Ageing behaviour of Al-Cu-Mg alloy matrix composites with SiCp of varying sizes

Scripta Mate&h,

Vol. 37, No. 4, pp. 485-489,1997 Elsevis Science Ltd Copyright 0 1997 Acte Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/97 $17.00 + .OO

Pergamon

PI1 s1359-6462(97)00107-3

AGEING BEHAVIOUR OF Al-Cu-Mg ALLOY MATRIX COMPOSITES WITH Sic, of VARYING SIZES Vijay K. Varma, Y.R. Mahajan and V.V. Kutumbarao* Defence Metallurgical Research Laboratory, P.O. Kanchan Bagh, Hyderabad 500058. (India). *Department of Metallurgical Engineering, Institute of Technology Banaras Hindu University, Varanasi 22 1005. (India). (Received January 6, 1997) Introduction Particulate reinforced aluminium alloy matrix composites with their improved yield and tensile strengths, high elastic modulus and isotropic properties are promising candidate materials for many structural applications. During the last decade, considerable efforts have been made to improve the strength of precipitation hardenable aluminium alloy matrix composites by suitable heat treatments (l15). It is also w~elldocumented that the composites show accelerated ageing behaviour because of the enhanced dislocation density resulting from the coefficients of thermal expansion (CTE) mismatch between ceramic reinforcement and the matrix (l-3,7 -10). The accelerated ageing behaviour of the composite eithelr because of the dislocation density or the residual stress, as a result of CTE mismatch, is dependent on the whisker’s size (8). The effect of volume traction of SIC particulate (SIC,) on the ageing behaviour of an aluminium alloy has also been studied systematically (10,16). The present study is an effort to characterize ageing behaviour of Al-Cu-Mg alloy-sic pariculate (Sic,) composites with varying sizes of the reinforcement. Experimental Al-Cu-Mg alloy matrix composite billets reinforced with three sizes of a-type Sic particles (average sizes -1 pm, -12 l.trn and -65 pm) were produced by a properietory powder metallurgy (P/M) process. The P/M billets were converted in to 16 mm diameter rods by hot extrusion with an extrusion ratio of 20: 1. The composites have a nearly constant 17 +/- 1% volume fraction of Sic, reinforcement irrespective of lthe size. Matrix alloy rods without any Sic, reinforcement were also produced in a similar fashion ,for comparison purpose. Chemical composition (in weight percent) of the matrix alloy used in the present study is: Cu: 4.03%, Mg: 1.54%, Mn: < 0.002%, Si: 0.04%, Fe: O.Ol%, Zn: 0.02% and Al: Balance. Extruded rods of the composites as well as the matrix alloy contained a high concentration of Curich intermetallic particles ranging in size from submicron to as big as 10 l.tm. A homogenization treatment developed earlier for the composites in vacuum hot pressed condition (17) was modified (18)

485

486

AGEING BEHAVIOUR

Vol. 37, No. 4

for the materials in as extruded condition. It involves soaking at 768 +/- 3 K for 4 hours in a salt bath furnace followed by cold water quenching. All the four materials of the present study were first homogenized as above and then aged at temperatures of 433 K, 453 K and 473 K respectively. Cylinderical specimens of 16 mm diameter and 15 mm height were used for the ageing study. After ageing, the specimens were levelled and polished for hardness measurement which was done using a Vickers hardness tester with 20 Kgs. load. The relatively large load was chosen in order to minimize scatter in the hardness readings due to the random distribution of Sic particles in the alloy. It has been reported that micro hardness measurement gave large scatter owing to variable impingement on subsurface Sic, (19). Other workers have successfully used the macro hardness measurement with large loads for Arrhenius analysis (14). Hardness values reported in the present paper are the mean value of at least 5 readings on each surface of the specimen. Microstructure of all the three types of composites was examined by optical metallography while a JEOL-840 scanning electron microscope under back scattered mode was used for the microstructure of the tmetched matrix alloy. Results and Discusssion Figure 1 shows the optical micrographs of the composites with an initial average Sic, reinforcement sizes of -1 l,trn, -12 l.trn and 65 pm respectively. In general, there appears to be a reasonably uniform distribution of the Sic, reinforcement within the matrix. Some clustering tendency observed in the finer SIC, (-1 pm) reinforced composite decreases as the size of the Sic, increases. Some of the coarse Sic particles have a tendency to fracture (Fig. lc). A high density of the white coloured Cu-rich

Figure 1. Optical micrographsof Al-G-Mg/SiC, reinforced composites with varying Sic, sizes;(a) -1 pm, (b) -12 pm and (c)d5 lm.

