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Fatigue crack growth behavior in inconel 706 at 297 K and 4.2 K
ml-6160188 $3.00+0.00
~cta meroll. Vol.36,No. 7.pp. 1731-1744, 1988 Printed in Great Britain. All rights reserved
Copyright G 1988Pergamon Press plc
FATIGUE CRACK GROWTH BEHAVIOR INCONEL 706 AT 297 K AND 4.2 K
IN
P. K. LIAW and W. A. LOGSDON Metals Technologies Department, Westinghouse R & D Center, Pittsburgh, PA 15235, U.S.A. (Received 18 Mu,r 1987; in
revised form
30 Seplember
1987)
Abstract-The near-threshold fatigue crack growth rate (FCGR) behavior of Inconel706 was investigated at ambient (297 K) and liquid helium (4.2 K) temperatures, respectively. Specimen orientation did not affect the FCGR properties of Inconel 706. At 297 K, a significant influence of R-ratio on the rates of crack propagation was observed while at 4.2 K, the R-ratio effect was minimal. The extent of oxide-induced crack closure was shown to be insignificant in influencing near-threshold crack growth kinetics of Inconel 706 at both temperatures of 297 and 4.2 K. Roughness-induced crack closure was believed to be the dominant mechanism responsible for the influence of R-ratio on the FCGR properties of Inconel706; this was proved quantitatively by direct crack closure measurements conducted at 297 and 4.2 K. A greater degree of roughness-induced crack closure was observed at 297 K than at 4.2 K; this correlated with the more pronounced R-ratio effect at 297 K. Decreasing the temperature from 297 to 4.2 K decreased the growth rates of fatigue cracks in Inconel 706. The effect of temperature on crack propagation behavior increased with increasing R-ratio. Crack closure could not rationalize this temperature effect. Moreover, the increase in material strength or Young’s modulus on cooling from 297 to 4.2K could not totally account for the influence of temperature on the near-threshold FCGR properties. Dislocation dynamics appears to offer a qualitative explanation for this temperature effect. R&am~&Nous avons etudit, a la temtirature ambiante (297 K) et a celle de l’htlium liquide (4.2 K). la vitesse de croissance des fissures de fatigue pms du seuil (VCFF) dans Hnconel 706. L’otientation des echantillons n’a pas d’effet sur la VCFF de l’Inconel706. A 297 K, le rapport R a une influence importante sur les vitesses de propagation des fissures, alors qu’a 4,2 K son effet est minimal. Nous montrons que la fermeture des fissures due a l’oxyde n’a guere d’influence sur la cinetique de croissance des fissures pms du seuil dans I’InconeI 706, que ce soit a 297 K ou a 4.2 K. Nous pensons que la fermeture des fissures induite par la rugosite est le mecanisme dominant, responsable de I’influence du rapport R sur les prop&es de la VCFF de fIncone 706; nous I’avons demontre quantitativement par des mesures directes de fermeture des fissures, malisees a 297 K et a 4,2 K. Le phtnomtne de fermeture des fissures induite par la rugosite est plus marque a 297 K qu’a 4.2 K, ce qui est en accord avec l’effet du rapport R qui est plus prononce a 297 K. Lorsque la temperature dbroit de 297 K a 4,2 K, la vitesse de croissance des fissures de rupture dans 1’Inconel 706 decroit. L’effet de la temperature sur la propagation des fissures augmente lorsque le rapport R augmente. La fermeture des fissures ne semblerait pas pouvoir rational&r cet effet de la temperature. Cependant, l’accroissement de la resistance mecanique ou du module d’Young du mat&au lorsqu’on le refroidit de 297 K a 4.2 K n’expliquerait pas non plus entibrement I’effet de la tempirature sur les propridtts de la VCFF pres du seuil. La dynamique des dislocations semble foumir une explication qualitative de c-et effet de la temperature. %aammenfaaaung--Die Wachstumsrate von Ermiidungsrissen in der Nahe des Bruches wurde an Inconel 706 bei 297 und 4,2 K untersucht. Die Orientierung der Probe beeinflugte das beobachtete Verhalten nicht. Bei 297 K wurde ein deuthcher Einflug des R-Verhaltnisses auf die Ausbreitungsrate des Risses beobachtet, dieser Einflug war minimal bei 4.2 K. Die Oxid-induzierte RiBschlieBung beeinfluBte die Kinetik dieser RiBausbreitung bei beiden Versuchstemperaturen nur unbedeutend. Die durch die Rauhigkeit induzierte RiDschlieDung wird als der bedeutende Mechanismus angesehen, durch den das R-Verhiltnis das Verhalten der Rillausbreitung bestimmt. Dieser Befund wurde mit direkten Messungen der RiBschlieBung bei 297 und 4.2 K quantitativ nachgewiesen. Der Beitrag der Rauhigkeits-induzierten RiDschlieDung war bei 297 K gr6l3er als bei 4.2 K; entsprechend war der Einflug des R-Verhlhnisses bei 297 K such griilkr. Mit absinkender Temperatur sanken zwischen 297 K und 4,2K such die Wachstumsraten der Ermiidungsrisse. Der EinfluB der Temperatur auf das Ausbreitungsverhalten der Risse nahm mit ansteigendem R-Verhihnis zu. Dieser Temperatureffekt konnte mit der RiBschlieBung nicht erkliirt werden. AuBerdem konnte der mit sinkender Temperatur ansteigende Elastizitltsmodul den EinfluB der Temperatur nicht vollsmndig erkllren. Es scheint, als ob die Versetzungsdynamik eine qualitative ErklPrung fib diesen TemperatureinfluB liefert.
1. INTRODUCTION
Near-threshold fatigue crack growth rate (FCGR) behaviour has been a subject of much interest [l-31].
Most of the threshold FCGR research, however, was performed at ambient temperature. Relatively few near-threshold crack propagation studies were conducted at cryogenic temperatures [12-15,25-28, 1731
1132
LIAW
and LOGSDON:
FATIGUE
CRACK
31-381. Furthermore, these cryogenic temperature crack growth investigations were rarely performed at extremely low temperatures approaching that of liquid helium 12%28,31,38], despite the fact that superconducting generators [39] and magnets [40] are designed to operate at temperatures as low as 4.2 K. To ensure the structural integrity of those machines that operate at cryogenic temperatures, it is necessary to develop low-tem~rature, near-thr~hold FCGR properties of the appropriate structural materials. Cryogenic temperature FCGR data are generally difficult to obtain because of the large cryogen and labor expenses as well as the complexity of the experimental setup. Moreover, to utilize and extrapolate these limited FCGR results effectively, it is im~rative that the mechanisms of crack growth at low temperatures be thoroughly understood. In this paper, the near-threshold FCGR behavior of Inconel 706 was examined at ambient (297 K) and liquid helium (4.2 K) temperatures. In addition, c~oge~c-tem~~ture near-threshold crack growth mechanisms were discussed in light of fracture surface characterizations, crack closure measurements and dislocation dynamics models. Specifically, the effects of specimen orientation, R-ratio (R = I’& /P_ , where Pti, and P_ are the minimum and maximum loads, respectively, in a fatigue cycle) and temperature on the near-threshold FCGR properties of Inconel 706 were emphasized.
2. ~XFERIME~AL
2.1. Material The material investigated was a 69.2cm diameter extrusion of Inconel 706. The chemical composition of this nickel-base superalloy in weight percent was O.O4C, O.I6Mn, O.l7Si, 41.4ONi, 15.7Cr, 0.14M0, 0.25 Al, 0.004 B, 0.08 Co, 0.1 Cu, 0.002 Mg, 2.96 Nb, 1.66 Ti and balance Fe. The Inconel706 was solution treated (1339 K), double aged (1033 K. hold 8 h and 922 K, hold 5 h) and air-cooled. The grain size was approximately equal to 150 pm. Tensile properties at 297 and 4.2 K are shown in Table 1. Tensile specimens were machined from the cylindrical extrusion in two loading orientations: radial and circumferential (Fig. 1). For both loading
297 297 4.2 4.2
Orientation Radial loading’ Circumferential loadi& Radial loading” Circumfcrenriat loading’
IN INCONEL
706
directions, decreasing the temperature from 297 to 4.2 K increased the material strength, elongation and reduction in area. Furthermore, at 297 K material strengths for the radial and circumferential loading directions were essentially comparable even though the elongation and reduction in area for the circumferential direction were greater than those for the radial direction (Table I). At 4.2 K, material strength, elongation and reduction in area in the radial direction were somewhat lower than those in the circumferential direction.
2.2. Fatigue testing 2.2.1. ~~~irne~ design. Near-threshold FCGR experiments were conducted at 297 and 4.2 K. Compact-type (CT) specimens, 6.4mm thick (B) by 51 mm wide (W) by 64mm high (2H), were used to develop the FCGR properties. The notch of each CT specimen was orientated in one of the three directions, C-R, R-C and C-L, in compliance with the ASTM Standard Test Method for Plane Strain Fracture Toughness of Metallic Materials (ASTM E399) [41]. A schematic of the specimen layout is presented in Fig. 1. The tensile specimen with a circumferential loading direction was designed for the CT specimens of the C-R and C-L notch orientations while that with the radial direction is for the R-C notch orientation (Fig. 1). 2.2.2. Automated testing technique. A computerized fatigue testing system was used to develop the near-threshold FCGR results. Details of this test technique have been previously described [25,42-431. Briefly, the stress intensity range, AK, was decreased according to the following equation AK = A& exp[c(a - ao)]
(1)
where A& is the initial stress intensity range, c is a negative value with a dimension of reciprocal length, a is the crack length and a, is the initial crack length. The expression for calculating AK for CT specimens can be found in Ref. [44]. In this investigation, the value of c equaled - 0.098 mm-‘. Test frequency was 90 Hz and the R-ratios investigated were 0.1 and 0.8. Crack length was dete~in~ by an unloading compliance technique f44]. A clip-on MTS extensometer (Model 630.05B-61), specially designed for cryogenic
Table I. Tensile properties of INCONEL Test temperature (K)
GROWTH
706 at 297 and 4.2 K
0.2% yield strength
Ultimate tensiie strength
Elongation’
(MPa)
(MPa)
(%)
979 959 1178 1201
II55 1162 1485 1529
9.02 13.08 18.45 23.70
‘For CT specimens of R-C notch orientation (Fig. 1). “For CT specimens of C-R and C-L notch orientations (Fig. 1). %age length = 25.4 mm
Reduction in area
(Oh) 12.90 19.42 19.30 22.92
LIAW and LOGSDON: FATIGUE CRACK GROWTH IN INCONEL 706 Tensile
1733
Specimen
Top View
5 ide View
Fig. 1. Schematic of specimen orientation.
temperature applications, was mounted on the front face of each CT specimen to measure crack opening displacement which, in turn, was converted into crack length by the compliance technique. The FCGR (da/dN) was determined by a seven-point incremental polynomial technique [45]. Data acquisition and analysis were automated. While at 297 K, the FCGR experiments were performed in laboratory air, a special test chamber was designed and utilized for 4.2 K tests, as described below. 2.2.3. Cryostat. At 4.2K, the FCGR tests were conducted using a specially designed cryostat that enclosed the CT specimen in liquid helium gas (Fig. 2). The cryostat was composed of a liquid helium inner chamber and a liquid nitrogen outer chamber that were separated by two Styrofoam filled chambers. To maintain the desired temperature of 4.2 K, liquid nitrogen was first placed in the outer chamber that were separated by two Styrofoam-filled chambers. To maintain the desired temperature of which contained the CT specimen. The helium coolant dispenser served as both a helium distributor and temperature stabilizer. Thermocouples were attached to the specimen to monitor test temperature and to control both the liquid nitrogen and liquid helium flow rates. To save the consumption of the expensive liquid helium coolant, the computerized FCGR tests
were conducted continuously through days and nights without interruption. During the FCGR experiments, crack closure levels were monitored using an unloading compliance technique at both 297 and 4.2 K [21,46,47]. To accomplish these crack closure measurements, strain gages were cemented near the crack plane of each CT specimen. Special strain gages (WK-06-1215TA350 from Micro-Measurement) were utilized for the liquid helium temperature crack closure measurements. Load vs strain curves were periodically recorded to determine crack closure levels. The unloading linear elastic strain was subtracted from the total strain to increase the sensitivity for measuring crack closure loads [46]. Details of the crack closure measurements have been documented in Refs [21, 46,471. 2.3. Fracture surface characterizations 2.3.1. Fractography. Following the fatigue tests, the CT specimen fracture surfaces were carefully examined using scanning electron microscopy (SEM); fracture morphology was determined along the crack growth direction so that the influence of AK level on fracture modes could be evaluated. Furthermore, the effects of R-ratio and temperature on fracture morphology were investigated.
