J. Steinmetz

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.

Aging and overaging processes in Pb-0.08%Ca-x%Sn alloys using transmission electron microscopy and differential scanning calorimetry

Abstract The precipitation hardening and overaging mechanisms in three Pb-0.08%Ca- x %Sn alloys were investigated by means of hardness and calorimetric measurements (DSC) and transmission electron microscopy (TEM) observations. It is found that the highest tin content, close to 2 wt.%, allows delaying the overaging phenomenon in ternary Pb–Ca–Sn alloys. DSC studies show that tin has a favourable effect on the aging by providing further nucleation sites. Furthermore, the precipitate coarsening due to overaging shifts to higher temperatures upon increasing the Sn content. Using TEM to observe the microstructural transformations shows the presence of an epitaxial relationship between precipitates and the matrix during both aging and overaging. In the course of overaging, coarse precipitates adopt a lamellar morphology and their growth occurs preferentially along the [111] compact atomic plane direction. The tin content of coarse precipitates of (Pb 1− x Sn x ) 3 Ca increases with rising overaging time and follows the tin content of the initial alloy.

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.

Single-crystal studies of non-stoichiometric CeNiSi2-type phases in the Ce-Co-Si and Lu-Co-Si systems

Two orthorhombic structures were studied using single-crystal X-ray diffraction data and electron microprobe analyses: Ce(Co1−xSix)Si2 with x = 0.17(1) (a = 4.143(1) A, b = 16.505(3) A, c = 4.076(1) A) and LuCo1−xSi2 with x = 0.36(2) (a = 3.928(2) A; b = 15.888(7) A, c = 3.857(1) A). Both crystallize in the Cmcm space group and are non-stoichiometric CeNiSi2 (or BaCuSn2)-type phases corresponding to two different types of defects. In LuCo1−xSi2, the transition metal site is only partially occupied by cobalt whereas in Ce(Co1−xSix)Si2 the same site is fully occupied by a mixture of cobalt and silicon atoms. Ce(Co1−xSix)Si2 compounds annealed at 1273 K have a homogeneity range which has been verified for 0 ⩽ x ⩽ 0.17. However, as-cast LuCo1−xSi2 compounds were only obtained in the range 0.15 ⩽ x ⩽ 0.36.

Precipitation hardening in Pb–0.08wt.%Ca–x%Sn alloys—the role of the pre-ageing

Abstract The precipitation hardening mechanism in both supersaturated and pre-aged Pb–0.08%Ca–x%Sn alloys was investigated by means of hardness and calorimetric measurements (differential scanning calorimetry, DSC) and transmission electron microscopy observations. DSC studies have shown that a pre-ageing treatment of the ternary alloys at 363 K has a favourable effect by reducing the driving force of the overaging process. Furthermore, this preliminary treatment seems to give a finer and denser particle distribution than for only quenched alloys. This latter effect could explain the higher hardness of the pre-aged alloys. For the tin-rich alloy (≈2.0 wt.%), the pre-ageing treatment considerably reduces the micro-segregation of Sn at the grain (sub-)boundary. This phenomenon is accompanied by an increase of the precipitate tin content and also by the appearance of a lattice parameter mismatch of 2.9% between the L12 precipitate and the lead matrix. From all of these experimental results, it appears that the processes leading to the reduction in interfacial energy are not the rate determining step for the overageing.

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 NbPd&Al and NbRuAl 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  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.

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.

Sputtered stainless steel-carbon coatings as a substitute for hard electrolytic chromium for potential applications in mechanics

Abstract The industrial feasibility of crystalline and amorphous stainless steel-carbon coating sprepared by reactive magnetron sputtering on flat and cylindrical steel substrates is clearly shown, especially concerning their carbon enrichment and their good mechanical properties (adhesion, internal stress, microhardness, wear and friction behaviour). The excellent intrinsic corroison resistance of such coatings deposited on glass slides leads one to seek optimized sputtering conditions that will allow the same excellent behaviour to be achieved in the case of coatings deposited on steel substrates.

Structure of “TaCo4Si3”, a new silicide closely related to the Y13Pd40Sn31 stannide

Abstract A new ternary compound “TaCo 4 Si 3 ” has been studied using single-crystal X-ray diffraction data. The crystal structure is closely related to that of Y 13 Pd 40 Sn 31 with space group P6/mmm and Z = 21, a = b = 17.141 (3) A , c = 7.906(2) A , D x = 9.14 g cm −3 , μ (MoKα) = 54.5 mm −1 , F(000) = 4924, R = 0.049 for 308 contributing unique reflections and 36 refined parameters. In the five compounds, Y 13 Pd 40 Sn 31 , Li 13 Ni 40 Si 31 , Sc 12.7 Ni 40.7 Ge 31 , NbCo 4 Si 3 and TaCo 4 Si 3 , the structures can be described as a stacking along the c -axis of two elemental “Zr 4 Al 3 ” and “CaCu 5 ”-type blocks, but the nature and the z -coordinate of the atom on the 2(e) site (0 0 z ) are not well determined. In the case of TaCo 4 Si 3 , these uncertainties are solved.