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A Study on the Precipitation Behavior of $L2_1$-type Precipitates in B2-NiAl  

Han, Chang-Suk (Dept. of Defense Science & Technology, Hoseo University)
Han, Seung-Oh (Institute of Fusion Technology, Hoseo University)
Lee, Ju-Hee (Dept. of Mechatronics Eng., Graduate School of M.T.&M., Hoseo University)
Publication Information
Journal of the Korean Society for Heat Treatment / v.22, no.6, 2009 , pp. 345-353 More about this Journal
Abstract
The precipitates of $L2_1$-type $Ni_2AlHf$ phase in B2-ordered NiAl system has been observed by using transmission electron microscope (TEM). The hardness of as-quenched NiAl-Hf alloys is high due to the larger strengthening. However, age hardening of this alloy is not main effect to increase hardness compared to the large microstructural variations during aging. At the beginning of aging, the $L2_1$-type $Ni_2AlHf$ precipitates keep a lattice coherency with the NiAl matrix. The orientation relationship between the $Ni_2 AlHf$ precipitate and the NiAl matrix is <100>$_{Ni2AlHf}$//<100>$_{NiAl}$, {001}$_{Ni2AlHf}$//{001}$_{NiAl}$. By aging treatment for long time $Ni_2AlHf$ precipitates lost their coherency and change their morphology to the spherical ones surrounded by misfit dislocations. The orientation relationship between the NiAl matrix and the $Ni_2AlHf$ precipitates, however, has been kept even after longer aging time. The lattice misfit between the $Ni_2AlHf$ precipitate and the NiAl matrix has been calculated by the selected electron diffraction patterns, and the spacings of misfit dislocations is about 4.5% at 1173 K.
Keywords
Intermetallic compound; Precipitation hardening; Orientation relationship; Transmission electron microscope;
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