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HIGH-TEMPERATURE processing of materials by microwave (MW) radiation has emerged as a novel technique and shown promise for sintering, brazing, melting, heat treatment, and synthesis of a wide variety of materials including powder metals.[1–4] To date, MW sintering is an established process for ceramics and has been realized in industrial production. In contrast, MW sintering of powder metals is only a recent development,[5] but it has attracted increasing attention because of its potential for both cost reduction and property improvements.[5–9] Magnetic and refractory powder metals appear to respond well to MW radiation compared with other powder metals which often show mild or weak responses.[10–13] Using the Heisenberg model, Tanaka et al.[14] have recently shown that MW radiation can heat magnetic metal oxides to temperatures well above their Curie temperatures while it is essentially ineffective for non-magnetic oxides. Their modeling identified that such fast heating ‘‘is caused by nonresonant response of electron spins in the unfilled 3d shell to the wave magnetic field,’’[14] and the effect ‘‘persists above the Curie temperature Tc S.D. LUO, Postdoctoral Research Fellow, C.L. GUAN, Ph.D. Candidate, Y.F. YANG, ARC Postdoctoral Research Fellow, G.B. SCHAFFER, Professor, and M. QIAN, Reader, are with The University of Queensland, School of Mechanical and Mining Engineering, ARC Centre of Excellence for Design in Light Metals, Brisbane, QLD 4072, Australia. Contact e-mail: [email protected] Manuscript submitted March 1, 2012. Article published online November 13, 2012 1842—VOLUME 44A, APRIL 2013

because each electron spin is able to respond to the alternating magnetic field of MWs even above Tc.’’[14] The same theory may apply to the MW heating and sintering of magnetic powder metals. The positive response of refractory powder metals to the MW radiation is not entirely surprising because these metals have a large covalent bonding component.[15] Titanium alloys are advanced structural materials, but the high cost of titanium components has impeded their wider adoptation for structural applications. A large proportion of the cost arises from the manufacturing process.[16] In this regard, powder metallurgy (PM) offers a promising route for both cost reduction and improved constitutional and microstructural capability for broad applications.[17,18] This makes MW heating and sintering particularly attractive, provided that the efficacy demonstrated on ceramics can be realized on titanium powder. The current understanding of MW heating and sintering of titanium is summarized as follows: (a) Response of titanium powder to MW heating Pure MW radiation appears to be ineffective in heating titanium powder.[12,19] No fundamental reason has been proposed, but this may be attributed to titanium being a paramagnetic metal where only some spins of its unpaired 3d electrons will be oriented by the external electromagnetic field[14] for limited magnetization.[20] Fortunately, the use of MW susceptors such as SiC can significantly