Thermal arrests

The temperature-composition phase diagram constructed from thermal arrests observed in the MoFe-UFa system is characteristic of a binary system forming solid solutions, a minimum-melting mixture (22 mole % UFe at 13.7°C.), and a solid-miscibility gap. The maximum solid solubility of MoFq in the UFe lattice is about 30 mole % MoFe, whereas the maximum solid solubility of UFe in the MoFe lattice is 12 to 18 mole % UFe- The temperature of the solid-state transformation of MoFe increases from ——lO C. in pure MoFe to 5°C. in mixtures with UFe, indicating that the solid solubility of UFe is greater in the low temperature form of MoFe than in the high temperature form of MoFe- This solid-solubility relationship is consistent with the crystal structures of the pure components The low temperature form of MoFe has an orthorhombic structure similar to that of UFe. 

This paper describes the results of an experimental study of condensed phase equilibria in the system MoFe-UFo carried out by thermal analysis and x-ray diffraction analysis. A temperature-composition phase diagram is constructed from the temperatures of observed thermal arrests in MoFe-UFe mixtures, and the basis for the formation of this particular type of diagram is traced to the physical properties of the pure components. The solid-solubility relations indicated by the diagram are traced to the crystal structures of the pure solids. 

After a sample tube had been charged, it was heated to 70°C. to melt both components. The tip of the tube was then placed against a block of dry ice. This procedure vibrated the tube, mixed the components, and condensed them into the bottom of the tube. The tube was warmed again and maintained at a temperature of 70°C. for an hour then the sample block assembly was heated or cooled at a programmed rate to observe the thermal arrests from which solidus and liquidus temperatures were obtained. Programmed thermal analysis was carried out two to five times on each mixture. Some of the thermal analyses were carried out at a heating rate of 0.6 °C. per minute most of the analyses were carried out at a heating rate of 0.3°C. per minute. 

Voltages of the sample thermocouple corresponding to thermal arrests were converted to temperatures using N.B.S. Circular No. 561 (17). Table I hsts the temperatures of thermal arrests for the entire range of composition between 0 and 100 mole % UFg. The values listed in Table I are averages of several measurements, and the uncertainty values are standard deviations of the averages. The uncertainty values associated with the liquidus points obtained by the extrapolation procedure (i) have not been estimated. 

The temperatures of thermal arrests are plotted as a function of composition in Figure 1. The lines have been drawn to suggest the location of equilibrium phase boundaries, and the best interpretation of the thermal analysis data. The resulting diagram is characteristic of a system exhibiting solid solubility with a minimum melting point and a solid-miscibility gap. 

The thermal arrests obtained at —5°C. in the region S2 are attributed to the transformation of Si to S3 in the nonequilibrium mixture of S2 plus MoFe-rich solid. 

Figure 7.10 The cooling behavior of a glass fusion specimen (a) too rapidly cooled results in a highly strained fusion bead (b) very slow cooling from the melt causes crystallization to occur, (c) ideal cooling behavior which includes a short thermal arrest at 500°C producing a tough, tempered fusion bead (d) intermediate-rate continuous cooling produces a satisfactory bead if conditions are controlled properly.

The specimen is heated stepwise with 0 = 0.1-10 K min. After every 10 K heating Step, a new quasi-balance state is established for the thermal arrest. Each individual measurement for one C, value corresponds to an enthalpy measurement. The time for heating between two temperature steps is fixed in these measurements as 25% of the total. Thus an average 0 is one quarter of the original 0. 

A schematic diagram of the measurement is shown in Figure 4.92. An advantage of the enthalpy method is that relaxation of amorphous alloys does not influence the measured specific heat (Cp because the relaxation arises not only during the heating, but also during the thermal arrest. Therefore, relaxation hardly changes C . 

The polymorphic transition between a- and P-UC2 is too rapid to allow the cubic form to be retained to room temperature. Using a high temperature x-ray camera, Wilson (1960) observed the transition at 1820°. However, thermal arrest work at Los Alamos has placed this temperature at first near 1800° (Witteman, 1963 Rupert, 1963), then at 1785° + 20°... 

Wallace et al., 1964), and finally, with a more sophisticated thermal arrest apparatus, 1765° + 20° was obtained (Witteman, 1966). This was confirmed by Cook (1965), using thermal analysis, with a temperature of 1768° 25°. All of this work was done in the presence of graphite. There is no doubt the transition is 15°-20° higher when UC is the second phase (Wilson, 1960 Witteman, 1963). 

Since the thermal arrests were within 1.5°C on both the heating and cooling curves, as is the case for pure metals, the liquidus-solidus line and the solvus line for the bcc hep transformation were drawn as single lines. The transformation temperature of terbium was found to be raised linearly by the addition of holmium but at a greater slope than the melting temperature. Extrapolation of the transformation temperature curve to the solidus showed that the two curves intersect at 90 at% Ho. This confirms the absence of the bcc form at high temperatures in holmium. 

Secondary carbides precipitate as the result of thermal exposures during fabrication operations or during component service life. These carbides precipitate preferentially at grain boundaries and internal structural defects such as twin boundaries and dislocations. The quantity of secondary carbides that precipitate depends on the amount of carbon in solutions, the exposure temperature, and the time at such temperature. Therefore, conditions that generate a supersaturated solution of carbon followed by slow cooling or thermal arrests below carbide solvus temperatures will produce heavy secondary carbide precipitation, which generally reduces ductility and toughness, and this adversely affects fabrication and service performance. 

Typical melting curve showing the thermal arrest. 

Singh and Raman (1968) furthermore claim the existence of two polymorphic forms of Nd3Co, whose Nd-rich ji-form has not yet been evaluated (a-Nd3Co was said to be Fe3C-type). These data concerning NdjCo and Nd2Co7 are at variance with Ray et al. (1973) who failed to observe indications for a thermal arrest associated with the proposed phase transformations. 

Heating and cooling curves are used in determining phase diagrams, since all (first-order) phase changes give rise to thermal arrests (the temperature remains constant for a period of time). Differential thermal analysis (DTA) is a refinement of the time-temperature method. In a DTA experiment, the sample is heated simultaneously with a standard sample, usually an inert material... 

The boride based on Fc3B was identified after rapid solidification as a constituent of metastable eutectic (Fe) + Fc3B [1995Go1, 1982Ber]. However, no thermal arrest of M3B separation was found in [1982Ber] before melting. 

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