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The terminal solid solubility of hydrogen and deuterium in Zr-2.5Nb alloys
ELSEVIER
Journal of Nuclear Materials 228 (1996) 227-237
jem'aalef nadear naterials
The terminal solid solubility of hydrogen and deuterium in Zr-2.5Nb alloys Z.L. Pan, I.G. Ritchie 1, M.P. Puls * Materials and Mechanics Branch, AECL, WhitesheU Laboratories, Pinawa, Manitoba, Canada ROE 1LO
Received 17 July 1995; accepted 6 October 1995
Abstract
The terminal solid solubility (TSS) of hydrogen or deuterium is an important parameter in Zr alloys that are used in the nuclear industry. If this solubility is exceeded, it can make the alloys susceptible to delayed hydride cracking. Accurate expressions for the TSS are necessary for modelling delayed hydride cracking as well as deuterium ingress into pressure tubes and blister formation in pressure tubes in contact with their calandria tubes. Measurements of the changes in the dynamic elastic modulus have been used to establish expressions for the TSS as a function of temperature and to study the hysteresis between hydride dissolution and precipitation. It is shown that the hysteresis is particularly sensitive to the thermal history of the sample (such as the prior maximum temperature, hold time at maximum temperature and cooling rate). As a result, the precipitation TSS (TSSP) has a range of values bounded by the solubility equations designated TSSP1 and TSSP2. These are obtained, respectively, by cooling from an upperand a lower-bound maximum temperature. The TSSD equation obtained in this study is very close to previously determined expressions, but the TSSP equations differ significantly from earlier results.
1. Introduction
When the concentration of hydrogen isotopes in a reactor core component exceeds the terminal solid solubility (TSS) at the temperature under consideration, hydrides may be present and embrittle the component. As a result, TSS data on hydrogen isotopes in Zr-2.5Nb pressure tubes are required to assess the potential for delayed hydride cracking (DHC) and hydride blister formation in CANDU 2 reactor pressure tubes. In the past, Kearns' TSSD (Dissolution) data for hydrogen in zirconium and the Zircaloys [1] have been
* Corresponding author. Tel.: +1-204 753 2311; e-mail: [email protected]. 1Present address: Nuclear Materials and Fuel Cycle Technology Section, Division of Nuclear Fuel Cycle and Waste Management, International Atomic Energy Agency, A-1400, Vienna, Austria. 2 CANDU is a registered trademark of Atomic Energy of Canada, Ltd.
Elsevier Science B.V. SSDI 0022-31 15(95)00217-0
used in fitness-for-service assessments of CANDU reactor pressure tubes as one of the threshold criteria that must be met for susceptibility to DHC initiation [2].
Similarly, embrittlement of zirconium-based alloy components in light water reactor systems (Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs)) occurs and can lead to problems during handling and manipulation of the components, both inand out-of-reactor. For example, difficulty in relocating control rods in a PWR reactor was recently experienced and traced to broken and deformed guide tubes caused by hydride embrittlement [3]. The pickup of hydrogen caused by radiolysis of the cooling water and corrosion mechanisms, and its subsequent redistribution to cold spots by thermal diffusion, will become more of a problem as fuel assemblies are pushed to increasingly higher burnups. It is well known that there is a large hysteresis between the phase boundaries for precipitation (TSSP) and dissolution (TSSD) of hydrides in Zr [4] and a
228
Z.L. Pan et al. /Journal of Nuclear Materials" 228 (1990) 227-237
theoretical model of this hysteresis has been proposed by Puls [5,6]. The hysteresis has important physical consequences for D H C , hydrogen ingress and blister formation [7,8]. Only limited experimental data is available on TSSP in Zr-2.5Nb. The situation is somewhat better regarding TSSD. Coleman and Ambler [9] compared the TSSD data for hydrogen and deuterium in Z r - 2 . 5 N b material, obtained by diffusion, dilatometry, and equilibrium pressure isotherms. It was found that the data scattered about Kearns' best fit line to TSSD data for the ~t-phase materials zirconium and the Zircaloys. However, there is considerable scatter, which was believed to be mainly caused by uncertainties in hydrogen analyses. In addition, Coleman and Ambler [9] compared the TSSD data with their data for the concentration limit for D H C initiation at sharp cracks. They found that the two limits coincided, within the scatter of the results. However, the close correspondence of the TSSD data to the concentration limit for D H C appears to be fortuitous, since the latter process clearly requires hydride precipitation at the crack tip, which is governed by TSSP. This point has been addressed in detail by Shi et al. [2] who show that the increase in hydrogen concentration - relative to the TSSD concentration - that is achieved at a sharp flaw tip due to the presence of a high, tensile hydrostatic stress, approximately compensates for the increase in hydrogen concentration needed for hydride precipitation to occur at the flaw tip. This accounts for the closeness of the D H C initiation 'solvus' at sharp flaws (under quasistatic cooling conditions) to the TSSD. To ensure that more accurate values for the TSS in Z r - 2 . 5 N b are available and to study in more detail the factors affecting, particularly, TSSP, a cooperative (round robin) study among three Canadian laboratories [10] was carried out - using a variety of techniques - to measure the TSS in deuterided Z r - 2 . 5 N b pressure tube material which was obtained from the same source pressure tube. The techniques used were differential scanning calorimetry (DSC), internal friction (specifically, dynamic elastic modulus (DEM)), small angle neutron scattering (SANS) and D H C . This paper reports on the D E M results and discusses their interpretation. A previous paper has presented the results obtained with the SANS technique [11].
2. Material preparation 2.1. Source pressure tube
A Z r - 2 . 5 N b pressure tube (R766), which was rejected for reactor use due to out-of-specification wall thickness, was selected for the cooperative TSS study. To correspond to tubes put in-reactor, the tube was
Table I Chemical composition of the pressure tube selected for test Element
Specification
Analysis
H N O Nb
< 25 ~xg/g < 80 txg/g 900-1300 Dxg/g 2.4-2.8 wt%
8.5 }xg/g 23.5 ixg/g 1060 ixg/g 2.73 wt%
autoclaved at 400°C. Chemical composition of tube R766 was determined as summarized in Table 1. The composition of the remaining alloy impurities were assumed to be the same as that of the ingot material used to produce tube R766 (and other tubes). The results of the ingot analysis are summarized in Table 2. The ingot was produced by preheating to 982°C and press forging. The ingot was then reheated to 1015 + 10°C for 1 + 0.25 h at temperature and quenched (13quenched). Afterwards, the ingot was rotary forged to the approximate final size. The microstructure of tube R766 was not specifically examined. However, it is expected to be similar to that of other tubes manufactured in the same way. The microstructure of such tubes consists of flattened grains of hexagonal-closepacked a-phase material containing about 0.8 wt% Nb in supersaturated solid solution surrounded by a thin layer of body-centred-cubic 13-phase material. The latter phase has an average, metastable composition of about 20 wt% Nb, which decomposes internally into a
Table 2 Chemical composition of ingot material close to selected pressure tube Element
Specification (Ixg/g)
Analysis (txg/g)
AI B C Cd Co Cr Cu Fe ttf Mg Mn Mo Ni Pb Si Sn Ta Ti U V W
75 0.5 140 0.5 20 200 50 1500 50 20 50 50 70 130 120 100 200 50 3.5 50 100
47 < 0.25 140 < 0.25 < 10 < 100 < 25 650 42 < 10 < 25 < 25 < 35 < 25 52 < 25 < 25 < 25 < 1.0 < 25 < 25
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237
229
Table 3 Summary of TSSD and TSSP results from DEM H + D/2 a
TSSD
TSSP1
Tmaxl
TSSP2
Tmax2
(p.g/g)
(°C)
(°C)
(°C)
(°C)
(°C)
99 88 86 86 79 68 63 63 61 53 52 52 47 36 36 31 27 25 15 b 11 b 7.5 7.3 7 7 7 7
350 340 335 336.2 329.2 314 304.2 313.3 303 288.4 295 296 274.3 258.7 265 256 248 234.3 201.6 190 177 173.9 173 172.4 174.9 174
286 279 274 273 267.6 259 246 252 238 228.5 234 235 219.2 200.8 204.8 198 188 177.4 145 124
450 450 450 450 450 420 385 430 450 430 440 430 420 400 430 450 420 420 430 450
107
430
111
430
302 299 294 292 286 273 266 272 262 249 258 257 243.2 222.7 228.0 221 213 205.6 167 154 129 126 125 126 127 126.5
368 360 360 350 330 320 310 320 320 310 310 310 300 280 280 280 260 260 220 300 250 250 250 250 250 220
Sample no. 90-6 90-5 90-4 90-3 90-1 67-4 67-1 67-3 67-5 58-3 58-4 58-5 58-1 42-1 42-3 42-4 30-3 30-1 10-1 10-2 7-3 7-1 7-2 7-4 7-5 7-6
a Uncertainty (two standard deviations) = 10%. b Ni-plated and gaseously hydrided at 250°C.
mixture of N b - d e p l e t e d o - p h a s e s u r r o u n d e d by Nb-rich 13-phase gradually increasing towards t h e stable c o m p o sition of a b o u t 80 w t % Nb, the extent of this d e p e n d ing o n t h e n u m b e r of autoclave t r e a t m e n t s and, for irradiated material, the irradiation t e m p e r a t u r e a n d fluence. T h e a - p h a s e grains are flat platelets, a b o u t 0.4 p,m thick, with an aspect ratio of t h e o r d e r of 1 : 10 : 40 (radial : transverse : longitudinal). T h e volume fraction of the 13-phase is approximately 8%. T h e m e c h a n i c a l p r o p e r t i e s of the selected t u b e at 300°C are: u l t i m a t e tensile stress (axial) of 529 M P a , yield stress (0.2% offset) of 374 M P a a n d e l o n g a t i o n of 17.8%.
