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Fatigue and crack growth behavior of VCrTi alloys
.__ __ FiiB
J-k!i% ELSEVIER
Journal of Nuclear Materials 233-237
journal of nuclear tnaterials
(1996) 488-491
Fatigue and crack growth behavior of V-Cr-Ti B.G. Gieseke 2, C.O. Stevens, M.L. Grossbeck Revised 30 September
1995; accepted 26 February
alloys
’
*
1996
Abstract The results of in-vacuum low cycle fatigue tests are presented for unirradiated V-SCr-5Ti tested at 25, 250, and 400°C. A pronounced environmental degradation of the fatigue properties is observed in this alloy at 25°C. Fatigue life was reduced by as much as 84% when testing was completed in a rough vacuum. Cyclic stress range data and SEM observations suggest that this reduction is due to a combination of increases in rates of crack initiation and subsequent growth. In high vacuum, the fatigue results also show a trend of increasing cyclic life with increasing temperature between 25 and 400°C. At 25O”C, life averages 1.7 times that at 25”C, and at 4oo”C, life averages 3.2 times that at 25°C. A comparison of low cycle fatigue results for V-SCr-5Ti was made to 20% cold-worked 316 stainless steel and several vanadium-base alloys. The results suggest that V-.5Cr-5Ti has better resistance to fatigue than 316.SS in the temperature range of 25 to 400°C. At 400°C. the data also show that V-SCr-5Ti out performs Vanstar alloys 7 and 8 over the entire range of strains investigated. Furthermore, the fatigue properties of the V-SCr-5Ti alloy compare favorably to V- 15Cr-5Ti (at 25°C) and Vanstar 9 (at 400°C) at strains greater than 1%. At lower strains, the lower fatigue resistance of V-SCr-5Ti is attributed to the higher strengths of the V-ISCr-5Ti and Vanstar 9 alloys.
1. Introduction
It is likely that most early fusion devices will operate in a cyclic mode. Additionally, plasma disruptions will produce cyclic stresses in the first wall and blanket structures. For these reasons, fatigue in steels has received attention and has been tested in both unirradiated and irradiated conditions. Favorable neutron activation characteristics, as well as low thermal fatigue strains, has drawn attention to vanadium alloys, in particular V-Cr-Ti alloys. In the present paper, the results of fatigue tests on V-5Cr-5Ti and fatigue crack growth tests on V-4Cr-4Ti are presented.
* Corresponding author. Tel.: + I-423-5745065; fax: + i-4235740641; e-mail: [email protected]. ’ Research sponsored by the Office of Fusion Energy. U.S. Department of Energy, under contract DE-AC05-840R21400 with Lockheed Martin Energy Systems. ’ Present address: Cincinnati Testing Laboratory, 417 Northland Blvd., Cincinnati, OH 45240, USA. 0022.3
2. Materials
and procedures
Specimens for low cycle fatigue (LCF) experiments were fabricated from a 6.35 mm thick plate of V-SCr-5Ti produced by Teledyne Wah Chang (Heat 832394). The chemical composition (wt%,) of the plate was 5.1 Ti, 4.5 Cr, 3 IO ppm Si, 35 ppm C, 364 ppm 0,52 ppm N, and 1. I ppm H with the balance being vanadium. The plate was received in the annealed condition (I h at 1125°C) and miniature hourglass specimens were fabricated with 3.18 mm diameters (cf. Ref. [2]). Following machining, specimens were wrapped in Ta foil, and annealed at 1125°C for 1 h in a vacuum of less than 1 X IO-’ Pa. The resulting microstructure was characterized by an equiaxed grain structure with an average ASTM grain size of 45 pm. LCF tests were conducted on a servo-hydraulic test frame using a strain computer to convert diametral strain to an equivalent axial strain. The tests were conducted in axial strain control at a constant axial strain rate of 4 X IO-‘/s. Heating was accomplished using radio frequency induction, and temperatures were maintained within & 2°C of the desired setpoint. Initial tests at room temperature
I I S/96/$1 5.00 Copyright 0 I996 Published by Elsevicr Science B.V. All rights reserved I 15(96)00236-X
P/I SOO22-3
B.G. Girseke et al./Journnl
oj’Nuclear Muterids
were conducted in a rough vacuum of y 2.6 X IO3 Pa. All other experiments were initiated after a pressure of less than 7.3 X 10m6 Pa was obtained while at the desired test temperature. For fatigue crack propagation tests, 3/4T compact specimens were fabricated from a 6.35 mm thick plate of V-4Cr-4Ti produced by Teledyne Wah Chang (Heat 832665). The chemical composition (wt%) of the plate was 4.0 Ti, 3.3 Cr, 800 ppm Si with the balance being vanadium. Specimens were annealed at 950°C for 2 h. Tests were conducted on a servo-hydraulic test frame using a load ratio of 0.10. Testing was in accordance with ASTM standard E647 and crack length was monitored using the dc potential drop technique.
