Papers by Keyword: Ni₃Al

Abstract: This paper presents the characteristic features of the catalytic properties of the cold-rolled Ni₃Al foils for methanol decomposition which were developed in our group. Methanol was effectively decomposed into H₂ and CO over the foils above 713 K. The production rates of H₂ and CO increased with an increase of time during the initial period of reaction, indicating that the Ni₃Al foils were spontaneously activated under the reaction conditions. Surface analyses revealed that fine Ni particles dispersed on carbon nanofibers formed on the foils during the reaction. The high catalytic performance of the foils can be attributed to the spontaneous formation of this nanostructure during the reaction.

Abstract: As a solid solution matrix alloy, nickel-based alloys have the shortcomings that the strength will decrease with increasing temperature. The significance of this experiment is forming the two-phase matrix, in which γ- Ni solid solution are replaced partly by Ni₃Al with potential performance at high temperature. In this paper, no visible cracks and pores Ni₃Al/γ- Ni composite matrix coatings were prepared on mild steel substrate by laser cladding with mixed powders of two common thermal spying powders of Ni60AAA alloy and Ni-coated Al. The microstructure and phase constitution of the coating were studied through SEM, EDX and x-Ray diffraction analysis technique. The results show the composite coating consists of γ- Ni, Ni₃Al, Cr₂₃C₆ and CrB, both the main matrix γ- Ni solid solution and the second matrix phase of Ni₃Al being free uniformity dendrite and containing some alloying elements such as Fe, Cr, Si, etc,with the dendrites surrounded continuously by the eutectic. It may be possible for improving high-temperature performance.

Abstract: A finite element approach is used to simulate the precipitation of Ni₃(Al,Ti) intermetallics in nickel-based superalloys containing a low volume fraction of spherical g’ precipitates, in which precipitation occurs following nucleation and growth mechanisms. Classical differential equations of nucleation and growth are implemented in the software Comsol (formerly Femlab), to compute the number of precipitates per unit volume and their mean size. Another originality of the model is the use of thermodynamic quantities coming from phase diagram computations (Thermo-Calc), like the temperature variation of the equilibrium g’ volume fraction, and the evolution of the concentration of g’ forming elements (Al, Ti) in the matrix with the volume fraction of precipitates. Once adjusted to experiment in the case of isothermal ageings, the model can be used to simulate precipitation during complex thermal histories. Finally, automatic heat treatment optimisation procedures are proposed and tested, which can reduce heat treatment times by a factor of more than five.

Abstract: Kinetic Monte Carlo (KMC) simulation is a valuable tool to investigate configu-ration changes in intermetallic compounds. The elementary process is the jump of an atomfrom a lattice site to a neighboring vacancy. In classical transition state theory the jump ratecontains the energy difference between the original equilibrium state and the saddle point (=transition) state. In traditional KMC the saddle point has mostly received rather careless treat-ment, setting it constant or relating it to the type of jumping atom. In the present work, saddlepoint heights were considered explicitly. Taking L1₂ ordered Ni3Al as an example, jump energyprofiles for various atom environments were calculated ab initio in relaxed configurations ofa 3x3x3 supercell, employing the Nudged Elastic Band method where necessary. From theseresults, effective ’pure’ saddle point heights were extracted. To show the effect on kinetics,simulations of order-order transitions were done with jump probabilities based on these results.When compared to the old assumption of constant saddle point heights, both overall kineticsand detailed jump statistics result considerably changed.

Abstract: Paramagnetic VKNA superalloy on the base of Ni3Al intermetallic phase (L12 superlattice) under cold rolling deformation demonstrates superparamagnetic behavior associated with the formation of nanosized ferromagnetic clusters within the paramagnetic matrix. It may be assumed that these clusters correspond to the long-period phase with DO22 superlattice.

Abstract: Process of nanostructure formation and amorphization by high pressure torsion (HPT) were studied for various intermetallic compounds. In ZrCu after HPT deformation, optical microscopy revealed that numerous shear bands formed running nearly parallel to the shear direction. Partial amorphization was confirmed by X-ray diffraction and TEM observations. Detailed TEM observations revealed localized amorphization within the nano-scale shear bands. For HPT deformation of zone-melted Zr50Cu40Al10 the preferential amorphization of ZrCu phase was observed. On the contrary, amorphization was not observed for Ni3Al even after HPT deformation of 100 turns; the sample remained to be disordered nanocrystalline of about 50 nm. The process and mechanism of the grain refinement and amorphization will be compared and discussed for these intermetallic compounds.

Abstract: The catalytic activity of oxidation-reduction pre-treated Ni3Al powder for methane steam reforming was examined. The oxidation-reduction pre-treatment consisted of two steps: oxidation in air at various temperatures from 973 to 1373 K, and then followed by reduction in H2 at 873 K. It was found that the oxidation-reduction treatments significantly reduced the onset temperature of activity, i.e., improved the activity of Ni3Al powder at low temperatures. The characterization of Ni3Al surface showed that an outer surface layer of fine NiO particles were formed on the surface of Ni3Al after oxidation. These NiO particles were reduced to metallic Ni by the subsequent reduction treatment, resulting in the high activity for methane steam reforming. These results indicate that the Ni3Al can form highly active surface structure with oxidation-reduction treatment, having excellent heat resistance.

Abstract: Porous cermets composites were prepared from mixed powders (Ni, Al, Ti and C) by SHS (Self-propagating High-temperature Synthesis) method. When Ni-Al intermetallics bonded TiC porous cermets substitute for TiC ceramic as perform in the process of manufacturing Ni-Al/TiC composites by pressureless melt infiltration, the wetting of the ceramic material and Ni-Al intermetallics can be improved. In this study, the microstructure and mechanical properties (density and compressive resistance strength) were reported. It is learnt from the study that the relation between the open porosity and the pore size does not exist, the former depends on the green density, and the latter depends on the particle size of the nickel powder. The porous cermets with porosity ranging from 40% to 50%, pore size from 1μm to 300μm, and the compressive resistance strength about 15 MPa can be prepared by SHS. SEM observations of fracture surface suggest the fracture mode is brittleness fracture, and the microstructures of the porous cermets are very homogenous.

Abstract: Starting from some fundamentals of solid-state diffusion, we remind the reader to the major techniques for lattice diffusion measurements. Self-diffusion is the most basic diffusion phenomenon in any solid. The paper covers main features of self-diffusion in pure fcc and bcc metals and some important facts about diffusion of substitutional solutes in metals. Binary intermetallics are compounds of two metals or of a metal and a semimetal. Their structures are different from those of the constituents. Some intermetallics are interesting functional materials others have attracted attention as high-temperature structural materials. The paper reviews some results mainly from our laboratory on diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si and Co-Nb, which have been published in detail elsewhere. Some results for the ternary system Ni-Fe-Al are also mentioned.

Abstract: In this paper, the synthesis of four-layered functionally graded materials (FGMs) samples of (TiB2)pNi/(TiB2)p(Ni+Ni3Al)/ Ni3Al with different metallic substrate (Ni, Cu and Cr-Ni steel) by field-activated and pressure-assisted synthesis process (FAPAS) was investigated. It was shown that the imposition of the electric field and the combustion reaction of Ni3Al is the key for the successful synthesis of the FGMs during process. The exothermal reaction of the powder of nickel and aluminum during combustion enhanced the diffusion bonding in the interfaces subsequently. The microstructure, phase composition of the interfaces and mechanical properties of the graded materials were characterized, which showed indicative formation of firm bonds by the combustion reactive process. The samples have an excellent thermal shock resistance ability and gradient distribution of micro-hardness across the substrate layers and the surface layer.