Dissimilar diffusion bonding of bulk metallic glass: Amorphous/crystalline atomic-scale interaction

Abstract

Dissimilar diffusion welding of a copper-based bulk metallic glass (BMG) with a stoichiometry of Cu 50 Zr 43 Al 7 and commercial pure aluminum was performed at evaluated different temperatures (below and higher than the glass transition temperature ~ 440 °C) and holding times (up to one hour). The main goal was to successfully bond these dissimilar materials without crystallizing the BMG part to avoid deteriorating properties. Optimum joining conditions were attained at a temperature of 430 °C (<T g ) and dwell time of 60 min. The diffusion-assisted atomic-scale interactions between the amorphous and crystalline structures were studied using electron microscopy. To this end, the interaction zone at the BMG/Al dissimilar interface was extracted and analyzed by focused ion beam/transmission electron microscopy (FIB/TEM) followed by elemental analysis and high resolution-transmission electron microscopy (HR-TEM) to reveal the nature of the diffused layer and orientation relationships within the structure. Based on microscopy observations, the formation of an interaction layer with a thickness in the range of 1–2 μm was observed, confirmed by diffusion of oxygen to this region. High-resolution atomic-scale observations supposed a gradient transition from crystallinity toward the amorphous state undergoing the formation of some semi-crystalline nano-scale layers. The activation energy for the thermally-assisted diffusion bonding process was estimated at ~486.7 kJ/mol, related to the substitutional diffusion of oxygen through the copper matrix after decomposition of the stable alumina oxide layer on the surface. Accordingly, an increased indentation hardness across the dissimilar interface was observed and attributed to the distribution of elements into the reaction layer and the formation of new phases. • Atomic-scale diffusion bonding of Cu-based BMG to Al was studied by FIB-TEM analysis. • Elemental inter-diffusion in amorphous-crystalline materials interaction characterized. • Decomposition of oxide surface layer and formation of a semi-crystalline phase at the interface associated bonding. • A gradual atomic structure transition from amorphous to crystallinity was revealed across the interface. • The activation energy of the bonding process was estimated at ~486.7 kJ/mol, nearby to substitutional diffusion of oxygen.