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Elevated temperature axial and torsional fatigue behavior of Haynes 188
programme for Mar-M24S Wee also carried out. Owing to the coarea-greined structure, the atrm-atrein response indicates variation from specimen to specimen in some c a m . The effect of this vldation on lifo does not appear significant, except at smaller strain ranges. A numerical method is IIlO devIdOINKI tO estimate the stress field due to a surface Inhomogenelty in an elestlc half space. The technique is hamKI on ElhalWs equivalent inclusion method. The results were used in developing a life prediction methodology for the coated superelloys. Crystallographic fatigue crack g r o w t h in Im:ompatible a l u m i n u m blcryatMs: ]te d ~ on m ~ 0 m d w y Mip. Li, C. Metall. Trana. A Dec. 1992 ~ (12) 3293-3301 The secondary slip behaviour eheed of cwstallngrephic fatigue cracks and its effect on the crack growth near the grain boundaries (GBI) in [12-1] tilt non-aymmatdcel AJ bicrysWle under constant-cycle stress amplitude have been systematically examined. The displacement field eheed of short crack tips near the interfaces in two specimens has been measured by using a microflduclel grid technique, it has been observed that the critical persistent slip hand (PSB) ahead of a short crack tip near the GB in a middle misodented blcrystal was able to develop as long as the primary one and resulted in • temporary stage II growth. As a longer crystallographic crack grew into the grain boundary affected zone (GBAZ), activation of the critical slip ahead of the crack front end crack branching along the critical PSB occurred in all groups of the AI blcrystels, which reveals a crucial role of the cdtical slip in increasing the crack opening and tdggering the slip in the adjacent grain. Conversely, crosa-sitp became the dominant slip mode ahead of the crystallographic crack front near the GB in • bicrystel of larger misfit angles and drove the crack along the cross PSB, • steep path with a remarkably high growth rate, until it propagated into the GBAZ. The resultant stress on the secondary slip system ahead of a crack front near the interface contributed by the intemsi stress due to both intergrenuler and intragrenular incompatible strain, ea well as the enhanced crack tip stress, has been evaluated end rationelizea the activation of the secondary slip systems. Graphs, photomicrographs, 33 ref. Influence o f prolonged t h e r m a l exposure on intergranuler fatigue crack g r o w t h behavior in A l l o y 7111 at 060°C. Zheng, D. and Ghonem, H. Metal/. Trans. A Nov. 1992 23A (11) 3169-3171 The relationship between microstructural changes during prolonged thermal exposure and the fatigue crack growth performance of Alloy 718 st 650~C in laboratory air environment is examined. Graphs, photomicrographs, 13 ref. Elevated t e m p e r a t u r e axial and torsional fatigue b c h a v l o r of H e y n ~
188. Bonacuse, P. J. and Kalluri, S. NASA Technical M e m o r a n d u m TM-105396 1992 24 pp The results of high-tempereture axial and torsional low-cycle fatigue experiments performed on Haynes 188, a wrought cohait-bese suberelioy, are reported. Fatigue tests were performed st 760°C in air on thin-walled tubular specimens at various strain ranges and under strain control. Data are also presented for coefficient of thermal expansion, elastic modulus, and shear modulus at various temperatures from room to 1000°C, end monotonic and cyclic strees-strein curves in tension aod in shear at 760"C. This data set is used to evaluate several multtexiel fatigue life models (most were odginslly developed for room temperature muitiexisi life prediction) including Von Miess equivalent strain range (ASME boiler and pressure vessel code), Manson-Halford, modified muitiexiality factor (proposed), modified Smith-Watson-Topper, end Fatemi-Socle-Kursth. At yon Mieas equivalent strain ranges (the torsional strain range divided by ~/3, taking the Poisaon's ratio to be 0.5), torsionally strained specimens lested, on average, factors two to three times longer than axially strained specimens. The modified muitiexisiity factor approach shows promise es a useful method of estimating torsional fatigue life from axial fatigue data et high temperatures. Several difficulties arose with the specimen geometry and exteosomatry used in these experiments, Cracking st extensomater probe indentations was a problem st smaller strain ranges. Also, es the largest axial and torsional strain range fatigue tests neared completion, a small amount of specimen buckling was observed. Graphs, 13 ref. Damage mechanisms in b i t h e n n a l end thennomenhanical fatigue of
Haynes 188. Kalluri, S. and Halford, G. R. NASA Technical M e m o r a n d u m NASA TM-105381 1992 18 pp Post-failure frectographic and metaliogrephic studies were conducted on Haynes 188 specimens fatigued under bithermal and thermomachanical loading conditions between 316 and 760°C. Bithermal fatigue specimens examined included those tested under high strain rate in-phase and out-of-phese, tensile cresp-in-phese, and compressive creep~ out-of-pheae loading conditions. Specimens tested under inphase and out-of-phase thermomachanicel fatigue were also examined. The nature of failure mode (trensgrenular vs intergrenular), the topography of the fracture surface, and the roles of oxidation end metallurgical changes were investigated for each type of bithermsi and thermomachanicel test. Photomicrographs, 15 ref.
