Methods of improving the high-tempera ture strength and corrosion resistance of vanadium and certain alloys used in reactors

We present new evidence for the high-temperature strength of vanadium and low-alloyed (up to 15 at.%) vanadium-based alloys under conditions of prolonged (up to 5000 h) high-temperature (973–1173 K) exposure to vacuum, helium, lithium and sodium melts, modeling reactor heat-transport media (10 Pa). The kinetics of phase transformations in alloys of the systems V−Zr−C, V−Nb−Zr−C, V−Mo−Zr−C, and V−Ti−O in the process of prolonged aging under stress is analyzed. It is established that the process of pre-decay of a solid solution has many stages and is intensified by the action of stresses. The corresponding isothermal diagrams of the decay of VTsU (V−Zr−C) alloy are constructed. We give recommendations as to the choice of alloying elements with the aim of enhancing the high-temperature strength of vanadium and vanadium alloys used for reactors. New alloys developed on the basis of the systems V−Ti−O, V−Nb−Zr, V−Ta−Hf, V−Cr−Sc, and V−Cr−Nd and a new method of hardening thermochemical treatment of vanadium-titanium alloys with the use of oxygen from air have found application in pilot-engineering developments for high-temperature nuclear power plants. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 3, pp. 91–104, May–June, 1999.     

Surface characterization

The biocompatibility of commercially pure titanium and its alloys is closely related to their surface properties, with both the composition of the protecting oxide film and the surface topography playing an important role. Surfaces of commercially pure titanium and of the two alloys Ti–6Al–7Nb and Ti–6Al–4V (wt %) have been investigated following three different pretreatments: polishing, nitric acid passivation and pickling in nitric acid–hydrogen fluoride. Nitric acid treatment is found to substantially reduce the concentration of surface contaminants present after polishing. The natural 4–6 nm thick oxide layer on commercially pure titanium is composed of titanium oxide in different oxidation states (TiO2, Ti2O3 and TiO), while for the alloys, aluminium and niobium or vanadium are additionally present in oxidized form (Al2O3, Nb2O5 or V-oxides). The concentrations of the alloying elements at the surface are shown to be strongly dependent on the pretreatment process. While pickling increases the surface roughness of both commercially pure titanium and the alloys, different mechanisms appear to be involved. In the case of commercially pure titanium, the dissolution rate depends on grain orientation, whereas in the case of the two alloys, selective -phase dissolution and enrichment of the -phase appears to occur. © 1999 Kluwer Academic Publishers     

Distribution of interstitial impurities in titanium-vanadium alloys after holding in liquid lithium

The distribution of interstitial impurities between liquid lithium and solid titanium-alloyed vanadium is investigated. It is shown that titanium alloying of vandium intensifies nitrogen and carbon transport from lithium into the solid metal. At the same time, the concentration of oxygen in the surface layers of the specimens increases as a result of penetration of lithium and formation of complex oxides of the Me _x Li _y O _z type. Increase in the titanium content leads to an increase in the carbon concentration on the surface and to the formation of protective carbon films that inhibit the diffusion of interstitial impurities into deep-seated layers.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 56–60, September–October, 1993.     

Delayed fracture and the nature of substructural defects formed in aluminum alloys due to sustained loading

We analyze and generalize experimental and theoretical data on substructural defects associated with dislocation pileups, hydrogenation, and excess vacancies formed in Al-Mg, Al-Mg-Li, and Al-Zn-Mg-(Cu) alloys subjected to long-term loading under normal conditions. It is shown that the key role in the formation of defects of the last two kinds is played by selective oxidation of magnesium and/or lithium. We investigate the interaction of defects of various kinds, including the interaction of dislocations with vacancies and vacancies with hydrogen in the processes of delayed fracture and deterioration of mechanical properties of alloys. One should especially mention the possibility of formation of substitutionally dissolved hydrogen (SDH) as a result of the interaction of the second type. The maximum possible amount of SDH accumulated in the entire service life of the alloy is equal to the concentration of the elements susceptible to selective oxidation. It is proposed that SDH embrittles grain boundaries of the alloy by the mechanism of diffusion creep and facilitates shifts of dislocations in pileups. The ideas suggested in the present work enabled us to explain, by using experimental data on the distribution of alloying elements in the vicinity of grain boundaries, all known facts about the influence of metallurgical factors on the susceptibility of alloys subjected to long-term loading under normal conditions to delayed fracture and embrittlement.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 5, pp. 27–33, September–October, 1994.     

