Niobium Aluminide Research Articles

Preparation of aluminium coated niobium aluminide by high speed impact treatment: K. Katoh et al.(National Industrial Research Inst. of Nagoya, Nagoya, Japan.) J. Japan Soc. Powder and Powder Metallurgy, Vol 42, No 3, 1995, 330–335. (In Japanese)

Preparation of aluminium coated niobium aluminide by high speed impact treatment: K. Katoh et al.(National Industrial Research Inst. of Nagoya, Nagoya, Japan.) J. Japan Soc. Powder and Powder Metallurgy, Vol 42, No 3, 1995, 330–335. (In Japanese)

Microstructure and strength of binary and tantalum alloyed two-phase Nb ss/Nb 3Al base alloys

Niobium aluminide (Nb 3Al) base intermetallic compounds are potential candidates for applications at temperatures above the operating temperatures of currently used superalloys, provided their low-temperature ductility and fracture toughness and high-temperature oxidation resistance are improved. Two-phase (Nb ss/Nb 3Al) materials with different volume fractions depending on the aluminium content were prepared by ingot metallurgy followed by isothermal forging and heat treatment. The addition of tantalum (about 10 mol%) has no influence on the phase content, volume fraction and grain size. The compressive strength of both the binary and ternary alloys investigated depends strongly on the aluminium content. Increasing the volume fraction of Nb ss phase decreases the strength at all temperatures tested. The addition of tantalum improves the high-temperature strength of Nb ss/Nb 3Al alloys.

Development of niobium aluminide-based composites with improved toughness using treated reinforcements

Many ordered intermetallic compounds exhibit attractive properties for high temperature structural aerospace applications. The major obstacles limiting the use of these materials are their low ductility and toughness at room temperature and poor creep resistance at elevated temperatures. In this investigation “treated” silicon carbide whisker and niobium particle reinforced niobium aluminide matrix composites have been fabricated by reaction hot pressing in vacuum. Elemental matrix powders react exothermally and form the niobium aluminide matrix in situ. A suitable hot pressing cycle has been established for the successful development of the niobium aluminide matrix, while reducing adverse reactions between the matrix and the reinforcements. The composites with “treated” reinforcements are expected to have high resistance to creep at elevated temperatures and they show relatively low hardness and higher toughness in comparison with the composites reinforced with untreated whiskers.

Preparation of niobium aluminide dispersion strengthened with niobium boride : T. Takahashi et al (Hyogo Prefecture Inst. of Industrial Research, Kobe, Japan), J. Japan Soc. Powder and Powder Metallurgy, vol 41, no 2, 1994, 166–169. (In Japanese)

Spray formed T15 high speed steel (HSS) billets were deposited using a state-of-the-art twin-atomiser spray forming facility. The effects of post thermo-mechanical processing (hot isostatic pressing and hot forging) and heat treatment on the microstructure and mechanical properties were investigated. As-deposited billet has a density over 99.3% of the theoretical value and no measurable macro-segregation was observed. The microstructure consists of the equiaxed grains with mean size of ∼18 μm and MC- and M6C-type carbides non-uniformly distributed inside the grains and along the grain boundaries. After optimal thermo-mechanical processing and heat treatment, the microstructure was composed of equiaxed fine tempered martensites, and refined M6C and MC spherical carbides particles uniformly distributed along the grain boundaries and inside the grains. The hardness reached HRC68 after thermo-mechanical processing, and the corresponding impact toughness and bending strength reached 27 J/cm2 and 4600 MPa respectively. Although HIP cannot increase the bending strength significantly, it can effectively improve the impact toughness through refining and globurizing carbides.

Mechanistic and processing studies in combustion synthesis of niobium aluminides

Combustion synthesis of NbAl3 and Nb2Al was studied using the volume combustion mode. The effects of heating rate and green density were examined for NbAl3 synthesis. The effect of green density was also investigated for Nb2Al. Greater reaction completion was achieved at higher heating rates and green densities. In both NbAl3 and Nb2Al samples, the reaction was initiated above the melting point of Al. Quenching (Nb+3Al) samples pressed at relatively high and low densities below the ignition temperature, and results of a particle-foil experiment, identified the spreading characteristic of molten Al over Nb, providing mechanistic details about niobium aluminide product formation.

