共查询到20条相似文献,搜索用时 15 毫秒
1.
R. J. Charles 《Metallurgical and Materials Transactions A》1976,7(8):1081-1089
The high temperature stress rupture process in polycrystalline solids is examined in terms of grain boundary parting by vacancy condensation at the tip of a crack within the boundary. The gradient for diffusion of these defects is considered in the model to arise from the stress gradient in advance of the flaw tip. A rupture stress function is derived which is shown to closely describe the stress rupture behavior of a number of metals and high temperature alloys. 相似文献
2.
DD6 single crystal superalloy slabs were prepared with seed method in the directionally solidified furnace with high temperature gradient. The transverse stress rupture properties and fracture behaviour of the alloy at 760 ℃/758 MPa, 850 ℃/550 MPa and 980 ℃/250 MPa were investigated and compared with those of longitudinal specimens. The transverse stress rupture lives are corresponding with the longitudinal stress rupture lives at 760 ℃/758 MPa and 850 ℃/550 MPa. The transverse stress rupture lives are slightly less than the longitudinal stress rupture lives at 980 ℃/250 MPa. The fracture mechanism of the transverse stress rupture of the alloy at 760 ℃/758 MPa shows quasi-cleavage mode and the fracture mechanism at 980 ℃/250 MPa shows dimple mode, while the fracture mechanism at 850 ℃/550 MPa shows quasi-cleavage and dimple mixture mode. At higher temperature and lower stress, the microcracks are easier to initiate and interconnect in the transverse specimen than those in longitudinal specimen because there are interdendritic regions perpendicular to the axis of stress. 相似文献
3.
4.
R. Viswanathan 《Metallurgical and Materials Transactions A》1977,8(6):877-884
The creep and stress rupture behavior of a normalized 1.25 pct chromium-0.5 pct molybdenum steel has been investigated over
a temperature (T) range of 510 to 620°C and a stress(σ) range of 65 to 425 MN/m2. The creep rate (
) and time to rupture (t
r
) data have been analyzed in terms of the general expression
ort
r
-A σn exp (Q/RT), whereA is a constant,n is the power exponent of stress,Q is an empirical activation energy for the rate controlling process andR is the universal gas constant. At each temperature, the logarithmic plots of creep rate and time to rupture as functions
of stress consist of two linear segments, separating the data into low stress and high stress regimes. The stress exponent
has approximate values of 4 and 10 in the low stress and high stress regimes respectively in the appropriate expressions for
both creep rate and for time to rupture. The activation energy has values of 367 and 420 kJ/mole in the low stress regime
for time to rupture and creep rate respectively. In the high stress regime, the respective values of activation energy are
581 and 670 kJ/mole. Fractographic observations show that the changes from low stress to high stress behavior in creep rate
and time to rupture approximately coincide with the transition in fracture mode from intergranular to transgranular cracking
as well as with the transition in the rupture ductility from a region of linear variation with stress to one of constant ductility.
These observations suggest that the transition from low stress to high stress behavior may be associated with a change in
deformation mode from predominantly grain boundary sliding at low stress to transgranular matrix deformation at high stress.
Analysis of the creep rate data based on this premise enables calculation of the ratio of the contributions of the grain boundary
sliding mode to the total deformation (ε
gb
/ε
T
) at various values of stress and temperature. Results of this analysis are consistent with numerous experimental observations
reported in the literature. 相似文献
5.
M. P. Puls 《Metallurgical and Materials Transactions A》1990,21(11):2905-2917
A model of slow crack propagation based on the delayed hydride cracking (DHC) mechanism in hydride-forming alloys has been
critically examined and evaluated to take account of recent experimental and theoretical advances in the understanding of
hydride fracture and terminal solid solubility (TSS). The model predicts that the DHC velocity is a sensitive function of
the hydrogen concentration induced in the bulk of the material as a result of the direction of approach to test temperature.
