Plastic instability in amorphous selenium near its glass transition temperature
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Yanfei Gaob) Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; and Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Warren C. Oliver Nanomechanics Inc., Oak Ridge, Tennessee 37830
George M. Pharr Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 (Received 21 December 2009; accepted 16 March 2010)
The deformation behavior of amorphous selenium near its glass transition temperature (31 C) has been investigated by uniaxial compression and nanoindentation creep tests. Cylindrical specimens compressed at high temperatures and low strain rates deform stably into barrel-like shapes, while tests at low temperatures and high strain rates lead to fragmentation. These results agree well with stress exponent and kinetic activation parameters extracted from nanoindentation creep tests using a similarity analysis. The dependence of the deformation modes on temperature and strain rate can be understood as a consequence of material instability and strain localization in rate-dependent solids. Amorphous materials usually display a ductile-to-brittle transition with a decrease in temperature, with the transition temperature depending on the applied strain rate. While tensile/compression tests are commonly used to characterize such behavior, the use of novel small-scale mechanical testing methods such as the nanoindentation may in some instances have distinct advantages, since the highly constrained deformation during indentation suppresses fracture and allows other deformation processes to be observed and measured.1,2 Indentation testing, however, produces a much more complex stress state and strain rate history, so that the analysis of rate-dependent material behavior is not straightforward. The primary objective of this work was to determine whether the kinetic activation parameters obtained from indentation creep tests can be correlated with the ductile-to-brittle transition observed in compression testing. To this end, a comparative study of uniaxial compression and nanoindentation creep tests was conducted on amorphous selenium. This material was chosen because its glass transition temperature, Tg ¼ 31.0 0.5 C, is near room a)
Present address: Alliant Techsystems Inc., Elkton, MD 21921. Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0141 b)
J. Mater. Res., Vol. 25, No. 6, Jun 2010
http://journals.cambridge.org
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temperature, so that simple modifications of a standard nanoindentation system can be used to explore the deformations behavior above and below Tg.3–5 Amorphous selenium samples were prepared by melting pure amorphous selenium shot (Alfa Aesar, Ward Hill, MA) and quenching into copper molds. The as-cast samples for nanoindentation (31.75 mm in diameter by 5 mm thick) had a mirror surface f
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