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1.
G. S. Was J. K. Sung T. M. Angeliu 《Metallurgical and Materials Transactions A》1992,23(12):3343-3359
An experimental program is conducted to determine the role of carbon, chromium, and phosphorus on the intergranular (IG) cracking
behavior of Ni-16Cr-9Fe in 360 °C argon and water. Both constant extension rate tensile (CERT) tests and constant load tensile
(CLT) tests are used to determine the susceptibility to IG cracking. Results show that carbon in solution strongly suppresses
IG cracking behavior through an increased resistance to power-law creep, which promotes failure by the formation and linkup
of grain boundary voids. The mechanical deformation at 360 °C is very time dependent, with slower extension rates resulting
in greater IG cracking and lower elongation due to the longer time afforded the creep process. Although creep-induced grain
boundary fracture is dominant in both water and argon, there is a substantial environmental enhancement in water. Grain boundary
carbides do not appear to play a primary role in the grain boundary deformation process. In both environments, addition of
P to Ni-16Cr-9Fe improves the IG cracking resistance, but chromium depletion has no effect. Results imply that carbon in solution
plays a critical role in strengthening and increasing resistance to creepinduced grain boundary void formation and fracture. 相似文献
2.
The influence of carbon and chromium on the creep and intergranular (IG) cracking behavior of controlled-purity Ni-xCr-9Fe-yC alloys in 360 °C argon was investigated using constant extension rate tension (CERT) and constant load tension (CLT) testing.
The CERT test results at 360 °C show that the degree of IG cracking increases with decreasing bulk chromium or carbon content.
The CLT test results at 360 °C and 430 °C reveal that, as the amounts of chromium and carbon in solution decrease, the steady-state
creep rate increases. The occurrence of severe IG cracking correlates with a high steady-state creep rate, suggesting that
creep plays a role in the IG cracking behavior in argon at 360 °C. The failure mode of IG cracking and the deformation mode
of creep are coupled through the formation of grain boundary voids that interlink to form grain boundary cavities, resulting
in eventual failure by IG cavitation and ductile overload of the remaining ligaments. Grain boundary sliding may be enhancing
grain boundary cavitation by redistributing the stress from inclined to more perpendicular boundaries and concentrating stress
at discontinuities for the boundaries oriented 45 deg with respect to the tensile axis. Additions of carbon or chromium, which
reduce the creep rate over all stress levels, also reduce the amount of IG fracture in CERT experiments. A damage accumulation
model was formulated and applied to CERT tests to determine whether creep damage during a CERT test controls failure. Results
show that, while creep plays a significant role in CERT experiments, failure is likely controlled by ductile overload caused
by reduction in area resulting from grain boundary void formation and interlinkage.
Thomas M. Angeliu, formerly Graduate Student Research Assistant, Department of Materials Science and Engineering, the University
of Michigan, Ann Arbor, MI,. 相似文献
3.
The influence of carbon and grain boundary carbides on intergranular stress corrosion cracking (IGSCC) of controlled-purity
Ni-16Cr-9Fe-xC alloys in 360 °C primary water was investigated using constant load tensile (CLT) and constant extension rate tensile (CERT)
tests. The CLT test results confirmed that carbon in solution decreases the creep rate by several orders of magnitude, while
grain boundary carbides serve to increase the creep susceptibility. Although carbon increases the work hardening rate, it
is demonstrated, using the Bailey-Orowan creep model, that the primary effect of carbon in solution is to delay the recovery
process of climb at the grain boundary, thereby reducing the creep rate. Grain boundary carbides produce a negligible contribution
to the internal stress and may increase the creep rate by acting as dislocation sources. Grain boundary carbide precipitation
increases IGSCC resistance in 360 °C primary water containing 0, 1, and 18 bar hydrogen, providing the highest overall resistance
to both environmentally induced creep and cracking. The magnitude of the beneficial effect of grain boundary carbides is extremely
sensitive to hydrogen overpressure, with the largest influence observed for 1 bar hydrogen. The detrimental effect of hydrogen
on IGSCC shows consistencies with aspects of both film rupture/slip dissolution and hydrogen embrittlement models. 相似文献
4.
