Effects of rare earth additions on structures and properties of rapidly solidified copper alloys

Copper based Cu-RE alloys (where RE represents lanthanum, neodymium, or samarium) with alloying content up to 16 wt-% were prepared by chill block melt spinning into ribbons of thickness between 40 and 100 μm. The melt spun ribbons were heat treated isochronally for 2 h at 300, 400, 500, 600, 700, and 800°C, respectively. The melt spun and heat treated ribbons were tested for microhardness and resistivity and were characterised by optical and transmission electron microscopy (TEM) and X-ray diffractometry (XRD). Microstructures were of the typical zone B type for Cu-1 RE and Cu-3RE ribbons and of the zone B/zone A type for Cu-5RE, Cu-8RE, and Cu-12RE ribbons. Only microstructures of the zone A type were found in Cu-15La ribbons. The metastable extended solid solubilities of the rare earth elements were evaluated by measurements of the lattice parameters of the supersaturated solid solutions and significant extension from the equilibrium solid solubility was found for all three alloys. The secondary phase was identified by TEM and XRD as Cu₆RE for all ribbons except Cu-15La ribbons in which metastable Cu₅La and Cu₁₃La phases were also found. Observations using TEM and XRD also showed a reduction in the α-Cu grain size of the as spun ribbons with increasing alloying content, producing nanosized α-Cu grains on the chill side of the ribbons. Heat treatment of the ribbons at 400°C for 2 h produced no significant coarsening of α-Cu grains as the size of these grains was still in the nanometer region. Both α-Cu and Cu₆RE grains coarsened as a result of heat treatment for 2 h at temperatures of 600°C and above for all the alloys.