Discovery of a thermally persistent h.c.p. solid-solution phase in the Ni-W system

Abstract

Although the accepted Ni-W phase diagram does not reveal the existence of h.c.p.-based phases, h.c.p.-like stacking sequences were observed in magnetron-co-sputtered Ni-W thin films at W contents of 20 to 25 at. %, by using transmission electron microscopy and X-ray diffraction. The occurrence of this h.c.p.-like solid-solution phase could be rationalized by first-principles calculations, showing that the vicinity of the system's ground-state line is populated with metastable h.c.p.-based superstructures in the intermediate concentration range from 20 to 50 at. % W. The h.c.p.-like stacking in Ni-W films was observed to be thermally persistent, up to temperatures as high as at least 850 K, as evidenced by extensive X-ray diffraction analyses on specimens before and after annealing treatments. The tendency of Ni-W for excessive planar faulting is discussed in the light of these new findings.

First principles, thermal stability and thermodynamic assessment of the binary Ni–W system

“…These energies were used for drawing the phase diagram at zero kelvin and Ni 4 W and Ni 8 W phases were found to be stable. These findings were confirmed by Kurz et al [14,15], who found Ni 8 W (bct Pt 8 Ti type) and Ni 4 W (Ni 4 Mo type) phases to be stable, but Ni 3 W phase (hcp-Cu 3 Ti type) to be at least metastable.…”

“…The result was that Ni 3 W compound was found from samples annealed after deformation, therefore this compound could be metastable, at least at 1 373 K. During annealing at 1 373 K the amount of Ni 3 W diminishes, which means metastability of the phase or that the temperature is over the dissociation temperature. The annealing temperature 1 373 K is over 850 K, where Ni 3 W forms according to Kurz [14,15]. The stability and metastability ratios of Ni 3 W phase are related to different temperatures and conditions: Ni 3 W is stable at lower temperatures and metastable at higher temperatures, which was reflected in the calculations made.…”

“…Hexagonal Ni 3 W phase in Ni-W films was observed to be thermally stable at temperatures as high as 850 K, as evidenced by (XRD) before and after annealing treatments [14,15] and was here also evaluated as a stoichiometric compound.…”

“…Liquid phase was assessed using one sublattice, fcc and bcc phases were assessed using two sublattices. According to Schindzielorz et al [3], Taylor [7] and Kurz [14,15] the phase Ni 8 W is very probably a bct phase like Pt 8 Ti. This structure is also considered as being similar to ordered tetragonal low temperature martensites a@-Fe 16 C 2 and Fe 16 N 2 [17 -19].Therefore, the Ni 8 W phase I. Isomäki et al: First principles, thermal stability and thermodynamic assessment of the binary Ni-W system was evaluated as Fe 16 C 2 with three sublattices: for the Fe-C system they are usually (Fe) 1 (C,Va) 1/8 (C,Va) 23/8 [19].…”

“…The stability and metastability ratios of Ni 3 W phase are related to different temperatures and conditions: Ni 3 W is stable at lower temperatures and metastable at higher temperatures, which was reflected in the calculations made. Also Kurz I. Isomäki et al: First principles, thermal stability and thermodynamic assessment of the binary Ni-W system [14,15] studied Ni-W thin film samples with XRD and transmission electron microscopy (TEM) and found that Ni 3 W is (at least meta-)stable. The thin films were annealed 12 h at 850 K.…”