealing
was done at temperatures from 1000 °C to 1500 °C to produce the unrecrystallized, duplex (gamma grains plus lamellar colonies)
and the fully lamellar microstructures. Samples of each of these microstructures were then tested in air at room temperature
and at various temperatures from 1000 °C to 1300 °C. Results indicate that successively higher temperature anneals produce
increasing grain or colony sizes from 138 jam in the unrecrystallized microstructure to 1017 ώm in the fully lamellar microstructure.
A sequentially lower yield stress was produced on samples tested at increasingly higher temperatures for a given microstructure.
In addition, a minimum yield stress on each yield stressvs temperature curve was recorded for the duplex microstructure with a colony size of 154ώm. One promising result was a sample of this microstructure tested at room temperature, where a yield stress of better than
800 MPa and a compressive strain at the cessation of testing of better than 14 pct were obtained. 相似文献
6.
D. M. Fegredo 《Metallurgical and Materials Transactions B》1972,3(7):1943-1949
Pure polycrystalline zinc specimens of varying grain size and pronounced texture were fatigued at different temperatures from
125°C (T
m = 0.57) to −196°C (T
m = 0.11). Conventional S/N curves were obtained under temperature and grain size conditions which rendered the specimens ductile
in tension, and unconventional, shallow-slope, S/N curves were obtained when the specimens were brittle. Marked grain size
effects appeared for the unconventional fatigue results with the finer grain sizes being superior. The larger grain sizes
were stronger at higher endurances at 125°C. As-machined specimens possessed a deformed, twinned surface layer, the depth
of which increased with increasing grain diameter, and were, in general, slightly superior to the electropolished specimens.
Tensile tests revealed that, for brittle specimens, the fracture stresses were in the vicinity of the fatigue stresses obtained
when the unconventional S/N curves were extrapolated to 10° cycles. Twinning occurred in tension at higher temperatures and
its extent increased with increase in grain size and temperature. The twinning stress at the first significant load drop was
substantially constant at different temperatures for the same grain size but decreased with increasing grain size. The experimental
results are primarily dependent upon the texture; and qualitative explanations are given for the grain size effects observed,
with various types of S/N curves obtained, and the twinning phenomena. 相似文献
7.
T. E. Howson D. A. Mervyn J. K. Tien 《Metallurgical and Materials Transactions A》1980,11(9):1609-1616
The creep and stress rupture behavior of a mechanically alloyed oxide dispersion strengthened (ODS) and γ′ precipitation strengthened
nickel-base alloy (alloy MA 6000E) was studied at intermediate and elevated temperatures. At 760 °C, MA 6000E exhibits the
high creep strength characteristic of nickel-base superalloys and at 1093 °C the creep strength is superior to other ODS nickel-base
alloys. The stress dependence of the creep rate is very sharp at both test temperatures and the apparent creep activation
energy measured around 760 °C is high, much larger in magnitude than the self-diffusion energy. Stress rupture in this large
grain size material is transgranular and crystallographic cracking is observed. The rupture ductility is dependent on creep
strain rate, but usually is low. These and accompanying microstructural results are discussed with respect to other ODS alloys
and superalloys and the creep behavior is rationalized by invoking a recently-developed resisting stress model of creep in
materials strengthened by second phase particles. The analysis indicates that at the intermediate temperature the creep strength
is controlled by the high volume fraction of γ′ precipitates and the contribution to the creep strength from the oxide dispersion
is small. At the elevated temperature, the creep strength is derived mainly from the inert oxide dispersoids.
Formerly at Columbia University. 相似文献
8.
The tensile deformation behavior of the intermetallic compound AuZn has been investigated over a range of temperature, composition, and grain size. Polycrystalline AuZn was found to behave in a ductile manner over the temperature range 77° to 533°K, and the composition range 48.0 to 52.0 at. pct Au, except for the alloys containing less than 49.5 at. pct Au at 77°K, which were brittle. The yield stress and flow stress were found to increase linearly with deviation from stoichiometry over the temperature range 133° to 463°K. A region of temperature independent work hardening was observed for all compositions at temperatures below 300°K. The nonstoichiometric alloys displayed a sharp increase in work hardening rate at temperatures below 150°K also. In addition to the previously accepted slip on the {ll0}(001) system, slip on the {21l}(lll) system has been postulated to account for the observed ductility and high work hardening rate. 相似文献
9.