For test temperatures approached from below, factors such as the hydridematrix accommodation energies, the stress state at
the crack tip, and the value of the yield stress have a strong effect on the DHC arrest temperature in the technologically
interesting temperature range of 400 to 600 K. A fracture criterion is explored based on the need to achieve a critical hydride
length in the plastic zone at the crack tip. A necessary condition for DHC is that the crack tip hydride must grow to this
critical length. An approximate estimate is made for the steady-state growth limit of the crack tip hydride as a function
of the direction of approach to temperature and the crack tip stress state. For temperatures approached from below, growth
of the crack tip hydride is limited to just outside the plastic zone boundary at low temperature, gradually receding toward
and inside the plastic zone boundary with increasing temperature. At lowK
I
values, this limits the crack tip hydride lengths to below their critical values for fracture. This could be one condition
forK
IH
. For test temperatures approaches from above, the growth limit is significantly increased, and the sensitivities to the above
parameters become less evident. 相似文献
6.
Zhihao Yao Maicang Zhang Jianxin Dong 《Metallurgical and Materials Transactions A》2013,44(7):3084-3098
Stress rupture behavior and microstructure evolution of nickel-based superalloy Waspaloy specimens from tenon teeth of an as-received 60,000-hour service-exposed gas turbine disk were studied between 923 K and 1088 K (650 °C and 815 °C) under initial applied stresses varying from 150 to 840 MPa. Good microstructure stability and performance were verified for this turbine disk prior to stress rupture testing. Microstructure instability, such as the coarsening and dissolution of γ′ precipitates at the varying test conditions, was observed to be increased with temperature and reduced stress. Little microstructure variation was observed at 923 K (650 °C). Only secondary γ′ instability occurred at 973 K (700 °C). Four fracture mechanisms were obtained. Transgranular creep fracture was exhibited up to 923 K (650 °C) and at high stress. A mixed mode of transgranular and intergranular creep fracture occurred with reduced stress as a transition to intergranular creep fracture (ICF) at low stress. ICF was dominated by grain boundary sliding at low temperature and by the nucleation and growth of grain boundary cavities due to microstructure instability at high temperature. The fracture mechanism map and microstructure-related fracture model were constructed. Residual lifetime was also evaluated by the Larson–Miller parameter method. 相似文献
7.
为了表征9CrMoWV耐热钢在实际应用中的力学行为,对9CrMoWV钢在不同温度和应力条件下进行持久试验,并对持久试样进行OM、SEM、TEM观察,分析不同温度、应力和持久试验时间对9CrMoWV钢显微组织和宏观、微观断口的影响,同时统计Laves相的尺寸变化及其他第二相分布情况。结果表明,随着试验时间和温度的增加,断口韧窝尺寸增大,板条状马氏体回复程度增大,Laves相的尺寸变化明显,分布于晶界的M23C6与Laves相均发生不同程度聚集粗化,这是高温低应力区试样断裂的原因之一。 相似文献
8.
The tensile and stress rupture properties of a Co(Cr)-TaC directionally solidified eutectic alloy have been investigated and
compared to those of a single phase, directionally solidified Co(Cr) alloy corresponding in composition to that of the eutectic
matrix. The temperature for 100 h stress rupture life at 20,000 psi (138 MN/m2) is about 200°F (111°C) better than that of any cast nickel-base superalloy now used in aircraft or land gas turbines. The
degree of superiority becomes progressively less at higher stresses, and at 50,000 psi (345 MN/m2), the temperature for 100 h stress rupture life in the eutectic is about 150°F (83°C) less than for several high strength
superalloys. This behavior is related to a bimodal stress rupture mechanism. A model predicts that at low stresses, failure
is controlled by the stress rupture behavior of the matrix; and at high stresses failure occurs by a stress relaxation mechanism
which causes early fiber failure. Fractographic observations are in agreement with the existence of two stress rupture mechanisms.
It was also observed that both stress rupture mechanisms can occur at the same temperature, with specimens failing by the
fiber-related mode having lives 1 to 2 orders of magnitude in time less than those which fail by the matrix-related mode at
the same stress level.
Both authors were associated with the General Electric Corporate Research and Development Center, Schenectady, N. Y. at the
time this study was performed 相似文献
9.
The specimens of the second generation single crystal superalloy DD6 with different Hf contents were prepared in the directionally solidified furnace with a high temperature gradient. The long term aging of the specimens after full heat treatment was performed at 1040 ℃ for 800 h. The effect of Hf on the microstructure and stress rupture properties under 980 ℃/250 MPa of the alloy after long term aging was investigated. The results show that the γ′ coarsening and rafting and no topologically close packed phase (TCP) are observed in the microstructures of DD6 alloy with different Hf contents after aged at 1040 ℃ for 800 h. It indicates that DD6 alloy with different Hf contents all possesses good microstructure stability. With increasing Hf content the rupture life after long term aging turns shorter and the elongation represents the increasing first and decreasing afterwards. The fracture mechanism of the alloy with different Hf contents at 980 ℃/250 MPa all shows dimple model. The influence of the microstructures on the stress rupture properties of the alloy is also discussed. 相似文献
10.
