Advanced Materials Processing and Analysis Center Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32816, United States.
ACS Comb Sci. 2020 Dec 14;22(12):757-767. doi: 10.1021/acscombsci.0c00096. Epub 2020 Oct 19.
Relative role of enthalpy and entropy in the stabilization of senary FCC Al-Co-Cr-Fe-Ni-Mn high entropy alloys was investigated via a high throughput combinatorial solid-to-solid diffusion couple approach. Many off-equiatomic compositions of FCC AlCoCrFeNiMn were generated by the diffusing Al and Ni in equiatomic CoCrFeNiMn alloy, i.e., the AlNi vs CoCrFeNiMn diffusion couple, annealed at 900°, 1000°, 1100°, and 1200 °C. Above 1000 °C, the solubility limit of Al in off-equiatomic AlCoCrFeNiMn alloy was determined to be higher than the solubility limit of Al in equiatomic AlCoCrFeNiMn alloy. Compositions corresponding to the highest solubility limit of Al in off-equiatomic AlCoCrFeNiMn alloy exhibited a lower free energy of mixing, i.e., higher thermodynamic stability, than equiatomic AlCoCrFeNiMn compositions, at 1100 °C and above. Therefore, the role of enthalpy was estimated to be significant in achieving higher thermodynamic stability in off-equiatomic alloys, since they always have lower entropy of mixing than their equiatomic counterparts. The magnitude of interdiffusion coefficients of individual elements in Al-Co-Cr-Fe-Ni-Mn alloys were compared to the interdiffusion coefficients in relevant quinary, quaternary, and ternary solvent-based alloys. Interdiffusion coefficients were not necessarily lower in FCC Al-Co-Cr-Fe-Ni-Mn alloys; therefore no sluggish diffusion was observed in FCC HEA, but diffusion of individual elements in BCC Al-Co-Cr-Fe-Ni-Mn alloy followed the sluggish diffusion hypothesis except for Ni. All compositions in the FCC Al-Co-Cr-Fe-Ni-Mn alloy were observed to comply with existing empirical single phase formation rules in high entropy alloys.
通过高通量组合固-固扩散偶方法研究了焓和熵在六元 FCC Al-Co-Cr-Fe-Ni-Mn 高熵合金稳定化中的相对作用。通过在等原子 CoCrFeNiMn 合金中扩散 Al 和 Ni，生成了许多非等原子 FCC AlCoCrFeNiMn 组成，即 AlNi 与 CoCrFeNiMn 扩散偶，在 900°C、1000°C、1100°C 和 1200°C 下进行退火。在 1000°C 以上，确定了非等原子 AlCoCrFeNiMn 合金中 Al 的溶解度极限高于等原子 AlCoCrFeNiMn 合金中 Al 的溶解度极限。在 1100°C 及以上，与非等原子 AlCoCrFeNiMn 合金中 Al 溶解度极限对应的组成表现出较低的混合自由能，即较高的热力学稳定性，而与等原子 AlCoCrFeNiMn 组成相比。因此，在非等原子合金中实现更高热力学稳定性的作用估计是焓的作用，因为它们的混合熵总是低于等原子对应物。比较了 Al-Co-Cr-Fe-Ni-Mn 合金中各元素的互扩散系数与相关五元、四元和三元溶剂基合金中的互扩散系数。FCC Al-Co-Cr-Fe-Ni-Mn 合金中的互扩散系数不一定较低；因此，在 FCC HEA 中没有观察到扩散缓慢，但 BCC Al-Co-Cr-Fe-Ni-Mn 合金中各元素的扩散符合除 Ni 外的扩散缓慢假设。观察到 FCC Al-Co-Cr-Fe-Ni-Mn 合金中的所有组成都符合高熵合金中现有的经验单相形成规则。
