Crystal structure and morphology of AIN precipitating on nitriding of an Fe-2at.% Al alloy

Crystal Structure and Morphology of Mixed Cr1–x Al x N Nitride Precipitates: Gaseous Nitriding of a Fe-1.5 Wt Pct Cr-1.5 Wt Pct Al Alloy

“…[5,[22][23][24][25][26][27][28] The results obtained in this work show unambiguously that nitride precipitates developing in Fe-Cr-Al alloys are of the mixed type: cubic, rock-salt structure type Cr 1-x Al x N precipitates (Sections III-C-1 through 3). This finding can be interpreted as a metastable, intermediate stage of nitride precipitation in this system: Indeed, annealing of the nitrided system at an elevated temperature (168 hours at 973 K) caused replacement of the mixed cubic, rock-salt structure type Cr 1-x Al x N precipitates by the equilibrium cubic, rock-salt structure type CrN and equilibrium hexagonal, wurtzite structure type AlN, involving a separation of the alloying elements (XRD, TEM, and STEM analyses presented and discussed in Reference 55).…”

“…[5,17,50] Tiny plateletlike structures are visible in the DF images taken from the rock-salt 002 MeN spots of two different variants of the Bain OR (the circles in the SADP) pertaining to nitride precipitates, which are oriented perpendicular to each other (Figures 3(a) and (b)). The length of the precipitate platelets can be estimated from the DF images in Figures 3(a) and (b) and is approximately 10 nm (and smaller).…”

“…[11,33] In annealed Fe-Al alloys, the nucleation of the thermodynamically stable hexagonal wurtzite structure type AlN is difficult. [23,25,26,34] The transmission electron microscopy (TEM) results of Biglari et al [5] from a Fe-2 at. pct Al alloy revealed the presence of two AlN modifications after long-time nitriding: (1) the hexagonal wurtzite structure type AlN exhibiting a PitschSchrader [35] OR with the ferrite matrix and (2) the cubic, rock-salt structure type AlN showing a Bain OR with the ferrite matrix.…”

“…Three modifications of chromium nitride can be distinguished, as follows: (1) the cubic, rock-salt structure type CrN, (2) the hexagonal structure type Cr 2 N, and (3) an orthorhombic CrN modification, which occurs only at very low temperatures (cryogenic modification) and is not relevant for the nitriding of iron-based alloys. [29] For aluminum nitride, three modifications also exist, as follows: (1) the at-ambient pressure thermodynamically stable hexagonal wurtzite structure type AlN, (2) the metastable cubic, rock-salt structure type AlN, [5] and (3) the metastable zinc blende structure type AlN, [31,32] which has never been observed in nitrided iron-based alloys. In ferritic Fe-Cr alloys, the formation of cubic, rock-salt structure type CrN platelets with a Bain-type OR with the ferrite matrix was observed.…”

“…[7,[11][12][13] The nitride-precipitation kinetics during (gaseous) nitriding has been classified according to the type of alloying element-nitrogen interaction (weak, intermediate, or strong). [14] The nitriding of binary Fe-Cr [6,11,[15][16][17][18][19][20][21] and Fe-Al [5,[22][23][24][25][26][27][28] alloys has been investigated rather extensively. Three modifications of chromium nitride can be distinguished, as follows: (1) the cubic, rock-salt structure type CrN, (2) the hexagonal structure type Cr 2 N, and (3) an orthorhombic CrN modification, which occurs only at very low temperatures (cryogenic modification) and is not relevant for the nitriding of iron-based alloys.…”