John Werner Cahn (January 9, 1928 – March 14, 2016) was an American scientist and recipient of the 1998 National Medal of Science.

Born in Cologne, Weimar Germany,[1] he was a professor in the department of metallurgy at the Massachusetts Institute of Technology (MIT) from 1964 to 1978.

In 1954, Cahn joined the chemical metallurgy research effort at the General Electric laboratory in Schenectady, New York, led by David Turnbull.

In 1964, Cahn became a professor in the Department of Metallurgy (now Materials Science) at the Massachusetts Institute of Technology.

In 1969, Cahn began a long professional relationship with his graduate student, Francis Larché, whose work focussed on the effect of mechanical stress on the thermodynamics of solids.

Good examples of this phenomenon are the regions near a coherent precipitate or the stress field around a dislocation.

[4][5][6][7] In the theory of crystal growth, Cahn concluded that the distinguishing feature is the ability of the surface to reach an equilibrium state in the presence of a thermodynamic driving force (typically in the form of the degree of undercooling).

Thus, for sufficiently large driving forces, the interface can move uniformly without the benefit of either a heterogeneous nucleation or screw dislocation mechanism.

Alternatively, for sharp interfaces, the critical driving force will be very large, and most growth will occur by the lateral step mechanism.

[8][9][10][11] In 1977, Cahn published a simple mathematical treatment of the thermodynamics of wetting: the interaction between a liquid in contact with a solid surface.

Cahn contributed to the theory of how such a structure could be thermodynamically stable and became co-author of the seminal paper which introduced quasicrystals.

In 2004, Cahn and Bendersky presented evidence that an "isotropic non-crystalline metallic phase" (dubbed "q-glass") can be grown from the melt.

TEM images show that the q-glass nucleates from the melt as discrete particles, which grow spherically with a uniform growth rate in all directions.

Yet there is a nucleation barrier, which implies an interfacial discontinuity (or internal surface) between the glass and the melt.

[12] In his retirement, Cahn accepted a position at the University of Washington as an affiliate professor in the Departments of Materials Science and Engineering and Physics.

[13] 2011 The Kyoto Prize, Inamori Foundation 2002 Bower Prize, Franklin Institute 2001 Emil Heyn Medal, German Metallurgical Society 2001 Honorary Life Member, American Ceramic Society 1999 Bakhuys Roozeboon Lecturer and Gold medal, Netherlands Academy of Sciences 1998 National Medal of Science[14] 1998 Member, National Academy of Engineering 1998 Distinguished GE Lecturer in Materials Science at RPI '69 & `98 MacDonald Lecturer, Canadian Metallurgical Society 1996 Doctor Honoris Causis, Universite d'Évry, France 1995 Harvey Prize, Technion.