Mad.robin of f u k m m = k m
foaow~ ak~e t e . d e
overloads in p o w d e r m e t o l l u r g y M u m i n u m adioys. Bray, G. H., Sterke,
E. A. Jr, and Reynolds, A. P. Metal/. Trans. A Nov. 1992 23A (11) 3055-3066 In ingot metallurgy (IM) alloys, the number of delay cycles following • single tensile overload typically increeaes from a minimum at an intermedllfle beealine stress intensity range, AKB, with decreasing AKe approaching thnmhold and increasing AKa approaching unstable fracture to produce • chamctedatic U-shaped curia. Two models have been proposed to explain this behaviour. One model is beaed on the interection between roughness- end plleticity-induead closure, while the other model only utJllsas pleaticity-induced closure. These models ere examined using exporimentel results from constant-amplltude and single-ovorloed fatigue performed on two powder memnurgy (PM) AI elloyl, AI-906XL and AAe000. The results indicate that the U-shaped curve is primarily due to pleatlc~IW-Indu¢~Kl closure, and that the plesticiW-Induced retardation effect is through-thicknees in nature, occurring in both the surface and interior regions. However, the retardation effect is greater at the surface, because the increase in plaat~ strain at the crack tip end overload plastic zone size ere larger in the plene-strmm surface regions than in the plans-strain interior regions. These results are not entirely consistent with either of the proposed models. Alloy 20Q0 is also discussed. Graphs, photomicrographs, 24 ref. The role o f annealing t w i n boundaries in t h e cyclic d e f o r m a t i o n o f foc nlatedaic, Lianas, L. and Laird, C. Mater. Sci. Eng. A Sept. 1992 A157 (1) 21-27 Detailed transmission electron microscopy (TEM) studies have been carried out to examine the role of annealing twin boundaries in the cyclic deformation of ramploaded polycrystelline Cu. it is found that regions adjacent to the twine are preferred sites for earlier formation of persistent slip bends (PSBs). Furthermore, eteps at twin boundaries and twin tips enhance early operation of secondary slip, and these features have been identified as critical places for promoting the nucleation of PSBs. Finally, • close relationship is established in high-cycle fatigue between cyclic deformation, in the bulk, and crack initiation, in the surface, in regions close to twins. Such beheviour is correlated with localization of deformation within the PSBs which form eadiest in life, and can be generalized to fcc polycrystels that contain twins. Graphs, photomicrographs, 25 ref. The apldication o f surface science t o fatigue: t h e role of surface chemistry and surface modification in fatigue crack Inltimion in Mlvor single crystals. Sriram, T. S., Fine, M. E. and Chung, Y. W. Acts Metafl. Mater. Oct 1992 40 (10) 2769-2780 The role of surface chemistry in the initiation of fatigue cracks was studied for AO single crystals. Fatigue tests up to crack initiation were carried out on pure AO specimens end specimens surface-alloyed with Au. The ambient used was oxygen. Concurrent investigations of the effect of Au and 0 adsorption on AO(111 )surfacas were carried out using X-ray photoelectron spactroacopy. A strong correlation was observed between the number of cycles to crack initiation and the reactivity of slip steps exposed by the fatigue prOCeSsand the rate of 0 adsorption on the exposed slip steps, it was alas found that 0 adsorbed on the slip steps was transported into subsurface regions during fatigue. When the same test was performed in Br, subsurface transport of Br was not observed; yet the fatigue life up to crack initiation was found to be the same as that in 0. The absence of Br transport is probably due to its larger atomic size. Baled on these studies, it Is concluded that the dominant mechanism for accelerating fatigue crack initiation in an active environment is the reduction of dislocation reversibility by strong adsorption of ambient species on exposed slip steps. Photomicrographs, graphs, 35 ref. Effects o f t e m p e r a t u r e and e n v i r o n m e n t on fatigue crack g r o w t h mechanisms in a 9% c h r o m i u m 1% m o l y b d e n u m steel. Cotterill, P. J.
