Texture and grain boundary character

Title: Correlations between the crystallographic texture and grain boundary character in polycrystalline materials

Authors: R. Edwin García, and Mark D. Vaudin

Source: Acta Materialia, Article in Press, Corrected Proof

Abstract:

A method is presented to determine the misorientation probability distribution function in polycrystalline materials based on a known, analytical or numerical, representation of the associated orientation probability distribution function, i.e., texture. The proposed formulation incorporates the local grain-to-grain orientation correlations by combining local or macroscopic statistical information, and finds a natural interpretation through the well-known stereographic projection (pole-figure) representation. The proposed formulation distinguishes between antiparallel crystallographic orientations, as well as cone-angle and polar angle misorientations. For fiber-textured samples, it is quantitatively shown that highly oriented samples are equivalent to polycrystals with a high density of low-angle misorientations, while completely random (untextured) materials are equivalent to microstructures with a high probability of large-angle misorientations.

Interface field based methods for multiphase transformations

Title: A generalized field method for multiphase transformations using interface fields

Authors: I Steinbach and F Pezzolla

Source: Physica D: Nonlinear Phenomena, Volume 134, Issue 4, 10 December 1999, Pages 385-393

Abstract: The recently developed multiphase field method, describing the interaction between an arbitrary number of individual phase fields with individual characteristics, is reformulated by the use of interface fields. This reformulation allows for the decomposition of the nonlinear multiphase field interactions into pairwise interaction of interface fields. This removes some difficulties in the treatment of triple points or higher order interactions that occurred in the original model. The interface fields being defined in a (2^Ñ) dimensional space, where Ñ is the order of the multiple point, can be interpreted being the generalized coordinates for this variational problem. The considered example of a multiphase change problem indicates clearly that a relaxation ansatz for the evolution of the field variables towards the minimum of the free energy is warranted only for generalized coordinates, while a relaxation ansatz using functionally dependent variables and the Lagrange formalism in general mixes time and energy scales.

Note: Hat Tip: Deep.

Polycrystalline solidifcation in 3D

Title: Modeling the formation and dynamics of polycrystals in 3D

Authors: Ryo Kobayashi and James A. Warren

Source: Physica A: Statistical Mechanics and its Applications, Volume 356, Issue 1, 1 October 2005, Pages 127-132

Abstract:
Phase field models of solidification have been extended to include grain boundaries, using a variety of techniques. A model developed by Kobayashi et al. [Physica D 140 (2000) 141] has been used to model a host of physical phenomena, but has so far been confined to two dimensions. In this letter we describe how to extend this model of polycrystalline solidification to three dimensions (3D).

In Nature this week

1. Formation of metallic glass in a pure metal:
   

   In order to form a glass by cooling a liquid, the normal process of solid crystallization must be bypassed. Achieving that for a pure metal had seemed impossible — until pressure was applied to liquid germanium.

2. Ageing and cancer:
   

   At first glance, cancer and ageing would seem to be unlikely bedfellows. Yet the origins for this improbable union can actually be traced back to a sequence of tragic—and some say unethical—events that unfolded more than half a century ago. Here we review the series of key observations that has led to a complex but growing convergence between our understanding of the biology of ageing and the mechanisms that underlie cancer.

Update: Abi on a news report in the Telegraph about the Germanium metallic glass work (and also on “probably the only instance in science in which confusion itself is elevated to the level of a principle”

In Nature this week

• Katharine Sandersonon “total synthesis”, and if such efforts are worthy or irrelevant;
• Femtosecond holography–looks very interesting:
  

  By cunningly diffracting X-rays twice from an exploding nanometre-scale sphere, holographic images can be made of a tiny system evolving at lightning speed. The technique could be used to picture atomic dynamics.

  and,

• Growing protein crystals using near-zero gravity conditions obtained using inhomogeneous magnetic fields.

