[en] The quasi-binary NiAl-Mo system exhibits a large two-phase field between NiAl and the terminal (Mo) solid solution, and offers the potential for producing in situ eutectic composites for high-temperature structural applications. The phase stability of this composite system was experimentally evaluated, following long-term exposures at elevated temperatures. Bend strengths as a function of temperature and room-temperature fracture toughness data are presented for selected NiAl-Mo alloys, together with results from fractography observations. 10 refs

[en] Steady progress made since the mid-1970's suggests that revolutionary engineering alloys may emerge, derived from gamma titanium-aluminides. Nearly twenty years of technical advance now shows signs that a materials technology exists, and there are indications that gamma alloy technology begins to satisfy the design and business related forces driving these advanced materials. Research and characterization documents the physical and mechanical properties of numerous alloy compositions, processes, and microstructures. These efforts provide sufficient experimental results on single alloys and process routes to suggest that damage tolerance, creep resistance, and oxidation resistance may be obtained and controlled simultaneously. Within these alloys, structural properties exist over a service-temperature range which compete with those of some nickel-based superalloys, although the gamma alloys introduce new groupings in the overall balance of properties available for structural alloys. Such new sets of properties and the research nature of the alloys reduces confidence in using conventional design practice in application of the alloys; some of which is linked to unknown reproducibility of processing and properties. This concern is addressed herein through global commentary on the synthesis-structure, structure-properties, properties-use relationships for gamma alloys. The commentary focuses on the question ''what is the current state of gamma alloy technology development, and what are the directions for the future of this technology?'' in discussing recent progress and technology needs

[en] Recent studies have shown that within the Nb-Si system, the two-phase Nb/Nb₅Si₃ microstructures exhibit excellent thermochemical stability and resistance to coarsening up to 1500 C. In addition, these Nb/Nb₅Si₃ 'in situ composites' exhibit a good balance in mechanical properties, having a reasonably high strength up to 1500 C and high fracture toughness at room temperature. These combined properties of the composites provide an attractive basis for developing high-temperature structural materials. Studies are currently underway on creep mechanisms and alloying to enhance the oxidation resistance of these composites. Most of the published results on this system pertain to the wrought Nb-10 at. pct Si alloy in which the large primary Nb particles provide toughening while the intermetallic Nb₅Si₃ phase in the eutectoid microconstituent provides high-temperature strength. The present work was undertaken to determine the strength and toughness of a wrought alloy with near-eutectic composition, Nb-1.65 at. pct Si, at temperatures from 25 C to 1500 C. The increased Si content is known to decrease the volume fraction of the primary Nb particles and increase the overall volume fraction of the Nb₅Si₃ intermetallic phase

[en] The Nb-Si system offers a possibility of ductile-phase toughening of the brittle Nb₅Si₃ intermetallic with the terminal niobium-silicon solid solution. Powder composites have been made in which the volume fraction of the terminal Nb-Si phase is systematically varied in a matrix of Nb₅Si₃ in order to study the extent of toughening. The Nb-Si solid-solution phase was observed to exhibit cleavage failure under both as hot pressed and heat treated conditions, thereby limiting the toughening attained by the presence of this phase. Hot working of the composite results in a dramatic improvement in toughness because of a change in the plastic behavior and fracture mode of the terminal Nb-Si phase from brittle cleavage to a mixed mode of cleavage and ductile microvoid growth and coalescence (dimpled) fracture

[en] Ion-beam mixing and thermal annealing of thin, alternating layers of Al and Nb, as well as Al and Ta, were investigated by selected area diffraction and Rutherford backscattering. The individual layer thicknesses were adjusted to obtain the overall compositions as Al₃Nb and Al₃Ta. The films were ion mixed with 1 MeV Au⁺ ions at a dose of 1 x 10¹⁶ ions cm/sup -2/ . Uniform mixing and amorphization were achieved for both Al--Nb and Al--Ta systems. Equilibrium crystalline Al₃Nb and Al₃Ta phases were formed after annealing of ion mixed amorphous films at 400 ⁰C for 6 h. Unmixed films, however, remained unreacted at 400 ⁰C for 1 h. Partial reaction was observed in the unmixed film of Al--Nb at 400 ⁰C for 6 h. After annealing at 500 ⁰C for 1 h, a complete reaction and formation of Al₃Nb and Al₃Ta phases in the respective films were observed. The influence of thermodynamics on the phase formation by ion mixing and thermal annealing is discussed