The effect of grain boundary misorientation on the intergranular cracking behavior of pure Ni-16Cr-9Fe was assessed by determining
if low-angle boundaries (LABs) or coincident site lattice boundaries (CSLBs) are more crack resistant than general high-angle
boundaries (GHABs) in argon and high-purity water. Cracking susceptibility of boundary types was determined using constant
extension rate tensile tests (CERTs) in 360 °C argon and in deaerated, high-purity water. Annealed samples contained 12 to
20 pct CSLBs, while CSLB-enhanced samples contained 27 to 44 pct CSLBs; GHAB proportions varied accordingly. Cracked boundary
fractions for CSLB-enhanced samples tested in either environment ranged from 0.01 to 0.08, while those for annealed samples
ranged from 0.07 to 0.10, indicating that samples with increased proportions of CSLBs are more crack resistant. No LABs cracked
in either environment. In annealed samples, the proportion of CSLBs that cracked in water was 6.7 pct compared to 1.5 pct
in argon; the proportion of GHABs that cracked in water was 9.3 pct compared to 6.6 pct for argon. Thus, CSLBs are more crack
resistant than GHABs in either environment, and both are more crack resistant in argon than in water. The higher amounts of
cracking and the higher CSLB cracking susceptibility in high-purity water indicate the presence of an environmental effect
on cracking behavior. The beneficial effect of LABs and CSLBs is likely due to the ability of these boundaries to induce slip
in neighboring grains by either transmitting or absorbing and re-emitting lattice dislocations, thereby reducing grain boundary
stresses and the propensity for crack initiation. The results indicate that control of grain boundary proportions can improve
the intergranular stress corrosion cracking susceptibility of pure Ni-16Cr-9Fe.
Formerly Graduate Research Assistant, The University of Michigan. 相似文献
5.
J. K. Sung J. Koch T. Angeliu G. S. Was 《Metallurgical and Materials Transactions A》1992,23(10):2887-2904
The role of chromium, carbon, chromium carbides, and phosphorus on the intergranular stress corrosion cracking (IGSCC) resistance
of Ni-Cr-Fe alloys in 50 pct NaOH at 140 °C is studied using controlled-purity alloys. The effect of carbon is studied using
heats in which the carbon level is varied between 0.002 and 0.063 wt pct while the Cr level is fixed at 16.8 wt pct. The effect
of Cr is studied using alloys with Cr concentrations between 5 and 30 wt pct. The effect of grain boundary Cr and C together
is studied by heat-treating the nominal alloy composition of Ni-16Cr-9Fe-0.035C, and the effect of P is studied using a high-purity,
P-doped alloy and a carbon-containing, P-doped alloy. Constant extension rate tensile (CERT) results show that the crack depth
increases with decreasing alloy Cr content and increasing alloy C content. Crack- ing severity also correlates inversely with
thermal treatment time at 700 °C, during which the grain boundary Cr content rises and the grain boundary C content falls.
Phosphorus is found to have a slightly beneficial effect on IG cracking susceptibility. Potentiodynamic polarization and potentiostatic
current decay experiments confirm that Cr depletion or grain boundary C enhances the dissolution at the grain boundary. Results
support a film rupture-anodic dissolution model in which Cr depletion or grain boundary C (independently or additively) enhances
dissolution of nickel from the grain boundary region and leads to increased IG cracking. 相似文献
6.
7.
A series of electrochemical, immersion, and constant extension rate tests was conducted on samples of Ni-16Cr-9Fe in sodium
tetrathionate at room temperature. Samples were heat treated to produce severe chromium depletion at the grain boundaries.
Titrimetric analysis of the tetrathionate solution, before and after exposure to a sensitized alloy, under an applied cathodic
current shows that the tetrathionate ion is reduced. The species primarily responsible for the observed IGA in immersion tests
and IG cracking in constant extension rate tests is the tetrathionate ion, S4O6
-, although elemental S also causes shallow IGA. The mechanism responsible for the observed IGA and IG cracking in sensitized
Ni-Cr-Fe alloys is stress assisted intergranular attack with the effect of stress being purely mechanical in nature. The degree
of IGA and IG cracking is directly related to the grain boundary chromium content. Samples with less than 5 wt pct Cr at the
grain boundary are rapidly attacked while those with 8 wt pct Cr are less susceptible and 12 wt pct Cr renders the grain boundary
immune to attack. Lower extension rates and higher Na2S4O6 concentrations represent more aggressive conditions for attack. 相似文献
8.
