Assessment of the As-Cu-Ni system: An example from archaeology

During the early phase of the Middle Horizon in the southern Andes (ca. AD 400-800), the majority of bronze objects at the site of Tiwanaku (Bolivia) were made of a ternary Cu-As-Ni alloy. This highly unusual bronze alloy was cast into the form of I-shaped cramps to clamp together rectangular stone building blocks in monumental constructions. The cramps are about 19cm long and about 7cm wide at the two extreme ends of the "I". The center section of the "I" is about 1.4 cm wide and 1.4 cm thick. In one design a cramp mad e of Cu-6.0 As-5.85 Ni-.27 Sb (wt.%) was cast in place into two abutting, " T-shaped" channels out into the top surfaces of adjacent blocks. The shrink age on solidification provided a clamping force to press the faces of the b locks together at the joint. The microstructure of the cramp shows substant ial porosity and consists of a two-phase, coarse-grained, highly cored cast ing. The primary phase (about 79%) is fee Cu-base solution with AsCuNi (21 %) in the grain boundaries, indicating that it formed from the mixture of l iquid and fee on cooling. An assessment of the ternary was conducted follow ing the CALPHAD method using Thermo-Calc software, an existing binary phase diagram, and thermochemical information to define the Cu-Ni rich portion o f the ternary. An existing description of the Cu-Ni system was used and the As-Cu and As-Ni assessments were derived and employed to define the ternar y compound AsCuNi. No ternary solution terms were employed. The Gibbs energ y of formation of this compound is assessed as -26000-2T(K) J/gm-atom and t he congruent melting point calculated as 1043 degreesC. The full binary and ternary assessment is presented.