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Glasses and amorphous materials

By Turnbull's guideline, the larger the reduced glass transition temperature Trg = Tg/Tl (Tg is the glass transition temperature and Tl the liquidus temperature, both absolute temperatures), the more readily a melt becomes glass on cooling. Which of the following alloys forms glass most readily by this guideline?

  1. AThe alloy with Tg = 625 K and Tl = 940 K
  2. BThe alloy with Tg = 420 K and Tl = 820 K
  3. CThe alloy with Tg = 700 K and Tl = 1300 K
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Answer: A. The alloy with Tg = 625 K and Tl = 940 K

Calculating Trg gives 625/940 ≈ 0.665 for the alloy with Tg = 625 K and Tl = 940 K, about 0.538 for the alloy with Tg = 700 K and Tl = 1300 K, and about 0.512 for the alloy with Tg = 420 K and Tl = 820 K. The narrower the temperature span from Tl to Tg, the more quickly the melt passes through the supercooled-liquid range in which crystals can nucleate and grow, and the liquid in that range is also more viscous, so glass forms more easily. Turnbull estimated that crystallization is strongly suppressed once Trg reaches about 2/3 or more. The alloy with the highest Tg or the lowest Tl is not necessarily the one chosen; what matters is their ratio. This is an empirical guideline with exceptions.

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