Properties of Mo—Ni—B phases in Mo—rich alloys and alloys with boron content of 40—43% (at.)

Abstract

Properties of phases, microhardness Hv and lattice parameters, were studied in Mo—Ni—B alloys, as-cast and annealed at subsolidus temperatures. The alloys were prepared by arc melting and their phase constituents were evaluated in the course of studying phase equilibria. The alloy compositions are located close to the Mo—B bounding system and on the B isoconcentrate of ~41% (at.). In the plotted dependences of the lattice parameters of phases on their compositions, usually there is their decrease with rise in the Ni content in accordance with the atomic radii. So, for the phase based on molybdenum (Mo), the lattice parameter a decreases for as-cast alloys from 314,7 pm (without Ni) to 313,8 pm (at 3,8% (at.) Ni). Some softening of Mo by Ni additions is observed (Hv drops from ~8,4 to ~5,5 GPa in annealed samples). The dissolution of Ni has a different effect on lattice parameters of Mo₂B and α-MoB borides: as the Ni content rises, parameters a increase from 554,3 to 555,1 and from 310,1 to 311,5 pm, and parameters c decrease (from 473,6 to 473,4 pm and from 1696 to 1692 pm respectively. For the compound Mo₃NiB₃ parameters a and b decrease with an increase in the Ni content (from 318,2 to 317,8 pm and from 841,8 to 840,9 pm respectively), while parameter с remains unchanged (1073,8— 1073,9 pm). The lattice parameters and Hv of the phase (o-Ni₄B₃) have barely increased with rise in the Mo content in it (1194—1196, 297,5—298,1 and 656,3—657,5 pm for periods a, b and c respectively). The values Hv of all the 5 studied boride phases fall in the range from 14 to ~23 GPa.