Nucleation of Fe-intermetallic phases in the Al-Si-Fe alloys
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I. INTRODUCTION
WITHOUT doubt, the binary Al-Fe and ternary Al-FeSi phases constitute an important part of the microstructure in aluminum alloys. Particles formed during casting may influence the material properties during subsequent fabrication steps or in service. For example, the -AlFeSi platelike phase has a detrimental influence on the alloy properties. The phase platelets act as potential sites for crack initiation that, consequently, results in decohesion failure.[1] Other phases such as Al3Fe and -AlFeSi are cathodic to the aluminum matrix and, when present on the surface, promote the pitting attack of the surface in conductive liquids.[2] Thus, control of these phases is of considerable technological importance. In commercial aluminum and aluminum alloys, a large variety of inclusions are present in small quantities.[3–6] The inclusions were observed to be responsible for the heterogeneous nucleation of different phases.[7,8,9] The nucleation of certain Fe-intermetallic phases was reported to be enhanced in the presence of particular inclusions such as the grain refiner particles introduced to the melt with the addition of Al-Ti-B master alloys.[10,11,12] Some of these works were based on the assumption that TiB2 particles added with the grain refiner nucleate the second-phase particles in the same manner as they do for the matrix phase, while others suggested that the promotion of certain phases in the presence W. KHALIFA, Postdoctoral Fellow, Departement des Sciences Appliquees, University of Quebec at Chicoutimi (UQAC), is Assistant Professor, Cairo University, Giza, Egypt. F.H. SAMUEL, Professor and GM-NSERC-UQAC Chair, is with the Departement des Sciences Appliquees, University of Quebec at Chicoutimi (UQAC), Chicoutimi, PQ, Canada G7H2B1. Contact e-mail: [email protected] J.E. GRUZLESKI, Dean, is with the Faculty of Engineering, McGill University, Montreal, PQ, Canada HA 2K6. H.W. DOTY, Senior Experimental Engineer, is with Materials Engineering, General Motors Powertrain Group, Metal Casting Technology, Inc., Milford, NH 03055 S. VALTIERRA, Research and Development Manager, is with Corporativo Nemak, S.A. de C.V., Garza Garcia, N.L. 66221, Mexico. Manuscript submitted October 12, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
of Al-Ti-B grain refiner is due to its effect on altering the growth conditions that stabilize some intermetallics over others.[12] Other workers attributed the preferential formation of certain Fe-intermetallic phases to the promotion of nucleation on certain inclusions[9] or unidentified potent catalysts in the melt (for example, the review of Allen et al.[12]). However, there is apparently no direct observation of the nucleation of Fe-intermetallics on the surface of TiB2 particles in these works.[13] The actual evidence that any one nucleant/inclusion may be associated with the formation of certain Fe-intermetallic phases is not extensive since no systematic study has been carried out for this purpose. In addition, it does not seem to have been clearly understood how
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