Effect of annealing on high temperature creep behaviour of a dispersion-strengthened A1 alloy

Scripta METALLURGICA

Vol. 23, pp. 1425-1430, 1989 Printed in the U.S.A.

Pergamon Press plc All rights reserved

EFFECT OF ANNEALING ON HIGH TEMPERATURE CREEP BEHAVIOUR

OF A DISPERSION-STRENGTHENED A1 ALLOY

A. O r l o v & , K. K u c h a # o v ~ , 3 . ~adek C z e c h o s l o v a k Aca0emy of S c i e n c e s , I n s t i t u t e of Physical 616 62 B r n e , C z e c h o s l o v a k i a

Metallurgy

(Received May 4, 1989) (Revised June 6, 1989) 1, I n t r o d u c t i o n

Ccmmerclally prooucec IN 9052 alloy belongs to the group of dispersion strengthened alumlnium alloys, which are prepared by mechanical alloying of the criginal pure aluminium or aluminium solid solution powder with carbon supplied in appropriate carbon containing additions. Its high strength even at elevated and high temperatures is ascribed to a very complicated defect structure resulting from mechanical alloying, including the influence of strengthening by f i n e c a r b i d e as w e l l as o x i d e d i s p e r s i o n s t h a t were formed i n t h e technological process of alloy production. R e c e n t l y , we i n v e s t i g a t e d the high temperature creep behaviour of the a l l o y IN 9052 a t t e m p e r a t u r e s 6 2 3 , 673 and 723 K w i t h i n t h e a p p l i e d s t r e s s i n terval from 1 7 . 5 t o 75 MPa [ 1 ] . Our e x p e r i m e n t a l range p a r t l y coincided with t h a t ol O t s u k a , Abe and H o r i u c h i [2] who i n v e s t i g a t e d the behaviour of this a l l o y ~n h i g h t e m p e r a t u r e c r e e p , t o o . The o n l y d i f f e r e n c e was p r o b a b l y i n t h e initial h e a t t r e a t m e n t - 2 h a n n e a l i n g at 823 K - t o w h i c h t h e a u t h o r s s u b j e c t e d t h e i r s p e c i m e n s p r i o r t o c r e e p . A c o m p a r i s o n o f b o t h d a t a s e t s has shown a great difference in high temperature creep behaviour, i.e., in the charactero f ti~e c r e e p c u r v e s and t h e a p p l i e d s t r e s s d e p e n d e n c e o f t h e minimum c r e e p r a t e To s u p p o r t t h e i d e a o f t h a t t h i s d i f f e r e n c e i s caused by t h e e f f e c t of anneali n g and t o get more k n o w l e d g e o f t h e changes i n s t r u c t u r e to which the diff e r e n t behavlour could be ascribed, we performed identical creep tests and structure investigations on specimens in the as received (AR) and heat treated (HT) conditions.

2. Experimental

techni,~u,e

The alloy IN 9052 produced by NOVAMET Co., USA, had the nominal chemical composition (mass. ~) 4 Mg, 0.2 Fe, 0.5 Si, 1 . 1 C and 0.8 O. It was received in the form of an extruded bar 60 mm21n diameter from which the creep specimens 50 mm in gauge length and 8.0x3.2 mm in cross-sectlon were machined. The majority of the specimens were creep tested in the as received (AR) condltlons,[l] , only a small part of them was subjected to 2 h annealing (HT) at 813 K (i.e. under the conditions applied by Otsuka et el. [2]) in silicon tubes, followed by cooling in air, prior to creep tests. Isothermal creep tests in tension were performed under the constant applied stress, the creep elongstlon was continuously recorded. The temperature 673 K and range of applied stresses were chosen with respect to a possible comparison with the data of Otsuka et el. E2]. The structure of the specimens was investigated by transmission electron microscopy and selected area electron diffraction. The microscopical observations were supplemented by measurements of Vickers hardness HV I0.

1425 0036-9748/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc

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CREEP OF A1 ALLOY

3.