Toughening mechanisms in ductile niobium-reinforced niobium aluminide (Nb/Nb3Al) in situ composites

Anin situ study has been performed in the scanning electron microscope (SEM) on a niobium ductilephase-toughened niobium aluminide (Nb/Nb3Al) intermetallic composite to examine the crack-growth resistance-curve (R-curve) behavior over very small initial crack extensions, in particular over the first ~500 μm of quasi-static crack growth, from a fatigue precrack. The rationale behind this work was to evaluate the role of toughening mechanisms, specifically from crack bridging, in the immediate vicinity of the crack tip and to define the size and nature of bridging zones. Although conventional test methods, where crack advance is monitored typically over dimensions of millimeters using compliance or similar techniques, do not show rising R-curve behavior in this material,in situ microscopic observations reveal that bridging zones resulting from both uncracked Nb3Al ligaments and intact Nb particles do exist, but primarily within ~300 to 400 μm of the crack tip. Accordingly, rising R-curve behavior in the form of an increase in fracture resistance with crack growth is observed for crack extensions of this magnitude; there is very little increase in toughness for crack extensions beyond these dimensions. Ductile-phase toughening induced by the addition of Nb particles, which enhances the toughness of Nb3Al from ~1 to 6 MPa√m, can thus be attributed to crack-tip shielding from nonplanar matrix and coplanar particle bridging effects over dimensions of a few hundred microns in the crack wake.

特集 粉末の成形・加工 高速気流中衝撃法によるＡｌコーティングＮｂ‐Ａｌ粉末の作製とその特性

Niobium aluminides are expected to be potentially structural materials for ultra-high temperature applications. We have fabricated Nb-Al powder into near net shape by injection molding process. In order to improve sinterability, surface modification of Nb-Al powder was carried out using the high speed impact treatment method. We got Nb-Al powder coated by Al layer with submicron thickness using surface-active agent. Mixing Al coated Nb-Al powder with organic binder was carried out. As few flaking of Al layer was observed, it was concluded that Al layer adhered to Nb-Al particle.

Nanophase structures in mechanically alloyed niobium rich niobium aluminide: M.J. Tracy, J.R. Groza (University of California, Davis, California, USA), Nanostructured Materials Vol 1, No 5, 1992, 369–378

Nanophase structures in mechanically alloyed niobium rich niobium aluminide: M.J. Tracy, J.R. Groza (University of California, Davis, California, USA), Nanostructured Materials Vol 1, No 5, 1992, 369–378

Production of niobium aluminide by plasma-melt-gas-atomization: N. Murahashi et al (Mitsubishi Metals Corp, Nagata, Japan)

Production of niobium aluminide by plasma-melt-gas-atomization: N. Murahashi et al (Mitsubishi Metals Corp, Nagata, Japan)

Fatigue Crack Growth In Niobium Aluminide Intermetallics

AbstractThe results of a recent study of the fatigue crack growth in a new class of niobium aluminide (Nb3Al) intermetallics alloyed with various levels of Ti are presented. The alloys studied have the following nominal compositions (note that all compositions are quoted in atomic &): Nb-15Al-10Ti(10Ti alloy), Nb-15Al-25Ti(25Ti alloy), Nb-15Al-40Ti(40Ti alloy) and Nb-12.5A1–41Ti-1.5Mo(41Ti alloy). Fatigue crack growth behavior was studied at room-temperature in all the alloys. Fracture mechanisms were determined by inspection of SEM images of fracture surfaces. Faceted cleavage fracture modes were apparent on the lOTi and 25Ti alloys. The 40Ti and 41Ti alloys exhibited clear evidence of ductile dimpled fracture. The ductile 40Ti and 41Ti alloys are shown to have comparable or better fatigue crack growth resistance than mill annealed Ti-6A1–4V. The effects of crack orientation (relative to elongated grains) on the fatigue crack growth behavior of the 41Ti alloy are also discussed.

