Laser cladding of quasi-crystal-forming Al-Cu-Fe-Bi on an Al-Si alloy substrate
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I. INTRODUCTION
QUASI-CRYSTALLINE materials were discovered in 1984 by Dan Shectmann et al.[1] to herald a new era in crystallography and atomic architecture of solids. A few years after the rush of the initial discovery, a few laboratories started looking at the properties of these materials and, thereby, exploring the possibility of the potential use of these materials. Quasi-crystalline materials are characterized by the longrange order without translational periodicity featuring noncrystallographic symmetry. The aperiodic arrangement of atoms in the lattice leads to several distinctive properties, such as very high hardness,[2] low friction coefficient,[3] low thermal conductivity,[4] low surface energy,[5] high corrosion and oxidation resistance,[6] etc. These properties make them potentially useful in many applications. However, due to their low room-temperature ductility,[7] these materials are often KRISHANU BISWAS, Graduate Student, and KAMANIO CHATTOPADHYAY, Professor, are with the Department of Metallurgy, Indian Institute of Science, Bangalore-560012, India. Contact e-mail: [email protected] ROLF GALUN, Scientist, and BARRY L. MORDIKE, Emeritus Professor, are with IWW, Technical University, Clausthal, D-38678, Clausthal-Zellerfeld, Germany. Manuscript submitted May 31, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
not found suitable for bulk applications. An attractive alternative is to use these materials in the form of coatings on soft metals and alloys, such as aluminum and Al-Si alloys. The Al-Cu-Fe system is known to be a stable quasicrystalforming system.[8] This ternary system has been extensively studied due to the possibility of a number of applications in the form of coatings.[9] However, their brittleness at room temperature and complex solidification pathways have made it difficult to use them. Therefore, it is important to study the effect of the addition of other elements on the morphological modifications of icosahedral phase– forming Al-Cu-Fe alloys. The quaternary addition of Si to this system significantly influences the phase formation. According to Lee et al.,[10] an Si addition up to 5 at. pct leads to an increase of the volume fraction of icosahedral phase in the microstructure. A further increase of Si content from 9 at. pct to 15 at. pct leads to the formation of the 1/1 cubic rational approximant to the icosahedral phase. These approximant phases are crystalline in nature with quasi-crystalline motifs and can be derived from the rational cut of six-dimensional hyperspace of the icosahedral quasi-crystals. Therefore, they have similar properties to those of the parent quasi-crystalline phases. According to Quivy et al.,[11] the cubic rational approximant phase exists over a large domain of compositions and temperatures, sometimes in coexistence with the VOLUME 36A, JULY 2005—1947
icosahedral phase. These authors have pointed out that the structural change from icosahedral phase to rational approximant can occur if the Si content in the quaternary alloy is more tha
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