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1.
Marcello Cabibbo Elisabetta Gariboldi Stefano Spigarelli Dario Ripamonti 《Journal of Materials Science》2008,43(8):2912-2921
The microstructural features of INCOLOY alloy 617 in the solution annealed condition and after long-term creep tests at 700
and 800 °C were characterized and correlated with hardness and creep strength. Major precipitates included (Cr,Mo,Fe)23C6 carbides and the δ-Ni3Mo phase. M6C and MC carbides were also detected within the austenitic grains. However, minor precipitates particularly γ′-Ni3(Al,Ti) was found to play an important role. At different exposure temperatures, the microstructural features of the Ni–22Cr–12Co–9Mo
alloy changed compared with the as-received condition. The presence of discontinuously precipitated (Cr,Mo,Fe)23C6 carbides and their coarsening until the formation of an intergranular film morphology could be responsible both for a reduction
in rupture strength and for enhanced intergranular embrittlement. The fraction and morphology of the γ′-phase, precipitated
during exposure to high temperature, also changed after 700 or 800 °C exposure. At the latter test temperature, a lower volume
fraction of coarsened and more cubic γ′ precipitates were observed. These microstructural modifications, together with the
presence of the δ-phase, detected only in specimens exposed to 700 °C, were clearly responsible for the substantially good
creep response observed at 700 °C, compared with that found at 800 °C. 相似文献
2.
Serrated flow in a Ni–Co–Cr-base superalloy was studied in three microstructural conditions (SUB, SUBA, and SUPER) from 25
to 750 °C by tensile test at initial strain rates ranging from 8 × 10−5 to 3 × 10−3 s−1. The results showed that the SUB and SUBA samples had fine grain size of about 9 μm, whereas the SUPER samples had coarse
grain size of about 600 μm. The tertiary γ′ fraction was about 0 in the SUB, 5% in the SUBA, and 15% in the SUPER samples,
respectively. The types and temperature ranges of serration were different in the alloy with SUB, SUBA, and SUPER microstructures.
It is proposed that the tertiary γ′ fraction and size had great effects on the serrated flow of the alloy with different microstructures. 相似文献
3.
Time-temperature dependence of the γ′ phase volume fraction was investigated for a second generation single crystal nickel-based
superalloy exposed to very short high temperature regimes (1,100–1,200 °C). In this temperature range, the dissolution of
the strengthening γ′ phase occurs. Evolution of the γ′ volume fraction in transient regimes has been established for each
temperature and activation energy of the dissolution phenomenon were determined. They directly attest from the activity of
the diffusing species involved during this phenomenon. From these analyses, the volume fraction at equilibrium was established
for the entire temperature range where dissolution occurs. A model, based on a time/temperature equivalence, is proposed to
quantify the γ′ volume fraction dissolved during short high temperature exposure. 相似文献
4.
Haifeng Zhang Ziqiao Zheng Yi Lin Xianfu Luo Jing Zhong 《Journal of Materials Science》2012,47(9):4101-4109
The effects of Ag addition on the microstructure and thermal stability of 6156 Al–Mg–Si–Cu alloy were investigated by means
of hardness measurement, tensile tests, differential scanning calorimetry, and transmission electron microscopy. The results
showed that addition of small amount of Ag to 6156 alloy did not change the precipitation sequence mainly β″ and Q′ strengthening phase but slightly accelerated the age-hardening rate and increased peak hardness at the same aging condition.
The tensile properties enhanced about 30 MPa at the room temperature or thermal exposure at lower temperature (<100 °C). With
the exposed temperature and time increased to 150 °C for 1000 h, almost no difference between the Ag-containing and Ag-free
alloys. When exposed at 200 °C, the tensile strength of Ag-containing alloy became lower than that of Ag-free alloy because
of the coarsening precipitations in matrix and boundary observed by TEM observed. For both alloys, thermal exposure at temperatures
100 °C for long time periods had no significant effect on tensile properties. Loss in strength was small and large with prolonging
the exposure time at 150 and 200 °C, respectively. 相似文献
5.