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Results

Fig. 1 illustrates the difference i n s t r e s s d e p e n g e n c e e o f minimum c r e e p r a t e o b s e r v e d on AR and HT s p e c i m e n s ; t h e d a t a o f O t s u k a e t e l , [2] a r e shown f o r a c o m p a r i s o n , I t i s e v i d e n t from t h e f i g u r e t h a t o u r HT s p e c i m e n s d a t a c o r r e s p o n d r a t h e r w e l l t o t h e c r e e p d a t a g i v e n by O t s u k s e t e l , [2], t,e,, it is primarily t h e a n n e a l i n g w h i c h c a u s e s a v e r y s t r o n g change i n t h e minimum c r e e p r a t e E~ a c h i e v e d i n c r e e p t e s t s i n t h e r a n g e o f i n v e s t i g a t e d creep conditions° The s t r o n g I n f l u e n c e o f a n n e a l i n g , on £m i s c l o s e l y related to the inf l u e n c e on t h e c r e e p c u r v e s o b t a i n e d on t h e AR and HT s p e c i m e n s ° A c o m p a r i s o n o f c r e e p c u r v e s c o r r e s p o n d i n g t o t h e a p p l i e d s t r e s s e s ¢ - 30 end 40 MPa, F i g , 2 and 3 , r e s p e c t i v e l y , shows • s t r o n g l y different character of creep curves o f t h e AR and HT m s t e r t a Z S o The m a j o r p a r t o f t h e c r e e p c u r v e s o f AR i s t h e primary stage which, after r e a c h i n g t h e minimum c r e e p r a t e , Cm, i s f o l l o w e d by v e r y s h o r t and u n i m p o r t a n t t e r t i a r y and f r a c t u r e , T h o s e o f HT, c o r r e s p o n d i n g

i

IN 9052 T = 673K

i"'1

.

i

S t r e s s d e p e n d e n c e s o f minimum c r e e p r a t e ~m o f s a m p l e s AR ( a s r e c e i v e d material) and HT ( h e a t t r e a t e d ) , Data o f O t s u k a e t e l ° [ 2 ] e r e shown f o r a comparison,

~,"

f/

,R

FIG. i

i

1¢"

~

1~e

~

ld?

~r

./ /'~

3b

Z~ Ofsuko et ol. [2] HT { 813K/2h)

do

STRESS 5 [MPQ]

Q25 u Q20

sobo ~ ,

T=B73K ~=40MPo

_z 0.15 002 HT

w Z (,')

T= 673K

U3

@= 3 0 M P o

/~ Q~

15~o

,

,

l

/

/

0.10

Q01

4

I

2

TIME t x lO'S[s] FIG. 2 Creep curves of samples AR and HT

for

~ ,= .30 MPe.

TIME t Es] FIG, 3 Creep curves of sempZes AR and HT f o r ¢ - 40 MPao

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CREEP OF A1 ALLOY

1427

to h i g h e r a p p l i e d s t r e s s e s ( e . g . F i g . 3 ) , have a s h o r t p r i m a r y , an i n f l e x i o n p o i n t of minimum creep r a t e and a long t e r t i a r y i n which a r e l a t i v e l y extensive s t r a i n a p p e a r s ; at the l o w e s t a p p l i e d s t r e s s ~ • 30 HPa ( F i g . 2 ) , however, the creep curve of HT m o s t l y c o n s i s t s of p r i m a r y c r e e p , f o l l o w e d r a t h e r s u d d e n l y by a s h o r t t e r t i a r y and f r a c t u r e , i . e . , the c h a r a c t e r i s s i m i l a r to t h a t of AR. As • r e s u l t of t h i s c h a r a c t e r , the creep s t r a i n to f r a c t u r e of HT at h i g h s t r e s s e s i s much g r e a t e r than t h a t of AR (see data summarized i n Tab. 1 ) .

ii

Tab.

1. T o t a l

creep s t r a i n

to f r a c t u r e

~, [°/=] IMPel

AR i

30 35 40 45

i

i

HT

1.10

1.84

1.08 1.40 1.96

11.00 23.28 47

The s t r u c t u r e i n the AR and HT specimens p r i o r - t o and a f t e r the creep exposures at 30, 35, 40 and 45 MPa (see examples in F i g s . 4 and 5) i s composed of v e r y f i n e ( s u b ) g r a i n s , c o n t a i n i n g a r e l a t i v e l y high d i s l o c a t i o n d e n s i t y and e l o n g a t e d p a r t i c l e s of AI.C~o Q u a l i t a t i v e changes in the s t r u c t u r e , which could be d e t e c t e d i n HT, ~r~ t h a t the ( s u b ) g r a i n s are more d i s t i n g u i s h e d , the d i s l o c a t i o n d e n s i t y seems to be s m a l l e r and the c o m p l i c a t e d , c h a o t i c s t r u c t u r e s , t h a t were b a d l y r e s o l v e d i n the "moderate" r e s o l u t i o n imaging mode of TEM i n AR ( F i g . 5 b ) , d i s a p p e a r e d ( p a r t i a l l y or t o t a l l y ) i n HT. ~n the HT specimen subJected to creep at 30 MPa ( t i m e to f r a c t u r e t~ = 4.04x10 s ) , a c o a r s e n i n g of A1AC ~ a c i c u l a r p a r t i c l e s was observed and a new t y p e of r a t h e r s p h e r i c a l p a r titles appeared, w i t h which the d i s l o c a t i o n s can i n t e r a c t perhaps by the mechanism suggested by A r z t [ 3 ] , F i g . 5d. On the o t h e r hand, no d i s l o c a t i o n c o n f i g u r a t i o n s u p p o r t i n g any mechanism of d i s l o c a t i o n i n t e r a c t i o n s w i t h the a c i c u l a r A14C 3 p a r t i c l e s was observed i n the m i c r o g r a p h s .

