Beneficial Effects of Rhenium Additions on the Cyclic-Oxidation Resistance of β-NiAl + α-Cr Alloys

This study reports the effects of up to 4 at.%rhenium addition on the cyclicoxidation behavior of-NiAl + -Cr alloys having a basecomposition (in at.%) Ni-40Al-17Cr. Tests were conductedin still air at 1100°C for up to 250 1-hr cycles.The ternary alloy (without rhenium addition) exhibitedpoor cyclic-oxidation resistance, undergoing extensivescale spallation and internal oxidation. Additions of rhenium considerably improved the oxidationbehavior, reducing the extent of both scale spallationand internal oxidation. These beneficial effectsincreased with increasing rhenium content. Rhenium additions improved cyclic-oxidation resistanceby both decreasing the solubility of chromium in the phase and causing the interdendritic -Crprecipitates in the alloy microstructure to become more spheroidized and disconnected. Theseeffects aided in preventing both interdendritic attackand the dissolution of the -Cr precipitates fromthe subsurface region of the alloy. The maintenance of -Cr precipitates at the alloy-scaleinterface decreased the extent of scale spallation byproviding a lower coefficient of thermal-expansion (CTE)mismatch between the alloy and theAl₂O₃-rich scale.     

Effect of lamellar structure parameters on the properties of titanium alloy VT3-1

Conclusion An increase in the ultimate breaking strength, stress-rupture strength, and fatigue limit of alloy VT3-1 with a lamellar structure may be achieved as a result of refining any parameter of the structure, particularly -phase platelet thickness, and increasing the volume fraction of secondary -phase. An increase in ductility characteristics, toughness, and creep resistance may be provided by increasing the dimensions of -colonies and primary -phase particles (up to 2.5–3.5 m) and reducing the volume fraction and dispersivity of secondary -phase lamellar precipitates. Coarsening of -grains leads to an increase in a_c, k_Q, and refinement leads to an increase in and a_n.Qualitative dependences for mechanical properties of alloy VT3-1 on lamellar structure parameters made it possible to isolate those structural parameters which have the most marked effect on properties.The properties of alloys with a finely lamellar structure (d25 m, b_{I, II}<2 m) are most sensitive to structure. In this case a change in -colony size by 10 m and -platelet thickness by 1 m affects the properties 3–20 times more strongly than a change in -grain size by 100 m. The effect of finely dispersed secondary -phase precipitates is greater, the coarser the primary -phase structure. Refinement of primary -phase structure with an increase in secondary phase platelet thickness to 1 m or more reduces the sensitivity of alloy mechanical properties to the effect of secondary -phase.With coarsening of the intragranular structure (d>25 m, b_{I, II}2 m) the effect of structural parameters d and b on properties is markedly weakened: on strength properties (_f, ₁₀₀ ⁴⁵⁰ ) by a factor of 100, on ductility (, ), by a factor of 10 to 20, and on impact strength and fracture toughness (a_n, a_c, K_Q) by a factor of five.The qualitative relationships obtained between structure and mechanical properties of alloy VT3-1 are fundamental for controlling the structure of semifinished titanium alloy products.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 52–55, July, 1986.     

Formation of a Rhenium-Base Coating on a Nb-Base Alloy

To protect Nb–5Mo–15W alloy against high-temperature oxidation a novel coating was developed involving electroplating of a Re–Ni film, followed by pack cementation with Cr and Al. The coating consisted of a duplex-layer structure, an inner (Re–Cr) or Re(Cr) layer and an outer Cr(Al) or NiAl layer. The Re–Ni film containing more than 70at.%Re, developed in the present investigation, is more useful than the conventional low Re–Ni film. The inner (Re–Cr) and Re(Cr) layer acts as a diffusion barrier between the Nb–5Mo–15W alloy substrate and the outer -Cr(Al) or -NiAl layer, which forms a protective -Al₂O₃ scale. The coated Nb–5Mo–15W alloy was oxidized in air at 1373K for up to 360ks, showing very good oxidation resistance.     

Evaluation of the Brittleness of Titanium Alloys in Terms of Mechanical Properties and Fracture Patterns

The phenomena of brittleness of structural -pseudo-- and ( + )-titanium alloys are analyzed and generalized using the results of tests of rupture specimens and various parts. The characteristics of ductility (elongation and contraction ) are used for evaluating the macroscopic manifestations of embrittlement, and the appearance of fracture surfaces as observed in electron microscopic diffraction patterns is used for estimating the micromanifestations of embrittlement.     

Structure formation in nitrided layers of titanium alloys

Conclusions Nitriding of -, pseudo-- and +-Ti alloys forms diffusion layers consisting of a nitride layer and a gas-saturated layer. In +-Ti alloys, a third additional transient zone forms. During the nitriding process, redistribution of the alloying elements present in the alloys takes place, which is caused by their differing affinities for nitrogen.Nitriding schedules also have an effect on the alloying element redistribution. Nitriding temperature or time increase causes either thickening of the diffusion layer zones or formation of a qualitatively new structure in them.Lvov Physicomechanical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 11–14, June, 1986.     

