Glass forming ability of (Hf,Cr)[sbnd]Co[sbnd]B alloys: Computational and experimental studies

Abstract

The quantities describing glass forming ability (GFA) of (Hf,Cr)CoB system were calculated. Enthalpies of formation of amorphous state, solid solution and intermetallic phases were determined and discussed along with normalized mismatch entropy and glass forming ability parameter. All thermodynamic quantities were confronted with the experimental results of the structural and thermal analysis of Hf2−xCrxCo11B (0≤x≤2) melt-spun alloys. Much emphasis was put on the alloy of the composition 2:11:1 because of its possible application as a permanent magnet material. Formation enthalpy of Hf2Co11B amorphous alloy ΔHam=−20.6kJ/mol, being in agreement with other determined parameters, indicates moderate glass forming ability. With increasing Cr content ΔHam becomes less negative and for Cr2Co11B it reaches ΔHam=−0.5kJ/mol. Cr-rich alloys are believed to not form amorphous state, which was confirmed by experimental results. Formation of solid solution is preferred in the latter case. Low GFA of Cr-containing alloy is connected with similar atomic radii of Cr and Co and smaller interfacial enthalpy of CrCo than HfCo pair. Fully amorphous diffraction patterns were observed for the alloys with high Hf content (0≤x≤1), while formation of crystalline phases at the expense of amorphous state was shown for 1.5≤x≤2 and indirectly confirmed by density measurements. Higher thermal stability of Hf-rich alloys is consistent with their moderate GFA.