Mechanical alloying of nb-al powders

The effect of mechanical alloying (MA) on solid solubility extension, nanostructure formation, amorphization, intermetallic compound formation, and the occurrence of a face-centered cubic (fcc) phase in the Nb-Al system has been studied. Solid solubility extension was observed in both the terminal compositions and intermetallic compounds: 15 pct Nb in Al and 60 pct Al in Nb, well beyond the equilibrium and even rapid solidification levels (2.4 pct Nb and 25 pct Al, respectively) and increased homogeneity range for the NbAl₃ phase. Nanostructured grains formed in all compositions. In the central part of the phase diagram, amorphization occurred predominantly. Only NbAl₃, the most stable intermetallic, formed during MA; in most cases, a subsequent anneal was required. On long milling time, an fcc phase, probably a nitride, formed as a result of contamination from the ambient atmosphere.     

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     

Fracture toughness and R-Curve behavior of laminated brittle-matrix composites

The fracture toughness and resistance-curve behavior of relatively coarse-scale, niobium/niobium aluminide (Nb/Nb₃Al) laminated composites have been examined and compared to other Nb/Nb₃Al composites with (in situ) Nb particulate or microlaminate reinforcements. The addition of high aspectratio Nb reinforcements, in the form of 20 vol. pct of 50- to 250-μm-thick layers, was seen to improve the toughness of the Nb₃Al intermetallic matrix by well over an order of magnitude, with the toughness increasing with Nb layer thickness. The orientation of the laminate had a small effect on crack-growth resistance with optimal properties being found in the crack arrester, as compared to the crack divider, orientation. The high fracture toughness of these laminates was primarily attributed to large (∼1- to 6-mm) crack-bridging zones formed by intact Nb layers in the crack wake; these zones were of sufficient size that large-scale bridging (LSB) conditions generally prevailed in the samples tested. Resistance-curve modeling using weight function methods permitted the determination of simple approximations for the bridging tractions, which were then used to make smallscale bridging (SSB) predictions for the steady-state toughness of each laminate.     

Dynamic compaction of titanium aluminides by explosively generated shock waves: Experimental and materials systems

Different approaches to compact cylinders of titanium aluminide powders by explosively generated shock waves were explored. Two basic compositions of the titanium aluminide powders produced by the rapid solidification rate (RSR) technique were used: Ti-21 wt pct Nb-14 wt pct Al and Ti-30.9 wt pct Al-14.2 wt pct Nb. A double-tube design utilizing a flyer tube was used in all experiments. Experimental parameters that were varied were initial temperature, explosive quantity, and explosive detonation velocity. The major problem encountered with shock consolidation of titanium aluminides was cracking. Titanium aluminide powders were also mechanically blended with niobium powders in one case and elemental mixtures of aluminum and titanium powders in the other case. Enhanced bonding and decreased cracking were observed in both cases. In the former case, the addition of niobium powder provided a ductile binder medium which assisted in consolidation. In the latter case, due to the additional heat generated and melting produced by the shock-induced reactions between Ti and Al, significant improvements in bonding of the titanium aluminide powders were observed.     

Structure and properties of Nb-Al alloys prepared by powder metallurgy

The structure and short-time strength of Nb-Al alloys of two compositions prepared by powder metallurgy are studied. The mechanical alloying of niobium with aluminum in a planetary ball mill in air is shown to result in simultaneous alloying of niobium with oxygen. During subsequent vacuum high-temperature sintering, disperse particles of a complex oxide, whose tentative composition is (AlNb)₂O₃, form in the alloy structure. The short-time strength at 1250°C of the prepared alloys exceeds that of nickel-aluminum superalloys.     

Causes of the refractory-phase formation during melting of Al-Nb-Si master alloys

Some causes of the formation of refractory phases during making of an Al-Nb-Si master alloy are considered. Chemical analysis, X-ray diffraction, and electron-probe microanalysis of master alloy ingots melted by aluminothermic and aluminocalciumthermic methods demonstrate that partial (up to 5%) substitution of calcium for aluminum in a charge does not degrade the quality of the master alloy. It is experimentally shown that the interaction of NbAl₃ and Nb₂Al with niobium pentoxide leads to the formation of elemental niobium. The presence of materials containing niobium aluminides in a charge can affect the perfection of the master alloy.     

