Intergranular creep fracture in a Cu20%Zn alloy under repetitive discontinuous changes of external stress

Scripta METALLURGICA et M A T E R I A L I A

INTERGRANULAR

CREEP

Czechoslovak

Vol.

26, pp. 1 8 5 7 - 1 8 6 2 , 1992 Printed in t h e U . S . A .

FRACTURE IN A C u - 2 0 ~ Z n ALLOY UNDER CHANGES OF EXTERNAL STRESS M. P a h u t o v & a n d V. S k l e n i ~ k a A c a d e m y of S c i e n c e s , Institute of 616 62 Brno. C2echoslovakia

Pergamon P r e s s Ltd. All rights reserved

REPETITIVE

Physical

DISCONTINUOUS

Metallurgy,

(Received February 27, 1992) ( R e v i s e d A p r i l 13, 1 9 9 2 ) Introduction Although fracture of a c r e e p i n g crystalline material c a n h a v e a n u m b e r of causes, intergranular cavitation remains one of the most prominent. The mechanisms of t h i s p h e n o m e n o n have been studied quite intensively over the past three decades, both experimentally and through theoretical modelling (for a recent review see Riedel (I)). W h i l e t h e m o r e s e a r c h i n g studies in t h i s a r e a have mostly concentrated on idealized constant external conditions of t h e applied stress (monotonic creep), there have been too few s t u d i e s of creep behavior under non-steady conditions. Most of the stress change tests have been conducted to e s t i m a t e the internal stress for recovery c r e e p , o r to s t u d y c r e e p at c o n s t a n t structure or backward anelastic straining ( 2 - 5 ) . T w o m a i n t y p e s of s t r e s s d i p L e s t e x i s t . In the first type, t h e s t r e s s is decreased from steady state c r e e p at a c o n s t a n t i n i t i a l s t r e s s to various final stress l e v e l s . In t h e s e c o n d t y p e of test, t h e stress is decreased from the steady state at various initial stresses to a constant final stress level. If the stress is c h a n g e d , a transient period follows, during which the dislocation structure adjusts to a n e w e q u i l i b r i u m configuration. A v a r i e t y of transient types have been observed (6) d e p e n d i n g on the material and on the sign and magnitude of t h e s t r e s s c h a n g e s . The creep rate after a stress change can be greater or less than the steady state rate appropriate to t h e n e w s t r e s s level and, f o l l o w i n g a s t r e s s dip, an i n c u b a t i o n p e r i o d c a n a r i s e w h e r e t h e c r e e p r a t e is t e m p o r a r i l y zero. In o t h e r c i r c u m s t a n c e s , the creep rate after a dip may be negative with respect to t h e o r i g i n a l c r e e p r a t e . Of course the stress changes can cause not only the changes in t h e c r e e p d e f o r m a t i o n behavior, b u t a l s o in the creep damage development. However. to the author's knowledge, no systematic investigation has been done of this phenomenon. Further, it is e x t r e m e l y Important to i n v e s t i g a t e the reponse in c r e e p b e h a v i o r to discontinuous changes of e x t e r n a l stress d u r i n g all t h e s t a g e s o f c r e e p , i.e. p r i m a r y , secondary and tertiary creep. The present investigation was undertaken to s t u d y t h e e f f e c t of r e p e t i t i v e discont,nuous changes of e x t e r n a l stress on high temperature creep behavior in a Cu-20%Zn alloy, with emphasis on characterizing the evolution of creep cavitation damage and the fracture process.

The Cu-2Oat.~Zn alloy investigated was prepared from component m e t a l s of 99.99~ purity. Flat tensile specimens with a gauge length of 50 mm and a cross-section of ( 9 . 0 x 3 . 2 ) mm z w e r e m a c h i n e d f r o m a h o t r o i l e d s t r i p and, t h e creep specimens were annealed at I O V g K f o r 10 h. T h e average linear intercept g r a i n s i z e , LL, o b t a i n e d from this heat treatment w a s 0 . 3 0 mm. The constant-stress uniaxia] creep tests were conducted in p u r i f i e d dried hydrogen at a t e m p e r a t u r e 773 K and a s t r e s s of 5 0 MPa. C r e e p s t r a i n incmements were measured by a linear variable differential transformer with a strain sensitivity of 10-s; t h e c r e e p c u r v e s w e r e c o n t i n u o u s l y recorded. 1857 0956-716X/92 $ 5 . 0 0 + .00 Copyright (c) 1 9 9 2 P e r g a m o n Press

Ltd.

