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.     

Diffusion of chromium and aluminum in Ni-20Cr and TDNiCr (Ni-20Cr-2ThO2)

Diffusion coefficients have been measured for⁵¹Cr in fine- and coarse-grained TDNiCr (Ni-20Cr-2ThO₂) and in fine-grained Ni-20Cr in the temperature range of 1038° to 1200°C. Selective diffusivities have also been determined for specimens of these alloys which were aluminized to give an initial surface concentration of 5.8 wt pct Al. Finally, diffusion coefficients for interdiffusion of aluminum in TDNiCr and Ni-20Cr have been obtained from electron probe microanalysis of the aluminized specimens. For a given grain size and temperature there is no difference in diffusivities for chromium diffusion in TDNiCr or Ni-20Cr. Diffusion coefficients increase with decreasing grain size for both alloys. Comparison of aluminum diffusion data obtained from electron microprobe profiles with radiotracer chromium diffusivities suggests that aluminum diffuses approximately three times faster than chromium in TDNiCr and Ni-20Cr.     

Hydrogen compatibility of dispersion-strengthened alloys

Hydrogen compatibility of Ni-2ThO₂, Ni-20Cr, Ni-20Cr-2ThO₂, and Inconel MA753 was investigated with both hydrogen charging and tests in high-pressure hydrogen gas. The former were conducted at 194 as well as at room temperature. Ductility and fractography results showed that recrystallized Ni-20Cr-2ThO₂ and MA753 were not embrittled by hydrogen, while Ni-2 ThO₂ showed small ductility losses with no change in fracture morphology. Annealed Ni-20 Cr-2 ThO₂ and as-received Ni-20 Cr-2 ThO₂ showed no ductility losses in charged hydrogen, but showed large ductility losses in high-pressure hydrogen; no changes in fracture morphology were observed. Ni-20 Cr was markedly embrittled by both sources of hydrogen. These results support the hypothesis of Thompson and Wilcox, that the presence of a dispersoid can confer improved performance in hydrogen.     

Production of chromium base alloys by ball milling in hydrogen iodide

The effects of processing variables on the tensile properties and ductile-to-brittle transition temperature (DBTT) of Cr+4 vol pct ThO₂ alloys and of pure Cr produced by ball milling in hydrogen iodide were investigated. Hot rolled Cr+ThO₂ was stronger than either hot pressed Cr+ThO₂ or pure Cr at temperatures up to 1540°C. Hot pressed Cr+ThO₂ had a DBTT of 500°C as compared with ?8°, to 24°C for the hot rolled Cr+ThO₂ and with 140°C for pure Cr. It is postulated that the dispersoid in the hot rolled alloys lowers the DBTT by inhibiting recovery and recrystallization of the strained structure.     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

An observation of vacancy sources during substitutional diffusion in thoriated nickel alloys

Various solid-solid diffusion couples were assembled from thoriated and nonthoriated alloys, welded, and diffusion annealed at 1530 K. Concentration profiles of the diffused couples indicated that a thoria dispersion does not affect diffusion in Cr(alloy): Ni and Ni-4.8A1: Ni types of couples unless a fine grain structure is retained by the thoria particles. Metallography revealed the presence of ThO₂-free bands in the thoriated nickel side of the diffusion zone. The bands formed at grain boundaries perpendicular to the direction of diffusion and, in some cases, contained non-Kirkendall porosity. A mechanism based on the operation of vacancy sources is proposed to explain the ThO₂-free bands.     