Break-down of Hall-Petch

Title: The strongest size

Authors: A. S. Argon; S. Yip

Source: Philosophical Magazine Letters, Volume 86, Issue 11 November 2006 , pages 713 – 720

Abstract:
The well known break-down of the Hall-Petch effect of the rise of the plastic resistance with decreasing grain size in polycrystalline metals, when the grain size drops into the nanometre range resulting in a peak plastic resistance at a grain size of about 12-15 nm, is explained by considering two alternative and complementary rate mechanisms of plasticity, grain boundary shear and dislocation plasticity, each contributing to the overall strain rate in proportion to the volume fraction of the material in which they operate. In the model for a given applied strain rate it is shown that the plastic resistance reaches a maximum at a grain size of about 12.2 nm in Cu when the two mechanisms contribute to the overall strain rate equally, defining the so-called strongest size.

Stochastic and curvature driven grain growth

Title: Grain growth as a stochastic and curvature-driven process

Authors: Y. G. Zheng; C. Lu; Y. -W. Mai; H. W. Zhang; Z. Chen

Source: Philosophical Magazine Letters, Volume 86, Issue 12 December 2006 , pages 787 – 794

Abstract:

Grain growth subjected to the interplay of stochastic and curvature-driven mechanisms in a single-phase system has been investigated. Numerical results have shown that when the grains are smaller than several tens of nanometres the dominating mechanism is stochastic diffusion control of boundaries. As the grains grow the influence of the deterministic curvature-driven mechanism increases and finally controls the process. In terms of finite-difference solutions to the Fokker-Planck continuity equation, the predicted grain size approaches a log-normal distribution, which agrees well with experimental observations.

High resolution electron microscopy of split patterns in Ni-base superalloys

Title: Statistical analysis of quartet split patterns in Ni alloys revealed by high resolution electron microscopy

Authors: H. A. Calderon; C. Kisielowski; T. Mori

Source: Philosophical Magazine Letters, Volume 87, Issue 1 January 2007 , pages 33 – 40

Abstract: The frequencies of adjacent pairs having in-phase and out-of-phase relationships in quartet configurations, determined by high resolution electron microscopy [Calderon et al., Phil. Mag. Lett. 85 51 (2005)], were examined in detail for particles belonging to the same translational order domain. Consequently, there were always at least two out-of-phase adjacent pairs in any quartet. It is concluded that the quartet split patterns in particles, but are produced by the migration of particles due to diffusion, these particles having been precipitated separately and independently prior to migration. An elasticity calculation is provided to show that two particles migrate to align along <100> when their initial position deviates from this direction.

Grain boundary–dislocation interactions

Title: Spacing defects and disconnections in grain boundaries

Authors: J.P. Hirth, R.C. Pond, and J. Lothe

Source: Acta Materialia, Article in Press, Corrected Proof

Abstract: The process of cutting of a grain boundary by a gliding or climbing dislocation is considered. Some planar dislocation arrays with long-range stress fields are also treated. Defects formed on the grain boundaries by these mechanisms include edge and screw disconnections, grain boundary dislocations, spacing defects and line forces. The cutting defects can also acquire kinks and jogs. The results have implications for emission of lattice dislocations from grain boundaries, trapping of dislocations at grain boundaries and grain boundary topography.

In Nature this week

1. Baboons in social situations, and what it tells about us:
   

   First, they lure the reader in with wonderful anecdotes, such as Ahla, the goat-herding baboon, who spontaneously recognized the relationships among her goats, and compulsively reunited any lost baby goats with their mothers. Or there’s the orphaned baboon who, when separated from his group, cleverly spent a few days under the protection of vigilant groups of impala, and later vervets, before a jubilant reunion with his fellow baboons.

   Next they provide experimental evidence to tease apart what baboons really know about social relationships and how they use this information to get ahead in the world.

2. Controlling intrinsic magnetic properties of materials:
   

   The presence of non-magnetic atoms can create a random internal field in magnetic crystals. Tuning that field from outside allows the intrinsic magnetic properties of the material to be precisely controlled.

3. Domain wall movement in magnetic nanowires:
   

   When two ‘bits’ of magnetic information race around a nanoscale wire, two factors determine whether or not they survive the course: the condition of the track, and how fast they respond to the starting signal.