FIG. 4 S t r u c t u r e of IN 9052 a l l o y p r i o r to creep ( a ) , (b) AR (as r e c e i v e d s t a t e ) ( c ) , (d) HT (2 h a n n e a l i n g at 813 K)

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FIG, 5 Structure of IN 9052 a l l o y a f t e r high temperature creep t e s t i n g : ( ~ ! AR ~ 35 MPa (~, AR; ~ : 30 MPa ( c ) HT, ~ = 35 MPa (d) HT, 6 = 30 MPa The q u a n t i t a t i v e characteristics of s t r u c t u r e i n the AR and HT specimens are summarized i n F i g . 6. The V l c k e r s h a r d n e s s , which can be understood as an over-all characteristic of a l l the changes, i n d i c a t e s a degree of s o f t e n i n g i n a n n e a l i n g . I n c r e e p , a s l i g h t f u r t h e r change ( h a r d e n i n g ) proceeds i n HT o n l y , w h i l e i n AR the hardness remains r e l a t i v e l y unchanged, except the sample subJ e c t e d to the l o n g e s t creep t e s t (6 = 30 MPa, t~ = 2 . 3 1 x 1 0 " s ) , which seems to be s o f t e n e d to the l e v e l of HT. The a n n e a l i n g r e s u l t s in a s m a l l i n c r e a s e of the ( s u b ) g r a i n s i z e ~ , but p r o b a b l y no f u r t h e r changes appear i n the course of c r e e p . The a n n e a l i n g p r o b a b l y m o d i f i e s a l s o the d i s p e r s o i d c o n t e n t - i n c r e a s i n g ( s l i g h t l y ) the number of AIAC~ p a r t i c l e s in the unit volume, Nv. While in AR Nv remains r e l a t i v e l y unchsngeB i n the process of creep, the changes in HT i n d i cate a continued p r e c i p i t a t i o n process - new p a r t i c l e s appear ( t h e i r number i s growing with increasing time t£ f r a c t u r e t i ) and coarsen while coagulating (creep at 30 MP~, t~ = 4,O,~x10~ s ) . The q u a n t i t a t i v e i n v e s t i g a t i o n of s t r u c t u r e was l i m i t e d o n l y to the above three characteristics that could be meaeureO with sufficient reliability in the very complex structure of the alloy, We bear in mind that the description of the structure is very incomplete, as not ell factors that could play a role in strengthening of the alloy could be evaluated. L i

l

'

FIG, 6 Dependences of hardness HV 10, ( s u b ) g r a i n s i z e d and number of A1.C_ d i s p e r s o t d p a r t i c l e s per u n i t volum~ ~v on a p p l i e d s t r e s s ~ end t i m e to f r a c t u r e i n creep t ~ .

1.00

o HT

bx I

I

@ 0 30 40 50I1 5 [MPa]

10 z

10~

tf IS]

10s

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CREEP OF AI ALLOY

~.

1429

Discussion

The present analysis of the influence of heat treatment on the creep behavlour of the IN go52 alloy has been based on the ideas about the properties of mechanically alloyed AI-M 9 alloys proposed by Kim [4]. Followin 9 them these types of alloys are strengthened primarily by the structure resulting from mechanical alloying, which consists of very fine (sub)gralns containing a rather dense end complex dislocation structure, while dispersion strengthening is only of a secondary importance. Let us suppose that the flow stress corresponding to a certain value of ~ is a sum of two components, ~* and ~ , where e" is the effective stress driving the deformation process. The internal stress 6~ opposing the deformation process consists of several components representing the contributions of various strengthening mechanisms, i.e.,

The c o m p o n e n t s o f ~; t a k e n i n t o a c c o u n t i n eq. ( 1 ) a r e as f o l l o w s : 6~ - d i s l o cation structure strengthening. ~ - precipitation strengthening. ~,~ s t r e n g t h e n i n @ by t h e m e c h a n i c a l l y a l l o y e d f i n e - g r a i n e d structure. A similar assumption of a flow stress consisting of several contributions has been a c c e p t e d a l s o by 3ang 9 et e l . [5]° In Fig. 7 the difference o f f l o w s t r e s s e s &~ = s.~- ~,, as o b t a i n e d from the ¢~) c u r v e s i n F i g . 1. i s shown i n d e p e n d e n c e on t h e minimum c r e e p r a t e . ~mo I f we s u p p o s e t h a t t h e minimum c r e e p r a t e c~ i s a u n i q u e f u n c t i o n o f t h e effective s t r e s s ¢ ' . &~ may be u n d e r s t o o d as a d i f f e r e n c e in ~ in both sets o f s a m p l e a . 1.e°, ~,~

=

A(~])

+

A~'I~

+

A~'MA

(2)

.

