The formation of AIN crystals in an Fe-Mn-Al-C alloy during the nitriding process
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I. INTRODUCTION
THE equilibrium shape (habit planes) of any isolated single crystal has the attribute that Aii is a minimum, where Ai is the area of the interface at a specific orientation (i) and i is the corresponding interface energy per unit area. Thus, the equilibrium shape can be predicted to be the inner envelope of the plot,[1] which satisfies the condition set out in the Wulff construction.[2] However, while a single crystal formed in the ambient atmosphere, the surface energy is primarily contributed from the extra energy associated with the broken bonds of the atoms at the surface. The adsorption of the gas molecules from the atmosphere to the surface atoms would occur as a result of the reduction of the surface energy from physical or chemical bonding with the surface atoms. If the entropy effect on the surface was negligible, it is reasonable to consider only the energy term of the surface energy for the growth of the single crystals. For the growth of AlN single crystals in the ambient atmosphere, most of the AlN crystals grow along the AlN 0001hcp direction with a morphology of long-straight whisker shapes, i.e., rod-, blade-, and platelike shapes, etc.[3,4] Nevertheless, while the single crystals form within the bulk material, the interface energy between the crystal and matrix is not simply like that in the ambient atmosphere. Variations in composition even at the interface happen, and the interface energies change. Moreover, when precipitation in matrix was considered, the coherency of the interface and misfit strain energy must also be taken into account. Therefore, the aforementioned energy term has to be modified to Aii Es, where Es is the misfit strain energy. When a single crystal nucleates and grows as a result of precipitation in the matrix, the interface energies at different contact areas between the precipitate and the matrix could be different. The equilibrium shape of the single crystal could differ from that forming in the ambient atmosphere. In our previous work,[5] the AlN JYH-HONG JAW, Postdoctoral Candidate, WEI-CHUN CHENG, Associate Professor, and CHAUR-JENG WANG, Professor, are with the Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan R.O.C. Contact e-mail: weicheng@mail. ntust.edu.tw Manuscript submitted August 10, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
crystal with the shape of a Widmanstätten side plate formed in the fcc matrix of a Fe-Mn-Al-C alloy. We observed that only the {0001} basal planes of the AlN crystal with an hcp crystal structure matched with the {111} planes of the fcc metal matrix. This matched interface is with the lowest interface energy, and the other contact interfaces of the AlN/matrix are with a much higher interface energy. Hence, the higher match degree leads to less “broken bonds,” which corresponds to a lower interface energy. The lower interface energy leads to a more stable interface. Since the {0001}AlN planes match to {111}fcc planes of the matrix, the interface e
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