《Table 1 EPMA analysis of marked position 1~5 in Fig.2》

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《高Nb-TiAl热变形过程中的晶界特征和显微组织演化（英文）》

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Fig.2 shows microstructure of the as-cast experimental alloy.BSE observation reveals that a white-contrast network is distributed throughout the matrix of the alloy.A part of the network exhibiting a faint outline is located at the lamellar matrix as indicated by arrow 3 in Fig.2a.EPMA analysis indicates that this region is enriched in Nb and W（Table 1）.It is caused by segregation of Nb and W to dendrite cores during solidification[17，18].The white irregular-shaped block at colony boundaries and triple junctions isβ/B2 phase which is confirmed by EPMA analysis（Table 1）and X-ray diffraction spectrum in Fig.1b.As can be seen in Fig.2b，theβ/B2 phase is occasionally intersected by smallγgrains.This phenomenon originates from the discontinuous reactionβ→β+γduring the cooling process[26].The EDS element liner scanning is shown in Fig.2d and composition analysis in Table1 indicates that theβ/B2 phase at boundaries（β-segregation）is rich in Ti，Nb and W but poor in Al.The observations suggest that the lamellar colonies，which come from priorαgrains，contain a lower content ofβ-phase stabilisers（Nb and W）.The volume fraction of theβ/B2 phase is about 8%.Theβ-segregation lies in a size range 10~30μm.The compositions at different positions in Fig.2 are summarized in Table 1.Except for the main microsegregation ofβphase，B-segregation and Y-segregation are observed as shown in the inset of Fig.2b and exist as borides and Y2O3，respectively.The borides in the form of particle or rod are distributed within the colonies or colony boundaries.It is well known that borides exist in different morphologies and crystal structures in Ti Al alloys，which is related to the boron content and solidification process[27].Yttrium oxide particles with bright contrast are mainly located at the boundaries，and the composition ratio of Y to O in the yttrium oxide particle is close to 2:3.The existence rate of O atom in the matrix is reduced by Y intrinsic oxidation.The volume fraction of Y2O3is considerably low considering the content of Y is only 0.02at%.It suggests that the solubility of Y is rather low in high Nb containing TiAl alloys.The reason for the absence of the borides and Y2O3 phase in XRD spectrum is that they are both too rare to be detected.