The d e p e n d e n c e o f ~ on C~ p r o b a b l y r e f l e c t s a l s o t h e d e p e n d e n c e on t h e time of creep exposure, which can be represented here by time to fracture %~ . Fig. 8 shows that in the interval of experimental data a single one-to-one correspondence between %~ and Em can be admitted for both AR and HT specimens. Taking this into account, we can identify the b~ value corresponding to the

107 I

l

- - T ~

i

10~

Q W

~'\ I

~

i

i

[

i

ii

T = 673 K

k-

10~

~

o

~o

,,, 10"~

• AR o HT

10~ I

I(]7

I

IG 6

I

105

- - ,

l

1(j4

MINIMUM CREEP RATE tm [ s'l]

Relation stresses ~rn"

FIG. 7 between difference of flow 1~61 and minimum c r e e p r a t e

10.9

16B

I~5

I04

MINIMUM CREEP RATE ~m [s'~] FIG, B Time to fracture %~ plotted against minimum creep rate &~,

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CREEP OF A1 ALLOY

Vol. 23, No. 8

shortest tests with the change caused by the annealing. This value, equal to 20 MPa. should essentlally represent the change in ~ A . as it is accompanled by a small change in (sub)graln size and also by some "recovery" in re91on of (sub)graln boundaries, resulting in narrower and more distinct boundaries in HT specimens. However. some contributions of ~s~ and ~6v in annealing should be admitted, too. According to Fig. 7. the difference in flow stress reduces its absolute value with decreasing Em and/or with the growing time t~ . This means that the effect of annealing disappears during creep, i.e.. AR and HT materials have a tendency to approach identical structures and creep behavlour. In accord with eq. (2) this may be realized by time dependent behavlour of =6~ and =~v . i.e.. different degrees of dislocation recovery and precipitation strengthening in HT and AR. while =e,~ may remain almost unchanged (the (sub)graln size remains constant). The present interpretation of the effect of prior-to-creep annealing and recovery and ageing in creep on the flow stress corresponding to the minimum creep rate Lm is in accord with the findings of Kim [4]. This author's data indicate a strong change in strength in connection with a very small change in (sub)grain size. too - the increase of (sub)graln size from 0.3 to 0.4 ~m in an extruded material is associated with a change in room temperature tensile strength from 580 to 510 HPa. Such a strong change in strength cannot be ascribed to the effect of grain or subgrain size as observed in conventional materials. An important point is probably the degree of recovery in the (sub)boundary region. The recovery decreases the stored deformation energy and thus it is connected with softening of the material [~ . The annealing leads to a substantial increase in ductility of the materlal. Very early an internal instability begins to develop which results in a rather long tertiary stage and finally in fracture. Qualitatively. this may be an indication that some type of barrier, which hinders the deformation process in the AR material, was removed, or its strength was weakened, by the annealing, and thus the elementary deformation process can apreao over longer distances. Unfortunately. our structural data do not give an answer to this proble¢. Following the present observation, we can suggest that the barriers in question might be related to the effect of carbon and oxygen (impurity clusters?) and carbides (ultraflne. not yet aclcular particles) and oxides in the complex mechanically alloyed structure. Annealing leads to transformation of these ultraflne clusters and particles to s form which is not so effective in strengthening the alloy structure, and thus the structure can be further weakened by processes of recovery. Re fere.nces I. A.Orlov~. K.Kuchafov~ and O.~adek. Metallic Mater. 27. 3 (1989). 2. M.Otsuka. Y.Abe and R.Horluchl. Proc. Third. Int. Conf. on Creep and Fracture of Engineering Materials. Ed. B.Wllshire end R.W.Evans. p. 307. The lnstA Metals. London (1987). 3. O.H.Schroder and E.Arzt. Scr. Metal. 19. 1129 (19B5). 4. Y.W.KIm. Proc. Conf. PM,Aerospsce Materials. Bern 1984. paper No. 35. NPR-Publ. Service Lid.. Shrewsbury. U.K. (1985). 5. G.Oangg. O.Va~gyura. K.Schr~der. M.~les~r and M.Bestercl. Proc. lnt. PN Conference. Dusseldorf 1986..p. 989. ed. V.A.Kayser and W.O.Hupman. Horizons of Powder Metallurgy. Dusseldorf (1986). 6. H.O.NcOueen and E.Evangellsta. Czech. O. Phys.. B 38. 359 (lgBB).