233-237
5 _
(19961488-491
, 1I , , I<
,
489
1 , . %, I I, ,.‘ /,
,
.
The results of the low cycle fatigue tests have been plotted in Fig. 1 and fitted to a strain-life relationship given by: AE, = AN,-” + BNfmp where, AE! = total strain range in %, N, = number of cycles to failure, and A, B, a and p are constants. The values of these constants can be found in Table I. The values of cy are on the same order of magnitude as those reported by Liu [I] for a V-15Cr-5Ti alloy. The values for /3 also compare well with those reported for V-ISCr5Ti, but the values of /3 for both V-15Cr-5Ti and V-SCr-5Ti differ from the 0.5 to 0.6 often reported in engineering alloys. Two observations can readily be drawn from Fig. 1. First, the fatigue lives in high vacuum increase with increasing temperature over the range investigated. This increase in life varies from a factor of I .5 to 4.4 times that observed at 25°C. Swindeman [2] has reported similar results on niobium alloy D-43 where he observed increasing plastic strain resistance (i.e., life) at temperatures up to 871°C. The second observation is that at 25°C there is a considerable effect of environment on the fatigue life. Between rough and high vacuum, there is an increase in life that ranges from a factor of about 5 to 6. Plots of the stress range as a function of cycle count (A(T versus N) indicate that the V-SCr-5Ti alloy cyclically hardens up to the point of crack initiation, which
Ht.
832394‘
(ANL
Ht.
BL-63)
-
0.1
- w -8 L--ni,d
1000
25°C 400°C 25°C 250°C
(Rough (High
-
._
Vat.) Vat)
(High WC.) (High Vat)
I,.,1111
I
I1,.1111 I
I I,
0.1
lo5 Cycles to Failure, N, lo4
tested at 25, 250, and
occurs near the maximum cyclic stress range. In high vacuum at 25”C, initiation takes place early and a considerable fraction of life is spent propagating the crack. Conversely, in rough vacuum crack initiation dominated and crack propagation was rapid. Crack propagation appears to have occurred in a predominantly transgranular fashion with small amounts of intergranular fracture, regardless of environment. The fracture morphology for the test conducted in rough vacuum showed a coarser striation spacing in combination with secondary cracking and a more brittle appearance than that observed for the test conducted in high vacuum. When viewed at a higher magnification, signs of ductility on a microscopic scale were observed for the test in high vacuum, but not for the test in rough vacuum. This suggests that crack face welding has occurred during the compressive portion of the strain cycle in vacuum. Several possible mechanisms exist which may explain the reduction in life observed in experiments conducted in a rough vacuum, including the pickup of 0 and N interstitials. Oxygen and nitrogen are potent hardeners, but diffusion rates of these species are not sufficiently high at 25°C to allow penetration into the alloy and any notable embrittlement. This was demonstrated by Korth and Schmunk [3], who conducted tests in air on Vanstar alloys 7, 8, and 9 at 400°C and found no change in oxygen content in post-test analyses of their samples.
