Effects of Fe and Al additions on the eutectoid transformation and its transformation products in Ti-5.9(wt.%)Cu

Novel metal feedstock materials, in particular titanium alloys, are urgently needed to meet the requirements of additive manufacturing processes. While substantial progress has been presented using powder-based processes, relatively few efforts have been made using wire feedstock and most literature in this field is on commercial welding wires. Alloys targeted for additive processing often exploit the beneficial effects of solid-state reactions. Available literature on Ti-alloys for AM, thereby, focuses on (a) alloy modifications of the established Ti-6Al-4V alloy and (b) binary variants such as Ti-Cu or Ti-Ni. In the present work, we investigate the effects of ternary additions of the sluggishly transforming element Fe and quaternary additions of Al on the active Ti-5.9(wt.%)Cu eutectoid system, with the objective of establishing an in-depth understanding on the microstructure formation phenomena and their impact on the mechanical properties. The interest in these systems is mainly based on their advantageous solidification behaviour regarding grain refinement and isotropy and microstructural design opportunities created by the eutectoid reaction. Microstructural and chemical analyses are performed using electron microscopy and atom probe tomography. Mechanical properties are assessed using microhardness measurements. Interpretation of the results is aided by use of thermodynamic simulations. The comprehensive analyses presented in this work suggests that the morphologies of the eutectoid transformation products can be modified using ternary elements. While the eutectoid transformation products in Ti-Cu are mostly lamellar resembling pearlite, the addition of Fe favours non-cooperative growth and an incomplete decomposition. Thereby, extremely fine microstructures can be generated that are further refined by additions of Al. The binary Ti-Cu alloy comprises α-phase and Cu-rich intermetallic phase only, whereas the ternary and quaternary alloys comprise α-phase, Cu-rich intermetallic phase, and β-phase. The β-phase is stabilized to room temperature by the addition of Fe. The microhardness of the material conditions investigated is substantially increased through the additions of Fe and Al.