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The time-dependent characteristics of incipient plasticity in Ni3Al during nanoindentation in the subcritical load regime were investigated statistically. The waiting time for incipient plasticity to occur at constant load was found to follow a Poisson-like distribution, with the peak shifting toward zero holding time as the load increased and eventually becoming an exponential distribution when the load was close to a critical value. The observed distribution of the strain burst waiting time at loads smaller than the critical value was inconsistent with the picture in which dislocations nucleated homogeneously out of the perfect crystal. The kinetics for the occurrence of strain burst in this case is thought to be governed by the accumulative growth of nucleation precursors.

I. INTRODUCTION

During nanoindentation, the onset of plasticity in a defect-free crystal is often associated with a sharp excursion in the indenter displacement when the load reaches a critical value, and this phenomenon is known as strain burst or pop-in.1–4 The occurrence of a strain burst is generally believed to be the result of the generation and fast multiplication of dense dislocation structures, marking the transition from elastic to plastic deformation overall.4–8 In constant ramping load experiments, the critical pop-in load was observed to increase with the loading rate.9 Moreover, it was also found that a strain burst may also occur when the indentation load is held below the critical value, after a certain waiting time.4,7 Figure 1 shows the load and displacement versus time during a nanoindentation test on a well-annealed Ni3Al crystal. After the load was held at about 397 ␮N for about 18 s, a sudden increase in the displacement and a sudden drop in the load were observed simultaneously, indicating a strain burst in the specimen. In previous studies of this time-delay effects of pop-in by Gerberich et al.7 and Chiu and Ngan,4 the waiting time before popin was observed to be longer as the indentation load decreased. The statistical scatter of the results also appeared to be larger as the load decreased, but the sample sizes used in these studies, typically 5 indentations at each condition, were too small to allow convincing statistical comparison. The aim of the present study was a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0056 J. Mater. Res., Vol. 20, No. 2, Feb 2005

therefore to investigate the statistical distribution of the waiting time before a pop-in occur at constant subcritical loads, with a goal of providing information on the mechanism of the delayed type of pop-in. II. EXPERIMENTAL

An ingot was prepared by melting pure Ni (>99.92%) and Al (>99.79%) in an induction furnace, which was then homogenized in a vacuum better than 10−5 mbar at 1200 °C for 120 h. A disk approximately 1 mm thick and 3 mm in diameter was cut from the ingot. Both sides of the disk were mechanically polished down to 1 ␮m. After that, one side of the disk was coated with lacquer, and the other side was