V. K. Portnoi

Formation of nickel carbide in the course of deformation treatment of Ni-C mixtures

Nonequilibrium Ni(C) solid solutions supersaturated with carbon to 10.2 at % were synthesized by mechanochemical method. An analysis of diffraction patterns showed that the formation of Ni(C) solid solutions is accompanied by an increase in the probability of appearance of deformation stacking faults. When the carbon content in the initial Ni-C mixtures is above 20 at %, the fcc Ni(C) solid solution resulting from the mechanical synthesis transforms into the metastable Ni3C nickel carbide with a hexagonal structure. The thermal stability of nonequilibrium Ni(C) solid solutions was determined. The solid solutions formed from the mixtures with carbon contents from 7 to 15 at % undergo partial decomposition accompanied by the carbide precipitation upon heating to 300°C. The decomposition of the metastable Ni3C carbide starts at a temperature Ts ~ 464.8°C; the thermal effect is —ΔH=10–13 kJ/mol. The effective radius of carbon atoms in the Ni(C) solid solutions was determined; it is equal to Rceff=0.061 nm.

First-principles study of point defects in Ni3Al

The energetics and structural properties of native, substitutional and interstitial defects in NiAl have been investigated by first-principles methods. In particular, we have determined the formation energies of composition conserving defects and established that the so-called penta defect, which consists of four vacancies on Ni sublattice and Ni antisite on the Al sublattice, is the main source of vacancies in NiAl. We show that this is due to the strong Ni-site preference of vacancies in NiAl. We have also calculated the site substitution behaviour of Cu, Pd, Pt, Si, Ti, Cr, V, Nb, Ta and Mo and their effect on the concentration expansion coefficient. We show the latter information can used for an indirect estimate of the site substitution behaviour of the alloying elements. The solution energy of carbon and its effect on the lattice constant of NiAl have been obtained in the dilute limit in the first-principles calculations. We have also determined the chemical and strain-induced carbon–carbon interactions in the interstitial positions of NiAl. These interactions have been subsequently used in the statistical thermodynamic simulations of carbon ordering in NiAl.

Mechanochemical synthesis in the Ni-Al-C system

Specific features of the phase formation during mechanochemical synthesis of mixtures of elementary components, Ni, Al, and graphite, in an atomic ratio of 2:1:1 and a mixture of the intermetallic compound Ni3Al and graphite (1:1) have been considered. It is shown that nanocrystalline (D = 4–6 nm) three-component solid solutions Ni(Al, C) with identical lattice constants (a = 0.366 nm) are formed during mechanosynthesis, independent of the initial components. Annealing at a temperature of 800°C for 2 h leads to decomposition of solid solutions into three phases: double carbide Ni3AlC0.46 (a = 0.3592 nm), solid solution Ni(Al, C) with the lattice constant 0.3546 nm, and graphite with the lattice constants a = 0.2461 nm and c = 0.660 nm.

Mechanochemical synthesis and heating-induced transformations of a high-entropy Cr-Fe-Co-Ni-Al-Ti alloy

An amorphous-crystalline two-phase (amorphous phase + BCC solid solution) powder alloy has been produced by mechanochemical synthesis (MS): by grinding an equiatomic mixture of Cr, Fe, Co, Ni, Al, and Ti metals in a Fritsch (P-7) ball mill at a powder-to-ball weight ratio of 1: 8. Using X-ray diffraction, X-ray microanalysis, and scanning electron microscopy, we have determined the sequence of reaction steps during milling of the mixture. In the early stages of milling (2 h), we observed the formation of an ordered phase (B2), Al + Ni → NiAl, and the CoHCP → CoFCC polymorphic transformation. Milling for 3 h led to the formation of a BCC solid solution. Further milling produced an amorphous phase (AP). In the range 6–25 h of milling, the percentage of the AP increased and that of the BCC solid solution decreased. The phase transformations induced by heating the alloy to 1200°C after MS have identified using differential thermal analysis and X-ray diffraction:

Chemical and deformational interactions in solid solution of carbon in nickel

MS (FCC + AP)xrightarrow{{450^ circ C}}(B2)xrightarrow{{650^ circ C}}(L2_1 + BCC)xrightarrow{{850^ circ C}}L2_1 + sigma - phase

Mechanochemical synthesis and heating-induced ordering of Ni-Al-Cr-C quaternary solid solutions

(FeCr structure). Prolonged milling has been shown to stabilize the metastable BCC solid solution at temperatures of ≃650°C.

Phase transformations in Ni + Al and Ni + Al + Cr mixtures during mechanochemical synthesis and subsequent heating

The method for the mechanical alloying of Ni-Al-C and Ni3Al-C mixtures was used to obtain nonequilibrium solid Ni(Al,C) solutions in which the carbon content varies from 2.9 to 8.5 at %. The relationship between carbon dissolution and the probability of appearance of deformation-induced stacking faults (SFs) in the formation of mixed (substitutional and interstitial) solid Ni(Al,C) solutions has been found based on an analysis of the diffraction spectra. SFs are assumed to serve as pathways of carbon penetration in nickel-based solid solutions. The effective carbon radius was found to be about 0.0616 nm in the formation of an antiperovskite phase Ni3AlCx. The method of calculating the amount of interstitial carbon was proposed based on the experimental lattice parameters of fcc solid Ni(Al,C) solutions and ordered phases L12 Ni3Al and E21 (Ni3AlCx). The temperature stability of the nonequilibrium solid Ni(Al,C) solutions was established. It was shown that the decomposition of the solid solutions proceeded according to a spinodal mechanism at a temperature of 400°C with separation into two phases, i.e., an antiperovskite carbide (Ni3AlCx) and Ni(Al,C). At higher temperatures (600–800°C), carbon precipitates from these phases with the formation of an antiperovskite Ni3AlC0.16, solid Ni(Al) solution, and nanocrystalline graphite.

Mechanochemical synthesis and compaction of intermetallic alloys containing nanocrystalline substructure elements

A first-principles stuy of ordering phenomena in hcp interstitial solid solutions of oxygen and nitrogen in Ti, Zr and Hf has demonstrated that the dominant contributions to the interaction energy of interstitial atoms are of chemical nature; thus, it is necessary to modify the previously established concepts about the priority role of deformational interactions in interstitial solutions. We have continued studies of the role of chemical and deformational interactions of interstitial atoms by the example of solid solutions of carbon in nickel. The results obtained also confirm a significant role of chemical interactions between carbon atoms in these solid solutions. The results were compared with the experimental data on the enthalpy of carbon dissolution in nickel and on the coefficient of solutal expansion of the lattice.