Temperature invariant flow stress during microcompression of a Zr-based bulk metallic glass J.M. Wheeler, ⇑ R. Raghavan and J. Michler EMPA – Swiss Federal Laboratories for Materials Testing and Research, Laboratory for Mechanics of Materials and Nanostructures, Feuerwerkerstrasse 39, Thun CH-3602, Switzerland Received 5 February 2012; revised 28 March 2012; accepted 29 March 2012 Available online 5 April 2012 The elevated temperature mechanical response of a Zr-based bulk metallic glass (BMG) was examined using in situ microcom- pression in a scanning electron microscope at temperatures up to 387 °C. The flow stress was found to remain constant with tem- perature at 2 GPa below the glass transition temperature, T g . The magnitude of the stress drops/serrations in the stress–strain curve was found to increase with temperature. Above T g , plastic flow was observed to be homogeneous without any shear band formation. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Micropillar; Scanning electron microscopy; Metallic glasses; High temperature deformation Bulk metallic glasses (BMGs) exhibit high yield strength akin to ceramics, fracture toughness equivalent to metals and formability like polymers, which are un- ique combinations of mechanical properties, placing them in the desirable regimes of Ashby maps [1]. Plastic deformation in BMGs occurs by the formation of local- ized “shear bands”, which are observed as offsets or sur- face steps at small length scales, high strain rates and low temperatures or under constrained loading cases [2–4]. A transition in the deformation mechanism from heterogeneous shear banding to Newtonian viscous homogeneous flow is observed at low strain rates and elevated temperatures [5,6]. One excellent methodology to study plastic deformation in brittle or quasi-brittle materials like BMGs is compression of micropillars machined by focused ion beams (FIBs) in displace- ment-controlled mode [7–12]. Indeed, microcompression testing has been shown to have superior accuracy over “bulk” testing for the determination of the intrinsic yield stress [7,13–15]. Much of the work using this technique has been concerned with a “size effect” where the yield strength or deformation mechanism is observed to change with decreasing sample size. A thorough review of the literature on the size effect can be found elsewhere [16]. Here, small scale, in situ testing (micropillar com- pression) of BMGs is used at elevated temperatures to study the change in deformation mechanics and mecha- nisms as a function of temperature in a size range which is representative of bulk behaviour [10]. By utilizing microcompression techniques at elevated temperatures in situ in a scanning electron microscope (SEM), it is also possible to directly correlate deforma- tion mechanism changes as a function of both tempera- ture and strain rate. This improves upon deformation mechanism mapping via elevated temperature nanoin- dentation [17,18] by eliminating strain gradients within the test volume and allowing observation of the defor- mation. Like nanoindentation, microcompression test- ing offers the potential of rapid, statistical testing on small quantities of material. The potential of this tech- nique for the development of materials is enormous, since the quantity of material required to generate a deformation mechanism map is literally microscopic. To demonstrate this technique’s application at elevated temperature, a Zr-based BMG (Vitreloy 1) of the nominal composition Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (at.%) was ob- tained in the form of 3 mm thick plates from Liquidmetal Technologies, USA. The glass transition temperature, T g , of this alloy, measured using differential scanning calorime- try, is 351.85 °C [19]. The tests were carried out on a 10  2 mm 2 block cut from the plate and metallographically prepared with a final polish using a 0.25 lm diamond suspension. Micropillars with diameters of 2 lm with as- pect ratios of 2–2.5 were fabricated using a Ga ion beam at an accelerating voltage of 30 kV in Tescan Vela FIB 1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.03.039 ⇑ Corresponding author. E-mail: Jeffrey.Wheeler@empa.ch Available online at www.sciencedirect.com Scripta Materialia 67 (2012) 125–128 www.elsevier.com/locate/scriptamat