Temperature (“C)
Press. (Pa)
A
CY
B
P
25 25 250 400
2.6 5 7.3 57.3 5 7.3
2.85 2.384 I .47 I .39
0.1464 0.0983 0.0646 0.0552
50093 35143 261 I 611
1.493 I.176 0.823 0.638
IO’ lo-” lo-6 Io-6
-
5
-..+
Table I Constants in strain-life equation for V-SCr-5Ti alloy
X x x x
94-13409 I ,
TWC
Fig. I. Cyclic fatigue data for V-SCr-5Ti 400°C in rough and high vacuums.
3. Results and discussion
ORNL-DWG , . , . . ,(
Fig. 2. Comparison of fatigue properties of V-5Cr-5Ti and V- IX-5Ti alloys to 7 16.SS at 25°C. Data for 316 stainless steel were obtained from Ref. [4] and those for V- 15Cr-5Ti were obtained from Ref. [I].
On the other hand, oxygen or moisture present in the vacuum chamber may have formed a brittle oxide layer on any freshly exposed surfaces, such as those created by the rupture of a surface oxide during plastic defomlation in each cycle or at persistent slip bands. The formation of an oxide layer on the crack faces may prevent rewelding of these surfaces during the compression portion of the strain cycle, again, leading to increased crack growth rates. In addition, hydrogen released during the formation of the oxides may have hardened the material at the crack tip [4]. In Fig. 2, a comparison of the room temperature low cycle fatigue properties of the V-SCr-5Ti alloy is made with both 316 stainless steel [S] and a V- l5Cr-5Ti alloy [I]. From the limited data, both the vanadium alloys appear to have better plastic strain resistance than does 316 stainless. The data may suggest that the endurance strain in the 316~SS is 0.25%, whereas estimates for the V-SCr-5Ti and V-ISCr-5Ti are 0.5% and 0.75%, respectively. At strains greater than I%, the data for the V-base alloys appear to converge, as would be expected since the ductilities do not differ by more than two percent [6] and strain controls life in this regime. The difference found at longer lives (lower strains) is attributed to increased strength of the V-ISCr-5Ti alloy and the resulting reduction in plastic strain ranges. Loomis 161 reports that both the yield and tensile strengths of the 5Cr alloy are 7 I-72% of those for the 15% alloy (oYI = 387 versus 545; u,,i, = 454 versus 634 MPa). Korth and Schmunk [3] tested Vanstar alloys 7, 8 and 9 in air at 400°C using a diametral extensometer and controlling the equivalent axial strain. Their data show little difference between the number of cycles, N,, at initiation and the number of cycles to failure. Ni, and they suggested that atmosphere had little effect on test results. In light of
the results for V-SCr-5Ti alloy, it is likely that Nt for tests conducted in vacuum would have been much greater. Notwithstanding this difficulty, the data are plotted in Fig. 3 and several observations can be drawn. First, at strain ranges of I% or more, the V-SCr-5Ti alloy shows better resistance to fatigue damage than Vanstar 9. At lower total strain ranges, the opposite is true. The better performance of Vanstar 9 at lower total strain ranges is, again, attributed to its higher yield strength (i.e., 360 MPa [3] versus 250 MPa [6]) and the resulting reduction in plastic strains. A second observation is that the V-SCr-5Ti out performs alloys Vanstar 7 and 8 at all strain ranges shown. A comparison of the tensile data of Korth and Schmunk [3] on the Vanstar alloys to that of Loomis [6] on V-SCr-STi indicates that the V-SCr-5Ti has greater strength than the Vanstar alloys 7 and X at 400°C. However, the authors believe that the testing of Vanstar alloys 7, X, and 9 in vacuum would result in higher cyclic lives and probably eliminate much of the observed differences, since these are vanadium-base alloys and should behave in a similar fashion to V-SCr-5Ti. In Fig. 3, a comparison of the low cycle fatigue properties of the V-SCr-STi alloy at 400°C is also made with 316 stainless steel [7] tested at 430°C. In this case, Grossbeck and Liu used the same specimen geometry and test method as was used in testing the V-SCr-5Ti alloy; thus the data should be very easily comparable with the exception of temperature. Again, the results suggest that the V-SCr-STi alloy has better low cycle fatigue properties than does the 316 stainless around 400°C. While insufficient data exist to allow an endurance strain to be determined from the V-5Cr-5Ti alloy, it is expected to be in the 0.550.6% range based upon the results of other studies
ORNL-DWG
94-13414
z5K-F]5
- 1
. V-5Cr-5T1 (Vat) Vanstar 7 (Air) Vanstar 8 (Air) Vanstar 9 (Air) 31655 (430WVac)
0 . 0.1
,
1000
.
