高Nb-TiAl热变形过程中的晶界特征和显微组织演化

Grain boundary character and microstructural evolution during hot deformation of a high Nb containing TiAl alloy

摘要:

本文研究了Ti-45Al-8.5Nb-(W,B,Y)合金热变形过程中的晶界特征和显微组织演化规律。采用等离子冷床炉熔炼制备的Ti-45Al-8.5Nb-(W,B,Y)合金具有典型的近片层组织并在晶界有高温β/B2相残留，富Nb、W、Ti等、贫Al的晶界β/B2相主要是熔炼过程中因冷速较快、β稳定元素的低扩散系数以及合金中各元素的分配系数差异导致β → α相变不能完全进行。合金中的 B 和Y等微量元素分别以硼化物和Y2O3的形式存在。晶界β/B2相的形态、尺寸、成分和稳定性等受后续热变形影响显著，高温和应力作用会使β相发生破碎细化并促进合金中的元素扩散会引起晶界β/B2相的成分变化。当合金在 (α + γ) 两相区进行热压缩变形时，会有部分β相向α相转变（β/B2 → α2），主要通过β相中二次α的析出和β相被相邻α相的蚕食等方式进行，热压缩变形会促进β/B2相向更为密排结构的α2相转变。

Abstract:

Grain boundary character and microstructural evolution during hot deformation of a high Nb-containing TiAl alloy with the composition of Ti-45Al-8.5Nb-(W,B,Y) have been investigated in this work. The plasma arc-melted alloy exhibits nearly lamellar microstructures with β/B2 phase precipitating along colony boundaries. The retained β/B2 phase rich in Ti, Nb and W and poor in Al is believed to be formed by the β → α transformation during nonequilibrium solidification. This β/B2 precipitation is caused by the relative high cooling rate, low atom diffusion rate of β stabilizers and different partition coefficient of the elements. B and Y elements exist mainly as borides and Y2O3, respectively. The morphology, size, composition and stability of β/B2 phase are determined by hot deformation. The high temperature and press enhance the elements diffusion, thus the variation of the composition of β/B2 phase in deformed samples is obviously observed. When the samples are deformed in (α + γ) phase region, the transformation β/B2 → α2 takes place. Considering the different crystal structures between β/B2 and α2 phases, there is a tendency for β/B2 phase to transform to an increased close packed structure of α2 during deformation. The β/B2 → α2 transformation is promoted by the hot deformation.