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Vibrational entropy of dislocations in Al
KTH, Superseded Departments, Physics.
KTH, Superseded Departments, Physics.
KTH, Superseded Departments, Physics.
2004 (English)In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 84, no 6, 521-532 p.Article in journal (Refereed) Published
Abstract [en]

The region nearest to a lattice defect must be described by an atomistic model, while a continuum model suffices further away from the defect. We study such a separation into two regions for an edge dislocation. In particular we focus on the excess defect energy and vibrational entropy, when the dislocation core is described by a cluster of about 500-100 atoms, embedded in a large discrete and relaxed, but static, lattice. The interaction between the atoms is given by a potential of the embedded-atom model type referring to Al. The dynamic matrix of the vibrations in the cluster is fully diagonalized. The excess entropy DeltaS near the core has positive and negative contributions, depending on the sign of the local strain. Typically, DeltaS/k(B) approximate to 2 per atomic repeat length along the dislocation core in fcc Al. In the elastic continuum region far from the dislocation core the excess entropy shows the same logarithmic divergence as the elastic energy. Although the work refers to a specific material and defect type, the results are of a generic nature.

Place, publisher, year, edition, pages
2004. Vol. 84, no 6, 521-532 p.
Keyword [en]
1st-principles calculations, aluminum, potentials, metals, copper
National Category
Physical Sciences
URN: urn:nbn:se:kth:diva-23106DOI: 10.1080/14786430310001635422ISI: 000188327500002ScopusID: 2-s2.0-1242352398OAI: diva2:341804
QC 20100525Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2010-10-13Bibliographically approved
In thesis
1. Atomistic simulations of lattice defects
Open this publication in new window or tab >>Atomistic simulations of lattice defects
2005 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Mechanical properties of solids are governed by crystal imperfections. Computational materials science is largely concerned with the modelling of such defects, e.g. their formation, migration, and interaction energies. Atomistic simulations of systems containing lattice defects are inherently difficult because of the generally complicated geometrical structure of the defects, the need for large simulation cells, etc.

In this thesis, the role of lattice defects in the mechanism behind homogeneous melting is demonstrated. Also, a generic calculational scheme for studying atomic vibrations close to extended defects (applied to a dislocation) has been considered. Furthermore, heat capacities in the solid and liquid phases of aluminium have been calculated, as well as various thermophysical defect properties.

The work was carried out using classical atomistic simulations, mainly molecular dynamics, of aluminium and copper. The interatomic forces were modelled with effective interactions of the embedded-atom type.

The main results of this thesis are the following:

• The thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies.

• In the initial stage of melting, no signs of a shear modulus melting mechanism, or the presence of line-like defects (dislocations), can be seen.

• The typical time interval from when melting initiates to the time at which the liquid phase is fully developed is of the order of 1000τ, where the period τ corresponds to the maximum vibrational frequency in the solid.

• The solid-liquid boundary advances at a pace comparable to that of thermal transport by vibrating atoms in the crystal at high temperatures.

• The seemingly small anharmonic effect in the heat capacity of aluminium is caused by a partial cancellation of the low-order term linear in the temperature and anharmonic terms of higher order in the temperature.

• The core region of an edge dislocation in face-centred cubic aluminium has compressed and expanded regions. The excess volume associated with the dislocation core is small, about 6 percent of the atomic volume, as a result of a partial cancellation between the volume changes of the compressed and expanded regions.

• The compressed and expanded regions of the edge dislocation core give negative and positive contributions, respectively, to the excess vibrational entropy. The overall effect is a positive vibrational excess entropy of the dislocation core which is about 2kB per atomic repeat length along the dislocation core.

• The atomic vibrations near the dislocation core are modelled by considering an atomic cluster with about 500-1000 atoms containing the core of dislocation, embedded in a large discrete, but relaxed, lattice of about 23 000 atoms. An atomic region that is four atomic layers thick and about 18 atomic diameters long in the direction parallel to the Burgers vector, accounts for most of the excess entropy.

• The constant-pressure heat capacity of aluminium shows a minimum as a function of temperature in the liquid phase.

Place, publisher, year, edition, pages
Stockholm: KTH, 2005. vii, 64 p.
Trita-FYS, ISSN 0280-316X ; 2005:26
Theoretical physics, Teoretisk fysik
National Category
Physical Sciences
urn:nbn:se:kth:diva-253 (URN)91-7178-042-4 (ISBN)
Public defence
2005-06-07, Sal FB42, AlbaNova, Roslagstullsbacken 21, Stockholm, 13:00
QC 20101013Available from: 2005-06-01 Created: 2005-06-01 Last updated: 2010-10-13Bibliographically approved

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