Irreversible Changes in Mechanical Properties of Nickel-based Alloys After Long-term High-temperature Service
After long-term service under high-temperature conditions, nickel-based alloys undergo a series of irreversible microstructural evolutions, which directly lead to the degradation of their mechanical properties. The main irreversible changes in mechanical properties are as follows:
1.Reduction in tensile strength and yield strength
During long-term high-temperature exposure, the strengthening phases (such as γ' phase, γ'' phase) in nickel-based alloys will experience processes like coarsening, aggregation, and even dissolution. These phases are the core components that maintain the high strength of the alloy. Their morphological and structural changes destroy the original dislocation pinning effect, resulting in a significant and irreversible decrease in the tensile strength and yield strength of the alloy at both room temperature and service temperature.
2.Deterioration of ductility and toughness
High-temperature long-term service will induce the precipitation of brittle phases (e.g., σ phase, Laves phase) at the grain boundaries of nickel-based alloys. Meanwhile, grain growth will occur due to the thermal activation effect, which reduces the number of grain boundaries that can hinder crack propagation. These factors together cause the alloy's elongation and reduction of area to decrease obviously, and the impact toughness will also be irreversibly degraded, making the alloy prone to brittle fracture under load.
3.Degradation of creep and fatigue resistance
Creep resistance is one of the key properties of nickel-based alloys for high-temperature applications. Long-term high-temperature stress will lead to the accumulation of creep damage, including dislocation climb, void formation and growth at grain boundaries, and intergranular cracking. These damages are irreversible. At the same time, high-temperature fatigue loading will accelerate the initiation and propagation of cracks. The combined effect of creep and fatigue makes the alloy's creep rupture life and fatigue life shorten sharply, and this performance degradation cannot be recovered by conventional heat treatment.
4.Loss of hardness
The decrease in the number of fine and dispersed strengthening phases and the softening of the matrix caused by high-temperature diffusion will lead to an irreversible reduction in the hardness of nickel-based alloys. Hardness is closely related to the wear resistance and contact fatigue resistance of the alloy, so the loss of hardness will further affect the comprehensive service performance of the alloy.
All the above mechanical property changes are driven by irreversible microstructural transformations, and cannot be restored to the original state through post-treatment processes such as reheating or reaging.
