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Ultra-high strength maraging stainless steels possess many important applications such as in aircraft landing gears owing to their excellent strength and good process ability. However, traditional ultra-high strength maraging stainless steels are facing the challenge of balancing strength and ductility while pursuing ultra-high strength. This is mainly due to the semi-coherent or non-coherent relationship between the precipitated nanoparticles and the body-centered cubic (BCC) martensitic matrix. In this work, a novel ultra-high strength maraging stainless steel (Fe-7.95Cr-13.47Ni-3.10Al-1.83Mo-0.03C-0.23Nb, weight percent, %) is designed using a cluster formula approach. Alloy ingots are prepared by vacuum induction melting under an argon atmosphere, followed by hot rolling at 950℃ and multiple passes of cold rolling. Finally, the alloy is aged at 500℃ for 288 h. Microstructural characterizations of the alloy in different aging states are performed using electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). As a result, the martensitic structure of the alloy is fragmented and elongated, with high-density dislocations (~ 1.8 × 10–3 nm–2) and a large number of coherent B2-NiAl nanoparticles (< 5 nm) observed in the BCC martensitic matrix after cold rolling and aging. In terms of mechanical properties, the alloy exhibits significant age-hardening, with a peak-aged hardness of 651 HV after ageing treatment. It also demonstrates an extraordinarily high yield strength (σYS = 2.3 GPa) and a decent elongation (El = 3.6%), indicating a well-balanced strength-ductility property. Finally, the origins of the ultra-high strength in the novel alloy are discussed in depth, showing that the ultra-high strength of this stainless steel comes from the strengthening effect of different microstructures. This study provides valuable guidance for designing high-performance ultra-high strength maraging stainless steels.
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Keywords:
- maraging stainless steel /
- ultra-high strength /
- coherent precipitation /
- strengthening mechanisms
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图 1 由Pandat软件计算得到的Fe-7.95Cr-13.47Ni-3.10Al-1.83Mo-0.23Nb-0.03C合金的平衡相图
Figure 1. Equilibrium phase diagram of the designed Fe-7.95Cr-13.47Ni-3.10Al-1.83Mo-0.23Nb-0.03C alloy calculated by Pandat software.
图 2 合金冷轧态(CR)的EBSD反极图(IPF)和相分布图(a), (b); 冷轧态合金TEM明场像及其对应的SAED花样(c); HRTEM图像及其对应的FTT图像(d)
Figure 2. EBSD IPF image and phase image of the designed alloy after cold-rolling (CR) treatment (a), (b); TEM bright-field (BF) image and corresponding SAED patterns (c) and High-resolution TEM image and its FTT pattern (d) of CR.
图 3 12 h时效后CRA合金的微观结构合金冷轧+时效态(CRA)的EBSD反极图(IPF)和相分布图(a), (b); CRA合金TEM明场、暗场像及其对应的SAED花样(c)(d), CRA合金HRTEM图像及其对应的FTT图像(e, e-1, e-2)以及衍射环图像(f)
Figure 3. Microstructures of 12 h-aged CRA alloys: EBSD IPF image and phase image of the CRA alloy (a), (b), TEM bright-field (BF) image and corresponding SAED patterns (c), TEM DF image (d), HRTEM image and its FTT patterns (e), (e-1), (e-2) and diffraction ring (f) of the CRA alloy.
图 4 冷轧态合金在时效48 h后的TEM明场(a)、暗场像(b)
Figure 4. TEM bright-field image (a), the corresponding dark-field image (b) of 48 h-aged CRA alloy.
图 5 合金经热轧(HR)处理(a)和热轧+500 ℃/12 h时效(HRA)处理(c)后的EBSD反极图(IPF), HR(b)和HRA(d)处理后的EBSD相图
Figure 5. EBSD inverse pole figures (IPFs) of the designed alloy after hot-rolling (HR) treatment (a) and hot-rolling + aging at 500 ℃/12 h (HRA) treatment (c), EBSD phase images of HR (b) and HRA (d).
图 6 12 h时效HRA合金(a)和12 h时效冷轧+时效态(CRA)合金(c)的晶界分布图, HRA(b)和CRA(d)的核平均取向差(KAM)
Figure 6. Grain boundary distribution maps of 12 h-aged HRA alloys (a) and 12 h-aged CRA alloys (c), kernel average misorientation (KAM) of HRA (b) and CRA (d).
图 7 CRA合金在500 ℃时效时显微硬度随时效时间的变化(a), 不同热处理状态下合金在室温拉伸下的工程应力-应变曲线(b)和室温三点弯曲下的载荷-位移曲线(c)
Figure 7. Variation tendency of microhardness of the CRA alloy with aging time at 500℃ (a), room-temperature engineering stress-strain tensile curves of the current alloy at different heat-treated (b), and 3-points bending test of the current alloy at different heat-treated (c).
图 8 根据不同强化机制计算得到的CR合金以及时效12 h CRA合金的屈服强度
Figure 8. Calculated strength increments from different strengthening mechanisms in CR and 12 h-aged CRA alloys, in which the measured yield strength values are also presented with black star symbols for comparison.
表 1 不同热处理状态下设计合金以及18 Ni(300)[45]的H, σYS, σUTS, El, 极限弯曲载荷(FB), 及极限弯曲角度(θB)
Table 1. Mechanical properties of the current stainless steel at different heat-treated states and the 18 Ni(350) steel[45], including microhardness (H), yield strength (σYS), ultimate tensile strength (σUTS), elongation to fracture (El), ultimate bending force (FB), and ultimate bending angle (θB).
States H/HV σYS/MPa σUTS/MPa El/% FB/kN θB/(°) HR 326±10 787 1100 9.3 8.5 35.5 CR 404±5 1408 1432 4.4 9.7 14.9 HRA: 5 h-aged 573±6 1704 1940 3.9 10.1 9.7 CRA: 5 h-aged 645±6 2326 2344 3.6 22.2 15.4 CRA: 12 h-aged 641±8 2254 2324 3.0 18.2 17.9 18 Ni(300) — 2000 2050 7.0 — — 
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