Toshio Kainuma

Effects of oxygen, nitrogen and carbon additions on the mechanical properties of vanadium and V/Mo alloys

Abstract Effects of oxygen, nitrogen and carbon additions on the mechanical properties at room temperature of vanadium and V-Mo alloys containing up to 25 at% molybdenum were studied. The V-O, N or C and V-Mo-O, N or C alloy systems, respectively, were prepared by heating vanadium and the V-Mo alloys with oxygen, nitrogen or propane gas in sealed quartz capsules. The prepared alloys were homogenized, argon-quenched and analyzed for oxygen, nitrogen and carbon content. Then they were examined by tensile and hardness tests at room temperature, SEM observations and TEM studies. Oxygen and nitrogen additions to vanadium and the V-Mo alloys raise the hardness by solid solution, with nitrogen being more effective. Carbon additions to vanadium form coarse precipatates of V2C along the grain boundaries, which do not raise the hardness, while those to the V-Mo alloys form fine precipitates of vanadium carbide, homogeneously distributed in the matrix, which raise the hardness. Threshold concentrations of oxygen, nitrogen and carbon for embrittlement (fracture without plastic deformation) of these alloy systems decrease with increasing molybdenum concentration, those of carbon being most detrimental. Also, it was shown that the embrittlement of the V-Mo alloys after exposure to liquid sodium may be explained not only by solid solution hardening due to absorbed oxygen but also by combined effects of absorbed oxygen, nitrogen and carbon.

Creep and creep rupture properties of unalloyed vanadium and solid-solution-strengthened vanadium-base alloys

Abstract The creep and creep rupture properties of vanadium and vanadium-base alloys were studied at 700 and 1000 °C. The alloys were vanadium-base binary alloys containing about 5–21 at.% Al, Ti, Nb, Ta, Cr, Mo or Fe, three V-20wt.%Nb-base ternary alloys containing 5 or 10 wt.% Al, Cr or Mo, V-10wt.%Ta-10wt.%Al and V-25wt.%Cr-0.8wt.%Zr. The creep rupture stress of the binary alloys, except the V-Al and V-Ti alloys, increased linearly with increasing concentration of the alloying elements. The V-Nb alloy had the best properties with respect to the rupture stress and creep rate at 700 °C and the rupture stress at 1000 °C, but the V-Mo alloy appeared likely to have better creep properties at longer times and higher temperatures. Of the five ternary alloys, V-20wt.%Nb-5wt.%Cr and V-20wt.%Nb-10wt.%Mo showed the best creep properties. The creep properties of these two alloys were compared with those of other vanadium alloys and of type 316 stainless steel.

Mechanical properties of vanadium-base binary alloys

Abstract The fabricability and tensile properties of solid-solution-strengthened vanadium-base binary alloys containing aluminium, titanium, niobium, tantalum, chromium, molybdenum or iron were studied. The alloys were prepared by non-consumable arc melting and contained 150–370 ppm O, 20–250 ppm N and 40–130 ppm C. The VTi, VNb and VTa alloys were superior to the other alloys in their fabricabilities into sheet. The tensile properties were examined between room temperature and 1000 °C. The tensile strength values of the VNb and the VTa alloys were higher than those of the VAl and the VCr alloys. The tensile strengths of the VAl, VCr and VMo alloys increased in proportion to the increase in atomic per cent of alloying element, while those of the VNb, VTa and VFe alloys increased with the square root of the atomic per cent of the alloying element.

Mechanical properties of molybdenum single crystals produced by means of secondary recrystallization

Abstract Tensile and creep-rupture characteristics of molybdenum single crystals, produced by the secondary recrystallization method, were measured. The crystals used had axial-orientations of 10–20° from the 〈100〉-direction. The yield strength of single crystals appreciably increased and simultaneously their ductility decreased abruptly with decreasing temperature. Ductile-to-brittle transition behaviour of single crystals was markedly improved by adding a small amount of carbon. Yield and tensile strengths and ductility of single crystals were insensitive to temperature at elevated temperatures. The creep resistance of single crystals at 700 and 1000°C was inferior to that of the recrystallized polycrystals. However, the difference in the creep resistance between these crystals tended to diminish at higher temperatures.

