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.     

Hydride precipitation and dislocation substructures in Ti-5 pct Al-2.5 pct Sn

Hydrides do not precipitate in Ti-5 pct Al-2.5 pct Sn stressed at room temperature at levels less than 80 pct of the yield stress. Above this level basal and γ-hydrides occur, the relative amounts of which are probably related to the relative amounts of basal and prism slip intersecting the stressed specimen surface.     

Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

The precipitation behavior of a Zr-2.5 wt pct Nb alloy

The morphology, crystallography, and nature of precipitates in a quenched and aged Zr-2.5 wt pct Nb alloy has been studied by transmission electron microscopy. The needle-shaped matrix precipitates and equiaxed twin boundary nucleated precipitates produced by aging at 500 °C were the equilibrium Nb-rich β₂ phase. On aging at 600 °C, the matrix precipitation was a mixture of β₂ needles and coarse metastable Zr-rich β₁ particles, while only β₁ particles were found at twin boundaries. The growth direction of the needle-shaped particles, 6.6 deg to 8.2 deg from (1-100)_h, and their orientation relationship can be predicted by an invariant line strain model. The β₁ precipitates have the Burgers orientation relationship. The formation of metastable β₁ and stable β₂ particles is considered from the free energy approach of Menon, Banerjee, and Krishnan.     

Interphase boundary precipitation in a Ti-1.7 at. pct Er alloy

Microstructures in a Ti-1.7 at. pct Er alloy were studied in the arc-cast, rapidly solidified, and annealed conditions. Transmission electron microscopy (TEM) of the rapidly solidified materials revealed 3- to 20-nm-diameter precipitates that were distributed in regularly spaced, approximately planar sheets throughout equiaxed α Ti grains. The precipitate sheet morphology is similar to the interphase boundary carbide sheets that have been documented in many alloy steels. In addition, precipitate fibers with cross sections of approximately 5 nm and up to 500 nm in length were often found adjacent to particle sheets. Electron diffraction experiments showed that the structure and lattice spacings of the sheet and fibrous particles are consistent with elemental erbium. Subsequent annealing treatments resulted in the formation of a face-centered cubic allotrope of Er₂O₃. The present work describes the precipitate morphologies and crystallography and discusses the applicability of current ledge growth models of interphase boundary precipitation to titanium-erbium alloys.     

Eddy current study of solidification in lead and lead 20 pct tin

The use of eddy currents to nondestructively evaluate metallic solidification is modelled in one dimension. Calculated responses of the reflected signals show characteristics that can be identified with different solidification morphologies, the extent of freezing, and temperature changes. Experimental verification of the calculated results is done with lead and a lead-20 tin alloy. Morphological details of the solidification for both lead and lead-20 tin alloy are extracted from the eddy current responses in real time. Features such as uniform freezing front motion in pure lead and phase formation in the alloy are detected. Finally, compensation for the thermal gradients in the copper mold and the phase shift in the balancing network allow for good agreement between the eddy current response and the calculated response for freezing.     

The solubility product of manganese sulfide in 3 pct silicon-iron at 1270 to 1670 K

The solubility product of manganese sulfide in 3 pct silicon-iron alloy was determined by two experimental methods. In one method, specimens of three alloys containing different amounts of manganese and sulfur were annealed at various temperatures and quenched; by transmission and scanning electron-microscopy, the lowest temperature at which manganese sulfide was completely dissolved was located for each alloy. In the other method, specimens of two of the three alloys were annealed together in capsules so that only one contained manganese-sulfide phase after equilibration. The solubility product of manganese sulfide was calculated from the final chemical analyses. The experimental results from both techniques could be fitted with a single line represented by an equation applicable to alloys with about 0.1 pct Mn from 1270 to 1670 K: Log [(pct Mn) · (pct S)] = •10,590/T + 4.092 whereT is in kelvins. This line lies between the values reported in two previous and nonconcordant investigations; at 1490 to 1540 K, the three sets of results roughly converge. Determination of the solubility product should help in understanding the application of manganese-sulfide precipitates to texture control in 3 pct silicon-iron alloys.     

The solubility product of manganese sulfide in 3 pct silicon-iron at 1270 to 1670 K

The solubility product of manganese sulfide in 3 pct silicon-iron alloy was determined by two experimental methods. In one method, specimens of three alloys containing different amounts of manganese and sulfur were annealed at various temperatures and quenched; by transmission and scanning electron-microscopy, the lowest temperature at which manganese sulfide was completely dissolved was located for each alloy. In the other method, specimens of two of the three alloys were annealed together in capsules so that only one contained manganese-sulfide phase after equilibration. The solubility product of manganese sulfide was calculated from the final chemical analyses. The experimental results from both techniques could be fitted with a single line represented by an equation applicable to alloys with about 0.1 pct Mn from 1270 to 1670 K: \(Log [(pct Mn) \cdot (pct S)] = - 10,590/T + 4.092\) whereT is in kelvins. This line lies between the values reported in two previous and non-concordant investigations; at 1490 to 1540 K, the three sets of results roughly converge. Determination of the solubility product should help in understanding the application of manganese-sulfide precipitates to texture control in 3 pct silicon-iron alloys.     

Cu precipitation in a prestrained Fe-1.5 wt pct Cu alloy during isothermal aging

The control of Cu precipitation at low temperatures, e.g., bake hardening of Cu bearing steels, has recently attracted considerable attention due to the potential of achieving good formability and high strength. An Fe-1.5 wt pct Cu alloy, solution treated and 10 pct prestrained, exhibits a two-step age-hardening behavior, i.e., a smaller, but substantial hardening around 200 °C to 300 °C and a major hardening around 500 °C, while only the latter hardening occurs in undeformed specimens. The precipitation behavior of nanoscale Cu particles or bcc Cu clusters that plays a major role in age hardening was simulated by Cahn-Hilliard nonclassical nucleation theory and the Langer-Schwartz model. Simulation results are compared with the distribution of Cu particles observed under three-dimensional atom probe field ion microscope (3-D APFIM) and transmission electron microscope (TEM), and age hardening behavior as well. The increase in hardness in prestrained specimens at low temperatures (≤400 °C) can be ascribed to Cu particles nucleated preferentially at dislocations or to Cu particles that were formed in the matrix as early as at dislocations presumably due to excess vacancies introduced by prestraining.     

Effect of stress during aging on the precipitation of θ′ in Al-4 Wt pct Cu

Experiments have been made on Al-4 wt pct Cu single crystals to determine the influence of stress, applied during aging, on precipitation. Without stress, θ platelets precipitate on all (100) planes. Transmission electron microscopy showed a marked effect of aging stress on the microstructure. A compressive stress of approximately 48 MPa (～ 7,000 psi) parallel to [001] favored precipitation on (010) and (100) and inhibited precipitation on (001). When a tensile stress of similar magnitude was applied parallel to [001], precipitation was favored on (001) and inhibited on (010) and (100).