Microsegregation in Al-Cu Alloys

A comparison has been made between the amount of microsegregation predicted by a numerical model and that found experimentally in Al-Cu alloys varying in composition between 1 and 8 wt pct Cu. A depleted region was predicted and observed experimentally near the Al[subscript 2]Cu. The depleted region was formed below the eutectic temperature and had a significant effect on the ordered composition-fraction plots, particularly for high alloy compositions. Although the fit between experiment and theory was reasonably good, it was concluded that it was necessary to propose that local equilibrium was not maintained between the phases in the solid-state reactions

Halo Formation During Solidification of Refractory Metal Aluminide Ternary Systems

The evolution of eutectic morphologies following primary solidification has been studied in the refractory metal aluminide (Ta-Al-Fe, Nb-Al-Co, and Nb-Al-Fe) ternary systems. The undercooling accompanying solid growth, as related to the extended solute solubility in the primary and secondary phases can be used to account for the evolution of phase morphologies during ternary eutectic solidification. For small undercooling, the conditions of interfacial equilibrium remain valid, while in the case of significant undercooling when nucleation constraints occur, there is a departure from equilibrium leading to unexpected phases. In Ta-Al-Fe, an extended solubility of Fe in σ was observed, which was consistent with the formation of a halo of μ phase on primary σ. In Nb-Al-Co, a halo of C14 is formed on primary CoAl, but very limited vice versa. However, in the absence of a solidus projection it was not possible to definitively determine the extended solute solubility in the primary phase. In Nb-Al-Fe when nucleation constraints arise, the inability to initiate coupled growth of NbAl3 + C14 leads to the occurrence of a two-phase halo of C14 + Nb2Al, indicating a large undercooling and departure from equilibrium

Solidification Reaction Sequence of Co-Rich Nb-Al-Co Alloys

The freezing reaction sequence of Co-rich Nb-Al-Co ternary alloys with emphasis on the formation of Laves and Heusler phases has been examined. For Co-rich alloys, the solidification reaction sequence is observed as primary freezing of α-Co and CoAl phases, subsequent [Co + C36] and [CoAl + C36] eutectics, and the final ternary eutectic reaction [L → α-Co + C36 + CoAl]. The compositions of solidified α-Co and C36 phases agree with the corresponding vertices of the tie-triangle at the solidus temperatures. When the Nb concentration is over 20 at. pct in Co-rich alloys, the quasi-peritectic reaction [L + Co2AlNb → C36 + CoAl] does not occur as equilibrium prediction. The formation of C36 and CoAl phases occurs through solid precipitation and must be distinguished from a solidification reaction