and Knott, J, F. Acts Metal/. Mater. Oct. 1992 40 (10) 2753-2764 The environmental contribution of laboratory air to fatigue crack growth in a 9% Cr 1% Me steel has been assessed by a compadasn of crack propagation rates in air and vacuum over a range of temperatures (25-625°C). In the Paris regime, growth rates in air are generally higher than those in vacuum, where there is liffie variation of da/dN with temperature. In contrast, the enhancing effect of the air atmosphere on crack growth rates is strongly tam beratu re-dependenL A variety of environmentalassisted crack growth mechanisms are found to be operative at different temperature ranges, end evidence of these is provided by both an analysis of activation energies end a frectogrephic investigation. The situation is different at lower stress intensities, where the threshold stress intensity range falls dramatically with increasing temperature in vacuum, and near-threshold growth rates st 525°C ere higher in vacuum than in air. This effect is attributed to the occurrence of severe oxideinduced closure in air st elevated temperatures, where the crack is blocked with oxide at low stress intensities, reducing the crack driving force to a level below the intrinsic msterisi threshold. Photomicrographs, graphs, 32 ref. A preliminary cheractedzatJon o f t h e tonMle end f a t i g u e behavior o f tungsten-flber/Waspaloy-matrix composite. Corner, R. E. and Larch, B.
Frequency interactions in high-temperature fatigue crack g r o w t h in superalloys. Ghonem, H. and Zheng, D. Metell. Trans. A Nov. 1992 23A (11) 3067-3072 The influence of high-frequency loading on the subsequent low-frequency crack growth behaviour in Ni-beesd Alloy 718 in laboratory air environment st 923 K has been investigated through the use of • sequential high/low frequency load waveform. The parameters that have been examined include the crack growth rate, fracture surface morphology, and slip line density at and below the fracture surface. Results indicate that prior application of high-frequency loading results in reduction of the subsequent low-frequency crack growth rate. An attempt is made to interpret this type of modification as being a result of the crack tip conditioning through the increase in the slip line density during the high-frequency part of the loading cycle. Furthermore, by linking the type of selective oxide formed at the crack tip to the degree of deformation in the crack tip zone, a correlation has been made between the increase in the slip line density in the crack tip zone during the preceding highfrequency loading and the increase of the crack resistance to environment degradation effects during the subsequent low-frequency loading. Graphs, photomicrographs, 18 ref.
A. NASA Technical M e m o r a n d u m NASA TM-105246 1992 18 pp A microstructural study and a preliminary characterization of the room-temperature tensile end fatigue behaviour of a continuous, tungsten-fibre Wespsioy-matrix composite was conducted. A heat treatment was chosen that would allow visibility of planar slip if it occurred during deformation but would not allow growth of the reaction zone. Tensile and fatigue tests showed that the failed specimens contained transverse cracks in the fibres. The cracks that occurred in the tensile specimen were observed at the fracture surface and up to approx 4.0 mm below the fracture surface. The crack spacing remained constant along the entire length of the cracked fibres. Conversely, the cracks that occurred in the fatigue specimen wore only observed in the vicinity of the fracture surface. In instances where two fibre cracks occurred in the same plane, the matrix often necked between the two cracked fibres. Large groups of slip bends were generated in the matrix near the fibre cracks. Slip bends in the matrix of the tensile specimen were alas observed in areas where there were no fibre cracks, at distances > 4 mm from the fracture surface. This suggests that the matrix plastically flows before fibre cracking occurs. Graphs, photomicrographs, 5 ref.