*
,1,,,1,,
,,,_I
IO4 Cycles
,
lo5
1
.“,UJo,,
lo6
to Failure, N,
Fig. 3. Comparison of fatigue data for vanadium-base alloys and 316 stainless steel at temperatures 400 to 43D”C. Data for 316 stainless were obtained from Ref. [6]. Data for Vanstar alloys were obtained from Ref. [il.
B.C. Gieseke et ul./Journul
c?~Nucleur Mutrriu1.s 233-237
AK (MPa+m)
1
o.8 5
6 7 8 910
20 AK
30
(ksiiin)
Fig. 4. Comparison of crack growth rate data of V-4Cr-4Ti to 3 16.SS at 25°C.
alloy
alloy [1,3]. In comparison, the endurance for 316 stainless is on the order of 0.3-0.4s [6]. The results of fatigue crack growth rate testing are plotted in Fig. 4 along with those for a 316 stainless steel. From this plot, it appears that the V-4Cr-4Ti alloy is inferior to 316 stainless in the moist atmosphere of &lSt Tennessee. The results are in close agreement with those of Van Witzenburg and De Bruyne for V-15Cr-5Ti at 200°C in air 181. It is expected that ongoing tests in high vacuum will show the curves to be closer together or even a reversal in the ranking of these two alloys. While the performance of 3 16 stainless will improve in high vacuum, the magnitude of change is small [9]. In contrast, the low cycle fatigue data on V-SCr-STi suggest that V-4Cr-4Ti will show significant reductions in crack growth rates.
on
vanadium
strain
4. Conclusions Low cycle fatigue tests have been conducted on unirradiated V-SCr-5Ti in vacuum at temperatures of 25, 250, and 400°C. A comparison of the fatigue data generated in rough and high vacuums shows that a pronounced environmental degradation of the fatigue properties exists in this
(1996) 488-491
491
alloy at 25°C. Fatigue life was reduced by as much as 84%. The cyclic stress range data and SEM observations suggest that this difference is due to a combination of differences in rates of crack initiation and subsequent crack growth. In high vacuum, the fatigue results show a trend of increasing cyclic life with increasing temperature between 25 and 400°C. From the limited data available, life at 250°C averages 1.7 times that at 25°C and at 400°C life averages 3.2 times that at 25°C. The low cycle fatigue results of V-Kr-5Ti were compared to 20% cold-worked 316 stainless steel and several vanadium-base alloys. In vacuum, V-SCr-5Ti has better resistance to fatigue than 316-SS in the temperature range of 25 to 400°C. At 4OO”C, the data also show that V-SCr-5Ti out performs Vanstar alloys 7, and 8 over the limited range of available data. Furthermore, the fatigue properties of the V-5Cr-5Ti alloy compare favordbty to V- 15Cr-5Ti (at 25°C) and Vanstar 9 (at 400°C) at strains greater than I%. Differences seen at higher lives (lower strains) are attributed to the higher strength of the V15Cr-5Ti and Vanstar 9 alloys.
