Rate dependence of serrated flow in a metallic glass

W. H. Jiang, M. Atzmon
2003 Journal of Materials Research  
Plastic deformation of amorphous Al 90 Fe 5 Gd 5 was investigated using nanoindentation and atomic force microscopy. While serrated flow was detected only at high loading rates, shear bands were observed for all loading rates, ranging from 1 to 100 nm/s. However, the details of shear-band formation depend on the loading rate. Plastic deformation of metallic glasses at low temperatures is known to be highly localized to narrow shear bands. 1 It is generally believed that shear-band formation is
more » ... -band formation is very weakly dependent on the strain rate or temperature. 2 However, some reports suggest that the strain rate affects shear-band formation. Mukai et al. 3 reported that the density of shear bands in a Pd-based bulk metallic glass increased with strain rate. Schuh et al. 4 recently studied the deformation behavior of Pd 40 Ni 40 P 20 bulk metallic glass by nanoindentation. They observed serrated flow only at strain rates below about 1-10/s. In a Zr-based bulk metallic glass, Wright et al. 5 attributed serrated flow behavior to formation of individual shear bands. Consequently, one might conclude that shear bands would form only at low strain rates. However, as Greer et al. 6 recently commented, shear-band formation is a high-strain-rate phenomenon. Recently, we also found serrated flow behavior of an Al-based amorphous alloy during nanoindentation at low strain rates. To conclusively determine the formation of shear bands, more direct observations are needed. Here, we present results of nanoindentation and observation of resulting shear bands by atomic force microscopy (AFM). We found shear bands at all strain rates and attribute the observations of Schuh et al. to the instrumental resolution of the nanoindenter. We note that the present discussion is restricted to the low-temperature regime, in which metallic glasses deform inhomogeneously. 1 Amorphous Al 90 Fe 5 Gd 5 ribbon, 22 m thick and 1 mm wide, was obtained by single-wheel melt-spinning in an argon atmosphere using a Cr-coated Cu wheel at a tangential velocity of 40 m/s. Prior to indentation, the ribbon was polished electrolytically using a solution of 25% nitric acid and 75% methanol, at 243 K, and a voltage of 90 V. Indentation experiments were performed using a Nanoinstruments Nanoindenter II (Oak Ridge, TN) with a diamond Berkovich indenter. At least ten indents were made on each of the multiple samples. The distance between adjacent indents was 20 m. The loading phase of indentation was carried out under displacement control, and the maximum (elastic plus plastic) indentation depth was 1 m. The thermal drift of the instrument was maintained below 0.2 nm/s. AFM observation on the indents was conducted using a Digital Instruments Nanoscope IIIa (Santa Barbara, CA) in contact mode. Figure 1 shows the loading portions of typical loaddisplacement (P-h) curves at loading rates of 100, 50, 10, and 1 nm/s. Serrated flow was observed at 1 nm/s and 10 nm/s only, which is similar to the results of Schuh et al. 4 Extensive AFM observation revealed that shear bands are the main morphological characteristics of indents formed at all loading rates. The typical morphologies of indents produced at the highest and lowest loading rates, 100 nm/s and 1 nm/s, are shown in illumination-mode AFM images in Fig. 2 . Surface steps due to shear bands are observed around and in the indents. Figure 3 shows the depth profiles of the indents in Fig. 2 . Evidently, the number of shear bands is smaller and the step size is larger outside the indent formed at the low-loading rate, indicating that a high deformation rate leads to a greater number of shear bands. For a standard sample of fused silica, relatively smooth load-displacement curves were obtained at all loading rates. This confirms that the serrated flow in the amorphous alloy is due to the material's intrinsic behavior. The present results show that at loading rates ranging from 1 nm/s to 100 nm/s, the characteristics of the load-displacement curves are dependent on the loading rate, and the shear bands formed at high rates are not reflected in load-displacement curves. Since the total strain used is the same for all rates, a smaller number of shear bands corresponds to larger slip steps. Thus, serrated flow is observed only when the step
doi:10.1557/jmr.2003.0103 fatcat:sz3ljnxtnbcfro6etbxqjl6lnm