Nitrogen solubility in liquid Fe-V and Fe-Cr-Ni-V alloys

Nitrogen solubility in liquid Fe, Fe-V, Fe-Cr-V, Fe-Ni-V and Fe-18 pct Cr-8 pet Ni-V alloys has been measured using the Sieverts’ method for vanadium contents up to 15 wt pct and over the temperature range from 1775 to 2040 K. Nitrogen solution obeyed Sieverts’ law for all alloys investigated. Nitride formation was observed in Fe-13 pet V, Fe-15 pet V and Fe-18 pet Cr-8 pet Ni-10 pet V alloys at lower temperatures. The nitrogen solubility increases with increasing vanadium content and for a given composition decreases with increasing temperature. In Fe-V alloys, the nitrogen solubility at 1 atm N₂ pressure is 0.72 wt pet at 1863 K and 15 pct V. The heat and entropy of solution of nitrogen in Fe-V alloys were determined as functions of vanadium content. The first and second order interaction parameters were determined as functions of temperature as: $$e_N^V = \frac{{ - 463.6}}{T} + 0.148 and e_N^{VV} = \frac{{17.72}}{T} - 0.0069$$ The effects of alloying elements on the activity coefficient of nitrogen were measured in Fe-5 pet and 10 pet Cr-V, Fe-5 pet and 10 pet Ni-V and Fe-18 pet Cr-8 pct Ni-V alloys. In Fe-18 pet Cr-8 pet Ni-10 pet V, the nitrogen solubility at 1 atm N₂ pressure is 0.97 wt pet at 1873 K. The second order cross interaction parameters, e _N ^Cr,V and e _N ^Ni,V , were determined at 1873 K as 0.00129 and ? 0.00038 respectively.     

Electrolytic recovery of molybdenum from molybdic oxide and molybenum sesquisulfide

An electrolytic process for molybdenum extraction in a KCl?K₃MoCl₄ electrolyte (containing approximately 7.5 wt pct molybdenum) was investigated using three types of anode feed —namely, molybdic oxide-graphite, molybdenum sesquisulfide-graphite, and molybdenum sesquisulfide without graphite. In the case of molybdic oxide-graphite anode, a maximum current efficiency of 99 pct was achieved at an operating voltage of 0.35V, a cathode current density of 5000 A/m² (450 A/f²) and a bath temperature of 1223 K. Electrolysis with molybdenum sesquisulfide-graphite, at an operating voltage of 1.2V, a cathode current density of 13,900 A/m² (1250 A/f²) and a bath temperature of 1173 K, yielded a maximum current efficiency of 84 pct. Electrolysis of molybdenum sesquisulfide without graphite incorporation, yielded under almost similar conditions, a maximum current efficiency of 87 pct. Extended electrolysis was carried out using molybdic oxide-graphite and molybdenum sesquisulfide cell charges and yielded metal with purity over 99.9 pct.     

The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys

Nickel-20 wt pet chromium alloys containing ThO₂, Y₂O₃, La₂O₃, Al₂O₃ and Li₂O, as prepared by the mechanical alloying technique, were examined for isothermal and cyclic oxidation resistance in dry air at 1000, 1100 and 1200°C. TDNiCr, a commercial electrical heating element alloy (Com Ni-20Cr) and a laboratory melted alloy⁹Lab Ni-20Cr) were also tested. It was found that Y₂O₃, La₂O₃, Al₂O₃ and ThO₂ dispersoids markedly increased both isothermal and cyclic oxidation resistance compared to Lab Ni-20Cr at all temperatures; in contrast Li₂O additions gave no improvement in protection. Com Ni-20Cr was in between Lab Ni-20Cr and the Y₂O₃, A1₂O₃ and ThO₂ containing alloys in both cyclic and isothermal oxidation performance. A mechanism based on alterations in the defect structure of Cr₂O₃ is proposed to explain these dispersed oxide effects on isothermal oxidation behavior. It is based on a reduction in cation transport rates which in turn alter the rate of oxide growth. ThO₂-containing alloys fabricated by the mechanical alloying technique were found to have oxidation resistance fully equal to commercial TDNiCr. Com Ni-20Cr performed better than Lab Ni-20Cr, but not as well as TDNiCr.     

