Nitride Formation and Excess Nitrogen Uptake After Nitriding Ferritic Fe-Ti-Cr Alloys

Abstract

The microstructure of the nitrided zone of Fe-Ti-Cr alloys, containing a total of 0.30 at. pct (Ti + Cr) alloying elements, with varying Ti/Cr atomic ratio (0.45, 0.87, and 1.90), was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The stable TiN and CrN nitrides did not precipitate after nitriding. Instead, ultrafine, metastable, mixed Ti 1-x Cr x N nitride precipitates developed in the nitrided zone: The precipitates were of platelet morphology (length £30 nm and thickness £3 nm) and of cubic, rock-salt, crystal-structure type. The misfit strain around the nitride platelets in the ferrite matrix increases with increasing Ti/Cr atomic ratio. As a consequence, most pronouncedly for the highest Ti/Cr atomic ratio, a tetragonally distorted ferrite matrix surrounds the precipitates, as evidenced both by XRD and TEM. The amount of nitrogen taken up was determined quantitatively by measuring the so-called nitrogen-absorption isotherms. It follows that the absorbed amount of so-called excess nitrogen dissolved in the matrix and adsorbed at the nitride-platelet faces increases distinctly with increasing Ti/Cr atomic ratio. The results were discussed in terms of the dependence of misfit strain on the Ti/Cr atomic ratio and the higher chemical affinity of Ti for N than of Cr for N.

Surface Hardening and Nitride Precipitation in the Nitriding of Fe-M1-M2 Ternary Alloys Containing Al, V, or Cr

“…It is rather difficult to unambiguously determine the crystal structures of alloy nitrides because the diffraction patterns (upper right corner insets) are not clear. According to the previous reports on the nitriding of Fe-Al-Cr [17] and Fe-Cr-Ti alloys, [20] these precipitates should have B1 structure and a Backer-Nutting orientation relationship with the a matrix. Small spots that cannot be indexed to either a or B1 precipitates may correspond to an oxide layer on the thin film.…”

“…[17][18][19] They also found that (Cr, Ti) mixed nitrides are formed in the nitriding of Fe-Cr-Ti alloys. [20] Both V and Ti atoms have a strong affinity with N atoms. Therefore, the similar nitriding behavior between the Fe-Cr-V and Fe-Cr-Ti alloys can be understood.…”

“…[16] It has been reported that mixed nitrides are precipitated in the nitriding of Fe-Cr-Al [17][18][19] and Fe-Cr-Ti alloys. [20] Schwarz et al [21] very recently investigated the nitriding behavior of an Fe-Si-V alloy and reported that VN forms in the early stage of nitriding, and after some incubation time, Si 3 N 4 is precipitated separately. Thus, they proposed that the precipitation of VN retards the precipitation of Si 3 N 4 due to elastic strain by coherent VN precipitation.…”

Continuous and discontinuous precipitation in Fe-1 at.%Cr-1 at.%Mo alloy upon nitriding; crystal structure and composition of ternary nitrides

“…The nitriding time required to fully precipitate both Cr and Mo as NaCl-type ternary nitride in the ternary Fe-1Cr-1Mo alloy is much shorter than for the precipitation of Mo as NaCl-type binary nitride in the binary Fe-1Mo alloy [7]. nitrides based on kinetic constraints, was given previously for the mixed nitrides formed in nitrided ternary Fe-Cr-Ti [70] and Fe-Cr-Al [44], [71], [72] alloys. Indeed, the mixed, cubic NaCl-type (Cr,Ti)N and (Cr,Al)N nitrides, as holds for the mixed (Cr,Mo)N x nitride of the present study as well, were found to be metastable in these systems: decomposition into the separate binary nitrides can take place at elevated temperature [72]- [74].…”

Recent Developments on Discontinuous Precipitation

“…Recent studies on ternary Fe-Cr-Al [63] and Fe-Cr-Ti [64] alloys showed that in the ternary alloys also cubic NaCl-type, ''mixed'' ternary CrxAl 1-X N, and Cr x Ti 1-X N nitride platelets develop along {100} lattice planes of the ferrite matrix and in accordance with a Baker-Nutting orientation relationship. Similar research performed by Kang et al [65] in quaternary Fe-based alloys also allowed to identify ''mixed'' nature of a quaternary (Cr X ,V Y , Mo 1-X-Y )N nitride…”