Superheating behavior of NiAl
- PDF / 182,949 Bytes
- 5 Pages / 612 x 792 pts (letter) Page_size
- 94 Downloads / 191 Views
mosphere control (Figure 1). The glass tube was housed in a suitable chamber, which facilitated rapid insertion and removal of samples. Since both heating and levitation de-
NAGRAJ S. KULKARNI, Graduate Student, is with the Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611. KYUNG TAE HONG is Principal Researcher with the Division of Metals, Korea Institute of Science and Technology (KIST), Seoul, Korea 136-791. Manuscript submitted September 3, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Schematic representation of the electromagnetic levitation melting apparatus.
pend on the strength of the magnetic field, independent control of temperature was done by varying the flow rate of He gas (a highly conductive gas) in a mixture of purified Ar, while maintaining a constant power level for the duration of the experiment. The gases (initial purity 5 ppm O) were ultrapurified by passing them through a Ti gettering furnace at 800 7C. The purity of the gases was determined at a location just before their entry into the levitation chamber using a solid-state oxygen monitor.* The oxygen *Centorr model 2D oxygen monitor, CENTORR Associates Inc., Suncook, NH.
monitor is a Voltaic cell of the concentration type and has yttria-stabilized zirconium dioxide as the cell electrolyte. The voltage of this cell depends on the ratio of the partial pressure of oxygen at the two electrodes and is given by the Nernst equation. The concentration of oxygen in air (20.95 pct or 209500 ppm) establishes the potential of the reference electrode. The monitor registered oxygen levels of ,10220 atm. (10214 ppm) for the ultrapurified gases used in this study. Such low levels of oxygen are achieved by the preferential oxidation of porous Ti in the gettering furnace (hence reducing the oxygen concentration in the gases) since the oxygen partial pressure of Ti in equilibrium with TiO2 is ,10235 atm at 800 7C, as seen from a standard Richardson–Ellingham stability diagram for the formation of oxides. This high-purity atmosphere protected the NiAl samples used in this study from oxidation. The gas flow lines and various fittings employed in the entire setup were made from stainless steel. Small button-head NiAl samples weighing 1 to 2 g, that were used for levitation processing, were obtained from arc-melted NiAl ingots of known composition. The ingots were prepared by arc melting high-purity (99.99 pct) Ni and Al pellets of weighed proportions in an Ar atmosphere that was purified by passing through the Ti gettering furnace. Wet chemistry* and inductively coupled plasma emis*Nickel by dimethylglyoxime hydroxyquinoline precipitation.
precipitation
and
Al
by
8-
VOLUME 29A, AUGUST 1998—2221
Fig. 2—Schematic of a typical output during levitation processing of NiAl alloys.
sion spectrometry (ICP)** analysis were employed to de**Perkin-Elmer ICP/5000 (Perkin-Elmer Physical Electronics, Eden Prairie, MN).
termine the Ni and Al contents. To ascertain the purity of the levitation chamber, combust
Data Loading...
