Boronizing of titanium-molybdenum alloys

Conclusions A study was made of the kinetics of the reaction of Ti-Mo alloys with boron. It is shown that the intensity of the reaction depends on the composition of the alloys. The rate of the reaction, which is a minimum at Mo contents of 10 and 20 at.%, is determined by the diffusion of atoms of the reacting components. Titanium alloys containing 10 or 20 at.% Mo may prove to be suitable for operation at high temperatures in boron-containing media.Translated from Poroshkovaya Metallurgiya, No. 12(180), pp. 66–69, December, 1977.     

Aqueous slurry erosion in some cobalt base superalloys

An orifice erosion test was used to study the influence of metallurgical variables in a series of cobalt base superalloys on their resistance to erosion by aqueous slurries of SiO₂. A slurry of 30 pct solids by weight of 4.5 μm silica was used to erode a variety of microstructures obtained by compositional and processing control. The time de-pendence of the pressure drop across an orifice (P) was found to follow the relationP = Kt^-@#@ n wheret is time (min), andK andn are constants. The erosion exponent,n, varied from 0.052 to 0.148 and was found to be dependent upon carbon content and processing variables.     

Hardening by ordered precipitates in some ternary ferritic alloys containing cobalt

Spheroidal ordered precipitates have been found to occur during aging of ferritic Fe-Co-Ti and Fe-Co-Al alloys. The fine uniformly distributed precipitates, which appear to have L2₁ and B2 structures respectively, give rise to a 100 pet increase in hardness. In the Fe-Co-Ti system, the precipitate composition has been established as approximately Fe₂CoTi by electron probe microanalysis. The ordered precipitate in this system is metastable with respect to the Fe₂Ti, C14, Laves phase, the latter growing as the former redissolves. In the Fe-Co-Al system, the ordered precipitates are stable, since no intermetallic phase occurs in the iron-rich corner of the Fe-Co-Al phase diagram. D. J. ABSON, formerly at Sheffield University, Sheffield, S1 3JD, England     

Structures and properties of cobalt base-TaC eutectic alloys

A series of cobalt base alloys with 15 and 20 pct by weight of TaC were directionally solidified to produce a high-temperature composite material. Tensile and stress-rupture properties were determined and related to composition and structure of the ingots. The volume fraction of TaC fibers and the degree of perfection of the composite structure was found to depend primarily upon the amount of chromium in the alloy and, to a lesser extent, upon the amount of TaC in the nominal composition. Tensile and stress-rupture properties of the alloy with the most uniform composite structure exceeded those of any commercial cobalt or nickel base alloy. The elongation to fracture is greater at room temperature than at high temperature because the matrix does not work harden at high temperatures.     

Fatigue deformation of TiAl base alloys

The fatigue behavior of Ti-36.3 wt pct Al and Ti-36.2 wt pct Al-4.65 wt pct Nb alloys was studied in the temperature range room temperature to 900°C. The microstructures of the alloys tested consisted predominantly of γ phase (TiAl) with a small volume fraction of γ phase (Ti₃Al) distributed in lamellar form. The alloys were tested to failure in alternate tension-compression fatigue at several constant load amplitudes with zero mean stress. Fracture modes and substructural changes resulting from fatigue deformation were studied by scanning electron microscopy and transmission electron miscroscopy respectively. The ratio of fatigue strength (at 10⁶ cycles) to ultimate tensile strength was found to be in the range 0.5 to 0.8 over the range of temperatures tested. The predominant mode of fracture changed from cleavage type at room temperature to intergranular type at temperatures above 600°C. The fatigue microstructure at low temperatures consisted of a high density of a/3 [111] faults and dislocation debris of predominantly a/2 [110] and a/2 [110] Burger's vectors with no preferential alignment of dislocations. At high temperatures, a dislocation braid structure consisting of all 〈110〉 slip vectors was observed. The changes in fracture behavior with temperature correlated well with changes in dislocation substructure developed during fatigue deformation. S. M. L. SASTRY was formerly NRC Research Associate in the Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH     

Systematic of the equivalent influence of alloying elements on the nitrogen and hydrogen solubility in iron,cobalt, and nickel base alloys

Starting with the demonstration of the equivalent behaviour of solutes in case of the solubility and the activity of nitrogen and hydrogen in iron, cobalt and nickel base ternary and multicomponent melts, it has been shown that the equivalence principle, which was proved for the solvents iron, cobalt and nickel earlier on, is also valid among these solvents. Therefore, common reference functions have to be introduced for the evaluation of the solubility of nitrogen and hydrogen respectively. The influence of the solutes on the solubility of gaseous components nitrogen and hydrogen is uniformly graduated in the three solvents iron, cobalt and nickel. That means there must exist a valid common basic systematic. The different courses of the isothermal saturation curves can be explained by different miscibility in the liquid and the solid state of the metallic components solvent and solute as well. Based on this systematic background, it can be shown that the relation between the ordinal numbers of the solutes and their equivalence factors is directly linear within the individual element groups.     

Diffusion in Bcc iron base alloys

Using radiotracer techniques, volume diffusion measurements have been carried out in the bcc phases of the following systems: Iron containing 0 to 20 wt pct Cr, Iron containing 0 to 20 wt pct V, and Iron containing 0 to 6 wt pct Ti The activation energies and frequency factors were found to decrease with increasing solute content. A comparison is made between the experimental results and theoretical calculations in terms of the various relationships between activation energy, entrogy, and frequency factor.     

