Date of Award

12-1963

Degree Type

Thesis

Degree Name

Master of Science

Major Professor

Dr. E.E. Stansbury

Abstract

This investigation is a continuation of research on the nickel-molybdenum binary alloy system which has been in progress in the Department of Chemical and Metallurgical Engineering of The University of Tennessee for the past several years in association with the Oak Ridge National Laboratory. The purpose of the present investigation was to develop an electrical resistivity method for studying the order-disorder transition in the nickel-molybdenum alloy of stoichiometric composition, Ni4Mo.

The alloy containing 29.1 weight per cent molybdenum in nickel was prepared by melting 250-gram ingots in a direct arc furnace under an inert gas. These were homogenized in argon for 100 hours at 1200 degrees Celsius and water quenched to retain the high temperature face-centered-cubic alpha phase. Samples for electrical resistivity measurements were prepared by swaging (or rolling) and machining the homogenized ingots to approximately 0.1 inch diameter rods about 6 inches in length. Pretreatment of the alloy by quenching from above 868 degrees Celsius produced an initial state of retained alpha phase for subsequent resistivity measurements. Heat treatments below this temperature produced the long range order Ni4Mo phase.

An apparatus was designed and constructed for the continuous measurement of electrical resistivity at high temperatures. This apparatus permitted the continuous recording of resistivity-temperature curves during heating and cooling through the transformation temperature of the alloy. It also provided a means to study the kinetics of ordering in the alloy using electrical resistivity as the index of order.

The principal results of this investigation can be summarized as follows:

1. The resistivity-temperature curves for the alloy in the initially ordered state exhibited positive slopes but with a marked negative deviation from linearity which increased rapidly with increasing temperature. The alloy undergoes a discontinuous change in resistivity at the critical temperature (868 degrees C), characteristic of an order-disorder transformation.

2. From 868 degrees C to 1000 degrees C, the alpha phase exhibits a negative temperature coefficient of resistivity which has been attributed to a decreasing degree of short range order with increasing temperature.

3. When the alloy is cooled below the critical temperature a sharp decrease in resistivity occurs at undercoolings of 60-90 degrees C due to the rapid onset of ordering. The immediate degree of order, however, is not complete and the resistivity-temperature curve on further cooling is cooling-rate dependent. Holding at temperatures below the transformation temperature resulted in a slow decrease in resistivity as equilibrium was approached.

4. The temperature coefficient of resistivity of the retained alpha phase is negative near room temperature. However, at higher temperatures the resistivity increases rapidly to a maximum near 650 degrees C and then decreases. This behavior has been attributed to the existence of a critical domain size and/or degree of long range order developed on heating.

5. Isothermal transformation of the retained alpha phase to the low temperature beta phase at temperatures below the critical temperature was followed by measuring the change in resistivity which accompanies it. The most rapid transformation was found to occur at a temperatures in the range 760 degrees to 800 degrees C.

6. Isothermal resistivity-time curves were used to construct a time-temperature-transformation diagram for the alloy. The "nose" of the diagram was found to lie within the temperature range 710 degrees C to 775 degrees C. The rate of transformation was extremely slow below 600 degrees C.

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