RETROSPECTIVE:
Robert W. Cahn (1924-2007) and David Turnbull (1915-2007)
Roger Doherty*
Sixty years ago, the precursor discipline to materials science--metallurgy--was largely empirical, necessarily so given its engineering importance and complexity. However, by the late 1940s, the field was about to be transformed. Robert W. Cahn and David Turnbull, who both died in April of this year, played vital roles in the development of this new science.
The two men came from remarkably different backgrounds. Cahn was born into a prosperous and artistic Jewish family in Fürth, Germany, on 9 September 1924. The family fled Nazi Germany in 1933, and in 1942, Robert entered Cambridge University (1). Turnbull was born on 18 February 1915 on a family farm in Elmira, Illinois. His family rarely ventured more than a few miles from their farm (2). He graduated from high school in 1932 at the height of the Depression, but was able to attend Monmouth College, which was governed by the Presbyterian sect to which his family belonged. Here he became interested in physical chemistry and went on to a scholarship and a Ph.D. at the University of Illinois in 1939.
Cahn's major contributions to the new science of physical metallurgy started during his graduate research, where his experiments demonstrated the reality of dislocations (line defects whose movement under stress allows metals to deform plastically). Dislocations had been proposed to explain the discrepancy between the very high predicted resistance to plasticity of perfect metal crystals, compared to the much smaller measured values of actual pure metal crystals. In pure metal crystals, dislocations can move under the influence of very small shear stresses. Metal processing techniques had always used solute atoms, second phases, and other obstacles (which impede the movement of dislocations) to improve the metals' resistance to plastic deformation and to control brittle fracture. Until the advent of dislocation theory, however, the question of what controls the properties of metal alloys such as steel could not be addressed scientifically.
From his graduate student work, Cahn also proposed a successful model for the nucleation of new crystals formed during recrystallization, a softening process that occurs when heavily deformed metals are heated, allowing them to be reshaped into new objects. In his model, the dislocations introduced by previous deformation rearranged to create small regions of dislocation-free crystals that can grow to become new grains. This model provided the basis of almost all subsequent research into this important industrial process, which is used to control the size and orientation of grains in metallic alloys to improve their strength and ability to be shaped, for example, in automobile bodies made from pressed steel sheets.
Turnbull made one of the major discoveries in materials science after joining the GE research laboratory in 1946. His elegant investigations into the nucleation of structural transformations (initially the solidification of liquid metals) showed that such complex processes could be quantitatively understood. The theory for this type of process had been developed a decade or so earlier by physical chemists studying crystallization from supersaturated liquid solutions. The initial formation of a crystal--for example, of salt from an aqueous salt solution--was known to require highly supersatured conditions, because the first small crystal to form, being chemically different from water solution, has a large interfacial energy. Yet, ingots of metals needed only a very small supercooling (a temperature below the melting temperature) to start to form solid metal crystals. The understanding was that the interfacial free energy between a metal crystal and its melt must therefore be small.
Turnbull carried out experiments on the freezing of a low-melting-point metal, gallium, and found that the small supercoolings usually seen resulted from particles of dirt, more politely called "heterogeneous catalysts." By breaking up the liquid gallium into small droplets, so that there were more droplets than there were pieces of dirt, he achieved the very large supercoolings characteristic of what is called "homogeneous" nucleation. His result indicated a large interfacial energy between crystalline and liquid gallium. In the early 18th century, Fahrenheit had reported similar observations for the freezing of water. Turnbull was now able to explain what was happening and show its universal application.
CREDIT: MATERIALS RESEARCH SOCIETY
To show that this principle also applied to metals with simple crystal structures, such as iron or copper, Turnbull, together with R. E. Cech, developed a simple experimental means of studying the phenomenon. By heating small droplets of high-melting-point metals, such as copper, on an inert substrate and then cooling them below their melting temperatures, they demonstrated very large undercoolings.
Through these experiments and theoretical insights, the previously empirical study of metal solidification had acquired a clear scientific foundation. Turnbull and his colleagues at GE went on to develop the scientific discipline of metal-alloy processing. In a particularly important later study, he and I. S. Servi (3) demonstrated the validity of homogeneous nucleation theory for a solid-solid transformation: the nucleation of coherent cobalt particles from dilute solid solutions of cobalt in copper. Solid-state nucleation processes form the basis of the technologically vital process of strengthening metallic alloys by precipitation hardening.
Both of these pioneers made many other vital contributions. Cahn studied the crystallography of uranium, mechanical twinning, and ordered intermetallic alloys. Turnbull made important contributions to solidstate diffusion and the structure of liquids. Both scientists also studied metallic glasses (metallic alloys that can be cooled to form rigid noncrystalline solids). Their research was instrumental in the development of today's approach to materials science, in which the structure of materials is modified to produce new and improved properties--such as higher strengths or improved electrical and magnetic properties--needed for applications ranging from aircraft engines to information-storage devices.
Cahn was a successful academic at several universities in the United Kingdom and in France, as was Turnbull at Harvard after he left GE in 1962. Cahn also made great contributions to publishing, particularly the encyclopedic tome Physical Metallurgy that he co-edited with Peter Haasen. Turnbull, in his research and teaching, was always a materials scientist curious about any material with interesting structural problems, independent of its chemical bonding. Sadly, Turnbull never translated his lecture notes on structural transformations into a textbook. Those of us who have written such textbooks are free of a major competitor, but his textbook would have been a contribution that we regret not having available to us.
References and Notes
- R. W. Cahn, The Art of Belonging (Book Guild Publishing, Sussex, UK, 2005).
- David Turnbull's autobiography can be found at www.mrs.org/s_mrs/sec.asp?CID=4746&DID=164491.
- I. S. Servi D. Turnbull, Acta Metal. 14, 161 (1966).
10.1126/science.1145490
The author is in the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104, USA. E-mail: dohertrd{at}drexel.edu