The effect of purity on high-temperature oxidation of zirconium

The oxidation kinetics of zirconium of different purities were studied over the temperature range of 600–1300°C (- and -phases). The structure of the oxidized specimens was examined. TGA, XRD, EPMA, SEM, metallographic analysis, and microhardness measurements were carried out. Impurity elements were found to increase the oxidation rate of technical zirconium. The mechanism of the effect of impurity elements on zirconium oxidation was shown to differ for the - and -phases. Activation energies were calculated for the parabolic and linear stages of oxidation.     

Development,growth, and adhesion of Al2O3 on platinum-aluminum alloys

The development, growth, and adhesion of -Al₂O₃ scales on platinum-aluminum alloys containing between 0.5 and 6 wt.% aluminum have been studied at temperatures in the interval between 1000 and 1450° C. The morphologies and microstructures of the -Al₂O₃ scales were found to be influenced by the temperature, oxygen pressure, and the microstructures of the alloys. The oxidation rates of the alloys appeared to be controlled by transport of oxygen along grain boundaries in the -Al₂O₃ scales. The -Al₂O₃ scales adhered to the platinum-aluminum substrates even after extensive periods of cyclic oxidation. The good adhesion of the -Al₂O₃ may result from mechanical keying of the oxide to the alloys due to the development of irregular oxide-alloy interfaces.This work was supported by the U.S. Army Research Office, Durham, under Contract Number DAHCO 4 73 C 0021.     

General laws governing variation in properties of cast alloys of the Al-Zn-Mg system

Among existing high-strength corrosion-resistant aluminum alloys, those of the Al-Mn-Zn system are most promising. They have a different phase composition, depending on the content of magnesium and Zinc: + (Al₃Mg₂), + + T(Al₂Mg₃Zn₃), + T, + T + (MgZn2) and + The majority of industrial Al-Mg-Zn alloys correspond to the phase regions + T and + T + with respect to composition. A high level of strength and satisfactory overall corrosion resistance are characteristic for these alloys. Al-Mg-Zn alloys may, however, tend to the most dangerous form of corrosion - stress-induced corrosion cracking. Using methods of experiment planning in the study, we investigated Al-Zn-Mg alloys of various compositions for the purpose of selecting alloy compositions with a high level of mechanical properties and stress-induced corrosion cracking.All-Union Scientific-Research Institute of Aviation Materials. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 12, pp. 24–28, December, 1994.     

On the oxidation ofα-Fe andε-Fe2N1−z : I. Oxidation kinetics and microstructural evolution of the oxide and nitride layers

The oxidation of -Fe and -Fe₂N_1–z at 573 K and 673 K in O₂ at 1 atm was investigated by thermogravimetrical analysis, X-ray diffraction, light-optical microscopy, scanning electron microscopy and electron probe X-ray microanalysis. Upon oxidation at 573 K and 673 K, on -Fe initially -Fe₂O₃ develops, whereas on -Fe₂N_1–z initially Fe₃O₄ develops. In an early stage of oxidation the oxidation rate of -Fe₂N_1–z appears to be much larger than of -Fe. This can be attributed largely to an effective surface area available for oxygen uptake, which is much larger for -Fe₂N_1–z than for -Fe due to the porous structure of -Fe₂N_1–z as prepared by gaseous nitriding of iron. The development of a magnetite layer in-between the hematite layer and the -Fe substrate, at a later stage of oxidation, enhances layer-growth kinetics. After 100 min oxidation at 673 K the (parabolic) oxidation rates for -Fe and -Fe₂N_1–z become about equal, indicating that on both substrates the oxide growth is controlled by the same rate limiting step which is attributed to short-circuit diffusion of iron cations. Oxidizing -Fe₂N_1–z increases the nitrogen concentration in the remaining -iron nitride, because the outward flux of iron cations, necessary for oxide growth, leads to an accumulation of nitrogen atoms left behind.     

Transient-Alumina Transformations During the Oxidation of Magnetron-Sputtered CoCrAl Nanocrystalline Coatings

The thermally-grown alumina formed at 1000, 1100, and 1200°C on magnetron-sputtered, nanocrystalline CoCrAl coatings, with and without yttrium, has been characterized using photostimulated-luminescence spectroscopy. The measurements enable the evolution of initially-formed transient alumina to its stable, phase to be followed, and in particular, the effect of yttrium on the transformation. Yttrium retards the transformation from gamma to theta alumina and also its subsequent transformation to alumina. The retardation of the transformation decreases with increasing oxidation temperature until at 1200°C the transformation is complete within minutes. The presence of yttrium in the coatings also affects the residual stress in the thermally-grown oxide. For samples oxidized at 1100 and 1200°C the residual stress is 0.3GPa higher in the oxide on the Y-containing coating, whereas the residual stresses are the same after oxidation at 1000°C.     

Austenitic stainless steels strengthened by phase strain hardening and aging

Conclusions Nonmagnetic stainless steels of the Kh12N12T3 and Kh12N14T3 type have good mechanical properties after phase strain hardening and aging (_0.2 = 685 - 785 MPa, _b = 1275 MPa, 20%) as compared with the properties of Fe-Ni-Ti austenitic steels with 26–30% Ni. After phase strain hardening and aging the stability of these steels is high with respect to the transformation during cold treatment.IFM UNTs AN SSSR. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 57–60, June, 1981.     