1734
LIAW
and LOGSDON:
FATIGUE
CRACK
L
ChrmberSub.. Fig, 2.
GROWTH
IN INCONEL
706
o-ring WI
Cryostat for low temperature testing.
2.32. Oxide thickness measurements. During the near-threshold FCGR tests, oxide deposits were often observed on the specimen fracture surfaces. These oxide deposits could inguence threshofd crack ~ro~ga~on behavior by the mechanism of oxideinduced crack closure 12,8,13-?O, 483. To quantify the oxide deposits present on the fracture surfaces,
Auger spectroscopy was used to determine the thickness of the oxides. The sputtering rate in the Auger spectrometer was calibrated to a known thickness of tantalum oxide and also by profilometry on sputtered craters. In tbe present Auger spectrometer, one minute of sputtering time was found to correspond to a sputtering depth of 80 A. As argon ions sputtered
LIAW and LOGSDON:
FATIGUE
CRACK GROWTH
4
t
6
,
Inconef h6 lem~rature : Wt K
t,1,1,
706
3.1. Fatigue crack growth rate properties The near-threshold FCGR properties of Inconel 706 at 4.2 and 297 K are presented in Figs 3-4. The levels of threshold stress intensity range (AK,,,) are shown in Table 2. The A& value is defined as the lowest AK level observed in each crack propagation experiment. The near-threshold FCGR data and A& levels are presented in light of the effects of specimen orientation, R-ratio and temperature. 3.i.f. E&et of specimen orientation. The FCGR properties at 297 K in those CT specimens with C-R, R-C and C-L notch orientations are illustrated in Fig. 3. The fatigue crack growth rates for the three orientations were essentially comparable regardless of R-ratio. Similarly, at both R-ratios, the values of A& at 297 K were in~nsitive to specimen o~entation, as shown in Table 2. Figure 4 shows the effect of specimen orientation on crack growth behaviour at 4.2 K. At the R-ratios of 0.1 and 0.8, specimen orientation did not significantly alter the crack propagation rates, which is in agreement with the results at 297 K. Consistently, at the cryogenic temperature, A& at a fixed R-ratio was essentially equivalent for the C-R and R-C notch orientations. 3.1.2. Efect of R-ratio. The effects of R-ratio on the FCGR properties of Inconel 706 are presented in
AK
ik?
8
10
6i 20
I
t
40
,
6080
I ,,,
Specimen Orientation R-Ratio Destgnation
Symbol 0 A
CR Rc
::
a
CL CR RC
a1
4 A
706R5 t R5) 7D6-C2l C21 7w5 ( I,!9 706-12( 128) 706X6 I 268) 0
-
.
.*
R-0.1
I
2
s
I
4
: I
lllll
6
1735
3. RESULTS
oxide debris on the fracture surface, the atomic weight percents of the compositional elements in the oxide were periodically measured and plotted. Therefore, a depth profile of the atomic percents of oxygen and nickel versus the average distance (d) from the fracture surface into the oxide was developed. Average thickness of the oxide deposit was defined as the value of d at which the atomic percent of oxygen equaled that of nickel. [21,22]. 2.3.3. Surface roughness measurements. At threshold levels, surface roughness can introduce roughnessinduced crack closure which greatly influences nearthreshold FCGR behavior [3,8,11, l&18]. Realizing the importance of surface roughness in governing FCGR properties, the levels of roughness were determined using a Surface Analyzer (Surfanalyzer 360 system, Clevite Corporation). On each specimen fracture surface, several roughness profiles were traced and recorded along the crack growth direction. The level of roughness at a fixed AK was defined as the average of the several measured roughness profiles. The roughness recorded in each trace was the arithmetic average (AA) deviation from the mean surface; this is the average of numerous measurements at the heights of the surface peaks and valleys (measured from the mean surface) [49]. The mean surface was defined as a perfect surface that would be formed if all of the roughness peaks were cut off, and used up in filling the valleys below this surface.
2
IN iNCONEL
I
8 10
2ll
I
40
‘Ill,
6D 80‘
AK (MPa m
Fig. 3. Effect of orientation on near-threshold fatigue crack growth behavior at 297 K.
LIAW and LOGSDON:
1736
FATIGUE
CRACK GROWTH
2
I
6
4
8
,
706
t/-iii!
AKfksl 1
IN INCONEL
8 10
40
20
I , , , I,
I
60
,
I
80100
I,,,,
Inconel706 Temperature : 4 2 K 10-7 c
0 A l
g Y
706-R4 I R4) 706x9 t C9J 706R4 I R48) 706c9 i C98)
aa:
Z RC
A
:
Specimen R-RatJo Designation
Orienteth -CR
Symbol -
9:
10-5
Y lo+
10-8 f-
E
P
2 10-7 3
B i
10-9 :
3
co R=Q8 *
R-O.1
0
- lo+ 10-10 :
c
I
1
2
I
I
on
Fig. 4. Effect of orientation
I III 6
4
8
I
near-threshold
AF lksi 1
2
I
I
4
6
,
I ,I,‘,
8
0 A 0
10-7 5
1 I I I I 60
lo+
80100
m 20
10
( K) R-Reti0
2
A
60
40
I
I
Temwature
I 40
fatigue crack growth behavior at 4.2 K.
Inconel706 Orientetlon: CR Symbol
1
10 20 AK tMPa di%iii)
,
80 100
I ,‘,I
Speclmen Designation
D.1
706R5
0.1 0.8
706R4 706-12 I(R41 128)
0.8
706-R4 ( R48)
: 1o-5
( R5)
-z 10-6
Ak
p 10%: L:
5 ,K -_ 1O-7 g
e E i 3
10-9 e 241 K R=(LE
: 10-8
lo-lo c F
I 2
I 4
I
I1111
6
8 10 AK (hIPa m
I
20
Fig. 5. Effects of load ratio and temperature on near-threshold orientation.
I
I 40
1I1IaL
60
80100
lo-9
fatigue crack growth behavior
in CR
LIAW and LOGSDON:
1
2
I
I
I
FATIGUE
4
,
CRACK GROWTH
AK(ksim 8 10
6
I,‘,‘,
a0
Inconel706 : RC
OrienWion
Tempetduref
5yllhOi
I
I
K)
::
z 42
0.1 0.8
aa
60
,
I ,I,,
1737
706
80100
Specimen Designation
R-Ratlo
291
IN INCONEL
706x2 706-Q 7Ow9 70639
f c21 t 2681 t C9) l C98)
A
2% K ,,t R=Q8 >
Fig. 6. Effects
A
of load ratio and temperature on near-threshold
fatigue crack growth behavior
in
RC
orientation.
Figs 5 and 6 for the C-R and R-C notch orientations, respectively. At 297 K and in the C-R orientation, increasing the R-ratio from 0.1 to 0.8 signifkzantly increased the rates of crack propagation. Correspondingly, the value of AK,,, at R = 0.1 is 1.9 times greater than that at R = 0.8. Furthermore, decreasing the AK level increased the influence of R-ratio on crack growth rates, which is characteristic of nearthreshold crack propagation behavior. Interestingly, at 4.2 K the crack growth rates were relatively insentive to R-ratio. For instance, at 4.2 K the value of AK,,, at R = 0.1 is only 1.2 times greater than that at R = 0.8. Thus, the effect of R-ratio on crack propagation behavior at 4.2 K is much less significant than at 297 K.
Results for the R-C orientated specimens are shown in Fig. 6. At 297 K, a significant effect of R-ratio on the rates of crack growth was observed while at 4.2 K, the influence of R-ratio was Mornay. Consistency, the ratio of AK,,, at R = 0.1 to that at R = 0.8 was lowered from 2.1 to 1.2 by decreasing the temperature from 297 to 4.2 K. Therefore, in the R-C notch orientation decreasing the temperature significantly reduced the R-ratio effect, as was also observed in the C-R notch orientation. 3.1.3. Effect of temperature. The influence of temperature on the FCGR properties of Inconel706 is also illustrated in Figs 5 and 6 for the C-R and R-C notch orientations, respectively. For both orientations at R = 0.1, decreasing the temperature
Tabk 2. Threshold stress intensity range (A&,), efftxWc threshold stress intensity range (A&.#),
tip opening dispkwment
&c&men dcsignatian 706*RS(RS) 706-12(128) 706~C2(C2) 706~CW268) 706-LS(LS) 706-R4(R4) 706-R4(R48) 706~Cs(C9) 706-cXC98)
TcS( temperature W) 297 297 297 297 297 4.2 4.2 4.2 4.2
R-ratio 0.1 0.8 0.1 0.8 0.1 0.1 0.8 0.1 0.8
oxide thickness at threshold and crack at threshold of Inconel 706 al 297 and 4.2 K
Notch orientation z::: R-C ::: :I:: R-C R-C
(h&t&)
(h&F&)
11.8 6.3 12.1 5.7 11.9 16.9 23.9 15.2 12.9
5.9 6.3 5.1 5.7 5.9 14.2 13.9 12.5 12.9
Oxide thickness (A) 64 54 30 :;: : 14 I2
Cyclic crack tip opening displacement (A)
Max. crack tip opening dispiacemcntb (A)
1721 491 1773 393 1750 2562 1733 2113 1522
4249 24528 4377 19668 4322 6327 a6674 5218 76109
“cyclic crack tip opening dispIa_t = 0.49 AK,,,*&E, where try is yield strength and E is Young’s modulus t&r]. bMaximum crack tip opzaing diit~ent = 0.49 K~~_z/ula,E, whm &,,, is maximum stress intensity at threshold [64].
