Microsc. Microanal. 23, 1076–1081, 2017 doi:10.1017/S1431927617012594


© MICROSCOPY SOCIETY OF AMERICA 2017


Micron-Scale Deformation: A Coupled In Situ Study of Strain Bursts and Acoustic Emission


Ádám István Hegyi,1 Péter Dusán Ispánovity,1,* Michal Knapek,2 Dániel Tüzes,1 Kristián Máthis,2 František Chmelík,2 Zoltán Dankházi,1 Gábor Varga,1 and István Groma1


1Department of Materials Physics, Eötvös Loránd University, Pázmány Péter sétány 1/a, H-1117 Budapest, Hungary 2Faculty of Mathematics and Physics, Department of Physics of Materials, Charles University in Prague, Ke Karlovu 5, 121 16 Prague 2, Czech Republic


Abstract: Plastic deformation of micron-scale crystalline materials differs considerably from bulk samples as it is characterized by stochastic strain bursts. To obtain a detailed picture of the intermittent deformation phenomena, numerous micron-sized specimens must be fabricated and tested. An improved focused ion beam fabrication method is proposed to prepare non-tapered micropillars with excellent control over their shape. Moreover, the fabrication time is less compared with other methods. The in situ compression device developed in our laboratory allows high-accuracy sample positioning and force/displacement measurements with high data sampling rates. The collective avalanche-like motion of the dislocations is observed as stress decreases on the stress–strain curves. An acoustic emission (AE) technique was employed for the first time to study the deformation behavior of micropillars. The AE technique provides important additional in situ information about the underlying processes during plastic deformation and is especially sensitive to the collective avalanche-like motion of the dislocations observed as the stress decreases on the deformation curves.


Key words: micropillar compression, in situ deformation, acoustic emission, microsample fabrication INTRODUCTION


In the past decades, the miniaturization of mechanical and electronic devices has inspired research to determine the mechanical properties of micron-sized specimens (Volkert& Lilleodden, 2006; Ng & Ngan, 2008; Zaiser et al., 2008; Kraft et al., 2010; Zhou et al., 2011). Many microelectromechanical devices (e.g., micromachined inertial sensors or cantilever transducer platforms for chemical and biological sensors) contain micrometer-sized components (Yazdi et al., 1998; Lavrik et al., 2004). To design progressively smaller devices, the detailed physical events underlying the deformation processes in the microparts must be understood. Plastic deformation of crystalline materials typically


occurs by the collective motion of dislocation ensembles. The stress–strain response of macroscopic samples is generally smooth and reproducible due to the large number of moving dislocations, thus allowing highly accurate predictions of the material properties. In contrast, at micrometer scales, the inhomogeneities in the dislocation structure can be on the order of the sample size, leading to a discontinuous response due to the stochastic activation of dislocation avalanches (Miguel et al., 2002; Weiss & Marsan, 2003; Zaiser et al., 2004; Zaiser & Moretti, 2005; Zapperi, 2012). Therefore, a statistical approach must be employed to assess the mechanical behavior of materials at this scale. The initial evidence of intermittent crystal plasticity was observed in single ice crystals by detecting strong acoustic


*Corresponding author. ispanovity@metal.elte.hu Received October 21, 2016; accepted August 29, 2017


emission (AE) signals during creep deformation (Weiss et al., 2000; Miguel et al., 2001). A decade ago, Dimiduk et al. found that instabilities in the form of strain jumps dominate micrometer-scale crystal plasticity by compressing pure single-crystalline Ni micropillars (Uchic et al., 2004; Dimiduk et al., 2006; Uchic et al., 2009), raising the questions (i) where is the limit between microscopic and macroscopic deformation, and (ii) how can material strength parameters, such as yield point or ultimate compressive/tensile strength, be defined for micron-scale objects (Arzt, 1998; Greer & De Hosson, 2011; Ispanovity et al., 2013). Due to their statistical nature, revealing the properties of


microdeformation requires numerous samples to be tested. One of the most typical and frequently applied methods for fabricating microsamples (micropillars) is focused ion beam (FIB)milling (Reyntjens & Puers, 2001). The main advantage of this method is the ability to continuously visually control the fabrication process. Moreover, there is practically no limitation on thematerials fromwhich the micropillars can be milled. Conversely, FIB-based methods are rather time consuming. To shorten the fabrication time of micropillars, several different (FIB-less) methods are being developed (Burek & Greer, 2009; Jennings et al., 2010). Unfortunately, FIB-less methods typically do not allow the production of pillars from any type of material. Furthermore, the initial dislocation density of the samples can be difficult to control, or the connecting force between the substrate and the grown micropillars can be rather weak (Moser et al., 2012). Two approaches are commonly applied to fabricate micropillars by FIB milling: “lathe” and “annular” milling


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