第三元素を添加したTiAl基合金の組織と結晶構造

Abstract

Microstructures and crystal structures of Ti-34, -36 and -38 mass%Al alloys containing Mn, Nb, V or Zr up to 10 at% as a third element has been examined using a optical microscope, an electron probe microanalyzer and an X-ray diffractometer. The alloying effects have been discussed from the viewpoint of ductility improvements of the alloys.Ternary phase diagrams around the TiAl phase have been determined. The structure is refined by the addition of each third element to the alloys. The Ti3Al phase is decreased and the annealing twins are increased by the addition of Mn or V. The microstructure is considered to be favourable for improving the ductility of the alloys.The lattice parameter of the TiAl (γ) phase increases with increasing Zr or Nb content and decreases with increasing Mn or V content, as can be expected from the atomic size factors of the third elements. On the other hand, the axial ratio c⁄a of the γ phase decreases with increasing Mn, Nb or Zr content and remains at a fixed value for the addition of V. This alloying effect on the axial ratio can be explained from the condition in which the increase in strain energy with the lattice parameter is minimized. It is recognized that in the γ phase Zr and Nb are substitute for the Ti site, Mn for the Al site and V for both sites. Also, it can be estimated that the antiphase boundary energy of γ is decreased by the addition of Mn, V or Nb, and that the stacking fault energy of γ is decreased by the addition of Mn or V.It is concluded that the third elements which improve the ductility of TiAl-based alloys are those which are smaller in atomic size than Ti and Al and replace the Al site to reduce the Al content in the γ phase.