The John Cahn Memorial Symposium: Session I
Sponsored by: TMS Materials Processing and Manufacturing Division
Program Organizers: James Warren, National Institute of Standards and Technology; W. Craig Carter, MIT; Carol Handwerker, Purdue University; Yuri Mishin, George Mason University

Wednesday 8:30 AM
March 1, 2017
Room: 22
Location: San Diego Convention Ctr

Session Chair: James Warren, National Institute of Standards and Technology; W. Craig Carter, MIT

8:30 AM Introductory Comments James A. Warren, chair

8:40 AM  Invited
Dislocations, Trijunctions and Grain Rotation: Kevin McReynolds1; Akinori Yamanaka2; Peter Voorhees1; 1Northwestern University; 2Tokyo University of Agriculture and Technology
    One of John Cahn’s many talents was his ability to identify important problems that were overlooked by past investigators in a field. John’s work with Jean Taylor on the rotation of grains during grain growth is a perfect example of this and, like usual, has stimulated much interest in the community. Building on his work, we have used a phase field crystal model to describe the dynamics grain growth in both two and three dimensions. We find that the atomic-scale structure of the boundary gives rise to qualitatively new grain growth kinetics as well as to both grain rotation and translation. The grain translation is a result of the climb, glide, and interactions of the dislocations that comprise the grain boundary, as well as dislocation interactions at trijunctions. The important role of the three-dimensional interfacial dislocation structure in the grain translation and rotation processes will be discussed.

9:10 AM  Invited
A DSC Model for Grain Boundary Migration and Properties: David Srolovitz1; Jian Han1; Spencer Thomas1; Vaclav Vitek1; 1University of Pennsylvania
    John Cahn and co-workers developed a predictive model for the coupling between stress and grain boundary (GB) migration that has since been validated by both atomistic simulation and experiment. In this presentation, we propose a generalization of this approach based on DSC disconnections. We demonstrate that the Cahn results represent a special case of the more general model. In particular, we will show how GB migration is coupled to a range of different driving forces, how GB migration can generate stress, and how these stresses can lead to switching between different GB migration mechanisms. The importance of these effects will be demonstrated by molecular dynamics simulation of the motion of individual GBs and within a polycrystal. We will then demonstrate the coupling between GB structure and the interactions between GB and plasticity (i.e., grain boundaries as barriers to dislocation motion and as sites for dislocation absorption, emission, and transmission). We briefly discuss how inclusion of thermal effects on GB structure can be related to GB roughening and GB sliding within the same model.

9:40 AM  Invited
Thin Film Grain Growth for Twin Related Orientations of Grains: John Blendell1; Jean Taylor2; John Cahn3; R. Edwin Garcia1; Daniel Lewis; 1Purdue University; 2Professor Emerita at Rutgers University and Visiting Faculty at Courant Institute, NYU; 3NIST and University of Washington
    More than 20 years ago Thangaraj and Dahmen (1992) produced Al films on (111) Si that contained only 3 twin related orientations. All grain boundaries were 30˚<100> tilt boundaries. Both symmetric and asymmetric triple junctions were observed. Cahn and Kalonji (1994) determined that there was a possibility of 7 different triple junctions. Recently we simulated the 2D growth of grains driven by surface curvature in such a system containing only three orientations. The nature of the growth was different as compared to the random orientations case. It was observed that the kinetics of growth were strongly influence by the grain boundary Wulff shape. For an isotropic Wulff shape the classical N-6 rule was observed. However, for a square or octagonal Wulff shape, grains with a large number of sides were stabilized. This may be a consequence of the grain growth due to topological changes. For example, neighbor switching does not occur, rather the grains merge into a larger grain. Also, grains tend to have an elongated structure and do not tend towards being equiaxed but there does not appear to be a tendency towards exaggerated growth.

10:10 AM Break

10:30 AM  Invited
Experimental Measures of Stress-coupled Boundary Migration and the Attendant Mechanical Behavior of Nanocrystalline Films: Paul Rottmann1; Suman Dasgupta1; Kevin Hemker1; 1Johns Hopkins University
    Stress-coupled grain boundary migration has been shown to trigger microstructural instability and accommodate plastic deformation in a wide number of nanocrystalline metals. The role of shear stress in promoting grain growth has been identified, but detailed information about local grain orientation and grain boundary character have been harder to obtain. TEM-based precession-assisted automated crystal orientation mapping (ACOM) now allows for nanoscale mapping of individual grains and grain boundaries. In situ mechanical tests, couypled with orientation mapping, have been used to make quantitative observations about twin formation, grain and twin boundary migration, and crack growth. The ability to observe and correlate these mechanisms with local orientation mapping, greatly facilitates comparisons with computer simulations and promotes better understanding of underlying deformation processes.

11:00 AM  Invited
Energetic Trends for Twin Boundaries in HCP Metals: Maarten de Jong1; Liang Qi2; Axel van de Walle3; Mark Asta1; 1University of California, Berkeley; 2University of Michigan; 3Brown University
    This talk will describe the results of computational studies exploring trends in the the energetics of twin boundaries in hcp transition metals and alloys. The calculations employ a recently developed method for modeling interfacial energies in substitutional alloys from first-principles calculations based on the special quasirandom structure approach. The calculations reveal anomalously low values of twin boundary energies for specific orientations, for hcp metals and alloys with d-band filling near that of the group VII metal rhenium. The origins of this behavior are investigated and linked to the theory of the trends in bulk structural energetics across the transition-metal series, which predicts the stability of topologically close-packed phases containing atomic coordination polyhedra similar to those found in the stable twins in this region of the periodic table. The results are discussed in light of experimental observations of deformation microstructures, and the implications for alloy design are described.

11:30 AM  Invited
Molecular Dynamics Simulations of Faceted, Incoherent Twin Boundaries: Elizabeth Holm1; Jonathan Humberson1; 1Carnegie Mellon University
    Although John Cahn was known primarily as a materials theorist, he was an early adopter of computational modeling as a tool to gain insight into physical phenomena. Collaborating with a number of computational scientists and applied mathematicians, John was particularly invested in phase field models (based of course on the Cahn-Hilliard and Allen-Cahn models) and in molecular dynamics simulations (mainly for grain boundary and dislocation physics). In this talk, we will present a few results from molecular dynamics simulations that we think might have interested John. In particular, we will discuss how two sets of Shockley partial dislocations, with the same Burgers vectors, gliding on the same plane under the same physical conditions, can evince fundamentally different temperature dependence of motion. The results elucidate the thermal behavior of faceted, incoherent twin boundaries.