Effect of strain rate on the cyclic hardening of Zircaloy-4 in the dynamic strain aging temperature range

ELSEVIER

Materials Science and Engineering A234-236

(1997) 834-837

Effect of strain rate on the cyclic hardening of Zircaloy-4 dynamic strain aging temperature range M.G. Moscato a, M. Avalos a, I. Alvarez-Armas a Institute de Fisica Rosario, b Forschungszentrum Karlsruhe,

in the

a, C. Petersen b, A.F. Armas a,*

CONICET-UNR, Bv. 27 de Febrero 210 Bis, 2000 Rosario, Argentina Institut fiir Materialforschung II, P.B. 3640, D-76021 Karlsruhe, Germany

Received 11 February 1997

Abstract Low cycle fatigue (LCF) tests were conducted in Zircaloy-4 with a total strain range of f 0.5% in the temperature range 573-873 K where dynamic strain aging (DSA) manifestations in uniaxial tensile tests were reported. A time dependent linear cyclic hardening stage is the principal feature observed in this alloy. The cyclic hardening rate exhibits a peak in this temperature range. In order to evaluate the strain rate effects on the cyclic hardening, tests at strain rates 2 x 10W3 sP ’ and 2 x 10W4 sP ’ were performed. The location of the cyclic hardening rate peak depends on the strain rate suggesting the operation of a thermally activated mechanism. Transmission electron microscopy (TEM) observations show that the linear hardening stage is characterized by a two-dimensional wall structure that evolves from the activation of two slip systems. DSA takes place during strain-controlled fatigue deformation at the same temperature range of that occurring during tensile deformation. 0 1997 Elsevier Science S.A. Keywords:

Strain aging; Fatigue; Zircaloy-4

1. Introduction The more

well

known

manifestations

of static

(SSA)

and dynamic strain aging (DSA) during monotonic tensile deformation of alloys are yield points in the stress-strain curve, appearance of plateaus or peaks in the flow stress-temperature diagram, serrations (Portevin-LeChatelier effect), abnormal strain rate sensitivity and anomalous strain hardening behavior. Information regarding the influence of DSA during low cycle fatigue of alloys is relatively limited. Nevertheless, some information exist on high-temperature Ni-base alloys and steels. Several authors have observed dramatic hardening occurring during cycling at intermediate temperatures in high-temperature alloys and stainlesssteels.A peak in the maximum tensile stressof the hysteresis loop in low-cycle fatigue (LCF) tests is the typical manifestation observed in the DSA temperature regime. Armas et al. [l] have investigated fatigue deformation in austenitic stainless steels at elevated temperatures. They have found that cyclic hardening * Corresponding author. Tel.: + 54 41 853200; fax: + 54 41 821772; e-mail: [email protected] 0921-5093/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved. PZI s0921-5093(97)00405-x

occurred since the beginning of the tests and also that, at intermediate temperatures, the hardening period had a longer duration than at lower or higher temperatures. As a consequenceof the higher duration of the hardening period, a hump in the temperature dependence of the saturation peak stress could be observed for this alloy. Similar behavior of the saturation stress was observed in a perlitic eutectoid steel [2] and in two high-temperature Ni-base alloys, namely, Hastelloy X [3] and Haynes 188 [4]. Abnormal cyclic hardening did occur for these materials always in the temperature range where DSA took place. Nevertheless, none of these authors reported a temperature dependence of the cyclic hardening rate. In a previous paper [5], it was reported that the alloy Zircaloyd showed a pronounced linear cyclic hardening in the temperature range where typical manifestations of DSA were present. The cyclic hardening rate is strongly temperature dependent and shows a peak in the temperature range 573-873 K. The objective of the present work is to examine strain rate effect on the cyclic hardening observed in Zircaloy4 at temperatures where DSA occurs. The analysis of the substructural changes occurring during the tests is also a purpose of this work. This article is part of a

M.G.

Moscato

et al. /Materials

Science

detailed investigation aimed at understanding the effect of DSA on the LCF behavior of materials with different crystal structures.

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p u 0.015E e

2. Experimental

procedure

The investigations were carried out on shallow hourglass shaped Zircaloy-4 samples prepared in accordance with ASTM B550 Grade 704, (for detailed testing conditions and the survey of specimens, see [6]). The samples were tested in the recrystallized condition with equiaxed grains and main grain diameter 20 mm. The chemical composition of the alloy is (in wt.%): Sn-1.37, Fe-0.14, Cr-0.10, C-0.01, o-0.14, N-0.004, H-20 ppm, Zr-balance. LCF tests under total strain control were performed in the temperature range 573-873 K. The tests were performed in air with a total strain range Ah = 0.01 and total strain rates 2 x 10 - 3 s - ’ and 2 x 10-4 s-i. In order to correlate the mechanical behavior with the dislocation structure of fatigued specimens, thin-foil discs were prepared from sections cut parallel to the tensile axes. The discs were electropolished with a solution of 10% perchloric acid, 35% n-butanol and 55% methanol. The foils were examined in a Philips EM 300 transmission electron microscope (TEM) operating at 100 kV.