The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys

Nickel-20 wt pet chromium alloys containing ThO₂, Y₂O₃, La₂O₃, Al₂O₃ and Li₂O, as prepared by the mechanical alloying technique, were examined for isothermal and cyclic oxidation resistance in dry air at 1000, 1100 and 1200°C. TDNiCr, a commercial electrical heating element alloy (Com Ni-20Cr) and a laboratory melted alloy⁹Lab Ni-20Cr) were also tested. It was found that Y₂O₃, La₂O₃, Al₂O₃ and ThO₂ dispersoids markedly increased both isothermal and cyclic oxidation resistance compared to Lab Ni-20Cr at all temperatures; in contrast Li₂O additions gave no improvement in protection. Com Ni-20Cr was in between Lab Ni-20Cr and the Y₂O₃, A1₂O₃ and ThO₂ containing alloys in both cyclic and isothermal oxidation performance. A mechanism based on alterations in the defect structure of Cr₂O₃ is proposed to explain these dispersed oxide effects on isothermal oxidation behavior. It is based on a reduction in cation transport rates which in turn alter the rate of oxide growth. ThO₂-containing alloys fabricated by the mechanical alloying technique were found to have oxidation resistance fully equal to commercial TDNiCr. Com Ni-20Cr performed better than Lab Ni-20Cr, but not as well as TDNiCr.     

Oxide Scales Formed on NiTi and NiPtTi Shape Memory Alloys

Ni-49Ti and Ni-30Pt-50Ti (nominal at. pct) shape memory alloys (SMAs) were isothermally oxidized in air over the temperature range of 773?K to 1173?K (500?°C to 900?°C) for 100?hours. The oxidation kinetics, presented in detail in a companion study, show ~4 times reduction in oxidation rate due to Pt.[1] The microstructure, composition, and phase content of the scales and depletion zones were determined by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). A relatively pure TiO₂ rutile structure was identified as the predominant scale surface feature, typified by a distinct highly striated and faceted crystal morphology, with crystal size proportional to oxidation temperature. The complex layered structure beneath these crystals was characterized by semiquantitative XRD of serial/taper polished sections and SEM/EDS of cross sections for samples oxidized at 973?K (700?°C). In general, graded mixtures of TiO₂, NiTiO₃, NiO, Ni(Ti), or Pt(Ni) metallic dispersoids, and continuous Ni₃Ti or Pt-rich metal depletion zones, were observed from the gas surface to the substrate interior. Overall, substantial depletion of Ti occurred due to the formation of predominantly TiO₂ scales. It is proposed that the Ni-30Pt-50Ti alloy oxidized more slowly than the binary Ni-49Ti alloy by decreasing oxygen and titanium diffusion through the thin Pt-rich layer.     

Thermodynamic properties of solid alloys of chromium with nickel and iron

The thermodynamic properties of chromium have been determined in the Ni-Cr and Fe-Cr binary systems and in the Fe-corner of the Fe-Ni-Cr system. These properties are based on experimental measurements using solid oxide electrolyte cells of the type: Cr, Cr₂O₃ I ThO₂-Y₂O₃Cr (alloy), Cr₂O₃. In the Ni-Cr system, between 900 and 1300°, the activity of chromium exhibits negative deviation from ideality up to about 25 at. pct chromium. For alloys higher in chromium content, the activity of chromium exhibits positive deviation from ideality. In the Fe-Cr system, between 900 and 1200°, and 0 and 63 at. pct Cr, the chromium activity when referred to solid pure chromium exhibits positive deviation from ideality in both the γ and α phases, approaching ideality with increasing temperature. The nickel and iron activities in these two respective binary systems were calculated by a Gibbs-Duhem integration. The activity of chromium, referred to solid pure chromium, was measured between 900 and 1200° in solid Fe-Ni-Cr alloys with chromium concentrations of 9, 20, and 30 at. pct and Ni concentrations of 8, 18, and 30 at. pct. Additions of nickel to Fe-Cr alloys in the above concentration range are found to increase the chromium activity. The effect of nickel in increasing the chromium activity is greater at both greater chromium contents and lower temperatures. Formerly Graduate Student at The University of Michigan, is Staff Associate, Gulf Energy and Environmental Systems, LaJolla, California. This paper is based on a portion of a thesis submitted by F. N. MAZANDARANY in partial fulfillment of the requirements for the degree Doctor of Philosophy at The University of Michigan.     

