The evolution of local mechanical properties of bulk metallic glasses caused by structural inhomogeneity

Abstract

The local mechanical properties and the serrated flow behavior of a Cu₃₆Zr₄₈Al₈Ag₈ bulk metallic glass (BMG) have been investigated using nanoindentation. The influence of structural inhomogeneity on the deformation behavior of the BMG is confirmed. The loading rate plays little role in the deviation degree of load–displacement (P–h) curves. It was found that the accumulation of different kinds of serrations leads to the deviation of the P–h curves under a low loading rate. The deviation degree is associated with the numbers and characteristics of the serrated flow. Under a high loading rate, the simultaneous operation of multiple shear bands leads to the disappearance of the serrated flows, and the deviation degree of the P-h curves with each other is assumed to be related to the accumulation of different inconspicuous serrated flows.

Introduction

Last two decades bulk metallic glasses (BMGs) have attracted extensive attentions in fundamental science and potential engineering applications due to their unique structure and promising properties [1], [2], [3]. However, owing to the low ductility at room temperature (near zero in tension and less than 2% under compression), practical structural applications of BMGs are severely limited [4], [5], [6]. In recent years, a lot of attempts [7], [8], [9], [10] have been made to improve the ductility of BMGs. Introducing inhomogeneous structure into the amorphous matrix is one of the effective approaches [7], [8]. It has been shown that the limited ductility of BMGs at room temperature is attributed to the shear localization and strain/thermal softening during deformation [11], [12]. Thus, it is believed that inhomogeneous structure affects significantly the ductility of BMGs by changing the degree of shear localization and strain/thermal softening.

Ye et al. [13] carried out high-frequency dynamic nanoindentation tests on micrometer-sized samples, which indicated that the basic structural unit in BMGs during deformation is atomistic core–shell structure, including a free volume site surrounded by elastic deformation region. Kanungo et al. [14] used position annihilation spectroscopy (PAS) examining a Cu-based bulk metallic glass, which showed that there was a distribution of free volume with different sizes and that the concentration of the sites with larger free volume size increased during deformation. Thus, the inhomogeneous structure of BMGs can be regarded as the random distribution of the basic structure with different sizes of the free volume. A typical distribution function of the free volume size introduced by Argon [12] is similar to normal distribution function, which means that the size of the free volume is concentrated in a range that is not too small and not too large. Although some investigations have been made to describe the deformation unit of BMGs and the evolution of local deformation, the relationship between inhomogeneous structure and the evolution of the local mechanical properties is still not well understood.

In recent years, nanoindentation has been widely used for the investigation of the inelastic deformation behavior of BMGs [15], [16], [17], [18]. Schuh and Nieh [15] demonstrated that the serrated flows of load–displacement (P–h) curves during nanoindentation depend significantly on the loading rate. At a high loading rate, multiple shear bands operate simultaneously instead of a single shear band that cannot accommodate the imposed strain. As the tip of the indenter is in the magnitude of nanoscale, nanoindentation testing method is particularly suitable for the investigation of the effects of structural inhomogeneity on the mechanical properties. Thus, in this paper, we conducted nanoindentation tests on a BMG to investigate the mechanical properties as a function of structural inhomogeneity.