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An improvement in creep resistance of Zr-2.5%Nb tubes
Journal of Nuclear Materials 57 (1975) 258-270 Q North-Hol~nd PubIi~ing Company
ANIMPROVEMENTINCREEPRESISTANCEOFZr-2S%NbTUBES LG. BELL1 Materials Development Branch, Atomic Energy of Canada Ltd., WhitesheN Nuclear Research Establishment, Finawa, Manitoba ROE ILO, Canada
Received 4 September 1974 Revised manuscript received 26 March 1975
Specimens from a Zr-2S%Nb pressure tube were heat-treated in the range 650 to 105O’C and then creep-tested at 450°C. Water quenching produced anisotropic behaviour for soaking temperatures from 650 to 85O”C, and isotropic behaviour above this range. A lower ‘intermediate cooling rate’ produced anisotropic behaviour for the whole soaking range. Creep resistance improved with increasing soaking temperature, particularly for transverse intermediate-cooled specimens. At soaking temperatures of 880°C and higher, a lOO-fold reduction in creep rate was achieved with these specimens compared with as-cold-worked. An attempt is made to relate the creep data to crystallographic texture. In the second part of the program, the 880°C intermediate-cool heat treatment was chosen for further testing. It was confirmed that the material exhibits superior creep resistance compared with cold-worked Zr-2S%Nb, at all stresses from 34 MN/m2 to the ultimate tensile strength, in the temperature range from 300 to 45O’C. As expected, the ultimate tensile strength was reduced by this heat treatment. At the upper end of the stress range, at 3OO”C,a discontinuity occurs in creep data. The instability causing the discontinuity and leading to failure at relatively low stress is attributed to a twinning mechanism. Large twins encompassing hundreds of grains are observed. Des Cchantillons p&eves sur des tubes de force en alliage Zr-2,.5%Nb ont ete trait& thermiquement dans le domaine de temperatures de 650- 1.OSO”C,puis soumis a des essais de fluage a 450°C. La trempe a l’eau produisait un comportement anisotrope pour les temperatures de traitement de 650 i 850°C et un comporteillent isotrope au-dessus de ces temperaturcs. Une ‘vitesse de refroidissement intermediaire’ plus basse provoquait un comportement anisotrope pour tout l’intervalle de temperatures de traitement. La resistance au fluage etait am&o&e avec la temperature de traitement croisSante, en particulier pour les ichantillons transverses refroidis a la vitesse intermediaire. Aux temperatures de traitement de 880°C et audessus, une reduction de 100 fois la vitesse de fluage etait obtenue avec ces ~chantillons par rapport a celle observee pour lesechantillons Bcrouis a froid. On a tenth de reher les don&es de tluage i la texture cristallographique. Dans la seconde partie du programme, le traitement thermique i 880°C suivant un refroidissement i la vitesse intermediaire a ete choisi pour d’autres essais. Ceux-ci ont confirm6 que le materiau presente une resistance au fluage superieure comparee a celle de l’alliage ecroui a froid et ceci pour toutes les contraintes allant de 34 MN/m* jusqu’a la charge de rupture dans l’intervalle de temperature de 300 a 400°C. Comme p&vu, la charge de rupture etait diminude par ce traitement thermique. A la limite superieure de l’intervalle de contraintes, 2i 3OO”C, une discontinuite apparait dans les don&es de fluage. L’instabilite causant la discontinuite et conduisant a la rupture pour des contraintes relativement faibles est attribuee a un mecanisme de maclage. De grandes macles embrassant des centaines de grains sont observees. Druckr~hrenabschrlitte aus Zr-2,5% Nb wurden zwischen 650 und 105O’C w~rmebehandelt und anschliessend bei 450°C verformt. Durch Abschrecken in Wasser wird ein anisotropes Verhalten der zwis,chen 650 und 85O’C warmebehandelten Proben erzielt, ein isotropes Verhalten oberhalb dieses Temperaturbereichs. Eine niedrigere Zwischenabkiihlgeschwindigkeit fuhrt zu einem anisotropen Verhalten im gesamten Temperaturbereich. Der Kriechwiderstand wird mit steigender W~rmebehandiungstemperatur verbessert, insbesondere bei senkrecht zur WaIzrichtung geschnittenen, zwischenwarmebehandelten Proben. Bei Warmebehandlungstemperaturen oberhalb 880°C wird eine lOO-fache Erniedrigung der Kriechgeschwindigkeit dieser Proben im Vergleich zu den kaltverformten erreicht. Es wird versucht, die Kriechdaten mit der kristallographischen Textur zu korrelieren. Im zweiten Teil des Programms wurde die Zwischenwarmebehandlung bei 880°C fir weitere Versuche ausgewahlt. Der hervorragende Kriechwiderstand des Materials im Vergleich zum kaltverformten Zr-2,5% Nb wurde bei allen Spannungen zwischen 34 MN/m* und der Zugfestigkeit zwischen 300 und 450°C bestlitigt. Die Zugfestigkeit wird durch diese Warmebehandlung erwartungsgemiss erniedrigt. Im oberen Spannungsbereich tritt bei 300°C eine Unstetigkeit in den Kriechwerten auf. Die Instabilitiit, die die Unstetigkeit verursacht und zu einem Versagen bei relativ niedrigen Spannungen fiihrt, beruht auf einer Zwillingsbildung. Es wurden Zwillinge beobachtet, die Hunderte von Kornern erfassen.
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2. Introduction Zr-2.5%Nb has been developed as a pressure-tube material for Canadian power reactors which employ water or boiling water as a coolant. There are two conditions in which it has been used; (1) extruded and cold-drawn about 25% and (2) heat-treated at about 88O”C, waterquenched, cold-worked to straighten, and aged 24 h at 500°C. One of the requirements of pressure-tube material for water reactors and the one which controls design, is a high tensile strength at the reactor-operating temperature of about 300°C. For an organic-cooled reactor, the higher operating temperatilre of 400 to 45O”C, and lower stress operation, due to the low-pressure coolant, change the controlling factor in design to long-term creep resistance of the pressure-tube material. A search was begun for a new matarial condition which would give better creep resistance at the higher temperature with perhaps a sacrifice of some of the short-term strength. It had been observed [l] that the braze zone of Zr-2.5%Nb sheath for fuel elements did not strain in service as much as the untreated portion. The fuel in the area of the braze seemed in fact to be restrained by the sheath and because of this it was thought that the braze heat treatment* might be conferring an improvement in high-temperature creep resistance. It was of particular interest that the improvement seemed to be maintained or enhanced in the presence of a neutron flux. Initially, an experiment was designed to investigate the effect, on creep resistance at 450°C, of variation of soaking temperature followed by cooling at an intermediate rate. The results led to a similar investigation of material subjected to a water quench and this in turn to a study of the effects of heat treatment on crystallographic texture. For the second part of the program, one heat treatment was chosen and material in this condition was creep-tested over a range of stress and temperature. 2. Procedure The Zr-2.5%Nb
available was a section of pressure
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tubing of 5 mm wall thickness which had been extruded and cold-worked about 14%. The tube was from the production order for the Bruce reactors thus met the composition of ASTM B350-73, grade R6090 1. Transverse creep blanks were straightened and both longitudinal and transverse blanks were heat-treated at 400°C for 24 h to stabilize the material for machining. Specimens of 25.4 mm gauge length and 4 mm diam gauge-length cross section, were sealed in quartz tubing, and heated in a furnace for one hour (soaking heat treatment). To achieve an intermediate cooling rate, which would simulate that received by the fuel sheath braze zone, specimens were removed from the furnace and allowed to cool in the quartz tubing?. The rate of cooling was measured by inserting a thermocouple inside a specimen and cooling as in an actual heat treatment. Over the temperature range 900 to 6OO”C, the rate varied from 1S”C/sec at the upper end, to 3’C/sec at the lower end. Water-quenched specimens were obtained by immersing the capsule in water and breaking the quartz tube immediately. Aging for 24 h at 450°C for IC specimens, and 500°C for water-quenched specimens, was done by reheating in evacuated quartz capsules. For the investigation of the effect of changing the soaking temperature, creep tests were performed in argon at 207 MN/m2 and 45O’C on constant -load creep machines. The tests were contifiued to rupture and the minimum or secondary creep rate was recorded. To help interpret the creep data, the ruptured specimens were examined metallographically. Crystallographic texture measurements using the X-ray diffraction, inverse-pole technique were also made on separately treated material. The final creep program on 880°C IC specimens, over a range of stress and temperature, was conducted on constant load machines in vacuum. In this instance, the strain rates recorded were those at 1000 h or the minimum achieved during the test. The 880°C IC treatment was chosen for further investigation rather than the simulated braze cycle of 1050°C IC because it showed the same improvement in transverse creep strength at 450°C, and 207 MN/m2,
‘Present address: Fuel Engineering Branch, Atomic Energy of Canada Ltd., Power Projects, Mississauga, Ontario, Canada. *The brazing cycle consists of heating to 1050°C for 10 set, followed by cooling at a rate between that of air cool and furnace cool.
tThe heat treatment and material heat-treated this way are referred to henceforth as IC for intermediate cool or intermediate cooled.
