Michel Vilasi

New silicides for new niobium protective coatings

Efforts to improve at high temperature the oxidation resistance of pure niobium or commercial niobium alloys have led to the development of a pack cementation process for the co-deposition of Si, Ti, Cr and Fe. Owing to the knowledge of the quaternary Nb(Ti)-T-Cr-Siphase diagrams (T5Fe or Co or Ni) and of the crystallographic features of phases present in the silicide coatings, new protective coatings have been applied on pure niobium and Cb752 alloy. The results of the crystallographic study of three new silicides isostructural with Nb Fe CrSi , in which Nb is substituted by Ti and Fe by Co or Ni are reported. The oxidation performances of two 33 6 new coatings mainly consisting of such a silicide are also outlined.

Phase equilibria in the Nb-Fe-Cr-Si system

Abstract Efforts to improve the CVD process used to elaborate protective coatings against oxidation for niobium alloys have needed a good knowledge of the quaternary Nb–Fe–Cr–Si phase diagram. Many phase equilibria have been established at 1473 K and a new quaternary, Nb 6.6 Fe 1.4 Cr 4 Si 8 , phase has been identified and studied using single X-ray diffraction data. The crystal structure is of a new type, derived from the Cr 11 Ge 8 -type structure (space group Pnma, Z =4, a =13.649(2) A, b =4.953(1) A, c =16.386(3) A, R = R w =5.7%). Higher temperatures stabilize the ternary Nb 2 Cr 3 Si 3 silicide in the Mn 5 Si 3 -type structure (space group P6 3 /mcm, Z =2, a =7.190(2) A, c =4.860(1) A, R =3.2%, R w =2.7%). The two structures are characterized by presence of Si octahedra sharing faces and edges.

Electrochemical study of chromium corrosion in Na2O-xSiO2 melts

The systematic study of the chromium corrosion in Na 2 O-xSiO 2 melts is performed with varying glass basicity using electrochemical techniques. The metal can exhibit several typical behaviors (active or passive state) depending on the melt composition. Each possibility is described in terms of corrosion layer morphologies and thicknesses. The effects of the melt basicity, preoxidation treatment of the metal, and temperature are studied. The conclusions are given by putting face-to-face the Na 2 O activities in the melt with quantitative data extracted from this work.

Experimental study of the SnO2–ZrO2 phase diagram

Abstract The study of the SnO 2 –ZrO 2 phase diagram in the 1230–1750xa0°C temperature range has shown the existence of an immiscibility gap, leading to two (Zr 1− x Sn x )O 2 and (Sn 1− y Zr y )O 2 limited solid solutions. Four compositions were synthesised for each solid solution, leading to pure phases, which were characterised by room-temperature and high-temperature X-ray diffraction. The unit-cell parameters of tetragonal (Sn 1− y Zr y )O 2 , monoclinic (Zr 1− x Sn x )O 2 and tetragonal (Zr 1− x Sn x )O 2 were determined and correlated with the content of the substituted atom. The monoclinic to tetragonal and reverse reactions for the (Zr 1− x Sn x )O 2 series were also characterised (transition temperatures) when varying the tin mole fraction.

Caractérisation de trois nouvelles phases ternaires, Nb(Pd, Al)2 et Nb(Ru, Al)2 de type MgZn2, et NbRu2Al de type BiF3, dans les systémes NbPdAl et NbRuAl

The phase diagrams of both Nbue5f8Pd&ue5f8Al and Nbue5f8Ruue5f8Al were investigated in order to elaborate new niobium base alloys, with two-phase structures A2-B2 or A2-L21. Three new aluminides were identified. The first, NbRu2Al, was studied using single-crystal X-ray diffraction data; NbRu2Al is a Heusler phase alloy with space group Fm3m and Z = 4, a = 6.135(1) A, Dx = 9.27 g cm−3, μ(Mo Kα) = 172.4 cm−1, F(000) = 568, R = 0.033 (Rw = 0.034) for 37 contributing unique reflections. The other phases, with formula Nb2(TxAl1−x)4 where T ue5fc Pd, Ru and 0.17(Pd) (0.19(Ru)) < x < 0.26(Pd) (0.29(Ru)), were characterized by the Laves phase MgZn2-type structure.

Isothermal section at 1673 K of the Mo–Ru–Si diagram and crystallographic structures of ternary phases

Abstract Efforts to improve the high temperature behavior of MoSi 2 in oxidizing environments led to the investigation of the Mo–Ru–Si phase diagram. The isothermal section at 1673 K was determined by X-ray diffraction, optical and scanning electron microscopies and EPMA. Five new silicides were identified and their crystallographic structure was characterized using conventional and synchrotron X-ray as well as neutron powder diffraction. Mo 15 Ru 35 Si 50 , denoted α-phase, is of FeSi-type structure, space group P2 1 3, a =4.7535 (5) A, D x =7.90 g. cm −3 , Bragg R=7.13. Mo 60 Ru 30 Si 10 is the ordered extension of the Mo 70 Ru 30 σ-phase with space group P4 2 /mnm, a =9.45940(8) A, c =4.94273(5) A, D x =6.14 g. cm −3 , Bragg R=5.75.

