On solid solution hardening in the zirconium-oxygen system

Journal of the Less-Common Metals, 40 (1975) 121 - 128 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

ON SOLID SOLUTION SYSTEM

HARDENING

121

IN THE ZIRCONIUM-OXYGEN

0. RUANO and G. ELSSNER Max-Planck-Znstitut fiir Metallforschung, 7 Stuttgart 1, Seestrasse 92 (Germany)

Znstitut fiir Werkstoffwissenschaften,

(Received September 27, 1974)

Summary Zirconium-oxygen alloys containing between 0.02 and 1.05 at.% oxygen were prepared by engassing /3-Zr at 1500 “C followed by an anneal at 800 “C. Conventional stress-strain measurements as well as strain-rate-change tests were made. The yield point was determined in the range from 77 to 500 K for wire samples with an a-Zr bamboo structure, and the hardening coefficient, AT/AC, was found to vary from 0.52 P at 77 K to 0.36 P at 300 K. Observations with the electron microscope under weak-beam conditions demonstrated the presence of clusters inside the Zr matrix. For the temperature range 0 - 500 K, two rate-controlling mechanisms are indicated. Below approximately 300 K, the flow stress is composed of a temperaturedependent component, caused by the interaction between dislocations and single, interstitial oxygen atoms, and a temperature-independent contribution due to an interaction with stable oxygen clusters. Above about 300 K, the onset of plastic deformation is determined by the overcoming of clusters. Zusammenfassuug ZrO-Legierungen im Konzentrationsbereich zwischen 0,02 und 1,05 At.-% Sauerstoff wurden durch Hochtemperaturbegasung im p-Zr-Gebiet und durch anschliessendes Gliihen bei 800 “C hergestellt. Die Streckgrenze dieser Drhhte mit a-Zr-Bambusgefiige wurde im Bereich von 77 bis 500 K ermittelt. Ausserdem wurden Geschwindigkeitswochselversuche durchgefiihrt. Die Hartungskoeffizienten AT/AC betragen bei 77 K 0,52 g und bei 300 K 0,36 P. Mit Hilfe der weak-beam-Technik wurden elektronenmikroskopisch Sauerstoffcluster innerhalb der Zr-Matrix nachgewiesen. Fiir den Bereich zwischen 0 und 500 K werden zwei verschiedene Verformungsmechanismen vorgeschlagen. Unterhalb etwa 300 K setzt sich demnach die Streckgrenze aus einem temperaturabhlngigen, durch die Wechselwirkung zwischen Versetzungen und einzelnen interstitiellen Sauerstoffatomen bestimmten Anteil und einem temperaturunabh5ingigen Clusteranteil zusammen. Oberhalb 300 K wird der Beginn der plastischen Verformung ausschliesslich durch die Uberwindung der clusterfijrmigen Hindernisse bestimmt.

122

1. In~oduction Solid-solution hardening in the Zr-0 system has already been investigated, using both polycrystalline and single-crystal samples [l - 61. Different specific models have evolved in attempts to interpret the increase in yield strength in the low-temperature range when oxygen is added; these previous efforts have all been based on the common assumption that the effect is due to interactions between dislocations and single, interstitial oxygen atoms. No generally acceptable interpretation has resulted as is evidenced by the fact that Soo and Higgins f 41 and Das Gupta and Arunachalam [ 5 J reported evidence for two rate-controlling mechanisms in the range up to 600 K, whilst other authors f2, 3, 61 have proposed that a single mechanism is adequate to describe the rate-controlling process for deformation. The principal goal of this work was to resolve the question about the deformation mechanism(s) alluded to above. This has been approached by investigating the dependence on oxygen concentration and temperature in the range from 77 to 500 K and by determining the activation parameters. These data were supplemented with transmission electron microscopy observations, which served to characterize the kind of obstacles present after engassing. Particular attention was paid to preparing samples with uniform oxygen content by engassing at high temperatures. 2. Experiments The two grades of starting materials used for the experiments were wires about one millimeter in diameter obtained from Materials Research Corporation (99.97 wt.% Zr; 0.003 wt.% 0) and from Heraeus GmbH (99.8 wt.% Zr; 0.087 wt.% 0). The wires were etched in a solution of 90 ml HNOs, 90 ml HzO, and 18 ml HF, heated to 1500 “C and volumetrically engassed by introducing oxygen at defined pressures into the sample chamber. The specimens were then homogenized at 1700 “C for 20 min in high vacuum and cooled to room temperature at about 200 “C/s by flooding the vacuum chamber with pure helium. In order to convert the tr~sformation structure obtained in this way into an cu-Zr bamboo structure, the specimens were annealed again in high vacuum at 800 “C for 70 h. The resulting bamboo structure [7] consisted of grains about 2 mm in length, each occupying the full diameter of the wire. The concentration of dissolved oxygen was determined by the vacuum-fusion method and checked by measuring the electrical resistivity. The homogeneity of the specimens and the absence of massive precipitates were verified by microhardness measurements and by optical metallographic investigations. All tensile testing was carried out with a model 1363 Zwick machine at a base strain rate of 6.9 X lo- 5 s- ‘, using samples with a gauge length of 6 cm. For strain-rate-change tests at a constant temperature, a change of a factor 10 was used. Tem~rat~es between 77 and 500 K were obtained with the aid of consent-temperature baths, controlled to f 2 “C.

