• Korean Journal of Metals and Materials
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• v.49 no.2
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• pp.93-103
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• 2011

Precipitation behavior of high-nitrogen duplex Fe-24Cr-7Mn-4Ni-4Mo-0.43N stainless steel aged at $850^{\circ}C$ was investigated using scanning transmission electron microscopy. Based on the analyses of selected area diffraction patterns, four kinds of precipitates (intermetallic sigma (${\sigma}$) and chi (${\chi}$), $Cr_2N$ and secondary austenite) were identified. At the ferrite/austenite phase boundary, the ${\sigma}$ phase and secondary austenite were formed via ${\alpha}{\rightarrow}{\gamma}+{\sigma}$ eutectoid reaction. The precipitation of $Cr_2N$ occurred at the austenite grain boundary as well as the interior of the ferrite. The intermetallic ${\chi}$ phase also formed within the ferrite and showed a cube-cube orientation relationship with the ferrite. Further aging produced a lamellar structure composed of $Cr_2N$ and austenite along the ferrite/austenite boundary and enhanced the precipitation of the ${\chi}$ phase. The crystallographic features of the precipitates were also examined in terms of the orientation relationship with the austenite or ferrite matrix.

• Journal of the Korean Society for Heat Treatment
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• v.29 no.5
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• pp.227-233
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• 2016

Phase transformation phenomenon at high temperature was investigated by using designed TiAl-Nb alloys with addition of the ${\beta}$ stabilizer. Examination of dendritic morphologies in arc-melted button ingot could reveal the crystallography of the primary solidification phase. It was found that the addition of ${\beta}$ stabilizer(Nb) shifted the high temperature region of the binary Ti-Al phase diagram to the high Al composition side so that ${\beta}$ phase forms as a primary crystal even at higher Al composition compared with the binary Ti-Al system. The ${\beta}$ was found to be the primary solidification phase for alloys with Al content less than about 52 at.%. The composition of ${\beta}$ solidification in Ti-Al-Nb ternary system could be determined from the partial liquidus projection which was constructed by observing the microstructure of arc-melted buttons. The Ti-46Al-(6, 8)Nb composition was selected for ${\beta}$ solidification and the directional solidification was performed by a floating zone-type DS apparatus at the growth rate 30 mm/hr respectively.