Pergamon Scripta Metallurgica et Materialia, Vol. 31, No. 11, pp. 1475-1480, 1994

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CYCLIC HARDENING AND SOFTENING BEHAVIOR

OF A FULLY ANNEALED ZIRCALOY4

D.H. Lee* and S.I. Kwun**

* Korea Atomic Energy Research Institute, Taejon, Korea 305-353

** Deptartment of Metallurgical Engineering, Korea University, Seoul, Korea 136-701

(Received May 24, 1994) (Revised July 5, 1994)

Introduction

Polycrystalline metals with a hexagonal close-packed (HCP) crystalline structure show a unique deformation behavior and fracture characteristics under cyclic loading conditions. As the HCP materials have limited slip systems and a tendency to develop a preferred orientation (texture), they show deformation anisotropy resulting from the texture during plastic deformation. Therefore, this anisotropy of the materials will affect not only the fatigue crack initiation/growth process but also the stress responses due to the increase in the slip irreversibility.

It is well known that the fully annealed metals usually cyclically harden not soften. However, the initial cyclic hardening followed by softening in the higher strain ranges and the only cyclic softening to fracture in the lower strain ranges were observed in the fully annealed Zircaloy-4. This paper is proposing a new model to explain such cyclic behavior in Zircaloy-4 in terms of texture change during fatigue.

Experimental

The chemical composition of the Zircaloy-4 plate used in this study is given in Table 1. The plate was cold-rolled to 3 mm thick and subsequently fully annealed at 650°C for 1 hour. The recrystallized grains were equiaxed with an average grain size of 10 ttm, which showed no mechanical fibering traces at all.

The low cycle fatigue specimens were of two types, as shown in Fig.1. The one has a gage length parallel to the rolling direction (designated as R specimen), and the other has a gage length parallel to the transverse direction (designated as T specimen). The fully reversed total strain ranges were controlled with a strain rate of 0.08%/s at room temperature.

Results & Discussion

Crystallographic Texture

The X-ray pole figure of the plate is shown in Fig.2. One observed that the density of the (0002) pole is the highest at the locations 1 0 - 2 0 ° tilted toward both transverse directions from the

plate normal. This type of texture is typical in the HCP materials with c / a ratio less than 1.633 [1,2].

It is well known that the main deformation systems in Zr and Zr alloys including Zircaloy-4 at room temperature are {10]0}<1120> type prismatic slip, and (10~2} type twinning in tension and

1475

1476 CYCLIC HARDENING / SOFTENING Vol. 31, No. 11

(11-22} type twinning in compression when stressed along c-axis [3,4]. It can be easily understood that both R and T specimens would slip on prismatic planes rather than twinning when they plastically deform, because the loading direction is almost normal to the (0002) pole in both specimens; that is, the loading direction is 90 ° and 7 0 - 8 0 ° to (0002) pole in R and T specimens, respectively. The Schmid factors on prismatic planes are higher for R specimen, resulting in the lower yield stress of 403 MPa than for T specimen of 453 MPa.

Fatigue Life

The cyclic stress responses with the number of cycles at given strain ranges are shown in Fig.3. As the plastic strain range became smaller, the fatigue life difference due to anisotropy became more pronounced, as can be seen in Coffin-Manson type plot (Fig.4). This observation tells us that the anisotropy due to texture affects the crack initiation process, because more life is spent for crack initiation at smaller strain range in low cycle fatigue. Therefore, it is important to investigate the fatigue crack initiation process in this material.

All the cracks initiated at the lateral sides of a gage in both R and T specimens, and then propagated along the direction normal to the loading axis. These results prove that most of the plastic deformation occurs by prismatic slip. As can be seen in Fig.5, the crack initiation sites were found to be the grain boundaries where one grain experienced large plastic deformation and the other neighboring grains showed very little plastic deformation. Some of the grains seem to protrude out toward the surface by a crack (Fig.5-b). Therefore, it can be said that cracks are initiated at steps on the grain boundary between the grain deformed easily by prismatic slip and the grain oriented in such a direction that slip is not favorable. This process is similar to the fatigue crack initiation model in pure Cu suggested by Laird [5]. The lateral side of T specimen has more grains in which (0002) poles are not oriented perpendicular to loading direction than that of R specimen (Fig.2). In such case, it is easier to initiate a crack in T specimen than in R specimen, because grains in T specimens are easier to form grain boundary steps, which cause earlier crack initiation during fatigue life.

