Papers by Keyword: Ni₃Al

Abstract: The high-temperature corrosion behavior of Ni₃Al+2.9 wt% Cr alloy was studied in SO₂-containing environment. Corrosion tests were carried out at 900, 1000, and 1100 °C for 100 h in atmospheric Ar-0.2% SO₂ gas. The alloy corroded relatively slowly due mainly to formation of Al₂O₃ in the scale. Its corrosion kinetics deviated from the parabolic corrosion rate law to a certain extent owing to ensuing scale spallation. This was attributed to (1) stress generated during scaling and the subsequent cooling period, (2) voids that formed due to the Kirkendall effect, and (3) incorporation of sulfur in the scale. The scale that formed after corrosion at 900 °C consisted of the outer NiO scale, middle NiAl₂O₄ scale, and inner Al₂O₃ scale. The increased corrosion rate at 1000 and 1100 °C led to formation of the outer NiO scale, and inner Al₂O₃ scale.

Abstract: AlMgB₁₄ is a very hard and brittle material which need be modified for utilizing as cutting tool materials. In present study, TiB₂ and Ni₃Al were used to strengthen and toughen AlMgB₁₄ material. The microstructure of the AlMgB₁₄-TiB₂-Ni₃Al composites was analysized by scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) and an X-ray diffractometer (XRD). The hardness and fracture toughness of the AlMgB₁₄-TiB₂Ni₃Al composites were also examined. The results showed that the major phases in AlMgB₁₄-Ni₃Al composites were TiB₂, AlMgB₁₄, MgAl₂O₄ , Ni₃Al and NiAl. With the increasing of the amount of Ni₃Al, more intergranular fractured features can be found in the fractured surface of the composites, the hardness and fracture toughness of the composites were both decreased comparing to the synthesised AlMgB₁₄. The AlMgB₁₄-TiB₂-4wt.% Ni₃Al composite has a hardness of 28.1 Gpa and a fracture toughness of 3.14 MPa·m^1/2.

Abstract: AlMgB₁₄ is a novel hard and brittle material which need be improved toughness for utilizing as cutting tool materials. In present study, intermetallic compound Ni₃Al were attempted to dope the synthesised AlMgB₁₄ materials. The microstructure of the AlMgB₁₄-Ni₃Al composites was analysized by scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) and an X-ray diffractometer (XRD). The density, hardness and fracture toughness of AlMgB₁₄-Ni₃Al composites were also measured. The results showed that the major phases in AlMgB₁₄-Ni₃Al composites were AlMgB₁₄, MgAl₂O₄ , Ni₃Al and NiAl as well as W₂B₅. With the increasing of the amount of Ni₃Al, more transgranular fractured features can be found in the fractured surface, the density of the AlMgB₁₄-Ni₃Al composites was increased, the hardness and fracture toughness of the AlMgB₁₄-Ni₃Al composites were decreased.

Abstract: To verify the assumption that the anelastic relaxation effect observed in Ni₃Al is due to stress-induced reorientation of antisite Al atoms [Numakura and Nishi, Mater. Sci. Eng. A 442 (2006) 59-62], the magnitudes of the anisotropic distortion produced by the intrinsic point defects have been evaluated by ab initio calculations. The anisotropy of the λ tensor (the strain per unit concentration of a particular defect) for the two candidate defect species, namely a Ni vacancy and an antisite Al atom, has been computed by full structure optimization of a supercell containing a single point defect: the difference in the principal values is +0.46 and −1.12, respectively. The relaxation strength estimated for antisite Al atoms agrees fairly well with experiment, while that for Ni vacancies is far too small because of their much lower concentration. The relaxation is, therefore, conclusively attributed to antisite Al atoms.

Abstract: 800x600 Intermetallics are compounds of two metals or of metal(s) and semimetal(s). Their structures are usually different from those of the constituents. Some intermetallics are interesting functional materials, others have attracted attention as high-temperature structural materials. We remind the reader of some fundamentals of solid-state diffusion and to the major techniques for tracer diffusion measurements, interdiffusion studies and the growth kinetics of layers in solid diffusion couples. Starting from self-diffusion, which is the most basic diffusion phenomenon in any solid, the paper covers the main features of diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Mg-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si, Co-Nb and Ni-Nb.. We illustrate the influence of phase transitions on diffusion and point out some common features of diffusion in intermetallics. We discuss in detail diffusion in silicides of iron, molybdenum and of silicides of refractory metals. We also consider aluminides of iron, nickel, and titanium and in the aluminium-magnesium system. We consider diffusion in intermetallics of the cobalt-niobium and nickel-niobium system and in in the Nb-Sn and V-Ga systems. We finish with some remarks about grain boundary diffusion in intermetallics. Normal 0 21 false false false UK X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Calibri","sans-serif";}

