Indentation creep revisited
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Indentation creep revisited In-Chul Choi, Byung-Gil Yoo, Yong-Jae Kim, and Jae-il Janga) Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea (Received 7 May 2011; accepted 17 June 2011)
Recent extensive nanomechanical experiments have revealed that the instantaneous strength and plasticity of a material can be significantly affected by the size (of sample, microstructure, or stressed zone). One more important property to be added into the list of size-dependent properties is time-dependent plastic deformation referred to as creep; it has been reported that the creep becomes more active at the small scale. Analyzing the creep in the small scale can be valuable not only for solving scientific curiosity but also for obtaining practical engineering information about the lifetime or durability of advanced small-scale structures. For the purpose, nanoindentation creep experiments have been widely performed by far. Here we critically review the existing nanoindentation creep methods and the related issues and finally suggest possible novel ways to better estimate the small-scale creep properties.
I. INTRODUCTION
Over the past 100 years, time-dependent plastic deformation of a material, referred to as creep, has been of great interest from both scientific and engineering viewpoints. Especially, for structural materials for hightemperature applications, evaluation of their creep properties has been essentially conducted since the creep is a thermally activated process and thus it plays an important role in mechanical performance at high temperatures because of high atomic mobility. Although the same terminology has been often used for describing the timedependent elastic (“viscoelastic”) deformation in some materials like polymers and glasses, the subject “creep” in this review will be limited to time-dependent plastic (“viscoplastic”) deformation for which a significant portion of the creep strain is permanent and unrecoverable after unloading. The creep curve describing the change in uniaxial strain of a metal under a constant load (or stress) and temperature can be clearly resolved into three stages1–3: primary (or transient) creep where the sample deforms rapidly but at a decreasing rate (which is typically described as e } t1/3; here, e is the creep strain and t is the hold time), secondary (or steady-state) creep where the creep strain rate reaches a minimum value and remains almost constant (i.e., e } t), and tertiary creep in which the creep strain rate accelerates rapidly until the sample fails. An important quantitative measure of the creep curve is the slope of secondary creep regime, that is, the socalled steady-state creep rate which can be empirically related with rupture time by Monkman-Grant equation.1,2 The steady-state creep strain rate in metals and alloys are a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.213 J. Mater. Res., Vol. 27, No. 1, Jan 14, 2012
known to be strongly dependent on the applied stress r, (absolute)
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