THE INFLUENCE OF AGEING IN THE MARTENSITE PHASE ON THE INTERNAL FRICTION IN Cu-Zn-Al ALLOYS

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THE INFLUENCE OF AGEING IN THE

MARTENSITE PHASE ON THE INTERNAL

FRICTION IN Cu-Zn-Al ALLOYS

J. van Humbeeck, A. Hulsbosch, L. Delaey, R. de Batist

To cite this version:

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JOURNAL D E P H Y S I Q U E

Colloque C10, supplement au n012, Tome 46, dBcembre 1985 page C10-633

THE INFLUENCE OF AGEING IN THE MARTENSITE PHASE ON THE INTERNAL FRICTION IN Cu-Zn-A1 ALLOYS

J. VAN HUMBEECK, A , HULSBOSCH, L . DELAEY AND R . D E B A T I S T * Dep. o f

Met.

and Mat. Eng. K. U. L e u v e n , d e C r o y l a a n 2 , 3030

Heverlee,

B e l g i u m

' D e p . o f M a t e r i a l s S c i e n c e , SCK-CEN, B o e r e t a n g 2 0 0 , 2400 MOL, B e l g i u m and RUCA, M i d d e l h e i m l a a n , 2020 A n t w e r p e n , B e l g i u m

Resume.

-

_{La c a p a c i t e d'amortissement dans l e s a l l i a g e s martensitiques Cu-Zn-A1 passe}

par un maximum pendant l e viei 11 i ssement dans 1 a phase martensitiaue. ~e processus e s t ' thermiquement a c t i v e . On a nomme c e t t e phenom~ne l e "peaking e f f e c t " du second t y p e par analogie au peaking e f f e c t du premier type d e j i p u b l i e .

Abstract.

During ageing below A,, t h e damping capacity o f CU-Zn-~'l m a r t e n s i t e goes through a maximum as a f u n c t i o n o f t h e ageing time. This process i s found t o be t h e r m a l l y a c t i v a t e d . The observed e f f e c t i s c a l l e d t h e peaking e f f e c t o f t h e second k i n d w h i l e r e f e r r i n g t o t h e peaking e f f e c t o f t h e f i r s t k i n d a l - ready described e a r l i e r .

I

-

INTRODUCTION

The m a r t e n s i t i c phase of t h e Cu-Zn-A1 a1 loys e x h i b i t s a very h i g h damping capacity which makes t h i s a l l o y very a t t r a c t i v e f o r i n d u s t r i a l a p p l i c a t i o n s . A l l o y s thus i n - v e s t i g a t e d have m a r t e n s i t i c t r a n s f o r m a t i o n temperatures Ws and As,well above room temperature. This c o n d i t i o n however, r e q u i r e s special heat-treatments i f one wants t o conserve t h e h i g h damping f o r a prolonged time. I n r e c e n t p u b l i c a t i o n s /1,2/ i t i s demonstrated t h a t t h e heat-treatment o f t h e a l l o y should be such t h a t t h e ex- cess vacancies r e t a i n e d a f t e r quenching t h e h i g h temperature beta-phase anneal o u t i n t h e beta-phase p r i o r t o transforming t o martensite. This i s done by r e t a i n i n g t h e samples a s u f f i c i e n t l y l o n g t i m e a t low temperatures i n t h e beta-phase /3/. Also r e c e n t l y l a r g e i n t e r e s t has oeen given t o t h e s t a b i l i s a t i o n o f t h e m a r t e n s i t i c phase r e s u l t i n g from ageing. During s t a b i l i s a t i o n t h e m a r t e n s i t i c phase becomes more s t a b l e r e l a t i v e t o t h e R-phase and thus t h e reverse t r a n s f o r m a t i o n temperatures A and Af increase. This s t a b i l i s a t i o n i s a t h e r m a l l y a c t i v a t e d process/4,5/ and n 8 t y e t completely understood/b/. The aim o f t h e present paper i s t o show whether o r not a study o f t h e i n t e r n a l f r i c t i o n could c o n t r i b u t e t o t h e understanding o f t h e s t a b i l i s a t i o n phenomenon. I n a d d i t i o n i t shows t h e e f f e c t o f ageing on t h e damping capacity.

