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Dynamic strain aging influence on the cyclic behavior of zircaloy-4
Scripta Materialia, Vol. 34, No. 2, pp.281-285,1996 Elsevier Science Ltd Copyright 8 1995 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/96 $12.00 + .OO
Pergamon 0956-716X(95)00502-1
DYNAMIC STRAIN AGING INFLUENCE ON THE CYCLIC BEHAVIOR OF ZIRCALOY-4 A. F. Armas, I. Alvarez-Armas
and G. Moscato
lnstituto de Fisica Rosario, CONICET-Universidad National de Rosario, Bv. 27 de Febrero 2 10 Bis, 2000 Rosario, Argentina (Received June 22, 1995) (Revised August 7, 1995) Introduction Dynamic strain aging is a very important factor in the plastic deformation of zirconium and zirconium alloys and its aspects in uniaxial tensile tests have been the subject of several studies. Evidence of yield points in the stmss-strain curve (l), appearance of plateaus or peaks in the flow stress-temperature diagram (2), discontinuous plastic flow (3), abnormal strain rate sensitivity (4) have been reported in the literature. These anomalous mechanical behaviors were observed in these metals within the temperature range 473 to 823 K . Little attention, however, has been paid to the influence of dynamic strain aging on the fatigue or cyclic behavior of these materials. Lee and Hill (5) reported, in their study of the effect of oxygen on the fatigue behavior of Zircaloy-2 at 623 K, that the stress range increased during cycling until fracture occurred. These authors attributed this effect to the normal fatigue hardening typical of annealed materials. In a previous work (6), it was reported that Zircaloy-4 samples show cyclic hardening, and it was attributed to an increased rate of dislocation accumulations due to dislocation-solute atoms interactions. The purpose of the present study is to examine the cyclic deformation characteristics of Zircaloy-4 in the temperature range 573-873 K and to show that the abnormal cyclic hardening observed in this material can be considered as a new aspect of dynamic strain aging. Experimental Procedures From Zircaloy-4 bars, prepared in accordance with ASTM B550 Grade 704, shallow hour-glass shaped specimens were machined (for detailed testing conditions and the survey of specimens, see (7)). The main grain diameter was 20 pm. The chemical composition of the alloy is (in wt.%): Sn-1.37, Fe-O.14, Cr-0.10, C-0.01, o-0.14, N-0.004, H-20 ppm, Zr-balance. Strain controlled cyclic tests were carried out under total axial strain control using a fully reversed triangular wave on a Type 1362 Instron machine. The total strain range used for most of the tests was AE, = 0.0 1 and the total strain rate 4 = 2x 10-3s-‘. The tests were performed in air and in the temperature range from room temperature to 873 K, and they were always started in tension. In order to correlate the mechanical behavior with the dislocation structure of fatigued specimens, thinfoil discs were prepared from sections cut parallel to the tensile axes. The discs were electropolished with a solution of 10 % perchloric acid, 35 % n-butanol and 55 % methanol. The foils were examined in a Philips EM 300 transmission electron microscope operating at 100 kV. 281
282
DYNAMIC STRAIN AGING
-
-F---r--y,....., 1
Figure 1. Stress response of Zircaloy-4
,,.,,.,I
10
100
Number
of Cycles,
Vol. 34. No. 2
.I
1000 N+%
cycled with a total strain range of 0.01 at various temperatures.
Results and Discussion Figure 1 shows the cyclic behavior of Zircaloy-4 samples at various temperatures. The variation in the peak tensile stress of the cycles is plotted versus N + 3/4 (here N is the number of cycles) for the temperatures 573,623,673,713,743,773, 823 and 873 K. In this way a 1 in the abscissa represents l/4 cycle, and the corresponding ordinate is the flow stress for the first tensile stroke (e, = 0.5%). A rapid hardening stage followed by a small softening that reaches a saturation state where the flow stress remains constant until fatigue failure sets in was observed at 573 K. At 873 K no hardening or softening effect takes place after the first cycle and nearly up to fracture. This saturation character observed for this material from the beginning of the test was rationalized by dislocations climb and cross slip processes activated at high temperature and that relieve all stress concentration (8).
