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Tensile and bend properties of nickel and iron-base amorphous alloys have been studied at 200 C in the presence of liquid lithium and in air. Nickel base alloys show severe embrittlement, except for Metglas 2826MB. The results are discussed on the basis of proposed mechanisms of LME and it is concluded that the embrittlement of amorphous alloys takes place by an enhanced shear mechanism. Hydrogen embrittlement also seems to occur by a similar process. (Author)

An infinite periodic heterophase dislocation array is used as a model of a tilt boundary near the melting point of a crystalline solid. The elastic interaction among dislocations is included via a traction-free-core boundary condition. The elastic energy is coupled to the chemical energy from the core by an energy minimization scheme of the core radius. The theory yields the total grain boundary energy as function of tilt misorientation, which is found to compare favorably with experiments on bismuth.

The potential theoretic methods recently developed at NRL for solving the diffusion equation are applied to the problem of lamellar eutectic solidification. This approach leads to a set of coupled nonlinear integro- differential equations for the shape of the solid-liquid interface and the solute concentration on the interface. The general characteristics of the solutions to these equations are discussed, and in particular it is shown that lamellar solutions may not be possible when the ratio of thermal gradient to freezing rate is less than some critical value which depends on the phase properties and the phase fraction. Selected numerical results are presented for the Pb-Sn eutectic system and compared to the results of Jackson and Hunt, who assumed a planar interface model. It is shown that the planar interface model can be a source of considerable error under certain conditions, and that the self-consistent model provides additional theoretical insights.

Periodic Peltier heating and cooling may be generated at a solid/ liquid interface by the passage of an alternating electric current. Melting and freezing will then occur at rates which will be influenced by the kinetic law of molecular attachment relating solidification velocity to interface temperature. The equation describing the interface attachment kinetics may be determined by measuring the response of a solid/liquid interface to square-wave Peltier heating and cooling. The mathematical methods required for extracting the kinetic equation by analysis of the waveform of interface motion show that this technique should give distinguishably different waveforms for linear, parabolic, and exponential kinetics. There are advantages of the method, but precautions must be taken to eliminate possible confusing effects., Sponsored in part by the DARPA.

Isenthalpic solidification of a pure supercooled liquid is shown to result in either a two-phase solid/liquid mixture in invariant equilibrium or a single-phase, totally solid material in univariant equilibrium, depending on the level of supercooling prior to solidification. The critical supercooling above which univariant equilibrium is obtained is large for metals (hundreds of centigrade degrees) but much smaller for certain molecular substances. Experiments on white phosphorus (alpha P4) show that the critical supercooling (25.6C) can be reached, and exceeded, easily. Solidification rate measurements taken above and below the critical supercooling for P4 show that the solid/ liquid interface temperature varies smoothly with melt supercooling, although light-scattering experiments indicate that rapid changes occur in the extent of the dendritic zone as the critical supercooling is approached and exceeded. A method for extracting interface attachment kinetics from solidification rate data was examined in detail and applied to our rate measurements on P4. We find that above about 9C supercooling, P4 solidifies with linear attachment kinetics having a rate constant of 17.7 plus or minus 0.4 cm/sec C. Below about 1C supercooling, P4 solidifies with a faceted morphology indicative of layer- passage limited kinetics. Between 1C and 9C supercooling, transitional growth kinetics occur. These results are in qualitative agreement with the crystal growth theory of Cahn, et al., which predicts that attachment kinetics should change as the driving force for crystal growth is varied by substantial amounts., Sponsored in part by the DARPA.

Convective mass transfer occurs during solidification because of the difference in mass density between the participating solid and liquid phases. The relationship between interphase mass transfer and the mechanical behavior of a bounded system undergoing rapid solidification is developed from an analysis of the kinematics and dynamics of dendritic freezing. The kinematic analysis yields theoretical expressions for the displacement, velocity, and acceleration imparted to the center of mass of a constrained melt by the solidificationinduced mass transfer, whereas the dynamical analysis relates the changes in external force and internal pressure with the accelerations of the solidifying body's mass center. These analyses reveal that a critical degree of supercooling exists, above which the dendrite velocity can increase without a concomitant increase in the acceleration of the center of mass, and that this condition coincides with the onset of cavitation in the specimen. Next, some new experiments which provide quantitative information on the mechanical behavior of rapidly freezing bismuth melts are discussed. Finally, a unified approach to solidification dynamics is presented in a discussion of the following effects: (a) the emission of acoustical disturbances during solidification, (b) the occurrence of an anomalous refinement in the as-cast grain size of nickel and cobalt specimens frozen from highly supercooled melts, (c) the development of cavitation pits on the constrained surfaces of rapidly frozen transition metal speciments, and (d) the presence of pressure pulses both during and after rapid dendritic growth.