Phase Transformations in Nb-Al-Ti alloys

Phase relationships as well as morphological and crystallographic features in Nb-rich Nb-Al and Nb-Al-Ti alloys have been investigated. The phase boundaries involving the bcc and Nb₃Al (A15 structure) were experimentally determined and several isothermal sections of the Nb-rich corner of the Nb-AI-Ti phase diagram established. The present findings show that (a) the solubility of Al in Nb is considerably less than that reported previously, (b) the high-temperature bcc phase undergoes an ordering transformation to the B2 structure, and (c) the ω phase also forms in these alloys. The sequence of decomposition of the high-temperature bcc phase during isothermal decomposition in the bcc + Nb₃Al phase field has been systematically studied in these alloys. A wide variety of morphological features were found to be associated with the Nb₃Al precipitates that formed in the bcc/B2 matrix during isothermal heat treatments. The lengthening kinetics of the plate-shaped Nb₃Al precipitates were also studied. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Production of niobium powder by direct electrochemical reduction of solid Nb2O5 in a eutectic CaCl2-NaCl melt

The method of electro-deoxidation was used to reduce solid Nb₂O₅ to niobium metal in a CaCl₂-NaCl eutectic melt. The direct electrochemical reduction of Nb₂O₅ was achieved by electrolysis in the eutectic melt at 1123 and 1173 K, respectively, at a controlled potential of 3.1 V, below the decomposition potential of the salts. Analysis of the anodic reaction gases carried by a flow of dry, high-purity argon confirmed that the preferred cathodic reaction is the oxygen ionization of the cathode to form oxygen ions that are successively dissolved in the chloride melt and then discharged at the graphite-rod anode. Chlorine ions are unlikely to be discharged at the anode. The niobium metal powders prepared contained as low as 2311 mass ppm oxygen, clearly demonstrating that the electrodeoxidation method is applicable to the reduction of solid Nb₂O₅ in chloride melts. The kinetics and mechanisms of reduction of the porous pellets of Nb₂O₅ are discussed in detail based on the measured current-time behavior, together with the microstructural analysis of the sintered Nb₂O₅ pellets and the phases present in the partially reduced samples obtained at various times of reduction.     

Shock densification/hot isostatic pressing of titanium aluminide

Consolidation of rapidly solidified titanium aluminide (Ti₃Al) powders employing explosive shock pressure followed by hot isostatic pressing (“hipping”) was carried out successfully. Shock densification was achieved by using a double tube design in which the flyer tube was explosively accelerated, impacting the powder container. Elemental mixtures of Ti (15 wt pet) and Al (15 wt pct) powders were added to intermetallic compound powders (Ti₃Al). Hipping was used to chemically induce bonding between Ti₃Al particles. The highly exothermic reactions were activated by hipping at 1000 ‡C and enhanced the bonding between the inert intermetallic powders. Compression tests indicated strong bonding between Ti₃Al particles. Well-bonded Ti₃Al compacts having an average ultimate compressive strength of 2 GPa and compressive fracture strain of 20 pct were produced by this technique. The ultimate tensile strengths, due to the presence of flaws in the microstructure (microcracks and voids) and intergranular fracture observed in the reacted regions, were much lower (~250 MPa).     

Influence of the precursor composition and the reduction conditions on the characteristics of magnesium-thermic niobium powders

The influence of the composition and the conditions of the reduction of niobium oxide compounds with magnesium vapor on the specific surface area of the produced metallic powder is studied. When magnesium niobate Mg₄Nb₂O₉ is used as a precursor, the specific surface of the powder increases by several times compared to that of an Nb₂O₅ precursor. Niobium powders with the specific surface up to 150 m² g^–1 (calculated particle size 4.7 nm) and a bulk density of 0.8 g cm^–3 are formed by the reduction of Mg₄Nb₂O₉. The powders are characterized by a mesoporous structure, and the most part of the specific surface consists of pores with a diameter less than 4 nm.     

Structural states and electrical conductivity of oxidized niobium nanopowders

The structure of niobium nanopowders (particle size 0.03–0.07 μm) oxidized in air is studied by X-ray diffraction. The nanopowder particles have a significant fraction of an amorphous phase. The amorphous component is likely to block the well-known mechanism of niobium oxidation Nb → Nb_s.s → Nb₆O → NbO → NbO_x, which was proposed on the basis of the results of studying the oxidation of niobium powders at high temperatures. Here, Nb_s.s is the solid solution of oxygen in niobium and NbO_x are the higher niobium oxides NbO₂ and Nb₂O₅. The amorphization of the surface of niobium nanopowders oxidized at 20°C can be one of the main causes of a rather high electrical resistivity (ρ ≈ 10⁸ Θ cm) of the samples compacted from these powders.     

