The microstructural evolution and alloying element partitioning in the α + β ↔ β phase transformation of Ti-17 alloy were explored under continuous heating and cooling conditions using the dilatometric method. Scanning electron microscopy and transmission electron microscopy were used to evaluate microstructural characteristics and trace alloying element partitioning behaviors occurring at different temperatures during heating and cooling. Results showed that the finer needle-like α phase first dissolved into the β phase in the matrix with increasing temperature, while the grain boundary α phase first coarsened and then transformed gradually into β phase during continuous heating. The dissolution of α phase of the alloy with the alloying element partitioning during continuous heating was observed. On the contrary, α_GB formed at the prior β grain of the alloy during continuous cooling, which might be the nuclei of α colony, thus resulting in the formation of α colony in the matrix. As the temperature decreased, the elements’ concentrations in the α and β phases became increasingly varied due to element partition. Moreover, Al and Cr, which had higher diffusion coefficients than Mo, easily reached the concentration equilibrium of alloying elements in the α and β phases, respectively. The shrinkage of dilatometric curves during heating in the Ti-17 alloy are mainly attributed to the change of α-HCP (hexagonal close-packed) lattice to β-BCC (body-centered cubic) lattice; while the element partitioning during the β → α + β transformation plays an important role in the shrinkage of the dilatometric curves of the Ti-17 alloy during cooling.

中文翻译：

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连续加热和冷却条件下Ti-17合金相变的组织演变和元素分配

利用膨胀法研究了在连续加热和冷却条件下Ti-17合金在α+β↔β相变中的组织演变和合金元素分配。扫描电子显微镜和透射电子显微镜用于评估在加热和冷却期间在不同温度下发生的微观结构特征和痕量合金元素分配行为。结果表明，随着温度的升高，较细的针状α相首先溶解在基体中的β相中，而晶界α相首先被粗化，然后在连续加热过程中逐渐转变为β相。观察到在连续加热过程中合金元素分配时合金的α相溶解。相反，_αGB在连续冷却过程中，在合金的先前β晶粒上会形成α菌落，这可能是α菌落的核，因此导致在基体中形成α菌落。随着温度降低，由于元素分配，α相和β相中元素的浓度变得越来越多。而且，具有比Mo高的扩散系数的Al和Cr分别容易地达到α相和β相中合金元素的浓度平衡。Ti-17合金在加热过程中膨胀曲线的收缩主要归因于α-HCP（六方密堆积）晶格向β-BCC（体心立方）晶格的变化。β→α+β转变过程中的元素分配在冷却过程中对Ti-17合金膨胀曲线的收缩起着重要作用。