Yoshiaki Shida

The effect of various ternary additives on the oxidation behavior of TiAl in high-temperature air

Twenty-four ternary element additions were made to a binary TiAl alloy (Ti−34.5 wt.% Al), and the oxidation behavior was studied. As a result of the oxidation tests in air at 1173 K for 360 ks, ternary elements were classified into three groups according to their effects, namely, (a) detrimental; V, Cr, Mn, Pd, Pt, Cu; (b) neutral; Y, Zr, Hf, Ta, Fe, Co, Ni, Ag, Au, Sn, O; (c) beneficial; Nb, Mo, W, Si, Al, C, B. This classification was valid for Cr, Mn, Mo, and W under several other temperature and time conditions. The influence of the additions was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. As a result of detailed examinations, it was confirmed that Cr and Mn additions caused linear-oxidation behavior from the outset at 1173 K, virtually no Al2O3 barrier being formed. This is probably due to the doping of those elements in TiO2. The beneficial elements, such as Mo, Nb, W, resulted in protectiveoxidation behavior. The characteristic features of the scale on those alloys were the presence of a continuous Al2O3 layer as the second layer from the outer surface and the relatively massive precipitation of Al2O3 in the vicinity of the scale-metal interface. Also, these alloys did not show any evidence of internal oxidation. The scale types and the proposed mechanism for the innerscale formation are described.

The influence of ternary element addition on the oxidation behaviour of TiAl intermetallic compound in high temperature air

Abstract A variety of ternary element additions were made to a binary TiAl (Ti-34.5 wt% Al) intermetallic compound and the oxidation behaviour was studied with particular interest in the influences of the ternary elements. As a result of the oxidation tests in air at temperatures between 1073 and 1273 K, the effect of various elements was classified into three groups, i.e. (a) detrimental—Cu, Y, V, Cr and Mn; (b) neutral—Sn, Zr, Hf, Ta, Ni and Co; (c) beneficial—Si, Nb, Mo and W. Their influence was very significant, the difference in the weight gain between the best and the worst alloys being approximately two orders of magnitude. Particularly, the W- and the Mn-modified alloys were approximately 200 and 100 K more resistant than the binary alloy. The mechanism for the improvement by the W and Mo additions may be explained as follows: the elements are enriched on the metal side of the scale/metal interface and may cause the formation of β and/or δ phases in which Al diffusion may be fast and oxygen solubility may be small. This leads to Al-enriched scale formation and/or Al 2 O 3 layer formation along the interface.

ENHANCED DIFFUSION OF OXYGEN DURING INTERNAL OXIDATION OF NICKEL-BASE ALLOYS

Abstract The depths of internal oxidation of Ni-Al (0.5–4 wt.%) alloys oxidized in the temperature range 800–1100°C in either Ni/NiO packs or in 1 atm oxygen are virtually independent of the alloy composition; with Ni-Cr (1–5 wt.%) alloys the depths decrease with increasing Cr content, in accord with classical theory. A model incorporating enhanced diffusion of oxygen along the incoherent interface between the internal oxide particles and the alloy matrix is proposed, since in the Ni-Al system the internal oxide particles show considerable elongation in the growth direction. In contrast, in the Ni-Cr system, the oxide particles are approximately spherical, and are not interconnected. Agreement between experiment and theory is satisfactory if the ratio of diffusion coefficients of oxygen along the interface and in the bulk lattice is of the order of 102−103. There are also differences between alloys oxidized in Ni/NiO packs, when no surface scale forms and samples oxidized in 1 atm oxygen when a surface Ni...

Internal-external transition for the oxidation of Fe-Cr-Ni austenitic stainless steels in steam

Several Fe-Cr-Ni austenitic stainless steels (Cr wt.%: 13–25, Ni wt.%: 15) were oxidized in steam for 1000 hr at 500–900°C. The oxide scales were examined and categorized with respect to the chromium concentration and the grain size of the base metal. Experiments showed three conditions for the critical bulk Cr concentration and the oxidation temperature at which the oxidation behavior changed drastically. Metallographic examination showed that two of these three conditions resulted from the internal-external transition of Cr2O3 either on the metal surface or along the grain boundaries of the base metal. Attempts were made to interpret these conditions from the available oxidation theories. Atkinsons treatment was employed with some modification to incorporate the grain-boundary diffusion of Cr in the base metal. The calculation basically explained the internal-external transition for the oxidation of these steels.

The morphological and structural development of internal oxides in nickel-aluminum alloys at high temperatures

The formation and development of internal oxides in Ni-Al alloys containing 1–4 wt.% Al in Ni-NiO packs and in 1 atm oxygen at 800 to 1100°C have been studied. The internal oxide particles were relatively fine, closely spaced, and mainly acicular, although more granular near the surface. They were identified as Al2O3 at the advancing front, but NiAl2O4 at the surface and at a significant distance from that surface. Growth of internal oxide particles resulted in the development of significant compressive stresses in the internal oxide zone when formed in Ni-NiO packs. These stresses led to grainboundary sliding at the higher temperatures and extrusion of weak, internal oxide-denuded zones adjacent to alloy grain boundaries. At the lower temperatures, these stresses also resulted in significant preferential penetration of oxides down grain boundaries and sub-grain boundaries. Stress development and resulting phenomena were much less significant during oxidation in 1 atm oxygen because vacancies injected from the external NiO scale accommodated the volume increase during growth of internal oxide particles.