Dynamic phase transformation during superplastic deformation of Nb/Nb<Subscript>3</Subscript>Al <Emphasis Type="Italic">in-situ</Emphasis> composite

Nb_ss/Nb₃Al in-situ composite with the nominal composition of Nb-16 mol pct Al-1 mol pct B, consisting of bcc niobium solid solution (Nb_ss) and A15 ordered Nb₃Al, was synthesized by arc melting, homogenization annealing, and isothermal forging, and their superplastic deformation behavior was investigated by tensile tests and microstructure observations. Maximum superplastic elongation over 750 pct was obtained at 1573 K and at a strain rate of 1.6 × 10⁻⁴ s⁻¹ for as-forged specimens. Phase transformation from Nb_ss to Nb₃Al was observed to occur during superplastic deformation. Dynamic phase transformation during superplastic deformation progresses more quickly than static phase transformation during annealing without applied stress. Dynamic phase transformation is accompanied by phase-boundary migration, which operates as an accommodation process of grain-boundary sliding. Dislocation creep dominates deformation and grain-boundary sliding is inhibited at a high strain rate, while grain-boundary sliding and cavity formation are promoted at a low strain rate because of insufficient accommodation of grain-boundary sliding arising from sluggish dynamic phase transformation. It is concluded that there exists an optimum strain rate that guarantees the grain-boundary sliding and the rapid dynamic phase transformation to achieve maximum superplastic elongation.     

Effect of LiF as Sintering Agent on the Densification and Phase Formation in Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-4 Wt Pct Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> Ceramic Compound

Different amounts of LiF were added to an Al₂O₃-4 pct Nb₂O₅ basic ceramic, as sintering agent. Improved new ceramics were obtained with LiF concentrations varying from 0.25 to 1.50 wt pct and three sintering temperatures of 1573 K, 1623 K, and 1673 K (1300 °C, 1350 °C, and 1400 °C). The addition of 0.5 wt pct LiF yielded the highest densification, 94 pct of the theoretical density, in association with a sintering temperature of 1673 K (1400 °C). Based on X-ray diffraction (XRD), this improvement was due not only to the presence of transformed phases, more precisely Nb₃O₇F, but also to the absence of LiAl₅O₈. The preferential interaction of LiF with Nb₂O₅, instead of Al₂O₃, contributed to increase the alumina sintering ability by liquid phase formation. Scanning electron microscopy (SEM) results revealed well-connected grains and isolated pores, whereas the chemical composition analysis by energy dispersive energy (EDX) indicated a preferential interaction of fluorine with niobium, in agreement with the results of XRD. It was also observed from thermal analysis that the polyethylene glycol binder burnout temperature increased for all LiF concentrations. This may be related to the formation of hydrogen bridge bonds.     

Redox Equilibrium of Niobium in Calcium Silicate Base Melts

The oxidation state of niobium has been determined at 1873 K (1600 °C) in CaO-SiO₂-NbO _x melts with CaO/SiO₂ ratios (mass pct) of 0.66, 0.93 and 1.10, and 5.72 to 11.44 pct Nb₂O₅ (initial). The slag samples were equilibrated with gas phases of controlled oxygen pressure, then quenched to room temperature and analyzed chemically. The niobium is mainly pentavalent with small amounts in the tetravalent state. It was found that the Nb⁵⁺/Nb⁴⁺ ratio increases with oxygen pressure at a constant CaO/SiO₂ ratio and constant content of total niobium, closely according to the ideal law of mass action, which is proportional to \textp_\textO2 ^1/4 . {\text{p}}_{{{\text{O}}_{2} }}^{1/4} . The ratio also increases with total niobium content, and it seems to have a maximum at a basicity of about 0.93. The color of the solidified slag samples is described and is explained with the help of transmission spectra.     

Reduction of CaO and MgO Slag Components by Al in Liquid Fe

This study documents laboratory-scale observations of reactions between Fe-Al alloys (0.1 to 2 wt pct Al) with slags and refractories. Al in steels is known to reduce oxide components in slag and refractory. With continued development of Al-containing Advanced High-Strength Steel (AHSS) grade, the effects of higher Al must be examined because reduction of components such as CaO and MgO could lead to uncontrolled modification of non-metallic inclusions. This may lead to castability or in-service performance problems. In this work, Fe-Al alloys and CaO-MgO-Al₂O₃ slags were melted in an MgO crucible and samples were taken at various times up to 60 minutes. Inclusions from these samples were characterized using an automated scanning electron microscope equipped with energy dispersive x-ray analysis (SEM/EDS). Initially Al₂O₃ inclusions were modified to MgAl₂O₄, then MgO, then MgO + CaO-Al₂O₃-MgO liquid inclusions. Modification of the inclusions was faster at higher Al levels. Very little Ca modification was observed except at 2 wt pct Al level. The thermodynamic feasibility of inclusion modification and some of the mass transfer considerations that may have led to the differences in the Mg and Ca modification behavior were discussed.     

