e/a-constant Hume-Rothery phases in an As-cast Zr₆₅Al_7.5Ni₁₀Cu_17.5 alloy

The microstructure of an as-cast Zr₆₅Al_7.5Ni₁₀Cu_17.5 ingot was studied by TEM and XRD analysis. Apart from the amorphous phase, five crystalline phases were identified, namely, face-centered cubic cF (0.5273 nm), body-centered tetragonal tI-Zr_67.0Al_1.7Ni_8.4Cu_22.9 (0.3216, 1.1120 nm), hexagonal hP¹-Zr_65.4Al_11.7Ni_11.6Cu_11.3 phase (0.8175, 0.3337 nm), hexagonal phase hP² (0.8175, 0.3337 nm), and orthorhombic oP (0.8210, 1.3187, 0.3315 nm). The latter two phases are superstructures of hP¹. Compared with the nominal sample composition, the tI phase is rich in Cu but poor in Al, the hP¹, hP² and oP phases are to the contrary. All the phases, including the amorphous phase, are Hume-Rothery phases having nearly the same valence electron concentration. Therefore the amorphous phase is strongly related with its related crystalline phases in electronic structure. This should constitute an empirical criterion for high glass forming ability, i.e. the compositions of the amorphous alloy and its crystalline counterparts should be near a constant valence electron concentration `surface' in the quaternary Zr-Al-Ni-Cu phase diagram.