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and the heat treatment was not as severe. Tensile tests were also done to determine the extentto which the 88O’C heat treatment reduced the ultimate tensile strength.
3. Observations 3.1. Investigation of changing heat treatment 3.1.1. Creep rate The effect on creep rate of varying soaking temperature for IC Zr-2S%Nb was very different for the two material directions (fig. 1). With transverse specimens, the creep rate showed a continuous reduction as the soaking temperature increased from 575 to 880°C then remained constant to 1050°C. After treatment at 88O”C, the creep rate was about 100 times lower than it had been in the cold-worked condition. With longitidunal specimens, the creep rate increased with increasing soaking temperature then decreased, forming a maximum at the soaking temperature of about 750°C. For all conditions tested, longitudinal specimens had a much higher creep rate than transverse, and heat-treated longitudinal specimens had a creep rate similar to cold-worked longitudinal specimens.
T-‘---l
Fig. 1. Minimum creep rate at 207 MN/m2 and 450°C for Zr-2S%Nb intermediate-rate cooled from various temperatures.
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Fig. 2. Minimum creep rate at 207 MN/m2 and 450°C for Zr-2S%Nb waterquenched from various temperatures.
The effect of soaking temperature followed by a rapid water quench was again different for the two material directions (fig. 2). Transverse specimens exhibited a low and decreasing creep rate as soaking temperature was increased from 650 to 760°C but, as temperature was increased further, the creep rate first increasedthen decreased forming a maximum at 85O’C. Longitudinal specimens had a high creep rate after a 65O’C soak but as the soaking temperature increased to 950°C, the creep rate continuously decreased. Longitudinal specimens were much less creep resistant than transverse specimens for soaking temperatures up to 85O’C but, above this temperature, creep properties were independent of direction. In general, with heat-treated Zr-2.5%Nb soaked in the range 600 to 76O”C, the cooling rate made little difference, but above this soaking temperature, the cooling rate had a large effect. With quenched specimens the creep rates converged for the two material directions as soaking temperature increased above 76O”C, whereas with intermediate-cooled specimens, the creep rates stayed well apart. Water-quenched specimens regardless of the direction did not achieve the 10~ creep rate of 1.5 X IO-5 h-1 which was exhibited by IC transverse specimens. Cold-worked specimens, unlike those heat-treated
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over the lower part of the soaking-temperature range, exhibited similar creep properties in the two directions. It is conceded that there was a factor of two differences in favour of the transverse direction but this is a long way from the factor of 100 found for heat-treated material.
faces for the two cooling rates with specimens subjected to the soaking temperature of 1050°C. Specimens quenched from this temperature had low ductility and exhibited a cleavage fracture possibly along prior 0 grain boundaries, whereas intermediate-cooled specimens retained high ductility.
3.1.2. Deformation characteristics Transverse creep specimens deformed such that the major axis of the elliptical fracture was in the radial tube direction. For both quenched and IC specimens, the ratio of radial axis to longitudinal axis of the fracture ellipse decreased as the soaking temperature increased. At 880°C soaking temperature, the ratio was one, indicating isotropic deformation, and it remained constant as the soaking temperature increased further to 1050°C (fig. 3). Longitudinal specimens also exhibited elliptical fracture and in this case the major axis of the fracture ellipse was in the tangential tube direction. The ratio of tangential axis to radial axis of the ellipse was larger than that for transverse specimens and decreased with increasing soaking temperature. At 1050°C the ratio became one. The deformation behaviour of both longitudinal and transverse specimens was independent of heat-treatment cooling rate. There was, however, a difference in appearance of the fracture sur-
3.1.3. Metallography Specimens cooled at an intermediate rate from 650°C were about 90% light coloured Q phase and 10% darker transformed 0. As the soaking temperature was increased, the amount of transformed fl increased at the expense of primary OL,and after an 880°C treatment, was about half the volume. At this latter condition, the matrix of transformed /I was darker and there was a hint of basketweave or (Y’ (fig. 4). Material cooled from 1050°C was made up of large prior 0 grains containing a coarse basketweave structure. The larger needles in the basketweave, and grain boundaries, were lighter in colour than the remainder of the material. Material quenched from 650°C was much the same as that which had been intermediate cooled from this temperature, but quenching from 880°C produced a structure which was about 5% (Y,rather than the 50% indicated above. The remainder was a very fine, needle-like, transformed 0. Quenching from 105O’C produced a structure that was completely fine, needlelike, transformed /.Iwith large prior p grains visible at lower magnification. In this structure, the needles
Fig. 3. Axis ratio of fracture ellipse of Zr-2S%Nb cimens tested at 207 MN/m2 and 450°C.
creep spe-
Fig. 4. Structure of 880°C intermediate-cooled Zr-2.5%Nb. Darker transformed p is about 50 ~01%. X 600. (The arrow above indicates the axial direction.)
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were much longer than those obtained by intermediate cooling from 105O’C. 3.1.4. Crystallographic texture It was found by X-ray diffraction that the pronounced cold-worked texture, with a large number of grains with basal poles oriented in the tangential direction, was retained for IC material over the soaking range 650 to 880°C and for waterquenched material over the range 650 to 770°C. Above these temperatures the textures tended to become more random with the introduction of a D component (see figs 5 and 6; in particular fig. 5 which shows the orientation of A, C and D texture components). The X-ray diffraction data were used to obtain the fraction of prism planes oriented for slip in two directions. The technique was that described by Ibrahim [2]. When soaking was in the range 650 to 880°C for IC material, and 650 to 800°C for water-quenched, the fractions differed by about a factor of 2. At higher soaking temperatures, 1050°C for IC material and 880 to 1050°C for water-quenched, the fraction for the two directions tended to approach one an-
Fig. 6. Basal pole-texture coefficient as a function of soaking temperature for waterquenched Zr-2.5%Nb.
other (figs. 7 and 8). As expected from the texture coefficients, the as-cold-worked tubing also had twice as many prism planes available for slip in the longitudinal direction as in the transverse.
01 ./ f
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I 700 SOI\KING
Fig. 5. Basal pole-texture coefficient as a function of soaking temperature for intermediate-cooled Zr-2S%Nb.
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900 TEMPERATURE,
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Fig. 7. Resolved fraction of prism planes oriented for slip for intermediate-cooled Zr-2S%Nb.
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Fig. 8. Resolved fraction of prism planas oriented for slip for waterquenched Zr-2.5%Nb.
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Fig. 9. Tensile properties of 880°C IntermedaiteCooled Zr-2S%Nb tube. 900-
3.2. Properties of 880°C intermediate-cooled Zr-2S%Nb
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3.2.1. Tensile There was a rapid decrease in tensile properties for both longitudinal and transverse specimens between room temperature and 300°C with 880°C IC Zr-2S%Nb. Above 300°C, the transverse property curves levelled out but then dropped rapidly again at 500°C. For all temperatures, longitudinal strength was considerably less than transverse; also, the level part of the curve above 300°C was shorter than that for transverse specimens (fig. 9). The tensile strength of heat-treated Zr-2.5%Nb was much lower than that of cold-worked material up to about 5OO’C at which temperature the strength for the two material conditions became about the same. The transverse UTS is compared with that of a typical cold-worked tube in fig. 10. During tensile testing at 300 and 400°C stress increased almost linearly with strain up to the UTS, then the curve flattened out. A distinct yield point was expected but was not observed.
WQ-
500-
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Fig. 10. Transverse ultimate tensile strength of 880°C intermediate-cooled Zr-2.5%Nb tube compared with that of typical cold-worked tube.