188. Bonacuse, P. J. and Kalluri, S. NASA Technical M e m o r a n d u m TM-105396 1992 24 pp The results of high-tempereture axial and torsional low-cycle fatigue experiments performed on Haynes 188, a wrought cohait-bese suberelioy, are reported. Fatigue tests were performed st 760°C in air on thin-walled tubular specimens at various strain ranges and under strain control. Data are also presented for coefficient of thermal expansion, elastic modulus, and shear modulus at various temperatures from room to 1000°C, end monotonic and cyclic strees-strein curves in tension aod in shear at 760"C. This data set is used to evaluate several multtexiel fatigue life models (most were odginslly developed for room temperature muitiexisi life prediction) including Von Miess equivalent strain range (ASME boiler and pressure vessel code), Manson-Halford, modified muitiexiality factor (proposed), modified Smith-Watson-Topper, end Fatemi-Socle-Kursth. At yon Mieas equivalent strain ranges (the torsional strain range divided by ~/3, taking the Poisaon's ratio to be 0.5), torsionally strained specimens lested, on average, factors two to three times longer than axially strained specimens. The modified muitiexisiity factor approach shows promise es a useful method of estimating torsional fatigue life from axial fatigue data et high temperatures. Several difficulties arose with the specimen geometry and exteosomatry used in these experiments, Cracking st extensomater probe indentations was a problem st smaller strain ranges. Also, es the largest axial and torsional strain range fatigue tests neared completion, a small amount of specimen buckling was observed. Graphs, 13 ref. Damage mechanisms in b i t h e n n a l end thennomenhanical fatigue of
Haynes 188. Kalluri, S. and Halford, G. R. NASA Technical M e m o r a n d u m NASA TM-105381 1992 18 pp Post-failure frectographic and metaliogrephic studies were conducted on Haynes 188 specimens fatigued under bithermal and thermomachanical loading conditions between 316 and 760°C. Bithermal fatigue specimens examined included those tested under high strain rate in-phase and out-of-phese, tensile cresp-in-phese, and compressive creep~ out-of-pheae loading conditions. Specimens tested under inphase and out-of-phase thermomachanicel fatigue were also examined. The nature of failure mode (trensgrenular vs intergrenular), the topography of the fracture surface, and the roles of oxidation end metallurgical changes were investigated for each type of bithermsi and thermomachanicel test. Photomicrographs, 15 ref.
Mad.robin of f u k m m = k m
foaow~ ak~e t e . d e
overloads in p o w d e r m e t o l l u r g y M u m i n u m adioys. Bray, G. H., Sterke,
E. A. Jr, and Reynolds, A. P. Metal/. Trans. A Nov. 1992 23A (11) 3055-3066 In ingot metallurgy (IM) alloys, the number of delay cycles following • single tensile overload typically increeaes from a minimum at an intermedllfle beealine stress intensity range, AKB, with decreasing AKe approaching thnmhold and increasing AKa approaching unstable fracture to produce • chamctedatic U-shaped curia. Two models have been proposed to explain this behaviour. One model is beaed on the interection between roughness- end plleticity-induead closure, while the other model only utJllsas pleaticity-induced closure. These models ere examined using exporimentel results from constant-amplltude and single-ovorloed fatigue performed on two powder memnurgy (PM) AI elloyl, AI-906XL and AAe000. The results indicate that the U-shaped curve is primarily due to pleatlc~IW-Indu¢~Kl closure, and that the plesticiW-Induced retardation effect is through-thicknees in nature, occurring in both the surface and interior regions. However, the retardation effect is greater at the surface, because the increase in plaat~ strain at the crack tip end overload plastic zone size ere larger in the plene-strmm surface regions than in the plans-strain interior regions. These results are not entirely consistent with either of the proposed models. Alloy 20Q0 is also discussed. Graphs, photomicrographs, 24 ref. The role o f annealing t w i n boundaries in t h e cyclic d e f o r m a t i o n o f foc nlatedaic, Lianas, L. and Laird, C. Mater. Sci. Eng. A Sept. 1992 A157 (1) 21-27 Detailed transmission electron microscopy (TEM) studies have been carried out to examine the role of annealing twin boundaries in the cyclic deformation of ramploaded polycrystelline Cu. it is found that regions adjacent to the twine are preferred sites for earlier formation of persistent slip bends (PSBs). Furthermore, eteps at twin boundaries and twin tips enhance early operation of secondary slip, and these features have been identified as critical places for promoting the nucleation of PSBs. Finally, • close relationship is established in high-cycle fatigue between cyclic deformation, in the bulk, and crack initiation, in the surface, in regions close to twins. Such beheviour is correlated with localization of deformation within the PSBs which form eadiest in life, and can be generalized to fcc polycrystels that contain twins. Graphs, photomicrographs, 25 ref. The apldication o f surface science t o fatigue: t h e role of surface chemistry and surface modification in fatigue crack Inltimion in Mlvor single crystals. Sriram, T. S., Fine, M. E. and Chung, Y. W. Acts Metafl. Mater. Oct 1992 40 (10) 2769-2780 The role of surface chemistry in the initiation of fatigue cracks was studied for AO single crystals. Fatigue tests up to crack initiation were carried out on pure AO specimens end specimens surface-alloyed with Au. The ambient used was oxygen. Concurrent investigations of the effect of Au and 0 adsorption on AO(111 )surfacas were carried out using X-ray photoelectron spactroacopy. A strong correlation was observed between the number of cycles to crack initiation and the reactivity of slip steps exposed by the fatigue prOCeSsand the rate of 0 adsorption on the exposed slip steps, it was alas found that 0 adsorbed on the slip steps was transported into subsurface regions during fatigue. When the same test was performed in Br, subsurface transport of Br was not observed; yet the fatigue life up to crack initiation was found to be the same as that in 0. The absence of Br transport is probably due to its larger atomic size. Baled on these studies, it Is concluded that the dominant mechanism for accelerating fatigue crack initiation in an active environment is the reduction of dislocation reversibility by strong adsorption of ambient species on exposed slip steps. Photomicrographs, graphs, 35 ref. Effects o f t e m p e r a t u r e and e n v i r o n m e n t on fatigue crack g r o w t h mechanisms in a 9% c h r o m i u m 1% m o l y b d e n u m steel. Cotterill, P. J.
