Much of the current understanding of the β → α + β phase transformation in the commercially important titanium alloy Ti-6Al-4V (Ti64) is by inference from more β-stabilised analogues, or post-mortem studies. In-situ cooling experiments have been conducted for the first time on Ti64 samples, in an adapted SEM with a high temperature heating stage, while performing real-time secondary electron video recording and sequential electron backscatter diffraction mapping. This has enabled direct observation of the development of grain boundary (GB) α colony microstructures, when cooling at relatively low rates (0.3–0.1 °C s⁻¹) through the β transus. It is shown that the α colonies develop discontinuously, with relatively widely spaced larger ‘primary’ α lamellae nucleating first at lower undercoolings at β GBs, which grow out rapidly into the grains until impingement. These primary laths form a ‘skeletal template’ for the subsequent colonies by determining the variant selected within their domain of influence, as the extent of transformation increases. This occurs through biasing the subsequent sympathetic autocatalytic nucleation of finer secondary α laths combined with branching and plate broadening, to ‘infill’ the remaining β matrix with laths of the same variant. Evidence has also been provided that is in agreement with previous studies of the crystallographic relationships between the nucleating GB α allotriomorph variants, colonies, and the β GBs. However, GB α allotriomorphs were not always found to be a pre-requirement for the development of single variant α colonies in Ti64 (at least when observed at a surface), with the primary α laths seen to nucleate directly off β GBs in competition with the GB α allotriomorphs in many cases.

目前对具有重要商业价值的钛合金 Ti-6Al-4V (Ti64) 中β  →  α  +  β相变的大部分理解是从更多β稳定类似物或验尸研究中推断出来的。首次在 Ti64 样品上进行了原位冷却实验，在具有高温加热台的改装 SEM 中，同时进行实时二次电子视频记录和顺序电子背散射衍射映射。当以相对低的速率 (0.3–0.1 °C s ^-1 )冷却通过β转换。结果表明，α 菌落不连续地发育，相对较宽的较大“初级” α薄片首先在β GB处的较低过冷度下成核，然后迅速生长到晶粒中直到受到撞击。随着转化程度的增加，这些初级板条通过确定在其影响范围内选择的变体，为后续菌落形成“骨架模板”。这是通过偏置随后的交感神经自催化成核与分支和板加宽相结合的更细的次级α板条，以“填充”剩余的β具有相同变体的板条的矩阵。还提供了与先前关于成核 GB α 同种异形变体、集落和β GB之间晶体关系的研究一致的证据。然而，GB α 同种异形体并不总是被发现是 Ti64 中单一变异 α 集落发展的先决条件（至少在表面观察时），主要的α板条被认为直接从β GB上成核，与在许多情况下，GB α 同素异形体。