123

3. Results Examples of the’stress-strain curves at 300 K for selected oxygen concentrations are given in Fig. 1. At the lower oxygen level (0.02 at.%), no yield point phenomenon is observed, and the flow stress, u, is measured at 0.1% offset from the modulus line. At an oxygen content of about 0.2 at.%, a region of constant stress appears which may be interpreted as a yield point. For higher oxygen concentrations, there are always well-defined upper and lower yield points, and, hence, the flow stress will be taken as that corresponding to the lower yield point.

I

,

93

I 96 Strain

1%)

I __

99

._

II

0

63 Oxygen

W

0.9

l2

Contentfat.%)

Fig. 1. Stress-strain curves for zirconium wires having a bamboo structure. Fig. 2. Effect of oxygen content on the flow stress of zirconium

The dependence of the flow stress on oxygen concentration at testing temperatures of 77 and 300 K is shown in Fig. 2. It may be noted that the strengthening due to oxygen is more pronounced at 77 than at 300 K; the hardening coefficient, AT/AC, is 0.52 P at 77 K and 0.36 P at 300 K. Here, p is the shear modulus, Css, according to Fisher and Renkin [ 81. In the following analysis, the critical resolved shear stress, 7, is taken as u1,./2. The flow stress as a function of testing temperature is shown in Fig. 3 for alloys with three different oxygen contents. Clearly, anomalous behavior is encountered in the region near to 300 K, as has been reported previously for Zr by Soo and Higgins [4] and for Ti by Levine [9]. The activation volume, u*, and activation energy, Q, were determined from the following relationships [ 10, 111:

and

(1)

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I ‘a

P

600‘

i= 6,s.10'sscc-'

\

P

A\_,

1,osat. % 0

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LOO Ih II

*T

z

i_

2a?-

h

at.% 0

0 402 qso

at.%

0 I,05

at.% 0

0

/” O/O\ 0 0

I25

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375

500

=8y--/.---oo:~-“\~/ 0

(Ki

125

L

1

250

Ibmpcmture

375

1 SW

fK,’

Fig. 3. Effect of temperature on the flow stress of zirconium. Fig. 4. Activation volume as a function of temperature.

0

100

200

Temperature

300

Lao

(K)

Fig. 5. Activation energy, Q, as a function of temperature.

Q

(21

where k and T have the usual meaning and d is the shear strain rate. Values for the activation parameters, u* and Q, were obtained from sag-ate-ch~ge tests and from the dependence of the flow stress on the testing temperature. The activation volumes for three alloys are presented in Fig. 4 where a distinct break is observed at about 300 K. Above this transition temperature the activation volume decreases and appears to exhibit a minimum. In Fig. 5, the activation energy determined at the different temperatures is shown. Below 300 K, Q seems to vary nearly linearly with temperature. Above 300 K it is not possible to assign a meaning to the values obtained, because of difficulties in obtaining physically realistic Ar/A T values. For the electron microscopy investigation, zirconium wires with oxygen

125

Fig. 6. Weak-beam

image of clusters

in zirconium

with 1.05 at.% oxygen.

U = 200 kV.

concentrations between 0.02 and 1.6 at.% were used. These wires were first ground to a thickness of 0.15 mm and then thinned electrolytically in a Tenupol instrument (available from Struers Co.) at 228 K and 30 V, using a polishing solution of 2 vol parts of 70% aqueous perchloric acid and 98 vol parts of ethanol. Weak-beam images [12, 131 were obtained with a JEOL200 electron microscope operated at 200 kV. Figure 6 shows a weak-beam micrograph of a foil containing 1.05 at.% oxygen in which bright spots are recognizable. These bright spots are due to localized strain-centers, which are attributed to clusters of interstitially dissolved oxygen atoms in the cr-Zr lattice; For this alloy, the average clusters are about 6 nm in diameter and are separated by about 76 nm in the glide plane. It will be shown in more detail elsewhere [ 141 that ,there is an increase in the number density of clusters with increasing oxygen content, but the cluster size remains nearly constant. 4. Discussion An elementary consideration of the temperature dependence of flow stress, activation volume and activation energy (see Figs. 3 - 5) is sufficient to convince one that a single deformation mechanism is incapable of explaining the behavior observed between 77 and 500 K. If one assumes, however, that the obstacle spectrum following engassing can be characterized approximately by two different kind of defects, then the present results are understandable. That is, below 300 K the dependence of the strengthening on temperature can be ascribed to an interaction between single interstitial atoms (acting as weak obstacles) and dislocations. The drop in the flow stress above this range of temperatures can be related to the presence of