3. Experimental

results and discussion

The changes in peak tensile stress of each hysteresis cycle with the number of cycles during low cycle fatigue are shown in Fig. 1. Tests were performed with a total strain range 0.01 and total strain rate 4 = 2 x lop3 s- 1 at temperatures from 573 to 873 K. To make clear the figure not all the tests were plotted. 220 _ As+ = 0.01 . strain

rate = 2 x 10%'

zooc$

/+-----Y

,,:

!-yfl c--+ &.-.4-o-*-*

.

a a, 160 $ 2 140

3

-m-*--t713K -A-.-O-

ryv--~y -5

.

\ .

.

fP---o-----o-o-,

573 K 623 K 758 K 823 K 873 K

‘0

---o I 0

I 1000

I 500 Number

1. Cyclic

tensile

of Cycles,

stress response

I

600

-

I-

650

I,

700 Temperature,

Fig. 2. Cyclic hardening modulus) as a function

rate (normalized of temperature

I,

I

750

800

a,,

850

s IO

K

with respect to the Young’s and strain rate.

Three stages are distinguished in all curves obtained from Zircaloy4 cycled up to failure: a transitional cyclic hardening with a high but decreasing hardening rate; a principal hardening with a noticeable linear dependence of the peak tensile stress with the number of cycles and a third stage which represents a period of decreasing rate of hardening where the stress goes through a maximum and then falls continuously due to the specimen failure. At the end of the transitional hardening stage and for temperatures 573-773 K Zircaloy-4 exhibited about 17 pet cyclic hardening. With increasing temperatures above 773 K the initial hardening is less pronounced. The cyclic hardening observed during this initial stage could be attributed to the typical hardening expected to take place in an annealed material [7]. At 573 K a very small softening that occupies most of the cyclic life and culminates in the failure of the sample is observed after the initial hardening stage. At 873 K the initial hardening is the smallest and no hardening or softening effect takes place after the first cycles and nearly up to fracture. This work is primarily concerned with the principal hardening stage characterized by a pronounced cyclic cycles. In order to determine the influence of the strain

rate on the cyclic hardening rate, tests were also per-

A

so-

Fig.

-

t

E --t” 2 120I p loo-

-I.

550

hardening that depends linearly with the number of

I-.

’ -.-.-.-0-e•-

z @ O.OlOJ 2 P 5 0.005e 9 g g o.ooo0

I 1500 N

at different

temperatures.

formed with total strain rate 4 = 2 x 10~ 4 s - ‘. Fig. 2 shows the variation with temperature of the cyclic hardening rate parameter AalAN for Zircaloy-4 cycled at two different strain rates. Aa is the peak tensile stress increase over the linear interval AN of the principal hardening stage. The curves were normalized with respect to the Young’s modulus, E, to account for its variation with temperature. The Young’s modulus and its temperature dependence were obtained from Rosinger et al. [8].

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As can be seen from Fig. 2 the cyclic hardening rate is markedly temperature and strain rate dependent. At the lower strain rate of 2 x 10 ~ 3 s ~ ’ the cyclic hardening rate increases with increasing temperature in the range from 573 to 728 K. A similar dependence of the cyclic hardening rate on temperature is seen in the temperature range from 573 to 773 K for the higher strain rate of 2 x lo- 3 sP i. Moreover, the cyclic hardening rate curve exhibits a peak at a certain temperature. Increasing the strain rate of the test increases the peak temperature, i.e. the cyclic hardening rate shows a peak at 728 K for a strain rate of 2 x lop4 s-’ and at 773 K for a higher strain rate of 2 x 10 - 3 s ~ ’ . At temperatures in the range from 573 K to about 730 K, the cyclic hardening rate for 2 x 10 P4 s ~ ’ is higher than that for 2 x 10W3 ss’. It was reported earlier that Zirconium and its alloys exhibits anomalous strain hardening behavior in the range from 573 to 873 K [9,10]. Slower strain rate produces faster strain hardening and vice versa. Such an inverse dependence has been well established as one of the typical features of tensile deformation accompanying DSA. The results shown in Fig. 2 clearly indicate that DSA also takes place during fatigue deformation in Zircaloy4, and that the prominent cyclic hardening shown in Fig. 1 is caused by DSA. It appears that this linear cyclic hardening and its strong temperature dependence is yet another important aspect of DSA in hexagonal alloys. Tsuzaki et al. [2] have investigated the LCF behavior of pearlitic steels and proposed a model according to which the temperature range for DSA to occur during fatigue deformation must be lower than that during monotonic tensile deformation. The plateaus which appear when both the yield stress and the strain hardening rate are plotted against temperature are well known manifestations of DSA in monotonic tensile tests. Armas et al. [5] have reported yield stress and strain hardening rate plateaus in uniaxial tensile tests of Zircaloy-4. They have reported a temperature region

