Recrystallization of zircaloy-4 during transient heating

Journal of Nuclear Materials 92 (1980) 184-190 0 North-Holland Publishing Company

RECRYSTALLIZATION

OF ZIRCALOY-4

DURING TRANSIENT HEATING

C.E.L. HUNT and E.M. SCHULSON * Atomic Energy of Canada Limited, Fuel Engineering Branch, Chalk River Nuclear Laboratories, 1JO, Canada

Chalk River, Ontario KOJ

Received 12 February 1980; in revised form 27 May 1980

Recrystallization of Zircaloy-4 has been studied during transient and short time isothermal (60 s) heating. Comparison of the results with published isothermal recrystallization data on Zircaloy-2, for hold periods from 9 X lo2 to 2.3 X 10’ s, shows that all the data can be described by the chemical rate equation dF/dt = -2.08 X 10’ 8 exp(-41 550/Q F, where f is the unrecrystallized fraction, I is time and T is temperature in degrees K.

1. Introduction

thinning. In this way we had material with two different starting conditions, cold-rolled and “as-received”. Two types of test were done. One consisted of heating and cooling at rates in the range 120 to 220 K/s with a 60 s isothermal hold at the peak temperature. In the other, the specimens were heated at either 0.5 or 25 K/s followed by a rapid quench with no hold time at the peak temperature. The test method has been described elsewhere [l]. Briefly, specimens were attached to water cooled copper electrodes, heated in a vacuum of about 10e4 Pa using electrical resistance heating, then quenched with helium. Temperatures were measured by unsheathed 0.127 mm diameter tungsten-5% rhenium versus tungsten-26% rhenium thermocouples welded to the specimen and were recorded by a high speed strip chart pen recorder. The degree to which recrystallization had occurred in each test was calculated from Vickers microhardness measurements using a 1 kg load. This load was selected in order to average the measurement over the largest possible number of grains, thus decreasing any effects which may be caused by differing grain orientations or adjacent recrystallized and unrecrystallized grains. The assumptions were: (1) The starting value of microhardness of the material with the additional 30% cold work represented no recrystallization (unrecrystallized fractionF= 1).

Recrystallization studies are traditionally isothermal tests where specimens are held at appropriate temperatures for times of the order of 1000 to 10000 s. However, in order to determine the behaviour of Zircaloy nuclear fuel sheaths during postulated high temperature transients, we are interested in recrytallization which may occur in 10 to 100 s. Experiments were therefore done to see if there is any difference in the recrystallization kinetics between short time (a few seconds) high temperature transients and long time isothermal experiments reported in the literature.

2. Experimental procedure Zircaloy-4 fuel sheathing, which is usually supplied in a cold-worked plus stress-relieved condition, was cut into strips about 85 mm long X 7 mm wide. To obtain rapid heating and cooling rates we thinned the strips from the nominal 0.45 nm to about 0.31 mm; some were thinned by cold-rolling, thus introducing an additional 30% cold work, and others by chemically

* Now with Thayer

School College, Hanover, NH, USA.

of Engineering,

Dartmouth

184

C.E.L. Hunt, EM. Schulson /Recrystallization of Zircaloy-4

185

3. Results and discussion

(2) The microhardness of fully annealed (1070 K, 15 min, slow cool) material presented complete recrystallization (unrecrystallized fraction F = 0). (3) The unrecrystallized fraction of any specimen was represented by linear interpolation between the microhardnesses representing F = 1 and F = 0. Each data point represents the average of eight or more microhardness readings. The error bars shown on the figures represent +2 standard deviations, the 95% confidence limits for each data point. Since the values of microhardness representing F = 1 and F = 0 were the average of eight readings, error bars can extend to values of F greater than 1 or less than 0. Transmission electron microscopy was done on selected specimens to compare the unrecrystallized fraction obtained by this method with that calculated from microhardness. The procedure was the same as that used by Gagne and Schulson [2]. As will become apparent, both methods yielded similar results, thus confirming the validity of the value of F inferred from the hardness measurements.

