Characterization of nanometer-scale defects in metallic glasses by quantitative high-resolution transmission electron microscopy Jing Li, 1 Z. L. Wang, 2 and T. C. Hufnagel 1, * 1 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218 2 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 Received 16 November 2001; revised manuscript received 9 January 2002; published 25 March 2002 Although defects can have a significant effect on the properties of amorphous materials, in many cases these defects are poorly characterized and understood. This is at least partly due to the difficulty of imaging defects in amorphous materials in the electron microscope. In this work, we demonstrate the utility of quantitative analysis of high-resolution transmission electron microscopy for the identification and characterization of nanometer-scale defects in metallic glasses. For a proper identification of such defects, it is important to carefully consider the effects of the imaging conditions and thickness variations in the sample, both of which we describe in detail. As an example, we show that regions of localized plastic deformation shear bands in bulk metallic glasses contain a high concentration of nanometer-scale voids. These voids apparently result from the coalescence of excess free volume once the applied stress is removed. DOI: 10.1103/PhysRevB.65.144201 PACS numbers: 61.72.Dd, 61.43.Dq, 62.20.Fe I. INTRODUCTION Transmission electron microscopy TEM is one of the most useful tools for studying defects in crystalline metals, with a wide variety of imaging and diffraction modes that can reveal defects such as dislocations. In most cases, these modes make use of the fact that defects create local disrup- tions in the otherwise perfect periodicity of the crystalline structure. Thus, it is the very existence of a lattice that allows us to detect the defects in the structure. Defects are also important in amorphous materials, but the very fact that these materials are noncrystalline makes the identification and characterization of defects in the TEM quite challenging. Despite the challenges, some progress has been made to- wards developing TEM techniques for imaging defects in amorphous materials. In this paper, we report on our use of quantitative high-resolution transmission electron micros- copy HRTEM to identify nanometer-scale defects in shear bands in bulk metallic glasses, using a technique previously described by Miller and Gibson. 1 Of particular importance is the influence of imaging conditions and variations in sample thickness on the results, which we describe in some detail. We observe a high concentration of defects in shear bands regions of the metallic glass which have undergone exten- sive local plastic deformation. We believe the defects form when excess free volume in the active shear band coalesces into voids. II. SAMPLE PREPARATION AND IMAGING We prepared 3 mm diameter rods of amorphous Zr 57 Ti 5 Cu 20 Ni 8 Al 10 by arc melting master alloy ingots from the pure elements, followed by casting into a copper mold. Samples for electron microscopy were prepared by elec- tropolishing sections of the rods in a solution of 30% per- chloric acid in ethanol at -30°C for 20–30 s until perfo- ration. We usually observed that samples electropolished in this way had some residual surface contamination, which we removed by a brief 1h ion milling process 3 kV, 0.1–0.25 mA, at 11° incidence. Ordinary handling of the TEM speci- mens produced small cracks at the edge of the electron- transparent region; ahead of and around these microcracks, shear bands regions of local plastic deformation were ob- served in ordinary bright-field TEM imaging. The contrast in bright-field imaging arises because the deformed regions are thinner than the undeformed material. For the particular ex- ample described below, the measured thickness of the unde- formed material is approximately 10 nm and that of the shear band region approximately 5 nm. We used a Philips CM300 field emission gun microscope operated at 300 kV; the im- ages were collected on a charge-coupled device CCD cam- era. III. DATA ANALYSIS A. Overview of an example of analysis The analysis procedure we used follows that of Miller and Gibson, who first described its use for identification of nanometer-scale voids in amorphous silica thin films. 1 In this section, we briefly outline the analysis procedure with com- ments relevant to our particular case and provide images of an example. Additional details on several critical points are provided in the following sections. 1 We begin by obtaining HRTEM images that contain both deformed and undeformed regions in a single micro- graph Fig. 1a. Using a single image ensures that the con- trast transfer function CTF is the same for both regions, eliminating any effect of the microscope conditions on the comparison of the two regions that might occur if they were imaged separately. For imaging defects in metallic glasses, it is convenient to chose a defocus value of around -200 nm. At this defocus the first zero of the contrast trans- fer function occurs at about k =1.8 nm -1 ; this allows us to examine the region of interest (0.5k 1.5 nm -1 ) without having to worry about the effect of contrast reversal on the image. This is discussed in more detail below. 2 We then calculate the two-dimensional Fourier trans- form of two portions of the image, one from an undeformed PHYSICAL REVIEW B, VOLUME 65, 144201 0163-1829/2002/6514/1442016/$20.00 ©2002 The American Physical Society 65 144201-1