Crystallization in Al₈₈RE₈Ni₄ glass-forming alloys

Abstract

The results from transmission electron microscopy (TEM) microstructural studies of devitrified amorphous alloys with a composition Al₈₈RE₈Ni₄ are presented as a function of average rare-earth atomic radii (between 1.76 and 1.87 Å). Rare-earth elements RE [tbnd] Gd, Er and La were co-substituted to vary the average atomic radius r_RE, while maintaining an 8 at.% rare-earth concentration. As reported previously for Al₈₈La₂Gd₆Ni₄ glasses, compositional phase separation prior to crystallization was observed in all alloys of this series. TEM examination of amorphous samples following low-temperature annealing treatments showed the development of strong image contrast prior to crystallization, corresponding to a segregation of the glassy matrix into aluminium-rich and solute-rich regions over a length scale of a few tens of nanometres. The subsequent crystallization behaviour depended on the average rare-earth radii, with metastable intermetallic phases forming first in glasses made with larger r _RE and with fee α-Al nanocrystallization in smaller r _RE glasses, showing preferential nucleation and growth of α-Al near the boundaries of the phase-separated regions.