1738
LIAW and LOGSDON:
FATIGUE
CRACK GROWTH
IN INCONEL
706
from 297 to 4.2 K decreased the crack propagation rates and increased AK&. Moreover, the difference in the crack growth rates at 297 and 4.2 K increased with decreasing AK level. At R = 0.8, the influence of temperature on the FCGR properties was much greater than that at R = 0.1 (Figs 5 and 6). For example, the ratio of AK,,, at 4.2 K to that at 297 K increased from 1.4, to 2.2 by increasing the R-ratio from 0.1 to 0.8 for the C-R notch orientation; the ratio changed from 1.3 to 2.3 for the R-C notch orientation. Thus, increasing the R-ratio amplified the influence of temperature on the FCGR properties of Inconel 706. 3.2 Fracture surface characterizations 3.2.1. Fractography. The SEM photos of fracture surfaces at 297 and 4.2 K are shown in Figs 7-10 (Figs 7-9 are for 297 K and Fig. 10 is for 4.2 K). At 297 K and R = 0.1 (Figs 7 and 8), crystallographic fracture facets were observed in the near-threshold AK range investigated [approximately from 20 to 11 MPa & (Fig. 3, 7 and S)]. The existence of c~stallo~p~c fracture facets has been found to be cha~cte~stic of nor-threshold crack propagation behavior [3,8, 10, 11, N-54]. Interestingly, clear-cut crystallographic facets became more pronounced as the value of AK de-creased (Figs 7 and 8). This trend indicated more distinct crystallographic crack growth with decreasing AK level. Moreover, surface rough-
(al
(bl
Low magnificaiion
Higher
mcgnificotion
Fig. 7. Fracture morphology at 297 K (AK = 20 MPa fi and R =O.l).
(bl
Higher magnification
(near-threshold)
Fig. 8. Fracture morphology at 297 K(AK = 11.5 MPa & and R =O.l).
ness associated with crystallographic fracture increased with decreasing AK values. The higher-magnification photos in Figs 7 and 8 indicated the presence of slip lines on the facet fracture surfaces. Previous investigation [SO,511 reported that the fracture facets of Inconel 706 were typi~lly along the (111) plane. At R = 0.8, crystallographic crack propagation was also visible (Fig, 9). The fracture modes seem to be independent of the AK level investigated. At R = 0.8, the presence of clear-cut crystallographic fracture was, however, not as pronounced as that at R = 0.1. Moreover, fracture surfaces at R = 0.8 appear to be smoother than those at R = 0.1. At 4.2 K, crystallographic crack growth was also observed and the fracture morphology was found to be insensitive to R-ratio and AK level. Slip lines were visible on fracture facets. At 4.2 K, the extent of clear-cut cr~tallographic fracture was much less significant than at 297 K. Furthermore, the fracture surfaces at 4.2 K were much smoother than those at 297 K. 3.2.2. Oxide thickness. The thicknesses of oxide debris present on the fracture surfaces were measured at threshold levels using Auger spectroscopy (see Table 2). At 297 K, the oxide deposits were fairly thin, ranging from 14 to 64 A in thickness. Increasing the R-ratio from 0.1 to 0.8 seemed to result in a decrease in oxide thickness. At 4.2 K, the measured oxide thickness ranged
LIAW and LOGSDON:
(0)
(b)
Low mognificotion
Higher
mognificotion
FATIGUE
CRACK GROWTH IN INCONEL
[near-threshold)
(nsor-threshold)
(0)
Low
(b)
Higher
mognificotion
mognificotion
706
1739
(near -threshold)
(near- threshold)
Fig. 9. Fracture morphology at 297 K(AK = 6.6 MPa ,/% and R =0.8).
Fig. 10. Fracture morphology at 4.2 K (AK = 19.1 MPa fi and R =O.f).
from 12 to 28 A which, as expected, was smaller than at 297 K. Oxide thickness at 4.2 K were found to be relatively insensitive to R-ratio. 3.2.3. Surface roughness measurements. Surface roughnesses, measured using a surface analyzer, are presented in Fig. 1I. At 297 K, the levels of surface roughness were typically much greater than those at 4.2 K. This trend was consistent with the rougher fracture surfaces observed at 297 K than at 4.2 K, as shown in Figs 7-10. Surface roughness at 297 K generally increased with decreasing AK levels (Fig. 11). This behavior was in compliance with the more pronounced crystallographic facet fracture at lower AK levels (Figs 7 and 8). At 4.2 K, surface roughness was insensitive to AK (Fig 11), which is in agreement with the insensitivity of the fracture mode to AK (Fig. 10). In Figs 7-11, it is interesting to note that decreasing the temperature from 297 to 4.2 K decreased the fracture surface roughness of Inconel 706. Other investigators [50] previously observed a decrease in the surface roughness of nickel-base alloys with a corresponding increase in the temperature from 297 to 873 K, Because cross slip is a thermally activated process, it has been suggested [!iO] that the ease of cross slip at higher temperatures reduce the distance of planar slip, thereby resulting in smoother fracture surfaces with increasing temperature. This argument would not be applicable, however, to the present
observation. Decreasing the temperature from 297 to 4.2 K makes cross slip more difficult which should, in turn, result in rougher fracture surfaces at 4.2 K; this trend is in contrast with the current finding (Figs 7-l 1). Consequently, the reason for the comparatively rougher fracture surfaces at 297 K than at 4.2 K is not understood at the present time; further research is needed to explain this phenomenon. 4. DISCUSSION The FCGR properties of Inconel 706 have been developed at 297 and 4.2 K. Specimen orientation produced little effect on crack propagation behavior.
R=O.l
Fig. 11 Surfaceroughness profiles at 297 and 4.2 K.
1740
LIAW and LOGSDON: FATIGUE CRACK GROWTH IN INCONEL 706
R-ratio and test temperature, however, influenced the rates of crack growth. In the following sections, these effects are discussed in light of oxide thickness, surface roughness and crack closure measurements. 4.1. Effect of R-ratio At 297 K, the influence of R-ratio on the growth rates of fatigue cracks in Inconel 706 is significant (Figs 5 and 6). The R-ratio effect at 4.2 K, however, is minimal. It has been reported that the influence of R-ratio on near-threshold FCGR properties can be rationalized by oxide and/or roughness-induced crack closure [5,21,22,55]. At lower R-ratios, the higher crack closure levels decrease the effective stress intensity ranges (A&), where AK8 = & - K&,,, and & and k;lOSurcare the maximum and crack closure stress intensity ranges, respectively; this yields slower crack propagation rates with decreasing R-ratio. At threshold levels, the presence of oxide-induced crack closure is supported by the fact that the thicknesses of oxide deposits are comparable to the crack tip opening displacements (CTOD) [2,5,21-301. These sizable oxide deposits wedge the crack tip and elevate crack closure levels, thereby resulting in oxideinduced crack closure. In the subject Inconel 706, oxide thicknesses at 297 and 4.2 K are fairly thin, and are approximately one to three orders of magnitude smaller than the CTOD, as presented in Table 2. Therefore, the extent of oxide-induced crack closure is minimal at both temperatures.
On the other hand, surface roughnesses are approximately one to three orders of magnitude greater than the CTOD (Table 2 and Fig. 11). Consequently, roughness-induced crack closure is suggested as the dominant mechanism responsible for the influence of R-ratio on the FCGR properties of Inconel 706. At 297 K, the rough fracture surfaces at R = 0.1 (Figs 7-9, 11) introduce crack closure and decrease A&. At R = 0.8, crack closure is insignificant because of the high mean load present at the crack tip, which prevents crack closure. Furthermore, the higher levels of surface roughness at R = 0.1 compared with those at R = 0.8 will elevate crack closure levels more significantly at R = 0.1, which, in turn, gives a smaller A& at R = 0.1. These trends result in slower crack propagation rates at R = 0.1 than at R = 0.8. At 4.2 K, the levels of surface roughness at R = 0.1 are much lower than those at 297 K. At R = 0.1, the much smoother fracture surfaces at 4.2 K compared with those at 297 K produce less crack closure (as also proved later by direct crack closure measurements), which, in turn, yields a decreased R-ratio effect at 4.2 K relative to the results at 297 K (Figs 5 and 6). Therefore, roughness-induced crack closure offers an explanation for the influence of R-ratio on the near-threshold FCGR behavior of Inconel 706. The fact that roughness-induced crack closure rationalizes the influences of R-ratio on the FCGR properties of Inconel706 can be further substantiated by direct crack closure measurements. In Figs 12 and
AK,,, ( kd
1
e I
I
4 ,
Inconel706 Orientation : CR
6 ,,I,,
m
8 10
20 I
Specimen Temperature(K) R-Ratio Designation 291 :i 706-R5(R5)
Symbol
0 0
E
A
42
:
:: Lcp A
C\ A
241K
-
Rr(Lland(l8 1 -
lo-lo r
10-5
706-12( 128) 706R4( R4) 706R4 l R48)
A
1o-7 F
40 HI en loo , I,‘,’
I
-5 lo+
42K
1 R=O.landas
Y lO"B
A 2
I
2
I
I,
4
Fig. 12 Crack growth rate (&/dN)
A 111, 6
8 10 AKd(MPa
I
20 m
I
40
vs effective stress intensity range
I
I,l,L
60
8Olul
lf9
(A&) in CR orientation.
LIAW and LOGSDON:
FATIGUE
CRACK
GROWTH
IN INCONEL
706
1741
AKeff ( ksi r/in) 1
2 I
I
4 ,
Inconel7Of1 Orientetion : RC
8
10
20 I
I
40
60 I,
Specimen Tempereture( Kb R-Ratio Designation
Symbol
10-I E-
6 f,I,1,
297 291 42 4.2
0
A 0 A
;
706-C2( C21 706C6 ( 2681 706-c9 ( C9) 706x9 ( C981
0.1 0.8 E
80 loo (,(
1o-5
= 10-6
0 z
lo-* F
0”
u” -Z :
rf
0 0
_
c D 0” lo-7 $
2 zi
2
10-9 F
#
b
: 1o-8
A
r
F
4.2K R=O.land0.8
f A
2¶ K R=O.land(18 if10-10
__(
I 2
I
I, 4
I, 6
I 8 10 2u AK,,, (MPa \riii)
I 40
I I I I 6 I 10-9 60
Fig. 13. Crack growth rate (da/dN) vs effective stress intensity range (A&)
80 100
in RC orientation
13, the rates of crack growth at R = 0.1 and 0.8 are ness-induced crack closure has been shown to be plotted versus AK,, in the C-R and R-C notch proportional to the levels of both surface roughness orientations, respectively. At both temperatures, the and slip irreversibility (or Mode II displacement) [8]. crack propagation rates at R = 0.1 and 0.8 are As mentioned before, the comparatively higher levels essentially identical when plotted versus AIcff regardof surface roughness at 297 than at 4.2 K have less of specimen orientation. Moreover, at a given contributed to the greater extent of roughnesstemperature and orientation, the values of effective induced crack closure at 297 K. It has been suggested threshold stress intensity range (AK,h.eR), where that the extent of slip irreversibility be related to test A&. c~ = Kth.max- Kh. cIosurc.and L manand Kth.closure environment [17,56,57]. In inert (vacuum or low are the maximum and crack closure stress intensities, temperature) environments, slip irreversibility is less respectively, are insentive to R-ratio (Table 2). Thus. significant than in oxidized environments [ 17,56,57]. crack closure can be used to explain the effect of Therefore, the greater extent of oxidation at 297 K R-ratio on the growth rates of fatigue cracks at each than at 4.2 K (Table 2) can be expected to result in temperature. greater slip irreversibility at 297 K, which also conIn Fig. 14, the ratios of Kc,,,,,, to Km,, at 297 and tributes to more significant crack closure at 297 K. 4.2 K are plotted as a function of AK. The levels of crack closure at 297 K are much greater than those at 4.2 K. This trend is consistent with the much greater surface roughness observed at 297 K (Figs 711). Moreover, at 297 K, crack closure levels tend to increase with decreasing AK, which is in agreement with the rougher fracture surfaces at lower AK values (Figs 7, 8 and 11). At 4.2 K, crack closure levels are insensitive to AK. This behavior is in compliance with the insensitivity of surface roughness to AK (Figs 10 and 11). These agreements between the crack closure and surface roughness results strongly support the fact that roughness-induced crack closure is the important mechanism governing the near-threshold AK lMPa V-d FCGR behavior of Inconel 706. Fig. 14. Crack closure results at 297 and 4.2 K. It is worth mentioning that the extent of rough-
I742 4.2.
