Internal friction in an aluminum metal matrix composite during thermal cycling

Scripta METALLURGICA et MATERIALIA

Vol. 29, pp. 165-170, 1993 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

I N T E R N A L F R I C T I O N IN AN A L U M I N U M M E T A L M A T R I X C O M P O S I T E D U R I N G T H E R M A L CYCLING S. Urreta * and R. Schaller Ecole Polytechnique Fdddrale de Lausanne, lnstitut de Genie Atomique, CH-1015 Lausanne

(Received January 12, 1993) (Revised April 23, 1993) I. Introduction

The difference in coefficient of thermal expansion (CTE) between reinforcement and matrix in metal matrix composites (MMC) leads to large internal stresses following temperature changes [1]. These stresses may be sufficient to generate plastic flow in the softer matrix phase and lead, in discontinuously reinforced MMC, to a shape change during thermal cycling even in the absence of any applied stress [2] .Different models describing this behaviour have been proposed. Taya and Mori [3] have developed a model where the misfit strain is accommodated by plastic deformation and the resulting residual elastic matrix stress may be further reduced by a time dependent, power law creep relaxation mechanism at the upper temperature of the cycle. Dislocation anelasticity and microplasticity in aluminum have been extensively studied by mechanical spectroscopy [4-7]. In the low and medium stress ranges and for frequencies of about 1 Hz, the internal friction spectrum of aluminum exhibits different maxima between room temperature and 800 K associated to relaxation mechanisms involving volume dislocations. At about 0.4 TM ( TM = melting point ), internal friction peaks arising in thermally activated movement of dislocations are reported. Depending on purity, jog dragging or impurity atoms dragging [4] are proposed to be the controlling mechanisms of these peaks which always exhibit activation energies close to those for pipe diffusion. At about 0.6-0.7 TM, internal friction maxima still arise in movement of longer dislocation loops inside the grains [5], but now activation energies are higher and close to those found for the two regimes of creep in aluminum that is, cross-slip and climb [6]. On the other hand, dislocations arranged in polygonization walls give an internal friction maximun close to TM [7] at frequencies near 1 Hz. On these bases, mechanical spectroscopy has been employed to investigate the dislocation properties in an aluminum matrix composite, reinforced with short non-oriented alumina fibers, during thermal cycles between 100K and 8OO K. 2. Exnerimental Procedure

Composite samples containing 27% volumetric fraction of alumina fibers (SaffilTM) were produced in the Materials Department, at the EPF Lausanne, by the method of Low Pressure Forming (LPF). Matrix composition is described in Table I and the elaboration conditions were the following: melting temperature TM = 1083 K, preform temperature T p = 1073 K, pressure P = 2 MPa. Infiltrated samples had a mean spacing between fibers ~. = 7 P-m, given by the ceramic preform, and a grain size d = 1500 I.tm. Aluminum samples were produced simultaneously with each composite, in the same solidification process in order to evaluate the effect of fibers on the solidification microstructures. Rod shaped specimens, 2 mm diameter and 40 mm in effective length, were obtained by machining and mounted in an inverted torsion pendulum. Mechanical spectroscopy provides a simultaneous measurement of internal friction (IF) and elastic torsion modulus G. Internal friction was measured automatically, by the method of free decay of the oscillations, during controlled heating and cooling thermal cycles between 100 K and 800 K. *On leave from : FAMAF , Universidad Nacional de Cordoba, Valparaiso y Rogelio Martinez, Ciudad Universitaria , 5000 Cordoba , Argentina.

165 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.

166

INTERNAL FRICTION

Vol.

29, No.

Strain amplitude of 5 x 10-5 was used. The shear modulus was determined from the frequency fmeasured, following the relation:

G= 8 x l I

f2

r4 where I and r are the sample length and radius respectively, and I is the pendulum inertia momentum.

