New insights into high-temperature deformation and phase transformation mechanisms of lamellar structures in high Nb-containing TiAl alloys

Characteristic effects of alloying elements on β solidifying titanium aluminides: A review

“…It was realized that the microstructures comprise of γ+ω 0 with minor precipitates of α 2 -phase after ageing treatment followed by quenching from high temperature. A unique mechanism of twinning by the α 2 -phase was observed after ageing that seems analogous to the described mechanism of phase transformation diffusive-displacive in deformed TiAl alloys [ 40 , 109 ]. This could provide a preferred site for ω 0 -phase initiation due to the twinned interface atomic structure being relative to the crystallographic arrangement of ω 0 -phase.…”

“…In TiAl-based alloys with high Nb content, the emergence of β 0 -phase leads to reduced RT ductility and the formation of ω-phase from the parent β 0 -phases. The ω”, ω 0 and ω (D8 8 structure) phases occur at separate stages during the β 0 →ω 0 or β 0 →D8 8 -ω transformations [ 25 , 40 , 97 ]. Depending on the cooling methods adopted, the ω-related phases vary in morphology, size, and structure around the β 0 -phases [ 24 ].…”

“…However, this causes increased hardness and leads to the formation of ω 0 -phase as an additional phase [ 19 , 24 , 99 ]. Typically, this ω 0 -phase precipitates in the β-matrix of β-solidifying TiAl-based alloys, but not the same for alloys where α 2 -phase precipitates [ 23 , 25 , 40 , 99 ]. It is understood that the B2-phase is a thermodynamically metastable phase susceptible to decompose into ω-related phases of trigonal-ω՛՛, D88-ω and B82-ω (ω 0 ) [ 25 , 61 ].…”

“…The occurrence of ω 0 (ordered ω) phase at intermediate temperatures in TiAl-based alloys is widely reported [ 40 , 61 , 109 ]. Song et al.…”

“…Jian et al [39], reported that the α 2 -phase shows strong metallic bond with low mechanical anisotropy; while γ-phase has a covalent bond with strong universal mechanical anisotropy. It is commonly known that to maintain the ordered D0 19 structure, a large interchange shuffling is required [32,40]. The appearance of α 2 -phase deformation twinning could play a key role in high Nb-containing TNM alloys' mechanical properties enhancement.…”