References [I] K.C. Liu, J. Nucl. Mater. 103-104 (1981) 913-918. [2] R.W. Swindeman, ASTM STP 4.59, Am. Sot. Test. Mater. (1969) 31-41. [3] G.E. Korth and R.E. Schmunk. ASTM STP 683, ed. J.A. Sprague and D. Krammer, Am. Sot. Test. Mater. (1979) 466-476. [4] A.L. Eustice and O.N. Carlson, Trans. ASME 221 (1961) 238. [5] C.E. Jaske and N.D. Frey, Rockwell International, Report No. ESG-DOE- 13307. [6] B.A. Loomis, Fusion Reactor Mater. Semiannu. Prog. Rep. for Period Ending March 3 1, 1991, U.S. Department of Energy, DOE/ER-0313/10 (1991) 145-155. [7] M.L. Grossbeck and K.C. Liu. Nucl. Technol. 58 (1982) 538-547. [S] W. van Witzenburs and H.J. de Brnyne, ASTM STP 1175, Am. Sot. Test. Mater. (1993) 1201. [9] L.F. Coffin, Met. Trans. 3 (1972) 1777.
J-k!i% ELSEVIER
Journal of Nuclear Materials 233-237
journal of nuclear tnaterials
(1996) 488-491
Fatigue and crack growth behavior of V-Cr-Ti B.G. Gieseke 2, C.O. Stevens, M.L. Grossbeck Revised 30 September
1995; accepted 26 February
alloys
’
*
1996
Abstract The results of in-vacuum low cycle fatigue tests are presented for unirradiated V-SCr-5Ti tested at 25, 250, and 400°C. A pronounced environmental degradation of the fatigue properties is observed in this alloy at 25°C. Fatigue life was reduced by as much as 84% when testing was completed in a rough vacuum. Cyclic stress range data and SEM observations suggest that this reduction is due to a combination of increases in rates of crack initiation and subsequent growth. In high vacuum, the fatigue results also show a trend of increasing cyclic life with increasing temperature between 25 and 400°C. At 25O”C, life averages 1.7 times that at 25”C, and at 4oo”C, life averages 3.2 times that at 25°C. A comparison of low cycle fatigue results for V-SCr-5Ti was made to 20% cold-worked 316 stainless steel and several vanadium-base alloys. The results suggest that V-.5Cr-5Ti has better resistance to fatigue than 316.SS in the temperature range of 25 to 400°C. At 400°C. the data also show that V-SCr-5Ti out performs Vanstar alloys 7 and 8 over the entire range of strains investigated. Furthermore, the fatigue properties of the V-SCr-5Ti alloy compare favorably to V- 15Cr-5Ti (at 25°C) and Vanstar 9 (at 400°C) at strains greater than 1%. At lower strains, the lower fatigue resistance of V-SCr-5Ti is attributed to the higher strengths of the V-ISCr-5Ti and Vanstar 9 alloys.
1. Introduction
It is likely that most early fusion devices will operate in a cyclic mode. Additionally, plasma disruptions will produce cyclic stresses in the first wall and blanket structures. For these reasons, fatigue in steels has received attention and has been tested in both unirradiated and irradiated conditions. Favorable neutron activation characteristics, as well as low thermal fatigue strains, has drawn attention to vanadium alloys, in particular V-Cr-Ti alloys. In the present paper, the results of fatigue tests on V-5Cr-5Ti and fatigue crack growth tests on V-4Cr-4Ti are presented.