LIAW and LOGSDON:
FATIGUE
Effect of temperature Decreasing the temperature from 297 to 4.2 K decreases the near-threshold FCGR properties of Inconel 706 at R = 0.1 and 0.8 (Figs 5 and 6). The influence of temperature on crack growth rates is much greater at R = 0.8 than at R = 0.1. In the higher temperature regions ( 2 297 K), it has been reported that oxide and/or roughness-induced crack closure can be used to explain the effect of temperature on the rates of near-threshold crack propagation [24,29,58-61]. As explained below, the concept of crack closure, however, cannot rationalize the effect of temperature on the near-threshold FCGR properties of Inconel706 in the temperature range of 297 to 4.2 K. Firstly, if crack closure could explain the influence of temperature on crack propagation behavior, it would be expected that at R = 0.8 the difference in crack growth rates between 297 and 4.2 K would be negligible since crack closure is minimal at this high R-ratio. This predicted trend is in direct contrast with the experimentally observed data, which shows a significant temperature effect on the rates of crack propagation at R = 0.8 (Figs 5 and 6). Secondly, if crack closure was the reason for the temperature effect, it would be expected that at R = 0.1, the growth rates of fatigue cracks in Inconel706 at 297 K would be slower than those at 4.2 K since crack closure (or surface roughness) levels are greater, and thus, AKcffis smaller at 297 K (Figs 11 and 14). This trend was not observed experimentally in Figs 5 and 6. Thirdly, if crack closure could explain the effect of temperature on the near-threshold FCGR properties of Inconel 706, no difference in the plot of crack propagation rates versus AhR would be observed at 297 and 4.2 K. In contrast, crack closure measurements demonstrate that by using AKeR,a large difference in the crack growth rates at 297 and 4.2 K still exists (Figs 12 and 13). Thus, crack closure cannot explain the effect of temperature on the rates of near-threshold crack propagation in Inconel 706. In the plots of da/dN vs A&, the crack propagation rates at 4.2 K are much slower than those at 297 K. Furthermore, AKth.cR at 4.2 K is approximately 2.3 times greater than at 297 K (Table 2). Thus, after taking crack closure into consideration, different FCGR properties at 297 and 4.2 K still exist. This trend indicates that there is an intrinsic temperature effect on the near-threshold FCGR behavior of Inconel 706. Recall, the yield and ultimate strengths of Inconel 706 increase by approximately twenty percent as the temperature is decreased from 297 to 4.2 K (see Table 1). Based on the literature data, Young’s modulus increases by no more than ten percent when cooling from 297 to 4.2 K [62]. Therefore, the variations in material strength or Young’s modulus between 297 and 4.2 K cannot totally account for the much greater differences in AK,h at these two temperatures. It is suggested that dislocation dynamics account
CRACK
GROWTH
IN INCONEL
706
for the effect of temperature on the FCGR properties in the low temperature regime. Recent investigations in Fe and Fe + Si alloys by Yu, Esaklul and Gerberich [15] revealed that the values of AK,,,,, on cooling from 297 to 123 K could not correlate with the increase in material strength, as observed in the subject Inconel 706. The increase in AK,,.,, for the Fe and Fe + Si alloys on cooling, however, could be related to the increase in the thermal component, u*, of material strength [ 151. Therefore, Yu et al. [I 51 reported that the improvement in near-threshold crack growth resistance with decreasing temperature could be associated with a thermally activated process in light of a dislocation dynamics model. In other words, the thermally activated energy available to mobilize the dislocations over the barriers near the fatigue crack tip generally decreases with decreasing temperature, thereby giving an increase in crack growth resistance at cryogenic temperatures. Moreover, Yu et al. [15] can relate AKth cR at different temperatures to the strain rate sensitivity (m*), i.e. AK,.,,a (1 + m*)“*, where m * = dlni/dlna* and i is the strain rate. Similarly, Fine et al. [12,63] derived the following equation using a dislocation dynamics model AK,,, a a,(2ns)‘~*
(2)
where 0, is the stress to activate a dislocation source at the crack tip and at a distance s from it. They suggest that Q, be related to thermally activated processes, such as, dislocation cross-clip. The rates of thermally activated processes decrease with decreasing temperature, thereby increasing cr, and thus, AK,,, at lower temperatures. Consequently, dislocation dynamics appears to provide a possible rationale for explaining the influence of temperature on the FCGR properties of Inconel 706.
5. CONCLUSIONS 1. The near-threshold FCGR properties of Inconel 706 were developed at 297 and 4.2 K. Specimen orientation did not influence crack propagation behavior. 2. Increasing the R-ratio from 0.1 to 0.8 generally increased near-threshold crack propagation rates. At 297 K, a significant R-ratio effect was observed, while at 4.2 K, the R-ratio effect was minimal. 3. Decreasing the test temperature from 297 to 4.2 K decreased the crack propagation rates. Increasing the R-ratio from 0.1 to 0.8 increased the temperature effect. 4. At 297 K, crystallographic crack growth was observed on fracture surfaces. Decreasing AK or decreasing R-ratio was found to increase the extent of clear-cut crystallographic fracture. At 4.2 K. crystallographic fracture was also observed. The extent of clear-cut crystallographic fracture at 4.2 K, however, was not as significant as at 297 K.
LIAW and LOGSDON:
FATIGUE
CRACK GROWTH
Crystallographic fracture at 4.2 K was found to be insensitive to AK or R-ratio. 5. The levels of surface roughness at 297 K were much greater than those at 4.2 K. At 297 K, surface roughness increased with decreasing AK, while at 4.2 K, it was insensitive to AK. 6. The thicknesses of oxide debris on the fracture surfaces of Inconel 706 near-threshold FCGR specimens tested at either 297 or 4.2 K were measured, and found to be less than 100 A. These thin oxide deposits were not expected to introduce significant oxideinduced crack closure. 7. Direct crack closure measurements indicated that there was a close relationship between crack closure level and surface roughness. Thus. roughnessinduced crack closure was believed to be the dominant mechanism governing the near-threshold FCGR behavior of Inconel 706. 8. Crack closure levels were greater at 297 K than at 4.2 K. At 297 K, the levels of crack closure increased with decreasing AK, while at 4.2 K, they were insensitive to AK. 9. At 297 and 4.2 K, the effects of R-ratio on the rates of near-threshold crack growth could be explained by the concept of roughness-induced crack closure. The more significant R-ratio effect at 297 K compared with that at 4.2 K correlated with the much rougher fracture surfaces observed at 297 K. IO. Crack closure could not account for the influence of temperature on the near-threshold FCGR properties of Inconel 706. The increase in material strength on cooling from 297 to 4.2 K was not significant enough to explain the effect of temperature on crack growth behavior. Dislocation dynamics offers a qualitative explanation for the temperature effect. Acknowledgements-The authors are very grateful to E. J. Helm. R. R. Hovan. G. McFetridee. A. R. Petrush and M. G. Peck for conducting the diffi< cryogenic temperature near-threshold FCGR experiments. The authors additionally wish to thank D. Detar. R. L. Conroy. T. Mullen and A. Karanovich for characterizing the specimen fracture surfaces. Financial support for this program was provided by the Advanced Programs Department. Westinghouse Steam Turbine-Generator Division.
IN INCONEL 706
1743
9. R. A. Smith. Proc. Inr. ConJ. Fatigue Thresholds, Stockholm. Vol. 1. 33 (1981). 10. K. Minakawa and A. J. McEvily, Scripra metoll. 15,633
(1981). Il. K. Minakawa. Y. Matsuo and A. J. McEvily, Metoll. Trans. 13A, 439 (1982). 12. J. McKittrick, P. K. Liaw. S. I. Kwun and M. E. Fine, Metal/. Trans. 12A, 1535 (1981). 13. D. H. Park and M. E. Fine. TMS-AIME Svmp. on Fatigue Crack Growth Threshold Concepts (edited by D. L. Davidson and S. Suresh). p. I45 (1983). 14. K. A. Esaklul. W. Yu and W. W. Gerberich. ASTM STP 857 (edited by R. I. Stephens), p. 63 (1985). 15. W. Yu. K. A. Ksaklul and W. W. Gerberich. Mefull. Trans. 15A, 889 (1984). 16. G. T. Gray III. J. C. Williams and A. W. Thompson, Metall. Trans. 14A, 421 (1983).
17. R. D. Carter, E. W. Lee. E. A. Starke and C. J. Beevers, Metal/. Trans. 15A, 555 (1984).
18. C. J. Beevers. Proc. Inr. Cotzf. Farigue Threshoh& Stockholm, Vol. I, p. 257 (1981). 19. T. C. Lindley and C. E. Richards, Proc. Int. Co& Fatigue Thresholds, Stockholm. Vol. 2, p. 1087 (1981). 20. R. P. Skelton and J. R. Haigh. Muter. Sci. Engng 36, I7 (1978).
21. P. K. Liaw, T. R. Leax, R. S. Williams and M. G. Peck, MeruN. Trans. 13A 1607 (1982).
22. P. K. Liaw, T. R. Leax, R. S. Williams and M. G. Peck, ACIU metall. 30, 2071 (1982). 23. P. K. Liaw. S. J. Hudak Jr and J. K. Donald, Merufl. Trans. 13A, 1633 (1982).
24. P. K. Liaw. A. Saxena, V. P. Swaminathan and T. T. Shih, Merul. Trans. 14A, 1631 (1983). 25. P. K. Liaw. W. A. Logsdon and M. H. Attaar, ASTM STP 857 (edited by R. I. Stephens), p. 173 (1985). 26. P. K. Liaw. W. A. Loasdon and M. H. Attaar. Crvogenies 23, IO (1983). 27. P. K. Liaw, W. A. Logsdon and M. H. Attaar, Ausrenific Steels at Low Temperatures (edited by R. P. Reed and T. Horiuchil. D. I7 I. Plenum Press. New York (1983). 28. P. K. Liaw and W. A. Logsdon. Engng. Fracr. Mech. 22, 585 (1985).
29. P. K. Liaw, ACIU me!&. 33, 1489 (1985). 30. P. K. Liaw. T. R. Leax and W. A. Logsdon, Acra melull. 31, 1581 (1983).
31. R. L. Tobler and Y. W. Cheng, Fully automatic nearthreshold fatigue crack growth rate measurements at liquid helium temperature. To be published. 32. E. Tschegg and S. Stanzl, Acra merull. 29, 33 (1981). 33. F. R. Stonesifer. Engng Frucf. Mech. 10, 305 (1978). 34. L. H. Burck and J. Weertman, Metall. Trans. 7A, 257 (1976).
35. P. K. Liaw and M. E. Fine. Melall. Trans. 12A. 1927 (1981). 36 J. P. Lucas and W. W. Gerberich. Ma/er Sci. Engng 51, 203 (1981).
