David Turnbull (1915-2007). Pioneer of the kinetics of phase transformations in condensed matter

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Entry by Emily Redston, AP 225, Fall 2011

Reference

David Turnbull (1915-2007). Pioneer of the kinetics of phase transformations in condensed matter by F. Spaepen and M.J. Aziz, Nature Materials 6: 556 (2007).

Keywords

metallic glasses, eutectics, phase transformation, liquid undercooling, liquid structure, crystal structure, free-volume model, viscosity, glass-transition temperature

Summary

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We have frequently discussed the importance of phase diagrams in class. However, while these equilibrium diagrams are undeniably useful in themselves, it is oftentimes important to think about the effect of kinetics on the system. David Turnbull (after whom McKay 402 is named) is one in a line of distinguished physical chemists, going back to J. W. Gibbs, who have profoundly influenced materials science. His work laid the basis of our understanding of the kinetics of phase transformations in condensed matter. In particular, he made seminal contribution to solidification theory and glass formation. He also made key contributions to grain-boundary diffusion, fast diffusion of noble metals in semiconductors and polyvalent metals, crystal growth, grain growth and recrystallization.

At the 1948 dinner meeting of the Science of Metals Club, Turnbull told his audience that he had undercooled liquid mercury several tens of degrees below its thermodynamic crystallization temperature by dispersing it into small droplets. At the end of the talk he speculated that, if similarly treated, other metals (especially those that crystallized into simple structures) would also exhibit large liquid undercoolings. Nonetheless, David Harker's challenge to David Turnbull, his colleague at the General Electric Research Laboratory, was clear: "If you can undercool molten copper by more than a few degrees, I'll eat my hat!". After the meeting, Turnbull quickly showed with a hot-stage microscope that small droplets of copper, silver, gold and many other metals could be cooled to temperatures far below their crystallization points. Harker graciously accepted the results: he appeared at the next meeting with a hat made of Swiss cheese.

Harker's skepticism was not unreasonable. Because the density and bonding in the crystalline and liquid phases of monatomic metals are similar, it was generally assumed that the short-range order in both phases was similar as well, and hence the liquid structure could easily collapse into the crystalline one at minimal undercooling. Turnbull's experiments radically changed this picture: the structure of a liquid differs profoundly from that of a crystal. His experiments inspired Charles Frank's proposal of icosahedral short-range order in liquids. This means that liquids need to be reconstructed to be transformed into the structures of close-packed crystals. This insight, combined with John Bernal's analysis of the dense random packing of hard spheres, became the basis of our current understanding of a simple liquid as a polytetrahedral structure.

Turnbull also realized that his undercooling results implied that it should be possible to cool liquid metals into the glassy state if the viscosity rose sufficiently sharply on cooling. With Morrel Cohen, he developed the now widely used free-volume model, the first and elegant microscopic explanation of viscosity rise on the basis of the probability of density fluctuations. These insights convinced him of the universality of the glass transition and led him to predict that alloys with deep eutectics were the best candidates for metallic glass formation. This was confirmed by Pol Duwez's discovery of an amorphous phase in an Au–Si eutectic alloy in 1959. Turnbull and his students later demonstrated that these amorphous metallic phases were true glasses, in that they exhibited the characteristic features of the fluid-glass transition: discontinuity in the specific heat and in the coefficient of thermal expansion, and a rapid change of the viscosity with temperature.

Based on his understanding of the kinetics of crystal nucleation and the viscosity, Turnbull proposed that the ratio of the glass-transition temperature and the liquidus temperature should be used as a criterion for determining the ease of glass formation. The importance of this criterion was demonstrated in 1982, when he, together with Lindsay Greer and their students, showed that a <math>Pd_{40}Ni_{40}P_{20}</math> liquid could be made into a glass of centimeter size: the first bulk metallic glass! Metallic glasses have found uses in a variety of applications: as high-efficiency transformer cores,as hard metals in coatings, and as high-strength-to-weight alloys for consumer electronics casings, medical implants, and sporting goods.

Turnbull joined Harvard University as the Gordon McKay Professor of Applied Physics, where he taught for 23 years. He received the Japan Prize in 1986, the second year the prize was awarded. In 1979, he received the Von Hippel Award, the highest award of the Materials Research Society. The Society later also established an annual Lectureship in his name. David Turnbull died on April 28, 2007, at the age of 92, in his house in Cambridge, Massachusetts.