Open cell Ti6Al4V foams (60% porosity) were prepared at sintering temperatures between 1,200 and 1,350 °C using ammonium bicarbonate
particles (315–500 μm) as space holder. The resulting cellular structure of the foams showed bimodal pore size distribution,
comprising macropores (300–500 μm) and micropores (1–30 μm). Compression tests have shown that increasing sintering temperature
increased the elastic modulus, yield and compressive strength, and failure strain of foams. The improvements in the mechanical
properties of foams prepared using smaller size Ti64 powder with bimodal particle distribution were attributed to the increased
number of sintering necks and contact areas between the particles. Finally, the strength of foams sintered at 1,350 °C was
found to satisfy the strength requirement for cancellous bone replacement. 相似文献
6.
In this study, hypoeutectoid Fe–N binary specimens have been prepared by gas nitriding pure iron in austenite domain at 840 °C.
The slow cooling of these specimens led to the α-ferrite + γ′-Fe4N pearlitic microstructure which is similar to the pearlite in Fe–C binary system. This pearlitic microstructure has been
characterized by electron microscopy. The crystal structure of the γ′-Fe4N nitride has been identified by electron microdiffraction and the Nishiyama–Wassermann (N–W) and near Kurdjumov–Sachs (K–S)
orientation relationships have been found between the α-ferrite and the γ′-Fe4N. 相似文献
7.
E. Çadırlı U. Böyük S. Engin H. Kaya N. Maraşlı M. Arı 《Journal of Materials Science: Materials in Electronics》2010,21(5):468-474
Sn–3 wt% Cu hypereutectic alloy was directionally solidified upward with different growth rates (2.24–133.33 μm/s) at a constant
temperature gradient (4.24 K/mm) and with different temperature gradients (4.24–8.09 K/mm) at a constant growth rate (7.64 μm/s)
in the Bridgman-type growth apparatus. The measurements of microhardness of directionally solidified samples were obtained
by using a microhardness test device. The dependence of microhardness HV on the growth rate (V) and temperature gradient (G) were analyzed. According to these results, it has been found that with the increasing the values of V and G the value of HV increases. Variations of electrical resistivity (ρ) and electrical conductivity (σ) for casting samples with
the temperature in the range of 300–500 K were also measured by using a standard dc four-point probe technique. The variation
of Lorenz coefficient with the temperature for Sn–3 wt% Cu hypereutectic alloy was determined by using the measured values
of electrical and thermal conductivities. The enthalpy of fusion for same alloy was determined by means of differential scanning
calorimeter from heating trace during the transformation from eutectic liquid to eutectic solid. 相似文献
8.
Jinjiang Yu Yanhong Yang Xiaofeng Sun Hengrong Guan Zhuangqi Hu 《Journal of Materials Science》2012,47(12):4805-4812
Smooth and notched specimens of single crystal superalloy DD32 were subjected to rotary bending high-cycle fatigue (HCF) loading
at different temperatures. The experimental results demonstrate that fatigue strengths of the smooth and notched specimens
reach the maxima and the minimum notch sensitivity displays at 760 °C. DD32 alloy exhibits excellent HCF properties compared
to SRR99 alloy under the same test condition. As for the smooth specimens, slip bands moving through γ and γ′ phases as well
as dislocation bowing are the main deformation modes. As for the notched specimens, the deformations are carried out by dislocation
loop bowing and shearing of PSBs mode at intermediate temperatures; at 900 °C, the minimum fatigue strength results from dislocation
climbing deformation and the degradation of γ′ precipitates. The fine secondary γ′ precipitates advantage the recovery of
dislocations and further deformation of the fatigue specimens. 相似文献
9.
Zn–1.26 wt% Al alloy was directionally solidified upward with a constant growth rate (V = 16.6 μm/s) in a wide range of temperature gradients (1.94–5.15 K/mm) and with a constant temperature gradient (G = 5.15 K/mm) in a wide range of growth rates (8.3–500 μm/s) with a Bridgman-type directional solidification furnace. The
microhardness (HV) and tensile strength (σ) of alloy were measured from directionally solidified samples. The dependency of
the microhardness, tensile strength for directionally solidified Zn–1.26 wt% Al alloy on the solidification parameters (G, V) and microstructure parameters (λ1, λ2) were investigated and the relationships between them were experimentally obtained using regression analysis. According to
present results, the microhardness and tensile strength of directionally solidified Zn–1.26 wt% Al alloy increase with increasing
solidification processing parameters and decrease with the microstructure parameters. Variations of electrical resistivity
(ρ) with the temperature in the range of 300–650 K were also measured using a standard dc four-point probe technique for cast
samples. The enthalpy of fusion and specific heat for same alloy was also determined by means of differential scanning calorimeter
(DSC) from heating trace during the transformation from solid to liquid. 相似文献
10.
