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Strain ageing in zirconium-2.5wt% niobium alloy
Journal of Nuclear Materials 67 (1977) 315-317 0 North-Holland Publishing Company
STRAIN AGEING IN ZIRCONIUM-2Swt%
NIOBIUM ALLOY
T.K.SINHAand M.K.ASUNDI Metallurgy Division, Bhabha Atomic Research Centre, Bombay-400
085, India
Received 25 January 1977
The strain-ageing phenomenon, which manifests itself in tensile testing, directly through the appearance of a yield point after interrupted loading and indirectly through the existence of plateau regions in the strength vs temperature curves, and through the appearance of ductility and strain-rate sensitivity minima, has been reported in zirconium and in Zircaloy-2 in the temperature range of 200-500°C [l-6]. In a cold-worked Zr-2Swt%Nb alloy, Steward [7] has observed inflections in the plots of the UTS and the yield strength versus temperature at around 25O”C, a temperature at which the total elongation is close to its minimum value. Lee [8] and Sinha et al. [9] have observed that around this same temperature, the strain-rate sensitivity parameter in this alloy has very small values. AIthough these results strongly indicate the occurrence of strain-ageing in this alloy, it appears that no direct
PLASTIC
Fig. 1. Strain-ageing
experimental observation of strain-ageing has as yet been reported in literature. In our laboratory, a detailed investigation of the strain-ageing behaviour of the heat-treated Zr-2 5 Nb alloy is in progress and this note describe some preliminary results. The Zr-2Swt%Nb alloy used in this investigation was obtained in the form of a doubly consumable-arcmelted ingot of 44 mm diameter, containing 2.55 wt% Nb and 1200 ppm oxygen. Flat tensile specimens were obtained with their axes parallel to the rolling direction, from hot-rolled sheets of 0.5 mm thickness. The specimens, sealed in evacuated silica capsules, were solution-annealed for 30 min at 88O”C, water-quenched and subsequently aged at 550°C for 24 hours. These heat-treated specimens had a hardness of 250 f 5 VPN and contained equiaxed alpha grains in a matrix of acicular ff’ martensite.
STRAIN
PERCENT
parameter as a function of plastic strain in zirconium-2.5wt% niobium alloy. 315
T.K. Sinha, M.K. Asundi /Strain ageing in Zr-Z.Swt%Nb alloys
316
Fig. 2. Strain-ageing
parameter
as a function
TIME 5 of ageing time at 250°C
Tensile tests were carried out in an Instron machine at nominal strain rates of 4 X 10e3 to 4 X lo-‘/min and at a temperature of 250°C. A salt-bath was used to achieve this temperature which was controlled to within ?2”C for each test. During a test, the specimen was strained into the plastic region, the stress reduced to the desired level (5 kg/mm*) and the cross-head stopped. After ageing for the desired lengths of time (TV = 30 to 600 s.), the specimen was reloaded. After yielding, the specimen work-hardened and the cycle was repeated at higher plastic strain. This procedure was continued until necking occurred. Strain-ageing was measured by the parameter Au/o (the ratio of the increase in flow stress to the flow stress at which the test was interrupted). In order to compare the strain ageing behaviour under different testing conditions, (Au/o) was plotted against plastic strain, en, and the values of Au/u at ep = 0.02 were used for comparison. Fig. 1 shows the relationship between Au/u and en. The strain-ageing parameter decreased from a value of 0.02 at 0.4% strain to a value of 0.013 at 2.9% strain at 250°C. Fig. 2 illustrates the results of strain-ageing tests on specimens aged for times ranging from 30 to 600 s. at a temperature of 250°C. It could be seen that strainageing first increased rapidly with ageing time. However, for ageing periods beyond about 300 s, it appeared to level off. The results of strain-ageing tests on specimens aged for 300 seconds at 250°C and deform-
in zirconium-2.5wt%
niobium
alloy.
ed at strain rates between 4 X 10d3 and 4 X lo-‘/min are given in fig. 3. It appeared that though Au/u was not very sensitive to strain rate, it showed a tendency to increase slowly as the strain rate was increased. The strain-ageing behaviour of Zircaloy-2 has been studied extensively by Veevers et al. [5,6] , who have suggested that strain-ageing is caused by the diffusion of interstitial oxygen atoms to glide dislocations. The results of the present experiment were similar in trend to those of Veevers et al., in that the strain-ageing decreased with increasing plastic strain, increased with ageing time and was almost insensitive to strain rates.