G. R. Chanani V. F. Zackay Earl R. Parker 《Metallurgical and Materials Transactions B》1971,2(1):133-139
The uniaxial tensile properties of a series of TRIP steels of varying carbon contents and processing histories were determined
over a wide range of test temperatures. The yield strengths at room temperature varied both with the deformation temperature
(over the range 250° to 550°C) and with the carbon content (0.05 to 0.20 pct). Possible reasons for these variations are advanced.
For all steels, the −100°C yield strengths were substantially lower than the 100°C yield strengths. The minima and maxima
in the yield strengths vs temperatures curves were especially pronounced for the steels processed at the lowest deformation
temperatures. Both the rate of work hardening and the elongation were influenced by the strain-induced austenite-to-martensite
transformation. The rate of strain hardening and the rate of production of strain-induced martensite (per unit strain) increased
with decreasing temperature.
Formerly Graduate Student, University of California, Berkeley, Calif. 相似文献
10.
N. Masahashi S. Watanabe S. Hanada K. Komatsu G. Kimura 《Metallurgical and Materials Transactions A》2006,37(5):1665-1673
Laminates of an iron-aluminum alloy (20Al) and three types of steel—chromium molybdenum (CrMo), high carbon (FeCMn), and precipitation
hardening steel with niobium carbide (FeCNb)—were fabricated at 600 °C and 1000 °C by clad rolling based on the compression
stress ratio of 20Al to steel. The laminates fabricated at 600 °C exhibit a deformation microstructure with partial recrystallization,
while those at 1000 °C reveal a refined microstructure. The 20Al layer of all the laminates exhibit a {001}〈110〉 texture,
and the intensity of the texture increases with a decrease in the fabrication temperature and an increase in the reduction.
The bending deformability of a laminate increases with a decrease in the compression stress ratio and by a reduction in the
intensity of the {001}〈110〉 texture. The clad plate is further rolled at room temperature to a thickness of approximately
150 μm, which enables winding without damage. It is concluded that a high-strength steel at high temperatures and a high Al
content in the Fe-Al alloy is beneficial for the fabrication of deformable laminates. 相似文献
11.
R. Mitra N. Eswara Prasad Sweety Kumari A. Venugopal Rao 《Metallurgical and Materials Transactions A》2003,34(5):1069-1088
The elevated-temperature deformation behavior of polycrystalline molybdenum disilicide (MoSi2), in the range of 1000 °C to 1350 °C at the strain rates of 10−3, 5×10−4, or 10−4 s−1, has been studied. The yield strength, post-yield flow behavior comprising strain hardening and serrations, as well as some
of the deformation microstructures of reaction-hot-pressed (RHP) MoSi2 samples, processed by hot pressing an elemental Mo + Si powder mixture and having a grain size of 5 μm and oxygen content of 0.06 wt pct, have been compared with those of samples prepared by hot pressing of commercial-grade
Starck MoSi2 powder, with a grain size of 27 μm and oxygen content of 0.89 wt pct. While the fine-grained RHP MoSi2 samples have shown higher yield strength at relatively lower temperatures and higher strain rates, the coarse-grained Starck
MoSi2 has a higher yield at decreasing strain rates and higher temperatures. The work-hardening or softening characteristics are
dependent on grain size, temperature, and strain rate. Enhanced dislocation activity and dynamic recovery, accomplished by
arrangement of dislocations in low-angle boundaries, characterize the deformation behavior of fine-grained RHP MoSi2 at a temperature of 1200 °C and above and are responsible for increased uniform plastic strain with increasing temperature.
The silica content appears to be less effective in degrading the high-temperature yield strength if the grain size is coarse,
but leads to plastic-flow localization and strain softening in Starck MoSi2. Serrated plastic flow has also been observed in a large number of samples, mostly when deformed at specific combinations
of strain rates and temperatures. 相似文献
12.
Microstructure evolution during annealing of a wrought near-gamma titanium aluminide alloy, Ti-45.5Al-2Nb-2Cr (at. pct), in
the temperature range 1200 °C to 1320 °C was investigated. The mean grain size of the alpha phase as well as the volume fraction
and size of the gamma particles were evaluated as a function of annealing temperature and time. Isothermal annealing at temperatures
above the alpha transus, T
α=1300 °C, led to rapid grain growth of the alpha phase, the kinetics of which could be described by a simple power-law type
expression with a grain growth exponent p=2.3. Alpha grain growth was significantly retarded during annealing at subtransus temperatures (1200 °C≤T≤1300 °C) by the pinning influence of gamma-phase particles. Limiting grain size values predicted by computer simulation models
applicable for high-volume fractions of precipitates/particles were in good agreement with experimental findings. The kinetics
of alpha grain growth in the presence of gamma particles were analyzed, and the results showed that a grain growth exponent
of p≈2.6 could satisfactorily account for the experimental results. 相似文献
13.