Nitrogen Addition to an O-1 Tool Steel

A new processing technique makes nitrogen alloying possible by adding nitrogen under elevated nitrogen pressure to prealloyed Fe-C ingots during continuous casting, producing a whole new class of precipitation-free, iron–carbon–nitrogen alloys. When both carbon and nitrogen bulk concentration levels exceeded 0.5 wt%, a duplex fcc-/(bcc-bct-) Fe microstructure resulted that is iron carbide- and nitride-free. With increasing carbon and nitrogen concentrations, there was an increase in the retained fcc-Fe phase. In cooling rate studies, increasing carbon and nitrogen concentrations shifted the knee of the fcc-Fe-to-bcc-Fe phase time–temperature–transformation (T–T–T) curve to longer times. Hardness, compression strength, and wear resistance increased with increasing carbon and nitrogen concentrations and were superior to iron–carbon alloys without the nitrogen addition.     

Comparison of the cytotoxicity of molybdenum as powder and as alloying element in a niobium–molybdenum alloy

Commercially pure metal niobium (c.p. Nb) as well as niobium–molybdenum (Nb–Mo) alloys were produced following several powder metallurgical routes. In brief, niobium and molybdenum powders were blended and milled in order to form Nb–Mo alloys. The alloy powders and the c.p. Nb were then either pressed and sintered, or cold isostatically pressed followed by hot isostatically pressing. In order to assess the cytotoxicity of the c.p. Nb and c.p. Mo powders, a 72 h minimal essential medium-extraction test was performed according to ISO/EN 10993–5. The cytotoxicity of the c.p. Nb metal and the Nb–Mo alloys was tested in a 72 h direct contact test. Compared to a negative control (UHMWPE), c.p. Nb was non-toxic, but c.p. Mo was moderately toxic. None of the powder metallurgically produced materials were toxic. Neither differences in molybdenum concentration, nor in porosity of the samples, due to different production routes, had any influence on the toxicity of the materials. Rat bone marrow cultures showed that only on c.p. Nb was a mineralized extracellular matrix formed, while on the more porous Nb–Mo alloys, cell growth was observed, but no mineralization. In conclusion, c.p. Mo powder is moderately toxic, however, as an alloying element it is non-toxic. Material porosity seems to influence differentiation of bone tissue in vitro. © 1998 Kluwer Academic Publishers     

Solid-State Reactions during Thermal Oxidation of Vanadium-Modified GaAs Surfaces

It is shown that vanadium layers deposited by magnetron sputtering onto GaAs accelerate surface oxide growth during thermal oxidation. The processes occurring during thermal oxidation on the surface and at the semiconductor–metal interface are interpreted under the assumption that vanadium deposition leads to the formation of an interfacial V _x Ga _y As _z layer, which plays a key role in determining the mechanism of V/GaAs oxidation.     

Simulation of processes of high-temperature gas corrosion of titanium alloys in vacuum

A physicomathematical model of the process of high-temperature interaction of titanium alloys of the system Ti-Al-Mn in a rarefied medium has been considered that takes into account gas saturation of the metal and sublimation of the alloying element. The example of the alloy OT4-1 illustrates the fact that the model representations and the experimental data are in satisfactory agreement. The model representations can be apparently extended also to other types of alloys. The model can be used for expert estimation of the effect of a vacuum in the heat treatment or the utilization of titanium alloys.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 6, pp. 29–34, November–December, 1990.     

Solidification microstructures and mechanical properties of high-vanadium Fe–C–V and Fe–C–V–Si alloys