NOMINAL
STRAIN
RATE mid
Fig. 3. Strain-ageing parameter at a function of strain rate in zirconium-2.5wt% niobium alloy (300 s at 250°C).
T.K. Sinha, M.K. Asundi /Strain ageing in Zr-2.5wt%Nb alloys
Oxygen has also been found to be responsible for strain-ageing in other metals such as niobium [lo], tantalum [ 1 l] and vanadium [ 121. Again, it is to be noted that the values of the strain-ageing parameter (Au/a) found in the present compared closely with those found in Zircaloy-2 by Veevers and Rotsey [6] at comparable temperatures. It seems likely, in view of these qualitative considerations, that the interaction of interstitial oxygen with glide dislocation is responsible for strain-ageing in the heat-treated Zr-2.5 Nb alloy. The authors are indebted to Shri C.V. Sundaram, Head, Metallurgy Division, for his interest in this work and are grateful to Shri R. Kishore for his assistance with the experimental work.
317
References [ l] J.H. Keeler, Trans. America Sot. Met. 47 (1955)
157. [2] B. Ramaswami and G.B. Criag, Trans. Met. Sot. AIME 239 (1967) 1226. [3] R.F. Mehan and W.E. Weisinger, Knolls Atomic Power Labs., Report KAPL-2110 (1961). [4] V. Ramachandran and R.E. Reed-Hill, Metallurg. Trans. 1 (1970) 2105. (51 K. Veevers, W.B. Rotsey and K.U. Snowden, ASTM-STP 458 (1970) 194. [6] K. Veevers and W.B. Rotsey, I. Nucl. Mater. 27 (1968) 108. [ 71 K.P. Steward, Atomic Energy of Canada Ltd., AECL2250 (1965). (81 D. Lee, Canad. Met. Quart. 11 (1972) 113. [9] T.K. Sinha and M.K. Asundi, J. Nucl. Mater. 67 (1977) 000. [lo] Z.C. Szkopiak and A.P. Miodownik, J. Nucl. Mater. 17 (1965) 20. [ll] S. Hartley, ActaMet. 14 (1966) 1237. [12] S.A. Bradford and O.B. Carlson, Trans. Met. Sot. AIME 224 (1962) 738.
STRAIN AGEING IN ZIRCONIUM-2Swt%
NIOBIUM ALLOY
T.K.SINHAand M.K.ASUNDI Metallurgy Division, Bhabha Atomic Research Centre, Bombay-400
085, India
Received 25 January 1977
The strain-ageing phenomenon, which manifests itself in tensile testing, directly through the appearance of a yield point after interrupted loading and indirectly through the existence of plateau regions in the strength vs temperature curves, and through the appearance of ductility and strain-rate sensitivity minima, has been reported in zirconium and in Zircaloy-2 in the temperature range of 200-500°C [l-6]. In a cold-worked Zr-2Swt%Nb alloy, Steward [7] has observed inflections in the plots of the UTS and the yield strength versus temperature at around 25O”C, a temperature at which the total elongation is close to its minimum value. Lee [8] and Sinha et al. [9] have observed that around this same temperature, the strain-rate sensitivity parameter in this alloy has very small values. AIthough these results strongly indicate the occurrence of strain-ageing in this alloy, it appears that no direct
PLASTIC
Fig. 1. Strain-ageing
experimental observation of strain-ageing has as yet been reported in literature. In our laboratory, a detailed investigation of the strain-ageing behaviour of the heat-treated Zr-2 5 Nb alloy is in progress and this note describe some preliminary results. The Zr-2Swt%Nb alloy used in this investigation was obtained in the form of a doubly consumable-arcmelted ingot of 44 mm diameter, containing 2.55 wt% Nb and 1200 ppm oxygen. Flat tensile specimens were obtained with their axes parallel to the rolling direction, from hot-rolled sheets of 0.5 mm thickness. The specimens, sealed in evacuated silica capsules, were solution-annealed for 30 min at 88O”C, water-quenched and subsequently aged at 550°C for 24 hours. These heat-treated specimens had a hardness of 250 f 5 VPN and contained equiaxed alpha grains in a matrix of acicular ff’ martensite.
