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Martin Heilmaier Karlsruhe Institute of Technology, Institute for Applied Materials (IAM), Karlsruhe D-76131, Germany (Received 15 May 2017; accepted 3 October 2017)

A customized-built high-temperature tensile creep setup is introduced. Dog-bone shaped miniaturized specimens made from Nimonic-75 were tested as reference materials at temperatures of 850 and 1000 °C under constant load to verify the setup’s accuracy. The results were compared to tensile creep tests with conventional (standard size) specimens at identical experimental conditions. The shape of the creep curves obtained in the miniaturized specimens exhibits a pronounced minimum creep rate, thus, being seemingly different from the ones obtained for the bulk samples which reveal a clear steady-state regime. This is partly due to the continuous increase of stress under constant load testing conditions and very likely affected by the much higher surface to volume ratio of the miniaturized specimens leading to the premature onset of tertiary creep. Still, a good agreement was obtained between the two specimen sizes with respect to the dependence of the steady-state (standard size) and minimum (miniaturized size) strain rate on applied stress at various temperatures leading to also comparable activation energies of the creep.

I. INTRODUCTION

The development of new materials applied in high temperature is a continual requirement, for example, the working temperature of a turbine blade is a key point for the power factor; therefore, the study of materials’ creep properties is an essential precondition of various engineering applications at elevated temperatures. With the development of microelectro-mechanical systems, mechanical testing of miniaturized specimens has become feasible to research the material’s properties influenced by microstructure, size-effect, and extreme external conditions, etc.1,2 Several research groups have developed tensile test methods3,4 of materials on the microscale down to several lm even at a high temperature as well as methods for tensile creep testing.5,6 With this article, a custom-built test setup7 for ceramic thin film micro-tensile testing is extended to the tensile creep testing of metallic specimens up to 1000 °C. The purpose of this manuscript is 2-fold: it firstly introduces a customized high-temperature micro-tensile-creep setup for miniaturized specimens with the application of (i) digital image correlation (DIC) for the contactless strain measurement and (ii) a customized system for load and temperature control. Mechanical testing by

Contributing Editor: Mathias Göken a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.414

using miniaturized specimens is both economically and scientifically attractive for materials that are expensive and difficult to manufacture. The second purpose of this article is the discussion of variations in specimen size, the resulting accuracy and limitations with the designed test setup with respect to temperature, strain, and load contr