Ar.,=l% 3
zoo-
I -
160-
v=O.l Hz
1 v) 160j7
140-
% 120&t , 1004 I,
.,
., 0
500
. 1000
Number
Figure 2. Cyclic hardening
,
( 1500 of Cycles,
2000
, 2500
. 3
N
curves at various temperatures
DYNAMIC STRAIN AGING
Vol. 34, No. 2
0.1
0.0
0.2
0.3
0.4
Total Strain
283
0.5
0.6
(%J
Figure 3. Tensile stress-strain curves in the temperature range 300-873 K. (q = 2x 10-k’)
This paper is primarily concerned with the intermediate temperatures range between 673 and 823 K characterized by a progressive cyclic hardening which is more pronounced between 673 and 773 K. Only one reference (!i) was found in the literature about this effect. It was attributed to the cyclic hardening expected to take place in an annealed material. When an annealed metal is cycled its flow stress increases rapidly in the first cycles of life. However, the rate of hardening falls to zero and a saturated stress condition is reached. The metal is then considered to be “in saturation”. As can be seen from Fig. 1, this is not the case for Zircaloy-4 cycled in the above temperature range. Plotting the number of cycles in a linear scale, a common feature of the curves is evident. Figure 2 shows curves for the temperatures 623, 673, 713 and 743 K. From the figure it can be inferred that the cyclic behavior of this material can be divided in three regions: a first stage with a high but decreasing hardening rate, a second stage characterized as a region where the peak tensile stress, u, varies linearly with the number of cycles and that is interrupted by the development of a third stage. The third stage represents a period of decreasing rate of hardening where the stress goes through a maximum and then falls continuously as a consequence of the specimen failure.
0.4
I
7
’
I
Tempentwe
I
,0.020&J
(K)
Figure 4. Yield stress, strain hardening rate and cyclic hardening rate (normalized with respect to the Young’s modulus) as a function of temperature.
DYNAMIC STRAIN AGING
284
For the second stage, a relationship
Vol.34,No.2
of the form o = a + bN
(uin MPa),
where N is the number of cycles, is obeyed. The parameters a and b are dependent on the temperature of the test. This linear dependence of the stress with the number of cycles occurs within a cycle range whose extremes are also dependent on temperature. In this expression b is the cyclic hardening rate. It is clear from these curves that a “saturation stage” is not observed in Zircaloy-4 cycled in this intermediate temperatures range. At the beginning of the curves in Fig. 1 one of the typical phenomenon of strain aging is evident, namely the anomalous flow stress dependence on temperature. In order to determine the interval of temperature where strain aging aspects are observed in uniaxial tests, the evolution of the stress-strain curves versus temperature was analyzed. Figure 3 shows stress-strain curves of Zircaloy-4 for the temperature range 300-873 K. Each of these curves corresponds to the first loading ramp in a cyclic test performed at the indicated temperature. They show the tensile behavior of the sample during the first tensile stroke up to the total strain amplitude of 0.5 % and with a total strain rate 4 = 2x 10”s’. It is evident from the figure that a strong influence of temperature exists in the range 300-573 K. In the range 673-773 K, the tensile stress-strain curves become almost independent of temperature, and at 873 K a small strain hardening rate is observed. The yield stress, ey, and the average strain hardening rate, AolAe, can be estimated from these curves. The yield stress was taken to be the flow stress at 0.1% plastic strain. The average strain hardening rate is the slope of an apparent linear fit for the plastic region of the curves. Points of the curves inside the total strain interval between 0.3 and 0.5% were selected for the linear fit. In Fig. 4 the yield stress and the average strain hardening rate, obtained as explained above, are plotted against temperature. Both curves were normalized with respect to the Young’s modulus, E, to account for its variation with temperature. The Young’s modulus and its temperature dependence were obtained from Rosinger et al. (9). Note that the yield stress shows a plateau between 673 and 823 K and the strain
Figure 5. Perpendicular dislocation walls structure observed by TEM in a failed specimen fatigued at 758 K.
Vol. 34, No. 2
DYNAMIC STRAIN AGING
285
hardening rate curve shows a shallow maximum in the same temperature range. As it is known, both aspects are manifestations of dynamic strain aging (4). The average cyclic hardening rate, b = Ao/AN, corresponding to the second stage of cyclic hardening curves obtained at temperatures within the temperature range 573-873 K is also plotted in Fig. 4. The striking feature of this curve is the presence of a sharp peak in the same temperature interval where dynamic strain aging aspects were observed. From this figure it is evident that the cyclic hardening rate is strongly influenced by strain aging phenomena and that the strongest effect occurs on samples cycled near 773 K. The dislocation arrangement observed by TEM in specimens showing the higher linear cyclic hardening rate is characterized by two-dimensional wall structures. These structures resemble the so called “labyrinth structure” observed in cyclically deformed fee materials (10). Figure 5, corresponding to a sample fatigued up to failure at 758 K, shows an example of these structures which consists of dislocations walls oriented perpendicular and parallel to the primary slip direction. The formation of these structures can be rationalized by an increase of dislocation accumulations due to stronger dislocation-solute atoms interactions. Their correlation with the linear cyclic hardening observed is being explored and will be dealt with in a more extensive paper. Conclusions
1. The cyclic behavior of Zircaloyd in the temperature range 573-873 K is strongly temperature dependent. A cyclic saturation stage was only observed at 573 and 873 K. 2. A new aspect of dynamic strain aging has been found. The striking feature of the cyclic behavior of Zircaloy-4 cycled in the temperature interval 623-823 K is a rapid hardening followed by a linear increase of the peak tensile stress with the number of cycles. On plotting the slope of this linear stage, the cyclic hardening rate, against temperature a sharp peak is observed in the above temperature interval where dynamic strain aging phenomena are present. References 1. 2. 3. 4. 5. 6. 7.