Etude de l'oxydation à 850°C de l'alliage NiCoCrAl(Y)Ta coulé et projeté au chalumeau plasma sous pression réduite

Resume Lalliage etudie (base nickel, 22–24 wt.% Co, 18–21 wt.% Cr, 7,5–9,5 wt.% Al, 0,7–0,8 wt.% Y, 3,5–4,5 wt.% Ta, specification Amdry 997) est utilise pour lelaboration de revetements par projection au chalumeau plasma sous pression reduite. Le choix de la temperature de 850°C a ete conditionne par une etude complementaire sur la corrosion a chaud par Na2SO4 de ce meme alliage. La nature et la composition des phases a lequilibre ont ete examinees en premier lieu. Les aluminiures γ, γ′, β et une phase σ sont presents dans les deux types dalliages (coule et projete), ainsi quun compose intermetallique M5Y (M ue5fc Ni + Co + Cr + Al). Dans les structures projetes, yttrium est present egalement sous forme dY2O3. Letude de loxydation de ces alliages a 850 °C, a lair, en conditions isotherme et de cyclage thermique, a montre linfluence determinante de leur structure. Dans les alliages coules, les dimensions importantes des grains des principaux constituants (γ et β) favorisent lheterogeneite de la couche doxyde. Dans ce cas en effet, elle est constituee principalement de Cr2O3 sur les grains de la phase γ et de Al2O3 sur ceux de β. La cristallisation beaucoup plus fine de lalliage projete favorise au contraire la formation dune couche homogene et compacte dAl2O3 modifiee par Cr2O3. Les deux alliages presentent une bonne tenue en oxydation cyclique avec des desquamations de couches protectrices tres limitees. Enfin leur comportement a ete compare dans les memes conditions a un alliage coule ne contenant pas dyttrium. Parallelement letude de loxydation des differentes phases constitutives de lalliage Niue5f8Coue5f8Crue5f8Alue5f8Yue5f8Ta a ete entreprise, a la meme temperature. Elle a permis de mieux mesurer linfluence de chacune dentre elles sur les processus doxydation de lalliage polyphase.

Protective Coatings for Niobium Alloys: Manufacture, Characterization and Oxidation Behaviour of (TiXCr) 7 Si 6 with X = Fe, Co and Ni

Intermetallic single phases (TiXCr)7Si6 (with Xxa0=xa0Fe, Co or Ni) were manufactured by uniaxial hot pressing. Their full characterization (SEMxa0+xa0XRD) highlighted that the nature of the metal constituent X influenced the chromium content in the silicide compositions. Both isothermal and cyclic oxidation studies at a temperature as high as 1300xa0°C evidenced very low degradation rate for all compounds and lifetime at 1100xa0°C exceeded 4700 cycles. All phases developed a duplex protective layer of chromia and silica. It was observed that the chromia:silica ratio depended on the nature of X, and, particularly for the Ni-containing specimen, also on the temperature. Co- and Ni-containing specimens developed pure chromia during oxidation whereas Fe ones develop (CrxFe1−x)2O3, reducing drastically the CrO3 volatilization during exposure at temperature higher than 1000xa0°C.

Effect of Water Vapor on the Oxidation Mechanisms of a Commercial Stainless Steel for Interconnect Application in High Temperature Water Vapor Electrolysis

High temperature water vapor electrolysis is one of the most promising methods for hydrogen production. The interconnect is a key component in the electrolyse technology. In a previous paper, the high temperature corrosion resistance and the electrical conductivity of a commercial ferritic stainless steel, K41X (AISI 441), were assessed in both anode (95xa0%O2–5xa0%H2O) and cathode atmospheres (10xa0%H2–90xa0%H2O). In cathode atmosphere, ageing tests performed up to 1,000xa0h revealed the formation of a duplex oxide scale: an inner layer consisting of protective chromia and an outer layer comprised of a magnetite-type iron oxide. In this study, we further investigated the oxidation mechanisms of K41X alloy in cathode atmosphere by means of marker experiments using an inert marker (Au) and isotopes. SEM-EDX and SIMS characterizations were combined in order to determine the oxide scale growth processes. The roles played by hydrogen and water vapor are discussed and a diffusion mechanism is postulated.

Crystal structure of triniobium triiron chromium hexasilicide Nb≈3Fe≈3Cr≈1Si6: an intergrowth of Zr4Co4Ge7 and Nb2Cr4Si5 blocks

Abstract A new quaternary compound Nb≈3Fe≈3Cr≈1Si6 was studied using single-crystal X-ray diffraction data. The crystal structure is of a new type with space group P4 2 /mbc and Z=8, a=b=16.558(3) A , c=4.940(2) A , D x =6.54 g cm −3 , μ( Mo Kα )=136.5 cm −1 , F(000)=2470 and R=0.034 (R w =0.044) for 515 contributing unique reflections and 41 parameters. The structure of Nb≈3Fe≈3Cr≈1Si6 can be characterized as a combination of elemental blocks of the Zr4Co4Ge7- and Nb2Cr4Si5-type structures.