and Knott, J, F. Acts Metal/. Mater. Oct. 1992 40 (10) 2753-2764 The environmental contribution of laboratory air to fatigue crack growth in a 9% Cr 1% Me steel has been assessed by a compadasn of crack propagation rates in air and vacuum over a range of temperatures (25-625°C). In the Paris regime, growth rates in air are generally higher than those in vacuum, where there is liffie variation of da/dN with temperature. In contrast, the enhancing effect of the air atmosphere on crack growth rates is strongly tam beratu re-dependenL A variety of environmentalassisted crack growth mechanisms are found to be operative at different temperature ranges, end evidence of these is provided by both an analysis of activation energies end a frectogrephic investigation. The situation is different at lower stress intensities, where the threshold stress intensity range falls dramatically with increasing temperature in vacuum, and near-threshold growth rates st 525°C ere higher in vacuum than in air. This effect is attributed to the occurrence of severe oxideinduced closure in air st elevated temperatures, where the crack is blocked with oxide at low stress intensities, reducing the crack driving force to a level below the intrinsic msterisi threshold. Photomicrographs, graphs, 32 ref. A preliminary cheractedzatJon o f t h e tonMle end f a t i g u e behavior o f tungsten-flber/Waspaloy-matrix composite. Corner, R. E. and Larch, B.
Frequency interactions in high-temperature fatigue crack g r o w t h in superalloys. Ghonem, H. and Zheng, D. Metell. Trans. A Nov. 1992 23A (11) 3067-3072 The influence of high-frequency loading on the subsequent low-frequency crack growth behaviour in Ni-beesd Alloy 718 in laboratory air environment st 923 K has been investigated through the use of • sequential high/low frequency load waveform. The parameters that have been examined include the crack growth rate, fracture surface morphology, and slip line density at and below the fracture surface. Results indicate that prior application of high-frequency loading results in reduction of the subsequent low-frequency crack growth rate. An attempt is made to interpret this type of modification as being a result of the crack tip conditioning through the increase in the slip line density during the high-frequency part of the loading cycle. Furthermore, by linking the type of selective oxide formed at the crack tip to the degree of deformation in the crack tip zone, a correlation has been made between the increase in the slip line density in the crack tip zone during the preceding highfrequency loading and the increase of the crack resistance to environment degradation effects during the subsequent low-frequency loading. Graphs, photomicrographs, 18 ref.
A. NASA Technical M e m o r a n d u m NASA TM-105246 1992 18 pp A microstructural study and a preliminary characterization of the room-temperature tensile end fatigue behaviour of a continuous, tungsten-fibre Wespsioy-matrix composite was conducted. A heat treatment was chosen that would allow visibility of planar slip if it occurred during deformation but would not allow growth of the reaction zone. Tensile and fatigue tests showed that the failed specimens contained transverse cracks in the fibres. The cracks that occurred in the tensile specimen were observed at the fracture surface and up to approx 4.0 mm below the fracture surface. The crack spacing remained constant along the entire length of the cracked fibres. Conversely, the cracks that occurred in the fatigue specimen wore only observed in the vicinity of the fracture surface. In instances where two fibre cracks occurred in the same plane, the matrix often necked between the two cracked fibres. Large groups of slip bends were generated in the matrix near the fibre cracks. Slip bends in the matrix of the tensile specimen were alas observed in areas where there were no fibre cracks, at distances > 4 mm from the fracture surface. This suggests that the matrix plastically flows before fibre cracking occurs. Graphs, photomicrographs, 5 ref.