B. V. Cockeram 《Metallurgical and Materials Transactions A》2005,36(7):1777-1791
The high-temperature strength and creep resistance of low carbon arc cast (LCAC) unalloyed molybdenum, oxide dispersion strengthened
(ODS) molybdenum, and molybdenum-0.5 pct titanium-0.1 pct zirconium (TZM) molybdenum have attracted interest in these alloys
for various high-temperature structural applications. Fracture toughness testing of wrought plate stock over a temperature
range of −150 °C to 1000 °C using bend, flexure, and compact tension (CT) specimens has shown that consistent fracture toughness
results and transition temperatures are obtained using subsized 0.5T bend and 0.18T disc-CT specimens. Although the fracture
toughness values are not strictly valid in accordance with all ASTM requirements, these values are considered to be a reasonable
measure of fracture toughness. Ductile-to-brittle transition temperature (DBTT) values were determined in the transverse and
longitudinal orientations for LCAC (200 °C and 150 °C, respectively), ODS (<room temperature and −150 °C), and TZM (150 °C
and 100 °C). At test temperatures > DBTT, the fracture toughness values for LCAC ranged from 45 to 175 MPa√m, TZM ranged from
74 to 215 MPa√m, and the values for ODS ranged from 56 to 149 MPa√m. No temperature dependence was resolved within the data
scatter for fracture toughness values between the DBTT and 1000 °C. Thin sheet toughening is shown to be the dominant toughening
mechanism, where crack initiation/propagation along grain boundaries leaves ligaments of sheetlike grains that are pulled
to failure by plastic necking. Specimen-to-specimen variation in the fraction of the microstructure that splits into thin
sheets is proposed to be responsible for the large scatter in toughness values at test temperatures > DBTT. A finer grain
size is shown to result in a higher fraction of thin sheet ligament features at the fracture surface. As a result finer grain
size materials such as ODS molybdenum have a lower DBTT. 相似文献
14.
P. Hidalgo C.M. Cepeda-Jiménez O.A. Ruano F. Carre?o 《Metallurgical and Materials Transactions A》2010,41(3):758-767
The 7075 alloy is an Al-Zn-Mg-Cu wrought age-hardenable aluminum alloy widely used in the aeronautical industry. The alloy
was accumulative roll bonded at 300 °C (573 K), 350 °C (623 K), and 400 °C (673 K), and the microstructure, texture, and hardness
were investigated. Cell/(sub)grain size in the nanostructured range, typical β-fiber rolling texture, and homogeneous hardness through thickness were determined in all cases. Misorientation was different
at each processing temperature. At 400 °C, the presence of elements in solid solution and the partial dissolution of the hardening
precipitates lead to a poorly misoriented microstructure with a high dislocation density and a homogeneous β-fiber texture of low intensity, typical of intermediate degrees of rolling. At 350 °C and 300 °C, highly misoriented microstructures
with smaller dislocation density and intense heterogeneous β-fiber rolling texture are observed, especially at 350 °C, wherein the degree of dynamic recovery (DRV) is higher. Hardness
of the accumulative roll bonded samples is smaller than that of the starting material due to particle coarsening, and it is
affected by solid solution and/or by fine precipitates produced by reprecipitation of the elements in solid solution. 相似文献
15.
Zhenhua Wang Wantang Fu Shuhua Sun Hui Li Zhiqing Lv Deli Zhao 《Metallurgical and Materials Transactions A》2010,41(4):1025-1032
The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900
to 1200 °C) and strain rate range 0.01 to 10 s−1 was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing
strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing
maps, flow instability was observed at higher strain rate regions (1 to 10 s−1) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at
higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower
temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain
size on Z value was obtained. 相似文献
16.
Effect of phase composition and hydrogen level on the deformation behavior of titanium-hydrogen alloys 总被引:4,自引:0,他引:4
Compression tests were carried out on a series of titanium-hydrogen alloys (containing up to 31 at. pct H) over the temperature
range 500 °C to 1000 °C within the α, α + β, and β phase fields. The effect of hydrogen content was studied on the flow stress,
rate of work hardening, and mechanical anisotropy. Hydrogen in solid solution produces significant softening of the α phase
and (α + β) phase mixture, while it induces hardening of the β phase. The occurrence of strain softening was detected in the
(α + β) phase field as well, the extent of which depends on temperature, strain rate, and hydrogen concentration. The work
required to deform the Ti-H alloys is sensitive to the hydrogen concentration and has the lowest value at interstitial levels
that correspond to the (α + β)/β phase boundary at the temperature of interest. 相似文献
17.
