Microstructure and Macrohardness of Ti2AlNb-Based Alloy with Different Cooling Modes AITranslate
Abstract AITranslate
With the rapid advancement of aerospace propulsion systems, the demand for high-performance materials capable of enduring extreme thermal and mechanical stresses in aeroengine components has reached unprecedented levels. Ti-22Al-25Nb （atom fraction） alloy, an orthorhombic titanium aluminide （Ti2AlNb-based system）, has emerged as a leading candidate for next-generation high-temperature structural applications due to its exceptional combination of low density, high specific strength, superior creep resistance, and remarkable stability under prolonged thermal exposure. This alloy system primarily consists of three distinct phases：the ordered body-centered cubic （B2） phase, the hexagonal close-packed α2 phase （derived from Ti3Al）, and the orthorhombic （O） phase. However, the intricate interplay between phase transformations and microstructural evolution during thermal processing poses significant challenges in optimizing its mechanical performance. To address this, the present study systematically investigated the phase transformation and microstructure evolution of Ti-22Al-25Nb alloy under varying cooling conditions, with a focus on elucidating the nucleation mechanisms, growth behavior, and crystallographic orientation relationships of O phase with the parent phase. Specimens were subjected to controlled heat treatments at two critical temperatures （1060 and 980 ℃） followed by cooling at rates ranging from rapid quenching （water and oil） to slow furnace cooling. Phase identification and structural characterization were performed using X-ray diffraction （XRD） with Cu-Kα radiation. Microstructural features were analyzed via optical microscope （OM） and scanning electron microscope （SEM）. Crystallographic orientation relationships between the precipitated O phase and the parent phase （B2 matrix or α2 particles） during the cooling process were resolved using electron backscatter diffraction （EBSD）. Macrohardness of the alloys were tested by using a macrohardness tester. The results indicated that the morphology of the precipitated O phase was significantly influenced by the cooling mode. When the alloy was cooled from a higher temperature （1060 ℃）, numerous acicular O phase precipitated formed in B2 matrix, along B2 grain boundaries, and around α2 particles. The specimen cooled at higher rates, such as water quenching and oil quenching, showed retention of the high-temperature B2 phase, in which B2 phase was an ordered cubic phase, due to the window for phase transformation to form new α2 phase was narrow and rapid cooling suppressed elemental diffusion, particularly of Nb elements. Moreover, the high niobium content in Ti-22Al-25Nb alloy decelerated diffusion processes and led to sluggish phase transformations. With the decrease of the cooling rate, such as air cooling and forced air cooling, the flocculent O phases consisting of numerous fine needle-like O plates gradually became the lamellar O phase. During furnace cooling, the grain boundary O phase became continuous and well-defined and the thickness and the length of acicular O phase changed greatly as well. The growth of lamellar O-phase structures developed a typical Widmanstätten structure in B2 grain. The formation of this microstructure was due to the fact that O phase had sufficient time for nucleation and growth at a slow cooling mode. In addition, Widmanstätten O phase precipitated directly from B2 phase maintained specific orientation relationships with B2 matrix, i.e., {001}O//{110}B2 and <110>O//<111>B2. When cooled from a lower temperature （980 ℃）, the increase of the phase boundary due to the presence of α2 particles provided favorable nucleation sites for O phase, and the acicular O-phase initially precipitated aroundα2 particles. The volume fraction and size of α2 particles during water quenching and oil quenching were similar with the particles in as-received Ti-22Al-25Nb alloy slab since the solution temperature was lower than the forging temperature. Compared with rapid cooling following heated at 1060 ℃, B2 grain size was smaller at 980 ℃ due to the combination of low heating temperature and pinning effect of α2 particle. During air cooling and forced air cooling, the acicular O lamellae were precipitated from B2 matrix, B2 grain boundaries and around α2 particles. Similar to 1060 ℃ cooled samples, the fine needle-like O lamellae precipitated into flocculent clusters. As the cooling rate further decreased （furnace cooling）, the volume fraction of the floccular O phase precipitated in B2 grains and around α2 particles greatly increased. Meanwhile, α2 particles appeared to be decomposed and the precipitated rim-O phaseappeared in the periphery of α2 particles. The rim O phase formed through a decomposition reaction of α2→α2 （Nb-lean） + O （Nb-rich） was controlled by a diffusional mechanism and maintain specific orientation relationship, i.e., {001}O//{0001}α2 and <110>O//<110>α2, with the parent α2 particles. Rapid cooling （water-quenching and oil-quenching） led to the lowest macrohardness values for both 1060 and 980 ℃ cooling conditions, attributed to the low strength of B2 phase. Aircooling and forcedair cooling enhanced alloy macrohardness due to extensive precipitation of acicular O-phase. However, partial dissolution of α2 particles at higher temperatures （1060 ℃） reduced nucleation sites for O phase, resulting in lower macrohardness compared to 980 ℃ cooled samples. Furnace cooling drastically reduced macrohardness owing to the increased volume fraction and coarsening of lamellar O phase. In summary, macrohardness under different cooling modes primarily correlated with Ophase precipitation strengthening, and the alloy macrohardness reached a maximum value under aircooling.
