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Glasses and amorphous materials

A Zr-based bulk metallic glass has a yield strength of 1.8 GPa and a Young's modulus of 90 GPa. Which correctly pairs the elastic strain limit estimated as σy/E with its comparison to crystalline metallic materials?

  1. AAbout 0.020%, smaller than most crystalline metallic materials
  2. BAbout 2.0%, larger than most crystalline metallic materials (below 1%)
  3. CAbout 2.0%, about the same as most crystalline metallic materials
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Answer: B. About 2.0%, larger than most crystalline metallic materials (below 1%)

σy/E = 1.8/90 = 0.020, that is, about 2.0%. For steel (yield stress 0.5 GPa, Young's modulus 200 GPa) it is about 0.25%, and even for a high-strength titanium alloy (0.9 GPa, 110 GPa) about 0.8%, so the elastic strain limit of the metallic glass is much larger. This is because metallic glasses have no dislocations and therefore high yield strength, while their Young's modulus is often about 30% lower than that of crystals of the same composition. The elastic energy stored per unit volume, σy²/(2E), is a large 18 MJ/m³ or so, which is exploited in springs, golf club faces and the like. About 0.020% is the value obtained by forgetting to convert 0.020 into a percentage.

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