Time-dependent twinning during ambient temperature compression creep of alpha Ti-0.4Mn alloy
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REFERENCES 1. R.E. Spjut: Noncontact Temperature Measurement, NASA Conf. Pub. 2503, M.C. Lee, ed., NASA, Washington, DC, 1987, pp. 182-213.
Discussion of “Superheating Behavior of NiAl”* WILLIAM HOFMEISTER The experimental observations of Kulkarni and Hong have a simple explanation that does not involve superheating behavior in NiAl. Similar time-temperature profile observations have been made in our laboratory on these alloys in electromagnetic levitation processing. We attribute this behavior to the formation of a surface oxide layer on solidified samples. This surface oxide has a much higher emissivity that the clean solid or the liquid melt, so that the apparent temperature on reheating of the solid is much higher than the apparent temperature of the liquid at the melting temperature. The apparent reduction in temperature on melting, shown in the authors’ Figure 2, is simply the melting or dissolution of this surface oxide to reveal the liquid at the melting temperature. We have processed NiAl alloys in a vacuum of 1 3 1025 torr for as long as 20 minutes at apparent temperatures several hundred degrees above the melting temperature in an effort to dissolve this oxide layer. Following one of these experiments, the sample was cooled and removed from the chamber, and a deep groove was cut in the sample with a mill bastard file. The sample was reinserted into the chamber and was melted at the normal apparent melting temperature during heating. Filing the oxide layer allows liquid from the interior to flow over the surface of the sample and dissolve the surface oxide. High-temperature alloys containing aluminum readily form surface oxides, and these oxides are difficult to reduce. In processing Nb3Al alloys, it was common to reach very high apparent temperatures (.2500 K) while maintaining a solid shell on the sample. When the oxide shell was broken at these temperatures, hot liquid was ejected and the sample was lost. We have concluded from these experiments that the interior of the samples was liquid and the surface was a solid oxide shell. The use of ratio pyrometry further complicates the analysis of the results. Spjut[1] pointed out that uncertainty in the spectral emissivity («l) has a much greater effect on ratio pyrometry than on single color pyrometry. In single color pyrometry, the uncertainty is proportional to ln («l), while in ratio pyrometry, the uncertainty is proportional to «l1/«l2. Certainly, the ratio method breaks down when comparing emissivity ratios of alloys to emissivity ratios of the oxides of the alloys. In short, the knowledge of spectral emissivity is essential to accurate temperature measurement with any pyrometric technique.
*NAGRAJ S. KULKARNI and KYUNG TAE HONG: Metall. Mater. Trans. A, 1998, vol. 29A, pp. 2221-25. WILLIAM HOFMEISTER, Research Associate Professor, is with the Department of Chemical Engineering, Vanderbilt University, Nashville, TN 37235. Discussion submitted September 1, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
Time-Dependent Twinning during Ambient Tempera
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