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.     

Kinetics and mechanisms of the oxidation of cobalt at 600–800°C

Two-phase layered scales comprising CoO and Co ₃O₄ formed on cobalt during oxidation at 600°, 700°, and 800°C and at oxygen partial pressures in the range 0.001–1 atm. The kinetics, which were obtained by thermogravimetric analysis, obeyed a parabolic rate law after an initial, non-parabolic stage of oxidation. The monoxide consisted of relatively large grains (10 ) and the spinel comprised small grains (3 ) for all conditions of oxidation. Grain boundary diffusion of cations played a significant role in the growth of the spinel layer. Thermogravimetric data and the steady-state ratio of the oxide layer thicknesses were employed to calculate the rates of thickening of the individual oxide layers and the rate of oxidation of CoO to Co₃O₄.     

Strengthening heat treatment of alloy VT16 in vacuum

Conclusions Alloy VT16 can be strengthened by heat treatment in vacuum under the following conditions: heating at 775–800° for 2 h, cooling in the container in water, and aging at 500° for 8 h.The alloy subjected to this treatment has the following properties; _b = 103–107 kgf/mm², =59–63%, ₅ = 15.1–16.1%.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 5, pp. 65–67, May, 1978.     

Mach 1 oxidation of thoriated nickel chromium at 1204°C (2200°F)

Electropolished and ground samples of TD-NiCr were exposed to a 1-atm, Mach 1 gas stream at 1204°C (2200°F)for times up to 50 hr. The samples were subjected to both cyclic and isothermal exposure. Weight change, metal loss, x-ray diffraction, metallographic, and electron microprobe analyses were performed. Neither surface preparation nor cyclic-against-isothermal-exposure conditions had a strong effect on the oxidation behavior of the alloy. Initially, a Cr ₂O₃ layer was formed whose volatilization resulted in a very rapid loss of metal—more than 40 m in the first hour. At about 1 hr the Cr₂O₃ layer broke down and NiO began to cover the surface. By 5 hr the NiO had covered the surface and the rate of loss slowed. The rate-controlling step was diffusion of Cr through NiO. By 50 hr the sample had lost approximately 200 m in thickness.     

Age hardening of VT35 titanium alloy

VT35 alloy belongs to -titanium alloys that preserve the body-centered lattice of the -phase in hardening from the -region. In an equilibrium state this alloy has an + structure. After hardening, VT35 alloy has a high ductility and a low strength. The subsequent single- or double-stage aging in the biphase region promotes considerable strengthening of the alloy due to segregation of a second phase. VT35 titanium alloy is hardened to a pure -phase by cooling from the single-phase region in water, in air, or with the furnace (at a rate of at least 3 -4 deg/min). This special feature of the alloy is caused by its chemical composition (Ti - 3% Al -15% V - 3% Cr - 3% Sn), which provides a high coefficient of -stabilizationK = 1.5. The present paper concerns the processes of age toughening of a hardened VT35 alloy and the kinetics of the structural transformations in such a treatment.     

Transport processes in the oxidation of Ni studied using tracers in growing NiO scales

Experimental techniques have been developed for determining Ni⁶³ and O¹⁸ tracer distributions in NiO scales ranging in thickness from 0.1 to 100 . These have been used to investigate Ni and O transport in scales on {100} Ni crystals and polycrystalline Ni in the temperature range 500–1300° C. NiO grown on {100} Ni crystals at 1000°C was uniform and compact and grew by the bulk diffusion of Ni in NiO by a vacancy mechanism. At temperatures below 800°C the principal transport mechanism was short-circuit diffusion of Ni in NiO. At all temperatures short-circuit diffusion of oxygen contributed to scale growth on polycrystalline Ni and was responsible for growth of the inner layer of duplex scales. The oxygen diffusion paths are believed to be micro-cracks induced by growth stresses.     

Efferct of the μ-phase on the properties of superhigh-temperature derofmable nickel alloy KhN60KVYuMB

One of the requirements imposed on high-temperature nickel alloys for long-term operation under high temperatures and stresses is structural stability, which ensures constant properties during operation of parts. It is known that the main strengthening phase of nickel alloys is a -phase of the type (Ni, Cr)₃(Al, N) or (Ni, Cr)₃(Al, Ti, Nb), which may amount to 50% in deformable nickel alloys. In addition, these alloys contain a certain amount of carbides, borides, and carbonitrides (up to 3 wt. %). An increase in the content of alloying elements in high-temperature alloys may cause formation of undesirable TCP-phases in their structure. For example, at high contents of molybdenum and tungsten a thin lamellar p-phase (Ni, Co, Fe)₇(Mo, W)₆ is segregated. The authors investigated the effect of the -phase on the set of chemical properties of industrial alloy KhN60KVYuMB (pressed rods 105 mm in diameter).Translated from Metailovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 15–19, September, 1995.     

