Formation of (Ti,W)Fe2 C14 Laves Phase in the W-Ti-Fe System and Impact on Mechanical Properties
Tungsten-based BCC superalloys 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 a (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 were annealed at 1400oC. 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 has a composition of 8.8±0.2W, 23.3±0.2Ti and 68.0±0.8Fe (at.%) consistent with (Ti,W)Fe2, which 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±0.41W, 13.94±0.73Ti and 1.94±0.73Fe (at.%). Thermodynamics 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 experimental findings. A revised calculation which reduced the stability of the μ phase at high temperatures led to an improved prediction, consistent with our experimental results. To investigate the mechanical impact of the C14 phase a combination of continuous stiffness measurement nanoindentation and high-speed nanoindentation mapping was used to study the mechanical properties 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 6.68±0.02 GPa was measured at depth of X using continuous stiffness nanoindentation, which is higher than comparable B2 reinforced WTiFe alloys. These findings confirm the ability of the TiFe2 phase to induce hardening in W-Ti-Fe alloys.