Ab-initio modeling of extended defects in body - centered cubic transition metals
Dr. Lorenz Romaner
Materials Center Leoben Forschung Gmbh, Roseggerstraße 12, A-8700 Leoben
17:00 - 18:00 Tuesday 21 April 2015 KFU HS 5.01 “We know our friends by their defects rather than by their merits”. These words by the British writer W. Somerset Maugham often conjure a smile in the face of material scientists since defects not only matter for understanding humans but also for understanding materials. Especially mechanical properties often arise only because of extended crystallographic defects. Important examples are plastic deformation which is governed by dislocation motion or crack propagation which for certain materials occurs predominantly at grain boundaries. Unrevealing and understanding the atomistic processes taking place in these defects is, hence, a topic of intense research.
In this seminar talk I will present ab-initio calculations of extended crystallographic defects in body-centered cubic transition metals. This material class encloses the refractory metals W and Mo which are of great interest for high-temperature applications and Fe, which is the most relevant element for structural materials. The first focus will be laid on screw dislocations which in this materials class exhibit a peculiar feature, namely a non-planar core whose atomistic structure has been heavily debated over decades. The use of density functional theory has recently shed new light on this topic and has shown that transformations in core structure can be induced by alloying. The current state of theoretical and experimental knowledge regarding the impact of such core transformations on plastic deformation will be reviewed. The second focus will be laid on grain boundaries, in particular on their geometric structure and segregation of solute elements towards the boundary plane. This phenomenon is of great technological interest as it provides a pathway for improving grain boundary strength which for the refractory metals W or Mo is of central relevance due to their high susceptibility to intergranular fracture. We have recently developed an ab-inito method for treating segregation also for high alloying contents and I present a detailed study on the dependency of segregation energies on the various atomic sites and types of grain boundaries. The obtained segregation anisotropy is compared to results obtained from atom probe field-ion microscopy.
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