Abstract <p>Isothermal forging is a crucial technique for finely tailoring the microstructure and achieving desirable mechanical properties in metastable β titanium alloys (β-Ti), enabling the formation of complex components. However, research on isothermal forging of metastable β-Ti based composites remains limited. Here, Ti-55531 alloys and TiB/Ti-55531 composites were isothermally forged in both (α + β) and β phase regions. The microstructure evolution, mechanical properties and strengthening/toughening mechanisms were thoroughly investigated. Isothermal forging promoted the refinement of matrix grains, homogenization of secondary α phase (α_s), and orderly arrangement of TiB whiskers. The forging temperature had a significant effect on the recrystallization behavior of β phase and the precipitation of primary α phase (α_p). The addition of TiB not only promoted discontinuous dynamic recrystallization but also facilitated the dispersion and precipitation of α_p around TiB. Consequently, the mechanical properties of isothermal forged samples were significantly improved compared to the as-casted ones, and the strength and ductility of the forged composites were both superior to those of the alloys. Specifically, the isothermal (α + β) forged composites achieved an excellent strength-ductility balance, with a tensile strength of 1514 ± 16&#xa0;MPa and a total elongation of 6.7% ± 0.2%. The strengthening and toughening mechanisms were mainly attributed to the hierarchical microstructure composed of intragranular α_p, α_s and α-precipitate-free zones, as well as the load-bearing strengthening of TiB whiskers. Moreover, the α_p closely adhered to TiB hindered crack propagation and relieved the stress concentration between transformed β structure and TiB by activating multi-slip systems, thereby improving ductility and altering the fracture mode.</p> Graphical abstract <p></p> 摘要 (Chinese abstract) <p>等温锻造是一种关键技术，它能在亚稳β钛合金中精细调控微观组织并获得理想的力学性能，从而能够制造复杂形状的部件。然而，关于亚稳β钛基复合材料等温锻造的研究仍显不足。在本文中，Ti-55531合金和TiB/Ti-55531复合材料分别在(α + β)相区和β相区进行了等温锻造，并对其微观组织演变、力学性能及强韧化机制进行了系统地研究。研究发现，等温锻造促进了基体晶粒细化、次生α相均匀化以及TiB晶须的有序排列。锻造温度对β相的再结晶行为和初生α相(αp)的析出具有显著影响。TiB的添加不仅加速了非连续动态再结晶，还促进了αp在TiB周围的弥散析出。因此，等温锻造样品的力学性能较铸态样品得到显著改善，且锻态复合材料的强度和塑性均优于锻态合金。总而言之，经(α + β)相区等温锻造的复合材料实现了优异的强塑性匹配，其拉伸强度为 1514 ± 16 MPa，总延伸率达到 6.7% ± 0.2%。其强化和韧化机制主要归因于由晶内初生α相、次生α相和无α相析出区构成的多尺度微观组织，以及TiB晶须的承载强化作用。此外，与TiB紧密贴合的αp通过激活多滑移系，阻碍了裂纹扩展并缓解了β转变组织与TiB之间的应力集中，从而提高了塑性并改变了断裂模式.</p>