REFERENCES
37 H. J. Choi and L. H. Schwartz. Merali. Trans. 14A, 1089 (1983).
1. R. 0. Ritchie. Inr. Merals Rec. Nos 5 and 6. 205 (1979).
38. P. K. Liaw and W. A. Logsdon.
2. S. Suresh, G. F. Zamiski and R. 0. Ritchie. Merall. Trans. 12A, 1435 (1981). 3. N. Walker and C. J. Beevers. Furigue Engng. Muter. Srrucr. 1, I35 (1979). 4. C. J. Beevers, K. Bell R. L. Carlson and E. A. Starke, Engng Fract. Mech. 19, 93 (1984). 5. A. T. Stewart, Engng Frucr. Mech. 13, 463 (1980). 6. Gu Haicheng and J. F. Knott. from. Inr. Cor$ Fatigue Thresholds. Stockholm, Vol. 2, p. 639 (1981). 7. M. N. James and J. F. Knott. Murer. Sci. Engng. 72, I I (1985). 8. S. Suresh and R. 0. Ritchie. Melal/. Trans. 13A. 1627 (1982).
R. 0. Ritchie and E. A. Starke Jr). Vol. II. pp. 899-908. EMAS (1987). 39. J. C. White and W. R. McCown, Power Engngs, p. 72 (1981). 40. Superconducring Magner Coils .for the Large Coil Program, Phase 2 Final Report to Union Carbide Corp., Oak Ridge, TN, by Westinghouse Electric Coru.. ’ Pittsburg, PA, Contract 22X-3 I7<7C (I 980). 41. ASTM E399-78a. 1980 Annual ASTM Standards. Part IO, p. 580. 42. R. S. Williams, P. K. Liaw, M. G. Peck and T. R. Leax, Engng Fracf. Mech. 18, 953 (1983).
Third Inl. Co@ on Fubgue and Fatigue Thresholds, Furigue 87(edited by
1744
LIAW and LOGSDON:
FATIGUE
CRACK GROWTH IN INCONEL
43. A. Saxena, S. J. Hudak Jr, J. K. Donald and D. W. Schmidt. J. Test. Euol. 6. 167 (1978). 44. A. Saxena and S. J. Hubak J;, Inf: J. Fracr. 14, 453 (1978). 45. W. G. Clark Jr and S. J. Hudak Jr, J. Test. Eual. 3,454 (1975). 46. M. Kikukawa, M. Jono and K. Tanaka, Proc. Second Rt. Conf. on Mech. Behavior of Mater. (edited by N. Promise1 and V. Weiss), p. 716. Am. Sot. Metals, Metals Park, Ohio (1976). 47. D. Gan and J. Weertman, Engng Fract. Mech. 15, 87 (1981). 48. K. Endo, K. Komai and Y. Matsuda, Mem. Fracr. Engng 31, Kyoto University, 25 (1969). 49. American Standard ASA B46 I-1955, UDC621.9.016, ASME (1955). 50. J. N. Vincent and L. Remy, Proc. Int. Con/. Farigue Threshok&, Stockholm, Vol. 1, p. 441 (1981). 51. M. Gel1 and G. R. Leverant, Acra merall. 16,553 (1968). 52. J. C. Runkle and R. M. Pelloux, ASTM STP 675, 501 (1979). 53. R. W. Hertzberg and W. J. Mills, ASTM STP 600,220 (1976).
706
54. R. B. Scarlin, ASTM STP 675, 396 (1969). 55. K. Minakawa, G. Clevan and A. J. McEvily, Merall. Trans. 17A, 1787 (1986). 56. J. Lindgkeit, A. Gysler and G. Liitjering, Merall. Trans. IZA, 1613 (1981). 57. S. Suresh, A. K. Vasudevan and P. E. Bretz, Merall. Trans. lSA, 369 (1984).
58. P. K. Liaw, A. Saxena, V. P. Swaminathan and T. T. Shih, Proc. TMS-AIME Symp. on Conceprs of Farigue Crack Growth Threshold (edited by D. L. Davidson and S. Suresh), p. 205 (1983). 59. P. K. Liaw, Proc. TMS-AIME Symp. on Synergism of Microstructure,
60. 61. 62. 63. 64.
Mechanisms and Mechanics in Fatigue
(edited by J. M. Wells and J. D. Landes), p. 479 (1984). J. L. Yuen, P. Roy and W. D. Nix, Merali. Trans. lSA, 1769 (1984). M. A. Hicks and J. E. King, Inr. J. Fatigue 5.67 (1983). W. F. Weston and H. M. Ledbetter, Murer, Sci. Engng 20, 287 (1975). M. E. Fine, Bull. J.I.M. 20, 668 (1981). D. M. Tracey, J. Engng Mater. Tech., Trans. A.S.M.E. 98, 146 (1976).
~cta meroll. Vol.36,No. 7.pp. 1731-1744, 1988 Printed in Great Britain. All rights reserved
Copyright G 1988Pergamon Press plc
FATIGUE CRACK GROWTH BEHAVIOR INCONEL 706 AT 297 K AND 4.2 K
IN
P. K. LIAW and W. A. LOGSDON Metals Technologies Department, Westinghouse R & D Center, Pittsburgh, PA 15235, U.S.A. (Received 18 Mu,r 1987; in
revised form
30 Seplember
1987)
Abstract-The near-threshold fatigue crack growth rate (FCGR) behavior of Inconel706 was investigated at ambient (297 K) and liquid helium (4.2 K) temperatures, respectively. Specimen orientation did not affect the FCGR properties of Inconel 706. At 297 K, a significant influence of R-ratio on the rates of crack propagation was observed while at 4.2 K, the R-ratio effect was minimal. The extent of oxide-induced crack closure was shown to be insignificant in influencing near-threshold crack growth kinetics of Inconel 706 at both temperatures of 297 and 4.2 K. Roughness-induced crack closure was believed to be the dominant mechanism responsible for the influence of R-ratio on the FCGR properties of Inconel706; this was proved quantitatively by direct crack closure measurements conducted at 297 and 4.2 K. A greater degree of roughness-induced crack closure was observed at 297 K than at 4.2 K; this correlated with the more pronounced R-ratio effect at 297 K. Decreasing the temperature from 297 to 4.2 K decreased the growth rates of fatigue cracks in Inconel 706. The effect of temperature on crack propagation behavior increased with increasing R-ratio. Crack closure could not rationalize this temperature effect. Moreover, the increase in material strength or Young’s modulus on cooling from 297 to 4.2K could not totally account for the influence of temperature on the near-threshold FCGR properties. Dislocation dynamics appears to offer a qualitative explanation for this temperature effect. R&am~&Nous avons etudit, a la temtirature ambiante (297 K) et a celle de l’htlium liquide (4.2 K). la vitesse de croissance des fissures de fatigue pms du seuil (VCFF) dans Hnconel 706. L’otientation des echantillons n’a pas d’effet sur la VCFF de l’Inconel706. A 297 K, le rapport R a une influence importante sur les vitesses de propagation des fissures, alors qu’a 4,2 K son effet est minimal. Nous montrons que la fermeture des fissures due a l’oxyde n’a guere d’influence sur la cinetique de croissance des fissures pms du seuil dans I’InconeI 706, que ce soit a 297 K ou a 4.2 K. Nous pensons que la fermeture des fissures induite par la rugosite est le mecanisme dominant, responsable de I’influence du rapport R sur les prop&es de la VCFF de fIncone 706; nous I’avons demontre quantitativement par des mesures directes de fermeture des fissures, malisees a 297 K et a 4,2 K. Le phtnomtne de fermeture des fissures induite par la rugosite est plus marque a 297 K qu’a 4.2 K, ce qui est en accord avec l’effet du rapport R qui est plus prononce a 297 K. Lorsque la temperature dbroit de 297 K a 4,2 K, la vitesse de croissance des fissures de rupture dans 1’Inconel 706 decroit. L’effet de la temperature sur la propagation des fissures augmente lorsque le rapport R augmente. La fermeture des fissures ne semblerait pas pouvoir rational&r cet effet de la temperature. Cependant, l’accroissement de la resistance mecanique ou du module d’Young du mat&au lorsqu’on le refroidit de 297 K a 4.2 K n’expliquerait pas non plus entibrement I’effet de la tempirature sur les propridtts de la VCFF pres du seuil. La dynamique des dislocations semble foumir une explication qualitative de c-et effet de la temperature. %aammenfaaaung--Die Wachstumsrate von Ermiidungsrissen in der Nahe des Bruches wurde an Inconel 706 bei 297 und 4,2 K untersucht. Die Orientierung der Probe beeinflugte das beobachtete Verhalten nicht. Bei 297 K wurde ein deuthcher Einflug des R-Verhaltnisses auf die Ausbreitungsrate des Risses beobachtet, dieser Einflug war minimal bei 4.2 K. Die Oxid-induzierte RiBschlieBung beeinfluBte die Kinetik dieser RiBausbreitung bei beiden Versuchstemperaturen nur unbedeutend. Die durch die Rauhigkeit induzierte RiDschlieDung wird als der bedeutende Mechanismus angesehen, durch den das R-Verhiltnis das Verhalten der Rillausbreitung bestimmt. Dieser Befund wurde mit direkten Messungen der RiBschlieBung bei 297 und 4.2 K quantitativ nachgewiesen. Der Beitrag der Rauhigkeits-induzierten RiDschlieDung war bei 297 K gr6l3er als bei 4.2 K; entsprechend war der Einflug des R-Verhlhnisses bei 297 K such griilkr. Mit absinkender Temperatur sanken zwischen 297 K und 4,2K such die Wachstumsraten der Ermiidungsrisse. Der EinfluB der Temperatur auf das Ausbreitungsverhalten der Risse nahm mit ansteigendem R-Verhihnis zu. Dieser Temperatureffekt konnte mit der RiBschlieBung nicht erkliirt werden. AuBerdem konnte der mit sinkender Temperatur ansteigende Elastizitltsmodul den EinfluB der Temperatur nicht vollsmndig erkllren. Es scheint, als ob die Versetzungsdynamik eine qualitative ErklPrung fib diesen TemperatureinfluB liefert.
1. INTRODUCTION
Near-threshold fatigue crack growth rate (FCGR) behaviour has been a subject of much interest [l-31].
Most of the threshold FCGR research, however, was performed at ambient temperature. Relatively few near-threshold crack propagation studies were conducted at cryogenic temperatures [12-15,25-28, 1731
1132
LIAW
and LOGSDON:
FATIGUE
CRACK
31-381. Furthermore, these cryogenic temperature crack growth investigations were rarely performed at extremely low temperatures approaching that of liquid helium 12%28,31,38], despite the fact that superconducting generators [39] and magnets [40] are designed to operate at temperatures as low as 4.2 K. To ensure the structural integrity of those machines that operate at cryogenic temperatures, it is necessary to develop low-tem~rature, near-thr~hold FCGR properties of the appropriate structural materials. Cryogenic temperature FCGR data are generally difficult to obtain because of the large cryogen and labor expenses as well as the complexity of the experimental setup. Moreover, to utilize and extrapolate these limited FCGR results effectively, it is im~rative that the mechanisms of crack growth at low temperatures be thoroughly understood. In this paper, the near-threshold FCGR behavior of Inconel 706 was examined at ambient (297 K) and liquid helium (4.2 K) temperatures. In addition, c~oge~c-tem~~ture near-threshold crack growth mechanisms were discussed in light of fracture surface characterizations, crack closure measurements and dislocation dynamics models. Specifically, the effects of specimen orientation, R-ratio (R = I’& /P_ , where Pti, and P_ are the minimum and maximum loads, respectively, in a fatigue cycle) and temperature on the near-threshold FCGR properties of Inconel 706 were emphasized.
