Microwave Combustion Synthesis and Sintering of Intermetallics and Alloys

The high melting points and high-temperature stability of many intermetallics=alloys make them useful for high-temperature applications, especially for corrosion and oxidation resistance. Some intermetallics ®nd applications in wear components and oxidation barriers as well as reinforcing phases in many metallurgical systems, including superalloys. Intermetallics and alloys have been prepared by a variety of methods including casting, mechanical alloying, gas atomization, sintering, combustion synthesis, etc. In the combustion synthesis process (also called self-propagating high-temperature synthesis), two or several mixed reactants react exothermically in a self-sustaining manner, to form products due to the large difference in free energy and enthalpy between the product and reactants [1, 2]. In the conventional combustion process, reactants are ignited in a resistance furnace or by a hot ®lament or laser. The novelty of the present method is the use of microwave energy to initiate the combustion reaction between metals to form intermetallic compounds or alloys. Investigations in the area of microwave combustion synthesis previously were successful only for ceramics and composites [3]. A second novelty of this method is the sintering of metal±metal systems using microwaves. Microwave sintering of white ceramics with marginal success has been studied for two decades (see reviews in [4, 5]). Recently, in our laboratories, it was used in a new con®guration for sintering to essentially theoretical density translucency and even transparency of a wide range of ceramics: alumina, mullite, spinel, etc. [6, 7]. The earlier disputed question ` Is there microwave effect?'' has been de®nitively answered not only in the radically enhanced (one to two orders of magnitude) kinetics of sintering but the totally different reaction pathways for reactions such as BaCO3  TiO2 ! BaTiO3  CO2. So far, there have been no reports of metal±metal chemical reaction processes or sintering through solid-state or liquid-state diffusion using microwave energy, possibly because metals invariably cause plasma discharges within a microwave cavity and are therefore unsuitable for microwave synthesis and sintering. It was recently demonstrated in this laboratory [8], however, that a wide variety of compacted metal powders and ordinary commercial powder metal compacts can couple effectively with microwave ®elds and be synthesized or sintered at least as well as or better than conventional heating. This letter reports on the use of this new method for the preparation of intermetallics and alloys by combustion synthesis and sintering using microwave energy. Microwave heating is fundamentally different from conventional heating because all the energy is directed at the workpiece, as is also the case in laser processing. In microwave processes, the heat is generated internally through material±microwave interaction instead of originating from an externalheating source. This internal volumetric heating results in an energy diffusion that is reverse to the direction of those observed in conventional heating. As noted, the large body of work existing on microwave processing of polymer and ceramic materials has been summarized elsewhere [4, 5] but will be repeated here. For metal processing, as expected, there is almost no work. Even ®nely powdered metals do not couple well with microwave energy at room temperature. Walkievwicz et al. [9] reported that 25 g samples of powder of a half-dozen metals (Al, Co, Fe, etc.) exposed for 1 h in a 2.45 GHz cavity reached temperatures of 120±770 8C and showed no sintering. One US patent [10] refers to composites of metals and oxides being sintered in microwaves. We have demonstrated that microwave coupling ef®ciency of metals is greatly increased by increasing the temperature of the sample. This can be achieved by starting with a hybrid method in which an external susceptor is used to trigger the reaction, but we established that exactly the same result can also be achieved in a pure microwave system but with slower heating rates. Once the material is heated to a ` critical'' temperature, enhanced microwave absorption becomes suf®cient to cause self-heating, and the susceptors can be removed. As the temperature of material increases, the absorption of microwave energy by the material also increases. Extensive work on commercial powder metal samples [8] has established the experimental feasibility of sintering virtually all powder metal compacts. Our focus was on intermetallic synthesis and sintering. Stoichiometric and nonstoichiometric mixtures of metal powders in systems Ti±Al, Ni±Al, Fe±Al, Nb±Al, Ta±Al, Ge±Al, Cu±Al, Co±Al, Mo±Al, W±Al, Pt±Al, Pd±Al, Sb±Al, Zr±Al, Cr± Al, Nb±Ge, Ti±Ni, Ti±Co, Ti±Fe, Cu±Ti, Cu±Zn, Fe±Ni, Ni±Mg, Ni±Zr, Ti±Cu±Al, Cu±Zn±Al, Nb±Ti±Al, Nb±V±Al, Nb±Zr±Al, Ta±Al±Fe, Ni±