Visit Thaveeprungsriporn Gary S. Was 《Metallurgical and Materials Transactions A》1997,28(10):2101-2112
The objective of this study is to understand and quantify the role of the coincidence-site-lattice boundary (CSLB) population
on creep deformation of Ni-16Cr-9Fe at 360 °C. It is hypothesized that an increase in the CSLB population decreases the annihilation
rate of dislocations in the grain boundary, leading to an increase in the internal stress and a decrease in the effective
stress. The result is a reduction in the creep strain rate. The role of CSLBs in deformation is, thus, to increase the internal
stress by trapping run-in lattice dislocations at the grain boundaries as extrinsic grain boundary dislocations (EGBDs), creating
backstresses on following dislocations rather than annihilating them, as in the case of high-angle boundaries (HABs). The
hypothesis was substantiated by showing (1) that dislocation absorption kinetics differ substantially between a CSLB and an
HAB, and (2) that the CSLB fraction strongly affects the internal stress in the solid. Dislocation absorption kinetics were
measured by comparing EGBD density in transmission electron microscopy (TEM). Results showed that CSLBs contain an EGBD density
which is 3 times higher than HABs at 1.25 pct strain. Internal stress was measured by the stress dip test and was found to
be ≈ 30 MPa higher in the CSLB-enhanced sample. Steady-state creep rates of Ni-16Cr-9Fe in 360 °C argon were also found to
be strongly affected by the grain boundary character distribution. Increasing the CSLB fraction by approximately a factor
of 2 resulted in a decrease in steady-state creep rates by a factor of 8 to 26 in coarse-grain (330 μm) samples and a factor of 40 to 66 in small-grain (35 μm) samples. It is postulated that annihilation of EGBDs only occurs at triple lines where at least two HABs intersect. By
using a geometric relationship to evaluate the probability of EGBDs annihilating at a triple line, the model predicts a non-linear
dependence of the creep rate with CSLB fraction, yielding excellent correlation with measurement. The model provides a physical
basis for measurements which show that increasing the CSLB fraction by only moderate amounts can greatly reduce the steady-state
creep rate in Ni-16Cr-9Fe. 相似文献
9.
The influence of carbon and grain boundary carbides on intergranular stress corrosion cracking (IGSCC) of controlled-purity
Ni−16Cr−9Fe−xC alloys in 360 °C primary water was investigated using constant load tensile (CLT) and constant extension rate tensile (CERT)
tests. The CLT test results confirmed that carbon in solution decreases the creep rate by several orders of magnitude, while
grain boundary carbides serve to increase the creep susceptibility. Although carbon increases the work hardening rate, it
is demonstrated, using the Bailey-Orowan creep model, that the primary effect of carbon in solution is to delay the recovery
process of climb at the grain boundary, thereby reducing the creep rate. Grain boundary carbides produce a negligible contribution
to the internal stress and may increase the creep rate by acting as dislocation sources. Grain boundary carbide precipitation
increases IGSCC resistance in 360 °C primary water containing 0, 1, and 18 bar hydrogen, providing the highest overall resistance
to both environmentally induced creep and cracking. The magnitude of the beneficial effect of grain boundary carbides is extremely
sensitive to hydrogen overpressure, with the largest influence observed for 1 bar hydrogen. The detrimental effect of hydrogen
on IGSCC shows consistencies with aspects of both film rupture/slip dissolution and hydrogen embrittlement models.
J.L. HERTZBERG, formerly Graduate Student Research Assistant, Department of Materials Science and Engineering, University
of Michigan 相似文献
10.
J. Daniel Whittenberger 《Metallurgical and Materials Transactions A》1977,8(7):1155-1163
The room temperature and 1365 K tensile properties and 1365 K tensile creep properties at low strain rates were measured for
several oxide dispersion strengthened (ODS) alloys. The alloys examined included ODS Ni, ODS Ni-20Cr and ODS Ni-16Cr-4J5Al.
