Microstructural stability as well as mechanical characteristics of Superni 625 (Indian equivalent grade of superalloy 625) during its thermal exposure in service are highly decisive to enlighten its performance and endurance. Hence, understanding the influence of prior lattice defects on microstructural phase evolution and/or transformation in a homogenized (at 1200 °C for 40 h) large-grained (>1 mm) Superni 625 alloy during aging treatment and its effects on mechanical properties (such as hardness) is vital prior to design a component for its high-temperature applications. Detailed microstructural characteristics of the homogenized, pre-strained (10 pct), and aged (at 700 °C up to 1000 h) samples were analyzed to understand various precipitation and phase transformation phenomena such as (i) precipitation kinetics for γ″ and δ phase, (ii) phase transformation of primary carbides, and (iii) grain boundary phase(s) evolution. Pre-straining of coarse-grained homogenized alloy resulted in non-uniform accumulation of strain (creating strained and strain-free zones) within the grain leading to variation in precipitation and growth kinetics of γ″ particles in it. It is noted that the average particle density is enhanced, and the average particle size is reduced in the pre-deformed sample. Further, precipitation kinetics of δ phase is boosted only in the presence of stacking faults in the plastically deformed matrix. Stacking fault region facilitates the segregation of solute atoms, mostly Mo and Nb, resulting in localized phase transformation to form an equilibrium δ phase (i.e., Ni3 (Nb, Mo)). Hardness value of the pre-strained samples found to increase dramatically in the initial stage up to 150 h of aging caused by the formation of fine γ″ strengthening phase mostly in the strained regions. However, the strengthening response becomes sluggish with further aging, resulting in the peak hardness of 297HV at 800 h of treatment followed by deteriorating it due to shearing of coarse γ″ particles (>60 nm) through twinning. The sluggish strengthening response beyond 150 h of treatment is ascribed to the coarsening of γ″ particles (beyond critical size, i.e., 60 nm) mainly in the defect-free zones. The phase transformation reactions at various locations in the microstructure of homogenized Superni 625 alloy are established as follows: (i) MC + γ → γ″ (Ni3Nb) + M23C6 at grain interior away from MC carbide particles, (ii) MC + γ → δ (Ni3Nb) + M23C6 at grain boundary, and (iii) MC + γ → γ″ (Ni3Nb) + δ (Ni3Nb) + M23C6 matrix adjacent to grain boundary and MC particles.

Influence of Pre-straining on Phase Transformation Kinetics in a Cast Homogenized Superni 625 During Thermal Exposure

期刊论文
发表日期：

2025年6月30日

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Sonika Chahar,
Suhrit Mula

摘要

Microstructural stability as well as mechanical characteristics of Superni 625 (Indian equivalent grade of superalloy 625) during its thermal exposure in service are highly decisive to enlighten its performance and endurance. Hence, understanding the influence of prior lattice defects on microstructural phase evolution and/or transformation in a homogenized (at 1200 °C for 40 h) large-grained (>1 mm) Superni 625 alloy during aging treatment and its effects on mechanical properties (such as hardness) is vital prior to design a component for its high-temperature applications. Detailed microstructural characteristics of the homogenized, pre-strained (10 pct), and aged (at 700 °C up to 1000 h) samples were analyzed to understand various precipitation and phase transformation phenomena such as (i) precipitation kinetics for γ″ and δ phase, (ii) phase transformation of primary carbides, and (iii) grain boundary phase(s) evolution. Pre-straining of coarse-grained homogenized alloy resulted in non-uniform accumulation of strain (creating strained and strain-free zones) within the grain leading to variation in precipitation and growth kinetics of γ″ particles in it. It is noted that the average particle density is enhanced, and the average particle size is reduced in the pre-deformed sample. Further, precipitation kinetics of δ phase is boosted only in the presence of stacking faults in the plastically deformed matrix. Stacking fault region facilitates the segregation of solute atoms, mostly Mo and Nb, resulting in localized phase transformation to form an equilibrium δ phase (i.e., Ni₃ (Nb, Mo)). Hardness value of the pre-strained samples found to increase dramatically in the initial stage up to 150 h of aging caused by the formation of fine γ″ strengthening phase mostly in the strained regions. However, the strengthening response becomes sluggish with further aging, resulting in the peak hardness of 297HV at 800 h of treatment followed by deteriorating it due to shearing of coarse γ″ particles (>60 nm) through twinning. The sluggish strengthening response beyond 150 h of treatment is ascribed to the coarsening of γ″ particles (beyond critical size, i.e., 60 nm) mainly in the defect-free zones. The phase transformation reactions at various locations in the microstructure of homogenized Superni 625 alloy are established as follows: (i) MC + γ → γ″ (Ni₃Nb) + M₂₃C₆ at grain interior away from MC carbide particles, (ii) MC + γ → δ (Ni₃Nb) + M₂₃C₆ at grain boundary, and (iii) MC + γ → γ″ (Ni₃Nb) + δ (Ni₃Nb) + M₂₃C₆ matrix adjacent to grain boundary and MC particles.