Nitridation effects on the oxidation mechanisms of an ods Fe–Al intermetallic alloy

Abstract

An ODS FeAl intermetallic alloy has been nitrided at low-energy high flux implantation diffusion leading to the formation of hexagonal AlN and α-Fe segregation. Its high temperature behaviour is subsequently evaluated under isothermal oxidation at 800º C. Microscopy techniques (SEM, TEM) will show that nitridation hinders aluminiumoutward diffusion and enhances iron outward diffusion giving rise to a layered structure. 1.-IntroductionUpon the high temperature oxidation of titanium aluminides, TiN and/or Ti 2 AlN subscales have been observed to develop at the substrate/oxide interface [1,2], which sometimes have been referred to provide an effective barrier for outward Al diffusion [3] or to prevent oxygen dissolution in the alloy [4] thus improving their oxidation resistance. When these alloys are nitrided, the same nitrogen containing phases appear [5] but only detrimental effects have been reported to occur compared to non nitrided specimens owing to a lower amount of Al 2 O 3 developed at the top surface [6][7][8].In the case of iron aluminides without oxygen-active elements, their oxidation behaviour is worse in air than in pure oxygen due to internal nitridation below a defective alumina scale [9]. Pint et al. polishing in SiC# 4000 emery paper was performed down to a thickness of about 50 μm. Then, Ar bombardment at 3 keV was carried out in a GATAN PIPS™ (precision ion polishing system) model 691 at different angles. 3.-Results 3.1.-NitridationThe surface of the nitrided specimens appeared to be homogeneous with protrusions at the top surface and remains of porosity. The former seemed to be pinned by yttria

Oxidation mechanisms of low energy-high flux nitrided ODS FeAl intermetallic alloy

“…The k p values (Fig 3b) from the first part of the oxidation curve is near of the measured values when pure iron is oxidised at 400°C [15]; then this observation could be consistent with the presence of α-Fe detected after the implantation process [16,17]. Conversely, the second parabolic constant is much lower and compares well to the one of the untreated FeAl.…”

Multiscale analysis of an ODS FeAl40 intermetallic after plasma-assisted nitriding

“…The APT analyses were conducted on a CAMECA-LEAP 4000 HR, with a base temperature of 80 K, a pulse repetition rate of 200 kHz and an average detection rate of 0.1%. Several peak overlaps in APT mass spectra were encountered, and treated as follows: the common isobar between 54 Cr ++ and 54 Fe ++ was assigned to Fe, because of very low Cr concentration, and the possible overlap between 27 Al + and 54 Fe ++ was neglected in favor of Fe, as the presence of 27 Al + is marginal in Fe based materials. In addition, peaks at 14, 14.5 and 15 Da were assigned to N + and N 2 ++ ions.…”

“…This additional handicap could slow down their future applications in which high loads and strong surface contacts are applied to the FeAl structural material. Thus, several studies have proposed surface engineering approaches such as thermal oxidation [19,24,25], boronizing [26] and nitriding treatments (gaseous atmosphere, ion implantation, plasma-assisted) [27][28][29] to improve the surface hardness. At the same time, as was proved by mechanical tests on preoxidized samples of B2 iron aluminides [19,25], these techniques should provide a physical protecting layer to avoid the moister-induced embrittlement originating from the interaction, at the crack tip, of the aluminium atoms with the hydrogen contained in the environment.…”

“…Among all the previously mentioned surface treatment processes, it has been shown that the nitriding treatments are the most promising for hardening and that they lead to thick and compact protective layers. In order to better understand the response of the B2 FeAl alloy to nitriding, particularly those containing large amounts of Al, few pioneer studies have been conducted to gain a deeper scientific understanding of the mechanisms involved during the growth of the nitride layers [27][28][29]. Owing to the complexity of the nitride microstructures in the B2 alloys, it is likely that the possible proposed mechanisms for nitriding remain still debateable mainly due to the fact that the nitrided FeAl microstructure has not yet been observed and described at very local scales.…”

Graded Nitrogen Ingress in FCC Metallic Structures and the Related Microstructures and High Temperature Oxidation Behaviour

“…However, the significant nitrided depths cannot be exclusively explained in terms of radiation-enhanced diffusion and an explanation based on quasiparticle-enhanced mobility has also been put forward [24]. The above mentioned studies relate to steels and only very few works previously discussed the nitriding mechanisms in Ni-base austenitic substrates [25][26][27][28][29][30], let alone reported on the high temperature oxidation resistance of nitrided materials [31][32][33] in contrast to corrosion resistance. Therefore, this work aims at studying the diffusion mechanisms and the resulting microstructures and composition of the nitrided layers in three austenitic substrates, namely pure Ni, Ni20Cr and a Fe20Cr10Ni (AISI 304L) austenitic steel.…”