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Influence of elemental B addition on the heat-treated cast structures of Ti–47Al–2Cr–(2–4)Nb alloys

Published online by Cambridge University Press:  31 January 2011

J. Y. Jung  and
J. K. Park
J. Y. Jung
Affiliation:
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-Dong Yosung-Gu, 305-701 Taejon, South Korea
J. K. Park
Affiliation:
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusung-Dong Yosung-Gu, 305-701 Taejon, South Korea
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Abstract

The influence of elemental B addition on the heat-treated cast structure of Ti–47Al–2Cr–(2–4)Nb alloys has been investigated using x-ray diffractometry, optical microscopy, scanning and transmission electron microscopy, and tensile testing. The phase sequence is β → β + α → α → α + γ → α + β + γ → β + γ. The addition of (0–2 at. %) B does not change the phase sequence. It, however, tends to stabilize α phase by shifting the (α + β + γ) three-phase region toward a higher (Cr + Nb) content. The B addition does not significantly alter the equilibrium composition within (α + γ) two-phase field. The B addition markedly accelerates the lamellar formation kinetics and enhances the thickening rate of γ plates, despite the fact that it increases both the misfit between α and γ plates and the α/γ interfacial energy. The acceleration of lamellar formation kinetics is thus believed to be primarily due to the enhancement of chemical diffusivity as a result of B addition. The B addition induces a significant refinement of heat-treated cast structure. This is primarily due to the role of boride to disperse the interdendritic γ regions to a fine network and to refine the dendrite cell size. Further refinement arises from the boride's role to act as the nucleation site for γ grains and from the intrinsic B effect to enhance the chemical diffusivity and the γ thickening rate. The addition of a small amount of B enhances both the strength and tensile ductility of near gamma structure. The strengthening arises from grain size refinement and from the boride dispersion. The calculation of fracture strain suggests that an enhancement of ductility for small B addition (up to ∼0.2 at. %) is mostly due to its effect to refine the γ grains.

Type
Articles
Information
Journal of Materials Research , Volume 12 , Issue 3 , March 1997 , pp. 665 - 680
Copyright
Copyright © Materials Research Society 1997

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