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3.2.2. CYeep Creep data at 450°C showed that the advantage in creep resistance over cold-worked material previously noted for transverse specimens at 207 MN/m2 was maintained over the stress range from 34 to 276 MN/m2 (fig. 11). Cold-worked Zr-2.5%Nb is virtually isotropic for high-temperature creep and since transverse data at 450” were not available, longitudinal data [3] were used for the comparison. At 207 MN/m2 and 450°C the longitudinal creep rate of IC material was about the same or slightly lower than that for cold-worked material. As stress was reduced, however, the heat-treated longitudinal material exhibited an increasing advantage in creep rate over coldworked. At 4OO”C, (fig. 12) a comparison with cold-worked transverse data [3] shows that the IC heat treatment reduced the creep rate by a factor of about 50 at 1000 h test time; not quite as much as at 450°C. As the stress was increased to about 345 MN/m2, the creep rates for the two materials became the same and presumably the curves would cross at higher stress. At 300°C (fig. 13), creep rates for IC material below 414 MN/m2 were similar to those for cold-worked [4] at 1000 h test time. However, at this time, creep rates were still decreasing, especially for IC specimens I
Fig. 11. Minimum creep rate of intermediate-cooled Zr-2S%Nb.
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where at 3000 h, the creep rates were about 5 times lower than those for cold-worked specimens. Thus, in long-term service at 300°C (and also at 400°C), transverse IC material would retain the advantage over cold-worked that it had at 45O’C. In addition to the above observation at 300°C, the creep rate of transverse IC material at a particular time went through a maximum at about half the ultimate tensile strength to produce a pronounced Sshaped curve of creep rate versus stress. The S-shape of this curve and the stress at maximum creep rate were similar to the curve and stress found for coldworked transverse Zr-2.5%Nb at 1000 h. There is presumably some variation in strength from one specimen to another. At 414 MN/m2, and 3OO”C, which is actually above the measured UTS of IC transverse material, there was a dramatic change in creep resistance with one specimen continuing for months at a very low creep rate and another failing on loading at a very slightly higher stress. An interesting result was obtained at the end of 3 128 h at 414 MN/m2 by increasing the load to 450 MN/m2. The specimen held this load without any noticeable deformation for several hours, then suddenly failed. Since this is well above the UTS at 300°C, it is suspected that some form of creep strain-ageing had strengthened the specimen, but once the specimen ,
I
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at 450°C compared with that of cold-worked longitudinal
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Fig. 12. Comparison of creep rates at 1000 h test time at 400°C for cold-worked and 880°C intermediate-cooled Zr-2.5%Nb.
transverse
A 01 10-9
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10-9
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10-4
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Fig. 13. Comparison of creep rates at 300°C for cold-worked and intermediate-cooled
started to deform, all strength was lost. Cold-worked material does not exhibit the discontinuity, having a continuous reduction in creep rate with increasing stress. Also, the discontinuity was not observed with transverse IC specimens at creep temperatures above 3oo”. Transverse IC specimens had an inflection point on
transverse Zrl2.5%Nb.
creep curves at about half the rupture time. This inflection usually occurs for zirconium alloys at a third to a quarter of the rupture time (see fig. 14). Localized deformation and relatively low elongations were observed to be a feature of ruptured transverse IC specimens.
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IO9-
Fig. 15. This area shows three sets of twins, one on tube radius, and the other two at about 38.5”. x 600. (The arrow indicates the stress and tranverse tube direction.)
Fig.
14.
Creep curves for transverse specimens of Zr-2.5%Nb tube in various conditions showing that IC material has inflection at relatively high fraction of rupture time.
3.2.3. Metallography The creep specimen which failed on loading at 3OO”C, a transverse tensile specimen which had been strained only 0.6% in a one-inch gauge length at 300°C, and ruptured 4OO’C creep specimens, were examined metallographically. The technique used was to chemically polish in a mixture of 30 parts H,S04,30 parts HNO,, 30 parts H20 and 8 parts HF, anodize electrolytically in Pickleseimer’s solution and examine under polarized light. Deformation twins were observed in all specimens. In most cases the twins were limited to a single grain, or perhaps two or three grains, but areas could be found where twins crossed many grains. Some single twins in fact included the volumes of hundreds of grains. The areas which contained the large twins were usually lens-shaped with the major lens axis in the tangential direction of the original tube. The lensshaped area that was untwinned was almost all one colour under polarized light, indicating that all grains in the area originally had almost the same orientation. In most instances, the twin boundaries were unaffected by grain boundaries and maintained a straight
course through both primary (o) and transformed (0) grains. Another feature of the macrotwinning was that there were occasionally three sets of twins, one approximately on the tube radius and two at an angle of 38.5” to the radius (fig. 15). Specimens were polished on the plane perpendicular to the tube axis and, with the major texture that is present, the polished surface was expected to coincide with the (lOTO) planes of most grains. A calculation of the angles for (1 OT2) twin boundaries, assuming lattice dimensions are the same as for pure zirconium, shows that they should be at 0 and about 38.5’ to the radius (fig. 16) which corresponds to what was observed. Since (1012) twins are those which most commonly occur in zirconium at temperatures above room temperature, it is fairly certain that the phenomenon is indeed twinning.
Fig._16. Intersection of 1012 twin planes with lOI plane. (1012) twmning produces three sets of twins one for each pair of 10 12 planes.
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The tensile specimen which was strained only 0.6% had been reduced in diameter by 0.02 mm at one position along its gauge length . Twins were observed in other places but there was one particularly large lensshaped area that was heavily twinned at the position of reduced diameter. Of the two specimens which had ruptured at 4OO”C, one had failed on loading and the other at 188.5 h test time. The specimen which failed rapidly had twins all along the gauge length. The longer-term specimen on the other hand had a twinned region near the heavily deformed portion while the major part of the test section had few or no twins.
4. Discussion 4.1. Texture effects The remarkable creep anisotropy of the heattreated Zr-2S%Nb tested here has not been observed before in zirconium alloys. To some extent it can be explained by the crystallographic texture and the preferred deformation mechanism during creep. Winton and Murgatroyd [S] found with Zr-2S%Nb that quenching from above the temperature of ‘about 775’C, where the equilibrium fl composition is about 7S%Nb, results in transformation of /3 to (Y’whereas quenching from below this temperature results in transformation of fl to w t retained p. Cheadle and Ells [6] observed that heat treatment in the (Y+ /I range, followed by slow cooling, results in the retention of the original cold-worked texture. They theorized that there was an interaction between 0 and cr phases with one influencing the texture of the other during transformation. When this information is added to that from the present work a picture begins to emerge. The cold-worked texture is retained when conditions are such that residual primary (Yphase is present and the /3phase transforms to cr, w or retained 0 during cooling from the heat-treatment temperature. A new ‘recrystallization’ texture develops only when cr’ is produced, i.e. when the material is cooled from above 775’C with water quenching or from a still higher temperature with intermediate cooling. It is well known that the preferred deformation mechanism for creep in close-packed hexagonal (11zirconium is slip on 1070 planes in 1120 direction. Cal-
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culations of the fraction of prism planes available for slip showed a difference approximating a factor of two for the two directions of interest. The fact that creep rates were found to differ by as much as a factor of 100 indicates that crystallographic texture alone does not explain creep anisotropy. An estimate from data available indicates that texture should only provide a factor of 10 difference. The theory developed by Cheadle and Ells to explain how the cold-worked texture is retained by heat treatment in the (Y+ fi range leads one to conclude that the primary (Ygrains have for the most part an A texture. Transformed /3will also to some extent retain the coldworked texture according to the Cheadle and Ells theory. However, the inducement to retain the texture seems to weaken as Nb concentration in the 0 phase increases and any material that does not have an A texture will be in the 0 last to transform and have a relatively high niobium concentration. Transverse specimens exhibit high creep resistance because on one hand the low Nb material, having A texture, is adversely oriented for easy slip and, on the other hand, that material which is oriented for easy slip has high Nb concentration and is thus also fairly strong. Longitudinal specimens will be able to deform relatively easily because the material which has low Nb is also oriented for easy slip. This provides an explanation for most of the anisotropy effects. It is difficult to explain why cold-worked material does not exhibit the same creep anisotropy as heattreated material with the same or similar texture. The cold-worked material had a factor of two difference in fraction of prism planes oriented for easy slip for the two material directions but in this instance the creep rate was different by only a factor of two. The difference between cold-worked and IC material must be due to segregation of Nb in IC material as described above. In a tube configuration with a factor of two difference in applied stress in the axial and tangential directions, the IC heat treatment should provide a component which is dimensionally very stable. The investigation has explained the reluctance of the sheath braze zone to deform, and this in turn suggests that the heat treatment may produce material which is resistant to in-reactor creep.