Abstract: The effects of Re on solidus/liquidus temperatures, solidification segregation characteristics of alloying elements, γ′ morphology and volume fraction as well as microstructural stability at 1150 °C have been investigated in two experimental Ni₃Al-based single crystal superalloys with and without Re (1.0 wt%) addition. The results indicated that Re addition increased the solidus/liquidus temperatures. The segregation characteristics of Ni₃Al-based single crystal superalloys were similar with Ni-based single crystal superalloys, and the solidification segregation ratios of W and Mo increased slightly due to Re addition. Meanwhile, Re addition resulted in more negative lattice misfit, more cuboidal γ′ phase and higher γ′ volume fraction. The kinetics of γ′ coarsening and rafting at 1150 °C was lowered substantially compared with the addition of Re.

Abstract: The effect of acid and alkali leaching on catalytic activity of an atomized Ni₃Al powder for methanol decomposition was investigated. It was found that a two-step process, an acid leaching followed by an alkali leaching, was most effective for improving the activity. The optimal conditions of acid and alkali leaching were examined. The effect of the acid and alkali leaching was attributed to the formation of fine Ni species and a porous surface structure.

Abstract: The creep behaviors of a Ni₃Al-base single crystal superalloy IC6SX prepared by spiral grain selection method was studied systematically under the testing condition of 760/540MPa. The microstructure evolution, movement of dislocations, formation of the dislocation networks and dislocation configuration during the creep process were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the creep performance of the single crystal alloy IC6SX is excellent under 760/540MPa. The experimental results showed that the creep curve of the Ni₃Al-base single crystal superalloy IC6SX was divided into three stages, including decelerating creep stage, steady-state creep stage and accelerated creep stage. The microstructure and the dislocation configuration were different at different stage during the creep and the raft microstructure has not been formed. The creep mechanism was main slipping characterized by dislocation glide mechanism.

Abstract: The Ni₃Al and Ni₃Al-B-Cr alloys were fabricated by the self-propagation high-temperature synthesis with hot extrusion method. Their microstructure and mechanical properties were studied by using combination of X-ray diffraction, optical microscopy, transmission electron microscopy and compression test. Analysis of X-ray spectra exhibited that the elemental powders had been transformed to the Ni₃Al phase after the self-propagation high-temperature synthesis processing. Microstructure examination showed that the alloy without extrusion consisted of coarse and fine grains, but the subsequent hot extrusion procedure homogenized the grain size and densified the alloy obviously. Transmission electron microscopy observations on the Ni₃Al alloy revealed that Ni₃Al, γ-Ni and Al₂O₃ particles were the main phases. When the boron and chromium were added, besides the β-NiAl phase, α-Cr phase and some Cr₇Ni₃ particles with stacking faults inside were observed. In addition, a lot of substructure and high-density dislocation arrays were observed in the extruded part, which indicated that the subsequent extrusion had led to great deformation and partly recrystallizing in the alloy. Moreover, the subsequent extrusion procedure redistributed the Al₂O₃ particles and eliminated the γ-Ni. These changes were helpful to refine the microstructure and weaken the misorientation. The mechanical test showed that the self-propagation high-temperature synthesis with hot extrusion improved the mechanical properties of the Ni₃Al alloy significantly. The addition of B and Cr in Ni₃Al alloy increased the mechanical properties further, but the compressive strength of the alloy was still lower than that synthesized by combustion. Finally, the self-propagation high-temperature synthesis with hot extrusion was a good method to prepare Ni₃Al alloy from powder.

Abstract: Two-phase single-crystal intermetallic alloys composed of Ni₃Al (L1₂) and Ni₃V (D0₂₂) with some orientations were compressed at various temperatures, and their deformation microstructures were observed by transmission electron microscopy (TEM). The deformation at room temperature was governed by the glide motion of dislocations in the primary Ni₃Al precipitates and the activation of the microtwins in the Ni₃V variant structures in the channel regions. The interfaces between the primary Ni₃Al precipitates and the Ni₃V variant structures are suggested to work as the barriers to the dislocation motion. While, at temperature above the peak temperature (873 K), the deformation microstructures of the two-phase intermetallic alloy exhibited the ribbon-like deformation microstructures penetrating the constituent phases i.e. through the interfaces between primary Ni₃Al precipitates and the Ni₃V variant structures in the channel regions. It was also suggested that the superior strength in the two-phase intermetallic alloys is due to the high flow strength of the Ni₃V phases and to the interfacial hardening receiving when the dislocations activated in the primary Ni₃Al precipitates propagate to the channel regions.