Bin Tang Hong-Chao Kou Yi-Hong Wang Zhi-Shou Zhu Feng-Shou Zhang Jin-Shan Li 《Journal of Materials Science》2012,47(1):521-529
The kinetics of martensite decomposition in TC21 alloy was investigated at isothermal conditions in the temperature range
500–850 °C. The dilatometry technique was utilized to trace the transformation process for different aging temperatures. Within
the framework of the Avrami theory, the analysis of the experimental data was made by means of the Johnson–Mehl–Avrami (JMA)
equation. A very good correspondence between the calculated and the experimental results was found. The JMA kinetic parameters
obtained from different aging temperatures implied different mechanisms of the transformation. The α″ phase transforms to
α + α″ (rich) at 500 °C and the transformation is incomplete. Further increase of the temperature to 800 and 850 °C results
in directly transformation α″ →α + β. The mechanism of the transformation alters during the course of the transformation for
550, 600, 650, 700, and 750 °C. Moreover, The TTT diagram was constructed for the martensite decomposition in TC21 alloy based
on the dilatometry analysis and JMA theory, respectively. Good agreement between experimental and calculated TTT diagram is
observed. 相似文献
11.
The effects of matrix/precipitate interface states on coarsening of Co and γ-Fe precipitates in a Cu–4 wt.%Co and a Cu–2 wt.%Fe
alloy aged at 500 and 700 °C have been examined by transmission electron microscopy (TEM) observations, electrical resistivity
measurements, and length-change measurements. Analyses of TEM images show that the average radius for coherent/semi-coherent
transition is 6–12 nm for the Co precipitates and 10–20 nm for the γ-Fe precipitates. The coarsening rates of the Co and γ-Fe
precipitates are unchanged by the transitions in coherency of the precipitates. The interface energies γ of coherent Co and γ-Fe precipitates are estimated from data on coarsening alone as 0.15 and 0.27 J m−2. From length-change measurements of the Cu–Co and Cu–Fe alloys during aging, the estimates of the isotropic misfit strains
of Co and γ-Fe precipitates are −0.018 and −0.016 for the coherent interfaces and −0.013 and −0.012 for the semi-coherent
interfaces. Free energy analyses for the coarsening of Co and γ-Fe precipitates reveal that the values of γ of semi-coherent Cu/Co and Cu/γ-Fe interfaces are 0.24 and 0.34 J m−2. 相似文献
12.
David Düsselberg Dominique Verreault Patrick Koelsch Claudia Staudt 《Journal of Materials Science》2011,46(14):4872-4879
High refractive index, colorless polymers with high temperature resistancy are of great industrial interest. In this study,
new sulfur-containing copolyimides with high refractive indices have been synthesized via polycondensation reaction using
4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride (DSDA)
as dianhydride monomers, and 4,4′-diaminodiphenyl sulfide (4,4′-SDA) and 2,3,5,6-tetramethyl-1,4-phenylene diamine (4MPD)
as well as 3,3′-diaminodiphenyl sulfone (3,3′-DDS) as diamino monomers. The resulting copolyimides are amorphous and soluble
in common organic solvents such as dimethylacetamide and N-methylpyrrolidone. They also exhibit very good optical properties such as high transparency (>88% at 589.6 nm) for samples
with a thickness of 120–150 μm. Furthermore, higher refractive indices ranging from 1.64 to 1.72 at visible wavelengths compared
to currently applied polymer materials, which show refractive indices between 1.5 and 1.6, are found. In addition, the copolyimides
synthesized in this study show high thermal stability with glass transition temperatures between 273 and 306 °C. 相似文献
13.
K. Wu G. Q. Liu B. F. Hu F. Li Y. W. Zhang Y. Tao J. T. Liu 《Journal of Materials Science》2012,47(11):4680-4688
The formation mechanism and coarsening behavior of fan-type structures in a new Ni–Cr–Co-based powder metallurgy superalloy
were investigated by means of field scanning electron microscope, transmission electron microscope, electron backscattered
diffraction, and differential scanning calorimetry. The results show that the fan-type structures consist of finger-shaped
γ′ dendrites and γ matrix between them. They nucleate in the chemical segregation regions on grain boundaries and grow by
diffusion. There are three types of solute atoms flow: (a) rapid diffusion along grain boundary; (b) the diffusion from supersaturated
γ matrix to fan-type γ′ phases; and (c) short-distance diffusion from the previous formed γ′ phases at high temperature to
γ′ phases formed at low temperature within the branches of fan-type structures. These γ′ dendrites are perpendicular to grain
boundaries and grow asymmetrically, resulting in grain boundary serration. In addition, the fan-type structures coarsen within
the γ′ depletion zone after the standard aging treatment. 相似文献
14.