Metallography of creep tested, large grain size ODS alloys indicated that creep of these alloys is an inhomogeneous process.
All alloys appear to possess a threshold stress for creep. It is believed that the threshold stress is associated with diffusional
creep in the large grain size ODS alloys and normal dislocation motion in perfect single crystalline ODS alloys. Threshold
stresses for large grain size ODS Ni-20Cr and Ni-16Cr-4J5A1 type alloys are dependent on the grain aspect ratio. Because of
the deleterious effect of prior creep on room temperature mechanical properties of large grain size ODS alloys, it is speculated
that the threshold stress may be the design-limiting creep strength property. 相似文献
11.
G. B. A. Schuster R. A. Yeske C. J. Altstetter 《Metallurgical and Materials Transactions A》1980,11(10):1657-1664
Creep tests were run on Fe-Ni alloys with nominal compositions of 100 pct Ni, 75 pct Ni-25 pct Fe, 50 pct Ni-50 pct Fe, 25
pet Ni-75 pet Fe and 100 pct Fe. Test temperatures were 898, 1073 and 1198 K, and the stress levels ranged from 6.5 to 80.1
MPa—varying with temperature and composition. Tests were conducted in a hydrogen or helium atmosphere, and creep rates, specimen
elongations and rupture lifetimes were recorded. Grain boundary sliding measurements were made on nickel specimens to determine
the fraction of strain due to grain boundary sliding at rupture. An alternating atmosphere test was also conducted on nickel
specimens to see if a change in test atmosphere while the test was in progress would change creep rates. Finally metallographic
studies were made of the fracture surfaces of all the test specimens using a light microscope and SEM. Results of the tests
showed that hydrogen reduced the creep rupture lifetime of the 100 pct Ni and 75 pct Ni-25 pct Fe by as much as 80 pct. The
lower Ni alloys showed little or no effect. The alternating atmosphere test showed no change in the creep rates of the Ni
specimens when the atmosphere was cycled. Grain boundary sliding measurements showed no significant difference in the fraction
of total strain due to grain boundary sliding. Metallography revealed no clear differences between fracture surfaces of specimens
tested in hydrogen or helium. Causes for the observed creep behavior modification are explored. 相似文献
12.
The influence of grain boundary precipitation on the measurement of chromium redistribution and phosphorus segregation in Ni-16Cr-9Fe 总被引:1,自引:0,他引:1
The redistribution of chromium at the grain boundary and the segregation of phosphorus to the grain boundary in Ni-16Cr-9Fe
is measured following thermal treatment at 700 °C for 1 to 100 hours. The addition of carbon to the base alloy results in
the formation of Cr7C3 precipitates at the grain boundary and the formation of a chromium depleted zone in the adjacent matrix. Measurement of the
Cr concentration is affected by the presence of Cr-rich carbides, and a technique of ratioing the Auger signal of the element
of interest to a sum of the signals of elements present in the carbide and the matrix is required to minimize the scatter
in the data. The presence of carbides does not affect the kinetics or extent of phosphorus segregation to the grain boundary,
and there is no evidence of co-segregation of phosphorus with any major alloying element. The free energy of segregation of
phosphorus is determined to be 46.2 KJ/mole at 1100 °C and 40.8 KJ/mole at 700 °C. Results show that the intergranular fracture
path is along the carbide-matrix interface as opposed to through the carbides or some distance into the matrix. These results
permit the calculation of the coverage of the grain boundary with carbides. 相似文献
13.
M. Cornet C. Bertrand M. Da Cunha Belo 《Metallurgical and Materials Transactions A》1982,13(1):141-144
The mechanical behavior of very high purity nickel base alloys of the Inconel 600 type that were simultaneously charged with
hydrogen and deformed in tension was investigated. Experimental results show that this procedure decreases markedly the fracture
strain of the pure 76 pct Ni-16 pct Cr-8 pct Fe alloy; cracks are observed after two to four pct elongation, and the fracture
is completely intercrystalline. Hydrogen embrittlement appears as an intrinsic property of the Ni-Cr-Fe system in the sense
that the grain boundary cohesion decreases when the purity of the alloy increases. The presence of carbon or phosphorus in
the alloys increases grain boundary cohesion. The addition of metallic elements such as antimony or tin has relatively little
effect on intergranular embrittlement. 相似文献
14.