2. ~XFERIME~AL
2.1. Material The material investigated was a 69.2cm diameter extrusion of Inconel 706. The chemical composition of this nickel-base superalloy in weight percent was O.O4C, O.I6Mn, O.l7Si, 41.4ONi, 15.7Cr, 0.14M0, 0.25 Al, 0.004 B, 0.08 Co, 0.1 Cu, 0.002 Mg, 2.96 Nb, 1.66 Ti and balance Fe. The Inconel706 was solution treated (1339 K), double aged (1033 K. hold 8 h and 922 K, hold 5 h) and air-cooled. The grain size was approximately equal to 150 pm. Tensile properties at 297 and 4.2 K are shown in Table 1. Tensile specimens were machined from the cylindrical extrusion in two loading orientations: radial and circumferential (Fig. 1). For both loading
297 297 4.2 4.2
Orientation Radial loading’ Circumferential loadi& Radial loading” Circumfcrenriat loading’
IN INCONEL
706
directions, decreasing the temperature from 297 to 4.2 K increased the material strength, elongation and reduction in area. Furthermore, at 297 K material strengths for the radial and circumferential loading directions were essentially comparable even though the elongation and reduction in area for the circumferential direction were greater than those for the radial direction (Table I). At 4.2 K, material strength, elongation and reduction in area in the radial direction were somewhat lower than those in the circumferential direction.
2.2. Fatigue testing 2.2.1. ~~~irne~ design. Near-threshold FCGR experiments were conducted at 297 and 4.2 K. Compact-type (CT) specimens, 6.4mm thick (B) by 51 mm wide (W) by 64mm high (2H), were used to develop the FCGR properties. The notch of each CT specimen was orientated in one of the three directions, C-R, R-C and C-L, in compliance with the ASTM Standard Test Method for Plane Strain Fracture Toughness of Metallic Materials (ASTM E399) [41]. A schematic of the specimen layout is presented in Fig. 1. The tensile specimen with a circumferential loading direction was designed for the CT specimens of the C-R and C-L notch orientations while that with the radial direction is for the R-C notch orientation (Fig. 1). 2.2.2. Automated testing technique. A computerized fatigue testing system was used to develop the near-threshold FCGR results. Details of this test technique have been previously described [25,42-431. Briefly, the stress intensity range, AK, was decreased according to the following equation AK = A& exp[c(a - ao)]
(1)
where A& is the initial stress intensity range, c is a negative value with a dimension of reciprocal length, a is the crack length and a, is the initial crack length. The expression for calculating AK for CT specimens can be found in Ref. [44]. In this investigation, the value of c equaled - 0.098 mm-‘. Test frequency was 90 Hz and the R-ratios investigated were 0.1 and 0.8. Crack length was dete~in~ by an unloading compliance technique f44]. A clip-on MTS extensometer (Model 630.05B-61), specially designed for cryogenic
Table I. Tensile properties of INCONEL Test temperature (K)
GROWTH
706 at 297 and 4.2 K
0.2% yield strength
Ultimate tensiie strength
Elongation’
(MPa)
(MPa)
(%)
979 959 1178 1201
II55 1162 1485 1529
9.02 13.08 18.45 23.70
‘For CT specimens of R-C notch orientation (Fig. 1). “For CT specimens of C-R and C-L notch orientations (Fig. 1). %age length = 25.4 mm
Reduction in area
(Oh) 12.90 19.42 19.30 22.92
LIAW and LOGSDON: FATIGUE CRACK GROWTH IN INCONEL 706 Tensile
1733
Specimen
Top View
5 ide View
Fig. 1. Schematic of specimen orientation.
temperature applications, was mounted on the front face of each CT specimen to measure crack opening displacement which, in turn, was converted into crack length by the compliance technique. The FCGR (da/dN) was determined by a seven-point incremental polynomial technique [45]. Data acquisition and analysis were automated. While at 297 K, the FCGR experiments were performed in laboratory air, a special test chamber was designed and utilized for 4.2 K tests, as described below. 2.2.3. Cryostat. At 4.2K, the FCGR tests were conducted using a specially designed cryostat that enclosed the CT specimen in liquid helium gas (Fig. 2). The cryostat was composed of a liquid helium inner chamber and a liquid nitrogen outer chamber that were separated by two Styrofoam filled chambers. To maintain the desired temperature of 4.2 K, liquid nitrogen was first placed in the outer chamber that were separated by two Styrofoam-filled chambers. To maintain the desired temperature of which contained the CT specimen. The helium coolant dispenser served as both a helium distributor and temperature stabilizer. Thermocouples were attached to the specimen to monitor test temperature and to control both the liquid nitrogen and liquid helium flow rates. To save the consumption of the expensive liquid helium coolant, the computerized FCGR tests
were conducted continuously through days and nights without interruption. During the FCGR experiments, crack closure levels were monitored using an unloading compliance technique at both 297 and 4.2 K [21,46,47]. To accomplish these crack closure measurements, strain gages were cemented near the crack plane of each CT specimen. Special strain gages (WK-06-1215TA350 from Micro-Measurement) were utilized for the liquid helium temperature crack closure measurements. Load vs strain curves were periodically recorded to determine crack closure levels. The unloading linear elastic strain was subtracted from the total strain to increase the sensitivity for measuring crack closure loads [46]. Details of the crack closure measurements have been documented in Refs [21, 46,471. 2.3. Fracture surface characterizations 2.3.1. Fractography. Following the fatigue tests, the CT specimen fracture surfaces were carefully examined using scanning electron microscopy (SEM); fracture morphology was determined along the crack growth direction so that the influence of AK level on fracture modes could be evaluated. Furthermore, the effects of R-ratio and temperature on fracture morphology were investigated.
1734
LIAW
and LOGSDON:
FATIGUE
CRACK
L
ChrmberSub.. Fig, 2.
GROWTH
IN INCONEL
706
o-ring WI
Cryostat for low temperature testing.
2.32. Oxide thickness measurements. During the near-threshold FCGR tests, oxide deposits were often observed on the specimen fracture surfaces. These oxide deposits could inguence threshofd crack ~ro~ga~on behavior by the mechanism of oxideinduced crack closure 12,8,13-?O, 483. To quantify the oxide deposits present on the fracture surfaces,
Auger spectroscopy was used to determine the thickness of the oxides. The sputtering rate in the Auger spectrometer was calibrated to a known thickness of tantalum oxide and also by profilometry on sputtered craters. In tbe present Auger spectrometer, one minute of sputtering time was found to correspond to a sputtering depth of 80 A. As argon ions sputtered
LIAW and LOGSDON:
FATIGUE
CRACK GROWTH
4
t
6
,
Inconef h6 lem~rature : Wt K
t,1,1,
706
3.1. Fatigue crack growth rate properties The near-threshold FCGR properties of Inconel 706 at 4.2 and 297 K are presented in Figs 3-4. The levels of threshold stress intensity range (AK,,,) are shown in Table 2. The A& value is defined as the lowest AK level observed in each crack propagation experiment. The near-threshold FCGR data and A& levels are presented in light of the effects of specimen orientation, R-ratio and temperature. 3.i.f. E&et of specimen orientation. The FCGR properties at 297 K in those CT specimens with C-R, R-C and C-L notch orientations are illustrated in Fig. 3. The fatigue crack growth rates for the three orientations were essentially comparable regardless of R-ratio. Similarly, at both R-ratios, the values of A& at 297 K were in~nsitive to specimen o~entation, as shown in Table 2. Figure 4 shows the effect of specimen orientation on crack growth behaviour at 4.2 K. At the R-ratios of 0.1 and 0.8, specimen orientation did not significantly alter the crack propagation rates, which is in agreement with the results at 297 K. Consistently, at the cryogenic temperature, A& at a fixed R-ratio was essentially equivalent for the C-R and R-C notch orientations. 3.1.2. Efect of R-ratio. The effects of R-ratio on the FCGR properties of Inconel 706 are presented in
AK
ik?
8
10
6i 20
I
t
40
,
6080
I ,,,
Specimen Orientation R-Ratio Destgnation
Symbol 0 A
CR Rc
::
a
CL CR RC
a1
4 A
706R5 t R5) 7D6-C2l C21 7w5 ( I,!9 706-12( 128) 706X6 I 268) 0
-
.
.*
R-0.1
I
2
s
I
4
: I
lllll
6
1735
3. RESULTS
oxide debris on the fracture surface, the atomic weight percents of the compositional elements in the oxide were periodically measured and plotted. Therefore, a depth profile of the atomic percents of oxygen and nickel versus the average distance (d) from the fracture surface into the oxide was developed. Average thickness of the oxide deposit was defined as the value of d at which the atomic percent of oxygen equaled that of nickel. [21,22]. 2.3.3. Surface roughness measurements. At threshold levels, surface roughness can introduce roughnessinduced crack closure which greatly influences nearthreshold FCGR behavior [3,8,11, l&18]. Realizing the importance of surface roughness in governing FCGR properties, the levels of roughness were determined using a Surface Analyzer (Surfanalyzer 360 system, Clevite Corporation). On each specimen fracture surface, several roughness profiles were traced and recorded along the crack growth direction. The level of roughness at a fixed AK was defined as the average of the several measured roughness profiles. The roughness recorded in each trace was the arithmetic average (AA) deviation from the mean surface; this is the average of numerous measurements at the heights of the surface peaks and valleys (measured from the mean surface) [49]. The mean surface was defined as a perfect surface that would be formed if all of the roughness peaks were cut off, and used up in filling the valleys below this surface.
2
IN iNCONEL
I
8 10
2ll
I
40
‘Ill,
6D 80‘
AK (MPa m
Fig. 3. Effect of orientation on near-threshold fatigue crack growth behavior at 297 K.
LIAW and LOGSDON:
1736
FATIGUE
CRACK GROWTH
2
I
6
4
8
,
706
t/-iii!
AKfksl 1
IN INCONEL
8 10
40
20
I , , , I,
I
60
,
I
80100
I,,,,
Inconel706 Temperature : 4 2 K 10-7 c
0 A l
g Y
706-R4 I R4) 706x9 t C9J 706R4 I R48) 706c9 i C98)
aa:
Z RC
A
:
Specimen R-RatJo Designation
Orienteth -CR
Symbol -
9:
10-5
Y lo+
10-8 f-
E
P
2 10-7 3
B i
10-9 :
3
co R=Q8 *
R-O.1
0
- lo+ 10-10 :
c
I
1
2
I
I
on
Fig. 4. Effect of orientation
I III 6
4
8
I
near-threshold
AF lksi 1
2
I
I
4
6
,
I ,I,‘,
8
0 A 0
10-7 5
1 I I I I 60
lo+
80100
m 20
10
( K) R-Reti0
2
A
60
40
I
I
Temwature
I 40
fatigue crack growth behavior at 4.2 K.
Inconel706 Orientetlon: CR Symbol
1
10 20 AK tMPa di%iii)
,
80 100
I ,‘,I
Speclmen Designation
D.1
706R5
0.1 0.8
706R4 706-12 I(R41 128)
0.8
706-R4 ( R48)
: 1o-5
( R5)
-z 10-6
Ak
p 10%: L:
5 ,K -_ 1O-7 g
e E i 3
10-9 e 241 K R=(LE
: 10-8
lo-lo c F
I 2
I 4
I
I1111
6
8 10 AK (hIPa m
I
20
Fig. 5. Effects of load ratio and temperature on near-threshold orientation.
I
I 40
1I1IaL
60
80100
lo-9
fatigue crack growth behavior
in CR
LIAW and LOGSDON:
1
2
I
I
I
FATIGUE
4
,
CRACK GROWTH
AK(ksim 8 10
6
I,‘,‘,
a0
Inconel706 : RC
OrienWion
Tempetduref
5yllhOi
I
I
K)
::
z 42
0.1 0.8
aa
60
,
I ,I,,
1737
706
80100
Specimen Designation
R-Ratlo
291
IN INCONEL
706x2 706-Q 7Ow9 70639
f c21 t 2681 t C9) l C98)
A
2% K ,,t R=Q8 >
Fig. 6. Effects
A
of load ratio and temperature on near-threshold
fatigue crack growth behavior
in
RC
orientation.
