Thermomechanical Anisotropy of Bulk Metallic Glasses Induced by Elasto-static Loading for Prolonged Time at Room Temperature - A Case Study of Copper Hafnium Aluminum Bulk Metallic Glass.

摘要

Bulk metallic glasses exhibit potential applications as structural and functional materials due to their excellent mechanical properties, high wear resistance, good corrosion resistance for many glassy alloys, and promising magnetic properties. Most existing and potential applications of metallic glasses entail elastic deformation---either in a cyclic manner or in a quasi-static mode. Recent reports have indicated that sustained elastic deformation of metallic glasses could improve their ductility. This rather unexpected result was explained with the free volume concept, but it is unclear if the free volume theory is adequate for capturing the effects of static elastic loading on metallic glasses. The dissertation centers on static elastic compression of a Cu50Hf41.5Al8.5 model bulk metallic glass. The objectives are to test if the free volume theory is useful in describing the elasto-static compression experiments; secondly, to test the hypothesis that the elastostatic compression induces anisotropy of thermomechanical properties. Bulk metallic Cu50Hf41.5Al8.5 glass samples were synthesized using suction casting and then uniaxially compressed at stresses between 60 % and 82 % of the yield stress for prolonged times. Results were analyzed mainly with thermomechanical analysis and modulated thermomechanical analysis in different sample directions. Thermomechanical polarization, i.e., an anisotropy in the thermomechanical behavior was observed after at least 20--25 hrs of loading at 60 % of the yield strength. With an increase in the compressive stress from 650 MPa to 812 MPa or 82 % of the yield strength, permanent deformation can be induced. The thermomechanical measurement results are discussed in light of an activation energy spectrum model. A decrease was determined in energy (0.057 eV/atom) in the loading direction and an increase in energy (0.037 eV/atom) in the direction perpendicular to the loading direction. These changes are interpreted as a coupling of the applied stress with defects in the metallic glass that emulates secondary-relaxation. The dissertation provides a basis for understanding configurational changes in metallic glasses during elasto-static compression. A main outcome of this dissertation is that the notion of elastic deformation of metallic glasses has to be modified. This finding should be highly relevant for applications of metallic glasses for springs or microparts.