KeyWords AITranslate
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Basic Information:
DOI：10.13373/j.cnki.cjrm.XY24120006
Chinese Library Classification Number:TK91
Citation Information:
With the rapid advancement of aerospace propulsion systems, the demand for high-performance materials capable of enduring extreme thermal and mechanical stresses in aeroengine components has reached unprecedented levels. Ti-22Al-25Nb （atom fraction） alloy, an orthorhombic titanium aluminide （Ti2AlNb-based system）, has emerged as a leading candidate for next-generation high-temperature structural applications due to its exceptional combination of low density, high specific strength, superior creep resistance, and remarkable stability under prolonged thermal exposure. This alloy system primarily consists of three distinct phases：the ordered body-centered cubic （B2） phase, the hexagonal close-packed α2 phase （derived from Ti3Al）, and the orthorhombic （O） phase. However, the intricate interplay between phase transformations and microstructural evolution during thermal processing poses significant challenges in optimizing its mechanical performance. To address this, the present study systematically investigated the phase transformation and microstructure evolution of Ti-22Al-25Nb alloy under varying cooling conditions, with a focus on elucidating the nucleation mechanisms, growth behavior, and crystallographic orientation relationships of O phase with the parent phase. Specimens were subjected to controlled heat treatments at two critical temperatures （1060 and 980 ℃） followed by cooling at rates ranging from rapid quenching （water and oil） to slow furnace cooling. Phase identification and structural characterization were performed using X-ray diffraction （XRD） with Cu-Kα radiation. Microstructural features were analyzed via optical microscope （OM） and scanning electron microscope （SEM）. Crystallographic orientation relationships between the precipitated O phase and the parent phase （B2 matrix or α2 particles） during the cooling process were resolved using electron backscatter diffraction （EBSD）. Macrohardness of the alloys were tested by using a macrohardness tester. The results indicated that the morphology of the precipitated O phase was significantly influenced by the cooling mode. When the alloy was cooled from a higher temperature （1060 ℃）, numerous acicular O phase precipitated formed in B2 matrix, along B2 grain boundaries, and around α2 particles. The specimen cooled at higher rates, such as water quenching and oil quenching, showed retention of the high-temperature B2 phase, in which B2 phase was an ordered cubic phase, due to the window for phase transformation to form new α2 phase was narrow and rapid cooling suppressed elemental diffusion, particularly of Nb elements. Moreover, the high niobium content in Ti-22Al-25Nb alloy decelerated diffusion processes and led to sluggish phase transformations. With the decrease of the cooling rate, such as air cooling and forced air cooling, the flocculent O phases consisting of numerous fine needle-like O plates gradually became the lamellar O phase. During furnace cooling, the grain boundary O phase became continuous and well-defined and the thickness and the length of acicular O phase changed greatly as well. The growth of lamellar O-phase structures developed a typical Widmanstätten structure in B2 grain. The formation of this microstructure was due to the fact that O phase had sufficient time for nucleation and growth at a slow cooling mode. In