Formation of Nanophase Niobium Aluminide by Mechanical Alloying

Mechanical alloying of an elemental powder mixture containing 51.5 mass%Nb; 43.0 mass%Al and 5.5 mass%Ti was carried out in an Argon-filled ball mill at ambient temperature for 90 to 1800 ks. MAed powders were examined by X-ray diffraction for changes in elemental particulate size and grain sizes of newly formed intermetallic phases. The elemental particulate size measured from the line-broadening decreased with increasing MA time up to 720 ks, while XRD identified the newly formed intermetallic phases, Nb 3 Al and NbAl 3 at and beyond 720 ks. Their grain sizes remained at a few nanometers level (3-8 nm). Differential scanning calorimetry (DSC) of MAed powders revealed both exothermic and endothermic reactions

Grain Growth and Its Kinetics of Nanophase Niobium Aluminide Produced by Mechanical Alloying

Grain growth and its kinetics studies were conducted on an intermetallic compound, NbAl 3 powders produced by mechanicaI alloying of elemental Nb and Al powders with 5.5 mass% Ti in a ball mill under Argon atmosphere for 1800 ks at ambient temperature. The initial grain size of as-MAed powders and grown grain size of annealed powders were measured from X-ray line broadening. Isochronal annealings of MAed powders were conducted from 653 to 1473 K, and it was found that a significant grain growth takes place only in a temperature range of 973 to 1373 K and temporarily ceases at 1473 K by reaching approximately 78 nm regardless of anneal time

Reactive sintering and reactive hot Isostatic Compaction of Niobium Aluminide NbAl3

NbAl3 has been synthesized from elemental powders by reactive sintering (RS) and reactive hot isostatic pressing (RHIP). Both processes involve a self-propagating exothermic reaction between the constituent powders to form an intermetallic compound. The RHIP approach uses simultaneous external pressurization to make a higher density product. This study focused on developing a method to use reactive synthesis to form high-density NbAl3 compacts. High RS and RHIP densities were possible with the appropriate raw materials and processing parameters. These include powder purity, particle sizes, degassing, heating rate, furnace temperature, and compaction pressures. Near full density was attained with RHIP, and up to 95 pct density was attained with RS.

A fundamental study on the preparation of niobium aluminide powders by calciothermic reduction

Fine niobium aluminide powders such as NbAl3 were produced directly from mixtures of Nb2O5 and aluminum powder by calciothermic reduction. Prior to the reduction experiment, phase equilibria between Nb-Al and Ca-Al alloys were studied. Isothermal annealing of the specimens in the Nb-Al-Ca system at 1273 K showed that NbAl3 is in equilibrium with CaAl2 and Al-rich Ca-Al liquid alloys and that Nb3Al and Nb2Al equilibrate with Ca-Al alloys containing around 9 to 18 and 18 to 36 mol pct Al, respectively. Based on these experimental phase equilibria and on reported thermodynamic data of the Al-Ca system, the activities of Al in Nb-Al alloys were evaluated. This information is necessary in determining suitable compositions and conditions for the coreduction and ultimate production of single-phase niobium aluminide. The procedure for preparation of niobium aluminide powders by calciothermic reduction of Nb2O5 consists of three steps: (1) blending of starting materials (Nb2O5 + Al); (2) high-temperature reaction with calcium; and (3) acid leaching. After the reduction of Nb2O5 with calcium and aluminum to produce niobium aluminide powders, by-products of Ca-Al alloy, CaAl2, and CaO were formed. These were removed by leaching in aqueous acid solution. The NbAl3 particles obtained were a few micrometers in size and contained about 0.15 wt pct oxygen.

