Strain Measurements within Lamellar (α₂+γ) Colonies of a γ-TiAlNb Alloy

It is well established that in γ-titanium aluminides that are alloyed with niobium the orthorhombic O-phase may form. In the present alloy, Ti-42Al-8.5Nb (at.-%), it was observed that at annealing temperatures between 450-680 °C or during slow cooling the hexagonal α₂ phase becomes distorted along three equivalent <11-20> directions and transforms to a disordered O-phase of identical chemical composition. The orientation relationship between the parent α₂ phase and O-phase is (0001) _α2 //(001)_O-phase and [11-20]_α2//[100]_O-phase. During further annealing at 550 °C the initially formed O-phase separates into a two-phase mixture of α₂-phase and niobium rich O-phase with the respective compositions Ti-37Al-6Nb and Ti-36Al-12Nb [1].
High resolution imaging of α₂/O-phase lamellae shows that the crystal lattice that is continuous across the α₂-O-phase boundaries is highly strained. These strains likely alter the mismatch stresses between (α₂/O)-phase lamellae and γ lamellae and thereby affect the deformation behavior of the alloy.
Internal stresses resulting from small differences in lattice parameters can be measured conveniently by high-energy X-ray diffraction (HEXRD), giving average results over a large volume. Our measurements have shown that in Ti-42Al-8.5Nb annealed at 550 °C the lattice parameter of γ is constant after different annealing times. On the other hand, the lattice spacings in the O-phase vary with annealing time. The spacing along the a-direction becomes larger while the b-lattice parameter shrinks. Due to peak overlap in the HEXRD patterns it is not possible to measure the difference between lattice parameters of the O-phase and the α₂ phase.
In the TEM, microstructural characteristics can be traced on a more localized scale. We monitor the relative strains between (α₂/O)-phase lamellae and γ-lamellae in different annealing states of the alloy. The strain measurements show an increase in the mismatch between (α₂/O)-phase lamellae and γ-lamellae with annealing time. The effect of the O-phase on the mechanical properties of the alloy, along with insights into the underlying deformation mechanisms, is being investigated by comparing the stress-strain behavior and dislocation/twinning activities in deformed micropillars.
The alloy of the composition Ti-42Al-8.5Nb (at.-%) has been produced by hot isostatic pressing of pre-alloyed powders as described in literature. Two annealing states are produced: the α₂-annealing state with all of the α₂ phase in hexagonal shape (DO₁₉) and the O-phase-state, where the α₂ phase has transformed to a mixture of α₂ and O-phase. The α₂-annealing-state is produced by heating to 1235°C for 2h followed by furnace cooling and a second heat treatment at 700°C for 72 h followed by water quenching. The O-phase-state is produced by a further heat treatment of 5000h at 550°C followed by furnace cooling. For the strain measurements, TEM foils are prepared electrolytically. The relative strain between (a₂/O)-phase lamellae and γ-lamellae is obtained from precession diffraction using the Nanomegas^© system in a Talos F200i Transmission Electron Microscope. Micro-pillars of lamellar colonies in selected orientations were milled in an FEI Nova 200 dualbeam FIB/SEM microscope and were deformed at room temperature under constant strain rate in a Hysitron TI 980 Nanoindenter. Cross-sections were also excised from the deformed micropillars by focused ion beam milling for observation of dislocations in the (α₂/O)-phase lamellae and γ-lamellae or the interface between both.

[1] H. Gabrisch, M. Janovská, M.W. Rackel, F. Pyczak, A. Stark, Impact of microstructure on elastic properties in the alloy Ti-42Al-8.5Nb, Journal of Alloys and Compounds, 932 (2023) 167578.