The isothermal-hold (60 s) results are shown on fig. 1. The chemically thinned “as-received” material was found, both from transmission electron microscopy and from the F value calculated from the microhardness, to be about 50% recrystallized. Softening begins around 800 to 850 K and is complete by about 950 K. The results from the transient-heated specimens are shown on fig. 2. As would be expected, recrystallization is dependent on the heating rate; the higher the heating rate the higher is the recrystallization temperature. At 0.5 K/s recrystallization starts around 840 K and is complete by about 940 K while at 25 K/s it starts at about 900 K and is completed by about 1020 K. Specimens heated quickly (120 K/s) and held isothermally for 60 s (fig. 1) recrystallized completely at 940 K. Specimens heated slowly (0.5 K/s) to 962 K, with 44 s spent between 940 and 962 K, did not

T

0.9

--

0

TRANSYISSION

ELECTRON

YICROSCOPE

ELECTRON

MICROSCOPE

YICROHARONESS 0

TRANSNlSSlON ERROR

=

0.2

-

0.1

-

290

800

820

840

860

900

920

BARS i

940

2 STANDARD

960

DEVIATIONS

980_ 1

ISOTHERMAL TEMPERATURE - K Fig. 1.60 second isothermal recrystallization

results. The curves are calculated from eq. (4).

1000

C.E.L. Hunt, E.M. Schulson /Recrystallization

186

I.0

1.: ,

09

-

0.8

-

e z

07

-

:

0.6

-

E 1

0.5

-

z z 0 0.3 4 = u :: 0 2 J 010.0

$" 290

,

:c

30% ADDITIONAL

0 25 KS-' X 0.5 KS-' 1 0

25 KS-'

l--c

COLD WORK

MICROHARONESS TRANSMISSION MICROSCOPE

ELECTRON

ERROR BARS ? 2 STANDARO DEVIATIONS ' 600

I 620

I 940

860

960

900

920

940

960

PEAK RAMP TEMPERATURE

1

0LA”

1560

I

1600

I

1640

1

I

I

I

1760

I600

1640

1660

I

1660

1720

960

I 31

I 32

I 33

1 34

I 35

/ 36

I 37

IO00

lOz,O

1040

1060

I060

_L

- K

I

1920

TEST TIME AT 0 5Ks

IA 0'

of Zircaloy-4

I

1960

1

2000

1

1

2040

I

2090

2120

'

I 39

I 39

1 40

I 41

I 42

I

2160 SECONOS

I 43 SECONOS

TEST TIME AT 25Ks-'

Fig. 2. Recrystallization

results

from specimens

heated

soften as much (fig. 2). The microhardness was equivalent to about 0.25 unrecrystallized fraction. The same behaviour was seen on specimens heated at 25 K/s to 1098 K. For specimens heated at 25 K/s electron microscopy showed that by 1020 K (fig. 2) specimens were fully recrystallized in spite of the contrary evidence of the microhardness measurements. The agreement between electron microscope and microhardness measurements on the 60 s isothermal specimens was good throughout the complete recrystallization range (fig. 1). One must therefore conclude that the microhardness readings on the high temperature transient specimens are in error. A 1 kg load on the hardness tester may have been too high for these specimens. For the lowest value of microhardness found in this work, the manufacturer of the testing machine would recommend a minimum specimen thickness of 0.25 mm. The average specimen thickness used was 0.30 mm. Some specimens could, therefore, have been less than 0.25 mm after being polished, with the result that the indenter depth may have been influ-

at 0.5 and 25 K/s. The curves are calculated

from eq. (4).

enced by the material on which the specimen was mounted. This material had a microhardness equivalent to that of specimens recrystallized about 50%. An alternative explanation is that the specimens may have become contaminated by oxygen. Based on the above arguments, we conclude that at 25”C/s, recrystallization is complete by 1020 K, in agreement with the electron microscopy, and that the microhardness readings at the higher temperature are in error. It is assumed that at 0.5”C/s recrystallization is complete by 940 K, as indicated by the line representing the equation, and that the final two points shown are in error for the same reasons as above. The result of this assumption is to increase the uncertainty in the fitted parameters of the equation. Fig. 3 shows typical transmission electron micrographs of cold rolled, partially recrystallized and fully recrystallized specimens. For isothermal temperatures between 885 and 940 K, the recrystallized grains rapidly reached a diameter of 1 to 2 pm. These data, plus data from Lee [3] for isothermal

C.E.L. Hunt, EM. Schulson /Recrystallization

187

of Zircaloy-4

anneals ranging from 15 min to 64 h, were fitted to the first order chemical rate equation dF/dt = -A0 exp [-Q/KT(t)]

,

(1)

where F = unrecrystallized fraction, t = time, he = a constant, Q = activation energy for the recrystallization process, K = Boltzmann’s constant, and T = absolute temperature. For a simple transient defined by a constant heating rate, an isothermal hold and a constant cooling rate, i.e.