Fe–C–V and Fe–C–V–Si alloys of various C, V and Si compositions were investigated in this work. It was found that the phases present in both of these alloy systems were alloyed ferrite, alloyed cementite, and VC_x carbides. Depending on the alloy composition the solidified microstructural constituents were granular pearlite-like, lamellar pearlite, or mixtures of alloyed ferrite + granular pearlite-like or granular pearlite-like + lamellar pearlite. In addition, it is shown that in Fe–C–V alloys the C/V ratio influences (a) the type of matrix, (b) the fraction of vanadium carbides, f_v and (c) the eutectic cell count, N_F. In Fe–C–V alloys, a relationship between the alloy content corresponding to the eutectic line was experimentally determined and can be described by where C_e and V_e are the carbon and vanadium composition of the eutectic. Moreover, in the Fe–C–V alloys (depending on the alloy chemistry), the primary VC_x carbides crystallize with non-faceted or non-faceted/faceted interfaces, while the eutectic morphology is non-faceted/non-faceted with regular fiber-like structures, or it possesses a dual morphology (non-faceted/non-faceted with regular fiber-like structures + non-faceted/faceted with complex regular structures). In the Fe–C–V–Si system, the primary VC_x carbides solidify with a non-faceted/faceted interface, while the eutectic is non-faceted/faceted with complex regular structures. In particular, spiral eutectic growth is observed when Si is present in the Fe–C–V alloys. In general, it is found that as the matrix constituent shifts from predominantly ferrite to lamellar pearlite, the hardness, yield and tensile strengths exhibit substantial increases at expenses of ductility. Moreover, Si additions lead to alloy strengthening by solid solution hardening of the ferrite phase and/or through a reduction in the eutectic fiber spacings with a decrease in the alloy ductility.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

Inelastic Properties of Amorphous and Nanocrystalline Fe–P–Mn–V Alloys

The processes occurring in amorphous Fe–P–Mn–V alloys at elevated temperatures, up to the crystallization temperature, are investigated by internal friction measurements and x-ray diffraction analysis. The results are used to evaluate the activation energy of defect migration and the particle size of the crystalline phases precipitating in the alloys during annealing. It is shown that increasing the vanadium content of amorphous Fe–P–Mn–V alloys increases the energy of defect migration and reduces the particle size of the precipitating phases.     

Mechanism and regularities of oxidation in Nb-Ti and Nb-Ti-Si systems

We study the processes of high-temperature oxidation in Nb-Ti, Nb-Ti-Si, and Nb-Ti-Si-Mo systems and propose a mechanism of oxidation of VN-10 niobium alloy highly alloyed with titanium (up to 50%), a Ti-Nb-Ti composite multilayer material, and unalloyed niobium and VN-10 alloy subjected to diffusion saturation of their surface with silicon and molybdenum. It is shown that high alloying and (to a lesser extent) Ti-cladding of niobium allow one to suppress the process of catastrophic high-temperature oxidation of it and increase significantly the heat resistance of the material. Additional formation of diffusion silicide and molybdenum-silicide layers on the surface of niobium alloys increases their heat resistance by more than a factor of ten.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 107–115, January–February, 1995.     

Evolution of the structure and phase composition of MeVa-MeIVa alloys under the action of liquid lithium

We present the results of a thorough investigation of the processes occurring on the surface and inside Me_VA-Me_IVA alloys as a result of their long-term contact with liquid lithium. With V-Ti alloys as an example, we determine the role of nonmetallic impurities (O, N, C) in corrosion processes and suggest a mechanism of the formation of structure and phase composition of the investigated alloys under the action of liquid lithium. In general, the redistribution of interstitials in Me_VA-Me_IVA alloys leads to the formation of a heterogeneous structure that consists of three zones: (I) a zone where lithium interacts with a base metal, (II) a single-phase zone of a V-Ti-O (C, N) solid solution formed as a result of the dissolution of the oxide phase, and (III) a two-phase zone of internal nitriding with particles of oxynitrides of variable stoichiometry. It is also demonstrated that the directions and rates of diffusive flows of nonmetallic atoms on the melt-metal interface are controllable by rational alloying.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 6, pp. 58–63, November – December, 1994.     

Concept of Formation and Work of CaO Coatings in the Mode of Self-Healing

On the basis of an analysis of obtained results and literature data, we propose a kinetic model of the interaction of components in the Li[Ca]–V[O] system. The model demonstrates the cyclicity of the process. Each cycle consists of the following stages: (1) transfer of oxygen from vanadium to lithium, (2) growth of an external CaO oxide layer with simultaneous dissolution of dispersed titanium oxides in the matrix and transfer of oxygen and titanium into the solid solution, (3) penetration of the liquid metal into the solid one by the reaction-diffusion mechanism, and (4) decay of metastable triple oxide and restoration of an external CaO oxide layer. Within the framework of the model proposed, we develop the concept of work of the CaO oxide coating under isothermal conditions and in the heating–cooling mode.     