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4.2. Structure effects
While the crystallographic texture remained constant the creep properties were found to change over the heat-treat soaking temperature range. This effect can be explained by the relative amounts and niobium concentration, of phases present. For the sake of brevity this argument is left out. 4.3. High stress deformation
The relatively low tensile strength and the sudden loss in creep resistance of transverse IC specimens at 300°C and about 414 MN/m2 can be related to a twinning mechanism. With the pronounced A texture (basal poles in the tangential tube direction), transverse specimens are resistant to deformation by slip because of a lack of resolved prism-plane stress. When twinning occurs, however, material within the twin becomes oriented so that it can slip easily. Large numbers of twins readily occur in IC material. Massive twins encompassing hundreds of grains have never been observed before. The fact that they occur here is another indication of the very high degree of preferred orientation in IC Zr-2.5%Nb. Grains themselves are very small, but large numbers have such similar orientation that boundaries do not provide an obstacle to twin propagation, and the material behaves to some extent as though it were made up of large grains. Considering the transverse tensile data as a function of temperature, it seems likely that a more or less constant stress is required for twinning. At temperatures below 300°C, a stress higher than the twinning stress is necessary for slip after twinning, whereas above 300°C once twinned, the material slips without further encouragement. A mechanism such as this would explain the knee in the curve of tensile strength versus temperature. Other mechanisms such as strain-agehardening are not expected to be sufficiently potent for such a pronounced effect. Twinning is deformation-rate sensitive such that an increasing rate of deformation increases the probability of twin formation. Increasing temperature has an opposite effect [7]. At a constant rate of deformation, such as in a tenSib test, the mechanism governing the UTS will therefore
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change, at a certain temperature, from twinning to slip. This explains the sudden drop in UTS at 500°C. Above this latter temperature the material strength is once again determined by a slip mechanism. Assuming the above postulation to be correct, it is necessary to explain why the curve for longitudinal tensile data also has a knee at 300°C. This knee occurs at considerably lower stress than that for transverse specimens and extends only to about 400 rather than 500°C. The probable explanation is that some grains of D orientation do exist in the material and it is the twinning of this D component in longitudinal specimens that produces the knee. A smaller amount of D oriented material compared with A makes the effect less pronounced. For transverse-creep tests at 300°C there is a sudden loss in creep resistance at the UTS. This temperature is at the transition from slip to twin-control of tensile strength. A critical stress for twinning may explain the sudden loss in strength and particular specimens may or may not fail due to a variability of material or slightly different loading rate from one specimen to the next. When an area starts to twin and hence to slip, there is a sudden increase in stress on adjacent grains so they also twin with the result that the specimen fails and deformation is restricted to a short section of the gauge length. Creep strain at a stress below the UTS at 300°C is not sufficient to trigger twinning. The more gradual transition at 400°C from longterm testing to immediate failure, and also the lack of twins away from the fracture in a specimen which ruptured at less than the UTS prove that, at this temperature, twinrring can be triggered by creep strain. At a stress lower than the UTS, twinning, once started, spreads through only that part of the gauge length which undergoes an increase in stress at the reduced section. At a certain set of creep conditions a specimen can be exhibiting superior creep resistance but once the material starts to deform locally, i.e., goes into tertiary creep, the stress and strain rate in the necked region increases so that twinning starts. This explains why time to failure of transverse IC specimens is relatively short once the material has passed the inflection point on the creep curve. Incidentally, this does not rule out the material for practical applications because the onset of tertiary creep occurs in the IC material at a strain in excess of 2%, which in turn is after a relatively long time.
L.G. Bell /Improvement in creep resistance
4.4. Low stress deformation Let us now consider deformation characteristics at stresses less than those required to produce twinning. In the transverse direction over the temperature range tested, the creep rate was considerably lower for intermediate-cooled than for cold-worked Zr-2S%Nb. At 45O”C, the reduction by a factor of 100 was achieved at 1000 h test time whereas at lower temperature the same reduction required a much longer time. This is an understandable result, in that creep requires a longer time at lower temperature to reach the same state. There is an indication, however, that the strain rate is being affected by the amount of creep strain and this may be making a contribution to the tempperature effect. At 300°C with increasing stress, a maximum creep rate was observed at about half the UTS. With stress increasing above the halfway point, the strain increased but the strain rate decreased, The remarkable creep strength of transverse IC Zr-2.5%Nb is believed to be the result of a preponderance of A texture grains which cannot deform by slip. Creep strain without twinning is thus limited to those grains which do not have an A texture and tends to have a limit. A maximum strain rate occurs at an intermediate stress simply because deformation at higher stress uses up the available strain in those grains which can deform. Of course, at sufficiently high stress, twinning begins to occur and this leads to rupture. This effect is pronounced at 300°C but probably also occurs to some extent at higher temperatures. From the point of view of out-reactor creep and tensile strength the intermediate-cool heat treatment could be used for organic-cooled reactor pressure tubes. The greater creep resistance compared with cold-worked tubes in both longitudinal and transverse directions would prolong the useful tube life.
5. Conclusions (1) Creep properties of Zr-2S%Nb tubing in the transverse direction are improved markedly by an intermediate-cool heat treatment from the upper a! + (3or 0 temperature. Creep rates are about 100 times lower than for cold-worked material.
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(2) For intermediate-cooled Zr-2.5%Nb, the high creep strength found for transverse specimens is attributed to crystallographic texture but may also depend to some extent on segregation of Nb to the /3 phase. (3) Water-quenched Zr-2.5%Nb exhibits isotropic creep properties when heat-treatment temperature is above about 800°C. Below this soaking temperature, the behaviour is similar to that of intermediate-cooled material. (4) Cold-worked Zr-2.5%Nb is much less anisotropit in creep at 450°C than material heat-treated to produce the same crystallographic texture. (5) The improvement in creep resistance conferred to transverse specimens by the intermediate-cool heat treatment is maintained for all stresses up to the UTS over the temperature range 300 to 450°C (6) The longitudinal creep strength of intermediatecooled Zr-2.5Nb, while inferior to that of transverse, exhibits relative improvement over cold-worked Zr-2.5%Nb (creep rate about 10 times lower) at 450°C as the stress is decreased below 207 MN/m2. (7) Cold-worked tensile strength in the range from room temperature to about 500°C is reduced by the intermediate-cool heat treatment. (8) The low tensile strength of intermediate-cooled Zr-2.5%Nb and the instability of creep specimens at high stress are related to a twinning mechanism. (9) Large twins encompassing hundreds of grains can occur in Zr-2.5%Nb.
Acknowledgements The author is indebted to B.F. Jones for the idea of a possible improvement in creep properties by the intermediate-cool heat treatment and also for discussion at the early stages on possible causes for the effect. Helpful discussions of deformation mechanisms were held with B.J.S. Wilkins. Many other individuals at WNRE helped with the experimental work; in particular G. Duncan conducted the short-term creep tests, F. Havelock the metallography and G.M. Omichinski with the help of M. Duclos did the crystallographictexture measurements. The Long-term creep test and most of the tensile tests were conducted at Orenda Ltd., under the supervision of M.C. Teeter.
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References [l] J. Walker, R.E. Dailly and M.H. Schankula, WNRE unpublished information. [2] E.F. Ibrahim, In-reactor Creep of Zirconium-Alloy Tubes and Its Correlation with Uniaxial Data, ASTM STP 458 (1969) p. 18. [3] LG. Bell, Creep Tests on Cold Worked Zr-2.5%Nb at 400°C and 500°C, Orenda Ltd. Report HSER-FCP67-003 (July 1967). [4] L.G. Bell, Creep Tests on Zr-2.5%Nb Pressure Tubing at 3OO”C, Orenda Ltd. Report HSER-FCP66-002 (August 1966).
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[S] J. Winton and R.A. Murgatroyd, The Effect of Variations in Composition and Heat Treatment on the Properties of Zr-Nb Alloys, Electrochemical Technology, vol. 4, no. 7-8 (July-Aug. 1966) p. 358. [6] B.A. Cheadle and C.E. Ells, The Effect of Heat Treatment on the Texture of Fabricated Zr-Rich Alloys, Electra- _ chemical Technology, vol. 4, no. 7-8 (July-Aug. 1966) p. 329. (71 S. Mahajan and D.F. Williams, Deformation Twinning in Metals and Alloys, Review 173, International Metallurgical Reviews, vol. 18, (1973) pp. 43-61.
