H. Mitsui

Phase equilibria among α (hcp), β (bcc) and γ (L10) phases in Ti–Al base ternary alloys

Phase equilibria between the α(A3), α2 (D019), β(A2 or B2) and the γ(L10) phases in the Ti–Al base ternary systems were investigated over the temperature range 1000–1300°C. The tie lines and the phase boundaries were determined by electron probe microanalysis using multiphase alloys. It was established that almost all the elements except Zr tended to partition into the β phase rather than into the α, α2 or the γ phase, while Zr mostly partitioned into the γ phase. At 1000°C, in the equilibrium state between the α2 and the γ phases, V, Cr, Mo, Ta and W partitioned to the α2 phase rather than to the γ phase, whereas Mn, Fe, Co, Ni, Cu and Zr tended to concentrate into the γ phase. The partition coefficients for the alloying elements were only slightly dependent on their concentration. Based on these data, the relative stabilizing effects of alloying elements on the α, α2, β and γ phases are discussed.

Recent progress in corrosion-resistant metastable alloys

Tailoring new corrosion-resistant alloys has recently been performed mostly by the sputter deposition technique. This technique is suitable for forming a single-phase solid solution even when the boiling point of one component is lower than the melting points of the other components and/or when one component is immiscible with another component in the liquid state. Aluminium-refractory metal, chromium-valve metal and molybdenum-chromium-nickel alloys have been successfully prepared in a single amorphous phase. Amorphous aluminium-refractory metal alloys are corrosion resistant in 1 M HCl and chromium-valve metal alloys are spontaneously passive in 12 M HCl, showing a better corrosion resistance in comparison with the alloy components. The amorphous aluminium-refractory metal alloys also have an extraordinarily high hot corrosion resistance. Their sulphidation resistance at higher temperatures is far higher than any other known metallic materials and their oxidation resistance is comparable to chromia- or alumina-forming alloys.

The sulfidation and oxidation behavior of sputter-deposited Al-Ta alloys at high temperatures

As a part of a systematic study to elucidate oxidation and sulfidation resistance of Al-refractory metal alloys at high temperatures, the behavior of sputter-deposited Al-(33–80) at.%Ta alloys has been examined at temperatures ranging from 1073 K to 1273 K in He-S2 atmosphere and in Ar-O2 atmosphere. The sulfidation kinetics of these alloys follow a parabolic rate law in an early sulfidation stage, although, in some cases, the sulfidation rates are decreased after prolonged sulfidation. The sulfidation resistance of these alloys is comparable to that of high purity tantalum and remarkably higher than those of typical high temperature alloys. The sulfide scales on these alloys comprise an outer aluminum-rich layer and an inner tantalum-rich layer. The formation of a protective inner tantalum sulfide layer is responsible for the excellent resistance to high temperature sulfidation. The oxidation kinetics of the Al-Ta alloys change with alloy composition and temperature. The oxidation of Al-33Ta initially follows a parabolic rate law, but after a particular period of oxidation, rapid oxidation is observed at high temperatures above 1173 K. In contrast, the oxidation rates of higher tantalum alloys decrease with oxidation time, although the oxidation rates in the early stage are higher than those of Al-33Ta. At the temperatures below 1123 K a rapid weight loss during oxidation was observed for the Al-Ta alloys. This seems to result from disintegration of these alloys due to the pest phenomenon.

The sulfidation of sputter-deposited niobium-base aluminum alloys

Abstract The sulfidation behavior of sputter-deposited niobium base Nb-Al alloys has been studied as a function of temperature (1073–1273 K), aluminum content (0.6–15.2 at%) and sulfur activity (40–4000 Pa) under isothermal-isobaric conditions. It has been found that under steady-state conditions the sulfidation process follows a parabolic rate law, the slowest step of the overall reaction rate being the outward diffusion of cations. The initial transient state of the reaction was longer when the aluminum content in the alloy and the temperature of sulfidation were higher. The sulfide scales on all the alloys consisted of the NbS 2 phase doped with aluminum and with a strongly developed growth texture, characterized by long columnar crystals situated perpendicular to the substrate surface. The e-axis of the hexagonal structure of niobium sulfide has been found to be parallel to the underlying alloy, i.e. the two-dimensional sulfide planes were situated perpendicular to the alloy surface. With increasing aluminum content in the alloy the structure of the scale changes from 3s-NbS 2 to 2s-NbS 2 . The surfaces of the sulfide scales on Nb-5.0A1 and Nb-15.2A1 alloys were transformed to Nb 2 O 5 and A1NbO 2 , respectively, as a result of interaction of the scale surface with trace amounts of oxygen.

Ordering and phase separation of BCC aluminides in (Ni, Co)–Al–Ti system

Abstract The phase equilibria and ordering reactions in the bcc aluminides of the Co–Al–Ti ternary and (Ni, Co)–Al–Ti quaternary systems were investigated mainly using the diffusion couple method. The phase boundaries between the (Ni, Co)Al (B2) and (Ni, Co) 2 AlTi (L2 1 ) phases, the critical compositions of the continuous ordering transition and the tricritical points were determined in the temperature range 1373–1573 K. It was found that the B2+L2 1 two-phase region in the composition square of NiAl–Ni 2 AlTi–Co 2 AlTi–CoAl narrows and then disappears due to increasing Co content or temperature, and that the maximum temperatures of the B2/L2 1 ordering reaction are lowered with the addition of Co. The phase stability of these aluminides is discussed and compared with those of the (Ni, Fe)–Al–Ti system which have been previously reported.