High-temperature thermoplastic strengthening of steels St3sp and 09G2S

Conclusions We worked out a regime of high-temperature thermoplastic treatment of steels St3sp and 09G2S consisting in austenization at 1020°C, strain (=200%, =5.7%) at 940°C with subsequent cooling at a rate >100°C/sec.As a result of such treatment the tensile strength of steel St3sp is increased to 1230 N/mm², of steel 09G2S to 960 N/mm², and the conventional yield strength to 1100 and 840 N/mm², respectively.N. É. Bauman Moscow State Technical University. All-Union Research Institute of Metallurgical Machinery Construction. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 18–19, July, 1991.     

Oxidation behavior of the aligned lamellar eutectic alloy Ni3Al-Ni3Nb

The oxidation behavior of the directionally aligned Ni₃Al-Ni₃Nb lamellar eutectic alloy has been examined as a function of lamellar spacing for two interlamellar spacings (1.7 and 3.3) in the temperature range from 600 to 1155°C. At the low and high temperatures examined, 600 and 1155°C, no dependency of the oxidation behavior on the interlamellar spacing is observed. However, in the intermediate temperature regime, 800 and 990°C, such a dependency is seen, with the finer spacing structure oxidizing more rapidly at 800°C and more slowly at 990°C. The oxide scale formed on these alloys is very complex and varies markedly as a function of oxidation temperature. The structure of the oxidized aligned samples and their thermogravimetric behavior during oxidation differ from the behavior reported for the cast alloys.     

Superplasticity of alloy V96Ts

Conclusions Alloy V96Ts with a fine-grained structure (grain diameter 5 ) exhibits superplasticity in tension at strain rates of 2.8·10^–4 to 5.5·10^–3 sec^–1 at 460–470°. In the superplastic condition the samples elongate evenly without necking.Ordzhonikidze UfaAviation Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metalov, No. 3, pp. 55–56, March, 1978.     

Improved Oxide Spallation Resistance of Microcrystalline Ni-Cr-Al Coatings

Microcrystalline Ni-20Cr-3Al coatings weredeposited on Ni-20Cr-3Al substrates by unbalancedmagnetron-sputter deposition. The grain size of thecoatings was varied by using different Ar pressures.Cyclic-oxidation testing was performed at 1100°C. It wasfound that (1) an external-Al₂O₃ scale formed oncoating A (4.7 m thick, 50 nm grain size); (2) anexternal Cr₂O₃ scale and internalAl₂O₃ oxide formed on coating B (14 m thick, 500 nm grain size);and (3) an outer layer scale ofCr₂O₃ +NiCr₂O₄ and interior layer ofAl₂O₃ formed on the as-cast alloy.Extensive spallation of the Cr₂O₃+ NiCr₂O₄ scale took place on the as-cast alloy, but no obviousspallation occurred on the two coatings. Improvement ofthe spallation resistance of the scale is explained byeffective diffusional creep of the coatings and the micropegging effect of the inward-grownoxides.     

The effect of the oxygen pressure and the growth of whiskers on the oxidation of pure Fe

The weight increase curves of pure iron specimens cold-worked by abrasion with SiC and 7- diamond paste have been registered as a function of pressure and temperature. The oxidation rate increases with temperature and pressure when the pressure is raised from 0.02 to 0.2 bar. When the pressure is further increased to 1.02 bar at 500 and 625°C the oxidation rate decreases. This decrease is attributed to an orientation of the oxide grains in the -Fe₂O₃ surface.     

Effect of the tempering temperature and time on the structure and phase composition of the AMg6 alloy

Conclusions It was determined that the decomposition of the solid solution in the AMg6 alloy begins at the grain boundaries. After a certain time interval the plate-like -phase precipitates within the grains. After a longer tempering time the platelets coagulate and take on a rounded form.At temperatures of 200 and 250°C the metastable -phase is not completely converted into the -phase, and even after a very long tempering time there is still a considerable amount of the -phase in the structure.At a temperature of 150°C or lower, only the metastable -phase occurs in the alloy even after tempering as long as 9 months.Translated from Metallovedenie i Termichesakaya Obrabotka Metallov, No. 9, pp. 59–61, September, 1966.     

