Key Takeaways & Executive Findings
- •• • At 560 °C, incomplete discontinuous precipitation (DP) restrains DRX, yielding necklace-like microstructures; above 640 °C, complete DP with fully lamellar structures promotes DRX, producing ultrafine-grained (UFG) microstructures. This threshold dictates whether hot deformation yields desirable UFG or heterogeneous necklace structures, directly impacting mechanical property uniformity. • • Increasing pre-ageing temperature reduces critical strain and peak strain for DRX initiation and increases DRX volume fraction. This acceleration of DRX lowers the deformation resistance and energy consumption during hot forming, enhancing industrial process efficiency. • • Pre-precipitated α-Cr lamellae undergo dissolution fragmentation and spheroidization into finer particles, stimulating DRX via continuous DRX (CDRX); coarse α-Cr particles enhance DRX through particle-stimulated nucleation (PSN) and discontinuous DRX (DDRX). The interplay of these mechanisms governs final grain size and texture, affecting component performance in aerospace and nuclear applications. • • For UFG microstructures, pre-ageing should precipitate fully lamellar structures. This processing recommendation enables targeted microstructure engineering, potentially improving high-temperature strength and corrosion resistance of Ni−38Cr−3.8Al alloy components.
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Abstract
The influence of pre-ageing temperature on dynamic recrystallization (DRX) and microstructure evolution during hot compression of Ni−38Cr−3.8Al alloy was investigated. Five samples with varying pre-precipitation states were fabricated. Pre-ageing treatment precipitates α-Cr phases in lamellar and particle forms. During subsequent hot deformation, pre-precipitated α-Cr lamellae undergo dissolution fragmentation and spheroidization, transforming into finer particles due to elevated temperature and high-density dislocations. At 560 °C, an incomplete discontinuous precipitation (DP) state restrains DRX, producing necklace-like microstructures. Above 640 °C, a complete DP state with fully lamellar structures promotes DRX, yielding ultrafine-grained (UFG) microstructures. Coarse α-Cr particles enhance DRX through particle-stimulated nucleation (PSN) and discontinuous DRX mechanisms, while dissolved α-Cr lamellae promote DRX via continuous DRX. DRX kinetics analysis indicates that increasing pre-ageing temperature accelerates DRX, evidenced by reduced critical strain and peak strain, and increased DRX volume fraction. To achieve UFG microstructures during hot deformation, fully lamellar structures should be precipitated during pre-ageing. These findings provide a processing pathway for tailoring microstructures in high-Cr nickel-based alloys.
1. Introduction
High-chromium nickel-based alloys are critical for aerospace, nuclear, and chemical industries due to their exceptional oxidation and corrosion resistance at elevated temperatures. However, alloys containing over 35 wt.% Cr suffer from Cr supersaturation, leading to inevitable precipitation of α-Cr phase (body-centered cubic) from the γ matrix. This two-phase microstructure significantly influences workability and final performance. The morphology and distribution of α-Cr precipitates—lamellar, acicular, or spherical—depend on Cr content, ageing temperature, and thermo-mechanical processing. These precipitates critically affect hot deformation behavior and microstructure evolution, yet the mechanisms by which pre-ageing states adjust dynamic recrystallization (DRX) and ultrafine grain formation remain insufficiently understood, hindering optimized processing routes.
Existing commercial approaches for Ni−38Cr−3.8Al alloy hot forming often yield heterogeneous microstructures or incomplete recrystallization, compromising mechanical properties. The lack of precise control over α-Cr precipitation during pre-ageing leads to inconsistent DRX kinetics and grain sizes. This study addresses the bottleneck by systematically varying pre-ageing temperature to tailor α-Cr precipitate morphology and distribution. Through hot compression tests, the work elucidates how pre-precipitated lamellae and particles influence DRX mechanisms—PSN, DDRX, and CDRX—and establishes quantitative relationships between pre-ageing temperature, DRX critical strain, peak strain, and volume fraction. The findings provide a processing window to achieve ultrafine-grained microstructures, offering a pathway to enhance alloy performance and manufacturing reliability.
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ZHANG Yu-qing, QUAN Guo-zheng, YU Yan-ze, LIU Ying-ying, XIONG Wei, DAI Wei-wei, JIANG Qian (2026). Adjusting mechanisms for ultrafine-grained microstructures during hot deformation of Ni−38Cr−3.8Al alloy via pre-ageing precipitation. Transactions of Nonferrous Metals Society of China (中国有色金属学报). https://doi.org/10.1016/S1003-6326(26)67062-8
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Frequently Asked Questions
What is the critical pre-ageing temperature threshold that determines whether DRX is inhibited or promoted during hot deformation?
At 560 °C, incomplete discontinuous precipitation (DP) restrains DRX, resulting in necklace-like microstructures. As temperature exceeds 640 °C, complete DP with fully lamellar structures promotes DRX, yielding ultrafine-grained microstructures. This threshold is critical for designing pre-ageing treatments to achieve desired grain structures.
How do pre-precipitated α-Cr lamellae and particles differently affect DRX mechanisms?
Pre-precipitated α-Cr lamellae undergo dissolution fragmentation and spheroidization into finer particles, stimulating DRX via continuous DRX (CDRX). Coarse α-Cr particles enhance DRX through particle-stimulated nucleation (PSN) and discontinuous DRX (DDRX). The balance between these mechanisms influences final grain size and homogeneity.
What are the quantitative effects of pre-ageing temperature on DRX kinetics?
Increasing pre-ageing temperature accelerates DRX, as demonstrated by reduced critical strain and peak strain, and increased DRX volume fraction. These metrics indicate that higher pre-ageing temperatures lower the deformation energy required for recrystallization, improving hot workability.
What processing recommendation is given to achieve ultrafine-grained microstructures during hot deformation?
To achieve ultrafine-grained microstructures, it is recommended to precipitate fully lamellar structures during pre-ageing treatment. This ensures complete DP state above 640 °C, which promotes DRX and results in finer grains.
What are the industrial implications of these findings for Ni−38Cr−3.8Al alloy components?
The findings enable tailored pre-ageing treatments to control DRX and grain size, potentially enhancing mechanical properties and corrosion resistance. This can lead to more reliable hot forming processes and improved performance in aerospace, nuclear, and chemical applications where high-temperature strength and durability are critical.
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