Nonequilibrium Alloys in Systems with Positive Heat of Mixing

Johns Hopkins University, Baltimore MD

Investigators

Abstract

0080361 Ma This research looks at nonequilibrium alloys created in binary systems exhibiting a positive heat of mixing. It builds on recent progress in understanding such phases alloyed in the solid state using mechanical milling. Several unresolved fundamental issues are identified regarding the structural and thermodynamic nature of alloys produced far from equilibrium. The alloys to be studied are amorphous phases or highly supersaturated crystalline solid solutions prepared through vapor quenching using a co-sputtering technique. The model systems include Ag-Ni, Cu-TA, Cu-Nb, Ag-Cu, Cu-Fe, and Cu-Cr. The plan is to systematically study the local atomic-level structure using Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) techniques. Short-range clustering or ordering tendencies and bond lengths are examined. In addition to proving true atomic-level alloying, local environment information will be correlated with the thermodynamic states quantitatively determined through extensive calorimetric characterizations of the deposited foils. %%% This work will identify the reasons for the stability of the phases formed relative to other competitors. The similarities and differences for alloys obtained in the same system through different processing routes are also considered. Possible decomposition and compositional modulations, effects on alloy energetics and stability, as well as competing mechanisms of spinodal decomposition versus nucleation and growth, will also be examined. ***