Precipitation of nanosized nitrides in plasma nitrided Fe–M (M = Al, Cr, Ti, V) alloys

Abstract

Nanosized alloy nitrides or alloying element (M)-Nitrogen(N) cluster formed in plasma nitriding of Fe-M binary alloys were investigated by means of high resolution TEM. Specimen surface of the Fe-Cr alloys was hardened by the formation of disk shaped CrN of NaCl structure, which is ,2 nm thick and 10 nm in diameter. In nitrided Fe-Al specimen, two kinds of AlN nitrides of metastable NaCl type and stable wurtzite type were formed near the specimen surface. Both of these were much larger than CrN. Contrasting to these specimens, in nitrided Fe-Ti and Fe-V alloys, high density M-N clusters of few nanometres diameter, many of which were monolayered lying on {001} a , were formed, leading to larger hardness increase than that in Fe-Cr and Fe-Al alloys. The formation of M-N clusters in nitriding is explained by the thermodynamical analysis of phase separation between M/N poor and M/N rich bcc phases.

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

“…Upon nitriding of binary Fe-Cr alloys, initially continuous precipitation of finely distributed, nano-sized, cubic NaCl-type CrN nitride platelets, with a Baker-Nutting orientation relationship (OR) with the ferrite matrix, occurs [8], [9], [11]- [13]. The precipitation of CrN platelets is very fast, due to a high (negative) Gibbs energy of formation of CrN (per mole N 2 ) 4 (-129.1 kJ mol -1 at 580 °C (853 K) and 1 atm).…”

“…During prolonged treatment time and given a sufficiently high Cr-content of the alloy (more than approx. 2 at.% Cr [9]), these fine precipitates can undergo a discontinuous coarsening reaction leading to the development of relatively small colonies/cells of alternating CrN and ferrite lamellae [9]- [13].…”

“…Upon nitriding of binary Fe-Cr [8]- [13] and Fe-Mo [7], [14] alloys initially cubic, NaCl-type CrN and cubic, NaCl-type γ-Mo 2 N (here 50 % of the octahedral-interstitial sublattice are not occupied by N), respectively, form as (semi-)coherent platelets with their broad faces parallel to the {100} α-Fe lattice planes and with a Baker-Nutting (BN) orientation relationship (OR) [15] with the ferrite matrix. The kinetics of formation of CrN is considerably faster than that of γ-Mo 2 N. This initial microstructure, composed of coherent nitrides finely dispersed in the ferrite matrix, is commonly referred to as continuously precipitated (CP).…”

“…The kinetics of formation of CrN is considerably faster than that of γ-Mo 2 N. This initial microstructure, composed of coherent nitrides finely dispersed in the ferrite matrix, is commonly referred to as continuously precipitated (CP). Upon nitriding for prolonged time, Fe-Cr alloys with sufficiently high Cr-content undergo a discontinuous coarsening (DC) reaction [8]- [13], whereas in Fe-Mo alloys a discontinuous precipitation (DP) reaction of γ-Mo 2 N to δ 3 -MoN occurs [7].…”

Effect of Pre-heat Treatment on Hardening Behavior in Gas Nitrocarburized Carbon Steel

“…On nitrocarburized steels containing a sufficient amount of alloying elements, such as Cr and Al, with high affinity for N, a lot of alloy nitrides are precipitated, and those nitrides contribute to hardening. [1][2][3] Low alloy steels which contain only a small quantity of nitride-forming elements also can be hardened by nitrocarburizing. [4][5][6][7] Quenching from nitrocarburizing temperature makes nitrogen into supersaturated solution.…”