Phase and structural transformations in deformation and heat treatment of 08Kh15N5D2T steel tubing

Conclusion Annealing in the intercritical area of the -transformation or hardening from a continuous furnace with subsequent tempering in the -transformation range included in the production plan for cold-rolled 08Kh15N5D2T tubing do not restore the original workability of the steel since the residual austenite is stabilized toward the -transformation in deformation. Annealing at 650°C does not lead to complete recrystallization and detexturing of the metal. To obtain this it is recommended that rapid electric heating for hardening with subsequent tempering be introduced into the production plan for rolling of tubing.Ural Polytechnic Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 8, pp. 7–11, August, 1990.     

X-ray topographic study of β-NiAl upon growth of an α-Al2O3 film

The Berg-Barrett X-ray topographic method was employed as a microstructural technique to seek correlations of the metal substructure to the morphological features of -Al₂O₃ films grown on -NiAl. An analysis of diffraction micrographs using {112} and {002} reflections from individual grains in -NiAl revealed its subgrain structure to a depth of 30 . The dimensions of these subgrains were directly related to the density of oxide ridges in the -Al₂O₃ films and to the dimensions and shapes of cavities at the NiAl-Al₂O₃ interface.     

On the Similarity Criteria for the Processes of High-Temperature Sulfur Corrosion of Heat-Resistant Alloys

A similarity parameter for the high-temperature sulfur corrosion of heat-resistant alloys is suggested. This parameter allows to put in order published experimental data of different authors. Original data on the weight loss by 826, -70, 26, and several materials for protective coatings also correlate with the similarity parameter. The dependences thus obtained allow to predict the operation life of materials (by their weight loss) and to rank nickel-based alloys by their corrosion resistance, depending on their elemental composition.     

Solubility of β-stabilizers in α-titanium

Conclusions We have determined the solubilities of iron, chromium, manganese, silicon, copper, molybdenum, vanadium, and tantalum in -titanium (Table 1) and in the -solid solution of Ti+6% Al (Table 2).Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 13–16, February, 1963     

Decomposition ofβ phase in titanium alloy VT22

Conclusion The low ductility of alloy VT22 after isothermal quenching and also after quenching in water and aging at 250–300° is due to the formation of phase, the formation of and phases at 350–400°, and the formation of phase at temperatures above 400°.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 47–48, September, 1971.     

Control of the structure of two-phase titanium alloys in thermomechanical working

Conclusions The embryos of - and -phases obtained in +-titanium alloys in hardening and aging are preserved in cold working and become centers of growth of these phases in subsequent aging. As a result it is possible to obtain -phase or more uniformly distributed -phase in cold-worked and aged alloys.Moscow Institute of Electronic Machine Building. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 53–55, May, 1984.     

Formation, Structure, and Corrosion Properties of Carbo-Nitrided High-Chromium Steels

Structure and properties of carbidized surface zones on 43, 45, and 2013 steels upon high-temperature nitro-cementation in atmospheres of common or vacuum pyrolysis of nitrogen- and carbon-containing components and charcoal mixtures are discussed.     

Improvement of cyclic high-temperature corrosion resistance of pack-aluminized heat-resistant stainless steels

Modification of pack aluminizing for heat-resistant stainless steels was studied to improve corrosion resistance by controlling the microstructure of the coating layer. The major process parameters examined include the pack powder composition, coating time, and temperature. Depending on the combination of these parameters, the microstructure of the coating layer can be controlled to form either a continuous layer of internal-diffusion barrier (IDB) or an interdiffusion zone (IZ). At the coating-process temperatures, the IDB forms as a mixture of - and -aluminide, whereas the IZ forms as a mixture of -ferrite and -aluminide. But the phase shown in the IZ at room temperature is formed by transformation from the phase during cooling. Even though the hardness of the IDB is higher than the other phases present in the coating layer, the aluminide coating layer with the IDB shows outstanding cyclic high-temperature corrosion resistance. As long as the stable IDB forms, the corrosion resistance increases with the thickness of the aluminide-coating layer.     

Effect of rapid cyclic electrothermal treatment on the structure of nickel steel

Conclusions The high dislocation density of austenite undergoing the transformation is due to the influence of fresh dislocations that occur during the transformation in virtue of its martensitic character and to dislocations inherited from the original phases.Since the increase of the dislocation density in austenite causes an increase in the number of martensite crystals, it can be assumed that the increase in the number of phase nuclei is due to an increase in the density of fresh dislocations. In this case the refining of martensite is due to an increase in the number of nuclei and to the barrier effect of elements of the substructure. The formation of atmospheres of impurity atoms at dislocations in alloys with carbon stabilizes the austenite and intensifies recrystallization processes. Cementite particles in phase are not inherited by austenite with heating to the transformation temperature.Institute of Metal Physics, Academy of Sciences Ukrainian SSR. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 1, pp. 20–23, January, 1975.