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ing two (a) systems of long straight screw dislocations. As a result of this, IV, may be formed by the interaction between these two active slip systems. Besides W,, Fig. 3(b) shows another set of parallel walls (W,) aligned to (a,). This two-wall dislocation structure resemble the so-called ‘labyrinth structure’ observed in cyclically deformed fee materials. The structure observed in Fig. 3(a) is similar to that found in failed samplesfatigued at 573 K where no linear cyclic hardening is observed. In this work it is proposed that the W, wall structure formed during the transitional hardening stage will stay during cycling (only the walls will become denser) if the principal hardening stage does not take place. However, as a consequence of strong dislocation trapping by solute atoms a second wall structure W, will be formed. The principal hardening stage (Fig. 1) and the negative strain rate dependence of the cyclic hardening rate

between 573 and 873 K for the occurrence of such plateaus. The results of the present work demonstrate that in Zircaloy-4 the temperature range for DSA to occur during fatigue deformation is coincident than that during monotonic tensile deformation. Microstructural observations by TEM were performed to characterize the dislocation structure occurring in specimens showing the higher linear cyclic hardening rate. Fig. 3a and b show the microstructure developed in samples fatigued at 713 K and strain rate 2 x 10 - 3 s - ’ up to 50 and 500 cycles, respectively. After the end of the transitional period and just at the beginning of the linear cyclic hardening (Fig. 3(a)) the structure consists mainly of edge dislocation walls (IV,) almost perpendicular to one primary slip system, i.e. (al) and channels of lower dislocation density contain-

Fig. 3. TEM micrographs of Zircaloy-4 cycled at 713 K and kt = 2 x lop3 sP ’ up to (a) 50 cycles and (b) 500 cycles.

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(Fig. 2) are considered to result from an increase in strain hardening rate taking place during each cycle of the test. An increase in dislocation density of (al) type may cause this effect. The matrix would be hardened by the presence of the family of dislocation walls IV,. During DSA, mobile screw dislocations of this type would become aged by solute atmospheres and additional dislocations have to be generated to impose the same total strain during succesive cycles. It is difficult to determine the rate controlling mechanism from the temperature and strain rate corresponding to the peak cyclic hardening rate. In fact, it has not already been proved whether the strain rate and temperature corresponding to these peaks are related by a simple Arrhenius equation of the type i: = A .exp( - Q/RT).

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References [I] A.F. Armas, 0. Bettin, LA. Armas, G.H. Rubiolo, J. Nucl. Mat. 155-157 (1988) 646. [2] K. Tsuzaki, Y. Matsuzaki, T. Maki, I. Tamura, Mater. Sci. Eng. Al42 (1991) 63. [3] R.V. Miner, M.G. Castelli, Metall. Trans. A 23A (1992) 551. [4] K. Bhanu Sankara Rao, M.G. Castelli, J.R. Ellis, Scripta Metall. 33 (1995) 1005. [5] A.F. Armas, I. Alvarez, G. Moscato, Scripta Metall. 34 (1996) 281. [6] A.F. Armas, I. Alvarez-Armas, Zirconium in the Nuclear Industry, ASTM STP 939 (1987) 617. [7] M. Klesnil and P. Lukas, Fatigue of Metallic Materials. Materials Science Monographs, vol. 7, Elseiver, Amsterdam, 1980, p. 17. [8] H.E. Rosinger, I.G. Ritchie, A.J. Shillinglaw, Atomic Energy of Canada Limited, Report AECL-5231, September 1975. [9] A.M. Garde, E. Aigeltinger, B.N. Woodruff, R.E. Reed-Hill, Metall. Trans. A 6A (1975) 1183. [lo] J.L. Derep, S. Ibrahim, R. Rouby, G. Fantozzi, Acta Metall. 28 (1980) 607.