Further studies on the nickel–aluminium system. I. β-NiAl and δ-Ni₂Al₃phase fields

Abstract

New lattice parameter and density results have been obtained for alloys in the β‐NiAl and 6‐Ni2Al3 phase fields of the nickel–aluminium system. The lattice parameter of the β‐NiAl phase (CsCl‐type) falls linearly from 2.8870 Å at 50 at.% Ni to 2.8618 Å at 66. at.% Ni, with 2.00 atoms per unit cell. On the other hand, the lattice parameter on the Al‐rich side of NiAl falls linearly from 2.8870 Å to 2.8652 Å, while the number of atoms per unit cell falls from 2.00 to 1.817 by the creation of vacancies in normally nickel sites. The trigonal 6‐Ni2Al3 phase‐structure, which is essentially an extension of cubic β‐NiAl, but with every third plane of nickel atoms perpendicular to the trigonal axis missing, shows a minimum in the a and c spacings at stoichiometric Ni2Al3. Density measurements indicate that the vacancies formed by the missing planes are progressively filled as nickel is added to Ni2Al3, but that a substitutional solid solution is formed on the aluminium‐rich side of stoichiometric Ni2Al3 with aluminium replacing nickel atom by atom. In the NiAl phase, the number of valence electrons increases from 2.28 per unit cell at 61.9 at.% Ni to 3.00 at stoichiometric NiAl, and remains constant at 3.00 as the vacancies form until Ni2Al3 is reached, at which stage the number of vacancies will have reached a maximum when there are only 1.67 atoms per pseudo‐cubic cell. The number of electrons per pseudo‐cubic unit cell then begins to rise and reaches the phase boundary value of 3.12 at 37.6 at.% Ni.

Constitutional and thermal point defects inB2NiAl

“…28,29 The best overall agreement of the theoretical results and experimental estimates is obtained using the GGA l max ϭ3 setup which yields the lowest values of the defect formation energies. On the other hand, all the three sets of theoretical results qualitatively agree with each other and there is only a numerical difference in defect formation energies obtained using different sets of parameters.…”

“…This minimum accounts for the structure of the low-temperature Ni 2 Al 3 phase, which may be viewed as a continuation of the B2 NiAl phase in which all Ni vacancies are separated by the distance a 0 ͗110͘ B2 in the ͕111͖ B2 plane so that each third ͕111͖ B2 plane of Ni atoms is missing. 28,29 As the separation between vacancies in the ͗111͘ direction is a 0 ͗111͘ B2 , the resulting rhombohedral structure is additionally stabilized by the relaxation of the c/a ratio. The recently observed Ni 3 Al 4 (Ni 3 Ga 4 prototype͒ phase contains constitutional vacancies separated by the a 0 ͗110͘ B2 and a 0 ͗210͘ B2 distances.…”

“…It is well known that the enthalpy of alloy formation 63 and the lattice parameter 28,29 of NiAl are essentially linear functions of the alloy composition. In such a case the WagnerSchottky model 56 exploiting the picture of a gas of noninteracting point defects on well-defined sublattices may be applied.…”

“…Short-range ordering of the constitutional vacancies in the Al-rich NiAl is also expected from experiment. [44][45][46] The crystal structures of the phases adjacent to NiAl: Ni 2 Al 3 , 28,29 Ni 3 Al 4 , 47 and Ni 5 Al 3 ͑Ref. 48͒ may be considered as ordered structures of constitutional defects on the B2 underlying lattice.…”

Site preference and effect of alloying on elastic properties of ternaryB2 NiAl-based alloys

“…40 (2.37 eV) and with experimental data (1.90 eV). 42 In Table I the calculated transfer energy and the site preferences of some ternary transition-metal additions to B2 NiAl in the dilute limit (c = 1 at%) at T = 0 K are presented. One can see that some substitutional metals X have a strong preference for either the Al or the Ni sublattice in B2 NiAl, but some of them do not have any particular site preference (i.e., atom X can substitute randomly the Al or the Ni sublattice).…”