Dislocation motion in the early stages of high-temperature low-stress creep in a single-crystal superalloy with a small lattice misfit 总被引:3,自引:0,他引:3
Dislocation configurations at different creep stages (1100 °C and 137 MPa) in a superalloy TMS-75(+Ru) were studied in transmission
electron microscopy (TEM) and the movement path of these creep-produced dislocations could be fully illustrated. Due to the
small value of γ/γ′ lattice misfit, these dislocations cannot glide in the horizontal γ matrix channels by cross slip, but
they mainly move by climbing around the γ′ cuboids. In the primary stage, the dislocations first move by slip in the γ-matrix
channels. When they reach the γ′ cuboids, they move by climbing along the γ′ cuboid surfaces. In the secondary creep stage,
dislocation reorientation in the (001) interfacial planes happens slowly, away from the deposition orientation of 〈110〉 to
the misfit orientation of 〈100〉. The velocity of the reorientation is lower and a perfect γ/γ′ interfacial dislocation network
cannot be formed quickly. This factor results in a large creep rate of the alloy during the secondary creep stage. The path
for dislocation motion during the early creep stages consists of the following sequences: (i) climbing along the γ′ cuboid
surface, (ii) deposition onto the (001) γ/γ′ interfacial plane, and (iii) reorientation from the 〈110〉 direction to the 〈100〉
direction. 相似文献
15.
Microstructural evolution during creep of a hot extruded Al–Cu–Mg–Fe–Ni (2D70) Al-alloy was investigated in this study using
transmission electron microscopy (TEM). The samples for creep test were carried out two-stage homogenization, followed by
extruding. The creep ultimate strength dropped and the temperature increased gradually from 312 to 117 MPa and from 423 to
513 K, respectively. The microstructural observation for the crept samples showed that the S′ phase coarsened with increased
creep temperature and the aging precipitates transformed from S″ phase to S′ phase during creep process. Meanwhile, excess
solute atoms in supersaturated solid solution dynamically precipitated to further form finer S′ phase and S″ phase, which
pinned the dislocations and impeded the dislocation movements. Large amount of dislocations piled up around the micron-scale
Al9FeNi phase, and a lot of dislocation walls were generated along 〈220〉 orientation. S phase accumulates around these defects.
The interaction between dislocations and precipitates was beneficial for the improved performances at elevated temperature. 相似文献
16.
The effect of heat treatments on the creep–rupture properties was investigated on a wrought Ni–Cr heat-resistant alloy at
973 K. Short-time aging (aging for 3.6 ks (1 h) at 973 K) was made on the solution-treated specimens with different grain
sizes. The fine-grained specimen (the grain diameter, d = 45.2 μm) produced by short-time solution treatment exhibited almost the same rupture life and superior creep ductility
as those of the medium-grained specimen (d = 108 μm) produced by normal solution treatment. The fine-grained specimen and medium-grained specimen showed the longer
rupture life compared with the specimen with recommended aging. The principal strengthening of specimens was attributed to
the precipitation hardening by γ′ phase particles. The fine-grained specimen had the highest hardness, and the increase of
the hardness was observed in both the fine-grained and the medium-grained specimens during creep at 973 K. However, coarse-grained
specimen (d = 286 μm) with high-temperature long-time solution treatment exhibited significantly short rupture life owing to insufficient
precipitation hardening after the short-time aging and during creep. Ductile intergranular fracture with dimples occurred
in the fine-grained specimen, while brittle intergranular fracture was observed in the medium-grained specimen and in the
specimen with recommended aging. Both transgranular fracture and brittle intergranular fracture were observed in the coarse-grained
specimen. A simple heat treatment composed of short-time solution treatment and short-time aging is applicable to high-temperature
components of wrought Ni–Cr alloys. 相似文献
17.