The internal friction and modulus as a function of temperature for monolithic AI and 27% vol. fibers composite samples are shown in Figure 1. In both cases, samples were previously annealed for 3 h at 800 K and slow cooled (2K/rain) to 100 K. A higher level of the elastic modulus in the composite in all the temperature range may be observed, as expected. Concerning the damping capacity at high temperature, it is lower in the composite. Internal friction in the aluminum samples continuously increases above 400 K indicating that only the low temperature side of the internal friction peak located near TM is observed. This peak appears in samples with polygonized dislocation substructure [7]. In addition, the absence of the low temperature peaks indicates that free dislocation density is low in aluminum samples after high temperature annealing. The spectra observed for the composite samples, on the other hand, show new features: during heating, a small maximun M1 at 450 K and a second one, M2, at about 650 K, are detected. After 30 rain at 800 K, the specmam found during cooling shows a lower level. The maximun M2 considerably reduces and M1 is not observable at all. The dependence of this hysteretic behaviour detected in the composite, on the extreme temperatures of the cycle, was further investigated. Figure 2 shows the internal friction evolution. It may be observed that the hysteresis depends on TH ( highest cycle temperature) for values above 400 K; TH particularly affects the cooling spectrum, but the heating one remains unchanged. The effect of changes in TL ( lowest cycle temperature ) was investigated and results are presented in Figure 3. Again, hysteresis is found to depend on TL but now through a strong sensitivity of the heating spectrum. The spectrum measured during cooling, after 30 min at 800 K, is completely reproducible. Interestingly, the hysteresis markedly increases when TL becomes lower than 400 K. We investigated the high temperature contributions to the heating and cooling internal friction composite spectra by plotting the function S(T)= - (l/G) dG/dT This function shows local maxima at the same temperatures where internal friction peaks take place. Figure 4 shows the function S(T) determined between 300 K and 800 K , during cycles 100K/800K at 2 K/rain. Three local maxima are observed during heating: PI located at about 450 K which is associated with the IF maximun MI, and two at higher temperature, P2A (600 K) and P2B (690 K) considered as the two components of the IF maximun M2, located at 650 K. During subsequent cooling after 30 rain at 800 K, these maxima are still observable at almost the same temperatures but their heights appear strongly reduced, especially that at the highest temperature. These maxima are located at temperatures which correspond well to those of the internal friction peaks observed in aluminum of the same purity [6] and attributed to movement of dislocations controlled by point defects dragging (PI), and cross slip and climb ( P2A and P2B ). Their heights are known to be proportional to the mean density of mobile dislocations undergoing such stress induced motions. Even when the microstructures responsible for energy dissipation seem to be reproducible in each thermal cycle imposed on the sample, these changes are not completely reversible in the sense that the overall sample microstructure does not remain unchanged after a complete cycle. This fact is evidenced by changes (a drop )in the elastic modulus as the number of thermal cycles, N, increases.

2

Vol.

29, No.

2

INTERNAL

FRICTION

167

The composite elastic modulus G depends on matrix (Gm) and fiber (Gf) modula, on the volumetric fraction of fibers (Vf) and on matrix/fiber interface microstructure and mechanical behaviour. In the actual conditions of our measurements, the observed drop in G during thermal cycling is likely to arise in defect accumulation in the A1 matrix (dislocations) or in defect accumulation in the matrix/fiber interfaces affecting the charge transference across it. From this evolution exhibited by the modulus, it is possible to define the damage D as: DT (N) = 1 - G(N)/G(0). Here G(N) is the shear modulus at temperature T, measured during heating, in the N th thermal cycle and G(0) is that measured at the same temperature during the first heating after annealing. Figure 5 shows the evolution of D when measured at 300 K, 500 K and 700 K, during the N th thermal cycle between 100 K and 800 K and also during thermal cycles between 300 K and 800 K. It may be observed that damage increases with N in a greater extent for cycles 100/800K than for cycles 300/800K. If these results are compared with those shown in figure 3, it appears that hysteresis and damage accumulation rates are correlated.