W.R.Thorpe and 1.0. Smith, J. Nucl. Mat., 78,49, (1978). V. Ramachandran and R.E. Reed-Hill, Met. Trans., 1,2105, (1970). B. Ramaswami and G.B. Craig, Trans. AIME, 239,1226, (1967). A.M. Garde, 13.Aigeltinger, B.N. Woodruff, and R.E. Reed-Hill, Met. Trans. 6A, 1183, (1975). D. Lee and P.T. Hill, J. Nucl. Mat., 60,227, (1976). A.F. Armas, I. Alvarez-Armas and G. Mansilla, J. Mater. Sci., 27, 1307, (1992). A.F. Armas and I. Alvarez-Armas, in “Zirconium in the Nuclear Industry”. ASTM STP 939, edited by R.B. Adamson and L.F.P. Van Swam, American Society for Testing and Materials, 617, (1987). 8. 1. Alvarez-Armas, A.F. Armas and R. Versaci, J. Mater. Sci., 25,2454, (1990). 9. H.E. Rosinger, LG. Ritchie and A.J. Shillinglaw, “Young’s Modulus of Crystal Bar Zirconium and Zirconium Alloys (Zircaloy-2,Zircaloy_4,Zirconium-2.5wt% Niobium) to lOOOK”,Atomic Energy of Canada Limited, Report AECL-523 1, September 1975. 10. F. Ackermann, L.P. Kubin, J. Lepinoux and H. Mughrabi, Acta Metall. 32, 715, (1984).
Pergamon 0956-716X(95)00502-1
DYNAMIC STRAIN AGING INFLUENCE ON THE CYCLIC BEHAVIOR OF ZIRCALOY-4 A. F. Armas, I. Alvarez-Armas
and G. Moscato
lnstituto de Fisica Rosario, CONICET-Universidad National de Rosario, Bv. 27 de Febrero 2 10 Bis, 2000 Rosario, Argentina (Received June 22, 1995) (Revised August 7, 1995) Introduction Dynamic strain aging is a very important factor in the plastic deformation of zirconium and zirconium alloys and its aspects in uniaxial tensile tests have been the subject of several studies. Evidence of yield points in the stmss-strain curve (l), appearance of plateaus or peaks in the flow stress-temperature diagram (2), discontinuous plastic flow (3), abnormal strain rate sensitivity (4) have been reported in the literature. These anomalous mechanical behaviors were observed in these metals within the temperature range 473 to 823 K . Little attention, however, has been paid to the influence of dynamic strain aging on the fatigue or cyclic behavior of these materials. Lee and Hill (5) reported, in their study of the effect of oxygen on the fatigue behavior of Zircaloy-2 at 623 K, that the stress range increased during cycling until fracture occurred. These authors attributed this effect to the normal fatigue hardening typical of annealed materials. In a previous work (6), it was reported that Zircaloy-4 samples show cyclic hardening, and it was attributed to an increased rate of dislocation accumulations due to dislocation-solute atoms interactions. The purpose of the present study is to examine the cyclic deformation characteristics of Zircaloy-4 in the temperature range 573-873 K and to show that the abnormal cyclic hardening observed in this material can be considered as a new aspect of dynamic strain aging. Experimental Procedures From Zircaloy-4 bars, prepared in accordance with ASTM B550 Grade 704, shallow hour-glass shaped specimens were machined (for detailed testing conditions and the survey of specimens, see (7)). The main grain diameter was 20 pm. The chemical composition of the alloy is (in wt.%): Sn-1.37, Fe-O.14, Cr-0.10, C-0.01, o-0.14, N-0.004, H-20 ppm, Zr-balance. Strain controlled cyclic tests were carried out under total axial strain control using a fully reversed triangular wave on a Type 1362 Instron machine. The total strain range used for most of the tests was AE, = 0.0 1 and the total strain rate 4 = 2x 10-3s-‘. The tests were performed in air and in the temperature range from room temperature to 873 K, and they were always started in tension. In order to correlate the mechanical behavior with the dislocation structure of fatigued specimens, thinfoil discs were prepared from sections cut parallel to the tensile axes. The discs were electropolished with a solution of 10 % perchloric acid, 35 % n-butanol and 55 % methanol. The foils were examined in a Philips EM 300 transmission electron microscope operating at 100 kV. 281
282
DYNAMIC STRAIN AGING
-
-F---r--y,....., 1
Figure 1. Stress response of Zircaloy-4
,,.,,.,I
10
100
Number
of Cycles,
Vol. 34. No. 2
.I
1000 N+%
cycled with a total strain range of 0.01 at various temperatures.
