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

Abstract

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‾]γT +1/9 [0 1‾1]γM. The adjacent atomic planes of the γ matrix and secondary γ twin can transformation to β0 through shear along 1/9 [0 1‾1]γM and 1/4 [11 1‾]γT, 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.