In Situ Rheological Characterization During Thermoplastic Joining of Novel Zr-based Bulk Metallic Glasses

Bulk Metallic Glasses (BMGs) are a captivating class of advanced materials that exhibit exceptional mechanical properties, such as high strength, large elastic limits, and excellent wear resistance, coupled with superior corrosion resistance. Unlike traditional crystalline alloys, BMGs lack long-range atomic order, and are characterized by an amorphous atomic structure. However, their broader engineering applications are restricted, largely due to limitations in the production process. The fabrication of these materials necessitates a specific chemical composition and high cooling rates. Rapid heat dissipation kinetically suppresses the nucleation and growth of crystalline phases. As a result, a metastable amorphous structure is formed. However, these production requirements lead to limitations in the critical sizes of as-cast objects that can be manufactured.<br/>A practical approach to obtain larger, ready-to-use objects out of BMGs and overcome the casting size limitation is to join the as-cast intermediates. One of the ways of joining is to utilize the distinguishing characteristics of amorphous BMGs – the presence of the glass transition temperature (T_g). This temperature signifies the shift from a supercooled liquid state to a glassy structure upon cooling, with reversibility observed upon heating. Therefore, in controlled heating, BMGs feature a supercooled liquid region (SCLR) below the actual melting point, specifically between T_g and the initial crystallization temperature (T_x). Within the SCLR, homogeneous deformation of material via viscous flow is possible, allowing for thermoplastic processing of BMGs. Thermoplastic joining method of BMGs in the SCLR targets the attainment of metallic bonding during superplastic flow and large deformation at the interface between the elements. However, this method has only been applied and verified for a small group of alloys.<br/>The success of the joining process is intrinsically linked to the rheology of the flow. The viscosity directly impacts the BMG's ability to flow, break up surface oxide layers and achieve bonding as well as determines the timescale of the process. Therefore, investigating and controlling viscosity is crucial for achieving strong, defect-free joints.<br/>Here we demonstrate the thermoplastic joining of novel highly stable BMGs from the ZrCuAgAlBe group with continuous viscosity measurement. Studied BMGs were manufactured by arc melting and conventional suction casting into 5 mm diameter copper molds. Two cylinders, each 4 mm in height, were used for joining by uniaxial compression at different temperatures within the SCLR under constant force. Microstructural and nanomechanical characterizations of the joining site cross-section revealed no distinctive bonding line for the compression strains above 50 % indicating proper bonding. In-process displacement measurement allowed for the estimation of viscosity over the entire procedure, which was found to be in the range 10⁹–10⁶ Pas. Continuous measurement provides valuable insights into the evolution of the process.<br/>Precise control over viscosity is essential for optimizing thermoplastic joining processes for BMGs. The viscosity should be low enough for sufficient flow and bonding at the interface, but high enough to prevent excessive deformation or crystallization. A comprehensive understanding of thermoplastic joining considering viscosity is then crucial for further, wider applications of this method and consequently the increasing industrial adoption of BMGs.<br/><br/><b>Acknowledgements:</b><br/><i>The work was supported by the project Minigrants for doctoral students of the Wroclaw University of Science and Technology</i>