2.2. Hydriding T h e source m a t e r i a l was d e u t e r i d e d by t h e electrolytic hydriding ( d e u t e r i d i n g ) / t h e r m a l diffusion technique. U s i n g this p r o c e d u r e , a solid d e u t e r i d e layer was electrolytically d e p o s i t e d o n t o t h e outside surface of the t u b e in a 0.1M D 2 S O 4 electrolyte at 90°C with a c u r r e n t density of 100-120 m A / c m 2. T h e d e u t e r i u m was diffused into t h e t u b e by equilibrating t h e deuteride layer with t h e p r e s s u r e t u b e matrix at K e a r n s ' T S S D t e m p e r a t u r e s c o r r e s p o n d i n g to t h e h y d r o g e n /
d e u t e r i u m 3 c o n c e n t r a t i o n s required. It was i n t e n d e d to d e u t e r i d e the source material to five c o n c e n t r a t i o n levels (in addition to using m a t e r i a l with as-received c o n c e n t r a t i o n ) up to m a x i m u m c o n c e n t r a t i o n of 100 i x g / g H - e q u i v a l e n t (0.9 at%). D e u t e r i u m was c h o s e n over h y d r o g e n b e c a u s e it provides a s t r o n g e r r e s p o n s e to n e u t r o n scattering for the S A N S p a r t of the cooperative study a n d b e c a u s e it is the same isotope t h a t is a b s o r b e d by the r e a c t o r p r e s s u r e tubes. A f t e r a diffusion anneal, t h e r e m a i n i n g surface hydride layer was r e m o v e d by machining. T h e s p e c i m e n s used for the D E M analysis were all chemically analyzed for h y d r o g e n by the H o t V a c u u m Extraction ( H V E ) m e t h o d . N o t e t h a t t h e H V E m e t h o d does n o t distinguish b e t w e e n h y d r o g e n a n d d e u t e r i u m a n d t h e r e f o r e provides only the total equivalent hydrogen c o n c e n t r a t i o n . In addition, for t h e TSS d e t e r m i n a tions using t h e D E M technique, f u r t h e r specimens were a d d e d to those specimens used for t h e c o o p e r a -
3 In the following, for simplicity, both hydrogen ad deuterium are referred to as hydrogen.
230
Z.L. Pan et al. /Journal of Nuclear Materials" 228 (1996) 227-237
tive study - with particular emphasis on specimens with the lowest hydrogen concentration. A few of the additional specimens that were added were hydrided using hydrogen only - by gaseously diffusing hydrogen into the specimen, the suface of which had been nickel plated. These specimens are explicitly identified in the table of results (Table 3).
,ruMMY SAMPLE COOLER HEATER
l OWER : SUPPLY
i
: TEMPERATURE CONTROLLER
FUSEI) QUARTZ
7 i GALGE CRYSTAL
COMPLrFER MONITOR PI.OTTER
t --~
3. Experimental procedure
The test procedures of DEM measurements are described in this section. The general guidelines were to control heating and cooling rates to 2°C/min and to have at least ten thermal cycles for each set of measurements.
]
i TI
DRIVE CRYSTAl
CLOSED LOOP t CRYSTALDR/VER
I ]
i VACUUM PUMP
Fig. 1. Schematic diagram of the APUCOT. 3.1. Internal friction and dynamic elastic modulus techniques
Elastic modulus and internal friction studies [ 12-17] have been reported previously to map H-TSS boundaries of hydrogen-doped, pure Zr. In the past few years, these investigations have been extended to the study of the TSS in Zr-2.5Nb alloy pressure tube materials [18-20] containing hydrogen or deuterium. Most recently, the same techniques have been used in the preliminary survey of the H-TSS in an experimental pressure tube alloy of Zr called Excel [21,22]. It should be noted that internal friction techniques are regarded as standard methods for the determination of the H-TSS in the body-centred-cubic hydride-formers V, Nb and Ta. Although an internal friction (Q 1) peak associated with hydride dissolution on heating and hydride precipitation on cooling, similar to those obtained in V, Nb and Ta, is indeed observed in pure Z r - H alloys, no such peak is observed in Zr-2.5Nb-H. This is attributed to the lack of long misfit dislocations in the pressure tube alloy compared with the copious numbers found in pure Zr-H alloys [23]. Nevertheless, a more or less sharp knee in the dynamic elastic modulus (proportional to F 2, where F is the resonant frequency) is observed at the expected TSS on both heating and cooling. For this reason, we chose the most precise method of determining dynamic elastic modulus as a function of temperature available to us for this study. 3.2. Apparatus
A general overview of the methods used to measure the elastic (Young's) modulus (E) and internal friction (Q 1) based on the automatic piezoelectric ultrasonic composite oscillator technique (APUCOT) has been given elsewhere [20,24,25]. With the APUCOT (see the schematic diagram in Fig. 1), the specimen tempera-
ture is measured by a thermocouple inserted in a calibrated dummy specimen of the same material as that being tested. The dummy specimen, of the same dimensions as the actual specimen, is located together with the real specimen so that they are symmetrically situated about the center of the cylindrical cavity of a small, non-inductively wound furnace. The furnace is 100 mm in length and 18 mm in internal diameter. A typical Zr-2.5Nb specimen with dimensions 3 x 3 × 46 mm is located in the center of the uniform temperature region of the furnace. This configuration allows temperature measurements to within an error of +0.5°C in the range of 50-500°C while the heating rate is less than 5°C/min. (Heating rates up to 100°C/min are available with the APUCOT for special tests in air.) The test chamber is normally evacuated to 10 3 torr or better before a test to prevent substantial oxidation of the specimen. The temperature measuring system was carefully recalibrated at heating and cooling rates of 2°C/min for the series of TSS tests described herein. A very stable heating (or cooling) rate is important for the precise determination of the temperature dependence of Young's modulus used to map the H-TSS boundaries. Although the sampling rate of the electronic instruments is every 0.1 s for T and F, and 1 ms for voltage, the computer is programmed to record the raw data collected only once each 6 s during a thermal cycle. At a cooling rate of 2°C/min, this corresponds to five raw data points collected for every I°C change in temperature. These points were averaged to give values of T, E and Q i for each I°C temperature interval. 3.3. Dynamic elastic modulus versus temperature
It has been shown [18-20] that a sharp 'knee' in a plot of dynamic elastic modulus versus temperature is
Z.L. Pan et aL / Journal of Nuclear Materials 228 (1996) 227-237
231
Zr-2.5Nb + 86 p,g/g H
85
Heating & Cooling at 2°C/min
Zr-2.5Nb + 86 ~g/g H Coolingfrom 370°Cat 2°C/min
6 [./" \ X
D \ 80
o r~
TSSP1
o
~ ° ° , o . ° TSSD °,
from TmB×~(460%)
75
-% ~
I
TSSP2
from Tmax2(370°C)
200
. . . .
. . . . .
3oo
. . . .
Temperature, °C Fig. 2. Typical curves of Young's modulus E versus temperature showing different hysteresis between hydride dissolution and precipitation during cooling from different maximum temperatures on a Zr-2.5Nb specimen containing 86 tJ-g/g
270
280
290
300
310
320
Temperature, °(2 Fig. 3. TSS temperature determined from the interception of straight lines extrapolated from the plots of frequency versus temperature above and below the 'knee'-point.
(0.77 at.%) of hydrogen. associated with the dissolution of hydrides during heating and their precipitation during cooling in Zr-2.5Nb alloys. The temperature at the knee-point can be used to identify the solvus temperature and, therefore, to map H-TSS boundaries. Young's modulus is proportional to the resonant frequency squared ( F 2) of the specimen in longitudinal vibration. With the APUCOT, resonant frequency measurements are much more precise than those obtained using other elastic modulus measuring techniques. Consequently, the A P U C O T was employed for this series of TSS tests, rather than the low frequency flexure pendulum technique, which we have employed to study hydrogen in pure Z r - H alloys [16,20]. The hysteresis between the hydrogen dissolution (TSSD) and precipitation (TSSP) is easily observed in the E versus T curves obtained on the same specimen during a thermal cycle through the TSS (see Fig. 2). Two methods were used to estimate the knee-point, i.e. the TSS temperature. In the first method, straight lines are drawn through the experimental data points of F versus T above and below the knee, and their intersection is taken as the TSS temperature, as shown in Fig. 3. This procedure is somewhat subjective. In the second method, curves of the derivative of frequency versus temperature (-dF/dT versus T), or the derivative of Young's modulus versus temperature (-dE/dT versus T) are calculated and plotted, as shown in Fig. 4. In this case, the TSS temperature is taken to be the point of maximum slope on the sharp peak in this plot. Using blind tests, we have found that both techniques are equivalent within an error of 0.5°C.
Even though the second method is more objective, the peak can be rather low and ragged, making the point of maximum slope difficult to determine with accuracy for the lowest hydrogen concentrations. The sharpness in the widths of the peaks in Fig. 4 may be indicative of the rate at which the phase transition proceeds, with the narrowest peak indicative of the fastest transition. The rate at which the transition proceeds may also provide a clue as to the type of transition. Thus, TSSP1, which has the narrowest peak, likely represents the nucleation stage of hydride formation, since nucleation is a process which is expected to proceed rapidly over a narrow temperature range. The
Zr-2.5Nb + 79 Ixg/g H
0.14l
TSSP1 /
0.12!
/
TSSP2 O~.. o.1
/
_ ~
~
TSSD
0.08 i
0.06] !
150
.
.
.
.
.
.
200
.
J
_J
•
i
.
.
250 300 Temperature, °C
.
.
.
.
.
350
400
Fig. 4. TSS temperature determined from the derivative of Young's modulus E versus temperature.
Z.L. Pan et aL /Journal of Nuclear Materials 228 (1996) 227-237
232
other two phase boundaries (TSSP2, representing some sort of hydride growth process and TSSD, representing hydride dissolution) have much broader peaks, which may be indicative of their slower rate. Based on this interpretion of the significance of the widths of the peaks, hydride dissolution appears to be the most sluggish of the three transformation processes.
352
Coolingfrom430°Cat 2~C/min
"\\\\
g -a
75 i
7 ~g
"
~
<
. . . . . . . 150
200
250 300 Temperature, °C
~
227
100 ~ . ~
40
X
3o
\
112
.
1.8
.
1
\
0.4 ~.
