镍基高温合金定向快速凝固的分子动力学模拟

Molecular Dynamics Simulation of Directional Rapid Solidification in Nickel-Based Superalloys

摘要:

运用分子动力学方法研究了Ni_xCr_yFe_z在定向快速凝固过程中的微观组织演化和凝固后的拉伸力学性能。研究发现，在固液转变时，温度先急剧下降，后由于潜热释放而轻微上升。在此过程中，晶核优先在低温区形成，展现了异质凝固的特性。凝固完成后，合金以面心立方（fcc）为主，同时包含少量密排六方（hcp）和体心立方（bcc）结构，非晶态晶界亦占有显著比例。Ni₆₀Cr₂₁Fe₁₉合金元素浓度分析进一步表明Cr原子在晶界处偏析。此外，合金凝固后形成了大量热致孪晶和堆垛层错，位错密度高达10¹⁶ m^–2量级，且位错线在低温区更为密集，呈现出明显的非均匀性。还研究了4种不同比例的Ni-Cr-Fe合金的形核过程，发现合金元素比例对形核率有显著影响。此外，还对凝固后的Ni₆₀Cr₂₁Fe₁₉合金进行了2个方向的拉伸测试模拟，结果表明，由于定向凝固组织的各向异性特性导致垂直凝固方向的抗拉强度低于平行凝固方向的抗拉强度。位错密度的变化与拉伸应力应变的变化趋势并不完全吻合，这一现象表明Ni₆₀Cr₂₁Fe₁₉合金定向快速凝固组织的强化机制更为复杂，可能涉及多种强化机制的协同作用。

Abstract:

The microstructural evolution of Ni_xCr_yFe_z alloy during the directional rapid solidification process and the tensile mechanical properties of Ni_xCr_yFe_z alloy after solidification were investigated using molecular dynamics simulations. The results reveal that during the solid-liquid phase transition, the temperature initially drops sharply and then rises slightly due to the release of latent heat. During this process, crystal nuclei preferentially form in low-temperature regions, exhibiting heterogeneous solidification characteristics. After solidification, the alloy is primarily composed of face-centered cubic (fcc) structures, with a small amount of hexagonal close-packed (hcp) and body-centered cubic (bcc) structures, and amorphous grain boundaries also occupy a significant proportion. The elemental concentration analysis of the Ni₆₀Cr₂₁Fe₁₉ alloy further indicates that Cr atoms segregate at the grain boundaries. Additionally, a large number of thermally induced twins and stacking faults are formed in the alloy after solidification, with a dislocation density reaching the order of 10¹⁶ m^–2. The dislocation lines become more concentrated in low-temperature regions, demonstrating significant heterogeneity. The nucleation process of four kinds of Ni-Cr-Fe alloys with different proportions was also studied, revealing that the alloy composition ratio has a significant impact on the nucleation rate. Furthermore, tensile tests were simulated on the Ni₆₀Cr₂₁Fe₁₉ alloy in two directions, showing that the tensile strength perpendicular to the solidification direction is lower than that parallel to the solidification direction due to the anisotropic nature of the directional solidification structure. The discrepancy between the changes in dislocation density and tensile stress-tensile strain indicates that the strengthening mechanism of directional rapid solidification structure of the Ni₆₀Cr₂₁Fe₁₉ alloy is more complex, potentially involving the synergistic effect of multiple strengthening mechanisms.