Elaine A. West Michael D. McMurtrey Zhijie Jiao Gary S. Was 《Metallurgical and Materials Transactions A》2012,43(1):136-146
Intergranular cracking of irradiated austenitic alloys depended on localized grain boundary stress and deformation in both
high-temperature aqueous and argon environments. Tensile specimens were irradiated with protons to doses of 1 to 7 dpa and
then strained in high-temperature argon, simulated boiling water reactor normal water chemistry, and supercritical water environments.
Quantitative measurements confirmed that the initiation of intergranular cracks was promoted by (1) the formation of coarse
dislocation channels, (2) discontinuous slip across grain boundaries, (3) a high inclination of the grain boundary to the
tensile axis, and (4) low-deformation propensity of grains as characterized by their Schmid and Taylor factors. The first
two correlations, as well as the formation of intergranular cracks at the precise locations of dislocation channel–grain boundary
intersections are evidence that localized deformation drives crack initiation. The latter two correlations are evidence that
intergranular cracking is promoted at grain boundaries experiencing elevated levels of normal stress. 相似文献
15.
J. H. Schneibel C. L. White M. H. Yoo 《Metallurgical and Materials Transactions A》1985,16(4):651-660
The creep and fracture properties of high-purity Ni-20 pct Cr and Ni-20 pct Cr-0.11 pct Zr alloys are compared at 1073 K in
vacuum. The Ni-20 pct Cr alloy cavitates at the grain boundaries and fractures intergranularly after strains of typically
20 pct. The observed cavity growth rates are in keeping with those predicted. Alloying with zirconium substantially increases
the creep strength and ductility. Creep rupture associated with dynamic recrystallization occurs, and voids are observed only
in heavily necked parts of the samples. In addition to Ni5Zr and ZrO2 inclusions, a Zr4C2S2 carbo-sulfide was identified. Thus, the sulfur-gettering effect of zirconium even at very low residual sulfur levels (20
wt ppm) was confirmed. The zirconium-induced increase in the creep strength is discussed, and the inhibition of creep cavitation
by zirconium is examined within the framework of thermal cavity nucleation. Lowering of the grain boundary diffusivity and
the grain boundary free energy as well as dynamic recrystallization are likely to reduce cavity nucleation and growth rates
in Ni-Cr-Zr and will thus increase its ductility. Finally, the results are used to illustrate the critical importance of minor
alloying additions in constructing and using fracture mechanism maps. 相似文献
16.
The effects of environment and grain size on the steady-state creep and creep rupture properties of a Ni-6 pct W solid solution
are examined by testing in vacuum and commercial purity argon at 5000 psi and 900°C. The steady-state creep rate is found
to decrease with increasing grain size at small grain sizes, both in vacuum and argon, owing to the effects of grain boundary
sliding. At large grain sizes the creep rate is independent of grain size in vacuum and increases with grain size in argon.
It is suggested that the increase in creep rate with increasing grain size is associated with fact that large-grained samples
tested in argon do not reach steady-state before rupture occurs.
Formerly Graduate Student, Stanford University, Stanford, Calif. 相似文献
17.
The effects of internal tritium and helium on the room-temperature tensile properties of a nitrogen-strengthened stainless
steel, forged 21Cr-6Ni-9Mn (NITRONIC 40), were investigated by thermally charging tritium into tensile specimens and aging
for selected times. The precipitation of helium as bubbles on dislocations greatly increased the yield strength, and as a
consequence of dislocation pinning, the deformation mode changed from long-range dislocation motion to deformation twinning.