[en] Recent research has shown that in the Nb-Si system, it is possible to produce composites consisting of a brittle Nb₅Si₃ intermetallic matrix and ductile Nb phase particles. These composites exhibit a good balance in mechanical properties, i.e., toughening is provided up to 1500 degrees C. The currently accepted Nb-Si phase diagram exhibits a wide two-phase field between the terminal Nb-Si solid solution and the intermetallic Nb₅Si₃ which is bounded by the eutectoid isotherm at 1783 degrees C. The Nb₅Si₃ phase shows no change in Si solubility with temperature and the terminal Nb phase shows only a small change in Si solubility at temperatures exceeding 1200 degrees C. These solubility characteristics indicate the possibility of producing stable two-phase microstructures with a slow rate of thermally induced morphological change. The results of current research have shown that the two phases are indeed thermochemically stable and exhibit little coarsening after a heat treatment of 100 hours at 1500 degrees C. The phase diagram shows the existence of a high-temperature β phase which is a line compound having a stoichiometric composition of Nb₃Si, and the β to (Nb + Nb₅Si₃) eutectoid transformation is shown at ∼ 1783 degrees C. A recent paper, however, shows the presence indicates that the β left-reversible Nb + Nb₅Si₃ eutectoid isotherm is at ∼ 1720 degrees C instead of 1783 degrees C. In addition to these conflicting results, the variation of Si solubility with temperature in the terminal Nb phase is not well established

[en] Recently, niobium based alloys have attracted much attention as potential high temperature materials. Nb-Al alloys have been examined, especially with a view to producing intermetallic matrix composite materials with a distribution of ductile bcc precipitates in the Nb₃Al matrix. The Nb₃Al phase exhibits the A15 crystal structure and is extremely brittle at temperatures below 1475 K. Attempts at alloy development are being carried out by several researchers and studies on Ti and other alloying additions to Nb-Al alloys are being done. At the same time, several studies on Nb additions to Ti-Al alloys are available and phase stability in this ternary system is still being critically examined. However, most of the phase boundaries in the Nb-rich corner of the ternary involving the bcc, Nb₃Al and the Nb₂Al phases, shown in reported isothermal sections of the Ti-Al-Nb phase diagram, are only speculative, based on judicious extrapolations of the binary Nb-Al phase diagram proposed by Jorda et al a number of years ago. In this paper phase boundaries involving the bcc and the Nb₃Al phase were experimentally determined and isothermal sections of the Nb rich corner of the Ti-Al-Nb system established at 1923 K, 1473 K and 1273 K

[en] Advanced intermetallic materials, such as refractory silicides, exhibit high melting points, high stiffness, low densities, and good strength retention at elevated temperatures. Further, some of these silicides are in equilibrium with terminal refractory solid solution (beta) phases, and therefore, offer the potential for ductile phase toughening. Studies were conducted to elucidate the compressive creep behavior of monolithic Nb₅Si₃ and to generate the constitutive creep law. This, in turn, is required for modeling the creep behavior of the Nb/Nb₅Si₃ two-phase system. Nb₅Si₃ has the ordered tetragonal structure with 32 atoms/cell in both its allotropic forms: αNb₅Si₃ (D8_l Cr₅Si₃-type; a ∼ 0.656 nm; c = 1.187 nm) and βNb₅Si₃ (D8_m W₅Si₃-type; a = 1.000 nm; c = 0.507 nm). αNb₅Si₃ is stable below 1,935 C, while βNb₅Si₃ is stable above 1,645 C. The large lattice parameters as well as the large number of atoms in the unit cell suggest that dislocation creep is unlikely to occur in Nb₅Si₃, because large Burgers vectors and complex dislocation core structures are expected in this material

[en] 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-Al-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

[en] In this study we present a new constitutive model for Ni3Al and Ni3(Al, X) alloys that was developed to represent many of the unusual plastic flow behavior found in L12 intermetallics while maintaining consistency with the experimentally-observed evolution of dislocation substructure. In particular, we sought to develop a model that would not only predict the anomalous increase of the yield strength with increasing temperature, but would also capture other important flow characteristics such as extremely high work-hardening rates that change anomalously with temperature, and a flow stress that is partially to fully reversible with temperature. For this model, we have treated work-hardening as arising from two different sources. Thermally-reversible work hardening is accounted for using the description of screw dislocation motion proposed by Caillard, which involves exhaustion of mobile dislocations by cross-slip locking of the dislocation core and athermal unlocking. Thermally-irreversible work hardening is accounted for using an approach consistent with the theoretical framework proposed by Ezz and Hirsch, which involves both the multiplication of Frank-Reed sources and the interaction of edge-dislocation segments with cross-slip locking events and the dislocation forest. Both work-hardening contributions were incorporated into the rate formulation for thermally-activated plastic flow proposed by Kocks, Argon and Ashby. We will show simulation results for the flow response of Ni3(Al, X) crystals over a wide range of temperatures in the anomalous flow regime, and we will compare these findings with experimental data