Effect of Y and Ti on FeCrAl resistance heating alloy oxidation at
There is recent discrepancy on the effect of reactive elements on the oxidation rate of alumina producers. While the reduction is nowhere close to 1-2 orders of content observed in chromia producers containing a reactive element, there is a repeated decrement in the oxidation rate for alumina producers comprising of Reactive element oxide dispersion. The particular case of an oxide dispersion is mentioned because in the case of ion implantation or particularly alloy inclusions, internal and external oxidation of the reactive element can show a significant weight gain. In the chromia and alumina producers, decrease in rate constant is due to an observed decrease of cation boundary diffusion. The decreased effect in alumina producers over chromia producers is attributed to the minor difference between cation and anio transport in the producer. It seems that the reduction in oxidation rate decreases with temperature.At 1400oC, the reduction factor is less than 2. It may be due to a reduction of the cation diffusion contribution in pure alumina or an increase in lattice diffusion relative to the boundary diffusion that increases at low temperatures.
The alloy inclusion of titanium seems to be most favourable reason for the observed spallation nature of MA956. Stresses linked with thick layers should have rooted similar failure in every FeCral resistance heating alloys not only in MA956. A similar argument implies to the significant effects of iron, chromium or reactive element enriched particles. Although, it is tough to create any direct assessments of Kanthal alloy APM and MA956. While possessing titanium lesser than MA956, Kanthal alloy APM also has a 0.3% silicon inclusion and a coarser oxide dispersion.
The influence of yttrium and titanium in this investigation seems to differentiate two types of failures. The initial kind of failure is that linked with the major scale exfoliation, exposing the base metal. There is a significant evidence to link this kind of failure to the segregation of the indigenous sulfur to the alloy- oxide interface. When sulphur seems to weaken the bond between oxide and metal, some stress, due to development of the oxide layer or thermal nonuniformity while quenching, is also needed to start failure. This kind of failure was found on the FeCrAl without reactive element inclusion. As an aside, the spallation of this alloy at 1400oC was less harsh than at temperature about 1200oC.
Another kind of failure can be found that doesn't seem to be directly connected o the interfacial sulphur segregation. This kind of failure generally takes place only while quenching and includes a second phase particle that could possess a thermal difference with the scale matrix or undergo a phase change. In specific cases, in which the particle is specifically large, the palling can be thoroughly on the metal interface. In this study, the relatively small TiN particles do not give rise to major failure of the thick scale but seem to start microcracks.
The source of nitrogen is significant, as is the stability of the nitride in the oxide layer. Varying the reaction condition from air to oxygen described no significant effect. A more preferable source is Inco use of air as a mechanical alloying condition that may promote the production of TiN that is more stable nitride than Iron, Chromium or Aluminium nitrides. Nitrides available in the alloy may develop in the growing scale and become more effective crack initiators, causing the final spallation.
At temperature limit of 1400oC, TiN is stable with nitrogen dissociation pressure of 10(-11) atm, although both TiO and TiO2 have a reduced free energy of production than TiN. While kinetic factors may restrict the transformation to the more stable oxide, oxygen activity in the scale could show a thermodynamic problem.
If the activity of oxygen decreased precipitously near the gas interface, a TiN particle in an Al2O3 matrix would be lower the dissociation pressure of TiO or TiO2. Aluminium and Yttrium producing more stable oxides, would avoid production of TiO.
TEM cross section study of the metal oxide interface is a significant method to search for segregation there. Y in MA956 and Zr in Kanthal alloy APM have been observed to segregate this interface at 1000 and 1200oC. XTEM samples are prepared from MA956 oxidized at 1400oC for further study.
A minor reduction in oxidation rate is ascribed to the segregation of Y to the oxide limit where it prevents cation diffusion. The reactive element inclusion also enhances oxide adherence at temperature about 1400oC.
Rather than at low temperatures where the titanium in the FrCrAl resistance heating alloy segregates strongly to oxide grain limits, at 1400oC Titanium segregates only slightly. Instead, Titanium is supposed in TiN particles in the scale.
After extended oxide times, these TiN particles seem to start intergranular cracking inside the oxide scale, but not spallation at the alloy oxide interface.
In the short term cyclic testing at 1400oC, these TiN particles do not appear to have any considerable effect on adherence and no spallation to the oxide metal interface was supposed.
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