METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 28A, APRIL 1997—1099 Table I. Mechanical Properties of Unreinforced and Reinforced EURA Al Alloys Material Treatment  S (MPa)  UTS (MPa) E (GPa) Elongation Percent Unreinforced peak aged 720 780 72 8.2 Unreinforced overaged 480 540 72 9.0 Reinforced 15 pct SiCp peak aged 660 720 97 2.1 Reinforced 15 pct SiCp overaged 610 660 97 3.2 Fig. 1—TEM micrograph showing the characteristic microstructure of the EURA composite in peak-aged conditions. Particles labeled A and B are both Mg 2 Zn 3 Al 2 precipitates. The diffraction pattern reported is from particle A, with B close to [110] pole. 17. A. Czyrska-Filemonowicz, M. Wrobel, B. Dubiel, and P.J. Ennis: Scripta Metall., 1995, vol. 32, pp. 331-35. 18. M.Q. Liu and J. Cowley: Scripta Metall., 1993, vol. 28, pp. 307-12. 19. R. Lagneborg: Scripta Metall., 1973, vol. 7, pp. 605-14. 20. J. Ro ¨sler and E. Arzt: Acta Metall., 1988, vol. 36, pp. 1043-51. Interfacial Bonding in Spray-Formed Metal Matrix Composites M. DE SANCTIS, R. VALENTINI, and A. SOLINA The modulus, strength, and ductility of Al alloys rein- forced with ceramic particulate (AMMCs) are influenced by many variables, such as the volume fraction, distribu- tion, and dimension of particulate, the matrix flow stress, and the debond resistance of the interface. As far as the failure mechanism of composites is concerned, different sources of fracture can be operative, and usually they are the reinforcement fracture, the matrix reinforcement inter- facial decohesion, and the ductile failure in the matrix by nucleation, growth, and coalescence of cavities around par- ticulates. The fracture of the reinforcement is particularly important for AMMCs based on high strength 2000 and 7000 series Al alloys. [1] For these composites, it has been shown that the particulate progressively fractures during de- formation of the material and that subsequent crack coales- cence is likely to control the ultimate strength and ductility of composites. [2,3] The high flow stress and low ductility of these metallic matrices are important factors in promoting particulate fracture, providing the interface bond is strong enough to allow an efficient mechanism of load transfer from the matrix to the reinforcement. Lewandowski et al. [2] stressed the importance of interface bonding in determining the failure mechanism for a SiCp-reinforced Al-Zn-Mg-Cu alloy produced by powder metallurgy (PM). They observed changes in the fracture surface morphology between un- deraged and overaged tempers of the composite, the former exhibiting extensive particulate fracture and the latter a fracture near the SiC/matrix interfaces. It was suggested that the change could reflect an embrittlement of the inter- face through extensive precipitation of secondary phases during heat exposure of the material. More recently, LLorca et al. [3] studied a spray-formed SiCp-reinforced 2618 Al alloy and observed a failure mechanism governed by par- ticulate fracturing also in peak-aged materials. They sug- gested that the composite was able to retain a good interfacial bond during heat exposure, and they stressed to this respect the beneficial effect of rapid solidification in minimizing solute segregation at interface regions. In the present work, we studied a spray-deposited (SD) SiCp-reinforced Al-Zn-Mg-(Cu) alloy and observed that the failure mechanism described previously was operative not only in peak-aged material, but also following strong over- agings, thus suggesting a substantially different behavior in respect to similar materials processed by the PM route. M. DE SANCTIS, Associate Researcher, R. VALENTINI, Scientist, and A. SOLINA, Associate Professor, are with the Department of Chemical Engineering, Industrial Chemistry and Materials Science Department, University of Pisa, 56100 Pisa, Italy. Manuscript submitted May 17, 1996. The composite studied here was provisioned by Osprey Metals Ltd. in the form of hot-extruded bars heat treated in peak-aged conditions (32 hours, 105 °C). The metallic ma- trix was constituted by a high solute content Al alloy with about 10 pct Zn, [4] which was codeposited with 15 pct SiCp particulate 10 m in average diameter. Tensile tests have been carried out following ASTM E 8-83 standards, and the results obtained are reported in Table I. In peak-aged conditions, an increase of about 30 pct in value for the modulus of elasticity and a significant loss in both ductility and strength were measured with respect to the unreinforced alloy. Transmission electron microscopy (TEM) showed a massive precipitation of metastable ' precipitates within Al grains (not shown) and the presence of relatively large particles mainly consisting of the Mg 2 Zn 3 Al 2 phase, cubic in structure with a lattice parameter close to 1.47 nm [5] (Figure 1). This phase formed at high temperature during homogenizing of the material (2 hours at 470 °C), which, as a matter of fact, was not fully suc- cessful. The precipitates were localized at the matrix grain boundaries and where different Al alloy grains intersected the particulate surfaces (Figure 1, particles A and B, re- spectively). Some -equilibrium particle also formed at