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Yield Point Phenomena and Dislocation Velocities Underneath Indentations Into BCC Crystals

Published online by Cambridge University Press:  21 February 2011

W. W. Gerberich ,
S. Venkataraman ,
J. Nelson ,
H. Huang ,
E. Lilleodden  and
W. Bonin
W. W. Gerberich
Affiliation:
Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, MN 55455
S. Venkataraman
Affiliation:
Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, MN 55455
J. Nelson
Affiliation:
Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, MN 55455
H. Huang
Affiliation:
Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, MN 55455
E. Lilleodden
Affiliation:
Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis, MN 55455
W. Bonin
Affiliation:
Hysitron, Inc., Minneapolis, MN 55413
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Abstract

In a single material, hardnesses can range from 3σys up to the theoretical strength or approximately E/10 where σys and E are tensile yield strength and modulus. This variation results with decreasing depth of penetration. Such indentation size effects may be associated with surface contamination, passivation films and dislocation phenomena. Even where dislocation nucleation is relatively difficult as in GaAs, the hardness varies from about 1.5 to 15 GPa as the indentation depth decreased from about 100 nm to 10 nm. Similar size effects in BCC metals can give hardnesses which range from about 1 to 30 GPa as the indentations decrease from 1000 nm to 10 nm. Here, there are two types of “yield” phenomena which can be related to an organic contamination film and a dislocation pile-up induced oxide film fracture. As measured in single crystals of Fe, Mo, W, Ta and NiAl, this typically gives lower and upper “yield” points which range from 0.6 to 10 raN. When a dislocation pile-up breaks through the oxide film, velocities can be reasonably large due to the stress at the head of the pile-up. The average dislocation velocity of this avalanche is controlled, to first order, by the Peierls’ energy. A more exhaustive study of NiAl, with a B2/BCC type crystal structure shows that dislocation velocity can be related to the local pile-up stress and a Peierls’ barrier of about 2.2 eV.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 356: Symposium B2 – Thin Films: Stresses and Mechanical Properties V , 1994 , 629
Copyright
Copyright © Materials Research Society 1995

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