An investigation was conducted into the impact of low cobalt content on the microstructure and mechanical properties of titanium alloys. Ti-4Al-6Cr-5Mo-8 Nb-Co ( = 0, 2, 3, 4, and 5) alloys were prepared by ultrasonic-assisted melting and casting. The changes in microstructure, mechanical properties, and the strengthening and toughening mechanisms were researched. Results show that, when the cobalt content increases from 0 to 5 wt%, the as-cast structure is all β phase. The lattice parameter of the β phase decreases from 0.3663 to 0.3114 nm, a 15% decrease. The size of primary β grains decreases from 676.8 to 310.7 μm, a 54% decrease. When the cobalt content exceeds 3 wt%, the segregation energy of cobalt increases and grain boundary segregation occurs. Grain refinement is due to constitutional supercooling caused by the β-stable element cobalt, as well as the solute drag caused by the differences in intragranular and grain boundary compositions. In addition, the size mismatch between cobalt and titanium atoms leads to lattice distortion. As the cobalt content increases from 0 to 3 wt%, the tensile strength increases from 918.1 to 1147.8 MPa, a 25% increase, while the fracture toughness increases from 51.7 to 59.7 MPa·m, a 15% increase. At 5 wt% cobalt, the tensile strength and fracture toughness diminished to 247.5 MPa and 14.8 MPa·m, respectively. The decrease in lattice parameters mainly lead to an increase in strength. The secondary crack leads to an increase in toughness. The decline in strength and toughness results from cobalt segregating at grain boundaries, leading to embrittlement.

中文翻译：

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Co对超声真空熔炼Ti-4Al-6Cr-5Mo-8 Nb合金显微组织演变和力学性能的影响

研究了低钴含量对钛合金显微组织和力学性能的影响。采用超声波辅助熔炼铸造方法制备了Ti-4Al-6Cr-5Mo-8 Nb-Co (=0、2、3、4、5)合金。研究了显微组织、力学性能以及强韧化机制的变化。结果表明，当钴含量从0 wt%增加到5 wt%时，铸态组织全部为β相。 β相的晶格参数从0.3663减小到0.3114 nm，降低了15%。初生β晶粒尺寸从676.8μm减小到310.7μm，减小了54%。当钴含量超过3wt％时，钴的偏析能增加并且发生晶界偏析。晶粒细化是由于β稳定元素钴引起的结构过冷，以及晶内和晶界成分差异引起的溶质阻力。此外，钴和钛原子之间的尺寸不匹配导致晶格畸变。随着钴含量从0 wt%增加到3 wt%，拉伸强度从918.1 MPa增加到1147.8 MPa，增加了25%，而断裂韧性从51.7 MPa·m增加到59.7 MPa·m，增加了15%。当钴含量为 5 wt% 时，拉伸强度和断裂韧性分别降低至 247.5 MPa 和 14.8 MPa·m。晶格参数的降低主要导致强度的增加。二次裂纹导致韧性增加。强度和韧性的下降是由于钴在晶界偏析导致脆化。