Reverse Plasticity in Nanoindentation
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1049-AA05-08
Reverse Plasticity in Nanoindentation Yongjiang Huang1,2, Nursiani Indah Tjahyono2, Jun Shen1, and Yu Lung Chiu2 1 School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China, People's Republic of 2 Chemical and Materials Engineering, University of Auckland, 20 Symonds Street, Auckland, 1142, New Zealand ABSTRACT This paper summarises our recent cyclic nanoindentation experiment studies on a range of materials including single crystal and nanocrystalline copper, single crystal aluminium and bulk metallic glasses with different glass transition temperatures. The unloading and reloading processes of the nanoindentation curves have been analysed. The reverse plasticity will be discussed in the context of plastic deformation mechanisms involved. The effect of loading rates on the mechanical properties of materials upon cyclic loading will also be discussed. INTRODUCTION Nanoindentation technique has been widely used to study the mechanical properties of various materials such as modulus and hardness from the load-displacement curves for its rapidity, simpleness and precision over the last two decades [1]. Traditionally, the elastic modulus of a testing material can be derived from the unloading curve assuming that the unloading is an elastic recovery. However, it has been noted that the measurement on metallic samples may result in elastic moduli higher than those measured by tensile tests. This has been ascribed to the reverse plasticity during unloading where the internal stress developed during loading becomes unstable [2]. In addition to the singular loading-unloading indentation test, cyclic indentation can be carried out at either constant loading rate or constant displacement rate [3]. The cyclic indentation can be used to study quantitatively the hardening or softening behaviour of materials under cyclic loading conditions [4] as well as the fatigue performance of materials [5]. The present work is an attempt to study systematically the reverse plasticity in the unloading/reloading process where a large range of materials were subjected to cyclic indentation. EXPERIMENT The testing materials used for the present study include single crystal aluminium, single crystal copper, nanocrystalline copper, and titanium-based and iron-based bulk metallic glasses (BMG). The single crystal aluminium sample was provided by MTS System Corp., USA. The single crystal copper was grown using the Bridgman method and the nanocrystalline copper with average grain size 70 nm was prepared using electrodeposition technique (see [6] for details). Three BMG samples, Ti40Zr25Ni3Cu12Be20 (BMG-1), Ti41.5Zr2.5Hf5Cu37.5Ni7.5Si1Sn5 (BMG-2)and Fe41Co7Cr15Mo14C15B6Y2 (BMG-3)were used in this study and the preparation details have been reported earlier [7-9].
All testing samples were mechanically polished to achieve a fine finishing before the nanoindentation experiments were performed at room temperature using a MTS nanoindenter XP system. Cyclic nanoindentation tests under constant peak loa
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