Iron NitrogenTitanium - FeNTi 1 Iron Nitrogen Titanium...

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Iron Nitrogen Titanium Pierre Perrot Introduction Titanium exhibits a strong af nity for non metallic elements (C, N, O, S) and its presence in steels gives rise to a multiplicity of effects. The Fe-N-Ti ternary system is of great interest to clarify the interplay of preci- pitations in steels. Experimental investigations on phase equilibria and thermodynamics, mainly related to the nitrogen solubilities in liquid, α and γ phases are gathered in Table 1 . No experimental ternary phase diagram is known. The only information on the ternary system comes from the thermodynamic assessment been carried out by [ 1991Oht2 , 1998Jon ] whereas [ 1999Dum2 , 2001Lee ] uses the available solubility mea- surements to evaluate the iron rich corner of the diagram. A review on the phase equilibria in the Fe-N-Ti system may be found in [ 1992Rag , 2003Rag ]. Binary Systems The Fe-Ti system has been carefully reviewed by [ 1981Mur ] and thermodynamically assessed by [ 1998Dum ]. The Fe-N phase diagram in the solid state is accepted from the review of [ 1987Wri1 ]. The Cal- phad assessment carried out by [ 1991Fri ] and justi ed by the model proposed by [ 1994Fer ] give an insight on the phase equilibria under high nitrogen pressures. The N-Ti phase diagram in the solid state given by [ Mas2 ] is reproduced from the extensive review of [ 1987Wri2 ]. It is accepted with the exception of the region surrounding the subnitride domain, around the δ Ti 2 N 3 and Ti 2 N phases. Indeed, [ 1991Len ] shows, by diffusion experiment that δ 2 N 3 is metastable and present a diagram in which appears two new stable phases Ti 4 N 3 x and Ti 3 N 2 x . The accepted diagram, showing the new equilibria is reproduced in [ 1993Oka ]. The Calphad assessment carried out by [ 1991Oht1 , 1999Dum1 ] takes only into account the Ti 2 N subnitride. A more complex assessment [ 1996Zen ] takes into account the 3 subnitrides Ti 4 N 3 ,T i 3 N 2 and Ti 2 N. Solid Phases The solid phases are shown in Table 2 . TiN is a very stable nitride which may be obtained under very low nitrogen potential. Fe 2 TiN is an intermediate phase which may be prepared as a thin layer on a FeTi alloy (< 2 mass% Ti) at 400 575°C under a 92H 2 -8NH 3 atmosphere [ 1976Jac ] whose nitrogen potential does not allow the iron nitridation. The TiN 2 phase, not shown in the table, was described by [ 1981Pod ] as the result of the adsorption of one N per mole of TiN at the interface α (Fe,Ti)-TiN. A more extensive investigation of [ 1986Ric ] shows that, under the same atmosphere, α (Fe,Ti) supersaturates before the pre- cipitation of TiN. Guinier-Preston zones are formed, in which the ratio N/Ti may take any value between 1 and 3. Above 0.5 at.% Ti, Guinier-Preston zones may have the ideal composition Fe 4 TiN 3 . Below 0.5 at.% Ti, Ti-N monolayers include Fe to give platelets of composition Fe 1 x x N. The excess of N, in a less stable environment, may be easily reduced by H 2 . Once the ratio N/Ti = 1, the nitrogen cannot be removed by reduction. Fe may replace Ti in the TiN lattice [ 1996Kir ], and a homogeneous layer of the Fe 0.25 0.75 N composition has been obtained. The diffusion experiment carried out by [
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Iron NitrogenTitanium - FeNTi 1 Iron Nitrogen Titanium...

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