1858

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6

,o,

I

I I TIME "¢

z

~0

......

Fig. ta,b Schematic illustrations of (a) repetitive stress reduction t e s t and (b) the strain transient f o l l o w i n g an abrupt r e d u c t i o n in t h e a p p l i e d stress o during creep deformation. Time h t represents a particular loading period.

TIME

(i)

(il)

The following two creep normal constant-stress creep fracture with no

testing procedures were utili=ed: c r e e p test; all t h e s p e c i m e n s were stress reductions during loading,

run

to

the

final

stress reduction test; during the creep exposure (Fig. la) t h e a p p l i e d stress was repetitively abruptly reduced by do = o and then increased again up to the initial value of a after the attainment of a period o£ zero creep with no deformation (Fig. lb). The strain increments A~ were comparable for all the stress reductions. These stress reductions were started at various predetermined fractions of the creep life and repeated up to t h e fracture of the specimens.

Assessment of the amount of intergranular creep cavitation damage was carried out using quantitative metallography on longitudinal sections taken from the gauge length of crept specimens. By means of lineal analysis along the grain boundary lines several quantitative parameters o£ intergranular cavitaLion were evaluated /7,8/. The extent of cavitation is quantitatively described by the fraction o£ grain boundary area occupied by cavities, Ao. T h e i n h o m o g e n e i t y o£ cavitation at v a r i o u s g r a i n b o u n d a r i e s can be characteri=ed by the parameter mo expressing the fraction of grain boundaries with observable cavitation. Knowing Ao a n d ao t h e m o s t i m p o r t a n t p a r a m e t e r s of the local extent of cavitation can be introduced, namely the parameter ~o - Ao/~ representing the areal fraction of cavities in cavitated grain boundaries.

Results

and

Discussion

Examples of creep curves recorded for both the creep testing procedures at 50 NPa and 773K are s h o w n in Fig. 2. F o r n o r m a l constant-stress creep tests ( c u r v e s A a n d B) it c a n b e s e e n t h a t t h e p r i m a r y s t a g e is r e p l a c e d by secondary creep in a relatively short time. In fact, the steady state region is well-established and represents the dominant stage with respect to both the fraction of time to fracture and the fraction of the total creep strain. The first indication of fracture is t h e i n c r e a s e in c r e e p rate which occurs at the onset o£ the tertiary stage. This suggests that secondary creep ends when the effect of creep damage processes become significant. Fractography showed that, under the loading conditions investigated, fracture is o f t h e i n t e r g r a n u l a r mode as a result o£ intergranular cavity formation and growth. To establish t h e e f f e c t of r e p e t i t i v e stress changes, three different types o£ stress reduction t e s t s w e r e c a r r i e d out. F i g u r e 2 s h o w s c r e e p c u r v e s 1,2 and 3 for specimens subjected to repetitive stress reductions starting at the beginning o f c r e e p t e s t ( c u r v e I), a t t h e m i d d l e of s t e a d y s t a g e ( c u r v e 2), a n d at t h e o n s e t of t e r t i a r y s t a g e ( c u r v e 3), r e s p e c t i v e l y . Thus, the total number of s t r e s s c h a n g e s was 58 for curve 1, 5 2 f o r c u r v e 2 a n d 15 f o r c u r v e 3. A g a i n independently of the testing regime, the fracture w a s of t h e i n t e r g r a n u l a r mode,

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I

1

I

I

Cu20Zn T =773K ~=50MPa

/~ #

A. B : 6 =const 1.2.3: AS=~. • frocture

z niv)

5

,,
1\

./

..f"

//

~ f

I

"S

I

10

Fig. 2 Strain/time plots showing the effect of repetitive stress change on creep behavior. Creep curves recorded fol, c o n s t a n t stress creep tests, curves A and B and for stress reduction tests ( c u r v e s I, 2 a n d 3).

