Spherulitic growth process in Ti-based metallic glass: Microstructure, phase identification, and growth mechanism

Ti40Cu36Zr10Pd14 metallic glass is often considered a promising candidate for biomedical applications, in particular for the manufacturing of small size dental implants. Nevertheless, its processing and limited glass-forming ability leads to the appearance of crystalline spheres called "spherulites". These spherulites play a major role in the deformation behaviour of this alloy. It is therefore a critical issue for its future industrial use to identify their nature and better understand their growth process to enable control of their size distribution and localisation. This article presents a detailed description of the microstructure of these crystalline defects, from the sub millimetre to nanometric scales, using transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS)/electron backscatter diffraction (EBSD) coupled with scanning electron microscopy and X-ray diffraction characterisation. Several phases are identified thanks to the combination of EDS/EBSD and TEM information, in particular a CuTi B2 phase, analogous to the ZrCu B2 phase usually present in other composite metallic glasses. The presence of other phases such as Ti2Cu3 and CuTi3 phases is also detected. The spherulites are studied at different times during their growth process, and a growth scenario is proposed based on the characterisation observations. However, the nucleation step of the spherulites remains unclear despite the in-depth investigation. Thus, different hypotheses are discussed.