N.J. Doyle

The solid-solubility of oxygen in Nb and Nb-rich Nb-Hf, Nb-Mo and Nb-W alloys: Part I: the Nb-O system☆

Abstract The interstitial solid-solubility of oxygen in niobium has been studied by means of X-ray diffraction, micrographic and thermal techniques using both a dynamic leak method and a Sieverts apparatus. The lattice parameter of degassed, zone-refined niobium was found to be 3.2986 A, which is much lower than hitherto published values. At pressures above 10−11 torr O2, Nb is in thermodynamic equilibrium with oxide vapor and can take up to 6 at.% O2 into solid solution at 1775°C. Above this temperature oxidation is “catastrophic” and the volatile oxides Nb2O, NbO and Nb2O5 form on the surface. The “degassing” of Nb at 2200°C and above at a pressure of 10−6 torr O2 is, in effect, brought about by the volatilization of the oxide and not by the de-adsorption of gaseous oxygen.

The solid-solubility of carbon in Nb and Nb-RICH Nb-Hf, Nb-Mo and Nb-W alloys

Abstract By comparison with oxygen and nitrogen, the amount of carbon which can be taken into interstitial solid solution in Nb and Nb-rich Nb-Hf, Nb-Mo and Nb-W alloys is very small. Over the temperature range 1600°–2180°C the solid-solubility limit of carbon in niobium is about 0.55 at.%, and this is reduced by the alloying elements Hf and Mo. At 2000°C the solid-solubility limit of carbon in 95 Nb 5 Hf (the numbers being at.%s) is less than 0.2 at.%. It is probably less than 0.1 at.% C at 2000°C in the alloy 95 Nb 5 Mo. However, tungsten additions in amounts up to 10 at.% do not seem materially to influence the solid-solubility of carbon, which remains at about 0.55 at.%, as in the case of niobium. In the Nb-Hf-C system, below 2 at.% Hf, the phase in equilibrium with α-Nb solid solution is Nb2C, switching over to the extensive (Nb,Hf)C phase above 2 at.% Hf. On the other hand, Nb2C is in equilibrium with α-Nb phase containing up to 48 at.% Mo, above which the equilibrium switches over to α-Nb + (Nb,Mo)C. The same holds true for the corresponding Nb-W-C system, the switchover now occurring at 40 at. % W, approximately. It is highly probable that the extensive f.c.c. (Nb,Hf)C, (Nb,Mo)C and (Nb,W)C phases contain lattice vacancies in lieu of carbon atoms when the metal content exceeds 50 at.%.

The constitution diagram of the system molybdenum-osmium☆

Abstract The constitution diagram of the molybdenum-osmium binary system has been elucidated using a combination of micrographiec and X-ray diffraction techniques. The system contains four single phase fields, namely: 1. (a) The body-centered cubic α-Mo primary solid solution which retains approximately 19.5 at.% Os at the eutectic temperature of 2,380°C, dropping to approximately 7.0 at.% Os at 1,000°C. 2. (b) a narrow intermediate phase field ranging from about 24 to 25.5 at.% Os and having a cubic phase structure similar to that of β-W (A15 type), the phase being formed via a peritectoid reaction at 2,210°C between α-Mo and σ-phase. 3. (c) a tetragonal σ-phase which forms peritectically at 2,430°C between liquid containing approximately 22 at. % Os and θ-Os primary solid solution containing 48 at.% Os, the σ-phase field broadening out and extending at 1,000°C from 30 to 39 at.% Os. 4. (d) a terminal solid solution, θ, which is based on the hexagonal close packed structure of osmium, the phase extending from 48 at.% Os at 2,430°C, the temperature of the σ-phase peritectic horizontal, to a solid solubility limit at 61 at. % Os at 1,000°C. Lattice parameters have been evaluated for α, β, σ, and θ-phase alloys along with their Vickers hardness values. Analogies are drawn between the Mo-Os and W-Os systems and with other Mo and W alloy systems containing the platinum metals.

The solid solubility of nitrogen in Nb and Nbrich NbHf, NbMo and NbW alloys: Part I: The binary system NbN

Abstract Using a dynamic leak method, a Sieverts apparatus and a combination of X-ray diffraction and microscopical techniques, a study has been made of the interstitial solid-solubility of nitrogen in niobium as a function of pressure and temperature. It has been found that Nb can accommodate 9.48 at.% N interstitially at 2200°C and 3 × 10−1torr N2, but the amount which can be retained in solution at room temperature is very much smaller, being about 1.0 at.% depending on the quenching rate. The results are fully in accord with those of Gebhardt, Fromm and Jakob , and of Cost and Wert who obtain a relationship x = 6.2 × 10 −4 (p N 2 ) 1 2 exp ( 46,000 RT ) for the amount x, in at.% of nitrogen in solid solution, pN2 being the nitrogen pressure in torr, R the gas constant and T in degrees Kelvin. The lattice parameter of Nb is increased from a =3.2986 A to 3.3060 A with 1.05 at.% N retained in the lattice.

