Calcium Based Metallic Glasses: Ductile by Design; Embrittlement by Environment

Metallic glasses made predominately from calcium were, for a short time, of interest for use within the transportation and aerospace industries. This was because of their low density, favorable strength to density ratio, and relatively low materials cost. When the other constituent elements of the Ca glasses are also biodegradable, e.g., Ca-Mg-Zn, then there is interest for their use in the biomedical industry.
For these applications, the glasses need to be strong and ductile, or at least less brittle. Most metallic glasses have a high tensile strength, when compared to their crystalline counterparts, but low ductility, and therefore low toughness. From a previous study at UVa, one way for increasing the toughness in Fe-based glass was to employ an alloying strategy where the addition of solute elements would tune the electron structure of the glass composition in a direction that leads to more metallic behavior of the host element, in this case iron. A similar electronic structure manipulation strategy model was proposed and used here to determine which calcium-based glass compositions might improve toughness. From this work it was determined that Ca-Mg was the most ductile system, while Ca-Al and Ca-Cu systems were very brittle. The binary system with Mg was more ductile than the Al or Cu systems because when alloyed, the lowest amount of charge was transferred away from Ca, the solvent metallic element.
Adjusting the compositions, Ca72Mg28 was determined to be the most ductile. Unfortunately, this predicted and experimentally verified ductile composition rapidly embrittled in ambient lab conditions, once removed from a vacuum. The time for embrittlement was on the order of minutes. Based on these results, a research project was established to determine, as clearly as possible, the atomic/electronic mechanism for this embrittlement. The goal was that if the process could be understood, then it might be possible to mitigate the deleterious effects. The research proposed was to isolate the atmospheric component(s) responsible for the embrittlement and then to follow the chemical reactions within the bulk of the Ca-glass in a detailed fashion. Further, the hypothesis to be tested is whether this atmospheric component is responsible for any charge transferred away from calcium, thus decreasing the metallic bonds present and in the process embrittling the Ca glass. This would be an independent test of the strategy applied for the Fe-based amorphous steels and demonstrate that it is applicable across alloy systems.
To study this process the alloy Ca75Mg15Al10 was selected. It was less ductile than Ca72Mg28 but its duration until becoming brittle was nearly 30 minutes long. Melt-spun ribbons of Ca75Mg15Al10 about 20 microns thick were tested. Ductility was monitored through a bend-over-mandrel critical failure test that was performed on these ribbons, after they were exposed to various ambient lab conditions and individual high purity components of the atmosphere. Following these tests it was determined that the condition leading to embrittlement was a reaction of the calcium ribbon surface with humidity, or water vapor, in the lab air.
Following the identification of the atmospheric component leading to rapid deterioration of the metallic glass ribbons, reactions of H2O with the ribbon were more closely examined. Here it was determined that the humidity dissociates into hydrogen as a byproduct of the reaction between H2O and Ca to produce Ca(OH)2. This is the initial reaction responsible for the embrittlement mechanism. The free hydrogen penetrates the Ca glass rapidly to bond with metal Ca in the ribbon bulk to form CaH2, a strong ionic salt. The ionic bonds replace the metallic Ca bonds leading to charge transfer, thus embrittling the calcium glass.
To verify this process, measurements of various calcium compound formation and determining the location of hydrogen over time was done using x-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Their results plus the growth kinetics of the Ca(OH)2 layer, that was grown in a 75% relative humidity (RH) environment, measured using scanning electron microscope (SEM), established that hydrogen is causing the embrittlement though electron transfer from calcium. Further, it is water vapor from the atmosphere that initiates the process.