Results and Discussion Figure 1 shows the cyclic behavior of Zircaloy-4 samples at various temperatures. The variation in the peak tensile stress of the cycles is plotted versus N + 3/4 (here N is the number of cycles) for the temperatures 573,623,673,713,743,773, 823 and 873 K. In this way a 1 in the abscissa represents l/4 cycle, and the corresponding ordinate is the flow stress for the first tensile stroke (e, = 0.5%). A rapid hardening stage followed by a small softening that reaches a saturation state where the flow stress remains constant until fatigue failure sets in was observed at 573 K. At 873 K no hardening or softening effect takes place after the first cycle and nearly up to fracture. This saturation character observed for this material from the beginning of the test was rationalized by dislocations climb and cross slip processes activated at high temperature and that relieve all stress concentration (8).
Ar.,=l% 3
zoo-
I -
160-
v=O.l Hz
1 v) 160j7
140-
% 120&t , 1004 I,
.,
., 0
500
. 1000
Number
Figure 2. Cyclic hardening
,
( 1500 of Cycles,
2000
, 2500
. 3
N
curves at various temperatures
DYNAMIC STRAIN AGING
Vol. 34, No. 2
0.1
0.0
0.2
0.3
0.4
Total Strain
283
0.5
0.6
(%J
Figure 3. Tensile stress-strain curves in the temperature range 300-873 K. (q = 2x 10-k’)
This paper is primarily concerned with the intermediate temperatures range between 673 and 823 K characterized by a progressive cyclic hardening which is more pronounced between 673 and 773 K. Only one reference (!i) was found in the literature about this effect. It was attributed to the cyclic hardening expected to take place in an annealed material. When an annealed metal is cycled its flow stress increases rapidly in the first cycles of life. However, the rate of hardening falls to zero and a saturated stress condition is reached. The metal is then considered to be “in saturation”. As can be seen from Fig. 1, this is not the case for Zircaloy-4 cycled in the above temperature range. Plotting the number of cycles in a linear scale, a common feature of the curves is evident. Figure 2 shows curves for the temperatures 623, 673, 713 and 743 K. From the figure it can be inferred that the cyclic behavior of this material can be divided in three regions: a first stage with a high but decreasing hardening rate, a second stage characterized as a region where the peak tensile stress, u, varies linearly with the number of cycles and that is interrupted by the development of a third stage. The third stage represents a period of decreasing rate of hardening where the stress goes through a maximum and then falls continuously as a consequence of the specimen failure.
0.4
I
7
’
I
Tempentwe
I
,0.020&J
(K)
Figure 4. Yield stress, strain hardening rate and cyclic hardening rate (normalized with respect to the Young’s modulus) as a function of temperature.