A first analysis of the internal friction exhibited by the MMC studied requires the identification of the different mechanisms responsible for energy dissipation which can arise : a) in the reinforcement, that is involving defects inside the fibers, b) in the matrix/fiber interface, and c) in the matrix, due to matrix defects. Mechanisms of type a) and b) would lead to new contributions to the spectrum, with magnitudes increasing with the volumetric fraction of fibers (or interfaces); such effects were not observed in the conditions of the present work. On the contrary, the spectra exhibited by the aluminum and composite samples are consistent with different microstructural matrix states resulting from solidification and treatment processes, indicating that the major contribution arises in the matrix, the effect of fibers being likely indirect. These indirect effects of fibers are mainly three: a) the high temperature internal friction background is lower in the composite, indicating that polygonized substructures are less developed, b) internal friction maxima associated with thermally activated motion of free volume dislocations are still present after 3 h at 800 K and cooling to 100 K, when they are not detected in monolithic aluminum sample; in addition, these maxima show hysteresis, c) elastic shear modulus depends on N. The hysteresis observed in the IF spectrum may be attributed to a different mobile dislocation density in the specimen during heating than during cooling. This is supported by the observation of different magnitudes of the two components (P2A and P2B in figure 4) of maximun M2 when measured during heating and during cooling. As peak temperatures remain essentially the same, with changes only in the peak heights, it may be concluded that mobile dislocation density in the composite is higher during heating than during cooling. A higher density of free mobile dislocations prior to heating may be achieved by the creation of dislocations during cooling. The elastic stress t~0 in the matrix due to different thermal expansion coefficients 0~ may be estimated [3] t~0 = (Gt f - Ct m) EfEm A T/(Em Vm-EfVf) where E is Young modulus, Vf the volume fraction of fibers, and subscripts m and f indicate values corresponding to matrix and fibers, respectively, and AT is the temperature change. In this way, here exists a critical change in temperature which induces plastic yielding ( t~0 = t~y ) throughout a region close to the fibers. This critical temperature variation is given by A T* = - (I/A t~ ) ( (~y item) ( I - V ) / V f with ay the matrix yield stress (30 - 60 Mpa ) and V the Poisson ratio.

168

I N T ERNAL

FRICTION

Vol.

29, No.

The value of A T* for the composite investigated is 70 K, well below the thermal amplitudes imposed in our experiences, so that plastic relaxation is likely to occur. Nevertheless, from figure 2 it becomes evident that thermal amplitudes larger than 70 K are required to increase free dislocation density, and so hysteresis, appreciably. This may be explained by the fact that the former analysis disregards other mechanisms of stress relaxation such as diffusional flux near fiber interface or creep of pre-existing dislocations in the high temperature region during cooling. All these factors lead to the higher value of A T* observed. Plastic relaxation (dislocation generation) is likely to predominate at temperatures where defect mobility becomes reduced that is, below 400 K in aluminum. The increased hysteresis observed when TL reaches values below this temperature is a further confn'mation that these new dislocations created by plastic relaxation during cooling are responsible for the higher relaxation maxima detected during heating and consequently for the hysteresis observed. At high temperature, as dislocation mobility increases, pileups relax by cross slip ; some free dislocations disappear and others are incorporated into cell walls ( by glide and climb). In this way, the resulting microstructure (low free dislocation density and increased cell wall thickness) leads to a reproducible internal friction spectrum during cooling but to a lower value of the elastic modulus as new defects have been incorporated into the sample. In this picture, the possibility of interface damage may not be excluded as long as it may develop during cycling without being detected by IF measurement (no interfacial relaxation mechanisms are observed). Concerning the indirect effects of the fibers on the damping capacity of the composite, they may be separated as those affecting microstructure during solidification and air quench (processing) and those affecting matrix microstructure during thermal cycling. Finally, from the results considered, it appears that during cooling not only the generation of new dislocations contribute to the misfit stress relaxation, but also movement of preexisting dislocations must be taken into account. On the other hand, stress relaxation during heating in the high temperature range of the cycle is likely to be accomplished not only by dislocation movement but also by diffusional creep. References