ANIMPROVEMENTINCREEPRESISTANCEOFZr-2S%NbTUBES LG. BELL1 Materials Development Branch, Atomic Energy of Canada Ltd., WhitesheN Nuclear Research Establishment, Finawa, Manitoba ROE ILO, Canada
Received 4 September 1974 Revised manuscript received 26 March 1975
Specimens from a Zr-2S%Nb pressure tube were heat-treated in the range 650 to 105O’C and then creep-tested at 450°C. Water quenching produced anisotropic behaviour for soaking temperatures from 650 to 85O”C, and isotropic behaviour above this range. A lower ‘intermediate cooling rate’ produced anisotropic behaviour for the whole soaking range. Creep resistance improved with increasing soaking temperature, particularly for transverse intermediate-cooled specimens. At soaking temperatures of 880°C and higher, a lOO-fold reduction in creep rate was achieved with these specimens compared with as-cold-worked. An attempt is made to relate the creep data to crystallographic texture. In the second part of the program, the 880°C intermediate-cool heat treatment was chosen for further testing. It was confirmed that the material exhibits superior creep resistance compared with cold-worked Zr-2S%Nb, at all stresses from 34 MN/m2 to the ultimate tensile strength, in the temperature range from 300 to 45O’C. As expected, the ultimate tensile strength was reduced by this heat treatment. At the upper end of the stress range, at 3OO”C,a discontinuity occurs in creep data. The instability causing the discontinuity and leading to failure at relatively low stress is attributed to a twinning mechanism. Large twins encompassing hundreds of grains are observed. Des Cchantillons p&eves sur des tubes de force en alliage Zr-2,.5%Nb ont ete trait& thermiquement dans le domaine de temperatures de 650- 1.OSO”C,puis soumis a des essais de fluage a 450°C. La trempe a l’eau produisait un comportement anisotrope pour les temperatures de traitement de 650 i 850°C et un comporteillent isotrope au-dessus de ces temperaturcs. Une ‘vitesse de refroidissement intermediaire’ plus basse provoquait un comportement anisotrope pour tout l’intervalle de temperatures de traitement. La resistance au fluage etait am&o&e avec la temperature de traitement croisSante, en particulier pour les ichantillons transverses refroidis a la vitesse intermediaire. Aux temperatures de traitement de 880°C et audessus, une reduction de 100 fois la vitesse de fluage etait obtenue avec ces ~chantillons par rapport a celle observee pour lesechantillons Bcrouis a froid. On a tenth de reher les don&es de tluage i la texture cristallographique. Dans la seconde partie du programme, le traitement thermique i 880°C suivant un refroidissement i la vitesse intermediaire a ete choisi pour d’autres essais. Ceux-ci ont confirm6 que le materiau presente une resistance au fluage superieure comparee a celle de l’alliage ecroui a froid et ceci pour toutes les contraintes allant de 34 MN/m* jusqu’a la charge de rupture dans l’intervalle de temperature de 300 a 400°C. Comme p&vu, la charge de rupture etait diminude par ce traitement thermique. A la limite superieure de l’intervalle de contraintes, 2i 3OO”C, une discontinuite apparait dans les don&es de fluage. L’instabilite causant la discontinuite et conduisant a la rupture pour des contraintes relativement faibles est attribuee a un mecanisme de maclage. De grandes macles embrassant des centaines de grains sont observees. Druckr~hrenabschrlitte aus Zr-2,5% Nb wurden zwischen 650 und 105O’C w~rmebehandelt und anschliessend bei 450°C verformt. Durch Abschrecken in Wasser wird ein anisotropes Verhalten der zwis,chen 650 und 85O’C warmebehandelten Proben erzielt, ein isotropes Verhalten oberhalb dieses Temperaturbereichs. Eine niedrigere Zwischenabkiihlgeschwindigkeit fuhrt zu einem anisotropen Verhalten im gesamten Temperaturbereich. Der Kriechwiderstand wird mit steigender W~rmebehandiungstemperatur verbessert, insbesondere bei senkrecht zur WaIzrichtung geschnittenen, zwischenwarmebehandelten Proben. Bei Warmebehandlungstemperaturen oberhalb 880°C wird eine lOO-fache Erniedrigung der Kriechgeschwindigkeit dieser Proben im Vergleich zu den kaltverformten erreicht. Es wird versucht, die Kriechdaten mit der kristallographischen Textur zu korrelieren. Im zweiten Teil des Programms wurde die Zwischenwarmebehandlung bei 880°C fir weitere Versuche ausgewahlt. Der hervorragende Kriechwiderstand des Materials im Vergleich zum kaltverformten Zr-2,5% Nb wurde bei allen Spannungen zwischen 34 MN/m* und der Zugfestigkeit zwischen 300 und 450°C bestlitigt. Die Zugfestigkeit wird durch diese Warmebehandlung erwartungsgemiss erniedrigt. Im oberen Spannungsbereich tritt bei 300°C eine Unstetigkeit in den Kriechwerten auf. Die Instabilitiit, die die Unstetigkeit verursacht und zu einem Versagen bei relativ niedrigen Spannungen fiihrt, beruht auf einer Zwillingsbildung. Es wurden Zwillinge beobachtet, die Hunderte von Kornern erfassen.
L.G. Bell /Improvement in creep resistance
2. Introduction Zr-2.5%Nb has been developed as a pressure-tube material for Canadian power reactors which employ water or boiling water as a coolant. There are two conditions in which it has been used; (1) extruded and cold-drawn about 25% and (2) heat-treated at about 88O”C, waterquenched, cold-worked to straighten, and aged 24 h at 500°C. One of the requirements of pressure-tube material for water reactors and the one which controls design, is a high tensile strength at the reactor-operating temperature of about 300°C. For an organic-cooled reactor, the higher operating temperatilre of 400 to 45O”C, and lower stress operation, due to the low-pressure coolant, change the controlling factor in design to long-term creep resistance of the pressure-tube material. A search was begun for a new matarial condition which would give better creep resistance at the higher temperature with perhaps a sacrifice of some of the short-term strength. It had been observed [l] that the braze zone of Zr-2.5%Nb sheath for fuel elements did not strain in service as much as the untreated portion. The fuel in the area of the braze seemed in fact to be restrained by the sheath and because of this it was thought that the braze heat treatment* might be conferring an improvement in high-temperature creep resistance. It was of particular interest that the improvement seemed to be maintained or enhanced in the presence of a neutron flux. Initially, an experiment was designed to investigate the effect, on creep resistance at 450°C, of variation of soaking temperature followed by cooling at an intermediate rate. The results led to a similar investigation of material subjected to a water quench and this in turn to a study of the effects of heat treatment on crystallographic texture. For the second part of the program, one heat treatment was chosen and material in this condition was creep-tested over a range of stress and temperature. 2. Procedure The Zr-2.5%Nb
available was a section of pressure
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tubing of 5 mm wall thickness which had been extruded and cold-worked about 14%. The tube was from the production order for the Bruce reactors thus met the composition of ASTM B350-73, grade R6090 1. Transverse creep blanks were straightened and both longitudinal and transverse blanks were heat-treated at 400°C for 24 h to stabilize the material for machining. Specimens of 25.4 mm gauge length and 4 mm diam gauge-length cross section, were sealed in quartz tubing, and heated in a furnace for one hour (soaking heat treatment). To achieve an intermediate cooling rate, which would simulate that received by the fuel sheath braze zone, specimens were removed from the furnace and allowed to cool in the quartz tubing?. The rate of cooling was measured by inserting a thermocouple inside a specimen and cooling as in an actual heat treatment. Over the temperature range 900 to 6OO”C, the rate varied from 1S”C/sec at the upper end, to 3’C/sec at the lower end. Water-quenched specimens were obtained by immersing the capsule in water and breaking the quartz tube immediately. Aging for 24 h at 450°C for IC specimens, and 500°C for water-quenched specimens, was done by reheating in evacuated quartz capsules. For the investigation of the effect of changing the soaking temperature, creep tests were performed in argon at 207 MN/m2 and 45O’C on constant -load creep machines. The tests were contifiued to rupture and the minimum or secondary creep rate was recorded. To help interpret the creep data, the ruptured specimens were examined metallographically. Crystallographic texture measurements using the X-ray diffraction, inverse-pole technique were also made on separately treated material. The final creep program on 880°C IC specimens, over a range of stress and temperature, was conducted on constant load machines in vacuum. In this instance, the strain rates recorded were those at 1000 h or the minimum achieved during the test. The 880°C IC treatment was chosen for further investigation rather than the simulated braze cycle of 1050°C IC because it showed the same improvement in transverse creep strength at 450°C, and 207 MN/m2,
‘Present address: Fuel Engineering Branch, Atomic Energy of Canada Ltd., Power Projects, Mississauga, Ontario, Canada. *The brazing cycle consists of heating to 1050°C for 10 set, followed by cooling at a rate between that of air cool and furnace cool.
tThe heat treatment and material heat-treated this way are referred to henceforth as IC for intermediate cool or intermediate cooled.