Effect of rare-earth elements on the microstructural characterization in rapidly quenched thermally strengthened aluminium alloys

The influence of rare-earth elements on the microstructural features of rapidly solidified Al93.3-xFe4.3V0.7Si1.7Mmx(x=0, 0.5, 1.0, 3.0, 6.0) alloy was systematically studied by differential scanning calorimetry, X-ray diffraction, transmission electron microscopy and energy dispersive X-ray analysis. Experimental results show that there are different type of phase transformation depending on mischmetal (Mm) concentration. For Al87.3Fe4.3V0.7Si1.7Mm6.0 metallic glass, a shoulder was observed on the high-angle side of the main peak in the X-ray diffraction patterns due to quenched-in aluminium nuclei and a prepeak resulting from Mm–Mm pairs. By means of particle extraction analysis, it has been proved that the -Al13(Fe, V)3Si phase existing in as-cast Al–Fe–V–Si alloy is wholly or partly inhibited for Al93.3-xFe4.3V0.7Si1.7Mmx (x=0.5, 1.0, 3.0) crystalline alloys. In addition, a new phenomenon has been reported that the lattice parameter of as-quenched Al–Fe–V–Si–Mm alloys decrease with increasing Mm content; the "cell lessening effect". This effect is presumably due to the results of composite interactions between rare-earth elements and alloy elements.     

Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu_0.5NiAlCo to septenary Cu_0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.     

Thermomechanical processing of Ti–5Al–5Mo–5V–3Cr

The constitutive behaviour and microstructural evolution of the near-β alloy Ti–5Al–5Mo–5V–3Cr in the α + β condition has been characterised during isothermal subtransus forging at a range of temperatures and strain rates. The results indicate that Ti–5Al–5Mo–5V–3Cr has a shallower approach curve, and therefore, offers a more controllable microstructure than the near-β alloy Ti–10V–2Fe–3Al. Flow softening is small in magnitude in both alloys in the α + β condition. The steady state flow stresses obey a Norton–Hoff constitutive law with an activation energy of Q = 183 kJ mol⁻¹, which is similar to the activation energy for self-diffusion in the β phase, suggesting deformation is dominated by dynamic recovery in the β matrix. Good evidence is found for the existence of ω phase after both air cooling and water quenching from above the β transus. In addition, dissolution of the α phase is found to be slow at near-transus temperatures.     

The wetting of alumina by copper alloyed with titanium and other elements

The wettability of alumina by ternary alloys of copper, titanium and aluminium, gallium gold, indium, nickel or silver has been investigated using sessile drop tests conducted in vacuum at 1050–1250° C. Substantial additions of titanium are known to induce copper to wet alumina due to the formation of a titanium rich reaction product at the alloy/ ceramic interface, but the present work has shown that the concentration of titanium can be reduced by the addition of ternary alloying elements. Additions of indium are very beneficial, of aluminium, gold or silver are moderately beneficial, and of gallium or nickel are of negligible benefit or detrimental. These observations, and previous work with copper-tin-titanium alloys [1] can be interpreted in terms of effects on the activity of titanium which it is argued will be enhanced if the ternary alloying element has a low surface energy and is readily saturated by titanium. The correlation of the experimental wetting observations with the surface energy and titanium solubility data for the ternary alloying elements provides a basis for the rational development of reactive metal brazes for joining unmetallized ceramics.     

Corrosion of nickel,iron, cobalt and their alloys in molten salt electrolytes

The processes of high-temperature corrosion, anodic dissolution and passivation of nickel, iron, cobalt and their alloys are reviewed to reveal the progress in understanding the reaction mechanisms defined in the last two decades. In the first part, the procedures of thermodynamical analysis of corrosion processes by potential — pO^2– diagrams are outlined. The second part is devoted to the electrochemical corrosion, anodic dissolution and passivation of the metals studied, the reaction mechanisms and composition of the corrosion layers formed. The effect of the alloying elements on the corrosion resistance and anodic behaviour of the base metal is treated in the third part. A brief summary of the kinetics of the so-called hot corrosion of the studied metals and their alloys in contact with thin molten salt films and aggressive atmospheres is then given. Finally, some conclusions are drawn and some future trends of investigation are indicated.