* Corresponding author. Tel.: + I-423-5745065; fax: + i-4235740641; e-mail: [email protected]. ’ Research sponsored by the Office of Fusion Energy. U.S. Department of Energy, under contract DE-AC05-840R21400 with Lockheed Martin Energy Systems. ’ Present address: Cincinnati Testing Laboratory, 417 Northland Blvd., Cincinnati, OH 45240, USA. 0022.3
2. Materials
and procedures
Specimens for low cycle fatigue (LCF) experiments were fabricated from a 6.35 mm thick plate of V-SCr-5Ti produced by Teledyne Wah Chang (Heat 832394). The chemical composition (wt%,) of the plate was 5.1 Ti, 4.5 Cr, 3 IO ppm Si, 35 ppm C, 364 ppm 0,52 ppm N, and 1. I ppm H with the balance being vanadium. The plate was received in the annealed condition (I h at 1125°C) and miniature hourglass specimens were fabricated with 3.18 mm diameters (cf. Ref. [2]). Following machining, specimens were wrapped in Ta foil, and annealed at 1125°C for 1 h in a vacuum of less than 1 X IO-’ Pa. The resulting microstructure was characterized by an equiaxed grain structure with an average ASTM grain size of 45 pm. LCF tests were conducted on a servo-hydraulic test frame using a strain computer to convert diametral strain to an equivalent axial strain. The tests were conducted in axial strain control at a constant axial strain rate of 4 X IO-‘/s. Heating was accomplished using radio frequency induction, and temperatures were maintained within & 2°C of the desired setpoint. Initial tests at room temperature
I I S/96/$1 5.00 Copyright 0 I996 Published by Elsevicr Science B.V. All rights reserved I 15(96)00236-X
P/I SOO22-3
B.G. Girseke et al./Journnl
oj’Nuclear Muterids
were conducted in a rough vacuum of y 2.6 X IO3 Pa. All other experiments were initiated after a pressure of less than 7.3 X 10m6 Pa was obtained while at the desired test temperature. For fatigue crack propagation tests, 3/4T compact specimens were fabricated from a 6.35 mm thick plate of V-4Cr-4Ti produced by Teledyne Wah Chang (Heat 832665). The chemical composition (wt%) of the plate was 4.0 Ti, 3.3 Cr, 800 ppm Si with the balance being vanadium. Specimens were annealed at 950°C for 2 h. Tests were conducted on a servo-hydraulic test frame using a load ratio of 0.10. Testing was in accordance with ASTM standard E647 and crack length was monitored using the dc potential drop technique.
233-237
5 _
(19961488-491
, 1I , , I<
,
489
1 , . %, I I, ,.‘ /,
,
.
The results of the low cycle fatigue tests have been plotted in Fig. 1 and fitted to a strain-life relationship given by: AE, = AN,-” + BNfmp where, AE! = total strain range in %, N, = number of cycles to failure, and A, B, a and p are constants. The values of these constants can be found in Table I. The values of cy are on the same order of magnitude as those reported by Liu [I] for a V-15Cr-5Ti alloy. The values for /3 also compare well with those reported for V-ISCr5Ti, but the values of /3 for both V-15Cr-5Ti and V-SCr-5Ti differ from the 0.5 to 0.6 often reported in engineering alloys. Two observations can readily be drawn from Fig. 1. First, the fatigue lives in high vacuum increase with increasing temperature over the range investigated. This increase in life varies from a factor of I .5 to 4.4 times that observed at 25°C. Swindeman [2] has reported similar results on niobium alloy D-43 where he observed increasing plastic strain resistance (i.e., life) at temperatures up to 871°C. The second observation is that at 25°C there is a considerable effect of environment on the fatigue life. Between rough and high vacuum, there is an increase in life that ranges from a factor of about 5 to 6. Plots of the stress range as a function of cycle count (A(T versus N) indicate that the V-SCr-5Ti alloy cyclically hardens up to the point of crack initiation, which
Ht.
832394‘
(ANL
Ht.
BL-63)
-
0.1
- w -8 L--ni,d
1000
25°C 400°C 25°C 250°C
(Rough (High
-
._
Vat.) Vat)
(High WC.) (High Vat)
I,.,1111
I
I1,.1111 I
I I,
0.1
lo5 Cycles to Failure, N, lo4
tested at 25, 250, and
occurs near the maximum cyclic stress range. In high vacuum at 25”C, initiation takes place early and a considerable fraction of life is spent propagating the crack. Conversely, in rough vacuum crack initiation dominated and crack propagation was rapid. Crack propagation appears to have occurred in a predominantly transgranular fashion with small amounts of intergranular fracture, regardless of environment. The fracture morphology for the test conducted in rough vacuum showed a coarser striation spacing in combination with secondary cracking and a more brittle appearance than that observed for the test conducted in high vacuum. When viewed at a higher magnification, signs of ductility on a microscopic scale were observed for the test in high vacuum, but not for the test in rough vacuum. This suggests that crack face welding has occurred during the compressive portion of the strain cycle in vacuum. Several possible mechanisms exist which may explain the reduction in life observed in experiments conducted in a rough vacuum, including the pickup of 0 and N interstitials. Oxygen and nitrogen are potent hardeners, but diffusion rates of these species are not sufficiently high at 25°C to allow penetration into the alloy and any notable embrittlement. This was demonstrated by Korth and Schmunk [3], who conducted tests in air on Vanstar alloys 7, 8, and 9 at 400°C and found no change in oxygen content in post-test analyses of their samples.
