Evolution of the as-solidified microstructures of Ti–45Al–8Nb, Ti–45Al–8Nb–2Cr-0.2B, Ti–45Al–8Nb–4Cr-0.2B alloys, and phase stability during high-temperature exposure (1000°, 1100°,1200 °C) have been analyzed in the present report. Macroscopic examinations of the as-cast pancakes indicate that Nb, Cr, and B alloying can be effectively utilized as potential grain/colony size refiners for Ti–45Al base TiAl alloys. However, the as-cast microstructure varies considerably from a nearly fully lamellar (α₂+γ) structure for 0 at.% Cr to a dominant coarse-γ laths+β(B2) structure for 4 at.% Cr addition to the baseline Ti–45Al–8Nb alloy. Examination of characteristics of prior-β dendritic morphology and associated crystallographic considerations and surface relief effects establish that all the alloys studied here solidify through the β-phase field. No peritectic α formation could be established. Formation of γ within β phase and lamellar microstructures are attributed to the reactions β→β+γ, and α→α₂+γ, respectively in the as-cast condition. It has been established that strong Cr partitioning in the β-phase significantly slows down the kinetics of β-phase decomposition into γ, and σ. This causes departure from the evolution of phases as predicted by thermodynamic calculations. The solidification microstructure resulting from the solid-state phase transformations are further discussed in terms of the micro-segregation and crystallographic characteristics of the parent and the product phases.

Ti–45Al–8Nb，Ti–45Al–8Nb–2Cr-0.2B，Ti–45Al–8Nb–4Cr-0.2B合金凝固态组织的演变以及高温暴露（1000°，1100）下的相稳定性°，1200°C）已在本报告中进行了分析。铸态薄煎饼的宏观检查表明，Nb，Cr和B合金可有效用作Ti–45Al基TiAl合金的潜在晶粒/菌落尺寸细化剂。但是，铸态显微组织从一个几乎完全的层状（α变化很大₂在基线Ti–45Al–8Nb合金中添加4 at。％Cr时，Cr含量为0 at。％的铬的+γ）结构+占主导地位的粗γ板条的β（B2）结构。对先验-β树枝状形态的特征以及相关的晶体学考虑和表面起伏效应的检查确定，此处研究的所有合金均通过β-相场凝固。不能建立包晶的α形成。β相和层状微观结构内γ的形成归因于反应β→β+γ和α→α ₂铸态时分别为+γ。已经确定，在β相中强的Cr分配会显着减慢β相分解为γ和σ的动力学。这导致偏离了热力学计算所预测的相演化。从母相和产物相的微观偏析和晶体学特征，进一步讨论了由固态相变产生的凝固微观结构。