Superheating behavior of NiAl

Abstract

The intermetallic compound NiAl has been widely studied for its possible use as a high-temperature structural material.[1] The preparation of NiAl alloys of controlled composition and stoichiometry has been recognized to be an important component in their continued development and understanding. Control of interstitial elements such as oxygen and carbon is necessary to minimize the effect of these elements on the mechanical properties of such intermetallics. A promising laboratory technique that can be used to circumvent these problems involves the containerless processing of metals and their alloys using electromagnetic levitation.[2,3] Compared to other conventional processing techniques, electromagnetic levitation processing offers several unique advantages,[2,3] such as (1) noncontamination of the melt since there is no contact with crucible materials; (2) homogeneity of the melt due to extremely efficient electromagnetic stirring and homogeneity of the solid due to rapid solidification; (3) rapid equilibration in metal-gas systems due to the large surface area of the melt exposed to the gas phase and to efficient stirring, thus permitting control of interstitial elements such as O, C, N, and H; and (4) laboratory survey work of unknown alloys and the investigation of basic phenomena such as melting, vaporization, nucleation, and solidification. Since only small samples weighing 1 to 2 g can be used during levitation processing, the application is limited to laboratory purposes. In the case of alloys that contain an element having a high vapor pressure (e.g., Al in NiAl), the technique is especially attractive since it permits control of the stoichiometry (Ni/Al atom ratio) by preferential evaporation of the high vapor pressure element. The present study describes an unusual superheating and transformation phenomenon observed during electromagnetic levitation processing of the intermetallic compound NiAl. A high frequency generator was used to supply a sinusoidally varying power with frequencies ranging from 50 kHz to 5 MHZ to a step-down transformer, which in turn was used to supply currents of several hundred amps to a specially designed coaxial levitation coil (1/8-in. Cu tubing and 0.02-in. wall thickness). The resulting electromagnetic field was used for levitating and simultaneously internally heating (due to eddy current losses) a given sample. The sample was contained within a PYREX* glass tube for at-