STRAIN
PERCENT
parameter as a function of plastic strain in zirconium-2.5wt% niobium alloy. 315
T.K. Sinha, M.K. Asundi /Strain ageing in Zr-Z.Swt%Nb alloys
316
Fig. 2. Strain-ageing
parameter
as a function
TIME 5 of ageing time at 250°C
Tensile tests were carried out in an Instron machine at nominal strain rates of 4 X 10e3 to 4 X lo-‘/min and at a temperature of 250°C. A salt-bath was used to achieve this temperature which was controlled to within ?2”C for each test. During a test, the specimen was strained into the plastic region, the stress reduced to the desired level (5 kg/mm*) and the cross-head stopped. After ageing for the desired lengths of time (TV = 30 to 600 s.), the specimen was reloaded. After yielding, the specimen work-hardened and the cycle was repeated at higher plastic strain. This procedure was continued until necking occurred. Strain-ageing was measured by the parameter Au/o (the ratio of the increase in flow stress to the flow stress at which the test was interrupted). In order to compare the strain ageing behaviour under different testing conditions, (Au/o) was plotted against plastic strain, en, and the values of Au/u at ep = 0.02 were used for comparison. Fig. 1 shows the relationship between Au/u and en. The strain-ageing parameter decreased from a value of 0.02 at 0.4% strain to a value of 0.013 at 2.9% strain at 250°C. Fig. 2 illustrates the results of strain-ageing tests on specimens aged for times ranging from 30 to 600 s. at a temperature of 250°C. It could be seen that strainageing first increased rapidly with ageing time. However, for ageing periods beyond about 300 s, it appeared to level off. The results of strain-ageing tests on specimens aged for 300 seconds at 250°C and deform-
in zirconium-2.5wt%
niobium
alloy.
ed at strain rates between 4 X 10d3 and 4 X lo-‘/min are given in fig. 3. It appeared that though Au/u was not very sensitive to strain rate, it showed a tendency to increase slowly as the strain rate was increased. The strain-ageing behaviour of Zircaloy-2 has been studied extensively by Veevers et al. [5,6] , who have suggested that strain-ageing is caused by the diffusion of interstitial oxygen atoms to glide dislocations. The results of the present experiment were similar in trend to those of Veevers et al., in that the strain-ageing decreased with increasing plastic strain, increased with ageing time and was almost insensitive to strain rates.
NOMINAL
STRAIN
RATE mid
Fig. 3. Strain-ageing parameter at a function of strain rate in zirconium-2.5wt% niobium alloy (300 s at 250°C).
T.K. Sinha, M.K. Asundi /Strain ageing in Zr-2.5wt%Nb alloys
Oxygen has also been found to be responsible for strain-ageing in other metals such as niobium [lo], tantalum [ 1 l] and vanadium [ 121. Again, it is to be noted that the values of the strain-ageing parameter (Au/a) found in the present compared closely with those found in Zircaloy-2 by Veevers and Rotsey [6] at comparable temperatures. It seems likely, in view of these qualitative considerations, that the interaction of interstitial oxygen with glide dislocation is responsible for strain-ageing in the heat-treated Zr-2.5 Nb alloy. The authors are indebted to Shri C.V. Sundaram, Head, Metallurgy Division, for his interest in this work and are grateful to Shri R. Kishore for his assistance with the experimental work.
317
References [ l] J.H. Keeler, Trans. America Sot. Met. 47 (1955)
157. [2] B. Ramaswami and G.B. Criag, Trans. Met. Sot. AIME 239 (1967) 1226. [3] R.F. Mehan and W.E. Weisinger, Knolls Atomic Power Labs., Report KAPL-2110 (1961). [4] V. Ramachandran and R.E. Reed-Hill, Metallurg. Trans. 1 (1970) 2105. (51 K. Veevers, W.B. Rotsey and K.U. Snowden, ASTM-STP 458 (1970) 194. [6] K. Veevers and W.B. Rotsey, I. Nucl. Mater. 27 (1968) 108. [ 71 K.P. Steward, Atomic Energy of Canada Ltd., AECL2250 (1965). (81 D. Lee, Canad. Met. Quart. 11 (1972) 113. [9] T.K. Sinha and M.K. Asundi, J. Nucl. Mater. 67 (1977) 000. [lo] Z.C. Szkopiak and A.P. Miodownik, J. Nucl. Mater. 17 (1965) 20. [ll] S. Hartley, ActaMet. 14 (1966) 1237. [12] S.A. Bradford and O.B. Carlson, Trans. Met. Sot. AIME 224 (1962) 738.