Effect of Fe and Cr on a eutectoid Ti-6.4(wt.%)Ni alloy developed for wire-arc additive manufacturing

Abstract

The development of novel metallic alloys tailored for additive manufacturing (AM) has been a research focus of recent years. In case of wire feedstock relatively little efforts have been presented, due to the complex feedstock preparation. The presented research addresses this need with particular focus on the requirements of space applications.

Alloys targeted for AM oftentimes exploit the beneficial effects of solid-state reactions as these enable advanced microstructural design. 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 slow transforming elements Fe and Cr on the active Ti-6.4(wt.%)Ni eutectoid system, with the objective of establishing sound process-structure-property relationships. This work focuses next to the optimization of processing behavior, on the tailored adjustment of intermetallic phases through heat treatment aiming at increasing the specific Young’s modulus to enable structural light-weighting. Additionally, the prevalence of high process-intrinsic cooling rates contributes to the avoidance of β-fleck formation, thereby, providing new alloy design opportunities. Microstructural analysis is performed using scanning electron microscopy and X-ray diffraction, facilitated using thermal analyses such as dilatometry and differential scanning calorimetry as well as thermodynamic calculations. Mechanical properties are assessed using microhardness measurements and compression testing.

The comprehensive analyses presented in this work suggest that the morphologies of the eutectoid transformation products are strongly affected by the addition of β-stabilizing elements. An incomplete decomposition into a non-lamellar structure is observed. These microstructures consist of Ti2Ni phases embedded in varying amounts of α- and β-phases with different morphologies. The obtained results are utilized to correlate microstructural features and the respective alloy compositions to the observed mechanical behaviors and contribute to the understanding of the microstructural evolution in advanced Ti alloys dedicated for wire-arc additive manufacturing.