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A Study on the Precipitation Behavior of $L2_1$-type $Ni_2AlTi$ Phase in B2-Ordered NiAl System  

Han, Chang-Suk (Dept. of Defense Science & Technology, Hoseo University)
Publication Information
Journal of the Korean Society for Heat Treatment / v.20, no.4, 2007 , pp. 187-194 More about this Journal
Abstract
A transmission electron microscope (TEM) investigation has been performed on the precipitation of $L2_1$-type $Ni_2AlTi$ phase in B2-ordered NiAl system. The hardness after solution treatment is high in NiAl-Ti alloys suggesting the large contribution of solid solution strengthening in this alloy system. However, the amount of age hardening is not large as compared to the large microstructural variations during aging. At the beginning of aging, the $L2_1$-type $Ni_2AlTi$ precipitates keep a lattice coherency with the NiAl matrix. By longer periods of aging $Ni_2AlTi$ precipitates lose their coherency and change their morphology to the globular ones surrounded by misfit dislocations. Misfit dislocations, which are observed on {100} planes of H-precipitates have the Burgers vector of a <100> with a pure edge type. The lattice misfits of NiAl-$Ni_2AlTi$ system is estimated from the spacings of misfit dislocations to be 1.1% at 1273 K. The lattice misfits decrease with increasing aging temperature in this system.
Keywords
intermetallic compound; precipitation; age hardening;
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