Temperature (“C)
Press. (Pa)
A
CY
B
P
25 25 250 400
2.6 5 7.3 57.3 5 7.3
2.85 2.384 I .47 I .39
0.1464 0.0983 0.0646 0.0552
50093 35143 261 I 611
1.493 I.176 0.823 0.638
IO’ lo-” lo-6 Io-6
-
5
-..+
Table I Constants in strain-life equation for V-SCr-5Ti alloy
X x x x
94-13409 I ,
TWC
Fig. I. Cyclic fatigue data for V-SCr-5Ti 400°C in rough and high vacuums.
3. Results and discussion
ORNL-DWG , . , . . ,(
Fig. 2. Comparison of fatigue properties of V-5Cr-5Ti and V- IX-5Ti alloys to 7 16.SS at 25°C. Data for 316 stainless steel were obtained from Ref. [4] and those for V- 15Cr-5Ti were obtained from Ref. [I].
On the other hand, oxygen or moisture present in the vacuum chamber may have formed a brittle oxide layer on any freshly exposed surfaces, such as those created by the rupture of a surface oxide during plastic defomlation in each cycle or at persistent slip bands. The formation of an oxide layer on the crack faces may prevent rewelding of these surfaces during the compression portion of the strain cycle, again, leading to increased crack growth rates. In addition, hydrogen released during the formation of the oxides may have hardened the material at the crack tip [4]. In Fig. 2, a comparison of the room temperature low cycle fatigue properties of the V-SCr-5Ti alloy is made with both 316 stainless steel [S] and a V- l5Cr-5Ti alloy [I]. From the limited data, both the vanadium alloys appear to have better plastic strain resistance than does 316 stainless. The data may suggest that the endurance strain in the 316~SS is 0.25%, whereas estimates for the V-SCr-5Ti and V-ISCr-5Ti are 0.5% and 0.75%, respectively. At strains greater than I%, the data for the V-base alloys appear to converge, as would be expected since the ductilities do not differ by more than two percent [6] and strain controls life in this regime. The difference found at longer lives (lower strains) is attributed to increased strength of the V-ISCr-5Ti alloy and the resulting reduction in plastic strain ranges. Loomis 161 reports that both the yield and tensile strengths of the 5Cr alloy are 7 I-72% of those for the 15% alloy (oYI = 387 versus 545; u,,i, = 454 versus 634 MPa). Korth and Schmunk [3] tested Vanstar alloys 7, 8 and 9 in air at 400°C using a diametral extensometer and controlling the equivalent axial strain. Their data show little difference between the number of cycles, N,, at initiation and the number of cycles to failure. Ni, and they suggested that atmosphere had little effect on test results. In light of
the results for V-SCr-5Ti alloy, it is likely that Nt for tests conducted in vacuum would have been much greater. Notwithstanding this difficulty, the data are plotted in Fig. 3 and several observations can be drawn. First, at strain ranges of I% or more, the V-SCr-5Ti alloy shows better resistance to fatigue damage than Vanstar 9. At lower total strain ranges, the opposite is true. The better performance of Vanstar 9 at lower total strain ranges is, again, attributed to its higher yield strength (i.e., 360 MPa [3] versus 250 MPa [6]) and the resulting reduction in plastic strains. A second observation is that the V-SCr-5Ti out performs alloys Vanstar 7 and 8 at all strain ranges shown. A comparison of the tensile data of Korth and Schmunk [3] on the Vanstar alloys to that of Loomis [6] on V-SCr-STi indicates that the V-SCr-5Ti has greater strength than the Vanstar alloys 7 and X at 400°C. However, the authors believe that the testing of Vanstar alloys 7, X, and 9 in vacuum would result in higher cyclic lives and probably eliminate much of the observed differences, since these are vanadium-base alloys and should behave in a similar fashion to V-SCr-5Ti. In Fig. 3, a comparison of the low cycle fatigue properties of the V-SCr-STi alloy at 400°C is also made with 316 stainless steel [7] tested at 430°C. In this case, Grossbeck and Liu used the same specimen geometry and test method as was used in testing the V-SCr-5Ti alloy; thus the data should be very easily comparable with the exception of temperature. Again, the results suggest that the V-SCr-STi alloy has better low cycle fatigue properties than does the 316 stainless around 400°C. While insufficient data exist to allow an endurance strain to be determined from the V-5Cr-5Ti alloy, it is expected to be in the 0.550.6% range based upon the results of other studies
ORNL-DWG
94-13414
z5K-F]5
- 1
. V-5Cr-5T1 (Vat) Vanstar 7 (Air) Vanstar 8 (Air) Vanstar 9 (Air) 31655 (430WVac)
0 . 0.1
,
1000
.
