Mo segregation and distribution in Ti–Mo alloy investigated using nanoindentation

Abstract

A titanium alloy with 12 mass% molybdenum was produced using the cold crucible levitation melting technique. By varying the process parameters, microstructures with and without Mo segregation could be obtained. The effects of Mo segregation and ω-phase precipitation on the mechanical properties of the β-type Ti-12 mass% Mo alloy were investigated through nanoindentation measurements. The homogeneous precipitation of an isothermal ω-phase occurred in the samples without Mo segregation, while denser ω-precipitates were detected in the Mo-lean regions of the heterogeneous microstructure. Microstructural examination of ingots was done by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD). The obtained results revealed the presence of bright and dark stripes in the heterogeneous microstructure; these corresponded to the Mo-rich and Mo-lean regions, respectively. A comprehensive analysis of Mo heterogeneity in the Ti-12Mo alloy suggested that Mo segregation had a significant effect on the mechanical properties as well as the distribution of the ω-phase within the grains. The regions with higher Mo contents (and lower amounts of the ω-precipitates) exhibited the lowest hardness and elastic modulus values. Similarly, the indentation marks made within the β-grains oriented almost along the <001> direction were characterised by lower elastic modulus values and in most cases, were followed by the formation of higher pile-ups (as compared to those corresponding to the indents made within grains with an orientation close to the <111> direction).