Materials Science Forum Vols. 475-479

Abstract: Oxidation protective silicide coatings for the new Nbss/Nb5Si3 in-situ composites were prepared by molten salt method at 900°C. The experiment results indicated that the majority phase in the coating was NbSi2. More Nb5Si3 was formed at the interface between the substrate and NbSi2 layer during the oxidation. The oxidation resistance of the composites was improved by the coating, significantly. The improvement in the oxidation resistance of the composites maybe mainly attributed to the formation of large amount of SiO2 and Al2O3 on surface of coating.

Abstract: The directionally solidified specimens of Nb-13.52 Si-22.60 Ti–6.88 Hf–2.54 Cr–2.24 Al alloy were prepared in an electron beam floating zone melting furnace at the withdrawing rate of 0.1, 0.3, 0.6, 1.0, 2.4 and 6.0 mm/min. All the primary Nb solid solution (Nbss) columns, Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies and divorced (Nb)3Si/(Nb)5Si3 plates or chains align well along the longitudinal axis of the specimens. With increasing of the withdrawing rate, the microstructure is gradually refined, and the amount of Nbss + (Nb)3Si/(Nb)5Si3 eutectic colonies increases. Both the room temperature ultimate tensile strength σb and fracture toughness KQ are improved for the directionally solidified specimens. The tensile fracture occurs in a cleavage way.

Abstract: During the past ten years, mechanical alloying/ball milling has been employed to successfully synthesize nanocrystalline NiAl intermetallic compounds (NiAl, NiAl (Cr,Co,Ti,Fe)) and composites (NiAl-TiC, NiAl-TiB2, NiAl-HfC, NiAl-HfB2) in our group, namely, by means of self-sustained reaction. During the mechanical alloying process, an abrupt temperature rise has been observed. The large heat release from the formation of NiAl, as well as HfC, HfB2, TiC, TiB2 compounds is suggested to be the driven force of the self-sustained reaction. It is interested to observe that nanocrystalline NiAl exhibits very well microstructure stability during high temperature annealing. The alloying additions can decrease reaction rate and prolong the reaction time. With the increase of alloying additions, supersaturated solid solution even amorphization of NiAl has been formed. The nanocrystalline NiAl compounds show a rise of the compressive strength and the ductility at room temperature compared with that of the cast coarse crystalline compound.

Abstract: Ni3Al has attractive high temperature properties, such as high strength and good oxidation/corrosion resistance, and is possible to be used for high temperature chemical processing and manufacture. Until now, the catalytic properties of Ni3Al were rarely investigated since the leaching of aluminum from Ni3Al is difficult to obtain a porous Raney-Ni compared to NiAl3 and Ni2Al3. In the present work, the catalytic properties of Ni3Al were examined for hydrogen production reactions from methanol. It was found that alkali-leached Ni3Al showed high activity for methanol decomposition (CH3OH→ 2H2+CO). Furthermore, Ni3Al catalysts suppress the formation of methane, i.e. they show higher selectivity for the methanol decomposition reaction than Ni catalyst. These results indicate a possibility for Ni3Al used as a catalyst for hydrogen production reaction.

Abstract: Mechanical properties of two stoichiometric NiAl intermetallic alloys processed by hot hydrostatic extrusion were investigated. The ingots for extrusion process were produced both by casting and by powder metallurgy route in which the self-sustained high-temperature synthesis (SHS) of elemental powders was initialized by the process of extrusion itself. Elastic anisotropy of the obtained products were determined using a resonant ultrasound spectroscopy (RUS) in measurements of the elastic constants. The anisotropy of plastic properties was estimated from uniaxial compression tests on specimens cut off from the material in the main directions of previous deformation. An acoustic emission (AE) method of monitoring of the microcracking process in the material was used for estimation the role of cataclastic mechanism of deformation. The effect of annealing of extruded materials on their plastic behavior was roughly investigated.

Abstract: The tensile creep behavior of NiAl-9Mo eutectic alloy has been investigated over a stress range of 50 to 100MPa at the temperatures ranging from 850 to 950°C. All of the creep curves exhibit the very long steady-state stage. The creep parameters and TEM observations indicates the kinetics of the steady-state creep deformation is governed by dislocation climb in the NiAl matrix phase. The crack origination and development at the colony boundary results in the onset of tertiary creep stage and final fracture of the alloy.

Abstract: The effect of ion implantation of yttrium on the oxidation behavior of Ni3Al base alloy IC6 at 1100°C was investigated by SEM/EDS, TEM and XRD methods. The results indicated that after Y ion implantation, the surface of IC6 alloy changed from large column crystals to fine distributed nano-crystals, and the density of dislocations in distorted region increased with the implanted ions dose, which can act as high-diffusivity path and oxide core. Comparing to un-implanted IC6 alloy, Y-implantation increased the oxidation rate of IC6 alloy in initial 25 h at 1100°C, but decreased the maximum weight gain and the oxidation rate in final period. Moreover, the oxide scales of Y-implanted IC6 alloy were more continuous and compact due to the benefit effect of yttrium, and had better coherence to substrate through reduction of oxide grain size resulting in oxide scale plasticity improvement. Hence the proper amount of implanted Y could improve the oxidation resistance of IC6 alloy at 1100°C in air slightly.

Abstract: The creep behavior and mechanisms of extruded NiAl-25Cr alloy at elevated temperatures have been studied in the paper. Analysis of the creep data over the temperature range 1073-1123 K reveals two distinct regions of creep behavior present in this material. At lower temperature, the creep characteristics are consistent with mobility-controlled deformation where viscous glide of dislocations controls creep. At higher temperature, the creep characteristics are consistent with a structure controlled creep process where some form of dislocation climb controls creep deformation.

Abstract: Small amounts of silver have been added to the intermetallic compound β-NiAl for the purpose of improving its mechanical properties. Four ternary NiAl-Ag alloys NiAl-0.5Ag, NiAl-1Ag, NiAl-5Ag and NiAl-10Ag (at.%), and an Ag-free β-NiAl have been oxidized at 900oC for 24 h in 1 atm O2 to study the effect of the presence of silver on the oxidation of β-NiAl. The kinetics of all the alloys were generally composed of two main parabolic stages with slightly larger parabolic rate constants for the second stage, except for NiAl-10Ag, which has an instantaneous parabolic rate constant decreasing with time. A continuous external layer of Al2O3 formed on all the alloys. In particular, the scales formed on NiAl-5Ag and NiAl-10Ag contained a thin and discontinuous layer of silver at the alloy/Al2O3 interface. Furthermore, NiAl-10Ag formed also isolated Ag particles or even a discontinuous Ag layer occasionally surmounting the Al2O3 scale. The addition of minor amounts of silver does not affect significantly the oxidation of β-NiAl, because silver is essentially present as a second phase due to its very small solubility in this intermetallic compound.

Abstract: In recent years, significant advances have been made in regard to the creep deformation behavior of two phase titanium alloys. It has been shown that the creep resistance depends on a number of factors, including the shape of the component phases, the strength difference between the phases, and the stability of the beta phase. For example, in two-phase materials with a similar volume fraction and morphology of phases, if the beta phase is less stable, then the creep resistance is lower. These developments will be reviewed and the reasons for such effects will be suggested.