The effect of compression and tension on shear-band structure and nanocrystallization in amorphous Al₉₀Fe₅Gd₅: a high-resolution transmission electron microscopy study

Abstract

Using both conventional and high-resolution transmission electron microscopy (HRTEM), the effect of bending at room temperature on the microstructure of amorphous Al₉₀Fe₅Gd₅ was investigated. In the compressive region, nanocrystallites formed at shear bands, along small cracks and at the fracture surface; in the tensile region, nanocrystallites were observed only at the fracture surface. Combining HRTEM with frequency filtering, low-density, nanoscale defects at shear bands were imaged. In the compressive region, both the shear bands and the undeformed matrix contain few defects. In the tensile region, there is a uniform distribution of defects within the shear bands. The preferential precipitation of nanocrystallites in the compressive region is attributed to a kinetic effect due to the uniformly distributed free volume in the shear bands. In contrast, the formation of the nanocrystallites at the fracture surfaces is likely due to adiabatic heating induced by fracture.

Introduction

Newly developed amorphous Al–TM–RE alloys, where TM is a transition metal and RE is yttrium or lanthanides, have higher mechanical strength than crystalline, high strength, Al alloys [1], [2]. Well-distributed nanocrystallites in the amorphous matrix, formed by partial crystallization, can strengthen it even further [3], [4], [5]. Much work on nanocrystallization at elevated temperatures has been reported [6], [7]. Kim et al. [4] found that well-distributed nanocrystalline Al particles formed in the amorphous matrix of an Al–Ni–Fe–Nd alloy during uniaxial tension at elevated temperatures. The resulting ultimate tensile strength was greater than 800 MPa at 580 K, three to four times that of crystalline, high strength, Al alloys. The authors attributed this result to both dispersion strengthening and enhanced thermal stability at the deformation temperature, caused by depletion of Al in the amorphous matrix due to precipitation. Nanocrystalline/amorphous composites show great promise as key structural materials. Lately, attention has been paid to mechanically induced nanocrystallization, such as by ball milling [8], [9], [10], bending [11], [12], rolling [13], tension [4], [14], nanoindentation [15], [16] as well as hydrostatic pressure [17], [18]. With the exception of [17], [18], these involve plastic deformation.

Generally, mechanical deformation can drive crystalline solids far from their equilibrium state by introducing structural or chemical defects [19]. Crystalline alloys may lose their long-range order and be amorphized as a result of deformation [20]. Deformation can also assist atomic transport and evolution toward equilibrium [10], [21]. The effect of deformation on phase evolution is of practical significance for synthesis of nanocrystalline/amorphous composites, their processing and service. In addition, the mechanisms of atom displacement and formation of crystalline phases under plastic deformation are of significant fundamental interest, and have yet to be conclusively explained.

Chen et al. [11], using transmission electron microscopy (TEM), were the first to report the formation of nanocrystallites at shear bands of Al-based amorphous alloys (Al₉₀Fe₅Gd₅, Al₉₀Fe₅Ce₅ and Al₈₇Ni_8.7Y_4.3) bent at room temperature. High-energy ball milling was also observed to lead to the formation of nanocrystallites in Al-based amorphous alloys [8]. Recently, Gao et al. [14] observed nanocrystal precipitation in amorphous Al₉₀Fe₅Gd₅, within vein protrusion on tensile fracture surfaces and along crack propagation paths, as well as within shear bands resulting from bending.

As of yet, no consensus concerning the microscopic mechanism of mechanically induced nanocrystallization has been reached. It has been argued that a temperature rise may play a crucial role in the formation of nanocrystallites. Csontos and Shiflet [22], using analytical electron microscopy, observed radial diffusion fields of Gd and Fe around nanocrystals in amorphous Al₉₀Fe₅Gd₅ bent at room temperature. Based on a comparison with similar diffusion fields for thermal crystallization, they argued that these diffusion fields formed due to a significant temperature increase during mechanical deformation. Indeed, the strain rates for both bending and high-energy ball milling are difficult to control. These may exert highly localized deformation on materials in a very short time. Therefore, it is difficult to rule out a temperature effect on nanocrystallization. Recently, we have combined nanoindentation with TEM to study microstructural evolution of amorphous Al₉₀Fe₅Gd₅ at a low strain rate, minimizing a temperature rise during mechanical deformation. We found that mechanical deformation at or near room temperature led to the precipitation of Al-rich nanocrystallites [16]. One expects the temperature and stress state to affect the microscopic details of deformation, and therefore of nanocrystallization. The small sample thickness, and the fact that uniaxial tensile samples typically undergo catastrophic failure, make it very difficult to obtain shear bands under purely compressive or tensile stresses. When a ribbon is bent, the stress ranges from maximum compressive at one surface to maximum tensile at the other. Chen et al. [11] did not address the location of the nanocrystallites within the ribbon. The aim of the present work is to investigate nanostructural features of shear bands, and the formation of nanocrystallites, induced by bending deformation in amorphous Al₉₀Fe₅Gd₅, in both the tensile and compressive regions.

Shear bands are the main microstructural effect of plastic deformation in amorphous alloys. Usually, they are identified and characterized morphologically, using a scanning electron microscope (SEM). However, less information is available on their nanoscale structure, the details of which are difficult to detect by TEM. Recently, Miller and Gibson [23] developed a method of characterizing the medium-range atomic structure of amorphous solids. This method includes quantitative analysis of electron images and their Fourier amplitudes, essentially combining small-angle scattering and high-resolution imaging from the same microscopic region. This method allowed the identification of nanometer-scale voids in amorphous silica thin films. Following Miller and Gibson, Li et al. [24] studied shear bands in bulk Zr-based metallic glasses and found that they contained a higher concentration of nanometer-scale voids than undeformed regions. We note that the authors did not address the nature of the applied stress. An investigation of the structure of shear bands in Al-based amorphous alloys is especially significant in the present context, as the shear bands are preferential sites for formation of nanocrystallites. Therefore, the present work employs high-resolution transmission electron microscopy (HRTEM) to examine nanodefects at shear bands in both the tensile and compressive regions. The results are used to elucidate the mechanism of mechanically-induced nanocrystallization.