Growth morphologies and mechanisms of non-equilibrium solidified MC carbide
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M. Wang Laboratory of Laser Materials Processing and Surface Engineering, School of Materials Science and Engineering, Beijinghang University, Beijing 100083, People’s Republic of China (Received 13 December 2004; accepted 8 August 2005)
Growth morphologies and mechanisms of the carbide of group IVB and VB elements (MC carbide), a typical faceted crystal, were studied with an estimated cooling rate from 102 to 105 K/s. Results showed that although the growth morphologies of the MC carbide vary remarkably with solidification cooling rate, the solid/liquid interface is always atomically smooth, and the growth mechanisms are always lateral growth. The growth mechanism transition from lateral to continuous growth mode, which was predicted by the classic crystal growth theory, was not observed for the TiC type MC carbide within the estimated cooling rate range of 102–105 K/s.
I. INTRODUCTION
The nature of the solid/liquid interface structure has a decisive influence on both the growth mechanisms and the growth morphologies of crystals. The Jackson’s factor ␣j1 can be used to predict the solid/liquid interface structure and growth mechanism under slow-cooling, near-equilibrium solidification conditions. For those phases with ␣j < 2, the solid/liquid interface structure is widely believed to be atomically rough, and the crystal growth occurs by continuous growth, leading to nonfaceted morphology. For those phases with ␣j > 2, the growth interface structure is atomically smooth, and the crystal growth occurs by lateral growth, leading to faceted growth morphologies with special crystallographic planes or facets on the crystal surface. It is predicted and widely accepted that the growth mechanism of a faceted crystal would change from lateral to continuous growth with increasing solidification cooling rate,2 as shown in Fig. 1. This prediction is confirmed by some metalloid elements’ faceted phases, such as silicon and germanium,4–9 with a Jackson’s factor slightly over 2 under slow-cooling, near-equilibrium solidification conditions. However, little information is available concerning the
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0043 J. Mater. Res., Vol. 21, No. 2, Feb 2006
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growth mechanism transition versus solidification conditions for typical faceted crystals with strong atomic bonds and very high Jackson’s factor. As an interstitial phase with high melting point, excellent high-temperature stability and wear resistance, the carbide of group IVB and VB elements (hereafter called MC carbide) such as TiC, VC, etc, is an important strengthening phase for nickel-based superalloys, alloy tool steels, and many metal matrix composite materials (MMCs). Also, TiC-type MC carbide is well known to be a typical faceted crystal due to high melting entropy and a large Jackson’s factor of approximately 5–7.10 Therefore, investigation on the non-equilibrium solidification growth morphologies of MC carbide is very important t
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