OPTICAL-PROPERTIES AND SPECTRAL EMISSIVITIES AT 632.8 NM IN THE TITANIUM-ALUMINUM SYSTEM

The spectral emissivities, refractive indices, and extinction coefficients of liquid titanium-aluminum alloys were measured vs temperature by He-Ne laser ellipsometry at 632.8 nm. Results were obtained on overheated liquids and on liquids undercooled by as much as 350 K. The experiments were conducted under containerless conditions using electromagnetic levitation and heating supplemented by CO2 laser beam heating. The undercooled melts exhibited spectral emissivities that were nearly independent of temperature, and emissivities varied slightly with temperature for the stable liquids. The spectral emissivities showed a maximum at near-equiatomic compositions and a minimum at a composition near pure liquid aluminum. The temperature coefficients of emissivity were negative for near-equiatomic compositions and positive for aluminum-rich alloys. The emissivity was determined to be independent of temperature at a composition of approximately 75 at. pct Al. This was termed the ''X-point composition,'' because it is the composition at which, for a wavelength of 632.8 nm, the spectral emissivity vs composition isotherms cross. Significant errors in noncontact temperature measurements would result if the spectral emissivities were estimated from a linear interpolation between the pure elements.