An evaluation of mechanical anisotropy of zircaloy using impression testing

Scripta METALLURGICA

Vol.

19, pp. 1045-1048, 1985 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

AN EVALUATION OF MECHANICAL ANISOTROPY OF ZIRCALOY USING IMPRESSION TESTING

K. Linga Murty, Sherief Hussein and Youn H. Jung North Carolina State University Raleigh, N.C. 27695-7909

(Received April 29, 1985) (Revised June 21, 1985) Zirconium alloys are important structural materials in nuclear applications used as thinwalled tubing in l i g h t water reactors, pressure tubes in steam generating heavy water reactors and calandria tubes in Canadian pressurized heavy water reactors [CANDU-PHW]. The properties of zirconium alloys are very sensitive to the thermo-mechanical treatment because zirconium has a hexagonal close packed crystal structure with a c/a ratio less than ideal [1]. Thus the inherent anisotropy of hcp crystals along with the limited number of s l i p systems in these materials render them highly textured, the degree of which depends on the fabrication details such as the Q-ratio* and the annealing treatment [2]. The high degree of crystallographic texture [ i . e . , the non-random distribution of grain orientations] is manifested in the anisotropic properties of these materials, in particular the mechanical properties [1,3,4]. A knowledge of the mechanical anisotropy of these materials is important for p r e d i c t a b i l i t y of dimensional changes and s t a b i l i t y of the fuel rods in service. In addition, the anisotropy parameters control the formability [5] and thus the ease with which Zircaloy tubing can be pilgered from tube-reduced extrusions [TREX]. The mechanical anisotropy parameters are usually determined using tensile and internal pressurization tests while monitoring the biaxial ~oop and a x i a l ] strains; the parameters are evaluated INDIRECTLY from stresses and strains [ i ] . We report here the application of the recently developed impression test method [6,7] in evaluating these anisotropy parameters relatively more DIRECTLY from yield and flow stresses along the three orthogonal directions, namely axial, hoop and radial directions of the tubing. In addition, this type of testing can be made on relatively small specimens, which is of great advantage in investigating radiation effects on materials.

Experimental Details The material used for the study is Zircaloy-4, obtained in the form of 63.5 mm outside diameter and 6.35 mm wall TREXwith the following chemical composition[w%]: 1.51 Sn, 0.21Fe, 0.12 Cr, 0.124 O, 0.003 Ni and balance Zr. The material was received as-pilgered and tested in the as-received condition. Fig. 1 depicts the basal pole intensity in units of times random as a function of the basal-pole angle in the transverse plane [perpendicular to a x i a l ] and exhibits the bimodal distribution of basal [0002] poles with peaks at ± 45° from the radial to the hoop direction typical of cold-worked Zircaloy tubing. The impression test method involves impressing a flat-ended cylindrical punch on the specimen surface and a closed loop hydaulic machine, Instron model 1350, was used for the testing. Penetration of the impressor machined from Unimar into the specimen was monitored using an LVDT in the load train attached to the central platens closest to the specimen and the impressor. The mechanical properties along the three orthogonal directions were evaluated on cube [6.35 mm side] samples cut according to Fig. 2. Experimental data were monitored using an x-y plotter along with an on-line data acquisition, storage and processing system provided by an Apple-If computer. Tests were performed at a constant cross-head speed of 2.5 x 10-3 mm/sec. *Q is the ratio of wall thinning to diametral decrease

1045 0036-9748/85 $3.00 + .00 Copyright (c) 1985 Pergamon Press Ltd.

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Results and Discussion Included in Fig. 3 for the as-received Zircaloy TREX are load-displacement curves tests along the radial, hoop and axial directions. These curves clearly exhibit directional dependence of the mechanical properties of these materials. Because of constant impression area, the impression test method used here is a constant stress test stress equal to the load divided by the cross-sectional area of the impressor: ~i = l°ad/Ai'

from the the with (I)

where Ai is the circular area of the cylindrical [1.5875 mm diameter] impressor. Yu et al [7] demonstrated the a p p l i c a b i l i t y of impression testing to enquire into the compressive properties of fcc [Al, Cu, Nil and bcc [mild steel] metals and noted that the impression stress values are l i n e a r l y related to the compressive stresses (°c): o i = k oc ,

(2)

where k has a value o f ~ 3. We found a value of 2.5 ± I for k for Zircaloy deformed along the axial direction. This value f a l l s in the range found by Yu et a l . [ 7 ] . The compressive strains are given by the normalized penetration depth (~) in the impression tests: c C = ~/D,

(3)

where D is the diameter of the impressor. The true stresses and strains in the plastic regime beyond the proportional l i m i t are f i t to the power-law work-hardening behavior and the derived results are l i s t e d in Table 1 along with the compressive y i e l d stresses and proportional limits. The yield stresses noted in Table 1 correspond to 0.5% off-set values and t~us the double-log plot of the compressive stress versus strain (normalized penetration depth) spans the range of stresses above and below the yield stresses (Fig. 4). The anisotropic nature of Zircaloy is evident in not only the strengths but also the strain-hardening behaviors. TABLE 1 Anisotropic Mechanical Properties of Zircaloy Radial (r) Proportional Limit (MPa) Yield Stress (Mpa) n K (MPa)

550 844 0.125 1,528 o=Kc

Hoop (B) 543 866 0.100 1,460

Axial (z)

418 804 0.094 1,305

n

These results are now used in evaluating the mechanical anisotropy parameters. H i l l [8] proposed a modified Von-Mises y i e l d criterion for an anisotropic material when the applied stress axes are coincident with the principal axes of mechanical anisotropy. When no shear stresses are present, the y i e l d condition is given by F ( o z - o@)2 + G(o e - O r ) 2