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and the heat treatment was not as severe. Tensile tests were also done to determine the extentto which the 88O’C heat treatment reduced the ultimate tensile strength.
3. Observations 3.1. Investigation of changing heat treatment 3.1.1. Creep rate The effect on creep rate of varying soaking temperature for IC Zr-2S%Nb was very different for the two material directions (fig. 1). With transverse specimens, the creep rate showed a continuous reduction as the soaking temperature increased from 575 to 880°C then remained constant to 1050°C. After treatment at 88O”C, the creep rate was about 100 times lower than it had been in the cold-worked condition. With longitidunal specimens, the creep rate increased with increasing soaking temperature then decreased, forming a maximum at the soaking temperature of about 750°C. For all conditions tested, longitudinal specimens had a much higher creep rate than transverse, and heat-treated longitudinal specimens had a creep rate similar to cold-worked longitudinal specimens.
T-‘---l
Fig. 1. Minimum creep rate at 207 MN/m2 and 450°C for Zr-2S%Nb intermediate-rate cooled from various temperatures.
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_il
Id6
_/
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700 SORK,:,G
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1000
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Fig. 2. Minimum creep rate at 207 MN/m2 and 450°C for Zr-2S%Nb waterquenched from various temperatures.
The effect of soaking temperature followed by a rapid water quench was again different for the two material directions (fig. 2). Transverse specimens exhibited a low and decreasing creep rate as soaking temperature was increased from 650 to 760°C but, as temperature was increased further, the creep rate first increasedthen decreased forming a maximum at 85O’C. Longitudinal specimens had a high creep rate after a 65O’C soak but as the soaking temperature increased to 950°C, the creep rate continuously decreased. Longitudinal specimens were much less creep resistant than transverse specimens for soaking temperatures up to 85O’C but, above this temperature, creep properties were independent of direction. In general, with heat-treated Zr-2.5%Nb soaked in the range 600 to 76O”C, the cooling rate made little difference, but above this soaking temperature, the cooling rate had a large effect. With quenched specimens the creep rates converged for the two material directions as soaking temperature increased above 76O”C, whereas with intermediate-cooled specimens, the creep rates stayed well apart. Water-quenched specimens regardless of the direction did not achieve the 10~ creep rate of 1.5 X IO-5 h-1 which was exhibited by IC transverse specimens. Cold-worked specimens, unlike those heat-treated
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over the lower part of the soaking-temperature range, exhibited similar creep properties in the two directions. It is conceded that there was a factor of two differences in favour of the transverse direction but this is a long way from the factor of 100 found for heat-treated material.
faces for the two cooling rates with specimens subjected to the soaking temperature of 1050°C. Specimens quenched from this temperature had low ductility and exhibited a cleavage fracture possibly along prior 0 grain boundaries, whereas intermediate-cooled specimens retained high ductility.
3.1.2. Deformation characteristics Transverse creep specimens deformed such that the major axis of the elliptical fracture was in the radial tube direction. For both quenched and IC specimens, the ratio of radial axis to longitudinal axis of the fracture ellipse decreased as the soaking temperature increased. At 880°C soaking temperature, the ratio was one, indicating isotropic deformation, and it remained constant as the soaking temperature increased further to 1050°C (fig. 3). Longitudinal specimens also exhibited elliptical fracture and in this case the major axis of the fracture ellipse was in the tangential tube direction. The ratio of tangential axis to radial axis of the ellipse was larger than that for transverse specimens and decreased with increasing soaking temperature. At 1050°C the ratio became one. The deformation behaviour of both longitudinal and transverse specimens was independent of heat-treatment cooling rate. There was, however, a difference in appearance of the fracture sur-
3.1.3. Metallography Specimens cooled at an intermediate rate from 650°C were about 90% light coloured Q phase and 10% darker transformed 0. As the soaking temperature was increased, the amount of transformed fl increased at the expense of primary OL,and after an 880°C treatment, was about half the volume. At this latter condition, the matrix of transformed /I was darker and there was a hint of basketweave or (Y’ (fig. 4). Material cooled from 1050°C was made up of large prior 0 grains containing a coarse basketweave structure. The larger needles in the basketweave, and grain boundaries, were lighter in colour than the remainder of the material. Material quenched from 650°C was much the same as that which had been intermediate cooled from this temperature, but quenching from 880°C produced a structure which was about 5% (Y,rather than the 50% indicated above. The remainder was a very fine, needle-like, transformed 0. Quenching from 105O’C produced a structure that was completely fine, needlelike, transformed /.Iwith large prior p grains visible at lower magnification. In this structure, the needles
Fig. 3. Axis ratio of fracture ellipse of Zr-2S%Nb cimens tested at 207 MN/m2 and 450°C.
creep spe-
Fig. 4. Structure of 880°C intermediate-cooled Zr-2.5%Nb. Darker transformed p is about 50 ~01%. X 600. (The arrow above indicates the axial direction.)
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were much longer than those obtained by intermediate cooling from 105O’C. 3.1.4. Crystallographic texture It was found by X-ray diffraction that the pronounced cold-worked texture, with a large number of grains with basal poles oriented in the tangential direction, was retained for IC material over the soaking range 650 to 880°C and for waterquenched material over the range 650 to 770°C. Above these temperatures the textures tended to become more random with the introduction of a D component (see figs 5 and 6; in particular fig. 5 which shows the orientation of A, C and D texture components). The X-ray diffraction data were used to obtain the fraction of prism planes oriented for slip in two directions. The technique was that described by Ibrahim [2]. When soaking was in the range 650 to 880°C for IC material, and 650 to 800°C for water-quenched, the fractions differed by about a factor of 2. At higher soaking temperatures, 1050°C for IC material and 880 to 1050°C for water-quenched, the fraction for the two directions tended to approach one an-
Fig. 6. Basal pole-texture coefficient as a function of soaking temperature for waterquenched Zr-2.5%Nb.
other (figs. 7 and 8). As expected from the texture coefficients, the as-cold-worked tubing also had twice as many prism planes available for slip in the longitudinal direction as in the transverse.
01 ./ f
I
900
I 700 SOI\KING
Fig. 5. Basal pole-texture coefficient as a function of soaking temperature for intermediate-cooled Zr-2S%Nb.
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900 TEMPERATURE,
I 900
I 1000
I 1100
Qc
Fig. 7. Resolved fraction of prism planes oriented for slip for intermediate-cooled Zr-2S%Nb.
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‘00 %
500 i
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’
100 -
zoo-
30
Fig. 8. Resolved fraction of prism planas oriented for slip for waterquenched Zr-2.5%Nb.
100 -
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O-
0
100
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I 200
300
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I
400
500
600
700
Fig. 9. Tensile properties of 880°C IntermedaiteCooled Zr-2S%Nb tube. 900-
3.2. Properties of 880°C intermediate-cooled Zr-2S%Nb
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3.2.1. Tensile There was a rapid decrease in tensile properties for both longitudinal and transverse specimens between room temperature and 300°C with 880°C IC Zr-2S%Nb. Above 300°C, the transverse property curves levelled out but then dropped rapidly again at 500°C. For all temperatures, longitudinal strength was considerably less than transverse; also, the level part of the curve above 300°C was shorter than that for transverse specimens (fig. 9). The tensile strength of heat-treated Zr-2.5%Nb was much lower than that of cold-worked material up to about 5OO’C at which temperature the strength for the two material conditions became about the same. The transverse UTS is compared with that of a typical cold-worked tube in fig. 10. During tensile testing at 300 and 400°C stress increased almost linearly with strain up to the UTS, then the curve flattened out. A distinct yield point was expected but was not observed.
WQ-
500-
400-
300-
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IOO-
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lco
200
300
400
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Fig. 10. Transverse ultimate tensile strength of 880°C intermediate-cooled Zr-2.5%Nb tube compared with that of typical cold-worked tube.