I I

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EXPERIMENTAL PROCEDURES

I n order t o i n v e s t i g a t e a broad temperature range f o r ageing experiments, an a l l o y w i t h h i g h t r a n s f o r m a t i o n temperatures was chosen.. The composition i n atomic percent

i s : 70,35 % Cu- 12,43 % Zn and 17,21 % Al. The As temperature o f t h e a l l o y is383K. The 3 1 ' ~ 7 x 3

m3

samples are s o l u t i o n h e a t - t r e a t e d a t 1023K f o r 15 min. subse- q u e n t l y quenched i n an o i l bath a t 423K and h e l d t h e r e f o r 10 min. The samples then a r e a i r - c o o l e d t o room temperature. D i f f e r e n t sets o f samples were stored i n o i l baths a t r e s p e c t i v e l y 313K,333K, 353K and 373K, f o r 2, 6, 20,60,and 120 days and p u t on t h e i n t e r n a l f r i c t i o n equipment immediately. The i n t e r n a l f r i c t i o n was measured a t room temperature f o r t h r e e hours under continous v i b r a t i o n a t resonance frequency on an apparatus DMA (Dynamic Mechanical Analyser) described e l swhere 111.

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C10-634 JOURNAL

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PHYSIQUE

111

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EXPERIMENTAL RESULTS.

When t h e internal f r i c t i o n i s measured continuously f o r some hours a peaking e f f e c t

i s observed (fig.1)/2,7/. The present study describes t h e most important items of

the damping curve as a function of t h e ageing parameters, time and temperature as

schematically indicated i n fig.1.

10

i s t h e s t a r t value of t h e internal f r i c t i o n ,

qmax

t h e peak-value,

tmax

the time when qmax i s reached, 11 t h e damping a f t e r one hour vibration and f o t h e i n i t i a l resonance frequency ( f 2 being proportional t o t h e

Young's modulus). The most important r e s u l t i s t h e occurrence of a new type of pea-

king e f f e c t . Indeed each of t h e defined 9-values,

o,

hax

and 91 a r e found t o show

a maximum a s a function of ageing time (frgures 2,

3

and 4 ) . The k i n e t i c s of t h i s

peaking e f f e c t are found t o be temperature dependent : t h e higher t h e temperature

the f a s t e r t h e maximum i s reached and the f a s t e r t h e decrease afterwards. In order

t o distinguish t h i s peaking e f f e c t from t h e previous one, we will c a l l i t t h e "peaking e f f e c t of the second kind", while the other will be called the "peaking e f f e c t

of t h e f i r s t kind". As also shown

i n

the f i g u r e s 2,3 and 4, the peaking e f f e c t of

the second kind

i s

only observed a t t h e ageing temperature of 333K. The maximum of

internal f r i c t i o n a t 313K i s not y e t reached a t maximum ageing time, whereas the maximum a t t h e ageing temperatures 353K and 373K occurs too soon t o be detected

within t h e experimental conditions. From f i g . 5 one can conclude t h a t t h e time a t

which t h e peaking e f f e c t of the second kind occurs also goes through a maximum while

t h e kinetics are temperature dependent. Another important r e s u l t

i s

t h a t t h e reso-

nance frequency and thus the e l a s t i c modulus increases with ageing time. This pro-

cess accelerates with increasing ageing temperature as i l l u s t r a t e d in fig.6.

Finally, f i g . 7 shows the behaviour of the r a t i o lmax/qo as a function of ageing time and temperature.

DISCUSSION

The main difference between the "peaking e f f e c t of the f i r s t kindn(PEl) and theUpea- king e f f e c t of the second kindU(PE2) i s t h a t t h e internal f r i c t i o n in t h e case of PEl i s e a s i l y restored by interruption of t h e vibration or an amplitude discontinuity /7/, while PE2 cannot be restored unless t h e material i s heated t o temperatures above

Af and cooled again below

Mf.

A second important difference i s t h e time s c a l e a t

which both phenomena occur. For PE1 t h e maximum i s reached w i t h i n minutes while i t

takes some days f o r PE2. In both cases the r o l e of the vacancies and t h e i r interac-

tion w i t h t h e interface boundaries cannot be denied. Indeed, the vacancy concentra-

t i o n i n t h e martensitic s t a t e i s higher than i t s equilibrium concentration. This can

be deduced from two observations : 1.The formation energy f o r vacancies i n martensite

i s higher than i n t h e O-phase and thus i t s concentration will be higher than t h e e- quilibrium concentration calculated from t h i s formation energy,since t h e vacancy-concentration i n t h e A-phase prior t o t h e transformation i s retained upon quenching /4/. 2. Positron annihilation studies revealed t h a t vacancies are produced during t h e

transformation

/8/.

