The thermochemistry of magnesium in nickel-base alloys: Part I. the determination of thermochemical parameters using the
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I.
INTRODUCTION
The determination of thermochemical parameters of metallic solutions has relied in the past very extensively on the various techniques of vapor pressure measurement and, hence, the application of representations based on Raoults' law. The direct methods of vapor pressure measurement become difficult at high temperatures due partly to the problems of construction materials and partly to those of assuring temperature uniformity over the large experimental volumes necessary. Methods which have been used are those of the Knudsen technique, mass spectrometry, and pseudoisopiestic equilibrium, which are briefly reviewed below, and that of direct measurement of vapor concentration using atomic absorption spectrophotometry. The objective of the present research was to develop a technique which could be used with reactive systems at high temperature, and the specific case chosen was that of magnesium in nickel-base alloys. A. Knudsen Cell Methods A general literature survey and discussion of the Knudsen cell or Effusion method is given by Cater, tl,21 and the mass transfer aspect is discussed by Geiger and Poirier. t3j The Knudsen method can be briefly described as follows: The evaporating substance is enclosed in a container with a small knife-edged orifice inside a vacuum sufficient to ensure molecular diffusion. The amount of gas escaping the container is representative of the vapor pressure of the substance, according to the Hertz-Knudsen equation: P =Att
[1]
where p = pressure of gas; w = weight loss;
E. SAMUELSSON, formerly Graduate Student, University of British Columbia, is Group Leader, Physical Metallurgy, Special Metals Corporation, New Hartford, NY 13413. A. MITCHELL, NiDI/NSERC Research Professor, is with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Manuscript submitted September 30, 1991. METALLURGICAL TRANSACTIONS B
t R T M
= = = =
time; gas constant; temperature; and molecular weight.
Knudsen t41 in his landmark measurements of mercury vapor pressure simply condensed the effusate and measured the volume. Subsequently, a number of detection methods have been developed. However, only mass spectrometry has provided direct intensity measurements of specific vapor species. The simplest approach to Knudsen experiments is to weigh the cell before and after heating to the experimental temperature. This of course assumes that significant evaporation of only one species occurs. The technique suffers from two major disadvantages. First, the vacuum has to be broken for each measurement. Second, weight loss during the heating and cooling cycle must be accounted for. Both of these problems are resolved by suspending the cell from a vacuum balance.t21 If the effusate is condensible, it may be collected on targets coaxial with the orifice. An example of such a system is shown in Figure 1. This technique has two major advantages. First, clausing correction factors are not needed (clausing factors correct for the use of a channel ty
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