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Tribute to David Turnbull

Prof. Michael J. Aziz

Gordon McKay Professor of Materials Science
Harvard School of Engineering and Applied Sciences
Harvard University

David Turnbull passed away in April at age 92 at his home in Cambridge. He laid the foundations of the quantitative study of the kinetics of phase transformations in condensed matter and was a giant in the development of the interdisciplinary field of Materials Science.

Turnbull has been associated with the MRS since its inception -- he found that its interdisciplinary charter matched the breadth of his interests. He was an active participant in early symposia on laser and ion beam-solid interactions where, for example, he presented crystallization enthalpy measurements of amorphous Si and Ge and successfully predicted, in collaboration with Frans Spaepen, a metastable first-order melting transition of these amorphous materials and the temperature at which it would occur.

But the breadth of his research spanned metals, semiconductors, insulators, polymers and molecular solids and he was a fixture at the MRS Fall Meeting, attending it to the limit of his physical capabilities up to the very end. And Ramesh, you know he would have been here today, congratulating you and looking forward to your talk.

Turnbull was the third recipient of the Materials Research Society's highest honor, the Von Hippel Award, in 1979, after Von Hippel himself and William Baker, director of Bell Labs. With characteristic modesty, in his presentation he remarked: "It is not clear that I belong in this progression. I feel that I am in a position like that of a certain Linus. I'm sure you all know two famous persons named Linus, but you may not have heard of the Linus I am referring to. He was [...] the first Bishop of Rome following the Apostles Peter and Paul". It was clear to all that his modesty was unwarranted, especially by the time he received the Japan Prize in 1986 and when, after his retirement, the MRS established the Turnbull Lectureship Award in his honor in 1992.

Turnbull received his Ph.D. in Physical Chemistry in 1939 from the University of Illinois. He then joined the Faculty at the Case School of Applied Science where, during WWII, he trained naval cadets and did research on synthetic rubber and on the improvement of additives for heavy-duty lubricants. During this period he became interested in what was then known as "solid state science", later to become the discipline of "materials science". As he was teaching the quantitative discipline of physical chemistry to metallurgists, he decided to learn metallurgy, which was a very empirical field at that time.

In 1946 he joined General Electric Research Laboratory as one of the first members of the group of Herb Hollomon, who was forming an interdisciplinary team with representation from metallurgy, applied mechanics, chemistry and physics to attack the problems posed by responses of metals to mechanical and thermal treatments.

By early 1950's Hollomon's group was one of 4 groups worldwide demonstrating the power of the interdisciplinary approach to materials research that led to the recognition of "Materials Science" as what he called a "superdiscipline". The others were William Shockley's group at Bell Labs, the University of Chicago's Institute for the Study of Metals under Cyril Smith's leadership, and Neville Mott's group at the University of Bristol.

Turnbull's most spectacular experimental result was the undercooling of pure liquid metals by up to 20% of the absolute melting temperature, demonstrating a shockingly high barrier to homogeneous nucleation. He did this by making a dispersion of small, isolated droplets that was sufficiently fine that heterophase or "dirt" impurities were confined to a small fraction of the individual droplets. When he first presented the large undercoolings obtained in liquid mercury to the Science of Metals club in Schenectady he speculated that, if similarly treated, not just mercury, but also metals that crystallized into simpler structures would exhibit such large liquid undercoolings. The response from his G.E. colleague David Harker was emphatic. He said, "If you can undercool molten copper by more than a few degrees below the melting point, I'll eat my hat!" After the meeting, Turnbull quickly showed that small droplets of molten copper, silver, gold and many other metals could be cooled to temperatures far below their melting points. Harker graciously accepted the results, appearing at the next meeting with a hat made of Swiss cheese.

Turnbull's undercooling experiments showed that the structure of simple melts is profoundly different than that of simple crystals and became the basis for the modern understanding of the liquid structure as polytetrahedral. Additionally, he developed the theory of homogeneous crystal nucleation and tested it with definitive quantitative experiments that remain models of their kind.