\ \ \
\
TSSD
"~
1.6
144
CoolingfromT~ax2(370'°C> CoolingfromTm~×.(450°C) 10.5
X \
5
182
- Zr-2.5Nb + 86 I.tg/g H
~ ~ "~.~. ~'~ 50 X X~
3.4. Relationship between dynamic elastic modulus and H concentration In Z r - 2 . 5 N b - H , Z r - H and T i - H , when all the hydrogen is in solution, the E versus T curve is extremely linear for temperatures below 550°C [12,20,21]. More importantly for this study, the presence of increased levels of hydrogen depresses the elastic modulus at any given temperature. Consequently, a set of E versus T curves for different levels of hydrogen arc nearly parallel, straight lines, when all the hydrogen is in solution. For example, the experimental results for four different levels of hydrogen are shown in Fig. 5. These observations strongly suggest that for a particular E versus T curve, such as the cooling branch of the 86 I.tg/g specimen in Fig. 5, the excess of the low temperature curved portion of the line, over the linear extrapolation of the high temperature region to low temperatures, simply represents the amount of hydrogen present as hydrides. This means that an analysis of this excess yields an estimate of the solubility of hydrogen at any point lower than the TSS in this specimen. Such an analysis has been carried out on the 86 Ixg/g specimen. Using this novel procedure, the results of Fig. 2 for TSSD on heatup and for TSSP on cool-down, using
Temperature, °C
283
.
2
.
o.s~ \
\
TSSP1
-%
2.2
\
I...X ' ? ~
2.4
a
~
,~
%.05
2.6
Reciprocal Temperature, 1000/K Fig. 6. Solvus boundaries derived from the analysis of Young's modulus as a function of temperature measured on a single specimen containing 86 Ixg/g (0.77 at.%) of hydrogen. The solid lines are the fits to the data shown in Fig. 7.
two different maximum temperatures to which the specimen was taken prior to cool-down, are shown in Fig. 6. In the important range of concentration from 10 to 80 Izg/g H (from the point of view of the operational conditions of pressure tubes), the analysed curve (log(concentration) versus l / T ) does indeed agree with a log(concentration) versus 1 / T line calculated from the TSS data of several specimens with different levels of hydrogen (Fig. 7, see Sections 4.1 and 4.2). At very low solubilities, the data are scattered because they represent very small differences between two large quantities and because they contain some contribution to the E versus T curve from the processes that govern the modulus defects of relaxation at low temperature. In addition, there could also be scatter because, below 100°C, the cooling rate cannot be kept at the target rate of 2°C/min and starts to decrease. In principle, the whole log(concentration) versus 1 / T plot for the TSS boundary can be estimated by this analysis technique using the data from only one thermal cycle on a specimen containing a suitably high level of H. This is an important advantage of the Young's modulus measuring technique in the field of TSS studies.
86ggtg %~ 350
Fig. 5. Typical curves of Young's modulus E versus temperature for Zr-2.5Nb specimens containing different concentrations of hydrogen.
4. Experimental results 4.1. TSSD The results of all of the TSSD temperatures obtained by the A P U C O T during heating at 2°C/min are
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237 listed in Table 3. There was always more than one specimen tested at each nominal hydrogen concentration level. No apparent influence of heating rate a n d / o r the maximum temperature reached during a thermal cycle was detected on the TSSD temperatures, as long as the previous cooling rate was the same as the heating rate. Preliminary tests had shown that previous thermal history can significantly influence the value of the dissolution temperature. This is believed to be because the typical sizes of the hydride particles, which resulted from the previous cool-down, need to be consistent with the sizes which will allow enough removal of hydrogen from them as they re-dissolve to maintain quasi-equilibrium at the imposed heating rate. If, for instance, the previous cooldown rate is much slower than the heatup rate, then the hydrides formed on the previous cooldown are too large for quasi-equilibrium to be maintained at a heatup rate which is faster than the previous cooldown rate. Therefore, to obtain reliable results, the heatup and cooldown rates must be identical. This requirement applies equally well to determinations of TSSP. For most of the specimens, ten or more thermal cycles were observed to check the repeatability of our technique. Therefore, the average value of multiple measurements on the same specimen is taken as the TSSD temperature of that specimen and listed in Table 3. The standard deviations of the TSSD temperatures for most specimens are less than or close to I°C. Thus, the reproducibility of our TSSD measurements is satisfactory, except for the 'as-received' specimens with the lowest levels of hydrogen. The TSSD data in Table 3 are well represented by the following solubility equation, assuming a log(C H) (where C H stands for hydroTemperature, °C
352 283 227 182 1 0 0 " ' . ~- ,.% ~ . ~ - ~ -'- ~ "\,\
[
Zr-2.5Nb + H
'\
\\
2so 301 r~O 20. -~
144 112 ' ' ~1
//
05
~,. " " \ X ~ XX
Slattery'sTSSP
10"3~
~'~TSSP1, ThisWo~ 0.2
TSSD,Th,sWo~,,,
X \'~.
I
I° i s
51 . . . . . . S,atterfs T S S D - ~ t S ~ 2 o ~ ~'0.05 1.6 1.8 2 2.2 2.4 2.6 Reciprocal Temperature, 1000/K Fig. 7. Solvus boundaries TSSD, TSSP1 and TSSP2 of hydrogen solubility versus reciprocal temperature derived from fits to the TSS data of Table 2.
233
Table 4 Summary of solubility equations for TSSD and TSSP from this work compared with data in the literature TSSD This work Kearns [1] Slattery [26]
C° = 8.080 X 10 4 exp[- 34520/RT] C° = 1.200x 105 exp[- 35900/RT] C° = 6.860x 104 exp[- 33570/RT]
Ixg/g ~g/g Ixg/g
CP1 = 2.473 x 10 4 exp[- 25840/RT] CH P2 = 3.150 X 10 4 exp[-27990/RT] CP = 4.110 x 104 exp[- 28000/RT]
Ixg/g ixg/g I~g/g
TSSP This work This work Slattery [26]
gen concentration) versus 1 / T dependence, calculated by linear regression (r 2 = 0.994): C D=8.080X
104
xexp[-34520/RT]
Ixg/g
for TSSD,
(1)
where R = 8.3144 J / m o l K and T is in K. This equation is plotted in Fig. 7 and compared with the experimental TSSD data. An analysis as described above from a single specimen containing 86 ixg/g hydrogen can also be used to derive a log(C D) versus 1 / T plot, as shown in Fig. 6. There is excellent agreement between the Iog(CHD) versus 1 / T plot from experimental results on one specimen and Eq. (1) calculated from TSSD data for 26 different specimens, representing a range of hydrogen concentration from 7 to 99 Izg/g. Also shown in Fig. 7 is Kearns' linear regression fit to a compilation of TSSD data for a-phase unalloyed Zr and Zr alloys [1] and Slattery's fit to TSSD data from heavily cold-worked Zr-2.5Nb rod material [26]. The comparison shows that there is good agreement between the three sets of data. However, because of slight differences in slope between the solvus lines, the solvus derived from the pressure tube Zr-2.5Nb material predicts, for a given temperature, progressively lower hydrogen solubility values, in the reactor operating range of 250 to 310°C, compared to the results from Kearns' solvus line. A summary of the solvus lines derived from this work as well as Kearns' and Slattery's results is given in Table 4. For a given hydrogen concentration, the DHC cracking temperatures, which were also obtained as part of the original cooperative study [10], were always found to be lower than the corresponding TSSD temperatures. Alternatively, for a given temperature, the hydrogen concentration necessary for DHC initiation is always greater than the solubility for hydride dissolution. A detailed analysis of these results has been given in a separate publication [2].
4.2. TSSP Previous studies [19,20] have confirmed that the processes of nucleation and precipitation of hydrides in
Z,L. Pan et al. /Journal ¢~fNuclear Materials 228 (1996) 227-237
234
Z r - 2 . 5 N b are very complex, and that the TSSP temperature depends sensitively on (i) the microstructure of the material in question, (ii) the maximum temperature (Tm~x) reached during a thermal cycle and (iii) the cooling rate. Even if Tm~x does not reach the dissolution TSS, i.e. Tmax < TSSD, a TSSP temperature can still be detected. Therefore, every datum of TSSP should always be associated with a given Tm~~. In the round robin series of TSS tests [10], we have defined TSSP1 and TSSP2 as the TSSP temperatures obtained after cooling from the highest and lowest Tm~~ temperature chosen, respectively. In practice, we usually chose both of the Tm~~ temperatures to be above the TSSD temperature. The experimental results for TSSP1 and TSSP2 and their corresponding Tm~x values for the six Z r - 2 . 5 N b materials are listed in Table 3. Observed differences between TSSP1 and TSSP2 vary from 14°C for the 68 Ixg/g specimen to 30°C for the 11 Ixg/g specimen. Both TSSP1 and TSSP2 are also plotted on a log(C H) versus 1/T graph as shown in Fig. 7. The corresponding solubility equations are C~ ] = 2.473 X 104 xexp[-25840/RT]
Ixg/g
for TSSP1
(2)
Ixg/g
for TSSP2.
(3)
and C~ 2 = 3.150 x 104
xexp[-27990/RT]
If the value of Tm~~ chosen in a particular test is between the highest and lowest Tm,x, then the corresponding TSSP temperature measured in the test will be between the TSSP1 and TSSP2 lines. The experiments on TSSP as a function of Tin,x were carried out on all five sets of specimens of Z r - 2 . 5 N b - H material consisting of different nominal hydrogen concentrations. The cooling rate was 2°C/rain and typical results are shown in Fig. 8. These show that TSSP decreases to an approximately minimum flat value at about 400°C in every case. However, at a lower cooling rate of 0.2°C/min, the TSSP temperature for the 50 Ixg/g specimen decreased to a minimum 220°C at a Tm~x value of 380°C. These TSSP Tmax values are significantly lower than those found at a cooling rate of 2 ° C / m i n . When the 50 I~g/g specimen was held for 4 days at a particular Tma~ value and subsequently cooled at 0.2°C/min, then as shown in Fig. 9, the TSSP temperature decreased to a minimum value, at a Tm~~ of 350°C, again significantly lower than that found without annealing for 4 days. In conclusion, it appears that the hysteresis between dissolution and precipitation of hydrides in Z r - 2 . 5 N b can be extended, and the maximum temperature needed to achieve this decreased, if the specimen is cooled very slowly or if the specimen is pre-annealed for a long time at a sufficiently high temperature.
Zr-2.5Nb + H 300 CoofingfromTma× ~ ' at 2°C/rain 280
~
61 tag/gH ¢~
53/.tg/
[~ 240 [_ 220 ~
31 pg/g H o~ S
P
2
i
2O0
I
TSSP1 '
~
..........................................
300
;
350 400 Maximum Temperature, °C
450
Fig. 8. Typical curves of TSSP temperature versus maximum temperature for thermal cycles to determine the TSSP temperature. The temperatures TSSPI and TSSP2 are obtained from the data, as indicated by the arrows.