*
,1,,,1,,
,,,_I
IO4 Cycles
,
lo5
1
.“,UJo,,
lo6
to Failure, N,
Fig. 3. Comparison of fatigue data for vanadium-base alloys and 316 stainless steel at temperatures 400 to 43D”C. Data for 316 stainless were obtained from Ref. [6]. Data for Vanstar alloys were obtained from Ref. [il.
B.C. Gieseke et ul./Journul
c?~Nucleur Mutrriu1.s 233-237
AK (MPa+m)
1
o.8 5
6 7 8 910
20 AK
30
(ksiiin)
Fig. 4. Comparison of crack growth rate data of V-4Cr-4Ti to 3 16.SS at 25°C.
alloy
alloy [1,3]. In comparison, the endurance for 316 stainless is on the order of 0.3-0.4s [6]. The results of fatigue crack growth rate testing are plotted in Fig. 4 along with those for a 316 stainless steel. From this plot, it appears that the V-4Cr-4Ti alloy is inferior to 316 stainless in the moist atmosphere of &lSt Tennessee. The results are in close agreement with those of Van Witzenburg and De Bruyne for V-15Cr-5Ti at 200°C in air 181. It is expected that ongoing tests in high vacuum will show the curves to be closer together or even a reversal in the ranking of these two alloys. While the performance of 3 16 stainless will improve in high vacuum, the magnitude of change is small [9]. In contrast, the low cycle fatigue data on V-SCr-STi suggest that V-4Cr-4Ti will show significant reductions in crack growth rates.
on
vanadium
strain
4. Conclusions Low cycle fatigue tests have been conducted on unirradiated V-SCr-5Ti in vacuum at temperatures of 25, 250, and 400°C. A comparison of the fatigue data generated in rough and high vacuums shows that a pronounced environmental degradation of the fatigue properties exists in this
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alloy at 25°C. Fatigue life was reduced by as much as 84%. The cyclic stress range data and SEM observations suggest that this difference is due to a combination of differences in rates of crack initiation and subsequent crack growth. In high vacuum, the fatigue results show a trend of increasing cyclic life with increasing temperature between 25 and 400°C. From the limited data available, life at 250°C averages 1.7 times that at 25°C and at 400°C life averages 3.2 times that at 25°C. The low cycle fatigue results of V-Kr-5Ti were compared to 20% cold-worked 316 stainless steel and several vanadium-base alloys. In vacuum, V-SCr-5Ti has better resistance to fatigue than 316-SS in the temperature range of 25 to 400°C. At 4OO”C, the data also show that V-SCr-5Ti out performs Vanstar alloys 7, and 8 over the limited range of available data. Furthermore, the fatigue properties of the V-5Cr-5Ti alloy compare favordbty to V- 15Cr-5Ti (at 25°C) and Vanstar 9 (at 400°C) at strains greater than I%. Differences seen at higher lives (lower strains) are attributed to the higher strength of the V15Cr-5Ti and Vanstar 9 alloys.
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