>'F

\A

5

1859

15

t

20

TIME t x10-2 [ s ] Comparing the results of monotonic creep t e s t s ( c u r v e s A a n d B) w i t h c u r v e 3, t h e r e a p p e a r s to be little effect of stress reductions. On the other hand comparing curves | and 2 w i t h c u r v e s A a n d B, it is e v i d e n t that the resulting strain to fracture for specimens undergoing repetitive stress changes increases significantly above the value obtained from monotonic creep. Further, there appears to be some beneficial effect of non-monotonic creep (curves I a n d 2) o n creep life extension (about 20%). There is, however, some ambiguity in evaluating this effect because the time to fracture of specimens with the stress reductions represents the sum of the particular loading periods. In order to understand the behavior of the stress reduction tests, it is relevant to present first the results of fractographic observations. Scanning electron microscopy (SEM) was used to characterize the features of cavltation structure in t h e f r a c t u r e s t a t e . Under the creep conditions of temperature and stress given, the SEM analysis clearly revealed that the pure intergranular creep fracture occurs via extensive cavitation. Figure 3a shows an SEM image of the creep fracture surface on the speclmen pulled to creep fracture without stress reductions. The SEM micrograph of the creep fracture specimen which was ex~,~sed to repetitive stress changes is s h o w n in F i g . 3b. B o t h f r a c t u r e s u r f a c e s consisted of heavily cavitated grain boundary areas, separated by ridges that appeared to be ruptured ligaments. No significant difference between the details of cavitation is evident when F*g. 3a and 3b are compared. This qualitatlve result suggests that the feature of critical cavitation corresponding to the creep fracture state does not reflect the effect of repetltive d|scontinuous stress changes.

Fig.

3.

SEM (b)

micrographs specimen I,

of creep fracture surface: stress reduction test

(a)

specimen

A,

d

= const

and

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The basic idea of the extent o£ cavitation in t h e f r a c t u r e s t a t e follows from the f r a c t i o n of g r a i n boundary area occupied by cavities (At)f, and the fraction of grain boundary area occupied bF cavities r e l a t e d o n l y to c a v i t a t e d grain boundaries, ( ~ c ) f ffi ( A o ) f l ( m c ) f , respectively.

Grain

size

and

damage

TABLE parameters

Creep condition (LL)f Specimen ...................................................................... O ~ COnSt. A 0.32 mm B 0 . 3 1 mm

1 of

creep

fractured

specimens

(mc)f

(Ac)f

(Ao)f

0.36 0.34

O. 13 0.11

0.36 0.32

Ac, ~ o I (primary stage) 0.20 mm 0.18 0.06 2 (secondary stage) 0.19 ms 0.22 0.08 3 (tertiary stage) 0.31 mm 0.31 0.12 ......................................................................

0.34 0.36 0.39

In Table I the results of q u a n t i t a t i v e damage investigations of creep fractured specimens are summarized. Examination of Tab. I r e v e a l s that both the area fraction of caviLated grain boundaries (Ao)f and the fraction of cavitated grain boundaries (m=)£ appear to decrease markedly for specimens ! and 2 subjected to repetitive stress reduction at the early stages of c r e e p . At t h e s a m e t i m e ( ~ = ) f r a n g e s f r o m 3 2 t o 39~. T h u s , c o n t r a r y to the foregoing results, no substantial effecL of creep testing procedure has been found in t h i s c a s e . Consequently, it c a n be expected that the final stage of s p e c i m e n life will be associated with the critical decrease of the grain boundary cohesion caused by the accumulation of oavitational damage along the prospective intergranular fracture path. The fact that approximately the same values of (~c)f characterize the fracture state need not result in t h e same creep life and/or the s t r a i n to fracture immediately. Clearly, t h e t l m e a n d s t r a i n to f r a c t u r e r e f l e c t the rates at which the deformation and the damage processes proceed. Moreover, it c a n b e eMpected Chat the final stage of the fracture process during the tertiary stage namely the formation and propagation of the main crack, is s t r o n g l y influenced by the long-range coalescence of d a m a g e inside the creeping body by the junction of internal cracks with surface cracks 18 /. T h u s , if t e s t c o n d i t i o n s can alter the morphology and distribution of c a v i t a t i o n , then this "configuration effect" would probably result in s o m e v a r i a t i o n of creep life and ductility.