In Fig. 9, two TSSP data were observed in thermal cycles for which Tmax w a s deliberately chosen to be below TSSD. These data are of interest because they reveal the effective TSSP for the nucleation of 'new' hydrides in the presence of pre-existing hydrides. Similar experimental results on Z r - 2 . 5 N b specimens containing 200 p~g/g of hydrogen were reported and discussed in detail elsewhere [19,20]. These results appear to show that the existing hydrides do not grow significantly for the cooling rate used in our tests. Rather, the supersaturation of hydrogen builds up until either
~60
o~
250
Zr4.5~ + 50 ~g/g H [] CoolingfromTna at0.2°C/m,nafter4daysatT~a~ Coolingf,omT,na~at0.2°C/mlr, O Coolingfromfna at2°C/m4r,
2401 ~D t~
230 1.~, < TSSD ~
O
,~] TSSD 22O . . . . T, , 200 250 300
350
400
'450
Maximum Temperature, °C Fig. 9. Curves of TSSP versus maximum temperature from thermal cycles to determine the TSSP temperature using different cooling rates and hold times on a Zr-2.5Nb specimen. Included in the figure are two TSSP temperatures for which the maximum temperature was below TSSD.
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237
nucleation occurs at new sites in the lattice, or growth on existing hydrides is possible. However, the results of Fig. 6 show that on continuous cooling, hydride formation follows the TSSP2 boundary after initial precipitation, regardless of whether the Tmax temperature was Tmaxl, or Tmax2. Assuming that TSSP2 represents the boundary for hydride growth rather than nucleation, this seems to suggest that hydride growth is favoured as cooling progresses in the presence of hydrides. Also plotted in Fig. 7, for comparison, is the linear regression fit to Slattery's TSSP data obtained from heavily cold-worked Zr-2.5Nb rod material [26]. The agreement among TSSP curves is not as good as that for the TSSD results. However, TSSP depends on thermal history such as the maximum sample temperature and the cooling rate. A possible explanation for this dependence is that the higher maximum temperature anneals out the dislocations on prior hydride precipitation sites (memory effect [20,27]) making nucleation more difficult and therefore lowering the precipitation temperatures because the hydride nucleation energy would be larger. The solvus lines derived from this work and Slattery's [26] are also summarized in Table 4.
5. TSS data and hysteresis The DEM results confirm previous observations that TSSD in Zr-2.5Nb is not very sensitive to heat-up rate and previous maximum temperatures. In contrast, TSSP is strongly affected by thermal history (maximum temperature, hold time, cooling rate, etc.). This could have significant effects on, for instance, DHC behaviour, since TSSP affects the amount of hydrogen supersaturation that is required at the flaw tip before a hydride can precipitate. In applications of the solvus data to physical processes such as DHC, blister growth and hydrogen ingress, we often need to know when the solubility limit has been achieved in a certain region where the hydrogen concentration level has increased by diffusion at constant temperature. Furthermore, to model the redistribution of hydrogen between different hydride-containing regions requires that the solvus boundaries for hydride nucleation, growth and dissolution are either the same for all stages of the nucleation, growth and dissolution processes, or are known at all stages of these processes (i.e., not necessarily just given at, respectively, the initial formation and final dissolution of the hydrides). As is demonstrated in this paper, the phase boundaries for hydride formation and dissolution are obtained by detecting the change in the elastic modulus as a function of temperature at a fixed hydrogen con-
235
centration. It is assumed that the change in the elastic modulus represents the initiation, or the end-point, of the phase transformation process for a given hydrogen concentration. Varying the hydrogen concentration of the sample will then produce a series of these phase transformation end-points as a function of hydrogen concentration. A fitted line through these points yields an equation for the phase boundary (assumed to be valid at all concentrations within range of the data points). Although this phase boundary is referred to as the terminal solid solubility (implying it represents only the end or initiation points of the phase transformation), it is generally assumed in applications to represent hydride/hydrogen local equilibrium at all other points of the phase transformation. For phase transformations for which there is no hysteresis between formation and dissolution, this is clearly a valid assumption. However, when there is hysteresis (due to plastic deformation) between formation and dissolution, this assumption is likely not correct because of the history dependence of the phase transition. In such a case, the applicability of the results may be limited to the physical process that was used to obtain them. Thus in assessing the applicability of the present experimentally derived solvus boundaries in theoretical models, we are faced with the resolution of two problems: (i) The solubility limit may be different depending on whether the solvus is approached by increasing the hydrogen concentration at constant temperature or by varying the temperature at constant hydrogen concentration. (ii) The solubility limit may be different depending on the size of the precipitate that is forming (growing) or dissolving (shrinking). That is, there may be a different solubility limit depending on how far the process has progressed from a given hydride-size distribution. A partial answer to the first problem can be found in the results of a recent study on isothermal hydrogen charging of an experimental pressure tube alloy of Zr, called Excel (Zr-3.8Sn-0.9Mo-0.8Nb) [21]. This study shows that hydride formation during isothermal charging occurring at a hydrogen concentration just below TSSP2 [22], which is the solvus boundary determined on cooling the specimen through the phase transition with Tmax --- Tmax2. This shows that the hydride formation boundaries detected by the two different processes are close, but not the same. Further work is needed to determine the reasons for this difference. The difference is not surprising in view of the many factors that have been shown to influence the hydride formation solvus on cooling. Similar isothermal hydrogen charging studies for Zr-2.5Nb are on-going. Insight into resolving the second, more vexing, problem may be obtained by comparing the TSS determined by continuous cooling for a given high H con-
236
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237
centration, with the TSS obtained by detecting the phase transition for a series of specimens of different H concentrations, In the former case, for hydride formation, once hydrides start to precipitate, the remainder of the TSS boundary represents the formation of hydrides in the presence of pre-existing ones. It is seen (Figs. 6 and 7) that the TSSP line obtained under continuous cooling approximates closely TSSP2, obtained for a series of hydrogen concentrations with a Tmax -= Tmaxpjust above TSSD. There is an initial overshoot of this boundary, the extent of which is consistent with a starting Tmax being between Tmaxl and Tmax2. Subsequently, the TSSP1 line joins with the TSSP2 line as shown in Figs. 2 and 6. It is not clear whether the TSSP solvus lines shown in Fig. 6 reflect continual formation of new hydride precipitates, or the growth of existing ones, or both. A possible answer may come from comparing Figs. 6 and 9. Fig. 9 shows that when Tma~ is below TSSD, at a given hydrogen concentration, TSSP is close to TSSP1. We have speculated that in the presence of pre-existing hydrides, no previous hydride nucleation sites are available and, therefore, at the start, hydride formation is difficult, as if a higher Tmax had been chosen, which would have wiped out vestiges of pre-existing hydride nucleation sites. The results of Fig. 6 seem to suggest that after the start of precipitation - during the continuous cooling stage - hydride growth a n d / o r hydride nucleation is made easier for some reason and, therefore, follows the solvus line, TSSP2. Ritchie and Pan [20] have suggested that the reason for this is that the growth of freshly precipitated hydrides may be easier. In Section 4.2 we speculate that the reason hydride precipitation, when all the hydrides have been dissolved, follows this boundary at low Zmax is because the vestiges of previously existing, dissolved hydrides create sites that make formation easier. This argument, however, cannot be invoked to explain the results of Fig. 6. Consequently, it is not entirely clear from the present results what actually governs the phase boundaries given by TSSP1 or TSSP2 or which of the two phase boundaries (if any) are appropriate to use in theoretical modelling. Further work is needed to resolve these important questions.
6. Conclusions There is fairly good agreement between the TSSD curves determined by DEM for Zr-2.5Nb pressure tube material with the curves determined from previous data compiled by Kearns for a-phase unalloyed Zr and Zr alloys and by Slattery for heavily cold-worked Zr-2.5Nb rod material. In Zr-2.5Nb, thermal history of the sample has a strong effect on TSSP and little effect on TSSD. The
result is that the hysteresis between dissolution and precipitation of hydrides in Zr alloys depends on the thermal history of the material. In addition, TSSP depends on whether or not the material already has hydride precipitates present, but this result is complicated by the prior history of the material. It is not clear whether the solvi derived from the present and extant data base adequately describe the hydride dissolution and formation processes under isothermal conditions which are required in models of DHC, blister formation and hydrogen ingress. Further experimental and theoretical work is required to resolve this important question.
Acknowledgements This study was funded by the CANDU Owners' Group (COG) under work package 2-31-6580. The authors acknowledge G.K. Shek of the Materials Technology Unit of Ontario Hydro Technologies (OHT), Toronto, Ontario, Canada for carrying out the hydriding of the specimens and the Analytical Science Branch at Whiteshell Laboratories, Pinawa, Manitoba, Canada for the hydrogen analyses.
References [1] J.T. Kearns, J. Nucl. Mater. 22 (1967) 292. [2] S.-Q. Shi, G.K. Shek and M.P. Puls, J. Nucl. Mater. 218 (1995) 189. [3] L. Bjornkvist, 'Oxidation and hydriding of fuel assembly guide tubes', presented at 1st Meeting of the PWR Working Sub-Group of the European Federation of Corrosion, September 1992. [4] M.P. Puls, Acta Metall. 29 (1981) 1961. [5] M.P. Puls, Acta Metall. 32 (1984) 1259. [6] M.P. Puls, J. Nucl. Mater. 165 (1989) 128. [7] M.P. Puls, Metall. Trans. A 21 (1990) 2905. [8] M. Leger and T.P. Byrne, Canadian Nuclear Society, 12th Simulation Symp. on Reactor Dynamics and Plant Control, Hamilton, Ontario, April 1986. [9] C.E. Coleman and J.F.R. Ambler, Scripta Metall. 17 (1983) 77. [10] G.K. Shek, M.P. Puls, R. Fong, Z.L. Pan, I.G. Ritchie and K. Tashiro, unpublished work, Ontario Hydro Technologies (GKS, KT) and -AECL-Whiteshell (MPP, ZLP, IGR) and Chalk River (RF) Laboratories, 1993. [11] R.W.L. Fong and S. Spooner, Scripta Metall. Mater. 30 (1994) 649-654. [12] K. Bungardt and H. Preisendanz, Z. Metallkd. 51 (1960) 280. [13] V. Provenzano, P. Schiller and A. Schneiders, J. Nucl. Mater. 52 (1974) 75. [14] S. Mishra and M.K. Asundi, ASTM-STP 551 (1974) 63. [15] F.M. Mazzolai, J. Ryll-Nardzewskiand C.J. Spears, Nuov. Cim. Soc. Ital. Fis. B 33B (1976) 251.