Also by positron studies i t was found t h a t t h e t o t a l defect con-

centration

i n

the martensitic s t a t e was not s i g n i f i c a n t l y altered a f t e r long term a-

geing (up t o s i x months) even a t high temperatures, but below As/3/. I t i s not c l e a r a t t h i s moment how the vacancies influence t h e mobility of t h e interface dislocations e i t h e r i n t h e case of PE1 as i n t h a t of PE2. For PEl a dragging model was proposed t o be responsible f o r t h e e f f e c t but then t h e vacancies should be mobile enough t o eva- porate when t h e dislocations are displaced from t h e i r steady-state positions /2/. In t h e case of PE2 one should now also consider t h e matrix s t r u c t u r e t o be able t o chan- ge due t o t h e ordering process t h a t i s considered as responsible f o r t h e s t a b i l i s a t i -

on/5/. In t h i s re-ordered martensite s t r u c t u r e t h e vacancies loose t h e i r mobility

and dragging can be excluded.

But

i f they a r e able t o concentrate a t t h e ( i n t e r f a c e - )

dislocations they can a c t as firm or weak pinning points. The increased elastic-modu- l u s i s a good indication f o r t h i s . Especially t h e binding force between the point defects and t h e dislocations and the distance between weak and firm points will l a r - gely determine the damping capacity as i s proposed by t h e Granato-Liicke model. Also, t h e influence of t h e matrix-structure should be considered when the mobility of t h e dislocation i s discussed. The occurrence of very small precipitates a t t h e interf a- ces a f t e r long term s t a b i l i s a t i o n may complicate t h e problem and reveal more infor-

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Acknowledgments : The authors wish t o thank t h e "Geconcerteerde Onderzoekakties" o f t h e government f o r f i n a n c i a1 support. Jan Van Humbeeck acknowledges t h e NFWO f o r t h e research grant.

REFERENCES

/I/ Van Humbeeck, J. and Delaey,

.L.,

Z e i t . f u r Wetllk.,75, (1984) 755-759. 121 Van Humbeeck, J I and Delaey, L., Z e i t - f u r M e t l l k . , E , (1984) 760-763. 131 Van Humbeeck, J., Segers, D. and Delaey, L. ,Scripts Met. ,19,(1985) 477- 80 /4/ Van Humbeeck, J., Janssens, J., Ngoie M, and Delaey, L., f i r r p t a Met., 18,

(1984) 893-898. -

/5/ Delaey, L.

,'

Suzuki

,

T. and Van Humbeeck, J., S c r i p t a Net.

18,

(1984) 899-903. 161 Proc. of "Discussion Meeting on Cu-Zn-A1 martensite, 19-21 June 1984''

Ed. Delaey, L. and Van Humbeeck J., K.U. Leuven Bep. Met. en Mat. Eng., R84- 1984, 86 pages.

/7/ Van Humbeeck, J. and Delaey, L., Journ. de Phys. 43, (1982), C4-691.

/8/ Segers, D., Van Humbeeck, J., Delaey, L. ,DorikensTM., and Dorikens-Van Praet, L., Appl. Phys. A 36, (1985) 179-182.

/9/

De Graaf, M., Van H a e e k , J., Andrade, M., and Delaey, L.,

scripta

M~~.,E,

( 1985) b43-646.

f l ~ ~

Schematic r e p r e s e n t a t i o n o f t h e i n t e r -

-_----

1 _{n a l f r i c t i o n and resonance frequency}

as a function o f t i m e d u r i n g c o n t i n u - ous v i b r a t i o n a t room temperature. The parameters under i n v e s t i g a t i o n are i n d i c a t e d on t h e f i g u r e . Figure 1. 0 0 h a x 1 hours 0 0 40 80 120 days Figure 2 .

The i n i t i a l value of the internal f r i c t i o n a t roan tenperature as a function o f t h e 2 t e i n g tim at the indicated temperatures.

t

0 ,

0 40 80 120 days

Figure 3.

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C10-6 36

JOURNAL

DE

PHYSIQUE

.

0 t

-

J 0 40 80 120 days 6- 2- 0,

Figure 4: The value o f t h e i n t e r n a l f r i c - Figure 5: The time a t which t h e maxi- t i o n a f t e r 1 hour v i b r a t i m a t room tempe- mun described i n f i g . 1 occurs as a r a t u r e ,

7

as a f u n c t i o n of ageing time a t f u l c t i o n

d ageing t i m e a t t h e i n d i c a -

t h e i n d i c i t e d temperatures. t& temperatures.

7-313K

L--

6-c

.\-

.---

----2-333 K

!

\.

I * l...

.\,

--'A.

\.,

.\.

-'"---.:.353~ L..

_-..

-.----

---..--- 7.-373K t D 0 40 8 0 120 days

F i g u r e 6: The value o f t h e i n i t i a l reso- Figure 7: The r a t i o

nance frequency,

f

a t room temperature

qmax/To

( f i g . 3 0 '

a f t e r ageing f o r 2 days(*) and 120 days ard f i g . 2 ) as a f u n c t i o n o f t h e ageing