Nonequilibrium synthesis of nbai3 and Nb-Ai-V

In this study, the isothermal oxidation behavior of laser-clad NbAl₃ has been investigated in the temperature range between 800 °C and 1400 °C in air. The effect on oxidation of vanadium microalloying, used to increase the ductility of the otherwise brittle NbAl₃ and discussed in Part I, [1] has also been considered. Bulk and surface oxide chemistry has been investigated using X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Oxidation kinetics have been determined from weight gain data. The XPS and X-ray diffraction data show that NbAl₃ does not exclusively form a protective A1₂O₃ layer when oxidized in air. The oxidation products at 800 °C are a mixture of Nb₂O₅ and A1₂O₃, while at 1200 °C, a mixture of NbAlO₄, Nb₂O₅, and Al₂O₃ is formed. At 1400 °C, a mixture of NbAlO₄, A1₂O₃, NbO₂, NbO_2.432, and Nb₂O₅ forms. Upon addition of vanadium, the oxidation rate is found to dramatically increase and may be related to the formation of (Nb, V)₂O₅ and VO₂, which grows in favor of protective A1₂O₃. Consequently, although vanadium may be a good additive in terms of its potential for improving ductility in NbAl₃, it is not in terms of its deleterious effects on oxidation.     

Interfacial Reaction Between CaO-SiO2-MgO-Al2O3 Flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) Steel at 1873 K (1600 °C)

This study investigated the interfacial reaction kinetics and related phenomena between CaO-SiO₂-MgO-Al₂O₃ flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) steel, which simulates transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels at 1873 K (1600 °C). It also examines the effect of changes in the composition of the steel and slag phases on the interfacial reaction rate and the reaction mechanisms. The content of Al and Si in the 1 mass pct Al-containing steel was found to change rapidly within the first 15 minutes of the reaction, but then it remained relatively constant. The content of Al and Si in the 3 to 6 mass pct Al-containing steels, in contrast, changed continuously throughout the entire reaction time. In addition, the content of Mn in the 1 mass pct Al-containing steels initially decreased with increasing time, but the content did not change in the 3 to 6 mass pct Al-containing steels. Furthermore, the mass transfer coefficient of Al, k _Al, in the 1 mass pct Al-containing systems was significantly higher than that in other systems; i.e., the k _Al can be arranged such that 1 mass pct Al systems >> 3 mass pct Al systems ≥ 6 mass pct Al systems. The compositions of the final slags were close to the saturation lines of the [Mg,Mn]Al₂O₄ and MgAl₂O₄ spinels when the slags reacted with 1 mass pct Al and 3 to 6 mass pct Al-containing steels, respectively. These results, which show the effect of Al content on the reaction phenomena, can be explained by the significant increase in the apparent viscosity of the slags that reacted with the 3 to 6 mass pct Al-containing steels. This reaction was likely caused by the precipitation of solid compounds such as MgAl₂O₄ spinel and CaAl₄O₇ grossite at locally alumina-enriched areas in the slag phase. This analysis is in good accordance with the combination of Higbie’s surface renewal model and the Eyring equation.     

A new phase in a rapidly solidified and consolidated NbAl3-1 Pct TiB2 alloy

Transmission electron microscopy (TEM) investigation was conducted to characterize the microstructure of a rapidly solidified and consolidated NbAl₃-l pct TiB₂ alloy. The study demonstrated that the alloy consisted of a two-phase microstructure with second-phase particles distributed uniformly in a NbAl₃ matrix. The dominant particle was identified to be an intermetallic phase having a chemical stoichiometry of Nb₃Al₂ and a hexagonal crystal structure (a = 0.78 andc = 0.52 nm). The structure was determined to belong to the P6₃/mcm space group. In addition, dispersion of fine NbB particles was also observed. Although the existing Al-Nb binary phase diagrams do not indicate a phase of the type Nb₃Al₂, it is suggested that formation of such a phase is likely based on analogy with Al-Zr and Al-Hf systems.