Instabilities in nonequilibrium solid solutions with respect to amorphization and melting

Citation

Krill, Carl Emil (1992) Instabilities in nonequilibrium solid solutions with respect to amorphization and melting. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/7vb7-vt33. https://resolver.caltech.edu/CaltechETD:etd-08072007-072811

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.

Recent years have witnessed the discovery of crystal-to-amorphous phase transformations induced by a variety of experimental processes. The kinetic and thermodynamic parallels between such amorphization transitions and ordinary melting are rather extensive, suggesting that the underlying physics are similar. In particular, the idea that melting occurs as the response of the crystalline phase to development of an intrinsic instability is quite controversial, but its analogue with respect to amorphization is gaining acceptance. This thesis investigates the validity of the instability concept for amorphization by searching for precursors to topological collapse in highly nonequilibrium solid solutions. In Chapter 2 the advantages and disadvantages of various means of synthesizing solid solutions beyond equilibrium solubility ranges are discussed, with particular attention being devoted to prediction of an alloy's potential for supersaturation from the characteristics of its equilibrium phase diagram. Hydrogenation and rapid quenching from the melt are found to achieve the best compromise between the kinetic requirements for sample preparation and the desired sample geometry for property measurements. Hydrogenation is applied in Chapter 3 to the progressive destabilization and amorphization of the intermetallic compound ErFe2. Through a combination of high-temperature neutron diffraction and differential scanning calorimetry, the enthalpy release and structural change associated with the amorphization transition are studied. The transition is found to be strongly exothermic and driven by short-range clustering of the metal atoms. Rapid quenching from the melt is used in Chapter 4 to prepare highly supersaturated alloys of Nb-Pd. Low-temperature neutron diffraction measurements made at several compositions of the bcc [alpha]-Nb solution find an anomalously large increase in the atomic root-mean-square displacement to about half of the value at which the Lindemann criterion predicts the lattice should melt. Low-temperature heat capacity measurements find a concomitant decrease in the Debye temperature, suggesting that supersaturation causes an elastic modulus to soften. Single crystals of the [alpha]-Nb solution at the highest supersaturation have a highly anisotropic structure visible in transmission electron microscopy images; it is consistent with the development of a local instability with respect to the bcc-to-[...] phase transformation. The ramifications of these experimental findings are considered in the final chapter. Violation of the polymorphous constraint with increasing supersaturation complicates quantification of the stability of a solid solution, but the results of this thesis definitely favor some proposed stability parameters over others. In particular, the anomalously high atomic mean-square-displacements found in supersaturated [alpha]-Nb solutions are most consistent with a mechanism of highly local topological fluctuations leading to shear softening as a precursor to--and probable cause of--amorphization.

Thesis Files