K. Sadananda 《Metallurgical and Materials Transactions A》1978,9(5):635-641
A theory of creep crack growth has been developed with the presumption that the crack growth occurs by the diffusion of vacancies
along the grain boundaries. This is consistent with many experimental results that show that creep fracture is generally of
intergranular type and the activation energies for crack growth rates fall within the range of grain boundary diffusion energies.
The theory is based on the concept that creep crack growth results from a balance of two competing processes-the diffusion
of point defects that contributes to the growth and the creep deformation process that retards the growth and causes even
its arrest. The present analysis shows that crack growth via grain boundary diffusion occurs within some temperature range.
The upper limiting temperature is determined by the bulk diffusion process which disperses the vacancies, that are diffusing
to the crack tip, to the plastic zone ahead of the crack front. The lower temperature limit is set by the fact that the grain
boundary diffusion rates decrease with the decrease in temperature and thus large stress intensities approaching the fracture
toughness value are required to accomplish crack growth by the grain boundary diffusion. Outside these limits creep crack
growth occurs via deformation which is significantly slower than growth by the grain boundary diffusion process. The importance
of the present analysis rests on the fact that service conditions for many high temperature structural materials fall within
the regime wherein creep crack growth occurs via grain boundary diffusion. 相似文献
11.
12.
R. C. Newman K. Sieradzki H. S. Isaacs 《Metallurgical and Materials Transactions A》1982,13(11):2015-2026
The stress corrosion cracking of a sensitized Type 304 stainless steel has been studied at room temperature using controlled
potentials and two concentrations of sodium thiosulfate. In both constant extension rate and constant load tests, the crack
velocities attain extremely high values, up to 8 μm s-1. Scratching electrode experiments conducted at various pH values on simulated grain boundary material show that both the
crack initiation frequency and crack velocity are closely related to the repassivation rate of the grain boundary material
as expected on a dissolution-controlled mechanism; however, the maximum crack velocity at any potential is consistently about
two orders of magnitude higher than that predicted from the electrochemical data. Frequent grain boundary separation ahead
of the crack tip is thought to occur, but retarded repassivation of the grain boundary material is a necessary feature of
the cracking. Effects of strain-generated martensite are discussed. 相似文献
13.
Dendrite growth is an important phenomenon during steel solidification. In the current paper, a numerical method was used to analyse and calculate the dendrite tip radius, dendrite growth velocity, liquid concentration, temperature gradient, cooling rate, secondary dendrite arm spacing, and the dendrite tip temperature in front of the solid/liquid (S/L) interface for the solidification process of medium carbon steels during continuous casting. The current model was well validated by published models and measurement data. The results show that in the continuous casting process, the dendrite growth rate is dominated by the casting speed. Dendrite growth rate, liquid concentration at the S/L interface, temperature gradient and cooling rate decrease with proceeding solidification and solid shell thickness growth, while other parameters such as dendrite tip radius, secondary dendrite arm spacing, and dendrite tip temperature in front of the S/L interface become larger with solidification progress and solid shell thickness growth. Parametric investigations were carried out. The effects of the stability coefficient, temperature gradient and casting speed on the micro‐structural parameters were discussed. Under the same conditions, higher casting speed promotes coarser secondary dendrite arm spacing and enlarges the dendrite tip radius, while decreasing temperature gradient, reducing the dendrite growth rate and making the solute distribute more uniform. 相似文献
14.
Yo Tomota Shin Daikuhara Shun Nagayama Masanori Sugawara Norihiko Ozawa Yoshitaka Adachi Stefanus Harjo Shigeo Hattori 《Metallurgical and Materials Transactions A》2014,45(13):6103-6117
Three-dimensional microstructure observations, macro- to micro-scopic residual stress measurements by three methods and creviced bent beam SCC tests were performed for Inconel/low alloy steel (LAS) weld samples. The possible reasons for the suppression of SCC crack propagation near the weld interface found at a nuclear power plant were estimated to include the crack branching at the grain boundary (GB) parallel to the interface, i.e., Type II GB, compressive residual stresses in the LAS region and crack tip oxidation in the LAS at the interface. The formation mechanism of Type II GB and stress gradient in individual grains in the Inconel are also discussed. 相似文献
15.
