Influence of Mo on microstructure and nanoindentation hardness of Ti-Al-Si-xMo alloy processed by Laser Engineered Net Shaping (LENS)

Abstract

In this work, the microstructure and nanoindentation hardness properties of Ti-Al-Si-xMo alloys produced through laser in-situ alloying using laser engineered net shaping (LENS) technology were investigated. The microstructures and phases present were examined by means of scanning electron microscopy (SEM) equipped with an electron dispersion spectrometer (EDS), while the mechanical properties were studied using a nanoindentation tester. The microstructures exhibited fine lamellar α₂-Ti₃Al/γ-TiAl colonies surrounded with ζ-Ti₅Si₃ and ordered β₀-TiAl phase in the as-produced state; while after heat treatments coarse β₀-phase was observed to be embedded within the lamellae colonies. Microstructural analysis showed that β₀-phase precipitated not only at the α₂/γ lamellae colony boundaries but also inside the lamellae owing to the relatively high content of the β₀-phase present. Nanoindentation testing showed that the indentation hardness of this current alloy is comparable to most TiAl alloys. This study also reveals that Mo additions generally increase hardness values, but only minor effects on hardness are observed at 1400 ^oC heat treatment temperature. Thus, Mo additions for TiAl alloys demonstrate positive effects on mechanical properties when less than 5 at.% of the alloy composition but the mechanical properties would either reduce or remains unchanged with further increase in Mo.