Structure,strength, and fracture of electrodeposited nickel and Ni−Co alloys

Massive electrodeposits of nickel and Ni?Co alloys ranging up to 43 pct Co were examined microstructurally and tested to determine tensile properties and static and dynamic fracture toughness. Specimens were also tested after being annealed at 575 K. Annealing increased grain size, decreased yield, and ultimate strengths, and increased ductility and dynamic toughness. The as-plated Ni-43 Co was the only material to exhibit validK _IC values, averaging about 38 MN/m^3/2. In instrumented dynamic tests on precracked Charpy bars, the same material exhibited aK _Id of 50 MN/m^3/2. The yield strength of the Ni-43 Co alloy was 1154 MN/m². All the materials tested showed dimpled, ductile rupture fracture surfaces. The Hall-Petch behavior of the nickel indicated that it is much easier to initiate flow in normal grain boundary structures than in structures composed of dislocation cell walls.     

Diffusion of cobalt,chromium, and titanium in Ni3Al

Diffusion studies of cobalt, chromium, and titanium in Ni₃Al (γ′) at temperatures between 1298 and 1573 K have been performed using diffusion couples of (Ni-24.2 at. pct Al/Ni-24.4 at. pct Al-2.91 at. pct Co), (Ni-24.2 at. pct Al/Ni-23.1 at. pct Al-2.84 at. pct Cr), and (Ni-24.2 at. pct Al/Ni-20.9 at. pct Al-3.17 at. pct Ti). The diffusion profiles were measured by an electron probe microanalyzer, and the diffusion coefficients of cobalt, chromium, and tita-nium in γ′ containing 24.2 at. pct Al were determined from those diffusion profiles by Hall’s method. The temperature dependencies of their diffusion coefficients (m[su2]/s) are as follows: ~D_(Co) = (4.2 ± 1.2) × 1O^-3exp {-325 ± 4 (kJ/mol)/RT} ~D_(Cr) = (1.1 ± 0.3) × 10^-1 exp {-366 ± 3 (kJ/mol)/RT} and D_(Ti) = (5.6 ± 3.1) × 10¹ exp {-468 ± 6 (kJ/mol)/RT} The values of activation energy increase in this order: cobalt, chromium, and titanium. These activation energies are closely related to the substitution behavior of cobalt, chromium, and titanium atoms in the Ll₂ lattice sites of γ′; the cobalt atoms occupying the face-centered sites in the Ll₂ structure diffuse with the normal activation energy, whereas the titanium atoms oc-cupying the cubic corner sites diffuse with a larger activation energy that includes the energy due to local disordering caused by the atomic jumps. The chromium atoms which can occupy both sites diffuse with an activation energy similar to that of cobalt atoms.     

Carbide morphology in p/m IN-792

Adequate tensile and smooth stress rupture properties were achieved by heat treating p/m IN-792 to produce serrated grain boundaries pinned with a globular M₂₃C₆ precipitate. A notch-strengthened condition with K_t = 3.5 at 1400?F (1033 K)/90 ksi (620 MN/m²) was achieved when the material was heat treated to produce a grain size larger than about 76 Μm (ASTM G.S. #6). To obtain this grain size using realistic solution time-temperature heat treatments, the carbon content of the IN-792 should be maintained below 0.1 pct and preferably about 0.04 pct.     

Nitrogen solubility and aluminum nitride precipitation in liquid iron,Fe-Cr,Fe-Cr-Ni,and Fe-Cr-Ni-Mo alloys

The nitrogen solubility and aluminum nitride formation in liquid Fe-Al, Fe-Cr-Al, Fe-18 pct Cr-8 pct Ni-Al and Fe-18 pct Cr-8 pct Ni-Mo-Al alloys were measured by the Sieverts' method. The temperature range extended from 1823 to 2073 K, and the aluminum contents from 1.01 to 3.85 wt pct Al. Increasing aluminum content increases the nitrogen solubility. The effect of molybdenum additions was determined for 2, 4 and 8 wt pct Mo levels. The first and second order effects of chromium, nickel, molybdenum and aluminum on the activity coefficient of nitrogen in iron were determined. The first and second order effects of chromium, nickel and molybdenum on the activity coefficient of aluminum also were determined. The nitride precipitates were identified as stoichiometric aluminum nitride, AIN, by X-ray diffraction analysis. The lattice spacing was in good agreement with the ASTM standard patterns for AIN in both higher and lower Al content solutions. The solubility product of AIN increases with increasing aluminum concentration and with temperature in liquid iron and the iron alloys studied. However, the magnitudes of the solubility products of AIN in those alloys are different because of the effects of chromium and nickel additions. Additions of molybdenum show little effect on the solubility product of AIN. The standard free energy of formation of AIN in liquid iron is: δG? = -245,990 + 107.59 \T J/g-molAIN, based on the standard state of the infinitely dilute solution in liquid iron for aluminum and nitrogen, referred to a hypothetical one wt pct solution, and on the pure compound for A1N.     

