Ti-base bulk nanostructure-dendrite composites: Microstructure and deformation

Bulk nanostructured/glassy Ti-base multicomponent alloys are attractive advanced high-strength materials for structural applications or as functional materials, e.g. for medical applications. Typically, the strength and elastic strain limits of such alloys are significantly higher than for conventional microcrystalline alloys. It is of strong interest to develop a one-step process that can directly produce such alloys in bulk form, e.g. through casting, with an in situ formed composite microstructure that combines phases with different desirable properties. Along this line, a series of (Ti–Zr–Ta)–Cu–Ni alloys designed from glass-forming alloy compositions with different volume fractions of β-Ti phase have been investigated. The introduction of the β-Ti dendritic solid solution phase enhances the ductility of the material. A further improvement can be achieved by adding isomorphous β-phase stabilizers in (Ti–Nb–Ta)–Si/In alloys. These alloys exhibit low elastic modulus (50 GPa) and very large deformability (>100% plastic strain) at room temperature without appearance of any microcracks. The microstructure of these alloys consists of a high volume fraction of bcc β-Ti phase coexisting with a nanostructured interdendritic matrix. The superplastic-like deformability of the near beta alloys is correlated with copious twinning in the micrometer-size β-Ti-dendrites leading to a nano/ultrafine filamentary structure.