Effect of Al content and Nb addition on the strength and fault energy of TiAl alloys
Introduction
The Al-lean TiAl-base alloys have received a great deal of attentions chiefly owing to their attractive combination of low density, good oxidation resistance and high temperature strength retention [1]. It has been recently recognized that Nb addition of 5–10 at.% can significantly improve the strength of TiAl-base alloys [2], [3], [4], [5], [6]. The origin of this strengthening effect is not altogether clear. It may arise from solid solution as has been suggested by Zhang et al. [2]. Alloying with Nb often leads to a refinement of the microstructure, as has been recognized in [5]. Thus, the strengthening effect may in terms of a Hall–Petch mechanism be attributed to dislocation interactions with internal boundaries [5]. Another hypothesis is that additions of Nb lead to a decrease of the stacking fault energy. This hypothesis is supported by the observation that deformed high Nb containing alloys exhibit an abundant activation of twinning [4], [5], [6]. This would increase the dissociation width of the dislocations, which in turn has significant implications on their resistance against glide and climb. To examine this hypothesis the stacking fault energies in Ti–(45, 49)Al alloys with and without Nb additions were measured in the present study using weak-beam TEM techniques. To the author's knowledge, the stacking fault energy in Ti-rich TiAl alloys has not been measured experimentally up to date.
Section snippets
Experimental details
Four kinds of TiAl alloys were investigated in this study with the nominal compositions of Ti–45Al, Ti–49Al, Ti–45Al–10Nb, and Ti–49Al–10Nb (at.%). The oxygen content of the materials was less than 900 ppm. All the alloys (that is cast Ti–49Al–xNb and forged Ti–45Al–xNb) were finally annealed at 1100 °C for 30 h. After annealing, all the studied materials exhibited a nearly-equiaxed gamma microstructure with the volume fraction of α2 grains ranging from 2 to 20 vol.% in different alloys. The
Effect of Al and Nb contents on the yield strength of TiAl
Fig. 1 compared the yield strengths of the four studied alloys at 900 °C. The γ grain sizes of the studied materials were indicated in the legend. The yield strengths of the ternary alloys are significantly higher than that of binary alloys, which are almost consistent with the results at room temperature [3]. Addition of Nb leads to a slight refinement of the microstructure as indicated by the grain sizes give in Fig. 1. With this data about 12–26% of the observed yield strength difference
Conclusions
The yield strengths and stacking fault energies were measured at 900 °C in binary Ti–(45, 49)Al and ternary Ti–(45, 49)Al–10Nb alloys in the form of nearly-equiaxed gamma microstructure. The yield strength of TiAl largely increases with Nb addition and is insensitive to the variation of Al content. On the other hand, the stacking fault energy decreases significantly with the decreasing of Al content in binary alloys and is practically independent of Al content in ternary alloys.
Acknowledgements
The authors would like to thank Professor G.L. Chen (University of Science and Technology Beijing, People's Republic of China) and Dr S.C. Deevi (Chrysalis Technology Inc., Richmond) for helpful discussions. One of the authors (WJZ) greatly acknowledges the Alexander von Humboldt Foundation and Chrysalis Technology Inc., for financial support.
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