T=

T1 + at,

0
T,,

tl
I T3 + /3(t3 - t),

t2 < t < t3

where T1, T,, T3 denote the starting, maximum and final temperatures respectively and QI and B are the heating and cooling rates respectively, the solution to the differential eq. (1) is [4] F = F. exp 1: (

[T2E2(Q/KT2)

- T1E2(Q/KTl)]

ev-Q/KT) - $ [TdMQIKTd

- hot

(2)

- T&dQIKTdl > 1

where t is the time at the maximum temperature. E,(x) is the exponential integral defined by E2(x) = 7

(eCXU/u2) du

,

1

and F. is the starting value of unrecrystallized fraction, normally 1 unless the specimen has been partially recrystallized before the test. For an isothermal test where the effect of heating and cooling may be neglected compared with the time at the isothermal temperature, eq. (2) reduces to F = F. exp [-ho t exp(-Q/KT)]

,

(3)

Fig. 3. Transmission electron micrographs illustrating (a) unrecrystallized, (b) partically recrystallized, (c) completely recrystallized Zircaloy4. The specimens had been coldrolled and then annealed for 60 s at (a) 838 K, (b) 885 K, and (c) 940 K.

188

C.E.L. Hunt, EM. Schulson /Recrystallization

of Zircaloy-4

Table 1 Data and fit to recrystallization equation Temp. W

Time at max. temp.

Heating rate

Cooling rate

(s)

(K/s)

(K/s)

Data from Lee [3] - isothermal 693 230400 0.1 + 301 a) 123 230400 0.1 + 301 748 230400 0.1 + 301 773 230400 0.1 + 301

F(l)

0.1 0.1 0.1 0.1

+ + + +

301 a) 301 301 301

I

1 1 0.450 0.200

0.996 0.949 0.698 0.115

0.004 0.051 -0.248 0.085

693 123 753 173 193

57600 57600 57600 57600 57600

0.1 0.1 0.1 0.1 0.1

+ + + + +

301 301 301 301 301

0.1 0.1 0.1 0.1 0.1

+ + + + +

301 301 301 301 301

1 1 1 1 1

1 0.95d 0.730 0.610 0.310

0.999 0.987 0.878 0.582 0.123

0.001 -0.037 -0.148 0.028 0.187

123 183 193 808 823

14400 14400 14400 14400 14400

0.1 0.1 0.1 0.1 0.1

+ + + + +

301 301 301 301 301

0.1 0.1 0.1 0.1 0.1

+ + + + +

301 301 301 301 301

1 1 1 1 1

0.960 0.760 0.800 0.330 0

0.997 0.764 0.592 0.249 0.029

-0.037 -0.004 0.208 0.081 -0.029

783 823 833 848 923

900 900 900 900 900

0.1 0.1 0.1 0.1 0.1

+ + + + +

301 301 301 301 301

0.1 0.1 0.1 0.1 0.1

+ + + + +

301 301 301 301 301

1 1 1 1 1

1 0.800 0.670 0.220 0

0.983 0.801 0.666 0.375 0

0.017 -0.001 0.004 -0.155 0

Data from this work - isothermal 838 60 125 862 60 125 813 60 125 885 60 125 903 60 125 923 60 125 952 60 125 831 60 125 849 60 125 873 60 125 893 60 125 898 60 125 904 60 125 918 60 125 934 60 125

220 220 220 220 220 220 220 220 220 220 220 220 220 220 220

1 1 1 1 1 1 1 0.460 0.460 0.460 0.460 0.460 0.460 0.460 0.460

1 0.820 0.690 0.550 0.160 0.050 0.090 0.410 0.390 0.260 0.240 0.090 0.130 0.070 0.040

0.964 0.865 0.766 0.601 0.273 0.030 0 0.449 0.429 0.352 0.212 0.168 0.117 0.029 0.001

0.036 -0.045 -0.076 -0.05 1 -0.113 0.020 0.090 -0.039 -0.039 -0.092 0.028 -0.078 0.013 0.041 0.039

Data from this work - transient 835 0 0.500 862 0 0.500 870 0 0.500 884 0 0.500 896 0 0.500 905 0 0.500 910 0 0.500 923 0 0.500 935 il 0.500

100 100 100 100 100 100 100 100 100

1 1 1 1 1 1 1 1 1

1 0.960 0.870 0.720 0.500 0.390 0.330 0.300 0.220

0.984 0.920 0.876 0.747 0.571 0.404 0.307 0.099 0.015

0.016 0.040 -0.006 -0.027 -0.071 -0.014 0.023 0.201 0.205

130 130

1 1

1 0.980

0.999 0.966

0.001 0.014

855 914

0 0

25 25

C.E.L. Hunt, E.M. Schulson /Recrystallization of Zircaloy-4

189

Table 1 (continued) Temp. (W

Time at max. temp.