Precipitation of chromium in copper and copper-nickel base alloys

The precipitation of chromium-rich particles in Cu-Cr and Cu-Ni-Cr alloys has been studied by electron microscopy, electrical resistivity measurements and tensile testing. The results show that chromium precipitate has an fcc structure and the bcc structure previously proposed was not observed. A strong tendency for alignment of adjacent precipitates was observed in the ternary alloy but preferential precipitation on dislocations and grain boundaries showed that this was not due to spinodal decomposition. An alternative explanation based on elastic interactions is discussed. The resistivity results on the binary alloy shows no anomalous peaks in the early stages and were analyzed by the Avrami or Johnson-Mehl method to give the parametern = 1.5 and activation energy for growth of ~1.35 eV. Formerly in the Metallurgy Department, Manchester University     

Nickel and cobalt eutectic alloys reinforced by refractory metal carbides

Eutectiferous behavior was observed within pseudobinary joins between the monocarbides of the group IVa and Va metals with nickel and cobalt. Coupled two phase growth normal to a macroscopically planar liquid-solid interface was noted even though the various carbide phases grew in a faceted manner. The composite structures formed by unidirectional freezing consisted of either a cobalt or nickel matrix with an aligned whisker-like or three-sided lamellar carbide dispersion. The nickel-niobium monocarbide system was examined in greatest detail and found to be stronger than TD nickel in tension from room temperature to 2100°F. Repeated fracture along the length of individual carbide whiskers was observed during tensile straining. This behavior, which is nontypical of previous eutectic whisker composites, was interpreted in terms of the distribution in whisker strengths. The strength of extracted NbC whiskers measured in bending was found to approach the theoretical failure stress. Creep rupture tests further indicated that reinforcement of a weak nickel or cobalt matrix with aligned monocarbide whiskers provides a new type of material for use at elevated temperatures.     

Hydrogen transport in nickel base stainless alloys

Hydrogen transport parameters have been measured in two nickel base stainless alloys, HASTELLOY Alloys C-276 and G. Hydrogen diffusivity and permeability were determined by means of the electrolytic permeability technique over the temperature range of 17 to 90 °C. Although the two alloys are similar in composition and structure, they exhibited dramatically different hydrogen behavior. For Alloy C-276, the diffusivity in both the cold worked and annealed conditions decreased by a factor of two following low temperature (500 °C) aging. That behavior was related to ordering in the alloy. Unexpectedly, hydrogen trapping was not observed in Alloy C-276. An analysis of hydrogen transport in Alloy G indicated reversible and irreversible trapping of hydrogen by niobium substitutional atoms and second phase carbides, respectively. The hydrogen transport results were related to the hydrogen embrittlement tendencies of the two nickel base alloys. DAVID A. MEZZANOTTE, formerly Graduate Assistant with the Department of Metallurgical Engineering and Materials Science, University of Notre Dame. NICHOLAS F. FIORE, formerly Professor and Chairman of the Department of Metallurgical Engineering and Materials Science, University of Notre Dame.     

Diffusional creep and creep-degradation in dispersion-strengthened Ni-Cr base alloys

Dispersoid-free regions were observed in the dispersion-strengthened alloy TD-NiCr (Ni-20 Cr-2 ThO₂) after, slow strain rate testing (stress rupture, creep, and fatigue) in air from 1145 to 1590 K. Formation of the dispersoid-free regions appears to be the result of diffusional creep. The net effect of creep in TD-NiCr is the degradation of the alloy to a duplex microstructure. Creep degradation of TD-NiCr is further enhanced by the formation of voids and intergranular oxidation in the dispersoid-free bands. Void formation was observed after as litte as 0.13 pct creep deformation at 1255 K. The dispersoid-free regions apparently provide sites for void formation and oxide growth since the strength and oxidation resistance of Ni-20 Cr is much less than Ni-20 Cr-2 ThO₂. This localized oxidation does not appear to reduce the static load bearing capacity of TD-NiCr since long stress-rupture lives were observed even with heavily oxidized microstructures, but this oxidation does significantly reduce the ductility and impact resistance of the material. Dispersoid-free bands and voids also were observed in two other dispersion-strengthened alloys, TD-NiCrAl (Ni-16Cr-4 Al-2 ThO₂) and IN-853 (Ni-20 Cr-2.5 Ti-1.5 Al-1.3 Y₂O₃). Thus, it appears that diffusional creep is characteristic of dispersion-strengthened alloys and can play a major role in the creep degradation of these materials.     

Preparation and properties of some ODS Fe-Cr-Al alloys

Several alloys based on Fe-25Cr-6Al and Fe-25Cr-11Al (wt pct) with additions of yttrium, Al₂O₃, and Y₂O₃ have been prepared by mechanical alloying of elemental, master alloy and oxide powders. The powders were consolidated by extrusion at 1000°C with a reduction ratio of 36:1. The resulting oxide contents were all approximately either 3 vol pct or 8 vol pct of mixed Al₂O₃-Y₂O₃ oxides or of Al₂O₃. The alloys exhibited substantial ductility at 600°C: an alloy containing 3 vol pct oxide could be readily warm worked to sheet without intermediate annealing; an 8 vol pct alloy required intermediate annealing at 1100°C. The 3 vol pct alloys could be recrystallized to produce large elongated grains by isothermal annealing of as-extruded material at 1450°C, but the high temperature strength properties were not improved. However, these alloys, together with some of the 8 vol pct materials, could be more readily recrystallized after rod (or sheet) rolling; sub-stantially improved tensile and stress rupture properties were obtained following 9 pct rod rolling at 620°C and isothermal annealing for 2 h at 1350°C. In this condition, the rup-ture strengths of selected alloys at 1000 and 1100°C were superior to those of competitive nickel-and cobalt-base superalloys. The oxidation resistance of all the alloys was ex-cellent. F. G. WILSON and C. D. DESFORGES, formerly with Fulmer Re-search Institute