Formation of (Ti,W)Fe₂ C14 laves phase in the W-Ti-Fe system and the impact on mechanical properties

Tungsten-based alloys utilising coherent intermetallic strengthening offer a route to improved high temperature strength. Recently proposed W-Ti-Fe alloys employ B2 TiFe phase in an A2 BCC matrix to achieve this strengthening, however, other phases such as the TiFe₂ Laves phase can form at higher temperatures. In this study, the formation of the (Ti,W)Fe₂ C14 phase in a BCC W-rich matrix is observed in a W–16Ti–4Fe (at%) alloy produced by vacuum arc-melting and annealing at 1400 °C. There was no evidence for the formation of either the TiFe B2 or W₆Fe₇ μ phase from analysis of the TEM diffraction patterns. The C14 Laves phase was found to have a composition of 8.8 ± 0.2 W, 23.3 ± 0.2Ti and 68.0 ± 0.8Fe (at%) consistent with (Ti,W)Fe₂. This formed a continuous region along the boundaries between the W-rich BCC prior-dendrites, as well as in smaller isolated precipitates. The prior dendritic regions consisted of W-rich BCC phase with a composition of 84.10 ± 0.41 W, 13.94 ± 0.73Ti and 1.94 ± 0.73Fe (at%). Thermodynamic calculations using CALPHAD predicted the formation of a W rich BCC phase and a Ti and Fe rich liquid phase initially, which was not consistent with the presented experimental findings. A revised calculation which reduced the stability of the μ phase at high temperatures led to an improved prediction, consistent with the experimental results. To investigate the mechanical impact of the C14 phase, a combination of Continuous Stiffness Measurement (CSM) nanoindentation and high-speed nanoindentation mapping was used to measure the comparative hardness of the matrix and precipitate phases, which showed that the C14 phase exhibits a very high hardness relative to the W-rich matrix phase. An average nanohardness of 6.25 ± 0.03 GPa was measured at depth of 1.5 μm using CSM nanoindentation, which is higher than comparable B2 reinforced WTiFe alloys.