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3.2.2. CYeep Creep data at 450°C showed that the advantage in creep resistance over cold-worked material previously noted for transverse specimens at 207 MN/m2 was maintained over the stress range from 34 to 276 MN/m2 (fig. 11). Cold-worked Zr-2.5%Nb is virtually isotropic for high-temperature creep and since transverse data at 450” were not available, longitudinal data [3] were used for the comparison. At 207 MN/m2 and 450°C the longitudinal creep rate of IC material was about the same or slightly lower than that for cold-worked material. As stress was reduced, however, the heat-treated longitudinal material exhibited an increasing advantage in creep rate over coldworked. At 4OO”C, (fig. 12) a comparison with cold-worked transverse data [3] shows that the IC heat treatment reduced the creep rate by a factor of about 50 at 1000 h test time; not quite as much as at 450°C. As the stress was increased to about 345 MN/m2, the creep rates for the two materials became the same and presumably the curves would cross at higher stress. At 300°C (fig. 13), creep rates for IC material below 414 MN/m2 were similar to those for cold-worked [4] at 1000 h test time. However, at this time, creep rates were still decreasing, especially for IC specimens I
Fig. 11. Minimum creep rate of intermediate-cooled Zr-2S%Nb.
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where at 3000 h, the creep rates were about 5 times lower than those for cold-worked specimens. Thus, in long-term service at 300°C (and also at 400°C), transverse IC material would retain the advantage over cold-worked that it had at 45O’C. In addition to the above observation at 300°C, the creep rate of transverse IC material at a particular time went through a maximum at about half the ultimate tensile strength to produce a pronounced Sshaped curve of creep rate versus stress. The S-shape of this curve and the stress at maximum creep rate were similar to the curve and stress found for coldworked transverse Zr-2.5%Nb at 1000 h. There is presumably some variation in strength from one specimen to another. At 414 MN/m2, and 3OO”C, which is actually above the measured UTS of IC transverse material, there was a dramatic change in creep resistance with one specimen continuing for months at a very low creep rate and another failing on loading at a very slightly higher stress. An interesting result was obtained at the end of 3 128 h at 414 MN/m2 by increasing the load to 450 MN/m2. The specimen held this load without any noticeable deformation for several hours, then suddenly failed. Since this is well above the UTS at 300°C, it is suspected that some form of creep strain-ageing had strengthened the specimen, but once the specimen ,
I
Zr-2S%hb
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I
at 450°C compared with that of cold-worked longitudinal
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Fig. 12. Comparison of creep rates at 1000 h test time at 400°C for cold-worked and 880°C intermediate-cooled Zr-2.5%Nb.
transverse
A 01 10-9
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10-9
I 10-7
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10-9
10-s
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10-4
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CREEP RPITE.h-1
Fig. 13. Comparison of creep rates at 300°C for cold-worked and intermediate-cooled
started to deform, all strength was lost. Cold-worked material does not exhibit the discontinuity, having a continuous reduction in creep rate with increasing stress. Also, the discontinuity was not observed with transverse IC specimens at creep temperatures above 3oo”. Transverse IC specimens had an inflection point on
transverse Zrl2.5%Nb.
creep curves at about half the rupture time. This inflection usually occurs for zirconium alloys at a third to a quarter of the rupture time (see fig. 14). Localized deformation and relatively low elongations were observed to be a feature of ruptured transverse IC specimens.
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IO9-
Fig. 15. This area shows three sets of twins, one on tube radius, and the other two at about 38.5”. x 600. (The arrow indicates the stress and tranverse tube direction.)
Fig.
14.
Creep curves for transverse specimens of Zr-2.5%Nb tube in various conditions showing that IC material has inflection at relatively high fraction of rupture time.
3.2.3. Metallography The creep specimen which failed on loading at 3OO”C, a transverse tensile specimen which had been strained only 0.6% in a one-inch gauge length at 300°C, and ruptured 4OO’C creep specimens, were examined metallographically. The technique used was to chemically polish in a mixture of 30 parts H,S04,30 parts HNO,, 30 parts H20 and 8 parts HF, anodize electrolytically in Pickleseimer’s solution and examine under polarized light. Deformation twins were observed in all specimens. In most cases the twins were limited to a single grain, or perhaps two or three grains, but areas could be found where twins crossed many grains. Some single twins in fact included the volumes of hundreds of grains. The areas which contained the large twins were usually lens-shaped with the major lens axis in the tangential direction of the original tube. The lensshaped area that was untwinned was almost all one colour under polarized light, indicating that all grains in the area originally had almost the same orientation. In most instances, the twin boundaries were unaffected by grain boundaries and maintained a straight
course through both primary (o) and transformed (0) grains. Another feature of the macrotwinning was that there were occasionally three sets of twins, one approximately on the tube radius and two at an angle of 38.5” to the radius (fig. 15). Specimens were polished on the plane perpendicular to the tube axis and, with the major texture that is present, the polished surface was expected to coincide with the (lOTO) planes of most grains. A calculation of the angles for (1 OT2) twin boundaries, assuming lattice dimensions are the same as for pure zirconium, shows that they should be at 0 and about 38.5’ to the radius (fig. 16) which corresponds to what was observed. Since (1012) twins are those which most commonly occur in zirconium at temperatures above room temperature, it is fairly certain that the phenomenon is indeed twinning.
Fig._16. Intersection of 1012 twin planes with lOI plane. (1012) twmning produces three sets of twins one for each pair of 10 12 planes.
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The tensile specimen which was strained only 0.6% had been reduced in diameter by 0.02 mm at one position along its gauge length . Twins were observed in other places but there was one particularly large lensshaped area that was heavily twinned at the position of reduced diameter. Of the two specimens which had ruptured at 4OO”C, one had failed on loading and the other at 188.5 h test time. The specimen which failed rapidly had twins all along the gauge length. The longer-term specimen on the other hand had a twinned region near the heavily deformed portion while the major part of the test section had few or no twins.
4. Discussion 4.1. Texture effects The remarkable creep anisotropy of the heattreated Zr-2S%Nb tested here has not been observed before in zirconium alloys. To some extent it can be explained by the crystallographic texture and the preferred deformation mechanism during creep. Winton and Murgatroyd [S] found with Zr-2S%Nb that quenching from above the temperature of ‘about 775’C, where the equilibrium fl composition is about 7S%Nb, results in transformation of /3 to (Y’whereas quenching from below this temperature results in transformation of fl to w t retained p. Cheadle and Ells [6] observed that heat treatment in the (Y+ /I range, followed by slow cooling, results in the retention of the original cold-worked texture. They theorized that there was an interaction between 0 and cr phases with one influencing the texture of the other during transformation. When this information is added to that from the present work a picture begins to emerge. The cold-worked texture is retained when conditions are such that residual primary (Yphase is present and the /3phase transforms to cr, w or retained 0 during cooling from the heat-treatment temperature. A new ‘recrystallization’ texture develops only when cr’ is produced, i.e. when the material is cooled from above 775’C with water quenching or from a still higher temperature with intermediate cooling. It is well known that the preferred deformation mechanism for creep in close-packed hexagonal (11zirconium is slip on 1070 planes in 1120 direction. Cal-
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culations of the fraction of prism planes available for slip showed a difference approximating a factor of two for the two directions of interest. The fact that creep rates were found to differ by as much as a factor of 100 indicates that crystallographic texture alone does not explain creep anisotropy. An estimate from data available indicates that texture should only provide a factor of 10 difference. The theory developed by Cheadle and Ells to explain how the cold-worked texture is retained by heat treatment in the (Y+ fi range leads one to conclude that the primary (Ygrains have for the most part an A texture. Transformed /3will also to some extent retain the coldworked texture according to the Cheadle and Ells theory. However, the inducement to retain the texture seems to weaken as Nb concentration in the 0 phase increases and any material that does not have an A texture will be in the 0 last to transform and have a relatively high niobium concentration. Transverse specimens exhibit high creep resistance because on one hand the low Nb material, having A texture, is adversely oriented for easy slip and, on the other hand, that material which is oriented for easy slip has high Nb concentration and is thus also fairly strong. Longitudinal specimens will be able to deform relatively easily because the material which has low Nb is also oriented for easy slip. This provides an explanation for most of the anisotropy effects. It is difficult to explain why cold-worked material does not exhibit the same creep anisotropy as heattreated material with the same or similar texture. The cold-worked material had a factor of two difference in fraction of prism planes oriented for easy slip for the two material directions but in this instance the creep rate was different by only a factor of two. The difference between cold-worked and IC material must be due to segregation of Nb in IC material as described above. In a tube configuration with a factor of two difference in applied stress in the axial and tangential directions, the IC heat treatment should provide a component which is dimensionally very stable. The investigation has explained the reluctance of the sheath braze zone to deform, and this in turn suggests that the heat treatment may produce material which is resistant to in-reactor creep.