Cyclic hardening and softening

Both R and T specimens showed initial hardening and subsequent softening in the strain ranges higher than 2.0%, and only cyclic softening from the beginning to fracture in the strain ranges lower than 1.6%. The cyclic flow stresses were always lower at the same strain ranges in R specimen than in T specimen with the same reason as mentioned to lower monotonic yield stress.

In a fully annealed metal, it is very scarce to find only cyclic softening even in low strain ranges. As this Zircaloy-4 plastically deforms by prismatic slip, the {10~0) pole figures of planes normal to loading directions in R and T specimens were compared before and after fatigue at 1.6% strain range (Fig. 6 & 7). After fatigue, both R and T specimens showed a developed texture tilted about 15 ° from the fatigue loading direction. Thus the cyclic softening can be explained in terms of these newly developed (1010) textures in the following way.

When we think of a crystal lying such that the <0001 > direction is perpendicular to both the rolling and transverse direction, and <11:20> direction is parallel to the rolling direction as shown in Fig.8-a, the maximum Schmid factors on prismatic planes under loading in either the rolling or transverse direction is calculated to be 0.433. However, when this crystal is rotated 15 ° about the <0001> axis as shown in Fig.8-b, the maximum Schmid factor becomes 0.5 which causes a 15%

increase in resolved shear stress on the (10]-0) planes. Therefore, as the cyclic deformation continues, the crystals rotate to the condition that increases the Schmid factor, resulting m easier prismatic slip and thereby cyclic softening. Even though these R and T specimens have characteristics of easy prismatic slip during plastic deformation due to texture, it is known that the crystal rotation in Zr is mainly caused by twinning [6]. However, twins were not observed after fatigue cycling at a 1.6% strain range in both R and T specimens (Fig.9 and Fig.10). From these results, it can be said that the fully annealed Zircaloy-4 cyclically soften by rotating the textured

Vol. 31, No. 11 CYCLIC HARDENING / SOFTENING 1477

crystals to an easier direction for prismatic slip by so called "texture-rotation induced cyclic softening," that is, slipping on prismatic planes. In order to visualize this model more clearly, a very large grained specimen was prepared by the strain-anneal method. This specimen also cyclically softened from the beginning under a 1.6% strain range but with lower stress responses than the ones with smaller grains (Fig.11). The uneven surface of the grains due to grain rotation during fatigue could be easily noted on the scratched surfaces as seen in Fig.12.

The specimen, which experienced initial cyclic hardening and subsequent softening at the strain ranges higher t h a n 2.0%, showed the evidence of twinning in some places after fatigue failure (Fig.13). The distribution of twins was not uniform through the gage and more twins were observed in T than in R specimens. In order to confirm that these twins contributed to initial cyclic hardening, fatigue was interrupted at the number of cycles showing the maximum stress responses (10 cycles and 40 cycles in R and T specimens, respectively), and then the microstructure of the specimens was examined (Fig.14). Twins were also observed and the amount of twins was almost the same as that observed in the fractured specimens. These observations suggest that as the strain range increases, the stress concentration at the grain boundary increases, thereby giving twinning more of a chance of occurring in grains which are not oriented favorably for prismatic slip. The twin boundaries will then act as barriers to the dislocation glide on prismatic slip planes. However, twins will form only at the beginning before not many grains rotate for easy prismatic slip, resulting in cyclic hardening. As the cycles increase, many grains will eventually rotate so that the slip on prismatic planes become easier. This explanation can be justified by the results that the T specimen which has a texture relatively not favorable for easier prismatic slip showed a longer hardening period and more twins as compared to R specimen.

Conclusions

The low cycle fatigue behavior in the rolling and transverse directions of the fully annealed Zircaloy-4 plate was investigated in terms of texture change during fatigue.

1. The anisotropy in the R and T directions due to texture resulted in larger differences in fatigue life in the lower strain ranges.

2. The fatigue crack was initiated at the grain boundary between the grain that went through large plastic deformation and the one with very little deformation. The anisotropy in fatigue crack initiation was also observed.

3. Only cyclic softening to fracture was observed in the strain ranges lower than 1.6% even in the fully annealed state. The "texture-rotation induced cyclic softening" model was proposed to explain this cyclic softening behavior.

4. The initial cyclic hardening and subsequent cyclic softening in the strain ranges higher than 2.0% were due to twinning in the grains aligned not favorably for easy prismatic slip.

