Influence of sulfur addition on the glass formation, phase transformation and mechanical properties of Cu50Zr50 alloy

In this work, a thorough study on the effects of sulfur-addition in the Cu$₅₀$Zr$₅₀$ glass-forming system was conducted using laboratory-based X-ray diffraction (XRD), synchrotron-based high-energy XRD (HESXRD), differential scanning calorimetry, and uniaxial compression testing. We report that the glass-forming ability (GFA), thermal stability, phase transformation sequence during heating and cooling, as well as the mechanical properties of the Cu$₅₀$Zr$₅₀$ system are highly sensitive to the sulfur concentration. Proper addition of sulfur can not only stabilize the B2-CuZr phase, but also enhances the GFA of the system. This enables the formation of BMG composites that solely contain a B2 phase, which results in a combination of high strength and substantial plastic strain under compressive loading. The total strains of the newly developed BMG composites with sulfur contents of 0.5 and 1.0 at.% are as large as ∼10 at.%, exceeding the previously reported strains of the Cu50Zr50 BMG by far. These superior properties make the Cu-Zr-S BMG composites a promising candidate material for industrial applications. In addition, it is found that excessive addition of sulfur leads to the precipitation of the Cu$₁₀$Zr$_7$ phase upon cooling from the alloy melt and a sulfide phase during heating from the glassy state. HESXRD experiments suggest that the preferential bonding between Zr and S atoms could be the determining structural mechanism on the atomic scale, leading to the changing primary crystallization sequence for heating and cooling. The findings in this work further strengthen the high relevance of sulfur in alloy development and tuning of properties in BMGs, especially considering the low cost, good availability, and non-toxicity of sulfur in the relevant modifications.