Orientation relationships between TiB (B27), B2, and Ti 3 Al phases
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W. Lu Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China; and Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, People’s Republic of China
L.L. Hea) and H.Q. Ye Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, People’s Republic of China (Received 27 August 2008; accepted 17 November 2008)
The orientation relationships among TiB (B27), B2, and Ti3Al phases have been investigated by transmission electron microscopy. By using the composite selected-area electron diffraction technique, the orientation relationship between TiB (B27) and B2 was determined to be [100]TiBjj[001]B2, (001)TiBjj(010)B2; and that between TiB (B27) and Ti3Al was ½010TiB jj½1120Ti3 Al , ð001ÞTiB jjð0001ÞTi3 Al . These orientation relationships have been predicted precisely by the method of coincidence of reciprocal lattice points. I. INTRODUCTION
TiAl alloys are promising materials for application in aerospace and automotive engine components due to their low-density and good high-temperature properties.1–3 Extensive efforts have been devoted to studying their possible technological applications in past decades.4–6 It is suggested that the addition of boron in TiAl has two effects that can significantly improve the mechanical properties of TiAl alloys7,8: one is grain refinement, and the other is dispersion strengthening by borides. The effect of grain refinement of boron in TiAl alloys can be summarized as follows9: on one hand, there is a minimum boron level required for grain refinement, and it is alloy composition-dependent. If the boron concentration is below this critical level, no grain refinement can be achieved: on the other hand, as soon as the alloys are fully grain refined, any excess boron concentration does not contribute to a further reduction in grain size. Because the morphologies and structures of borides have a significant impact on the properties of TiAl alloys with boron addition, much effort has been devoted to investigating the morphologies and structures of borides in TiAl alloys. The morphologies ranging from needle through flake to block particles have been reported in different alloy systems.10–12 For the categories of borides, both monoborides and diborides have been found in TiAl alloys with the addition of boron. It has been a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0220
1688
http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 03 Apr 2015
proposed that the strong boride formers, such as W, Ta, and Nb, change the prevalent titanium boride form from TiB2 to TiB.13 In the past, the orientation relationships of borides with a, b, and g phases have been widely investigated.11,14 However, although it is also of significant importance, the orientation relationship between TiB and a2-Ti3Al is still unknown. On t
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