References

1. W.F. Hosford, Jr. and W.A. Backofen, Fundamentals of Deformation Processing, p. 259, Syracuse Univ. Press, Syracuse, N.Y. (1964)

2. E. Tenckhoff, in Zirconium in the Nuclear Industry: Fifth Conference, ASTM STP 754, D.G. Franklin, Ed., p. 5, ASTM (1982)

3. D.O. Hobson, Trans. TMS-AIME, 243, 1105 (1968) 4. R.E. Reed-Hill, in Deformation Twinning, R.E. Reed-Hill, J.P. Hirth, and H.C. Rogers, eds, Vol.

25, p. 295, Gordon and Breach, N.Y. (1964) 5. W.H. Kim and C. Laird, Acta Metall., 26, 789 (1978) 6. E. Tenckhoff, Metall. Trans. A, 9A, 1401 (1978)

TABLE 1. Chemical Composition of Zircaloy-4 Plate. (wt.%)

i 1.52 ~ 0.21 I 0.11 0.15 Balance . . . . . . . x . . . . . . . . . . .

1478 CYCLIC HARDENING / SOFTENING Vol. 31, No. 11

#£

T r a n s v e r s e / ~ j ' ~ _ / / / + R - S p e c i m e n

D i r e c t i o n / /_// r / : - - . ' : / / ( i ~ - - ? ~ / /~ Transverse

D i r e c t i o n T - S p e c i m e n

FIG. 1. Odentation of the low cycle fatigue specimen.

R o

LT. FIG. 2. (0002) pole figure of the annealed Zircaloy-4 plate.

1 O0

E E

~ 80

~ 60

40

. . . . . . . . ~ . . . . . . . . L . . . . . . . . . . . . . . . . . . . . 1000

2~%J12, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . t , ,

I 0 I O0 I000 10000

C y c l e s , N

I O0

E E

~oo~ ~o C E

ooo ~ ~ oo

400 40 . . . . . . . . . . .

. . . . . . . . , . . . . i . . . . . . . . , . . . . . . . . , , ,

2.o~ 1.&~ , . . . . . . . . . . . . . . . . . . . . . . , . , ,

I 0 I O0 1000 10000

C y c l e s , N

FIG. 3. Stress range vs. number of cycles for R and T specimens.

1000

E

z 800

v

& E

600

400

.V. z

E

~_0.1 o

1000 10000 Number of Cycles ot Frocture, Nf

FIG. 4. Effects of plastic strain on cycles to failure for R and T specimens.

a)

b)

FIG. 5. Micrographs showing the fatigue crack initiation on the specimen surface after Aet=1.6% fatigue. (arrow shows loading direction.) a) lateral side of R specimen, b) lateral side of T specimen.

Vol. 31, No. 11 CYCLIC HARDENING / SOFTENING 1479

a)

a)

STO

FIG. 6.

b)

ST~

( 1 0 ] 0 ) pole figures of the plane normal to rolling direction for R specimen. a) before cyclic deformation, b) after cyclic deformation at ~=1.6%.

STD

FIG. 7. ( 1 0 ] 0 ) pole figures of the plane normal to transverse direction for T specimen. a) before cyclic deformation, b) after cyclic deformation at ~t,=1.6%.

RD RD

FIG. 8. Effect of crystal rotation on the change of angle between prismatic slip plane and stress axis.

a) b)

FIG. 9. Microstructures of Zircaloy-4 specimens before cyclic deformation. a) the plane normal to rolling direction for R specimen, b) the plane normal to transverse direction for T specimen.

1480 CYCLIC HARDENING/SOFTENING Vet. 31, No. 11

a) b)

FIG. I0. Microstructures of Zircaloy-4 specimens after failure under AE,=l.6% fatigue. a) the plane normal to rolling direction for R specimen, b) the plane normal to transverse direction for T specimen.

,. 100~1 ~ . . . . . . . . . . . . . . . . . i . . . . . . . . , . . . . . . . . . . . 1000

~E f A~,= 1 6~ ~'~ E ~ E

o - " 600 o~

¢00 1 10 100 1000 10000

Cyc les , N

FIG. 11. Stress range vs. number of cycles for large grained specimen.

FIG. 12. Surface morphology of large grained specimen after A~,=1.6% fatigue.

a) b)

FIG. 13. Microstructures of Zircaloy-4 specimen after failure under Ae,=2.6% fatigue. a) the plane normal to rolling direction for R specimen, b) the plane normal to transverse direction for T specimen.

a) b)

FIG. 14. Mierostructures of Zircaloy-4 specimen after A~,=2.6% fatigue. a) the plane normal to rolling direction for R specimen after 10 cycle~% b) the plane normal to transverse direction for T specimen after 40 cycles.