addition, Widmanstätten O phase precipitated directly from B2 phase maintained specific orientation relationships with B2 matrix, i.e., {001}O//{110}B2 and <110>O//<111>B2. When cooled from a lower temperature （980 ℃）, the increase of the phase boundary due to the presence of α2 particles provided favorable nucleation sites for O phase, and the acicular O-phase initially precipitated aroundα2 particles. The volume fraction and size of α2 particles during water quenching and oil quenching were similar with the particles in as-received Ti-22Al-25Nb alloy slab since the solution temperature was lower than the forging temperature. Compared with rapid cooling following heated at 1060 ℃, B2 grain size was smaller at 980 ℃ due to the combination of low heating temperature and pinning effect of α2 particle. During air cooling and forced air cooling, the acicular O lamellae were precipitated from B2 matrix, B2 grain boundaries and around α2 particles. Similar to 1060 ℃ cooled samples, the fine needle-like O lamellae precipitated into flocculent clusters. As the cooling rate further decreased （furnace cooling）, the volume fraction of the floccular O phase precipitated in B2 grains and around α2 particles greatly increased. Meanwhile, α2 particles appeared to be decomposed and the precipitated rim-O phaseappeared in the periphery of α2 particles. The rim O phase formed through a decomposition reaction of α2→α2 （Nb-lean） + O （Nb-rich） was controlled by a diffusional mechanism and maintain specific orientation relationship, i.e., {001}O//{0001}α2 and <110>O//<110>α2, with the parent α2 particles. Rapid cooling （water-quenching and oil-quenching） led to the lowest macrohardness values for both 1060 and 980 ℃ cooling conditions, attributed to the low strength of B2 phase. Aircooling and forcedair cooling enhanced alloy macrohardness due to extensive precipitation of acicular O-phase. However, partial dissolution of α2 particles at higher temperatures （1060 ℃） reduced nucleation sites for O phase, resulting in lower macrohardness compared to 980 ℃ cooled samples. Furnace cooling drastically reduced macrohardness owing to the increased volume fraction and coarsening of lamellar O phase. In summary, macrohardness under different cooling modes primarily correlated with Ophase precipitation strengthening, and the alloy macrohardness reached a maximum value under aircooling.
quote
| GB/T 7714-2015 | [1] Dong Li, Yi Dai, Yingjie Xie, et al. Microstructure and Macrohardness of Ti2AlNb-Based Alloy with Different Cooling Modes[J]. Chinese Journal of Rare Metals, 2026, 50(8): 1385-1392. DOI:10.13373/j.cnki.cjrm.XY24120006. |
| MLA | [1] Dong Li, et al., "Microstructure and Macrohardness of Ti2AlNb-Based Alloy with Different Cooling Modes." Chinese Journal of Rare Metals, vol. 50, no. 8, 2026, pp. 1385-1392, https://doi.org/10.13373/j.cnki.cjrm.XY24120006. |
| APA | [1] Dong Li, Yi Dai, Yingjie Xie, Wenjie Fu, Peng Li, Zhimin Hou, & Xiong Ma. (2026). Microstructure and Macrohardness of Ti2AlNb-Based Alloy with Different Cooling Modes. Chinese Journal of Rare Metals, 50(8), 1385-1392. https://doi.org/10.13373/j.cnki.cjrm.XY24120006 |
| IEEE | [1] Dong Li, Yi Dai, Yingjie Xie, Wenjie Fu, Peng Li, Zhimin Hou, and Xiong Ma, "Microstructure and Macrohardness of Ti2AlNb-Based Alloy with Different Cooling Modes," Chinese Journal of Rare Metals, vol. 50, no. 8, pp. 1385-1392, 2026, doi: 10.13373/j.cnki.cjrm.XY24120006. keywords: {Ti-22Al-25Nb alloy;cooling modes;microstructure evolution;orientation relationship;macrohardness} |