Heating rate

Cooling rate

(s)

(K/s)

(K/s)

25 25 25 25 25 25

130 130 130 130 130 130

0 0 0 0 0 0

934 955 970 994 1023 1050

F(1)

F(2)

F(3)

F(4)

1 1 1 1 1 1

0.850 0.830 0.490 0.310 0.200 0.200

0.909 0.767 0.586 0.206 0.004 0

-0.059 0.063 -0.096 0.104 0.196 0.200

F(1) = initial F, F(2) = final F measured, F(3) = final F fitted, F(4) = (F(3)-F(2)). a) Values assumed very large so that heating and cooling times negligible compared

which is basically another form of the Johnson-Mehl equation [5], with the time exponent taken as 1.0. Lee’s work falls into this category since his isothermal tests ranged from 15 min to 64 h. A multiple least squares computer subroutine, with a weighting factor of one, was used to simultaneously fit the pre-exponential, he, and the exponential term, Q/K. First, Lee’s data and the data from this work were treated separately. Similar, overlapping results were obtained. Consequently, we combined the two data sets and recalculated the constants. All the data and the measured and calculated values of the unrecrystallized fractions are given in table 1. The equation which best fits all the data is dF’ldt = -2.08

X lo’* exp(-41

550/T)

F.

(4)

The fit of this equation to the data is shown by the curves on figs. 1 and 2 and the final column of table 1. The 95% confidence contour for the constants in eq. (4) is a long narrow ellipse on log-linear paper extending from 2.4 X 10” and 35 824, to 4.9 X lo*’ and ,48 168 for the two constants respec-

Table 2 Differences Zircaloy4 Alloying

in alloy

elements

Fe Ni Total Fe + Cr + Ni

composition

between

Zircaloy-2 0.07-0.20 0.03-0.08 0.18-0.38

Zircaloy-2

Zircaloy-4 wt% wt% wt%

0.18-0.24 70 ppm 0.28-0.37

and

with isothermal

time.

tively. The best value is at the centre of the ellipse at 2.08 X lo”, 41 550. The minor axis of the ellipse at this point is from 41 150 to 41950. The softening of cold worked metals is usually considered to occur by two sequential processes, by recrystallization. In some recovery followed metals these can be distinguished as two separate processes. Other metals retain full hardness up to the point where recrystallization starts. Lee [3] found that Zircaloy-2 was of this latter type. This work confirms his findings for Zircaloy-4. Figs. 1 and 2 show that cold worked Zircaloy-4 retains its initial hardness up to a temperature at which softening occurs quite abruptly. The onset of recrystallization for the chemically thinned material was not detected: both microhardness and transmission electron microscopy showed that this material was approximately 50% recrystallized at the start of the tests. The most important finding from this work was that the recrystallization behaviour of transiently heated specimens and isothermal specimens over the range of 60 s to 2.3 X 10’ s hold times, could all be represented by a single equation. Another finding is that neither the small differences in alloy composition (table 2) between Zircaloy-2 (Lee’s data) and Zircaloy-4 (the present data) nor the thermal cycle affect the recrystallization kinetics.

4. Conclusions wt% wt%

(1) The recrystallization kinetics of Zircaloy-2 and Zircaloy-4 during both transient and isothermal heat-

C.E.L. Hunt, EM

190

Schulson /Recrystallization

References

ing can be described by the equation U/dt

= -2.08

X lo’* exp(-41550/T)

or in the integrated

F= F. exp[-2.08

F,

form for isothermal behaviour by X lo’* t exp(-41

of Zircaloy4

550/T)],

where F = unrecrystallized fraction, F. = initial unrecrystallized fraction, t = time (s), and T = absolute temperature (K). (2) There is no observable recovery process prior to the onset of recrystallization. (3) The recrystallized grains rapidly reach a diameter at 1 to 2 pm.

Acknowledgements The authors wish to acknowledge B.C. Robinson for the preparation of the specimens and J.A. Roy for the electron microscope measurements.

[l-]-C.E.L. Hunt and P. Niessen, J. Nucl. Mater. 58 (1971) 17. [2] M. Gagne and E.M. Schulson, Met. Trans. 7A (1976) 1775. [3] D. Lee, J. Nucl. Mater. 37 (1970) 159. 141 J.M. Blair and K.R. Chaplin, private communication, Chalk River Nuclear Laboratories (1979). [5] W.A. Johnson and RF. Mehl, Trans. AIME 135 (1939) 416.