低成本中熵合金时效强化研究

自 2004 年研究发表以来，高/中熵合金作为“多主元”合金设计新理念 的载体一直受到广泛关注，在力学性能方面也展现出巨大的潜力。早期开发 的面心立方（FCC）结构高/中熵合金，具有较好的延展性，但其强度较低。 另外，贵重金属元素（如 Co 等）的大量使用也导致了合金成本较高的问题， 严重阻碍了其应用。针对上述问题，本文开发了 Fe45Cr25Ni10Mn15Al4C（1 at.%） 的低成本中熵合金，并探索了其时效强化的潜力。结果表明，该合金时效前 具有 FCC/体心立方（BCC）双相组织，强塑性较好（抗拉强度：940 MPa， 总延伸率：20.2%；极限抗压强度：1123.08 MPa，真实应变：41%）。当时 效温度在 600 ℃以下时，材料强度随时效温度逐渐升高（抗拉强度：940→ 1065 MPa），但塑性逐渐降低（总延伸率：20.2→6.6%）。特别是经过 700 和 800 ℃时效处理后时，材料出现显著脆化现象，在屈服前就发生断裂。对 800 ℃时效后的微观组织进行表征后发现，在 BCC 基体中形成了大量微米 尺寸树枝晶状 σ 相。σ 相具有本征脆性，同时树枝状的形态容易引起应力集 中并提供裂纹扩展路径，因此导致了材料的显著脆化。为了克服 σ 相的脆化 作用，本文提出通过快速凝固后再进行时效处理的方法，以获得纳米尺寸弥 散分布的 σ 相。结果表明，通过快速凝固（冷却速率：2x103 ℃/s）制备的样 品同样具有 FCC/BCC 双相组织，但两相的合金成分与热轧态不同，另外晶 粒得到细化，并具有一定的织构。快速凝固的样品表现良好，抗压强度达到 1304 MPa，真实应变为 58%。经过 700 ℃时效处理后，材料强度稍微下降到 1095 MPa，但塑性几乎不变。微观组织表征显示，700 ℃时效处理后并未形 成大量 σ 相，这主要是由于快速凝固控制了元素的扩散，提高了 BCC→σ 相 变的阻力，从而控制了相变动力学。

High/medium entropy alloys (HEAs/MEAs) have been receiving extensive attention as carriers for the new concept of "multi-principal element" alloy design, and have shown great potential in terms of mechanical properties since its publication in 2004. The previously developed HEAs/MEAs have good ductility but low strength with face-centered cubic (FCC). In addition, the extensive use of precious metal elements (such as Co) also leads to the high cost of alloys, which seriously hinders their application. To solve these problems, a low-cost Fe₄₅Cr₂₅Ni₁₀Mn₁₅Al₄C₁ (at.%) MEA was developed and its potential for aging strengthening was explored. The present work shows that the alloy has a FCC/body-centered cubic (BCC) dual-phase structure before aging, and has good combination of strength and ductility (tensile strength: 940 MPa, total elongation: 20.2%; ultimate compressive strength: 1123.08 MPa, true strain: 41%). The strength of the material increases gradually with the aging temperature (tensile strength: 940→1065 MPa) when the aging temperature is below 600 ℃, but the plasticity decreases gradually (total elongation: 20.2→6.6%). Significant embrittlement occurred and fracture occurred before yield, especially after aging treatment at 700 and 800 ℃. The microstructure after aging at 800 ℃ was thoroughly characterized. It was found that a large number of micron-sized dendritic σ phases were formed in the BCC matrix. σ phase is intrinsically brittle. Moreover, its dendritic morphology tends to cause stress concentration and provide crack propagation paths. Consequently, the presence of σ phase with dendritic morphology leads to significant embrittlement of the present MEA. To overcome the embrittlement of σ phase, a method of rapid solidification followed by aging treatment is proposed in this thesis to obtain nano-sized dispersed σ phase. The results show that the sample prepared by rapid solidification (cooling rate: 2x10³ ℃/s) also has FCC/BCC dual-phase structure, but the alloy composition of the two phases is different from that of the hot rolled state. In addition, the grain is refined and has a certain texture. The rapidly solidified sample showed better compressive strength of 1304 MPa and true strain of 58%. The strength of the material decreased slightly to 1095 MPa, without noticeable change of the plasticity after aging treatment at 700 ℃. Microstructure characterization showed that a large number of σ phases did not form after aging treatment at 700 ℃, which was mainly due to the fact that rapid solidification controls the diffusion of elements and increased the resistance of BCC → σ phase transition, thus controlling the dynamics of phase transition.

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