Figs 5 and 6 for the C-R and R-C notch orientations, respectively. At 297 K and in the C-R orientation, increasing the R-ratio from 0.1 to 0.8 signifkzantly increased the rates of crack propagation. Correspondingly, the value of AK,,, at R = 0.1 is 1.9 times greater than that at R = 0.8. Furthermore, decreasing the AK level increased the influence of R-ratio on crack growth rates, which is characteristic of nearthreshold crack propagation behavior. Interestingly, at 4.2 K the crack growth rates were relatively insentive to R-ratio. For instance, at 4.2 K the value of AK,,, at R = 0.1 is only 1.2 times greater than that at R = 0.8. Thus, the effect of R-ratio on crack propagation behavior at 4.2 K is much less significant than at 297 K.
Results for the R-C orientated specimens are shown in Fig. 6. At 297 K, a significant effect of R-ratio on the rates of crack growth was observed while at 4.2 K, the influence of R-ratio was Mornay. Consistency, the ratio of AK,,, at R = 0.1 to that at R = 0.8 was lowered from 2.1 to 1.2 by decreasing the temperature from 297 to 4.2 K. Therefore, in the R-C notch orientation decreasing the temperature significantly reduced the R-ratio effect, as was also observed in the C-R notch orientation. 3.1.3. Effect of temperature. The influence of temperature on the FCGR properties of Inconel706 is also illustrated in Figs 5 and 6 for the C-R and R-C notch orientations, respectively. For both orientations at R = 0.1, decreasing the temperature
Tabk 2. Threshold stress intensity range (A&,), efftxWc threshold stress intensity range (A&.#),
tip opening dispkwment
&c&men dcsignatian 706*RS(RS) 706-12(128) 706~C2(C2) 706~CW268) 706-LS(LS) 706-R4(R4) 706-R4(R48) 706~Cs(C9) 706-cXC98)
TcS( temperature W) 297 297 297 297 297 4.2 4.2 4.2 4.2
R-ratio 0.1 0.8 0.1 0.8 0.1 0.1 0.8 0.1 0.8
oxide thickness at threshold and crack at threshold of Inconel 706 al 297 and 4.2 K
Notch orientation z::: R-C ::: :I:: R-C R-C
(h&t&)
(h&F&)
11.8 6.3 12.1 5.7 11.9 16.9 23.9 15.2 12.9
5.9 6.3 5.1 5.7 5.9 14.2 13.9 12.5 12.9
Oxide thickness (A) 64 54 30 :;: : 14 I2
Cyclic crack tip opening displacement (A)
Max. crack tip opening dispiacemcntb (A)
1721 491 1773 393 1750 2562 1733 2113 1522
4249 24528 4377 19668 4322 6327 a6674 5218 76109
“cyclic crack tip opening dispIa_t = 0.49 AK,,,*&E, where try is yield strength and E is Young’s modulus t&r]. bMaximum crack tip opzaing diit~ent = 0.49 K~~_z/ula,E, whm &,,, is maximum stress intensity at threshold [64].
1738
LIAW and LOGSDON:
FATIGUE
CRACK GROWTH
IN INCONEL
706
from 297 to 4.2 K decreased the crack propagation rates and increased AK&. Moreover, the difference in the crack growth rates at 297 and 4.2 K increased with decreasing AK level. At R = 0.8, the influence of temperature on the FCGR properties was much greater than that at R = 0.1 (Figs 5 and 6). For example, the ratio of AK,,, at 4.2 K to that at 297 K increased from 1.4, to 2.2 by increasing the R-ratio from 0.1 to 0.8 for the C-R notch orientation; the ratio changed from 1.3 to 2.3 for the R-C notch orientation. Thus, increasing the R-ratio amplified the influence of temperature on the FCGR properties of Inconel 706. 3.2 Fracture surface characterizations 3.2.1. Fractography. The SEM photos of fracture surfaces at 297 and 4.2 K are shown in Figs 7-10 (Figs 7-9 are for 297 K and Fig. 10 is for 4.2 K). At 297 K and R = 0.1 (Figs 7 and 8), crystallographic fracture facets were observed in the near-threshold AK range investigated [approximately from 20 to 11 MPa & (Fig. 3, 7 and S)]. The existence of c~stallo~p~c fracture facets has been found to be cha~cte~stic of nor-threshold crack propagation behavior [3,8, 10, 11, N-54]. Interestingly, clear-cut crystallographic facets became more pronounced as the value of AK de-creased (Figs 7 and 8). This trend indicated more distinct crystallographic crack growth with decreasing AK level. Moreover, surface rough-
(al
(bl
Low magnificaiion
Higher
mcgnificotion
Fig. 7. Fracture morphology at 297 K (AK = 20 MPa fi and R =O.l).
(bl
Higher magnification
(near-threshold)
Fig. 8. Fracture morphology at 297 K(AK = 11.5 MPa & and R =O.l).
ness associated with crystallographic fracture increased with decreasing AK values. The higher-magnification photos in Figs 7 and 8 indicated the presence of slip lines on the facet fracture surfaces. Previous investigation [SO,511 reported that the fracture facets of Inconel 706 were typi~lly along the (111) plane. At R = 0.8, crystallographic crack propagation was also visible (Fig, 9). The fracture modes seem to be independent of the AK level investigated. At R = 0.8, the presence of clear-cut crystallographic fracture was, however, not as pronounced as that at R = 0.1. Moreover, fracture surfaces at R = 0.8 appear to be smoother than those at R = 0.1. At 4.2 K, crystallographic crack growth was also observed and the fracture morphology was found to be insensitive to R-ratio and AK level. Slip lines were visible on fracture facets. At 4.2 K, the extent of clear-cut cr~tallographic fracture was much less significant than at 297 K. Furthermore, the fracture surfaces at 4.2 K were much smoother than those at 297 K. 3.2.2. Oxide thickness. The thicknesses of oxide debris present on the fracture surfaces were measured at threshold levels using Auger spectroscopy (see Table 2). At 297 K, the oxide deposits were fairly thin, ranging from 14 to 64 A in thickness. Increasing the R-ratio from 0.1 to 0.8 seemed to result in a decrease in oxide thickness. At 4.2 K, the measured oxide thickness ranged
LIAW and LOGSDON:
(0)
(b)
Low mognificotion
Higher
mognificotion
FATIGUE
CRACK GROWTH IN INCONEL
[near-threshold)
(nsor-threshold)
(0)
Low
(b)
Higher
mognificotion
mognificotion
706
1739
(near -threshold)
(near- threshold)
Fig. 9. Fracture morphology at 297 K(AK = 6.6 MPa ,/% and R =0.8).
Fig. 10. Fracture morphology at 4.2 K (AK = 19.1 MPa fi and R =O.f).
from 12 to 28 A which, as expected, was smaller than at 297 K. Oxide thickness at 4.2 K were found to be relatively insensitive to R-ratio. 3.2.3. Surface roughness measurements. Surface roughnesses, measured using a surface analyzer, are presented in Fig. 1I. At 297 K, the levels of surface roughness were typically much greater than those at 4.2 K. This trend was consistent with the rougher fracture surfaces observed at 297 K than at 4.2 K, as shown in Figs 7-10. Surface roughness at 297 K generally increased with decreasing AK levels (Fig. 11). This behavior was in compliance with the more pronounced crystallographic facet fracture at lower AK levels (Figs 7 and 8). At 4.2 K, surface roughness was insensitive to AK (Fig 11), which is in agreement with the insensitivity of the fracture mode to AK (Fig. 10). In Figs 7-11, it is interesting to note that decreasing the temperature from 297 to 4.2 K decreased the fracture surface roughness of Inconel 706. Other investigators [50] previously observed a decrease in the surface roughness of nickel-base alloys with a corresponding increase in the temperature from 297 to 873 K, Because cross slip is a thermally activated process, it has been suggested [!iO] that the ease of cross slip at higher temperatures reduce the distance of planar slip, thereby resulting in smoother fracture surfaces with increasing temperature. This argument would not be applicable, however, to the present
observation. Decreasing the temperature from 297 to 4.2 K makes cross slip more difficult which should, in turn, result in rougher fracture surfaces at 4.2 K; this trend is in contrast with the current finding (Figs 7-l 1). Consequently, the reason for the comparatively rougher fracture surfaces at 297 K than at 4.2 K is not understood at the present time; further research is needed to explain this phenomenon. 4. DISCUSSION The FCGR properties of Inconel 706 have been developed at 297 and 4.2 K. Specimen orientation produced little effect on crack propagation behavior.
R=O.l
Fig. 11 Surfaceroughness profiles at 297 and 4.2 K.
1740
LIAW and LOGSDON: FATIGUE CRACK GROWTH IN INCONEL 706
R-ratio and test temperature, however, influenced the rates of crack growth. In the following sections, these effects are discussed in light of oxide thickness, surface roughness and crack closure measurements. 4.1. Effect of R-ratio At 297 K, the influence of R-ratio on the growth rates of fatigue cracks in Inconel 706 is significant (Figs 5 and 6). The R-ratio effect at 4.2 K, however, is minimal. It has been reported that the influence of R-ratio on near-threshold FCGR properties can be rationalized by oxide and/or roughness-induced crack closure [5,21,22,55]. At lower R-ratios, the higher crack closure levels decrease the effective stress intensity ranges (A&), where AK8 = & - K&,,, and & and k;lOSurcare the maximum and crack closure stress intensity ranges, respectively; this yields slower crack propagation rates with decreasing R-ratio. At threshold levels, the presence of oxide-induced crack closure is supported by the fact that the thicknesses of oxide deposits are comparable to the crack tip opening displacements (CTOD) [2,5,21-301. These sizable oxide deposits wedge the crack tip and elevate crack closure levels, thereby resulting in oxideinduced crack closure. In the subject Inconel 706, oxide thicknesses at 297 and 4.2 K are fairly thin, and are approximately one to three orders of magnitude smaller than the CTOD, as presented in Table 2. Therefore, the extent of oxide-induced crack closure is minimal at both temperatures.
On the other hand, surface roughnesses are approximately one to three orders of magnitude greater than the CTOD (Table 2 and Fig. 11). Consequently, roughness-induced crack closure is suggested as the dominant mechanism responsible for the influence of R-ratio on the FCGR properties of Inconel 706. At 297 K, the rough fracture surfaces at R = 0.1 (Figs 7-9, 11) introduce crack closure and decrease A&. At R = 0.8, crack closure is insignificant because of the high mean load present at the crack tip, which prevents crack closure. Furthermore, the higher levels of surface roughness at R = 0.1 compared with those at R = 0.8 will elevate crack closure levels more significantly at R = 0.1, which, in turn, gives a smaller A& at R = 0.1. These trends result in slower crack propagation rates at R = 0.1 than at R = 0.8. At 4.2 K, the levels of surface roughness at R = 0.1 are much lower than those at 297 K. At R = 0.1, the much smoother fracture surfaces at 4.2 K compared with those at 297 K produce less crack closure (as also proved later by direct crack closure measurements), which, in turn, yields a decreased R-ratio effect at 4.2 K relative to the results at 297 K (Figs 5 and 6). Therefore, roughness-induced crack closure offers an explanation for the influence of R-ratio on the near-threshold FCGR behavior of Inconel 706. The fact that roughness-induced crack closure rationalizes the influences of R-ratio on the FCGR properties of Inconel706 can be further substantiated by direct crack closure measurements. In Figs 12 and
AK,,, ( kd
1
e I
I
4 ,
Inconel706 Orientation : CR
6 ,,I,,
m
8 10
20 I
Specimen Temperature(K) R-Ratio Designation 291 :i 706-R5(R5)
Symbol
0 0
E
A
42
:
:: Lcp A
C\ A
241K
-
Rr(Lland(l8 1 -
lo-lo r
10-5
706-12( 128) 706R4( R4) 706R4 l R48)
A
1o-7 F
40 HI en loo , I,‘,’
I
-5 lo+
42K
1 R=O.landas
Y lO"B
A 2
I
2
I
I,
4
Fig. 12 Crack growth rate (&/dN)
A 111, 6
8 10 AKd(MPa
I
20 m
I
40
vs effective stress intensity range
I
I,l,L
60
8Olul
lf9
(A&) in CR orientation.
