Effect of fictive temperature on tribological properties of Zr44Ti11Cu10Ni10Be25 bulk metallic glasses

Zirconium (Zr) based bulk metallic glasses (BMGs) have emerged as a potential material for various engineering applications due to their unique and tunable mechanical properties. Heat treatment of the BMGs plays a vital role in changing the mechanical as well as tribological properties. In the present work, the effect of the fictive temperature (T-f) on the mechanical and tribological behavior of Zr based BMGs with a glass transition temperature (T-g) of 352 degrees C has been studied by nanoindentation and micro-scratch testing at constant (load 10 N) and progressive (0.5-25 N) loading conditions. The BMGs were heat treated at three different fictive temperatures ranging from 0.91T(g) (320 degrees C) to 1.05T(g) (370 degrees C). Hardness and effective elastic modulus were observed to be highest (8.3 GPa and 128 GPa, respectively) for the metallic glass with a fictive temperature of 0.91T(g) (320 degrees C). With an increase in fictive temperature, hardness and effective elastic modulus decrease due to decreased short-range ordering and more free volume (density fluctuations) in the internal structure, which enhances atomic and plastic flow in the material. For constant loading, wear resistance is observed to be similar for as-cast BMG and BMG with T-f = 1.05T(g), but increases by 65% and 40% from as-cast to BMGs with T-f's of 0.91 T-g and 0.99T(g), respectively. For progressive loading, the wear resistance increases by 10% from as-cast to 0.91T(g), whereas it decreases by 75% and 76% as T-f increases to 0.99T(g) and 1.05T(g), respectively. BMGs with T-f = 0.91T(g) are observed to be the most wear resistant in both the loading conditions, making it a viable candidate for biomedical applications, machine tools, and engine parts where wear resistance of the component during service is a dominating factor.