Effects of alloying and heat treatment on the endurance limit of molybdenum in high cycle fatigue tests

The endurance limit (δσw) of molybdenum was investigated on four different materials in push-pull type high cycle fatigue tests at room temperature. The ΔσW of recrystallized (AN) specimens was determined as 780 MN/cm2 in stress range, Independent of materials, while that of stress relieved (SR) specimens was larger for TZM alloy than for other materials. The fracture toughness was estimated as 22.5 and 11.5 MN/m32 for SR and AN specimens, respectively. Intergranular fracture of AN specimens indicated that the ΔσW can be governed by the decohesion strength of grain boundaries which is not affected by alloying.

Effects of oxygen on corrosion of V-Nb-Mo, V-Nb-Cr, and V-Ti alloys in liquid sodium

Abstract The corrosion of V/20wt%Nb/10wt%Mo, V/20wt%Nb/5wt%Cr, and V/20wt%Ti alloys exposed to high-purity liquid sodium has been examined. The oxygen absorption, the dissolution of solid metals into the sodium, and the deterioration of mechanical properties of the alloys were suppressed by decreasing the oxygen concentration in the sodium. The changes of tensile properties of the V/20wt%Nb/10wt%Mo and V/20wt%Nb/5wr%Cr alloys exposed to the sodium are interpreted in terms of oxygen in solid solution, and those of the V/20wt%Ti alloy in terms of internal oxidation of titanium in the alloy. The 700°C tensile elongation values of the V/20wt%Nb/10wt%Mo and V/20wt%Nb/5wt%Cr alloys were more than 15 and 20 percent, respectively. The room-temperature ductility of the three alloys was substantially reduced.

Mechanical properties of high strength vanadium-base ternary alloys

Abstract High strength V-20wt.%Nb and V-10wt.%Ta base ternary alloys containing aluminium, chromium and molybdenum were studied. The fabricability and tensile properties at room temperature and in a temperature range 700–1000 °C were measured. The tensile strength values were estimated on the basis of the values for the corresponding vanadium-base binary alloys and compared with the experimental data. The estimated values are in good agreement with the experimental data. From these results, V-20wt.%Nb-5wt.%Al, V-20wt.%Nb-5wt.%Cr and V-20wt.%Nb 10wt.%Mo alloys were selected as alloys that could be recommended for their good high temperature mechanical properties.

Ductility of V-Cr and V-Cr-Zr alloy ingots

Abstract The ductility and the fracture modes of V-(5–20wt.%)Cr, V-15wt.%Cr-(up to 1.09wt.%)Zr and V-20wt.%Cr-0.2wt.%Zr alloy ingots containing about 1000 ppm O were studied at temperatures between −50 and 1000 °C. Elongation-temperature curves and maps of the fracture modes of these alloy ingots were obtained from the results of tensile tests and fractography. The elongations of the V-Cr alloy ingots were improved by the addition of small quantities of zirconium and the optimum addition for the V-15wt.%Cr alloy was 0.15–0.3 wt.%.

Improvement of mechanical properties of Mo alloys in CTR gaseous environment

Abstract The effects of vacuum annealing and the addition of carbon or boron on the tensile properties of Mo alloys exposed to helium containing oxygen of 0.1 vpm at 1273 K were studied. The vacuum annealing was effective in desorbing oxygen which penetrated into the grain boundaries and to recover the mechanical properties of molybdenum if the molybdenum did not contain active elements. The carbon additions to molybdenum improve the tensile properties of molybdenum exposed to helium containing impurity oxygen. Molybdenum containing carbon more than 0.1 % showed a good resistance against embrittlement due to the oxygen contamination. The carbide layer formed along the grain boundaries was considered to prevent the oxygen penetration into the grain boundaries.