Subnanometer-scale chemistry and structure of α-iron/molybdenum nitride heterophase interfaces

Abstract

The local chemistry and structure of ␣-iron/molybdenum nitride heterophase interfaces is studied on a subnanometer scale by atom-probe field-ion microscopy (APFIM), three-dimensional atom-probe microscopy (3DAPM) and both conventional transmission electron microscopy (CTEM) and highresolution electron microscopy (HREM). Molybdenum nitride precipitates are generated by annealing Fe-2 at. pct Mo-X, where X ϭ 0.4 at. pct Sb or 0.5 at. pct Sn, at 550 ЊC or 600 ЊC, in an ammonia/ hydrogen mixture. Internal nitridation at 550 ЊC produces thin, coherent platelet-shaped molybdenum nitride precipitates. Nitridation at 600 ЊC generates a much coarser structure with semicoherent thick plate-shaped and spheroidal precipitates in addition to the thin-platelet structure. The APFIM and 3DAPM analyses of the heterophase interfaces show substantial segregation of the solute species Sn and Sb only at the coarse precipitates, with Gibbsian interfacial excesses of up to 7 Ϯ 3 nm Ϫ2 , whereas the broad faces of the thin platelets have no detectable segregation. The TEM and HREM analyses show that the coarse precipitates are semicoherent, whereas the thin platelets are either coherent or have much fewer misfit dislocations than geometrically necessary. This demonstrates that Sn and Sb segregation is related to the presence of misfit dislocations at the interfaces of the coarse precipitates.

Low temperature precipitation kinetics of niobium nitride platelets in Fe

“…At lower temperature, niobium nitrides have received recently a renewed interest [1,2,8] because a precursor phase forming at low temperature from the solid solution has been identified as being single plane NbN platelets, very similar to GP zones found e.g. in Aluminium alloys, and similar in shape to the early precipitates formed when nitriding steels [9][10][11][12][13]. Subsequently to these observations, their stability has been recently investigated by ab initio calculations [14].…”

Surface and Interface Analysis

“…Platelets of thickness <1 nm, similar to those produced by nitridation at 550°C, coexist with much coarser spherical precipitates 20-30 nm in diameter and large plate-shaped precipitates 2-10 nm thick. 2 The thin platelet structure is characteristic of a homogeneous reaction, but the size and distribution of the coarse precipitates are typical of a heterogeneously initiated discontinous reaction. 2 Selected-area one-dimensional atom probe analysis with the direction of analysis exactly perpendicular to the habit plane of the platelets allowed for accurate determination of the composition (in at.%) of these small platelets (Fe 7.7š0.1 Mo 50.0š2.1 Sn 0.2š0.2 N 42.1š2.1 (at.%)…”

Surface and Interface Analysis

“…Platelets of thickness <1 nm, similar to those produced by nitridation at 550°C, coexist with much coarser spherical precipitates 20-30 nm in diameter and large plate-shaped precipitates 2-10 nm thick. 2 The thin platelet structure is characteristic of a homogeneous reaction, but the size and distribution of the coarse precipitates are typical of a heterogeneously initiated discontinous reaction. 2 Selected-area one-dimensional atom probe analysis with the direction of analysis exactly perpendicular to the habit plane of the platelets allowed for accurate determination of the composition (in at.%) of these small platelets (Fe 7.7š0.1 Mo 50.0š2.1 Sn 0.2š0.2 N 42.1š2.1 (at.%) 2 ) with a metal/nitrogen ratio close to the ideal ratio of 1.5 for the Mo 3 N 2 phase.…”

“…2 The thin platelet structure is characteristic of a homogeneous reaction, but the size and distribution of the coarse precipitates are typical of a heterogeneously initiated discontinous reaction. 2 Selected-area one-dimensional atom probe analysis with the direction of analysis exactly perpendicular to the habit plane of the platelets allowed for accurate determination of the composition (in at.%) of these small platelets (Fe 7.7š0.1 Mo 50.0š2.1 Sn 0.2š0.2 N 42.1š2.1 (at.%) 2 ) with a metal/nitrogen ratio close to the ideal ratio of 1.5 for the Mo 3 N 2 phase. Because the solute component (Sn or Sb) does not influence the formation and composition of the nitride significantly, this composition is used for the Sb-containing alloy investigated here.…”