DYNAMIC STRAIN AGING
284
For the second stage, a relationship
Vol.34,No.2
of the form o = a + bN
(uin MPa),
where N is the number of cycles, is obeyed. The parameters a and b are dependent on the temperature of the test. This linear dependence of the stress with the number of cycles occurs within a cycle range whose extremes are also dependent on temperature. In this expression b is the cyclic hardening rate. It is clear from these curves that a “saturation stage” is not observed in Zircaloy-4 cycled in this intermediate temperatures range. At the beginning of the curves in Fig. 1 one of the typical phenomenon of strain aging is evident, namely the anomalous flow stress dependence on temperature. In order to determine the interval of temperature where strain aging aspects are observed in uniaxial tests, the evolution of the stress-strain curves versus temperature was analyzed. Figure 3 shows stress-strain curves of Zircaloy-4 for the temperature range 300-873 K. Each of these curves corresponds to the first loading ramp in a cyclic test performed at the indicated temperature. They show the tensile behavior of the sample during the first tensile stroke up to the total strain amplitude of 0.5 % and with a total strain rate 4 = 2x 10”s’. It is evident from the figure that a strong influence of temperature exists in the range 300-573 K. In the range 673-773 K, the tensile stress-strain curves become almost independent of temperature, and at 873 K a small strain hardening rate is observed. The yield stress, ey, and the average strain hardening rate, AolAe, can be estimated from these curves. The yield stress was taken to be the flow stress at 0.1% plastic strain. The average strain hardening rate is the slope of an apparent linear fit for the plastic region of the curves. Points of the curves inside the total strain interval between 0.3 and 0.5% were selected for the linear fit. In Fig. 4 the yield stress and the average strain hardening rate, obtained as explained above, are plotted against temperature. Both curves were normalized with respect to the Young’s modulus, E, to account for its variation with temperature. The Young’s modulus and its temperature dependence were obtained from Rosinger et al. (9). Note that the yield stress shows a plateau between 673 and 823 K and the strain
Figure 5. Perpendicular dislocation walls structure observed by TEM in a failed specimen fatigued at 758 K.
Vol. 34, No. 2
DYNAMIC STRAIN AGING
285
hardening rate curve shows a shallow maximum in the same temperature range. As it is known, both aspects are manifestations of dynamic strain aging (4). The average cyclic hardening rate, b = Ao/AN, corresponding to the second stage of cyclic hardening curves obtained at temperatures within the temperature range 573-873 K is also plotted in Fig. 4. The striking feature of this curve is the presence of a sharp peak in the same temperature interval where dynamic strain aging aspects were observed. From this figure it is evident that the cyclic hardening rate is strongly influenced by strain aging phenomena and that the strongest effect occurs on samples cycled near 773 K. The dislocation arrangement observed by TEM in specimens showing the higher linear cyclic hardening rate is characterized by two-dimensional wall structures. These structures resemble the so called “labyrinth structure” observed in cyclically deformed fee materials (10). Figure 5, corresponding to a sample fatigued up to failure at 758 K, shows an example of these structures which consists of dislocations walls oriented perpendicular and parallel to the primary slip direction. The formation of these structures can be rationalized by an increase of dislocation accumulations due to stronger dislocation-solute atoms interactions. Their correlation with the linear cyclic hardening observed is being explored and will be dealt with in a more extensive paper. Conclusions
1. The cyclic behavior of Zircaloyd in the temperature range 573-873 K is strongly temperature dependent. A cyclic saturation stage was only observed at 573 and 873 K. 2. A new aspect of dynamic strain aging has been found. The striking feature of the cyclic behavior of Zircaloy-4 cycled in the temperature interval 623-823 K is a rapid hardening followed by a linear increase of the peak tensile stress with the number of cycles. On plotting the slope of this linear stage, the cyclic hardening rate, against temperature a sharp peak is observed in the above temperature interval where dynamic strain aging phenomena are present. References 1. 2. 3. 4. 5. 6. 7.
W.R.Thorpe and 1.0. Smith, J. Nucl. Mat., 78,49, (1978). V. Ramachandran and R.E. Reed-Hill, Met. Trans., 1,2105, (1970). B. Ramaswami and G.B. Craig, Trans. AIME, 239,1226, (1967). A.M. Garde, 13.Aigeltinger, B.N. Woodruff, and R.E. Reed-Hill, Met. Trans. 6A, 1183, (1975). D. Lee and P.T. Hill, J. Nucl. Mat., 60,227, (1976). A.F. Armas, I. Alvarez-Armas and G. Mansilla, J. Mater. Sci., 27, 1307, (1992). A.F. Armas and I. Alvarez-Armas, in “Zirconium in the Nuclear Industry”. ASTM STP 939, edited by R.B. Adamson and L.F.P. Van Swam, American Society for Testing and Materials, 617, (1987). 8. 1. Alvarez-Armas, A.F. Armas and R. Versaci, J. Mater. Sci., 25,2454, (1990). 9. H.E. Rosinger, LG. Ritchie and A.J. Shillinglaw, “Young’s Modulus of Crystal Bar Zirconium and Zirconium Alloys (Zircaloy-2,Zircaloy_4,Zirconium-2.5wt% Niobium) to lOOOK”,Atomic Energy of Canada Limited, Report AECL-523 1, September 1975. 10. F. Ackermann, L.P. Kubin, J. Lepinoux and H. Mughrabi, Acta Metall. 32, 715, (1984).