The high-temperature internal oxidation and intergranular oxidation of nickel-chromium alloys

Abstract The development of internal oxides and intergranular oxides in dilute NiCr alloys, containing 1–5% Cr, in NiNiO packs and in 1 atm oxygen at 800–1100°C has been investigated. The internal oxide particles were relatively coarse and widely spaced and were Cr2O3, except for a narrow band adjacent to the surface where NiCr2O4 particles were also present. Several types of intergranular oxide were developed in the Ni/NiO packs, with preferential penetration being more extensive in the higher chromium-containing alloys at the lower temperatures. Discrete intergranular oxide particles were formed deep in the alloy beneath bands of Cr2O3 which developed over intersections of the alloy grain boundaries with the surface, or beneath continuous or discontinuous grain-boundary oxides near the surface, possibly due to the development of a relatively flat oxygen profile and a steep chromium gradient in the subjacent alloy. In the presence of a thickening NiO external scale, preferential intergranular oxidation was much less extensive than in the Ni/NiO packs as the rapid growth of the scale prevented development of Cr2O3-rich surface bands.

Development of preferential intergranular oxides in nickel-aluminum alloys at high temperatures

The development of intergranular oxides in dilute Ni-Al alloys containing 0.55–4.10% Al in Ni-NiO packs and in 1 atm oxygen at 800–1100°C has been examined. In the Ni-NiO packs, preferential intergranular oxide penetration as well as internal oxidation occurs in every case, except in the higher aluminum-containing alloys at 1100°C. Several different types of intergranular oxide morphology were observed, depending on alloy aluminum concentration and on temperature. The oxides in the more dilute alloys are thin and relatively continuous and are accompanied by preferential penetration of internal oxide particles in the adjacent grains. Thicker intergranular oxides are precipitated in the more concentrated alloys while, in some situations, numerous fine oxide particles are formed well ahead of the main intergranular oxide. The intergranular oxidation is facilitated by high stress development in the specimens due to increases in volume as internal and intergranular oxides are formed. These stresses create microvoids in the grain boundaries immediately ahead of the advancing internal and intergranular oxides, resulting in preferential nucleation and growth of further intergranular oxides. This is the case particularly at the lower temperatures where other stress-relief processes cannot operate. The resulting relatively continuous, incoherent intergranular oxide-metal interface allows a high flux of oxygen to the advancing intergranular oxide front. Preferential intergranular oxidation is much less extensive in the presence of a thickening external NiO scale, due to accommodation of the volume increases on internal oxide formation by vacancies injected into the alloy from the growing cationdeficient scale.

The development of internal and intergranular oxides in nickel-chromium-aluminium alloys at high temperature

The development of internal oxides, intergranular oxides and internal voids in Ni-15.1Cr-1.1Al and Ni-28.8Cr-1.0Al during oxidation in 1 atm oxygen at 1000° to 1200°C has been studied. In both cases, the formation of an external Cr2O3-rich scale causes vacancies to be generated in the alloy due to the different diffusion rates of chromium towards the alloy-scale interface and of nickel back into the bulk alloy. At 1000°C, condensation of these vacancies at the alloy grain boundaries facilitates formation of intergranular oxides while, at 1200°C, the vacancies condense to give voids in the grains and grain boundaries. Internal oxides are formed at both temperatures. The internal and intergranular oxides are mainly α-Al2O3, although some Cr2O3-rich oxides are produced near the alloy-scale interface. Possible mechanisms for the development of the internal and intergranular oxides in these alloys are discussed and related to the observed oxide morphologies and compositions.

Effect of aluminum and titanium additions to Fe-21%Cr-32%Ni on the oxidation behavior in an impure helium atmosphere at high temperatures

The effects of aluminum and titanium additions on the oxidation and carburization behavior of Fe-21%Cr-32%Ni in an HTGR-simulated impure helium environment at high temperatures were studied. Intragranular and intergranular oxidation were the principal forms of degradation, with the effect of aluminum being major and that of titanium relatively minor. As the aluminum content in the alloy increased, the mode of degradation changed from both uniform intragranular and intergranular oxidation, to one involving only uniform intragranular oxidation. This transition in the degradation mode was explained by the volume changes in the alloy resulting from a combination of both a volume increase due to internal-oxide precipitation and shrinkage due to the condensation of vacancies formed as a result of selective removal of alloying elements to the external scale. Intergranular oxidation was observed only when the resultant volume change was due to shrinkage. When the resultant volume change was positive, only uniform intragranular oxidation occurred and at the same time, extensive carburization was observed probably due to the deterioration of the surface scale caused by the deformation of the alloy substrate. A small amount of titanium, ca. 0.4%, appeared to modify the phenomena caused by aluminum additions, e.g., causing increased intergranular penetration for the 0.4% Al alloy and internal-to-external transition for the 2.1% Al alloy. External scale formation without any internal oxidation was observed for alloys containing more than 1.9% Al at 973 and 1023 K and for an alloy containing 2.1% Al and 0.4% Ti at 1073 and 1123 K. In these cases, carburization was almost completely eliminated.

Oxidation behaviour of alloy 800 in an impure helium atmosphere at high temperatures

Abstract The high temperature behaviour of Alloy 800 in impure helium (containing H 2 , H 2 O, CH 4 CO, CO 2 ) typical of the environment to be encountered in High Temperature Gas-cooled Reactors was studied, with particular emphasis on the carbon transfer and the selective oxidation behaviour. Under the conditions of the present study, only limited carbon transport was observed, as was expected from a thermodynamic study. The most important mode of damage was intergranular oxidation which was observed to develop relatively deeply into the substrate. The intergranular oxidation resulted from selective oxidation. The chromium depletion developed during the selective oxidation may have caused vacancies to be injected into the alloy. The vacancies may have diffused and finally condensed as voids in the grain boundary area, which may have provided a fast oxygen supply path, allowing oxide formation along them.