Mechanical behavior and tensile/compressive strength asymmetry of ultrafine structured Ti–Nb–Ni–Co–Al alloys with bi-modal grain size distribution

• A new family of high strength bi-modal Ti-based alloys was designed.
• Microstructure consists of coarse β-Ti and ultrafine structured intermetallics.
• Correlation between microstructure and mechanical behavior was studied.
• Tough β-Ti provides high plasticity while intermetallics strengthen the alloys.
• Distribution of brittle intermetallics significantly affects tensile ductility.

Bulk ultrafine structured metallic materials with the bi-modal grain size distribution exhibit both high strength and good ductility. Here we show a new family of bi-modal Ti–Nb–Ni–Co–Al alloys. Their microstructure consists of an ultrafine structured eutectic matrix and relatively coarse β-Ti dendrites. Chemical modification of the parent Ti–Nb–Ni–Cu–Al system significantly affected type and volume fraction of matrix phases. In its turn, this influenced the deformation behavior. The unexpected “double yielding” behavior as well as tensile/compressive fracture strength asymmetry of the designed alloys are discussed in detail. For that, the microstructure alterations under compressive and tensile loading was in situ and ex situ analyzed by scanning electron microscopy.