Hetallographic observations have revealed that, d u r i n g creep exposure of specimens t a n d 2, e x t e n s i v e grain boundary migration (GMB) occurred - Fig. 4. H o w e v e r , it is n o t e x p e c t e d t h a t G H B is fully responsible for the observable decrease in g r a i n s i z e ( L L ) f o f t h e s e specimens (Table I). T h i s phenomenon is evidently caused by dynamic recrystallization, which proceeds by repeated nucleation and limited growth of the new grains (Fig. 5). T h i s contrasts with the mechanism of static recrystallization in w h i c h limited nucleation takes place e a r l y in the process, and then recrystallization proceeds by continuous growth of these nucleated grains until impingement with other growth grains takes place. The difference in mechanism arises directly from the concurrent creep deformation taking place during dynamic recrystallization. This causes work hardening of the growing grains so that, w i t h increasing strain, the driving force for growth is r e d u c e d until growth ceases when the grains reach a size w h i c h is characteristic of the deformation conditions. The concurrent deformation also continually modifies the dislocation structure in t h e d e f o r m e d matrix so that new nuclei may be formed. This mechanism of repeated nucleation and restricted growth leads to two important features of dynamically recrystallimed structures. T h e f i r s t is t h a t if t h e d e f o r m a t i o n conditions change within the unrecrystallized f r a c t i o n of t h e

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Fig.

4.

Optical micrographs showing the appearance of (a) at a specimen fmee surface and, (b) ( s p e c i m e n I, t e n s i o n a x i s is h o r i z o n t a l ) .

Fig.

5.

Optical Specimen

Fig.

6.

Optical mlcPographs s h o w i n g t h e e f f e c t of recrystallization on i n t e r g m a n u l a r creep S p e c i m e n 2 ( t e n s i o n a x i s is h o r i z o n t a l ) .

micrographs showing the appearance |, (b) in S p e c i m e n 2 ( t e n s i o n a x i s

grain boundary migration: inside a creep specimen

of recrystallization is h o r i 2 o n t a l ) .

(a)

grain boundary migration damage: (a) S p e c i m e n I,

in

and (b)

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material, either by a change in t h e externally applied stress or strain rate or because the deformation becomes substantially accommodated iF t h e r e c r y s t a l l i 2 e d fraction of the material, then further nucleation may cease in the recrystalli2ed material. This means that recrystallized grains surrounding the r e m n a n t s of t h e original grains become stabilized. The second feature is t h a t the continuing deformation can result in the critical stored energy being attained in t h e a l r e a d y r e c r y s t a l l i z e d material so that recrystallization takes place again. Repeated cycles of recrystallization may therefore occur with increasing strain. In v i e w o f t h e f a c t t h a t dynamic recrystallization provides an additional softening process to d y n a m i c recovery, its o c c u r r e n c e can be reflected by an increase in creep rate (c.f. Fig. 2 curves t and 2). No GBM and recrystalli2ation have been found for specimens without stress changing or for specimen 3, w h i c h underwent stress change only when the tertiary stage was reached. This point requires further study. Finally, a question naturally arises about the role of GBM and recrystallization in the damage progress. White and Rossard /9/ f i r s t p o i n t e d out that the migration of grain boundaries during recrystallization t e n d s to blunt growing cracks and results in the development of m o r e i r r e g u l a r cavities w i t h an a s s o c i a t e d increase in d u c t i l i t y . This work indicates that the situation is more complex and that the refinement of g r a i n size produced by dynamic recrystalli2ation is itself an important contributory factor. Further, GBM and recrystallization quickly isolate some grain boundary cavities and microcracks within the grain interiors and prevent their g r o w t h (Fig. 6). A c c o r d i n g l y , these inLragranular cavities do not Lake part in the formation of the intergranu]ar f r a c t u r e p a t h in t h e f i n a l s t a g e o f f r a c t u r e p r o c e s s . On the other hand, an advanced stage of cavity coalescence a s t r i n g of c a v i t i e s or microcracks may pin down the grain boundary against migration or recrystalli2ation.

No. for

This work 24 156-92. technical

was

The

supported by authors wish

the C2echoslovak A c a d e m y of S c i e n c e s u n d e r G r a n t to e x p r e s s t h e i r a p p r e c i a t i o n to Dr. V. S u s t e k

assistance.

Re.fergnces 1. 2. 3. 4. 5. 6. 7. 8. 9.

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