1 Vogelsang, M ; Arsenault, R.J. ; Fisher, R.M. : Metall. Trans. 17A 379 (1986). 2 Derby, D. : Metal Matrix Composites- Processing, Microstructure and Properties, p. 31 12 th Rise International Symposium on Material Science, (1991) 3 Taya, M ; Mori,T. : Proc. IUTAM 147 (1987) 4 No, M. L., Esnouf, C., San Juan, J., Fantozzi, G. Acta Met 36 4 837 (1988) 5 Woirgard, J., Amirault, J, de Fouquet, J. ICIFUAS-3 Aachen Springer, Belin (1975) 6 Bonetti, E., Evangelism, E., Gondi, P., ECIFUAS-3 Manch., Pergamon Press (1981) 7 Woirgard, J., Gerland, M., Riviere, A . , ECIFUAS-3 Manch. Pergamon Press (1981) Acknowledtqnent

This work was partially supported by the Swiss National Science Foundation. One of the authors (S.U.) gratefully acknowledges to the Consejo Nacional de Investigaciones Cientificas y Tecnicas, Republica Argentina, and to FAMAF Universidad Nacional de Cordoba, Argentina, for financial support.

Ta~eI

%W

A1 99.99

Si .0014

Mg .0014

Zn .0019

Fe .0010

Cu .0011

Mn,Cr,Ti < 10-5

2

Vol.

29, No.

2

INTERNAL

FRICTION

169

80

-32 8

-30

b '

l.: -t

60

-28 o

a

-26

v

~40

• - _1

~~/ e° ~ •

• ." M2

a , a'

0 % nbcrs

b , b'

27 % fibers

j/~

.\

M1 0 -

I

-

4 I

100

~

200

I

I

300

~,o-

I

400 T K 500

":...IZ 0 ".,

~-18

I

I

600

700

'lP•

~-16

800

Fig. 1 : Aluminum and Composite IF spectra ( a, b ) and modula (a', b') as function of temperature, after annealing 3 h at 800 K. F = 2 Hz. Cycle : 100 K -> 800 K (30 rain ) -> 100 K. Composite spectrum shows a large hysteresis.

60-

50-

40-

b', a~, T~-800 ~ K "30-

/

c, c' Tn=600K

b ~ b ~

20C

10-

0 -

I

200

I

400 T °K

a'

Fig.2 : Effect of TH ( the highest cycle temperature) on the hysteresis observed in the composite spectrum. Sample previously annealed 3 h at 800 K and cooled to 100 K. Cycles : 100 K -> TH (30 rain ) -> 100 K. Heating spectra remain unchanged.

/i/ I

I

600

800

170

INTERNAL FRICTION

Vol.

29, No.

1.4-

607 50 7

/

40,

J]" j~ c ~;

a - T L ,4OOK b-TL=200K

1 2i •

"'

°" :

t

. ~o

E°

/P

207

h - heat. c - cool.

'7

0.6"~

h:"~, h

l

/c

"

°41- / 200

400

600

800

T K

0.2

t 300

t 400

t 500

I 600

t 700

I 800

T K

Fig. 3 : Effect of TL ( the lowest cycle temperature ) on the hysteresis observed in the composite spectrum. Sample previously annealed 3 h at 8OOK. Cycles : TL -> 800 K ( 30 min ) -> TL. Cooling spectra remain unchanged.

Fig. 4 : Function S(T) for a cycle 100K/8OOK (only shown between 300 K and 800 K). l.x~al maximun P1 associated with IF peak M1 and maxima P2A and P2B considered as components of IF maximun M2.

0.25 -

Cycles 100K/800K O 300K tX 5OOK 7OOK

0

0.20 -

Cycles 300K/8OOK • 3OOK • 500K • 7OOK

0.15-

6 0

0.10 [] [] []

6

0.05

0.00 0

"7 2

_

= A

•

"

? , 4

N

•

•

•

•

6

"

y 8

,

•

10

Fig. 5: Function D determined from modulus values at 300 K, 500 K and 700 K during the Nth heating when the specimen in cycled between 300 K and 800 K (full symbols ) and 100K and 800 K (open symbols). Damage increases at a higher rate for 1OOK/8OOKcycles, where plastic relaxation becomes important.

2