+ H(o r - Oz)2 : 1,

(4)

where F, G and H, called the coefficients of anisotropy are constant functions of the material yield strengths, and Oz, o@~ and o r are the principal stresses along the a x i a l , hoop and radial directions respectively of the tubing. Following later modifications by Backofen [5] and Duncombe et al [3,9], a generalized stress, oq, is defined as the uniaxial yield stress along the axial direction so that the yield criterion reads as follows: R(°B - ~r )2 + RP(°z - °B)2 + P(~r - °@)2 = P(R+I)Y~

(5)

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M E C H A N I C A L A N I S O T R O P Y OF ZIRCALOY

1047

where R and P are the coefficients of anisotropy. As has been demonstrated by Murty and Adams [3], these anisotropy parameters P and R are given by the transverse contractile strain ratios in uniaxial hoop and axial tests respectively. Generally, these parameters are evaluated from yield stress and s t r a i n - r a t i o in axial tensile tests along with the hoop yield stress in biaxial closed-end internal pressurization tests [ I ] . The impression testing, however, enables a rather direct evaluation of these parameters from the yield stresses along the three orthogonal directions of the tubing: R:

(YrYz)2 + (YrYB)2 - (YeYz)2 ,

(6a)

,

(6b)

(YBYz)2 + (YrYe)2 - (YrYz)2 P=

(YrYz)2 + (YrYe)2 - (YeYz)2 (YeYz)2 + (YrYz)2 - (YeYr)2

where Ys correspond to the y i e l d stress values. R = 0.95,

Using the present data, we find that P=1.21.

(7)

I t is i n t e r e s t i n g to note that the R value is very close to that predicted from the basal pole angle [~] corresponding to the basal pole peak i n t e n s i t y in the transverse plane. Following van Swam et al [ I 0 ] and Murty and Adams [ i i ] , we note t h a t , R : COt2(~).

(8)

With the peak angle of 45° for ~, we predict that R should be unity, a value very close to the result of the present experiment. The predictions of the P parameter, however, require a quantitative evaluation of orientation distributions in three-dimensional space such as the c r y s t a l l i t e orientation d i s t r i b u t i o n functions [CODF] which u t i l i z e three or more pole figures [4] and are not available for this material at the present time. Conclusions The impression test method is applied in evaluating the anisotropic mechanical properties of textured Zircaloy and is shown to yield the mechanical anisotropy parameters directly from the yield stress values along the three orthogonal directions of the tubing. Financial support of the National Science Foundation through Grant #DMR 8313157 is gratefully acknowledged. We express our sincere appreciation to Mr. D. J. Oh for assistance and useful discussions. References 1.

R. J. Beauregard, G. S, C1evinger and K.L. Murty, "Effect of Annealing Temperature on the Mechanical Properties of Zircaloy-4 Cladding," in Proceedings of the 4th International Conference on Structural Mechanics in Reactor Technology, San Francisco [1977], paper C3/5. 2. E. Tenckhoff, "A Review of Texture and Texture Formation in Zircaloy Tubing," in 'Zirconium in the Nuclear Industry,' ASTMSTP 754, 5 [1982]. 3. K. L. Murty and B. L. Adams, "Biaxial Creep of Textured Zircaloy I: Experimental and Phenomenological Descriptions," Mat. Sci. Eng., 70, 169 [1985]. 4. B. L. Adams and K. L. Murty, "Biaxial Creep of T~tured Zircaloy I I : Crystal Mean Plastic Modelling," Mat. Sci. Eng., 70, 181 [1985]. 5. W. A. Backofen, 'Deformatio~rocessing,' Addison-Wesley, Reading, MA. [1972]. 6. S. N. G. Chu and J. C. M. Li, J. Mat. Sci., 1__22,2200 [1977]. 7. H. Y. Yu, M. A. Imam and B. B. Rath, J. Mat. Sci., 20, 636 [1985]. 8. R. H i l l , Proc. Roy. Soc. (Lond.), 193A, 281 [1948].-9. E. Duncombe, E. M. Friedrich and W. H. Guilinger, Nucl. Tech., 12, 194 [1971]. 10. L. F. P. Van Swam, D. B. Knorr, R. M. Pelloux and J. F. Shewbr~ge, Met. Trans., IOA, 483 [1979]. 11. K. L. Murty and B. L. Adams, "Multiaxial Creep of Textured Zircaloy-4," in 'Mechanical Testing for Deformation Model Development,' ASTMSTP 765, 382 [1982].

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M E C H A N I C A L A N I S O T R O P Y OF ZIRCALOY

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9

Z

•

5 4

I

-

[

A

I

'

I

^

^

'

Zircaloy-4 TREX

3 ~/~o

2

../ 1 I

0 -90

-60

J

l -30

~

I 0

J

I 30

(?)

,

I 60

,

.I/

90

(~)

3/

Basal Pole Angle ( ¢ ) , Deg.

FIG. 1 Basal Pole Intensity Distribution in r-8 Plane

Y

6.35mm

FIG. 2 Impression Test Specimens With Respect to TREX Coordinates

7,0 R

z

4

load, kN

"~

6.5 /~,f

2

ZiRCALOY -4 TREX (Cold-Worked)

ZIRCALOY-4 -

)

6.0 I

I

0.025

0.05

-8 .075

impression depth, mm

J -6

i -4

-2

en (~o}

FIG. 4 FIG. 3 Impression Load Versus Depth Curves

Double-Log Plot of Compression Stress Versus Strain