High-temperature oxidation of pure titanium in CO2 and Ar-10%CO2 atmospheres

The oxidation behavior of pure titanium has been investigated in the temperature range of 1000 K to 1300 K in CO₂ or Ar-10%CO₂. Optical microscopy, electron probe microanalyses, and X-ray measurements on the oxide scales formed during oxidation indicate that their structures are nearly independent of temperature and the corrosion atmosphere. The scales consisted of two layers, an external one and an internal one, having a rutile (TiO₂) structure. The parabolic rate law was confirmed for growth of the external scale and the permeation depth of oxygen in titanium with apparent activation energies of 266 and 226 kJ/mol, respectively. The rate-determining diffusion species in the oxidation processes are discussed.     

Gas entrainment rate and flow pattern of vertical plunging liquid jets

For a vertical plunging liquid jet system using various liquids, the effects of the operating conditions including the nozzle length-to-diameter ratio on the gas entrainment rate Q_g were evaluated experimentally. The differences in Q_g were related to the changes in the jet shape before plunging and the velocity of the plunging liquid jet at the point where the gas sheath breaks up. Empirical relationships were also presented to predict the maximum depth Z of bubbles entrained by the diffusing jet. The changes of Z were further discussed in terms of the downward liquid velocity distribution in the submerged two-phase region, which depends mainly on the size of entrained bubbles.     

Effects of cold rolling and annealing on the structure of γ″ precipitates in a Ni-18Cr-16Fe-5Nb-3Mo alloy

A Ni-18Cr-16Fe-5Nb-3Mo alloy (a modified alloy 718, referred to herein as 718M), in which the γ″ phase is the only precipitation-strengthening phase, was made by substituting most of the aluminum and titanium in INCONEL 718 with niobium. The specimens of alloy 718M were solid-solution heat treated, aged at 1033 to 1073 K for up to 360 ks, and cold rolled to a reduction of 10 to 15 pct at room temperature. After subsequent annealing heat treatments, at the same temperatures used for aging, the morphological and structural changes of the γ″ precipitates were investigated by transmission electron microscopic observation using carbon extraction replicas. Although stacking faults were not noticed in the γ″ precipitates extracted from the aged 718M alloy, stacking faults introduced in the {112} _γ″ planes by cold rolling of the samples were confirmed. The stripelike contrast due to the stacking faults becomes less clear with increasing annealing time. With longer annealing times, necking occurs between the stripes, which eventually separate into plural particles. The selected-area electron diffraction (SAED) patterns of the cold-rolled and annealed particles show that the metastable γ″ precipitates with stacking faults are transformed gradually into a stable δ phase. In this article, we discuss the basis of morphological and structural changes in detail. INCONEL is a trademark of INCO Alloys International, Inc., Huntington, WV.     

Gas entrainment behavior of vertical plunging liquid jets in terms of changes in jet surface length

For a vertical jet system using various liquids, the shape change of liquid jets discharged from nozzles having different nozzle aspect ratios was evaluated in terms of the surface length l_m of the jet flow, and the relation between the change of l_m and the gas entrainment behavior was investigated. It was found that the way of changing of l_m with varying operating conditions or the liquids corresponded well to that of changing of the gas entrainment rate Q_g, that is, the change of l_m of the jet before plunging could be closely related to the change of Q_g.     

DIRECTIONALLY SOLIDIFIED MICROSTRUCTURE OF AN ULTRA-HIGH TEMPERATURE Nb-Si-Ti-Hf-Cr-Al ALLOY

The directionally solidified samples of an ultra-high temperature Nb-Si- Ti-Hf-Cr-Al alloy have been prepared with the use of an electron beam floating zone melting (EBFZM)furnace, and their microstructural characteristics have been analyzed. All the primary dendrites of Nb solid solution (Nbss), euteetic colonies of Nbss plus (Nb, Ti)3 Si//(Nb, Ti)5 Si3 and chains of (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates align along the growth direction of the samples. With increasing of the withdrawing rate, the microstructure is refined, and the amounts of Nbss (Nb, Ti)3 Si/(Nb, Ti)5 Si3 euteetic colonies and (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates increase. There appear nodes in the (Nb, Ti)3 Si/(Nb, Ti)5 Si3 plates.     

Deformed microstructure of the single-crystal superalloy NASAIR 100 at 1050 °C

The microstructure of the [001] oriented single-crystal superalloy NASAIR 100, solidified respectively with the planar, cellular, coarse-dendritic, and fine-dendritic solid/liquid (S/L) interfaces, deformed at 1050 °C and 160 MPa, was investigated with the attempt to reveal the variation of γ′ structures in the necked zones of creep specimens and, furthermore, to explain that the dendritic single crystals constantly possessed superior stress-rupture lives. During the stress-rupture tests, it was found that directional coarsening of γ′ precipitates occurred and that γ′ rafts perpendicular to the tensile load axis, [001] orientation, formed. The γ′ rafts in the necked zones of tensile creep specimens were thickened significantly and were no longer perpendicular to, but gradually inclined to, the tensile load axis. Additionally, the γ′ rafts were much thicker and showed less perfect morphology in both the planar and cellular single crystals than in the dendritic single crystals. From the observation of dislocation configurations, it was evident that the shearing of γ′ precipitates was the main movement of dislocations when passing the γ′ particles. At the primary creep stages of both the planar and cellular single crystals, the slip of dislocations was inhomogeneous. However, the shearing of γ′ precipitates in the dendritic single crystals was homogeneous, resulting in higher creep resistance.     

Fast amorphization and crystallization in Al-Fe binary system by high-energy ball milling

Large amount of amorphous phase of Al-Fe binary system was obtained by MA of elemental powders using a high-energy ball mill at milling intensity of 150G (G is the gravitational acceleration). XRD, HRTEM and DSC were used to analyze the process of amorphization and crystallization. The time required achieving almost complete amorphous state is only 4.2 ks for Al-25 at.%Fe system and 3 ks for Al-30 at.%Fe system, respectively. The time of amorphous formation is very shorter than that of previous reports on Al-Fe binary system. Further milling causes rapid crystallization of the amorphous phase. By analysis of S(Q), the presence of a strong Al-Fe chemical short-range order in the amorphous matrix is suggested. Moreover, the superstructure of these Al-Fe clusters in the amorphous matrix is similar to the solid structure of Al₅Fe₂, and the clusters transform into the nucleus of Al₅Fe₂ intermetallic compound under the action of milling energy.     

High-Temperature Scale Formation of Fe–36% Ni Bicrystals in Air

Fe–36% Ni bicrystals were oxidized at temperatures from 1100 to 1250 K in air, both under and without tensile stress. The scales were divided into three categories: external scales, intragranular subscales, and intergranular subscales. A linear relationship exists beween the thickness of scales and subscales and the square root of oxidation time. Tensile stress significantly accelerates intergranular oxidation, but has essentially no influence on either the formation of intragranular subscales or that of external scales. The external scales are composed of -Fe₂O₃ and Fe₃O₄ after a longer oxidation time. The intergranular and intragranular subscales consist of Fe₃O₄ and FeO particles and an Ni-enriched matrix. Cellular or dendritic oxide nodules sometimes exist in the intragranular subscales, causing their frontier interfaces to significantly undulate.