Comparison of the substructures and properties of nickel,TD-Ni,chromel-A,inconel 600, and TD-NiCr following explosive-shock deformation

The substructures of Ni, TD-Ni, Chromel-A (80/20 NiCr), Inconel 600, and TD-NiCr following simultaneous explosive shock loading at pressures of 80, 180, 240, and 460 kbars were observed by transmission electron microscopy. Well defined dislocation cells having average diameters of 0.8, 0.3, 0.2, and 0.1 μ for the respective pressure level were observed in pure nickel, but were not developed to the same extent in annealed TD-Ni due to the presence of the ThO₂ particles having a mean particle diameter of 340Å. Deformation microtwins having an average width of 175Å occupied approximately 1 vol pct of the nickel substructure at 460 kbars, while twinning was inhibited by the ThO₂ particle distribution in the TD-Ni. Planar dislocation arrays observed in the Chromel-A and Inconel 600 were generally not observed to form in TD-NiCr following shock loading at 80 and 180 kbars. In addition, deformation microtwins having an average width of 150Å occupied 5 pct of the Chromel-A microstructure at the same pressure. At 460 kbars, twins occupied 18 pct of the Chromel-A and 24 pct of the Inconel 600 microstructure. No evidence of deformation twinning was observed in the TD-NiCr microstructure because of the inhibition afforded by the ThO₂ particle distribution having a mean particle diameter of 145Å in the annealed material. A relationship was observed for the residual hardness and dislocation density; and stacking-fault energy and the character of the microstructures.     

Interdiffusion in the Ni/TD-NiCr and Cr/TD-NiCr Systems

The diffusion of Ni and Cr into TD-NiCr has been studied over the 900 to 1100°C temperature range. The diffusion couples were prepared by electroplating Cr and Ni on polished TD-NiCr wafers. Concentration profiles produced as a result of isothermal diffusion at 905, 1000, and 1100°C were determined by electron microprobe analysis. The Boltzmann-Matano analysis was used to determine concentration dependent diffusion coefficients which were found to compare favorably with previously reported values. These data suggest that 2 vol pct ThO₂ distribution has no appreciable effect on the rates of diffusion in TD-NiCr with a large grain size. This supports the view that an inert dispersoid in an alloy matrix will not in itself lead to enhanced diffusion unless a short-circuit diffusion structure is stabilized. Formerly Asst. Prof, of Metallurgical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Va.     

Effects of Si and Ni powder refining on the formation mechanism of nickel silicides induced by mechanical alloying

The synthesis of the Ni₂Si, Ni₅Si₂, and NiSi phases has been investigated by mechanical alloying (MA) of Ni-33.3 at. pct Si, Ni-28.6 at. pct Si, and Ni-50 at. pct Si powder mixtures. As-received and 60-minute premilled elemental powders were subjected to MA. The average surface area of the premilled Ni powder particles, which had a flaky shape, was 3.5 times larger than that of the as-received Ni powder particles, which had a spherical shape. The as-received Si powder was angular in shape and the mean particle size was 19.1 μm, whereas the mean particle size of the premilled Si powder was 10 μm. A self-propagating high-temperature synthesis (SHS) reaction, followed by a slow solid-state diffusion reaction, was observed to produce Ni silicide phases during MA of the elemental powders. The reactants and the product, however, coexisted for a long period of MA time. On the other hand, only the SHS reaction was observed to produce Ni silicides during MA of the premilled elemental powders, indicating that Ni silicides formed rather abruptly in a short period of MA time. The mechanisms and reaction rates for the formation of Ni silicides via MA appeared to be influenced by the elemental powder particle size and shape as well as the heat of formation of the products.