Turnbull 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 upon cooling. With Morrel Cohen, he developed the free-volume model, the first microscopic explanation of super-Arrhenius viscosity rise based on 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 formation of a metallic glass. His prediction was confirmed by Pol Duwez's discovery of an amorphous phase in a splat-quenched foil of gold-silicon eutectic alloy. Turnbull and his students then demonstrated that amorphous metallic phases were true glasses, in that they exhibited the characteristic features of the glass transition: discontinuity in the specific heat and in the coefficient of thermal expansion, and a rapid change of the viscosity with temperature.

Turnbull used his understanding of nucleation and the glass transition to propose the ratio of glass transition temperature to liquidus temperature as a figure of merit for glass formability. He demonstrated the value of this criterion in 1982 when, with his colleague Lindsay Greer and their students, he developed the first bulk metallic glass, a palladium-nickel-phosphorus alloy. (Symposium Z at this meeting is on bulk metallic glasses.) Metallic glasses have found uses in a variety of applications, perhaps the most noteworthy being as the highest-efficiency transformer cores, which are currently saving billions of kWh a year in energy losses.

Besides showing how a quirky, complex phenomenon like crystal nucleation could yield exciting, rigorous, and applicable science, Turnbull also made key contributions to a wide variety of materials phenomena, especially short circuit atomic diffusion along extended defects, spectacularly fast diffusion of noble metals in semiconductors and in polyvalent metals, crystal growth, grain growth and recrystallization.

In 1962 he moved to Harvard University, where the interdisciplinary atmosphere attracted him with no organizational boundaries between applied physics, materials science, and applied mechanics.

David Turnbull was the master of conceiving of and performing the simple but critical experiment, and wrote in his autobiography, "my experimental designs were always quite simple and fell far short of fully utilizing the magnificent facilities for sophisticated experimentation provided by G.E." and he wrote "I emphasize to my students that while we can hardly achieve distinction with our hardware, we may do so with our wits."

At Harvard he taught a graduate course called "kinetics of condensed phase processes", which was taken or faithfully audited by more than 200 people from Harvard and MIT including graduate students, postdocs, and faculty members from physics, metallurgy, chemistry, geology, ceramics and applied mechanics.

His homework sets were legendary. Here I present an example from when I took the course, the final problem set of the term. It had just three problems, but I felt compelled to make this annotation. First I quote Professor Turnbull: " 'I tried to cut it down some, so you could work on your talks.' [Our final exam was to give a lecture to the class and answer his penetrating questions.] Problem #2 is a personal landmark. It has consumed more of my time than any other single problem during the past 22+ years. And problem #3 is a close second." I'm obviously feeling sorry for myself in my oppression as a first-year graduate student.

It all made sense when I read what he wrote in his autobiography: "It seemed that I learned most from having to solve difficult problems and rather little from classroom exposition, however excellent it was. Because of this, the emphasis of my own teaching has been on posing meaningful and challenging problems." There are at least 200 of us out there - and a large number at this Meeting - who truly understand what he meant.

At Harvard he supervised 40 Ph.D. students, of which I had the good fortune to be one, and 10 postdocs. He was unsurpassed in recognizing and bringing out our individual strengths, as well as those of each of his associates at General Electric.

In his autobiography he relates how he would have liked to continue to work the family farm in western Illinois. A childhood asthmatic condition put him on his way to Monmouth College to become the scientist, leader, and mentor many of us are deeply grateful to have had among us.

So now I would like to lead a moment of silence to honor his memory.

This is a transcript of an oral tribute, which drew from David Turnbull's autobiography published on the MRS website, published obituaries by Frans Spaepen in the MRS Bulletin (Vol. 32, No.8, August 2007, 659) and by Frans Spaepen and Michael Aziz in Nature Materials (6 (2007) 556-557).