Effect of Alloying on the Hydrogen Sorption in Ti–Zr–Mn-Based Alloys. Pt. 2: Eutectic Alloys with Laves Phase and B.C.C. Solid-Solution Structure

Abstract

After reviewing and analysing literature data, the microstructure and phase composition of the Ti47.5Zr28Mn(22.5–х)V2Niх (where х = 5.0 and 17.5 at.%) eutectic alloys in the as-cast and annealed states as well as the phase composition of the hydrogenation products of these alloys are investigated using the x-ray phase analysis and scanning electron microscopy methods. As found, nickel is distributed in the alloy between the b.c.c. solid solution and the intermetallic compound with a significantly higher content in the latter. In addition, the phase composition of the initial alloy is changed with the substitution of Mn by 5.0–17.5 at.% of Ni: the ternary NiTiZr ψ-phase, which is isostructural to the Laves phase (C14), is formed. As shown, the size of the intermetallic crystallites is a critical factor for the activation of the H absorption at room temperature and a hydrogen pressure of 0.6 MPa. At a grain surface area of 1–10 µm2 in the initial eutectic microstructure, active interaction of the alloy with hydrogen at the first hydrogenation occurs exclusively during the heating and isobaric–isothermal exposure at 510±10 °C, whereas this process starts at room temperature, when the area of the crystallites is increased up to 100–300 µm2 after annealing. An increase in the size of each of the phase components cause greater volume misfits at the interphase boundaries between the adjacent crystallites at the initial stages of hydrogenation that leads to the cracking of the alloy surface and subsequent activation of interaction with hydrogen.

Keywords

• nickel
• eutectic
• b.c.c. solid solution
• intermetallic compound
• hydrogenation
• dehydrogenation
• hydrogen capacity