Vol. 37, No. 4

AGEING BEHAVIOUR

487

intermetallic particles were observed in the as-extruded matrix alloy (Fig. 2a). The modified homogenization treatment has resulted in redissolution of the tine Cu-rich particles in the matrix (Fig. 2b), but not the coarse one. Variation of hardness with ageing time is plotted in Figure 3. At all the ageing temperatures (433 K, 453 K and 473 K) Sic, (-1 ym) reinforced composite exhibits superior age hardening followed by the composite with -12 pm Sic, reinforcement, composite with coarse Sic, (-65 pm) and the matrix alloy in that order. The time to achieve peak hardness increases in the same order. It has been observed (8,12,20) that the dislocation density of the matrix increases as a result of the difference in the coefficient of thermal expansion (CTE) between Sic, reinforcement and the matrix. This increase in dislocation density depends upon the reinforcement size (21). As the particle size increases at a constant volume fraction, the inter particle spacing increases and the dislocation density due to the CTE mismatch decreases. Particle size and inter particle spacing are related by the expression (22): h = [(7t/6f)“*- 2/x]D

(I)

where, h = mean edge to edge free path, f = volume fraction of reinforcement and D = size of reinforcement. Dislocations generated due to the CTE mismatch act as heterogeneous nucleation sites for the precipitates (8,12). The decrease in dislocation density due to increase in inter particle spacing with increasing Sic, size would decrease the number of heterogeneous precipitation sites. This would account for the slower kinetics of ageing and lower peak hardness with increasing Sic,, size (Fig. 3). Activation energy (Q) values for precipitation have been calculated from the Arrhenius plots in Figure 4 for all the four materials and are listed in the Table 1. The activation energy for the matrix alloy is comparable to that reported in literature for a 2124Al alloy (Q = 147 kJ/mole) (14). The activation energy for precipitation in the composites is less than that for the matrix alloy and nearly independent of the Sic, size averaging 130 +/- 3 kJ/mole. A slightly lower activation energy (118 kJ/mole), has been reported in the literature (14) for a composite of 2 124 AV17.9 volume percent Sic, of 3-5 pm size. This small difference may be attributed to a difference in the matrix microstructure. The matrix of all the three composites in the present study contains a significant quantity of coarse Cu-rich intermetallic particles even after the homogenization treatment (Fig. 2b), and is thus depleted of solutes unlike the literature alloy. The lower activation energy in the composites as compared to that in the matrix alloy indicates that the diffusion of solute atoms in the composites is relatively easier than in the matrix.

Figure 2. Back scattered emission scanning electron micrographs of the matrix alloy showing Cu-rich intermetallic particles (a) before and (b) after homogenizationtreatment.

488

AGEING BEHAVIOUR

Vol. 37, No. 4

200

200 ACE” IBU

-

T‘ vi [3

160

-

3

140

-

12U

-

ACE”

AT 433 K

AT 453 K

s

200

,

I

I AGED

AT 473 K

IUO

i

.

HMC ,-12pml

m

MMC I-lpm)

IU’

NJ*

IU’

‘I IME,

Hrs

Figure 3. Varition of hardness with ageing time.

0.5

* *pese--- CONTROL ( REF.11 ) + .. . . *. MMC (REF. I1 ) 0 -_CONTROL

j: *. . . **.* **.*

0.0021

I. . .

*.

0 -

MMC (-lum)

A- - -

MMC (-12um)

m- - - -

MMC (-65um)

0.0022

0.0023

0.0024

l/T,l/K Figure 4. Activation energy plot showing the time to reach peak hardness as a k&n

of temperature.