The tensile specimens exhibited a 90 pct decrease in tensile ductility at 1438 appm 3 He, accompanied by a severe change in
fracture mode from ductile rupture to a dominantly intergranular fracture mode. Grain-boundary facets showed multiple striations
where deformation twins had intersected the boundaries. Twinning began immediately upon yielding, and at small strains, the
microstructure evolved into a fully hardened state, normally observed at 40 pct or greater strain in unexposed or hydrogen-charged
21-6-9. Fracture occurs at low strains in tritium-charged and aged 21-6-9, in part, because helium bubble precipitation causes
the deformed microstructure to evolve to a heavily deformation-twinned state at a lower strain. Helium bubble precipitation
on the grain boundaries may have caused a further loss in ductility. Fracture appeared to nucleate at the intersection of
deformation twins with the grain boundary. 相似文献
18.
The crack growth behavior of a gamma titanium aluminide alloy, K5S, was investigated at 650 °C and 800 °C under constant load
conditions in a nearly fully-lamellar microstructural form. Crack growth at both temperatures ensues at stress intensities
(K) much higher than anticipated from the R curves. At 650 °C, creep crack extension occurs through the formation of microcracks (interlamellar (IL) separation) and
their joining to the main crack tip through ligament fracture. This results in a mainly transgranular (TG) fracture with occasional
IL separation. This process features a rapid initial crack growth but at decreasing growth rate, followed by a nearly no-growth
stage. At 800 °C, crack extension is accompanied by extensive plastic deformation and consists of an initial rapid transition
period and a subsequent steady state. For similar K’s, crack extension and growth rate are greater at 800 °C than at 650 °C, but even these are very slow processes for this
alloy. The resistance to crack propagation at 650 °C is explained in terms of work hardening that arises during the extended
primary creep deformation occurring ahead of the crack tip. Increased crack propagation at 800 °C is accredited to grain boundary
and lamellar-interface weakening and extensive post primary creep damage in the plastic zone. The resulting fracture at 800
°C is mainly boundary fracture, which consists of IG fracture involving formation and coalescence of voids, and IL separation. 相似文献
19.
《Acta Metallurgica Materialia》1991,39(12):3063-3070
High temperature creep behavior of carbide precipitation strengthened Fe-15 Cr-25 Ni alloys with different carbon content have been investigated. Grain boundary carbides obstruct dislocation annihilation at the grain boundary and, therefore, increase the dislocation density near the grain boundary. This gives rise to formation of a hard grain boundary region and significantly increase creep resistance of the alloy. The grain boundary precipitation strengthening and combined matrix/boundary strengthening are modeled following the concept of hard-soft composite structure, and a unified creep equation is derived by taking account of back stress from intergranular carbide particles, “boundary obstacle stress”. The models and analysis show that grain boundary precipitation strengthening is predominant for soft matrix but decreases with the increase of matrix strength, indicating the existence of coupled matrix/boundary strengthening. 相似文献
20.
This paper describes a study carried out at room temperature on an Fe-21 pct Cr-11 pct Ni heat resisting alloy under tensile
and fatigue deformation. Specific microstructures were developed by heat treating the as-received alloy at different temperatures
and times. The surface condition of all specimens displayed surface grain boundary oxidation to a maximum depth of 0.16 mm.
In addition, the microstructure of specimens in one batch (B) contained intergranular chromium carbides. The major conclusions
drawn from this study are that different microstructures respond differently to monotonie and cyclic modes of deformation.
In particular, the embrittling effect of intergranular chromium carbides observed during the monotonie mode of deformation
was different from that found when deformation was cyclic. During cyclic deformation these chromium carbides assisted in reducing
the damaging effects of the surface grain boundary oxidation. Also during cyclic deformation, the overall fatigue life was
found to depend on the mode of both fatigue crack initiation and Stage I crack growth. Fatigue life was reduced when crack
initiation and Stage I crack growth were intergranular while it was enhanced when crack initiation occurred at slip bands
and subsequent Stage I crack growth was transgranular. It was observed that surface grain boundary oxidation is a most deleterious
micro-structural feature especially under fatigue loading but, if this feature is unavoidable then the presence of intergranular
chromium carbides is considered to be highly beneficial in increasing the overall fatigue resistance of the material.
Formerly a Postgraduate Student, School of Materials Science and Engineering, University of New South Wales, Kensington, New
South Wales 2033. 相似文献