16.
The creep behavior of 0.018 cm diam doped tungsten wire has been studied over a range of stress from 30 to 90 MPa and temperature
from 2400 to 2800 K. Grain aspect ratio (gar) had a strong influence on creep and rupture of the recrystallized wires, and
separated the creep behavior into two regimes with a transitional gar of about 11 between the two. The low gar regime showed
lower strength and characteristics typical of grain boundary sliding. In the high gar regime, properties were independent
of gar, and evi-dence is presented to show that creep is governed by dislocation-bubble dispersion strengthening.
Formerly with the Lamp Business Division of General Electric Company, E. Cleveland, Ohio 相似文献
17.
The nonequilibrium grain boundary state which has a high energy state is the result of absorption of a certain density of
extrinsic grain boundary dislocations (EGBD’s). The equilibrium of such a boundary occurs by annealing at higher temperatures.
A model has been proposed in this paper which assumes that the equilibrium of a nonequilibrium grain boundary involves the
annihilation of EGBD’s by climbvia lattice diffusion of vacancies at the triple points. Due to the stress field of the EGBD’s, there is a vacancy concentration
gradient around the triple points. The profile of the vacancy concentration gradient is derived by assuming a steady state
flux of vacancies. Using this vacancy concentration profile, the expressions for the rate of climb of EGBD’s are derived.
The proposed model predicts that the time required for the equilibration of nonequilibrium grain boundaries is dependent not
only on the annealing temperature but also on the initial density of EGBD’s and the boundary length (which is related to the
grain size). It has also been shown that the equilibrium behavior predicted by our model is in good agreement with the experimental
results obtained for 316L stainless steel. 相似文献
18.
19.
The influence of orientation on the stress rupture properties of nickel-base superalloy single crystals 总被引:1,自引:0,他引:1
The influence of orientation on the stress rapture properties of MAR-M247 single crystals was studied. Stress rupture tests
were performed at 724 MPa and 774 °C where the effect of anisotropy is prominent. The mechanical behavior of the single crystals
was rationalized on the basis of the Schmid factors for the operative slip systems and the lattice rotations which the crystals
underwent during deformation. The stress rupture lives at 774 °C were found to be greatly influenced by the lattice rotations
required to produce intersecting slip, because second-stage creep does not begin until after the onset of intersecting slip.
Crystals which required large rotations to become oriented for intersecting slip exhibited a large primary creep strain, a
large effective stress level at the onset of steady-state creep, and consequently, a short stress rupture life. Those crystals
having orientations within about 25° of the [001] exhibited significantly longer lives when their orientations were closer
to the [001]-[011] boundary of the stereographic triangle than to the [001]-[1l 1] boundary, because they required smaller
rotations to produce intersecting slip and the onset of second-stage creep. Thus, the direction off the [001], as well as
the number of degrees off the [001], has a major influence on the stress rapture lives of single crystals in this temperature
regime.
REBECCA A. MacKAY, formerly Graduate Assistant, Case Western Reserve University, Cleveland, OH
RALPH D. MAIER, formerly Assistant Professor, Case Western Reserve University 相似文献
20.
D. A. Woodford 《Metallurgical and Materials Transactions A》1981,12(2):299-308
Exposure in air in the temperature range 900 to 1100 °C produces a major loss in stress rupture life and ductility of IN738.
The sensitivity to this environmental damage increases with decreasing test temperature in the range 1000 to 700 °C. Oxygen
is identified as the source of the damage for air exposure and indirect evidence supports grain boundary penetration of the
gas to considerable depth. It is argued that oxygen segregation can lead to grain boundary immobilization and unstable intergranular
fracture at intermediate temperatures. It is shown that compositional modifications, particularly boron and hafnium additions,
may reduce the oxygen damage susceptibility, and that a cobalt base coating effectively eliminates the susceptibility. The
relevance of these observations in understanding the effect of test environment on creep-rupture and fatigue crack propagation
is considered. 相似文献