Vol. 37, No. 4

AGEING BEHAVIOUR

489

TABLE 1 Activation Energy Values for Al-Cu-Mg Alloy Matrix Composites with Different Sic, Sizes (-1 pm, -12 urn and -65 urn) and the Matrix Alloy ____ _______________._____________ __________ ________ _______.~~-~~~~~~_~~~_~~~~~~~~~~~~~~~~~~~~~~~~~~~.--~-~-~-----Activation Energy. Q ( W / mole ) MtU‘Zlid ________________________________________________________~______~_~________~._~__~_.~.~~~~~~~~~_~__~~~~~~~~~~~~~~~~~ Maix

142

MMC (- l)tm )

127

MMC (- 12pm )

131

MMC (- 65pm )

133

Conclusions An Al-Cu-Mg alloy reinforced with Sic, of -1 p,rn size has shown superior ageing behaviour as compared to the composites containing coarser Sic, reinforcements (-12 pm and -65 pm) as well as the matrix alloy. It is also observed that the time to reach peak hardness decreases with decreasing Sic, size. This is the result of faster diffusion of solutes in the matrix of the composites due to enhanced dislocation density. Acknowledgement The authors would like to thank Dr. D. Banerjee, Director, Defence Metallurgical Research Laboratory (DMRL), Hyderabad, India, for his encouragement and permission to publish this paper. Technical discussion with Dr. S.V. Kamat and Dr. M. Srinivas, Scientists, Defence Metallurgical Research Laboratory, Hyderabad, India, is gratefully acknowledged. The authors are grateful of Mr. V.V. Bhanuprasad, S’cientist, DMRL, Hyderabad, India for providing the materials. References 1.

2. 3. 4. 5. 6. I. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

20. 21. 22.

T.G. Nieh and R.F. Karl&, Scripta Metall., 18,25 (1984). J.M Papazian, Metall. Trans., Vol19A, 2945 (1988). T. Christman and S. Sure& Mater. Sci.Eng., A 102,211(1988). J.J. Lewandowski, C. Liu and W.H. Hunt; Mater.Sci.Eng., A107,241 (1989). J.M. Papaxian and P.N. Adler; Metall. Trans., 2lA, 401(1990). D.M. Knowles and J.E. King; Acta MetaRMater., 39,793 (1991). I. Dutta, S.M. Allen and J.L. Hafley, MetaRTrans., Z!A, 2553 (1992). I. Duttaand D.L. Bourell, Mat.Sci.Eng.,Ali2.67 (1989). P. Appendino, C. Badini, F. Marino and A. Tom&, Mat Sci. Eng., A135,275 (1991). S. Suresh, T. Christman and Y. Sugimura, Scripta Metal., 23,1599 (1989). R.D. Schuellerr and F.E. Wawner; J. Mater. Sci., 26,3287 (1991). K.K. Chawala, A.H. Esmaeili, A.K. Datye and A.K. Vasudevan; ScriptaMetall. Mater., 25,131s (1991). M.P. Thomas and J.E. King; Mater. Sci. and Tech., 9,742 (1993). M.P. Thomas and J.E. King; J. Mater. Sci., 29,5272 (1994). T.J.A. Doel and P. Bowen., Mater. Sci. and Tech., 12A, 586 (1996). Tun-shan Lin, Peng-xiny Li and Ren-Jic Wu, Scripta Metall. Mater., 28,281(1993). V.K. Varma, S.V. Kamat, M.K. Jain,V.V. Bhanu Prasad and Y.R. Mahajan.; J.Mater.Sci., 28,477 (1993). V.K. Varma et al., unpublished work. M.P. Thomas, D.M. Knowels and J.E. King “Euromet ‘91”, Proceeding of 2nd European Confenncc on Advanced Materials and Processes, Cambridge, UK., vol.2 (1991), edited by T.W. Clyne and P.J. Withers, p 105, Institute of Materials, London (1993). Mary Vogelsang, R.J. Arsenaultand RM. Fisher; Metall. Trans., 17 A, 379 (1986). R.J. Arsenault andN. Shi; Mater. Sci. and Eng., 81,175 (1986). S.V. Kamat, A.D. Rollet and J.P. Hirth, Scripta Metall., 25,27 (1991).