LIAW and LOGSDON:
FATIGUE
CRACK
GROWTH
IN INCONEL
706
1741
AKeff ( ksi r/in) 1
2 I
I
4 ,
Inconel7Of1 Orientetion : RC
8
10
20 I
I
40
60 I,
Specimen Tempereture( Kb R-Ratio Designation
Symbol
10-I E-
6 f,I,1,
297 291 42 4.2
0
A 0 A
;
706-C2( C21 706C6 ( 2681 706-c9 ( C9) 706x9 ( C981
0.1 0.8 E
80 loo (,(
1o-5
= 10-6
0 z
lo-* F
0”
u” -Z :
rf
0 0
_
c D 0” lo-7 $
2 zi
2
10-9 F
#
b
: 1o-8
A
r
F
4.2K R=O.land0.8
f A
2¶ K R=O.land(18 if10-10
__(
I 2
I
I, 4
I, 6
I 8 10 2u AK,,, (MPa \riii)
I 40
I I I I 6 I 10-9 60
Fig. 13. Crack growth rate (da/dN) vs effective stress intensity range (A&)
80 100
in RC orientation
13, the rates of crack growth at R = 0.1 and 0.8 are ness-induced crack closure has been shown to be plotted versus AK,, in the C-R and R-C notch proportional to the levels of both surface roughness orientations, respectively. At both temperatures, the and slip irreversibility (or Mode II displacement) [8]. crack propagation rates at R = 0.1 and 0.8 are As mentioned before, the comparatively higher levels essentially identical when plotted versus AIcff regardof surface roughness at 297 than at 4.2 K have less of specimen orientation. Moreover, at a given contributed to the greater extent of roughnesstemperature and orientation, the values of effective induced crack closure at 297 K. It has been suggested threshold stress intensity range (AK,h.eR), where that the extent of slip irreversibility be related to test A&. c~ = Kth.max- Kh. cIosurc.and L manand Kth.closure environment [17,56,57]. In inert (vacuum or low are the maximum and crack closure stress intensities, temperature) environments, slip irreversibility is less respectively, are insentive to R-ratio (Table 2). Thus. significant than in oxidized environments [ 17,56,57]. crack closure can be used to explain the effect of Therefore, the greater extent of oxidation at 297 K R-ratio on the growth rates of fatigue cracks at each than at 4.2 K (Table 2) can be expected to result in temperature. greater slip irreversibility at 297 K, which also conIn Fig. 14, the ratios of Kc,,,,,, to Km,, at 297 and tributes to more significant crack closure at 297 K. 4.2 K are plotted as a function of AK. The levels of crack closure at 297 K are much greater than those at 4.2 K. This trend is consistent with the much greater surface roughness observed at 297 K (Figs 711). Moreover, at 297 K, crack closure levels tend to increase with decreasing AK, which is in agreement with the rougher fracture surfaces at lower AK values (Figs 7, 8 and 11). At 4.2 K, crack closure levels are insensitive to AK. This behavior is in compliance with the insensitivity of surface roughness to AK (Figs 10 and 11). These agreements between the crack closure and surface roughness results strongly support the fact that roughness-induced crack closure is the important mechanism governing the near-threshold AK lMPa V-d FCGR behavior of Inconel 706. Fig. 14. Crack closure results at 297 and 4.2 K. It is worth mentioning that the extent of rough-
I742 4.2.
LIAW and LOGSDON:
FATIGUE
Effect of temperature Decreasing the temperature from 297 to 4.2 K decreases the near-threshold FCGR properties of Inconel 706 at R = 0.1 and 0.8 (Figs 5 and 6). The influence of temperature on crack growth rates is much greater at R = 0.8 than at R = 0.1. In the higher temperature regions ( 2 297 K), it has been reported that oxide and/or roughness-induced crack closure can be used to explain the effect of temperature on the rates of near-threshold crack propagation [24,29,58-61]. As explained below, the concept of crack closure, however, cannot rationalize the effect of temperature on the near-threshold FCGR properties of Inconel706 in the temperature range of 297 to 4.2 K. Firstly, if crack closure could explain the influence of temperature on crack propagation behavior, it would be expected that at R = 0.8 the difference in crack growth rates between 297 and 4.2 K would be negligible since crack closure is minimal at this high R-ratio. This predicted trend is in direct contrast with the experimentally observed data, which shows a significant temperature effect on the rates of crack propagation at R = 0.8 (Figs 5 and 6). Secondly, if crack closure was the reason for the temperature effect, it would be expected that at R = 0.1, the growth rates of fatigue cracks in Inconel706 at 297 K would be slower than those at 4.2 K since crack closure (or surface roughness) levels are greater, and thus, AKcffis smaller at 297 K (Figs 11 and 14). This trend was not observed experimentally in Figs 5 and 6. Thirdly, if crack closure could explain the effect of temperature on the near-threshold FCGR properties of Inconel 706, no difference in the plot of crack propagation rates versus AhR would be observed at 297 and 4.2 K. In contrast, crack closure measurements demonstrate that by using AKeR,a large difference in the crack growth rates at 297 and 4.2 K still exists (Figs 12 and 13). Thus, crack closure cannot explain the effect of temperature on the rates of near-threshold crack propagation in Inconel 706. In the plots of da/dN vs A&, the crack propagation rates at 4.2 K are much slower than those at 297 K. Furthermore, AKth.cR at 4.2 K is approximately 2.3 times greater than at 297 K (Table 2). Thus, after taking crack closure into consideration, different FCGR properties at 297 and 4.2 K still exist. This trend indicates that there is an intrinsic temperature effect on the near-threshold FCGR behavior of Inconel 706. Recall, the yield and ultimate strengths of Inconel 706 increase by approximately twenty percent as the temperature is decreased from 297 to 4.2 K (see Table 1). Based on the literature data, Young’s modulus increases by no more than ten percent when cooling from 297 to 4.2 K [62]. Therefore, the variations in material strength or Young’s modulus between 297 and 4.2 K cannot totally account for the much greater differences in AK,h at these two temperatures. It is suggested that dislocation dynamics account
CRACK
GROWTH
IN INCONEL
706
for the effect of temperature on the FCGR properties in the low temperature regime. Recent investigations in Fe and Fe + Si alloys by Yu, Esaklul and Gerberich [15] revealed that the values of AK,,,,, on cooling from 297 to 123 K could not correlate with the increase in material strength, as observed in the subject Inconel 706. The increase in AK,,.,, for the Fe and Fe + Si alloys on cooling, however, could be related to the increase in the thermal component, u*, of material strength [ 151. Therefore, Yu et al. [I 51 reported that the improvement in near-threshold crack growth resistance with decreasing temperature could be associated with a thermally activated process in light of a dislocation dynamics model. In other words, the thermally activated energy available to mobilize the dislocations over the barriers near the fatigue crack tip generally decreases with decreasing temperature, thereby giving an increase in crack growth resistance at cryogenic temperatures. Moreover, Yu et al. [15] can relate AKth cR at different temperatures to the strain rate sensitivity (m*), i.e. AK,.,,a (1 + m*)“*, where m * = dlni/dlna* and i is the strain rate. Similarly, Fine et al. [12,63] derived the following equation using a dislocation dynamics model AK,,, a a,(2ns)‘~*
(2)
where 0, is the stress to activate a dislocation source at the crack tip and at a distance s from it. They suggest that Q, be related to thermally activated processes, such as, dislocation cross-clip. The rates of thermally activated processes decrease with decreasing temperature, thereby increasing cr, and thus, AK,,, at lower temperatures. Consequently, dislocation dynamics appears to provide a possible rationale for explaining the influence of temperature on the FCGR properties of Inconel 706.
5. CONCLUSIONS 1. The near-threshold FCGR properties of Inconel 706 were developed at 297 and 4.2 K. Specimen orientation did not influence crack propagation behavior. 2. Increasing the R-ratio from 0.1 to 0.8 generally increased near-threshold crack propagation rates. At 297 K, a significant R-ratio effect was observed, while at 4.2 K, the R-ratio effect was minimal. 3. Decreasing the test temperature from 297 to 4.2 K decreased the crack propagation rates. Increasing the R-ratio from 0.1 to 0.8 increased the temperature effect. 4. At 297 K, crystallographic crack growth was observed on fracture surfaces. Decreasing AK or decreasing R-ratio was found to increase the extent of clear-cut crystallographic fracture. At 4.2 K. crystallographic fracture was also observed. The extent of clear-cut crystallographic fracture at 4.2 K, however, was not as significant as at 297 K.
LIAW and LOGSDON:
FATIGUE
CRACK GROWTH
Crystallographic fracture at 4.2 K was found to be insensitive to AK or R-ratio. 5. The levels of surface roughness at 297 K were much greater than those at 4.2 K. At 297 K, surface roughness increased with decreasing AK, while at 4.2 K, it was insensitive to AK. 6. The thicknesses of oxide debris on the fracture surfaces of Inconel 706 near-threshold FCGR specimens tested at either 297 or 4.2 K were measured, and found to be less than 100 A. These thin oxide deposits were not expected to introduce significant oxideinduced crack closure. 7. Direct crack closure measurements indicated that there was a close relationship between crack closure level and surface roughness. Thus. roughnessinduced crack closure was believed to be the dominant mechanism governing the near-threshold FCGR behavior of Inconel 706. 8. Crack closure levels were greater at 297 K than at 4.2 K. At 297 K, the levels of crack closure increased with decreasing AK, while at 4.2 K, they were insensitive to AK. 9. At 297 and 4.2 K, the effects of R-ratio on the rates of near-threshold crack growth could be explained by the concept of roughness-induced crack closure. The more significant R-ratio effect at 297 K compared with that at 4.2 K correlated with the much rougher fracture surfaces observed at 297 K. IO. Crack closure could not account for the influence of temperature on the near-threshold FCGR properties of Inconel 706. The increase in material strength on cooling from 297 to 4.2 K was not significant enough to explain the effect of temperature on crack growth behavior. Dislocation dynamics offers a qualitative explanation for the temperature effect. Acknowledgements-The authors are very grateful to E. J. Helm. R. R. Hovan. G. McFetridee. A. R. Petrush and M. G. Peck for conducting the diffi< cryogenic temperature near-threshold FCGR experiments. The authors additionally wish to thank D. Detar. R. L. Conroy. T. Mullen and A. Karanovich for characterizing the specimen fracture surfaces. Financial support for this program was provided by the Advanced Programs Department. Westinghouse Steam Turbine-Generator Division.
IN INCONEL 706
1743
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