Oxidation-Rate Excursions During the Oxidation of Copper in Gaseous Environments at Moderate Temperatures

The kinetics of oxidation of copper powders in oxygen and in dry and humid air was investigated using thermogravimetric analysis (TGA). The extent of oxidation grew linearly with time until the weight-based thickness of the oxide film reached 0.13–1.22 nm, depending on the temperature. Between 30 and 90°C there was little difference between the kinetic curves observed in air and in oxygen, respectively. Higher humidity of the air resulted in an increased oxidation rate. Following the initial linear segment, the oxidation kinetics could be best described in terms of a logarithmic rate law between 30 and 45°C and in terms of a power law between 60 and 90°C. The activation energy for the initial linear stage was (44±2) kJ and for the subsequent oxidation (102±12) kJ. Delayed increases in oxidation rate were observed with a ca. 0.1-m powder around 100°C, with a ca. 1-m powder around 320°C, and with a < 10m powder around 360°C. A three-stage model consisting of an initial linear stage, parabolic growth culminating in cracking of the oxide film, and subsequent re-start of the parabolic growth, gave good agreement with the experimental data. Whenever the powder is relatively uniform and the distribution of film-cracking times among the powder grains is narrow, e.g., within 23% of the median cracking time, an increase in the oxidation rate of the entire sample can be observed.     

Environmental Effects on Orthorhombic Alloy Ti-22Al-25Nb in Air Between 650 and 1000°C

The environmental behavior of an orthorhombictitanium-aluminide alloy, Ti-22Al-25Nb, was studied indry and humid air between 650 and 1000^°Cby scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction.Microhardness measurements were performed after exposureto gage hardening due to nitrogen and oxygen ingress.The parabolic rate constant of Ti-22Al-25Nb was of the same order as conventional titanium alloys andTi₃ Al-base titanium aluminides at and below750^°C. Between 800 and1000^°C, the oxidation resistance ofTi-22Al-25Nb was as good as -TiAl base aluminides;however, the growth rate changed from parabolic tolinear after several tens of hours at 900 and1000^°C. The mixed oxide scale consistedof TiO₂, AlNbO₄, andAl₂O₃ with TiO₂ beingthe dominant oxide phase. Underneath the oxide scale, anitride layer formed in the temperature rangeinvestigated and, at 1000^°C, internal oxidation was observed below thislayer. In all cases, oxygen diffused deeply into thesubsurface zone and caused severe embrittlement.Microhardness measurements revealed that Ti-22Al-25Nbwas hardened in a zone as far as 300 m belowthe oxide scale when exposed to air at900^°C for 500 hr. The peak hardnessdepended on exposure time and reached five times theaverage hardness of the bulk material under the aboveconditions.     

The formation of α-Al2O3 scales at 1500°C

The growth of Al₂O₃ scales on -NiAl was studied at 1500°C. Oxidation rates, diffusion mechanisms, and microstructures were examined in order to achieve a complete understanding of the scale development. Variation of the Al content within the phase field had little effect on the oxidation behavior. Ionimplanted yttrium (2×10¹⁶/cm²) was observed to provide a short-term improvement in scale adhesion but little long-term effect. When doped with Y or Zr, the first 1 m of -Al₂O₃ was observed to grow mainly by an inward oxygen growth mechanism. At longer times when the implant was ineffective, microstructural observations indicate a mixed-growth mode.     

The Oxidation Behavior of Oxide-Dispersed β-NiAl: I. Short-Term Performance at 1200°C

Oxide dispersions were added to -NiAlalloys using a powder-metallurgy technique. During 20,2-hr cycles at 1200°C, scale adhesion of theexternal alpha Al₃O₂ was improvedby the addition of Y₂O₃ and ZrO₂. Relative to an undoped, castNiAl alloy, no improvement was observed forTiO₂ and HfO₂ additions andnegative effects were observed forAl₂O₃ andLa₂O₃ additions. The variouscation additions also had differing effects on the scale morphologyand isothermal growth rates at 1200°C. The effect ofthe dopants added as oxide dispersions was compared tosimilar alloy additions of the dopants to -NiAl.     

The influence of thermal cycling on the oxidation and oxide spallation of a 1%Cr–0.5%Mo low-carbon steel

The oxidation and oxide spallation of 1%Cr–0.5%Mo low carbon steel disks in dry oxygen was studied isothermally at 800°C (1073 K) and in thermal cycling between 800 and 600°C (1073 and 873K) followed by cooling at rates from 3 to 100°C/min. Mostly parabolic oxidation kinetics were observed. Thin scales (10 ) were more prone to spalling than thicker scales (20 ). The thickness and growth imperfections of an inner scale layer enriched in chromium, molybdenum, and silicon strongly influenced the probability of cohesive failure exceeding that of adhesive failure of the scale. Cohesive failures in the bulk scale during thermal cycling were probably nucleated at voids and microcracks produced in the initial isothermal period of scale growth. The number of segmented scale layers that became detached during cycling was governed by the number of parallel rows of voids in the scale and not necessarily by the number of cycles.