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237 [16] I.G. Ritchie and K.W. Sprungmann, J. Phys. (Paris) 44 (1983) C9-313. [17] H. Numakura, T. Ito and M. Koiwa, J. Less-Common Met. 141 (1988) 285. [18] K. Nutall, R. Dutton and A.J. Shillinglaw, Proc. 3th Int. Congress on Hydrog~ne et Materiaux, Paris, vol. 1 (1982) pp. 167-172. [19] I.G. Ritchie and Z.L. Pan, Philos. Mag. A63 (1991) 1105. [20] I.G. Ritchie and Z.L. Pan, ASTM-STP 1169 (1992) 385. [21] Z.L. Pan, M.P. Puls and I.G. Ritchie, J. Alloys Comp. 211&212 (1994) 245. [22] D. Khatamian, Z.L. Pan, M.P. Puls and C.D. Cann,
[23] [24] [25] [26] [27]
237
Hydrogen solubility limits in Excel - an experimental zirconium-based alloy, Proc. Int. Symp. on Metal-Hydrogen Systems, Fuji-Yoshida, Yamanashi, Japan, 6-11 November, 1994 to be published in J. Alloys Comp. G.J.C. Carpenter and J.F. Watters, J. Nucl. Mater. 73 (1978) 190. J. Marx, Rev. Sci. Instrum. 22 (1951) 503. W.H. Robinson and A. Edgar, IEEE Trans. Sonics Ultrasonics SU-21 (2) (1974) 98. G.F. Slattery, J. Inst. Met. 95 (1967) 43. D.J. Cameron and R.G. Duncan, J. Nucl. Mater. 68 (1977) 340.
Journal of Nuclear Materials 228 (1996) 227-237
jem'aalef nadear naterials
The terminal solid solubility of hydrogen and deuterium in Zr-2.5Nb alloys Z.L. Pan, I.G. Ritchie 1, M.P. Puls * Materials and Mechanics Branch, AECL, WhitesheU Laboratories, Pinawa, Manitoba, Canada ROE 1LO
Received 17 July 1995; accepted 6 October 1995
Abstract
The terminal solid solubility (TSS) of hydrogen or deuterium is an important parameter in Zr alloys that are used in the nuclear industry. If this solubility is exceeded, it can make the alloys susceptible to delayed hydride cracking. Accurate expressions for the TSS are necessary for modelling delayed hydride cracking as well as deuterium ingress into pressure tubes and blister formation in pressure tubes in contact with their calandria tubes. Measurements of the changes in the dynamic elastic modulus have been used to establish expressions for the TSS as a function of temperature and to study the hysteresis between hydride dissolution and precipitation. It is shown that the hysteresis is particularly sensitive to the thermal history of the sample (such as the prior maximum temperature, hold time at maximum temperature and cooling rate). As a result, the precipitation TSS (TSSP) has a range of values bounded by the solubility equations designated TSSP1 and TSSP2. These are obtained, respectively, by cooling from an upperand a lower-bound maximum temperature. The TSSD equation obtained in this study is very close to previously determined expressions, but the TSSP equations differ significantly from earlier results.
1. Introduction
When the concentration of hydrogen isotopes in a reactor core component exceeds the terminal solid solubility (TSS) at the temperature under consideration, hydrides may be present and embrittle the component. As a result, TSS data on hydrogen isotopes in Zr-2.5Nb pressure tubes are required to assess the potential for delayed hydride cracking (DHC) and hydride blister formation in CANDU 2 reactor pressure tubes. In the past, Kearns' TSSD (Dissolution) data for hydrogen in zirconium and the Zircaloys [1] have been
* Corresponding author. Tel.: +1-204 753 2311; e-mail: [email protected]. 1Present address: Nuclear Materials and Fuel Cycle Technology Section, Division of Nuclear Fuel Cycle and Waste Management, International Atomic Energy Agency, A-1400, Vienna, Austria. 2 CANDU is a registered trademark of Atomic Energy of Canada, Ltd.
Elsevier Science B.V. SSDI 0022-31 15(95)00217-0
used in fitness-for-service assessments of CANDU reactor pressure tubes as one of the threshold criteria that must be met for susceptibility to DHC initiation [2].
Similarly, embrittlement of zirconium-based alloy components in light water reactor systems (Pressurized Water Reactors (PWRs) and Boiling Water Reactors (BWRs)) occurs and can lead to problems during handling and manipulation of the components, both inand out-of-reactor. For example, difficulty in relocating control rods in a PWR reactor was recently experienced and traced to broken and deformed guide tubes caused by hydride embrittlement [3]. The pickup of hydrogen caused by radiolysis of the cooling water and corrosion mechanisms, and its subsequent redistribution to cold spots by thermal diffusion, will become more of a problem as fuel assemblies are pushed to increasingly higher burnups. It is well known that there is a large hysteresis between the phase boundaries for precipitation (TSSP) and dissolution (TSSD) of hydrides in Zr [4] and a
228
Z.L. Pan et al. /Journal of Nuclear Materials" 228 (1990) 227-237
theoretical model of this hysteresis has been proposed by Puls [5,6]. The hysteresis has important physical consequences for D H C , hydrogen ingress and blister formation [7,8]. Only limited experimental data is available on TSSP in Zr-2.5Nb. The situation is somewhat better regarding TSSD. Coleman and Ambler [9] compared the TSSD data for hydrogen and deuterium in Z r - 2 . 5 N b material, obtained by diffusion, dilatometry, and equilibrium pressure isotherms. It was found that the data scattered about Kearns' best fit line to TSSD data for the ~t-phase materials zirconium and the Zircaloys. However, there is considerable scatter, which was believed to be mainly caused by uncertainties in hydrogen analyses. In addition, Coleman and Ambler [9] compared the TSSD data with their data for the concentration limit for D H C initiation at sharp cracks. They found that the two limits coincided, within the scatter of the results. However, the close correspondence of the TSSD data to the concentration limit for D H C appears to be fortuitous, since the latter process clearly requires hydride precipitation at the crack tip, which is governed by TSSP. This point has been addressed in detail by Shi et al. [2] who show that the increase in hydrogen concentration - relative to the TSSD concentration - that is achieved at a sharp flaw tip due to the presence of a high, tensile hydrostatic stress, approximately compensates for the increase in hydrogen concentration needed for hydride precipitation to occur at the flaw tip. This accounts for the closeness of the D H C initiation 'solvus' at sharp flaws (under quasistatic cooling conditions) to the TSSD. To ensure that more accurate values for the TSS in Z r - 2 . 5 N b are available and to study in more detail the factors affecting, particularly, TSSP, a cooperative (round robin) study among three Canadian laboratories [10] was carried out - using a variety of techniques - to measure the TSS in deuterided Z r - 2 . 5 N b pressure tube material which was obtained from the same source pressure tube. The techniques used were differential scanning calorimetry (DSC), internal friction (specifically, dynamic elastic modulus (DEM)), small angle neutron scattering (SANS) and D H C . This paper reports on the D E M results and discusses their interpretation. A previous paper has presented the results obtained with the SANS technique [11].
2. Material preparation 2.1. Source pressure tube
A Z r - 2 . 5 N b pressure tube (R766), which was rejected for reactor use due to out-of-specification wall thickness, was selected for the cooperative TSS study. To correspond to tubes put in-reactor, the tube was
Table I Chemical composition of the pressure tube selected for test Element
Specification
Analysis
H N O Nb
< 25 ~xg/g < 80 txg/g 900-1300 Dxg/g 2.4-2.8 wt%
8.5 }xg/g 23.5 ixg/g 1060 ixg/g 2.73 wt%
autoclaved at 400°C. Chemical composition of tube R766 was determined as summarized in Table 1. The composition of the remaining alloy impurities were assumed to be the same as that of the ingot material used to produce tube R766 (and other tubes). The results of the ingot analysis are summarized in Table 2. The ingot was produced by preheating to 982°C and press forging. The ingot was then reheated to 1015 + 10°C for 1 + 0.25 h at temperature and quenched (13quenched). Afterwards, the ingot was rotary forged to the approximate final size. The microstructure of tube R766 was not specifically examined. However, it is expected to be similar to that of other tubes manufactured in the same way. The microstructure of such tubes consists of flattened grains of hexagonal-closepacked a-phase material containing about 0.8 wt% Nb in supersaturated solid solution surrounded by a thin layer of body-centred-cubic 13-phase material. The latter phase has an average, metastable composition of about 20 wt% Nb, which decomposes internally into a
Table 2 Chemical composition of ingot material close to selected pressure tube Element
Specification (Ixg/g)
Analysis (txg/g)
AI B C Cd Co Cr Cu Fe ttf Mg Mn Mo Ni Pb Si Sn Ta Ti U V W
75 0.5 140 0.5 20 200 50 1500 50 20 50 50 70 130 120 100 200 50 3.5 50 100
47 < 0.25 140 < 0.25 < 10 < 100 < 25 650 42 < 10 < 25 < 25 < 35 < 25 52 < 25 < 25 < 25 < 1.0 < 25 < 25
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237
229
Table 3 Summary of TSSD and TSSP results from DEM H + D/2 a
TSSD
TSSP1
Tmaxl
TSSP2
Tmax2
(p.g/g)
(°C)
(°C)
(°C)
(°C)
(°C)
99 88 86 86 79 68 63 63 61 53 52 52 47 36 36 31 27 25 15 b 11 b 7.5 7.3 7 7 7 7
350 340 335 336.2 329.2 314 304.2 313.3 303 288.4 295 296 274.3 258.7 265 256 248 234.3 201.6 190 177 173.9 173 172.4 174.9 174
286 279 274 273 267.6 259 246 252 238 228.5 234 235 219.2 200.8 204.8 198 188 177.4 145 124
450 450 450 450 450 420 385 430 450 430 440 430 420 400 430 450 420 420 430 450
107
430
111
430
302 299 294 292 286 273 266 272 262 249 258 257 243.2 222.7 228.0 221 213 205.6 167 154 129 126 125 126 127 126.5
368 360 360 350 330 320 310 320 320 310 310 310 300 280 280 280 260 260 220 300 250 250 250 250 250 220
Sample no. 90-6 90-5 90-4 90-3 90-1 67-4 67-1 67-3 67-5 58-3 58-4 58-5 58-1 42-1 42-3 42-4 30-3 30-1 10-1 10-2 7-3 7-1 7-2 7-4 7-5 7-6
a Uncertainty (two standard deviations) = 10%. b Ni-plated and gaseously hydrided at 250°C.
mixture of N b - d e p l e t e d o - p h a s e s u r r o u n d e d by Nb-rich 13-phase gradually increasing towards t h e stable c o m p o sition of a b o u t 80 w t % Nb, the extent of this d e p e n d ing o n t h e n u m b e r of autoclave t r e a t m e n t s and, for irradiated material, the irradiation t e m p e r a t u r e a n d fluence. T h e a - p h a s e grains are flat platelets, a b o u t 0.4 p,m thick, with an aspect ratio of t h e o r d e r of 1 : 10 : 40 (radial : transverse : longitudinal). T h e volume fraction of the 13-phase is approximately 8%. T h e m e c h a n i c a l p r o p e r t i e s of the selected t u b e at 300°C are: u l t i m a t e tensile stress (axial) of 529 M P a , yield stress (0.2% offset) of 374 M P a a n d e l o n g a t i o n of 17.8%.