The constitution diagram of the tungsten-osmium binary system

Abstract The constitution diagram of the refractory alloy binary system tungsten-osmium has been determined using a combination of X-ray and micrographic techniques. The system contains three single-phase fields, namely: 1. (a) a body-centered cubic α-W primary solid solution which retains approximately 18.5 at. % Os at the peritectic temperature of 2945°C, dropping to approximately 6 at. % Os at 1200°C; 2. (b) a tetragonal σ-phase which forms peritectically at 22 at. % Os and at a temperature of 2945°C by reaction between α-W solid solution and liquid, the σ-phase field opening out and extending at 1200°C from 21 to 35 at. % Os; 3. (c) a hexagonal close-packed terminal solid solution based on θ-Os, whose solid solubility limit ranges from 46 at. % Os at the eutectic temperature of 2725°C to 68 at. % Os at 1200°C. Lattice parameters have been evaluated for α-, σ-, and θ-phase alloys along with their Vickers hardness values. Sigma-phase alloys have been found to be extremely hard and brittle and scratch glass very easily.

The constitution diagram of the molybdenum-hafnium binary system☆

Abstract The molybdenum-hafnium binary system has been determined. The body-centered cubic α-Mo primary solid solution extends to approximately 16.5 at.% Hf at 1000°C and 28 at.% Hf at 2165°C at which temperature it reacts peritectically with liquid to form e-MO2Hf, a hexagonal Laves phase of the C36-Cu2Mg type. A eutectic forms at 1915°C between e-Mo2Hf and the high-temperature β-Hf body-centered cubic primary solid solution which extends to 58.5 at.% Hf at the eutectic temperature. Between 1910° and 1750° the e-MO2Hf structure transforms to the cubic η C15-Cu2Mg form which reverts back to the original e-MO2Hf C36 form below 700°C. β-Hf transforms martensitically to hexagonal close-packed α-Hf at 1950°C, the phase field of the latter being very narrow, ranging from approximately 99.2 to 100 at. % Hf. β-Hf decomposes eutectoidally at 73.5 at.% Hf and 1215°C to form η-MO2Hf + α-Hf.

Solid solubility limits of Y and Sc in the elements W, Ta, Mo, Nb, and Cr☆

Abstract Alloys and diffusion couples of yttrium and scandium with the refractory elements Nb and Ta of Group V, and Cr, Mo, and W of Group VI have been examined by means of the optical microscope, X-ray diffraction, and mass spectrometry. The results indicate that the solid solubility of Y and Sc in these elements is less than I p.p.m. This is in conformity with the Hume-Rothery Rule that only a very restricted range of solid solution can be expected when the atomic diameter of solvent and solute differ by more than 14–15%.

The solid-solubility of oxygen in Nb and Nb-rich Nb-Hf, Nb-Mo and Nb-W alloys: Part III: The ternary systems Nb-Mo-O and Nb-W-O

Abstract Both molybdenum and tungsten additions reduce the interstitial solidsolubility of oxygen in niobium. Thus, at 1500°C, the solid-solubility of O 2 in Nb, which is 4.75 at.% at zero Mo, drops to about 1.75 at.% at 10.5 at.% Mo. Similarly, the addition of 10.5 at.% W causes the solid-solubility of O 2 at 1500°C to fall to 2.3 at.%. The lattice parameter of Nb is increased by the addition of O 2 but decreased by the additions of either Mo or W. However, unlike the Nb-Hf-O system, the mutual effects of O 2 , Mo and W on the lattice parameter of Nb are additive so that the isoparametric contours run as straight lines across the α-Nb phase field in the two ternary systems Nb-Mo-O and Nb-W-O.

The solid-solubility of oxygen in Nb and Nb-rich, Nb-Hf, Nb-Mo and Nb-W alloys: Part II: the ternary system Nb-Hf-O

Abstract The lattice parameter of b.c.c. niobium is increased when oxygen is taken into interstitial solid solution. It is also appreciably increased when hafnium is taken into solid solution substitutionally. Surprisingly enough, the addition of oxygen to a hafnium-containing niobium alloy does not necessarily produce an additional increase in lattice parameter as expected, but first produces a sharp decrease followed by an increase. It is found that a minimum occurs in the isoparametric curves at an O2to-Hf ratio of 2 : 1 and with a lattice parameter almost identical with that of pure Nb, namely 3.2986 A, this minimum passing through the Nb corner of the diagram and continuing into the tie-line linking the α-Nb solid-solubility limit with the phase HfO2. A possible reason for this behavior is the formation of Preston-Guinier zones of effective composition HfO2, or alternatively, a form of clustering occurs in which randomly dispersed “molecules” of HfO2 form within the Nb lattice.

The constitution diagram of the rhenium-hafnium system☆

Abstract The niobium-hafnium binary system has been studied using a combination of X-ray diffraction and microscopical techniques along with hardness and melting point determinations. The phase diagram is closely analogous to those of the niobium-titanium and niobium-zirconium systems. It comprises two single-phase fields, namely, that of the body-centered cubic Nb primary phase which forms an unbroken series of solid solutions with β-Hf above 1950°C, and the narrow hexagonal close-packed α-Hf phase which exists below 1950°C and forms via a martensitic transformation from β-Hf. The β α + β phase boundary stretches from pure Hf at 1950°C across the diagram until at 1000°C only 34 at. % Hf is in solution, whereas the α α + β boundary is almost vertical, α-Hf taking, at most, 2–2.5 at. % Nb into solid solution, the precise amount being uncertain owing to the Zr content of the Hf (approx. 4.41 at. % Zr).