Alloy-718 and Udimet alloy 720 are gamma prime strengthened superalloys with excellent mechanical and thermal properties at elevated temperatures, as well as at cryogenic temperatures. The nickel-based alloys were improved to be resistant to creep and become stronger by changing the heat-treatment conditions. The measurement of the specific heat capacity of a nickel-based alloy is a very useful tool to investigate the effect of heat treatment. The specific heat capacity of nickel-based alloys Alloy-718 and Udimet alloy 720 were measured using a differential scanning calorimeter in the temperature range of 100 – 1000 K. The specific heat capacity of the nickel-based alloys increases monotonically with temperature; however, above 800 K, it is strongly dependent on the heat treatment conditions and it is thought to be influenced by the precipitation phase (γ′, γ′′). Optical and scanning electron microscopies are used to investigate the microstructure of the phases. The microstructures of the precipitates are examined.Paper presented at the Seventh Asian Thermophysical Properties Conference, August 23–28, 2004, Hefei and Huangshan, Anhui, P. R. China. 相似文献
18.
Hossein Mohammadi Mostafa Ketabchi Ali Kalaki 《International Journal of Material Forming》2012,5(2):109-119
The thixoforming process is a new method for manufacturing complicated and net shape components through which high strength
materials can be formed more easily. In this study 7075 Al alloy which has low extrudability has been thixoformed by backward
extrusion process. The recrystallisation and partial melting (RAP) route was used to obtain the semi-solid feedstocks for
thixoforming. Microstructural evolution during partial remelting was studied at temperatures for times. Results showed that
a fine and globular microstructure can be obtained by the RAP route. The results showed that high semisolid isothermal temperature
would increase the liquid volume fraction and accelerate the spherical processing of the solid particles. Furthermore at long
holding time, the globular grains coarsened slightly and the average grains size are increased. The experimental results showed
that when the semisolid billet is hold at 580°C with the holding time, less than 30 min, the microstructure of the billet
is composed of spherical grains and remnant liquids, the average grain size are smaller than 100 μm. So the remelted billet
is suitable for thixoforming. In this paper, a back-extruding of 7075 Al alloy with a high solid fraction in the semi-solid
state at 580°C for 10 min was performed. Mechanical properties of thixoformed components at room temperature were examined.
Tempering treatment T6 has been applied after thixoforming to investigate the effects of heat treatment on mechanical properties
of thixoformed parts. The tensile properties and low hardness values in the as-thixoformed 7075 Al alloy were improved by
subsequent heat treatment. Post-forming heat treatment is one of the key parameters for improving the mechanical properties
of thixoformed parts. 相似文献
19.
The Sn–3.5 wt% Ag eutectic alloy was directionally solidified upward with a constant growth rate (V = 16.5 μm/s) at different temperature gradients (G = 1.43–4.28 K/mm) and with a constant temperature gradient (G = 3.93 K/mm) at different growth rates (V = 8.3–500 μm/s) in a Bridgman-type directional solidification furnace. The rod spacings (longitudinal section, λ
L
and transverse section, λ
T
) and mechanical properties (microhardness, HV and ultimate tensile strength, σ
UTS
) of Sn–3.5 wt% Ag eutectic alloy were measured. The dependency of the microhardness, ultimate tensile strength on the temperature
gradient, growth rate and rod spacings were determined. According to experimental results, the microhardness and ultimate
tensile strength of the solidified samples increase with increasing G and V, but decrease with the increasing the rod spacing. 相似文献
20.
Ultrafine-grained (UFG) Al6063 alloy reinforced with 0.8 vol% nanometric alumina particles (25 nm) was prepared by reactive
mechanical alloying and direct powder extrusion. Transmission electron microscopy and electron backscatter diffraction analysis
showed that the grain structure of the nanocomposite composed of nanosize grains (<0.1 μm), ultrafine grains (0.1–1 μm) and
micronsize grains (>1 μm) with random orientations. Mechanical properties of the material were examined at room and high temperatures
by compression test. It was found that the yield strength of the UFG composite material is mainly controlled by the Orowan
mechanism rather than the grain boundaries. The deformation activation energy at temperature ranges of T < 300 °C and 300 °C ≤ T < 450 °C was determined to be 74 and 264 kJ mol−1, respectively. This observation indicated a change in the deformation mechanism at around 300 °C. At the higher temperatures,
significant deformation softening was observed due to dynamic recrystallization of non-equilibrium grain boundaries. The reinforcement
nanoparticles, however, renders the high strength of the material at the elevated temperatures mainly by dislocation pinning. 相似文献