2.2. Hydriding T h e source m a t e r i a l was d e u t e r i d e d by t h e electrolytic hydriding ( d e u t e r i d i n g ) / t h e r m a l diffusion technique. U s i n g this p r o c e d u r e , a solid d e u t e r i d e layer was electrolytically d e p o s i t e d o n t o t h e outside surface of the t u b e in a 0.1M D 2 S O 4 electrolyte at 90°C with a c u r r e n t density of 100-120 m A / c m 2. T h e d e u t e r i u m was diffused into t h e t u b e by equilibrating t h e deuteride layer with t h e p r e s s u r e t u b e matrix at K e a r n s ' T S S D t e m p e r a t u r e s c o r r e s p o n d i n g to t h e h y d r o g e n /
d e u t e r i u m 3 c o n c e n t r a t i o n s required. It was i n t e n d e d to d e u t e r i d e the source material to five c o n c e n t r a t i o n levels (in addition to using m a t e r i a l with as-received c o n c e n t r a t i o n ) up to m a x i m u m c o n c e n t r a t i o n of 100 i x g / g H - e q u i v a l e n t (0.9 at%). D e u t e r i u m was c h o s e n over h y d r o g e n b e c a u s e it provides a s t r o n g e r r e s p o n s e to n e u t r o n scattering for the S A N S p a r t of the cooperative study a n d b e c a u s e it is the same isotope t h a t is a b s o r b e d by the r e a c t o r p r e s s u r e tubes. A f t e r a diffusion anneal, t h e r e m a i n i n g surface hydride layer was r e m o v e d by machining. T h e s p e c i m e n s used for the D E M analysis were all chemically analyzed for h y d r o g e n by the H o t V a c u u m Extraction ( H V E ) m e t h o d . N o t e t h a t t h e H V E m e t h o d does n o t distinguish b e t w e e n h y d r o g e n a n d d e u t e r i u m a n d t h e r e f o r e provides only the total equivalent hydrogen c o n c e n t r a t i o n . In addition, for t h e TSS d e t e r m i n a tions using t h e D E M technique, f u r t h e r specimens were a d d e d to those specimens used for t h e c o o p e r a -
3 In the following, for simplicity, both hydrogen ad deuterium are referred to as hydrogen.
230
Z.L. Pan et al. /Journal of Nuclear Materials" 228 (1996) 227-237
tive study - with particular emphasis on specimens with the lowest hydrogen concentration. A few of the additional specimens that were added were hydrided using hydrogen only - by gaseously diffusing hydrogen into the specimen, the suface of which had been nickel plated. These specimens are explicitly identified in the table of results (Table 3).
,ruMMY SAMPLE COOLER HEATER
l OWER : SUPPLY
i
: TEMPERATURE CONTROLLER
FUSEI) QUARTZ
7 i GALGE CRYSTAL
COMPLrFER MONITOR PI.OTTER
t --~
3. Experimental procedure
The test procedures of DEM measurements are described in this section. The general guidelines were to control heating and cooling rates to 2°C/min and to have at least ten thermal cycles for each set of measurements.
]
i TI
DRIVE CRYSTAl
CLOSED LOOP t CRYSTALDR/VER
I ]
i VACUUM PUMP
Fig. 1. Schematic diagram of the APUCOT. 3.1. Internal friction and dynamic elastic modulus techniques
Elastic modulus and internal friction studies [ 12-17] have been reported previously to map H-TSS boundaries of hydrogen-doped, pure Zr. In the past few years, these investigations have been extended to the study of the TSS in Zr-2.5Nb alloy pressure tube materials [18-20] containing hydrogen or deuterium. Most recently, the same techniques have been used in the preliminary survey of the H-TSS in an experimental pressure tube alloy of Zr called Excel [21,22]. It should be noted that internal friction techniques are regarded as standard methods for the determination of the H-TSS in the body-centred-cubic hydride-formers V, Nb and Ta. Although an internal friction (Q 1) peak associated with hydride dissolution on heating and hydride precipitation on cooling, similar to those obtained in V, Nb and Ta, is indeed observed in pure Z r - H alloys, no such peak is observed in Zr-2.5Nb-H. This is attributed to the lack of long misfit dislocations in the pressure tube alloy compared with the copious numbers found in pure Zr-H alloys [23]. Nevertheless, a more or less sharp knee in the dynamic elastic modulus (proportional to F 2, where F is the resonant frequency) is observed at the expected TSS on both heating and cooling. For this reason, we chose the most precise method of determining dynamic elastic modulus as a function of temperature available to us for this study. 3.2. Apparatus
A general overview of the methods used to measure the elastic (Young's) modulus (E) and internal friction (Q 1) based on the automatic piezoelectric ultrasonic composite oscillator technique (APUCOT) has been given elsewhere [20,24,25]. With the APUCOT (see the schematic diagram in Fig. 1), the specimen tempera-
ture is measured by a thermocouple inserted in a calibrated dummy specimen of the same material as that being tested. The dummy specimen, of the same dimensions as the actual specimen, is located together with the real specimen so that they are symmetrically situated about the center of the cylindrical cavity of a small, non-inductively wound furnace. The furnace is 100 mm in length and 18 mm in internal diameter. A typical Zr-2.5Nb specimen with dimensions 3 x 3 × 46 mm is located in the center of the uniform temperature region of the furnace. This configuration allows temperature measurements to within an error of +0.5°C in the range of 50-500°C while the heating rate is less than 5°C/min. (Heating rates up to 100°C/min are available with the APUCOT for special tests in air.) The test chamber is normally evacuated to 10 3 torr or better before a test to prevent substantial oxidation of the specimen. The temperature measuring system was carefully recalibrated at heating and cooling rates of 2°C/min for the series of TSS tests described herein. A very stable heating (or cooling) rate is important for the precise determination of the temperature dependence of Young's modulus used to map the H-TSS boundaries. Although the sampling rate of the electronic instruments is every 0.1 s for T and F, and 1 ms for voltage, the computer is programmed to record the raw data collected only once each 6 s during a thermal cycle. At a cooling rate of 2°C/min, this corresponds to five raw data points collected for every I°C change in temperature. These points were averaged to give values of T, E and Q i for each I°C temperature interval. 3.3. Dynamic elastic modulus versus temperature
It has been shown [18-20] that a sharp 'knee' in a plot of dynamic elastic modulus versus temperature is
Z.L. Pan et aL / Journal of Nuclear Materials 228 (1996) 227-237
231
Zr-2.5Nb + 86 p,g/g H
85
Heating & Cooling at 2°C/min
Zr-2.5Nb + 86 ~g/g H Coolingfrom 370°Cat 2°C/min
6 [./" \ X
D \ 80
o r~
TSSP1
o
~ ° ° , o . ° TSSD °,
from TmB×~(460%)
75
-% ~
I
TSSP2
from Tmax2(370°C)
200
. . . .
. . . . .
3oo
. . . .
Temperature, °C Fig. 2. Typical curves of Young's modulus E versus temperature showing different hysteresis between hydride dissolution and precipitation during cooling from different maximum temperatures on a Zr-2.5Nb specimen containing 86 tJ-g/g
270
280
290
300
310
320
Temperature, °(2 Fig. 3. TSS temperature determined from the interception of straight lines extrapolated from the plots of frequency versus temperature above and below the 'knee'-point.
(0.77 at.%) of hydrogen. associated with the dissolution of hydrides during heating and their precipitation during cooling in Zr-2.5Nb alloys. The temperature at the knee-point can be used to identify the solvus temperature and, therefore, to map H-TSS boundaries. Young's modulus is proportional to the resonant frequency squared ( F 2) of the specimen in longitudinal vibration. With the APUCOT, resonant frequency measurements are much more precise than those obtained using other elastic modulus measuring techniques. Consequently, the A P U C O T was employed for this series of TSS tests, rather than the low frequency flexure pendulum technique, which we have employed to study hydrogen in pure Z r - H alloys [16,20]. The hysteresis between the hydrogen dissolution (TSSD) and precipitation (TSSP) is easily observed in the E versus T curves obtained on the same specimen during a thermal cycle through the TSS (see Fig. 2). Two methods were used to estimate the knee-point, i.e. the TSS temperature. In the first method, straight lines are drawn through the experimental data points of F versus T above and below the knee, and their intersection is taken as the TSS temperature, as shown in Fig. 3. This procedure is somewhat subjective. In the second method, curves of the derivative of frequency versus temperature (-dF/dT versus T), or the derivative of Young's modulus versus temperature (-dE/dT versus T) are calculated and plotted, as shown in Fig. 4. In this case, the TSS temperature is taken to be the point of maximum slope on the sharp peak in this plot. Using blind tests, we have found that both techniques are equivalent within an error of 0.5°C.
Even though the second method is more objective, the peak can be rather low and ragged, making the point of maximum slope difficult to determine with accuracy for the lowest hydrogen concentrations. The sharpness in the widths of the peaks in Fig. 4 may be indicative of the rate at which the phase transition proceeds, with the narrowest peak indicative of the fastest transition. The rate at which the transition proceeds may also provide a clue as to the type of transition. Thus, TSSP1, which has the narrowest peak, likely represents the nucleation stage of hydride formation, since nucleation is a process which is expected to proceed rapidly over a narrow temperature range. The
Zr-2.5Nb + 79 Ixg/g H
0.14l
TSSP1 /
0.12!