The flow properties of β-Zr-Nb (Cb) alloys were investigated by means of compression testing in the strain rate range 10-1 to 10-5 s-1 and from 725 to 1025°C. The flow curves obtained on Zr-Nb alloys containing 10, 15 and 20 pct Nb exhibited flow softening,
and the magnitude of this effect decreased as the temperature was increased. All three alloys also exhibited anneal hardening,
i.e. an increase in flow stress at 825°C with annealing time at 1000°C. Neither the flow softening, nor the anneal hardening
could be associated with environmental effects, as in Zr-Mo alloys, nor could they be attributed to texture changes or to
the occurrence of dynamic recrystallization. On the basis of X-ray and microprobe investigations, as well as grain size measurements,
it is concluded that the anneal hardening is due to the combined effect of grain growth and the formation of solute clusters
during annealing. The occurrence of flow softening is attributed to the destruction of the solute clusters by straining. Stress-strain
curves were also determined for Zr-2.5 pct Nb. Unlike the high Nb alloys, these materials exhibited neither flow softening
nor anneal hardening. The flow stresses were found to be highly strain rate dependent, with stress sensitivities of about
5.5 for yielding and 4.5 for steady state flow. 相似文献
18.
D. M. Moon 《Metallurgical and Materials Transactions B》1972,3(12):3097-3102
The increasing interest in the application of fine wires as high strength structural components,e.g., in high temperature composite materials, makes an understanding of the differences between the creep behavior of large specimens
and that of fine wires desirable. In this investigation, the creep properties of fine wires of recrystallized W-1 wt pct ThO2 were studied over the temperature range between 1800° to 2750°C. In tungsten wires in which the dispersion of fine thoria
particles stabilized a fine grained structure, the stress dependence of the creep rate varies with test temperature and stress.
For test temperatures below 2500°C, a stress dependence ofn ~ 5 was found, indicative of creep deformation due to dislocation climb processes, while for temperatures above 2500°C and
low stresses, values ofn < 2 were obtained, indicative of deformation by grain boundary sliding and diffusional creep processes. In wires which recrystallized
to a large-grained structure, having a large aspect ratio, a high stress dependence of 15 was found when tested at 1800°C. 相似文献
19.
V. K. Nosov O. A. Polyakov Yu. Yu. Shchugorev N. A. Grachev P. A. Nesterov 《Russian Metallurgy (Metally)》2012,(1):82-89
The effect of the initial hydrogen concentration, warm rolling, and vacuum annealing conditions on the formation of the phase
composition, structure, and mechanical properties of rolled sheet workpieces made of a Ti-6Al α alloy is studied. When the
initial hydrogen concentration increases to C
Hini = 0.3–0.9%, the grain size decreases and the phase composition of the alloy is complicated. In the grain size range 27–5
μm, the yield strength of the alloy obeys the Hall-Petch relation with the lattice friction stress σ
i
= 662 MPa. When the initial hydrogen concentration increases, the grain-boundary hardening intensity and the yield strength
increase. At an average α grain size of 5 μm, the yield strength increases from 770 MPa in the alloy with C
Hini = 0.004% to 970 MPa in the alloy with C
Hini = 0.7%. The maximum yield strength (σy = 1064 MPa) is obtained for the alloy with C
Hini= 0.5% after vacuum annealing at 650°C. The conditions and contributions of solid-solution hardening, grain-boundary hardening,
precipitation hardening induced by the formation of the α2 phase, and strain hardening to the total hardening of the alloy are considered. 相似文献
20.
Gerhard Welsch Gerd Lütjering Kanay Gazioglu Wolfgang Bunk 《Metallurgical and Materials Transactions A》1977,8(1):169-177
A commercial Ti−6Al−4V alloy with an equiaxed grain shape was investigated after solution annealing at 810°C and after aging
at 550 and 350°C. Age hardening at both temperatures produced significant increases in Young's modulus and yield strength.
Finely dispersed α2(Ti3Al) precipitates formed within the α phase upon aging at 550°C, but not when aging at 350°C. However, there is evidence of
order, probably of oxygen, in the α grains of specimens which were aged at 350°C. The formation of the ordered Ti3Al precipitates at 550°C and the occurrence of oxygen ordering at 350°C can account for the increases in Young's modulus and
yield strength.
since January 1977 with General Electric Co., Lighting Research Division, Nela Park, Cleveland, OH.
KANAY GAZIOGLU, formerly with DFVLR, is deceased. 相似文献