Synthesis of metastable phases in Al-Nb powders by mechanical alloying

Recently there have been many investigations on the synthesis and properties of transition metal trialuminides based on titanium, zirconium, vanadium, niobium and tantalum for use as structural materials in an elevated temperature environment. This interest is due to their high strength-to-density ratios, high melting points and excellent oxidation resistance. Amongst these, niobium trialuminide (NbAl{sub 3}) has a high melting point (1605{degrees} C), possesses adequate oxidation resistance (a result of the formation of a protective alumina over-layer and a density (4.54 g/cc) which is lower than that of advanced Ni{sub 3}Al-based compounds (7.6 g/cc). However, NbAl{sub 3} melts congruently and since it has an extremely limited homogeneity range, it is difficult to ensure that the chemistry falls in this desired narrow range. Further, due to the intrinsic brittleness, niobium aluminide ingots crack during solidification. Some of these problems can be overcome by producing the NbAl{sub 3} compound through the powder metallurgy route. This paper reports on the successful synthesis of homogeneous NbAl{sub 3} and amorphous phases by mechanical alloying starting from elemental powders.

In Situ Ductile Toughening of Nb2Al by Combustion Synthesis

Ductile toughening of niobium aluminide (Nb2Al) has been accomplished in situ while combustion synthesizing the compound from a mixture of Nb, Al, and 3 wt% B powders. An enhancement of the fracture toughness of the resulting Nb2Al has been shown to be primarily caused by the interaction of propagating cracks and cores of ductile Nb phases inside the partly combusted Nb powders.

Microstructures and mechanical properties of rapidly solidified niobium aluminide (NbAl3)

The principles of rapid solidification technology were applied to cause dispersion strengthening in the intermetallic niobium aluminide (NbAl3). Two niobium aluminide compositions, NbAl3 and NbAl3 containing 1% TiB2 were prepared as rapidly solidified powders by an advanced melt spinning-pulverization technique. Powders were consolidated by hot isostatic pressing (HIP). HIPed materials were characterized for microstructures and high-temperature deformation behaviour. The microstructure of the HIPed niobium aluminides consisted of a recrystallized grain structure with grain size in the range of 5–10 μm. As HIPed NbAl3+ 1% TiB2 material additionally exhibited the presence of multiple phases in a finely, dispersed state. The grain boundaries in this material were populated by 1–2.5 μm particles of intermetallic phase identified as Nb3Al2 with a hexagonal crystal structure, while 0.1–0.2 μm sized dispersoids of a NbB-base phase with orthorhombic crystal structure formed in the interior of the grains. The flow characteristics of the niobium aluminide materials were investigated by constant-velocity compression testing at 1300 and 1477 K with strain rates ranging between 10−2 and 10−6 s−1. NbAl3 + 1% TiB2 alloy containing a dispersion of second phases, notably borides, showed superior strength compared with the baseline NbAl3 composition. This suggests possible interaction of the grain boundaries and dislocation with the second-phase particles (i.e. borides).

Combustion synthesis of niobium aluminide and its mechanical properties

Combustion synthesis of niobium aluminide and its mechanical properties

高融点金属間化合物（Ｎｂ３Ａｌ）のプラズマ溶解ガスアトマイズ

Rapidly solidified niobium aluminides (Nb3Al) have been studied to understand their ability as structural materials for ultra-high temperature applications. A new gas atomization process combined with plasma-skull-melting without any refractory, refractory metal and ceramic, has been developed to produce ultra-clean powders. In this process, named Plasma-Melt-Gas-Atomization (PMGA), a pouring nozzle for molten metal stream is made of water-cooled copper with a bottom hole. Rapidly solidified powders of Nb3Al have been successfully obtained. Mean diameter of the powders is around 100μm, and oxygen content is 300ppm. PMGA'ed powders of Nb-22at%Al mostly consists of supersaturated solid solution of Al in Nb (bcc), although cast ingot of same composition wholly consists of ordered A15. Because microstructure of powder is identical to melt-spun ribbon, solidification rate of PMGA can be considered very high.

Dispersion strengthened niobium aluminide alloys via advanced melt spinning technology

New approaches are currently being sought by materials scientists to develop and characterize advanced high temperature structural intermetallic materials and composites. These materials have potential for aircraft applications in the temperature range of 1090–1650°C in advanced gas turbine engines. Many ordered intermetallic compounds appear to have great potential for applications as high temperature, load-bearing structural materials because of their high melting points, the possibility of improved strength combined with fracture toughness and stability in oxidizing environments.