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INTRODUCTION

THE unabated thirst for fuel saving and cost-efficient materials in automotive, defence, and aerospace industries has made the researchers to focus on metal matrix composites (MMCs). Over the last few decades, MMCs reinforced with nano-dispersoids are gaining particular attention.[1–3] Such metal matrix nanocomposites (MMNCs) are isotropic in nature and possess enhanced mechanical, thermal, and structural properties. MMNCs are produced via several routes.[4–9] Casting and solidification processing method is found to be not only an economical way but also an efficient way for the production of bulk composites. However, achieving uniform distribution of the nano-dispersoids in the metal matrix through the solidification processing method remains a major challenge due to the density difference between the liquid metal and the solid ceramic particles. The characteristic nature of the high surface-to-volume ratio and strong Van der Waals forces between the nano-dispersoids keep it in agglomerated state. Thus, H.M. VISHWANATHA, Doctoral Student, JAYAKUMAR ERAVELLY, MS Research Scholar, and SUDIPTO GHOSH, Professor, are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology-Kharagpur, Kharagpur, 721302, India. Contact e-mail: [email protected] CHERUVU SIVA KUMAR, Professor, is with the Department of Mechanical Engineering, Indian Institute of Technology-Kharagpur, Kharagpur, WB, 721302, India. Manuscript submitted March 3, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A

very high forces are required to break the bonds and thereby deagglomerate the clusters.[10,11] Neither mechanical stirring nor such other type process can result in complete deagglomeration and uniform distribution of nano-dispersoids in the matrix. However, it is found that ultrasonic waves transmitted into the liquid melt (containing agglomerates of nano-dispersoids) can break the strong bonds and deagglomerate clusters of nano-dispersoids. [12] When the solidification follows the dispersion, the process can be referred to as ultrasonic casting. Ultrasonic casting can be of two types: contact type[13–18] and non-contact type.[1–3] In the case of contact type ultrasonic casting, the ultrasonic probe is in direct contact with the liquid melt present inside the furnace. The advantage of this method is that the ultrasonication (the process of melt treatment using high-energy ultrasonic waves) can be carried out for a longer duration of time. On the other hand, the contamination of the liquid melt as a result of direct contact between the probe tip and the liquid melt is a severe problem. An alternative method was proposed to resolve the problem of contamination, called as non-contact type ultrasonic casting[1] to produce bulk MMNCs. In this type of ultrasonic casting, the probe is not in direct contact with the liquid melt, but the acoustic energy to the liquid melt is transmitted through the mold wall. The drawback of this method is that there is a lack of sufficient time for complete deagglomeratio