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4.2. Structure effects
While the crystallographic texture remained constant the creep properties were found to change over the heat-treat soaking temperature range. This effect can be explained by the relative amounts and niobium concentration, of phases present. For the sake of brevity this argument is left out. 4.3. High stress deformation
The relatively low tensile strength and the sudden loss in creep resistance of transverse IC specimens at 300°C and about 414 MN/m2 can be related to a twinning mechanism. With the pronounced A texture (basal poles in the tangential tube direction), transverse specimens are resistant to deformation by slip because of a lack of resolved prism-plane stress. When twinning occurs, however, material within the twin becomes oriented so that it can slip easily. Large numbers of twins readily occur in IC material. Massive twins encompassing hundreds of grains have never been observed before. The fact that they occur here is another indication of the very high degree of preferred orientation in IC Zr-2.5%Nb. Grains themselves are very small, but large numbers have such similar orientation that boundaries do not provide an obstacle to twin propagation, and the material behaves to some extent as though it were made up of large grains. Considering the transverse tensile data as a function of temperature, it seems likely that a more or less constant stress is required for twinning. At temperatures below 300°C, a stress higher than the twinning stress is necessary for slip after twinning, whereas above 300°C once twinned, the material slips without further encouragement. A mechanism such as this would explain the knee in the curve of tensile strength versus temperature. Other mechanisms such as strain-agehardening are not expected to be sufficiently potent for such a pronounced effect. Twinning is deformation-rate sensitive such that an increasing rate of deformation increases the probability of twin formation. Increasing temperature has an opposite effect [7]. At a constant rate of deformation, such as in a tenSib test, the mechanism governing the UTS will therefore
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change, at a certain temperature, from twinning to slip. This explains the sudden drop in UTS at 500°C. Above this latter temperature the material strength is once again determined by a slip mechanism. Assuming the above postulation to be correct, it is necessary to explain why the curve for longitudinal tensile data also has a knee at 300°C. This knee occurs at considerably lower stress than that for transverse specimens and extends only to about 400 rather than 500°C. The probable explanation is that some grains of D orientation do exist in the material and it is the twinning of this D component in longitudinal specimens that produces the knee. A smaller amount of D oriented material compared with A makes the effect less pronounced. For transverse-creep tests at 300°C there is a sudden loss in creep resistance at the UTS. This temperature is at the transition from slip to twin-control of tensile strength. A critical stress for twinning may explain the sudden loss in strength and particular specimens may or may not fail due to a variability of material or slightly different loading rate from one specimen to the next. When an area starts to twin and hence to slip, there is a sudden increase in stress on adjacent grains so they also twin with the result that the specimen fails and deformation is restricted to a short section of the gauge length. Creep strain at a stress below the UTS at 300°C is not sufficient to trigger twinning. The more gradual transition at 400°C from longterm testing to immediate failure, and also the lack of twins away from the fracture in a specimen which ruptured at less than the UTS prove that, at this temperature, twinrring can be triggered by creep strain. At a stress lower than the UTS, twinning, once started, spreads through only that part of the gauge length which undergoes an increase in stress at the reduced section. At a certain set of creep conditions a specimen can be exhibiting superior creep resistance but once the material starts to deform locally, i.e., goes into tertiary creep, the stress and strain rate in the necked region increases so that twinning starts. This explains why time to failure of transverse IC specimens is relatively short once the material has passed the inflection point on the creep curve. Incidentally, this does not rule out the material for practical applications because the onset of tertiary creep occurs in the IC material at a strain in excess of 2%, which in turn is after a relatively long time.
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4.4. Low stress deformation Let us now consider deformation characteristics at stresses less than those required to produce twinning. In the transverse direction over the temperature range tested, the creep rate was considerably lower for intermediate-cooled than for cold-worked Zr-2S%Nb. At 45O”C, the reduction by a factor of 100 was achieved at 1000 h test time whereas at lower temperature the same reduction required a much longer time. This is an understandable result, in that creep requires a longer time at lower temperature to reach the same state. There is an indication, however, that the strain rate is being affected by the amount of creep strain and this may be making a contribution to the tempperature effect. At 300°C with increasing stress, a maximum creep rate was observed at about half the UTS. With stress increasing above the halfway point, the strain increased but the strain rate decreased, The remarkable creep strength of transverse IC Zr-2.5%Nb is believed to be the result of a preponderance of A texture grains which cannot deform by slip. Creep strain without twinning is thus limited to those grains which do not have an A texture and tends to have a limit. A maximum strain rate occurs at an intermediate stress simply because deformation at higher stress uses up the available strain in those grains which can deform. Of course, at sufficiently high stress, twinning begins to occur and this leads to rupture. This effect is pronounced at 300°C but probably also occurs to some extent at higher temperatures. From the point of view of out-reactor creep and tensile strength the intermediate-cool heat treatment could be used for organic-cooled reactor pressure tubes. The greater creep resistance compared with cold-worked tubes in both longitudinal and transverse directions would prolong the useful tube life.
5. Conclusions (1) Creep properties of Zr-2S%Nb tubing in the transverse direction are improved markedly by an intermediate-cool heat treatment from the upper a! + (3or 0 temperature. Creep rates are about 100 times lower than for cold-worked material.
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(2) For intermediate-cooled Zr-2.5%Nb, the high creep strength found for transverse specimens is attributed to crystallographic texture but may also depend to some extent on segregation of Nb to the /3 phase. (3) Water-quenched Zr-2.5%Nb exhibits isotropic creep properties when heat-treatment temperature is above about 800°C. Below this soaking temperature, the behaviour is similar to that of intermediate-cooled material. (4) Cold-worked Zr-2.5%Nb is much less anisotropit in creep at 450°C than material heat-treated to produce the same crystallographic texture. (5) The improvement in creep resistance conferred to transverse specimens by the intermediate-cool heat treatment is maintained for all stresses up to the UTS over the temperature range 300 to 450°C (6) The longitudinal creep strength of intermediatecooled Zr-2.5Nb, while inferior to that of transverse, exhibits relative improvement over cold-worked Zr-2.5%Nb (creep rate about 10 times lower) at 450°C as the stress is decreased below 207 MN/m2. (7) Cold-worked tensile strength in the range from room temperature to about 500°C is reduced by the intermediate-cool heat treatment. (8) The low tensile strength of intermediate-cooled Zr-2.5%Nb and the instability of creep specimens at high stress are related to a twinning mechanism. (9) Large twins encompassing hundreds of grains can occur in Zr-2.5%Nb.
Acknowledgements The author is indebted to B.F. Jones for the idea of a possible improvement in creep properties by the intermediate-cool heat treatment and also for discussion at the early stages on possible causes for the effect. Helpful discussions of deformation mechanisms were held with B.J.S. Wilkins. Many other individuals at WNRE helped with the experimental work; in particular G. Duncan conducted the short-term creep tests, F. Havelock the metallography and G.M. Omichinski with the help of M. Duclos did the crystallographictexture measurements. The Long-term creep test and most of the tensile tests were conducted at Orenda Ltd., under the supervision of M.C. Teeter.
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References [l] J. Walker, R.E. Dailly and M.H. Schankula, WNRE unpublished information. [2] E.F. Ibrahim, In-reactor Creep of Zirconium-Alloy Tubes and Its Correlation with Uniaxial Data, ASTM STP 458 (1969) p. 18. [3] LG. Bell, Creep Tests on Cold Worked Zr-2.5%Nb at 400°C and 500°C, Orenda Ltd. Report HSER-FCP67-003 (July 1967). [4] L.G. Bell, Creep Tests on Zr-2.5%Nb Pressure Tubing at 3OO”C, Orenda Ltd. Report HSER-FCP66-002 (August 1966).
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[S] J. Winton and R.A. Murgatroyd, The Effect of Variations in Composition and Heat Treatment on the Properties of Zr-Nb Alloys, Electrochemical Technology, vol. 4, no. 7-8 (July-Aug. 1966) p. 358. [6] B.A. Cheadle and C.E. Ells, The Effect of Heat Treatment on the Texture of Fabricated Zr-Rich Alloys, Electra- _ chemical Technology, vol. 4, no. 7-8 (July-Aug. 1966) p. 329. (71 S. Mahajan and D.F. Williams, Deformation Twinning in Metals and Alloys, Review 173, International Metallurgical Reviews, vol. 18, (1973) pp. 43-61.