The interaction of dislocations with precipitates is an essential strengthening mechanism in metals, as exemplified by the superior high-temperature strength of Ni-base superalloys. Here we use atomistic simulation samples generated from atom probe tomography data of a single crystal superalloy to study the interactions of matrix dislocations with a gamma' precipitate in molecular dynamics simulations. It is shown that the precipitate morphology, in particular its local curvature, and the local chemical composition significantly alter both, the misfit dislocation network which forms at the precipitate interface, and the core structure of the misfit dislocations. Simulated tensile tests reveal the atomic scale details of many experimentally observed dislocation–precipitate interaction mechanisms, which cannot be reproduced by idealized simulation setups with planar interfaces. We thus demonstrate the need to include interface curvature in the study of semicoherent precipitates and introduce as an enabling method atom probe tomography-informed atomistic simulations.

A. Prakash, J. Guénolé, J. Wang, J. Müller, E. Spiecker, M.J. Mills, I. Povstugar, P. Choi, D. Raabe, E. Bitzek

doi:10.1016/j.actamat.2015.03.050

Multiple PhD positions are available for the fall 2012/winter 2013 semesters in microstructure-mechanics relationships for structural materials at high temperatures in the Center for Advanced Vehicular Systems (http://www.cavs.msstate.edu) at Mississippi State University. The project is funded through Air Force Office for Scientific Research (AFOSR) and will involve multiscale materials characterization, in situ SEM experiments, and high temperature mechanical testing for turbine blade structural materials (nickel-based superalloys).

Students with interests in experimental solid mechanics and materials science are encouraged to contact me with a resume/CV at mtschopp@cavs.msstate.edu . These positions are restricted to US citizens only and will involve working with university, national laboratory and/or industry collaborators on mechanics-related problems for the aerospace industry.

The Institute for General Material Properties of the University Erlangen-Nürnberg
is seeking outstanding candidates for a collaborative research project
(SFB/Transregio 103) on high-temperature deformation of super alloys.

The aim of this four-year project is to provide the scientific
foundation for a new generation of single-crystalline super alloys.
For this purpose, atomistic simulations will be carried out to
determine the structure and properties of the relevant defects (dislocations,
stacking faults, heterophase boundaries) and to study the interaction between
these defects. The atomistic simulations will be part of a multiscale modeling
approach in close collaboration with experimental groups.

The successful candidate will hold a degree in Materials Science,
Physics or Mechanical Engineering and have a solid background in physical
metallurgy and mechanical behavior of materials. Experience with numerical
simulations (preferably Molecular Dynamics) and scientific programming is
required. In addition, experience in the simulation of crystal defects on the
atomistic scale is highly desirable.
Excellent oral and written communication skills and the ability to work
well in a dynamic and collaborative research environment are essential.

The department of materials science and engineering of the
Friedrich-Alexander University Erlangen-Nürnberg is the largest in Germany. It
is located in the metropolitan area of Nuremberg (3.5 Mio inhabitants), in the
northern part of Bavaria. The region is famous for its high-tech industry,
hiking and rock-climbing as well as for the world’s highest density of
breweries.

Applicants are asked to submit their curriculum vitae, a list of
publications and names and contact information of two references to:

Prof. Dr.-Ing. Erik Bitzek
erik.bitzek@ww.uni-erlangen.de

Simulation of crack bifurcation in single crystal nickel base superalloys under mixed mode conditions

PhD position available at the Centre des Matériaux , ParisTech , starting fall 2009.

This 3-year project is fully funded by Mines ParisTech, SNECMA and ONERA.

More details are given in the attached file.

Wanted skills: Applicants should possess a degree in Mechanical Engineering, Materials Science or a related discipline and have an outstanding academic track record. The candidate should have a solid understanding of mechanics of materials and continuum mechanics and be prepared to work on a mostly numerical project using non-linear finite element techniques.

Contact: Send a CV and a motivation letter to Samuel Forest.

During solid-solid phase transformations elastic stresses arise due to a difference in lattice parameters between the constituent phases. These stresses have a strong influence on the resultant microstructure and its evolution; more specifically, if there be externally applied stresses, the interaction between the applied and the transformation stresses can lead to rafting.

Rafting is the preferential coarsening of (dilatationally) misfitting precipitates in a direction parallel (P-type) or perpendicular (N-type) to an applied stress. In the materials literature, it is sometimes argued that rafting is an elasto-plastic phenomenon, and that plastic pre-strains are essential for rafting. In this paper (which we have submitted to Acta Materialia) we show that purely elastic stress driven rafting is a distinct possibility.

Abstract:

We examine rafting of two-phase microstructures under a uniaxial applied stress, a process in which a mismatch in elastic moduli (elastic inhomogeneity) plays a central role. For this purpose, we have used a phase field model of an elastically inhomogeneous alloy; elastic stress and strain fields are calculated using a method adapted from the homogenization literature. We have characterized the efficiency of the resulting iterative algorithm based on Fourier transforms. Our simulations of rafting in two-dimensional systems show that rafting (unidirectionally elongated microstructures) is promoted when the precipitate phase is softer than the matrix and when the applied stress has the same sign as the eigenstrain. They also show that migration (for both hard and soft precipitates) and coalescence (for soft precipitates) have significant contributions to rafting.