Effects of Chemistry on Toughness and Temperature on Structural Evolution in Metallic Glasses

The notch toughness of various Bulk Metallic Glasses (BMGs) were investigated. Systematic changes in composition to change the Poisson’s ratio wereemployed to increase the notch toughness of a variety of Fe-based BMGs. The Fe₅₀Mn₁₀Mo₁₄Cr₄C₁₆B₆ BMG possessed very high hardness (e.g. 12 GPa) but very low notch toughness (e.g. 5.7 MPam^1/2) at room temperature, consistent with fracture surface observations of brittle features. Many of the other Fe-BMG variants, created to change the Poisson’s ratio, exhibited higher toughness but more scatter in the data, reflected in a lower Weibull modulus. SEM examination revealed fracture initiation always occurred at inclusions in samples exhibiting lower toughness and/or Weibull modulus for a given chemistry. Implications of these observations on reliability of BMGs are presented.

In addition, high-temperature micro-hardness testing on fully amorphous Fe₄₈Mo₁₄Cr₁₅Y₂C₁₅B₆ was performed in order to determine the behavior and structure evolution under a variety test conditions. The effects of changes in test temperature on the micro-hardness/strength were determined over the temperature range from 25°C to 620°C. Although high (e.g. >12 GPa) micro-hardness was exhibited at 25°C, significant hardness reductions were exhibited near Tg. In addition, the effect of exposure time (up to 300 minutes)at elevated temperature on the evolution of micro-hardness/strength was also evaluated for selected temperatures between 25°C and 620°C. The micro-hardness results were complemented with X-ray diffraction (XRD), conventional transmission electron microscopy (TEM), and an in-situ heating TEM study in order to evaluate any structural evolution that could explain the large differences in hardness evolution under different test conditions.