126

clusters (acting as strong obstacles). Accordingly, the low-temperature range, the single interstitial ed and that the clusters supply a contribution to dependent on the oxygen concentration but not in the range below 300 K, the flow stress, 7, can

it will be supposed that, in atoms are thermally activatthe yield point which is on the temperature. Thus, be expressed generally by:

7 = T* (T, C) + T; (C),

(3)

where r* and 7; are the contributions from single interstitial oxygen atoms and clusters, respectively. The temperature dependence of the flow stress below 300 K can be described by the h~dening model proposed by Fleischer [15,16] in which

This has been evaluated using the plateau values for the flow stress, T, at 300 K, and the results are plotted in Fig. 7. The linear behavior for each of the concentrations and the dependence of 71)with the square root of the total oxygen toncentration shown in Fig. 8 are indications that the criteria for Fleischer’s theory are satisfied. These curves also allow the critical temperature, Ye, to be determined. The resulting value for To is 320 K, in agreement with the experimental observations shown in Fig. 5. proposed by Fleischer [16] between the flow stress, 7, and the activation energy

Q=Qo

p-(z)

‘*]‘,

(5)

a value of 1.1 eV was obtained for QO. According to the interstitial hardening model given by Fleischer [ 161, the activation energy is related to the shear stress, T*, and the interstitial solute content, C, by the relationship, u* =

(6) (3 Ci)%

where b is the Burgers vector. Using this relationship for a rough calculation of the proportion of single interstitial atoms of oxygen in the total concentration, a value of 40% is obtained. An estimate of the proportion of oxygen atoms in clusters can be obtained using electron microscopy data (co = 1.05 at.%, number of clusters per unit volume 3.1 X 1016 cmS3 and cluster diameter 6 nm). Assuming a close packing of oxygen atoms in the zirconium lattice, it appears as if about 50% of the total concentration is present in clusters. Considering the. crudeness of the methods used in arriving at these estimates, the materials balance is quite satisfactory. One can estimate the contribution of the clusters to the shear stress in the region below approximately 300 K using the Orowan relationship,

127 50

’

10

b

e-

0>02at:/.

0

0 0.50 at. % 0 ~osaL*%O

10

-

30

-

q

0 ? E <

*0 h-J s

0

I

8

12

Temperature

Fig. 7. r*l”

\

20

0

T”?KJ’”

as a function

Fig. 8. 78 as a function I5

16

20lo>/, 0

,

0

42

qL

ofi

c(ya

(at.%P

0.8

1.0

of T”. of the square

root

of total oxygen

concentration

Co”.

a

1,os at.%0 _\

lo

qsoat

%O 0

-

‘\

lo\ -ALA

I

I

LB

56

0 LO

Temperature

Fig. 9. T as a function

I

F!

002at.%o A 64

T”fK)*”

of T213.

_clb

Tp---_,

A

where A is the effective spacing between clusters. Above about 300 K, the single oxygen atoms have become transparent to the glide dislocations (‘I’, 21 300 K), and the temperature dependence of the flow stress shows a T213 relationship as shown in Fig. 9. This decline in flow stress can be explained by the dislocations overcoming the resistance of the clusters by a thermally-activated process, as described by Kelly [17] for clusters of substitutional atoms and/or by a gradual dissolution of the obstacles with increasing temperature. Since it has been argued that above 300 K the resistance to flow offered by the single interstitial oxygen atoms is negligible, one can use the value of the stress at the 300 K plateau in eqn. (7) to calculate the spacing between clusters. For the 1.05 at.% oxygen alloy, A is found to be 82 nm, in reasonable agreement with the value of 76 nm for the cluster spacing obtained from the electron microscopy observations.

128

5. Conclusions (1) The introduction of oxygen produces a marked increase in the yield point of zirconium, whereby the hardening effect decreases with increasing testing temperature. The hardening coefficient A r/AC has a value of 0.36 cc at 300 K and 0.52 P at 77 K. (2) For wire samples with a bamboo structure containing above approximately 0.2 at.% oxygen, there is always a well-defined upper and lower yield point. (3) The weak-beam electron microscopy technique revealed the presence of clusters in engassed zirconium specimens. In an alloy with a total oxygen content of 1.05 at.%, roughly 50% of the solute was found to be in about 6 nm diameter clusters separated by about 76 nm in the slip plane. (4) Between 77 and 500 K, two deformation mechanisms are inferred in Zr-0 alloys with the transition from one to the other occurring at approximately 300 K. Below this temperature, the rate-controlling mechanism is the thermally-activated overcoming of single interstitial oxygen atoms by moving dislocations. The activation energy, QO, for this process has a value of 1.1 eV. In the low-temperature range, the flow stress consists of this temperature-dependent component and a temperature-independent component due to the presence of clusters. Above 300 K the drop in the flow stress can be explained in terms of a therm~ly-activa~d overcoming of clusters by moving dislocations and/or of a dissolution of the clusters. Acknowledgements The authors thank Dr. M. Riihle for his assistance with the electron microscope investigations. References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

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