Microstructure Evolution of Bulk Metallic Glasses via Laser Processing

Bulk metallic glasses (BMGs) have attracted tremendous attention as structural materials because of their remarkable mechanical properties. However, the critical cooling rates required to produce an amorphous atomic structure limit the dimensions of as-cast BMG components and therefore restrict their widespread use. Therefore it is not only of fundamental scientific interest but also of great practical importance to investigate the microstructure evolution of glass forming alloy systems during laser processing, which provides the potential opportunity for the production and repair of large-scale amorphous metallic components because of their localized heat input and inherently rapid heating and cooling.;In the present work, we use the Laser Engineering Net Shaping (LENS(TM)) process to deposit pre-alloyed Zr-based Vitreloy 106a (Zr58.5Cu 15.6Ni12.8Al10.3Nb2.8, nominal at. %) powders on both glassy and crystalline substrates with the same nominal composition. In all surveyed laser deposition conditions, amorphous melt zones are observed surrounded by distinct crystalline heat-affected zones (HAZs). The morphology and microstructure of the melt zones and the HAZs were characterized as functions of the LENS(TM) processing parameters. To address this, we investigate the thermal histories of the melt zones and HAZs using a combination of a three-dimensional finite element method (FEM) simulation and in situ thermal imaging measurement. It is demonstrated that amorphous nature in the melt zones is ensured by the extremely fast cooling rates inherent to LENS(TM) process. On the other hand, rapid heating at a rate on the order of 104 K/s up to the temperature right below the melting point of the substrate is not sufficient to avoid solid state phase transformation in the HAZs and results in the formation of polycrystalline spherulites.;Detailed microstructural and compositional examination of the HAZ reveals that the spherulites have a different crystal morphology and structure from the nanocrystalline phases formed following non-isothermal crystallization at a low heating rate using different scanning calorometry (DSC). The FEM simulation results indicate that the critical temperature for spherulitic crystallization exceeds 900 K, ∼150 K higher than that observed during DSC experiments. Consequently rapid heating appears to suppress decomposition and nucleation, resulting in a growth dominant crystallization behavior. This is consistent with the investigation on the thermodynamic and kinetic mechanisms based on the calculations of Avrami exponent and the activation energy for the spherulitic crystallization. In addition, a numerical model was developed to predict the formation of the amorphous melt zones and the crystalline HAZs.;Finally, preliminary investigation was performed to use the LENS(TM) technique as a combinatorial tool for the design and development of glass forming systems, by incorporating the Cu-based metallic glassy powder into the Zr-based amorphous substrate. A new amorphous phase with intermediate composition different from either the alloy powder or the substrate was successfully synthesized by laser deposition.