The high temperature sulfidation behavior of Nb-Al-Si coatings sputter-deposited on a stainless steel

Abstract The sulfidation behavior of amorphous 58at.%Nb-38at.%Al-4at.%Si and 59at.%Nb-35at.%Al-6 at.%Si coatings sputter-deposited on a stainless steel substrate of SUS 304 has been studied at 1 kPa sulfur vapor pressure over the temperature range of 973–1173 K. The scales formed on the coating were bilayered, primarily consisting of an outer non-protective layer of A1 2 S 3 and an inner barrier layer of NbS 2 . Beneath the coating the formation of (Fe, Cr, Ni) depleted, Al enriched zone was observed, which resulted from the mutual diffusion between the coating and the substrate. The coating improved effectively the sulfidation resistance of the stainless steel, although elements in the substrate diffused outwards in the coating and small amounts of their ions were contained in the sulfide scales. The high sulfidation resistance of the coated specimens seems attributable to the formation of a protective NbS 2 scale, instead of less protective FeNb 2 S 4 , even in the presence of iron, incorporated by outward diffusion from the substrate, in the coating.

Sulfidation- and oxidation-resistant alloys prepared by sputter deposition

Abstract In order to tailor novel alloys resistant to high temperature corrosion in multicomponent sulfidizing–oxidizing environments, amorphous or nanocrystalline Al-refractory metal alloys with and without silicon and Cr-refractory metal alloys have been prepared by sputter deposition. The sulfidation and oxidation behavior of the alloys has been studied as a function of temperature in sulfur vapor pressure of 10 3 Pa and in oxygen of 2×10 4 Pa, respectively. The sulfidation of these alloys generally follows a parabolic rate law, being thus diffusion controlled. The sulfidation rates of Al-refractory metal and Cr-refractory metal alloys are several orders of magnitude lower than those of conventional high temperature alloys and comparable to or even lower than those of the corresponding refractory metals. The sulfide scales formed on these alloys consist of two layers, comprising an outer Al 2 S 3 or Cr 2 S 3 layer and an inner refractory metal disulfide layer. The formation of the inner layer is attributed to the excellent sulfidation resistance of these alloys. The oxidation resistance of Al-refractory metal alloys is not sufficiently high, but the addition of silicon improves remarkably their oxidation resistance by synergistic effect of aluminum and silicon. Although the CrMo alloys possess poor oxidation resistance, due to the formation of volatile molybdenum oxide, the oxidation resistance of the CrNb and CrTa alloys is as high as that of typical chromia-forming alloys.

The sulfidation and oxidation behavior of sputter-deposited Cr-refractory metal alloys at high temperatures

Abstract Cr-Nb and Cr-Mo alloys have been sputter-deposited on to quartz substrate, and their sulfidation and oxidation behavior has been studied as a function of temperature and alloy composition in He-S 2 and Ar-O 2 atmospheres. The sulfidation of these alloys follows a parabolic rate law, being diffusion controlled. The sulfidation rates of Cr-Nb alloys decrease with increasing niobium content in the alloy, and the sulfidation resistance of the high niobium alloys is comparable with that of niobium. The sulfidation resistance of Cr-Mo alloys is independent of alloy composition, being comprable with that of molybdenum. The sulfide scales formed on these alloys consist of two layers, comprising an outer chromium sulfide layer and an inner layer composed mainly of refractory metal sulfides. The formation of the refractory metal sulfide scales is responsible for the high sulfidation resistance to sulfide corrosion. Under the oxidation condition the Cr-Mo alloys are rapidly oxidized due to the formation of volatile molybdenum oxide. The oxidation of Cr-Nb alloys proceeds accompanying partial breakdown and the restoration of the scales, but the average oxidation rates are almost the same as their sulfidation rates. Consequently, the Cr-Nb alloys possess high resistance to both sulfidation and oxidation at high temperatures.

High temperature corrosion of sputter-deposited Al–Nb alloys

Sputter-deposited Al-Nb alloys have excellent sulfidation resistance, although their initial sulfidation rate is high due to the preferential sulfidation of aluminum. The preferential sulfidation of aluminum leads to the enrichment of niobium at the metal/scale interface, and consequently, the protective NbS2 scale is formed on the Al-Nb alloys. The sulfidation rate of the Al-Nb alloys is lower than that of niobium. The resistance of these alloys to high temperature oxidation is higher than that of niobium. The oxide scale formed on the Al-Nb alloys at high temperatures is composed of a double aluminumniobium oxide and alumina. Because of the formation of the less protective double oxide, the oxidation resistance of Al-Nb alloys is lower than that of typical alumina-forming alloys.

Sputter-deposited amorphous Al–Mo–Si alloys resistant to high temperature sulfidation and oxidation

Sputter-deposited amorphous Al-(31–45)Mo-(6–21)Si ternary alloys were prepared to improve the oxidation resistance of the sulfidation-resistant Al-Mo binary alloys. The ternary alloys showed excellent oxidation resistance in air even at temperatures above the melting point of MoO3 (1069 K) when molybdenum content of the alloys was about 30at%. The oxidation rate of the Al-Mo-Si ternary alloys was almost comparable with those of typical alumina-forming materials. XPS analysis revealed that the addition of silicon depressed molybdenum oxidation and assisted the formation of alumina scale without molybdenum oxide on these alloys. The high oxidation resistance of the Al-Mo-Si alloys resulted from the fact that during the heating of the ternary alloys, in addition to the Al8Mo3 intermetallic compound, the silicon rich Mo5Si3, was formed instead of molybdenum rich AlMo3 formed in Al-Mo binary alloys. The Al-Mo-Si ternary alloy containing 6at% of silicon also showed the significantly high sulfidation resistance, but excess amount of silicon addition is detrimental for sulfidation resistance.