/
TSSP2 O~.. o.1
/
_ ~
~
TSSD
0.08 i
0.06] !
150
.
.
.
.
.
.
200
.
J
_J
•
i
.
.
250 300 Temperature, °C
.
.
.
.
.
350
400
Fig. 4. TSS temperature determined from the derivative of Young's modulus E versus temperature.
Z.L. Pan et aL /Journal of Nuclear Materials 228 (1996) 227-237
232
other two phase boundaries (TSSP2, representing some sort of hydride growth process and TSSD, representing hydride dissolution) have much broader peaks, which may be indicative of their slower rate. Based on this interpretion of the significance of the widths of the peaks, hydride dissolution appears to be the most sluggish of the three transformation processes.
352
Coolingfrom430°Cat 2~C/min
"\\\\
g -a
75 i
7 ~g
"
~
<
. . . . . . . 150
200
250 300 Temperature, °C
~
227
100 ~ . ~
40
X
3o
\
112
.
1.8
.
1
\
0.4 ~.
\ \ \
\
TSSD
"~
1.6
144
CoolingfromT~ax2(370'°C> CoolingfromTm~×.(450°C) 10.5
X \
5
182
- Zr-2.5Nb + 86 I.tg/g H
~ ~ "~.~. ~'~ 50 X X~
3.4. Relationship between dynamic elastic modulus and H concentration In Z r - 2 . 5 N b - H , Z r - H and T i - H , when all the hydrogen is in solution, the E versus T curve is extremely linear for temperatures below 550°C [12,20,21]. More importantly for this study, the presence of increased levels of hydrogen depresses the elastic modulus at any given temperature. Consequently, a set of E versus T curves for different levels of hydrogen arc nearly parallel, straight lines, when all the hydrogen is in solution. For example, the experimental results for four different levels of hydrogen are shown in Fig. 5. These observations strongly suggest that for a particular E versus T curve, such as the cooling branch of the 86 I.tg/g specimen in Fig. 5, the excess of the low temperature curved portion of the line, over the linear extrapolation of the high temperature region to low temperatures, simply represents the amount of hydrogen present as hydrides. This means that an analysis of this excess yields an estimate of the solubility of hydrogen at any point lower than the TSS in this specimen. Such an analysis has been carried out on the 86 Ixg/g specimen. Using this novel procedure, the results of Fig. 2 for TSSD on heatup and for TSSP on cool-down, using
Temperature, °C
283
.
2
.
o.s~ \
\
TSSP1
-%
2.2
\
I...X ' ? ~
2.4
a
~
,~
%.05
2.6
Reciprocal Temperature, 1000/K Fig. 6. Solvus boundaries derived from the analysis of Young's modulus as a function of temperature measured on a single specimen containing 86 Ixg/g (0.77 at.%) of hydrogen. The solid lines are the fits to the data shown in Fig. 7.
two different maximum temperatures to which the specimen was taken prior to cool-down, are shown in Fig. 6. In the important range of concentration from 10 to 80 Izg/g H (from the point of view of the operational conditions of pressure tubes), the analysed curve (log(concentration) versus l / T ) does indeed agree with a log(concentration) versus 1 / T line calculated from the TSS data of several specimens with different levels of hydrogen (Fig. 7, see Sections 4.1 and 4.2). At very low solubilities, the data are scattered because they represent very small differences between two large quantities and because they contain some contribution to the E versus T curve from the processes that govern the modulus defects of relaxation at low temperature. In addition, there could also be scatter because, below 100°C, the cooling rate cannot be kept at the target rate of 2°C/min and starts to decrease. In principle, the whole log(concentration) versus 1 / T plot for the TSS boundary can be estimated by this analysis technique using the data from only one thermal cycle on a specimen containing a suitably high level of H. This is an important advantage of the Young's modulus measuring technique in the field of TSS studies.
86ggtg %~ 350
Fig. 5. Typical curves of Young's modulus E versus temperature for Zr-2.5Nb specimens containing different concentrations of hydrogen.
4. Experimental results 4.1. TSSD The results of all of the TSSD temperatures obtained by the A P U C O T during heating at 2°C/min are
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237 listed in Table 3. There was always more than one specimen tested at each nominal hydrogen concentration level. No apparent influence of heating rate a n d / o r the maximum temperature reached during a thermal cycle was detected on the TSSD temperatures, as long as the previous cooling rate was the same as the heating rate. Preliminary tests had shown that previous thermal history can significantly influence the value of the dissolution temperature. This is believed to be because the typical sizes of the hydride particles, which resulted from the previous cool-down, need to be consistent with the sizes which will allow enough removal of hydrogen from them as they re-dissolve to maintain quasi-equilibrium at the imposed heating rate. If, for instance, the previous cooldown rate is much slower than the heatup rate, then the hydrides formed on the previous cooldown are too large for quasi-equilibrium to be maintained at a heatup rate which is faster than the previous cooldown rate. Therefore, to obtain reliable results, the heatup and cooldown rates must be identical. This requirement applies equally well to determinations of TSSP. For most of the specimens, ten or more thermal cycles were observed to check the repeatability of our technique. Therefore, the average value of multiple measurements on the same specimen is taken as the TSSD temperature of that specimen and listed in Table 3. The standard deviations of the TSSD temperatures for most specimens are less than or close to I°C. Thus, the reproducibility of our TSSD measurements is satisfactory, except for the 'as-received' specimens with the lowest levels of hydrogen. The TSSD data in Table 3 are well represented by the following solubility equation, assuming a log(C H) (where C H stands for hydroTemperature, °C
352 283 227 182 1 0 0 " ' . ~- ,.% ~ . ~ - ~ -'- ~ "\,\
[
Zr-2.5Nb + H
'\
\\
2so 301 r~O 20. -~
144 112 ' ' ~1
//
05
~,. " " \ X ~ XX
Slattery'sTSSP
10"3~
~'~TSSP1, ThisWo~ 0.2
TSSD,Th,sWo~,,,
X \'~.
I
I° i s
51 . . . . . . S,atterfs T S S D - ~ t S ~ 2 o ~ ~'0.05 1.6 1.8 2 2.2 2.4 2.6 Reciprocal Temperature, 1000/K Fig. 7. Solvus boundaries TSSD, TSSP1 and TSSP2 of hydrogen solubility versus reciprocal temperature derived from fits to the TSS data of Table 2.
233
Table 4 Summary of solubility equations for TSSD and TSSP from this work compared with data in the literature TSSD This work Kearns [1] Slattery [26]
C° = 8.080 X 10 4 exp[- 34520/RT] C° = 1.200x 105 exp[- 35900/RT] C° = 6.860x 104 exp[- 33570/RT]
Ixg/g ~g/g Ixg/g
CP1 = 2.473 x 10 4 exp[- 25840/RT] CH P2 = 3.150 X 10 4 exp[-27990/RT] CP = 4.110 x 104 exp[- 28000/RT]
Ixg/g ixg/g I~g/g
TSSP This work This work Slattery [26]
gen concentration) versus 1 / T dependence, calculated by linear regression (r 2 = 0.994): C D=8.080X
104
xexp[-34520/RT]
Ixg/g
for TSSD,
(1)
where R = 8.3144 J / m o l K and T is in K. This equation is plotted in Fig. 7 and compared with the experimental TSSD data. An analysis as described above from a single specimen containing 86 ixg/g hydrogen can also be used to derive a log(C D) versus 1 / T plot, as shown in Fig. 6. There is excellent agreement between the Iog(CHD) versus 1 / T plot from experimental results on one specimen and Eq. (1) calculated from TSSD data for 26 different specimens, representing a range of hydrogen concentration from 7 to 99 Izg/g. Also shown in Fig. 7 is Kearns' linear regression fit to a compilation of TSSD data for a-phase unalloyed Zr and Zr alloys [1] and Slattery's fit to TSSD data from heavily cold-worked Zr-2.5Nb rod material [26]. The comparison shows that there is good agreement between the three sets of data. However, because of slight differences in slope between the solvus lines, the solvus derived from the pressure tube Zr-2.5Nb material predicts, for a given temperature, progressively lower hydrogen solubility values, in the reactor operating range of 250 to 310°C, compared to the results from Kearns' solvus line. A summary of the solvus lines derived from this work as well as Kearns' and Slattery's results is given in Table 4. For a given hydrogen concentration, the DHC cracking temperatures, which were also obtained as part of the original cooperative study [10], were always found to be lower than the corresponding TSSD temperatures. Alternatively, for a given temperature, the hydrogen concentration necessary for DHC initiation is always greater than the solubility for hydride dissolution. A detailed analysis of these results has been given in a separate publication [2].
4.2. TSSP Previous studies [19,20] have confirmed that the processes of nucleation and precipitation of hydrides in
Z,L. Pan et al. /Journal ¢~fNuclear Materials 228 (1996) 227-237
234
Z r - 2 . 5 N b are very complex, and that the TSSP temperature depends sensitively on (i) the microstructure of the material in question, (ii) the maximum temperature (Tm~x) reached during a thermal cycle and (iii) the cooling rate. Even if Tm~x does not reach the dissolution TSS, i.e. Tmax < TSSD, a TSSP temperature can still be detected. Therefore, every datum of TSSP should always be associated with a given Tm~~. In the round robin series of TSS tests [10], we have defined TSSP1 and TSSP2 as the TSSP temperatures obtained after cooling from the highest and lowest Tm~~ temperature chosen, respectively. In practice, we usually chose both of the Tm~~ temperatures to be above the TSSD temperature. The experimental results for TSSP1 and TSSP2 and their corresponding Tm~x values for the six Z r - 2 . 5 N b materials are listed in Table 3. Observed differences between TSSP1 and TSSP2 vary from 14°C for the 68 Ixg/g specimen to 30°C for the 11 Ixg/g specimen. Both TSSP1 and TSSP2 are also plotted on a log(C H) versus 1/T graph as shown in Fig. 7. The corresponding solubility equations are C~ ] = 2.473 X 104 xexp[-25840/RT]
Ixg/g
for TSSP1
(2)
Ixg/g
for TSSP2.
