Twins and β₀ precipitation effects on the lamellar degradation in TNM alloy during creep

In this paper, the microstructure instability of the TNM alloy during creep at 800 °C under 150 MPa and 250 MPa was investigated. The results indicate that, both β₀ precipitation (inside lamellae) and primary γ twins formation are initiated during the second stage of creep. Compared to low creep stress conditions (150 MPa), high creep stress conditions (250 MPa) accelerate the initiation of the third stage of creep, inducing the precipitation of ω₀ phase and secondary twins. The nanoscale ω₀ particles formed inside β₀ or at the interface of the β₀/β₀. The intersection of secondary γ twins and α₂ lamellae leads to stress concentration and dislocation accumulation, providing nucleation points for β₀ phase and promoting the precipitation of β₀ within α₂ lamellae. The precipitation of the secondary γ twin introduces a unit dislocation of 1/3 [111]γ in the (111) atomic planes, promoting the movement of adjacent atomic planes within the (111) atomic planes. The 1/3 [111]γ dislocations will dissociate into two partial dislocations via 1/3 [111]γ→1/4 [11 1‾]γ_T +1/9 [0 1‾1]γ_M. The adjacent atomic planes of the γ matrix and secondary γ twin can transformation to β₀ through shear along 1/9 [0 1‾1]γ_M and 1/4 [11 1‾]γ_T, respectively. The size and content of β₀ phase significantly increase in the third stage of high creep stress, accelerating the decomposition and fragmentation of the α₂ lamellae, leading to rapid creep failure.