(3)
and C~ 2 = 3.150 x 104
xexp[-27990/RT]
If the value of Tm~~ chosen in a particular test is between the highest and lowest Tm,x, then the corresponding TSSP temperature measured in the test will be between the TSSP1 and TSSP2 lines. The experiments on TSSP as a function of Tin,x were carried out on all five sets of specimens of Z r - 2 . 5 N b - H material consisting of different nominal hydrogen concentrations. The cooling rate was 2°C/rain and typical results are shown in Fig. 8. These show that TSSP decreases to an approximately minimum flat value at about 400°C in every case. However, at a lower cooling rate of 0.2°C/min, the TSSP temperature for the 50 Ixg/g specimen decreased to a minimum 220°C at a Tm~x value of 380°C. These TSSP Tmax values are significantly lower than those found at a cooling rate of 2 ° C / m i n . When the 50 I~g/g specimen was held for 4 days at a particular Tma~ value and subsequently cooled at 0.2°C/min, then as shown in Fig. 9, the TSSP temperature decreased to a minimum value, at a Tm~~ of 350°C, again significantly lower than that found without annealing for 4 days. In conclusion, it appears that the hysteresis between dissolution and precipitation of hydrides in Z r - 2 . 5 N b can be extended, and the maximum temperature needed to achieve this decreased, if the specimen is cooled very slowly or if the specimen is pre-annealed for a long time at a sufficiently high temperature.
Zr-2.5Nb + H 300 CoofingfromTma× ~ ' at 2°C/rain 280
~
61 tag/gH ¢~
53/.tg/
[~ 240 [_ 220 ~
31 pg/g H o~ S
P
2
i
2O0
I
TSSP1 '
~
..........................................
300
;
350 400 Maximum Temperature, °C
450
Fig. 8. Typical curves of TSSP temperature versus maximum temperature for thermal cycles to determine the TSSP temperature. The temperatures TSSPI and TSSP2 are obtained from the data, as indicated by the arrows.
In Fig. 9, two TSSP data were observed in thermal cycles for which Tmax w a s deliberately chosen to be below TSSD. These data are of interest because they reveal the effective TSSP for the nucleation of 'new' hydrides in the presence of pre-existing hydrides. Similar experimental results on Z r - 2 . 5 N b specimens containing 200 p~g/g of hydrogen were reported and discussed in detail elsewhere [19,20]. These results appear to show that the existing hydrides do not grow significantly for the cooling rate used in our tests. Rather, the supersaturation of hydrogen builds up until either
~60
o~
250
Zr4.5~ + 50 ~g/g H [] CoolingfromTna at0.2°C/m,nafter4daysatT~a~ Coolingf,omT,na~at0.2°C/mlr, O Coolingfromfna at2°C/m4r,
2401 ~D t~
230 1.~, < TSSD ~
O
,~] TSSD 22O . . . . T, , 200 250 300
350
400
'450
Maximum Temperature, °C Fig. 9. Curves of TSSP versus maximum temperature from thermal cycles to determine the TSSP temperature using different cooling rates and hold times on a Zr-2.5Nb specimen. Included in the figure are two TSSP temperatures for which the maximum temperature was below TSSD.
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237
nucleation occurs at new sites in the lattice, or growth on existing hydrides is possible. However, the results of Fig. 6 show that on continuous cooling, hydride formation follows the TSSP2 boundary after initial precipitation, regardless of whether the Tmax temperature was Tmaxl, or Tmax2. Assuming that TSSP2 represents the boundary for hydride growth rather than nucleation, this seems to suggest that hydride growth is favoured as cooling progresses in the presence of hydrides. Also plotted in Fig. 7, for comparison, is the linear regression fit to Slattery's TSSP data obtained from heavily cold-worked Zr-2.5Nb rod material [26]. The agreement among TSSP curves is not as good as that for the TSSD results. However, TSSP depends on thermal history such as the maximum sample temperature and the cooling rate. A possible explanation for this dependence is that the higher maximum temperature anneals out the dislocations on prior hydride precipitation sites (memory effect [20,27]) making nucleation more difficult and therefore lowering the precipitation temperatures because the hydride nucleation energy would be larger. The solvus lines derived from this work and Slattery's [26] are also summarized in Table 4.
5. TSS data and hysteresis The DEM results confirm previous observations that TSSD in Zr-2.5Nb is not very sensitive to heat-up rate and previous maximum temperatures. In contrast, TSSP is strongly affected by thermal history (maximum temperature, hold time, cooling rate, etc.). This could have significant effects on, for instance, DHC behaviour, since TSSP affects the amount of hydrogen supersaturation that is required at the flaw tip before a hydride can precipitate. In applications of the solvus data to physical processes such as DHC, blister growth and hydrogen ingress, we often need to know when the solubility limit has been achieved in a certain region where the hydrogen concentration level has increased by diffusion at constant temperature. Furthermore, to model the redistribution of hydrogen between different hydride-containing regions requires that the solvus boundaries for hydride nucleation, growth and dissolution are either the same for all stages of the nucleation, growth and dissolution processes, or are known at all stages of these processes (i.e., not necessarily just given at, respectively, the initial formation and final dissolution of the hydrides). As is demonstrated in this paper, the phase boundaries for hydride formation and dissolution are obtained by detecting the change in the elastic modulus as a function of temperature at a fixed hydrogen con-
235
centration. It is assumed that the change in the elastic modulus represents the initiation, or the end-point, of the phase transformation process for a given hydrogen concentration. Varying the hydrogen concentration of the sample will then produce a series of these phase transformation end-points as a function of hydrogen concentration. A fitted line through these points yields an equation for the phase boundary (assumed to be valid at all concentrations within range of the data points). Although this phase boundary is referred to as the terminal solid solubility (implying it represents only the end or initiation points of the phase transformation), it is generally assumed in applications to represent hydride/hydrogen local equilibrium at all other points of the phase transformation. For phase transformations for which there is no hysteresis between formation and dissolution, this is clearly a valid assumption. However, when there is hysteresis (due to plastic deformation) between formation and dissolution, this assumption is likely not correct because of the history dependence of the phase transition. In such a case, the applicability of the results may be limited to the physical process that was used to obtain them. Thus in assessing the applicability of the present experimentally derived solvus boundaries in theoretical models, we are faced with the resolution of two problems: (i) The solubility limit may be different depending on whether the solvus is approached by increasing the hydrogen concentration at constant temperature or by varying the temperature at constant hydrogen concentration. (ii) The solubility limit may be different depending on the size of the precipitate that is forming (growing) or dissolving (shrinking). That is, there may be a different solubility limit depending on how far the process has progressed from a given hydride-size distribution. A partial answer to the first problem can be found in the results of a recent study on isothermal hydrogen charging of an experimental pressure tube alloy of Zr, called Excel (Zr-3.8Sn-0.9Mo-0.8Nb) [21]. This study shows that hydride formation during isothermal charging occurring at a hydrogen concentration just below TSSP2 [22], which is the solvus boundary determined on cooling the specimen through the phase transition with Tmax --- Tmax2. This shows that the hydride formation boundaries detected by the two different processes are close, but not the same. Further work is needed to determine the reasons for this difference. The difference is not surprising in view of the many factors that have been shown to influence the hydride formation solvus on cooling. Similar isothermal hydrogen charging studies for Zr-2.5Nb are on-going. Insight into resolving the second, more vexing, problem may be obtained by comparing the TSS determined by continuous cooling for a given high H con-
236
Z.L. Pan et al. /Journal of Nuclear Materials 228 (1996) 227-237
centration, with the TSS obtained by detecting the phase transition for a series of specimens of different H concentrations, In the former case, for hydride formation, once hydrides start to precipitate, the remainder of the TSS boundary represents the formation of hydrides in the presence of pre-existing ones. It is seen (Figs. 6 and 7) that the TSSP line obtained under continuous cooling approximates closely TSSP2, obtained for a series of hydrogen concentrations with a Tmax -= Tmaxpjust above TSSD. There is an initial overshoot of this boundary, the extent of which is consistent with a starting Tmax being between Tmaxl and Tmax2. Subsequently, the TSSP1 line joins with the TSSP2 line as shown in Figs. 2 and 6. It is not clear whether the TSSP solvus lines shown in Fig. 6 reflect continual formation of new hydride precipitates, or the growth of existing ones, or both. A possible answer may come from comparing Figs. 6 and 9. Fig. 9 shows that when Tma~ is below TSSD, at a given hydrogen concentration, TSSP is close to TSSP1. We have speculated that in the presence of pre-existing hydrides, no previous hydride nucleation sites are available and, therefore, at the start, hydride formation is difficult, as if a higher Tmax had been chosen, which would have wiped out vestiges of pre-existing hydride nucleation sites. The results of Fig. 6 seem to suggest that after the start of precipitation - during the continuous cooling stage - hydride growth a n d / o r hydride nucleation is made easier for some reason and, therefore, follows the solvus line, TSSP2. Ritchie and Pan [20] have suggested that the reason for this is that the growth of freshly precipitated hydrides may be easier. In Section 4.2 we speculate that the reason hydride precipitation, when all the hydrides have been dissolved, follows this boundary at low Zmax is because the vestiges of previously existing, dissolved hydrides create sites that make formation easier. This argument, however, cannot be invoked to explain the results of Fig. 6. Consequently, it is not entirely clear from the present results what actually governs the phase boundaries given by TSSP1 or TSSP2 or which of the two phase boundaries (if any) are appropriate to use in theoretical modelling. Further work is needed to resolve these important questions.
6. Conclusions There is fairly good agreement between the TSSD curves determined by DEM for Zr-2.5Nb pressure tube material with the curves determined from previous data compiled by Kearns for a-phase unalloyed Zr and Zr alloys and by Slattery for heavily cold-worked Zr-2.5Nb rod material. In Zr-2.5Nb, thermal history of the sample has a strong effect on TSSP and little effect on TSSD. The
result is that the hysteresis between dissolution and precipitation of hydrides in Zr alloys depends on the thermal history of the material. In addition, TSSP depends on whether or not the material already has hydride precipitates present, but this result is complicated by the prior history of the material. It is not clear whether the solvi derived from the present and extant data base adequately describe the hydride dissolution and formation processes under isothermal conditions which are required in models of DHC, blister formation and hydrogen ingress. Further experimental and theoretical work is required to resolve this important question.
Acknowledgements This study was funded by the CANDU Owners' Group (COG) under work package 2-31-6580. The authors acknowledge G.K. Shek of the Materials Technology Unit of Ontario Hydro Technologies (OHT), Toronto, Ontario, Canada for carrying out the hydriding of the specimens and the Analytical Science Branch at Whiteshell Laboratories, Pinawa, Manitoba, Canada for the hydrogen analyses.
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