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 made of Cu-6.0 As-5.85 Ni-.27 Sb (wt.%) was cast in place into two abutting, “T-shaped” channels cut into the top surfaces of adjacent blocks. The shrinkage on solidification provided a clamping force to press the faces of the blocks together at the joint. The microstructure of the cramp shows substantial porosity and consists of a two-phase, coarse-grained, highly cored casting. The primary phase (about 79%0 is fcc Cu-base solution with AsCuNi (21%) in the grain boundaries, indicating that it formed from the mixture of liquid and fcc on cooling. An assessment of the ternary was conducted following the CALPHAD method using Thermo-Calc software, an existing binary phase diagram, and thermochemical information to define the Cu-Ni rich portion of 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 ternary compound AsCuNi. No ternary solution terms were employed. The Gibbs energy of formation of this compound is assessed as -26000-2T(K) J/gm-atom and the congruent melting point calculated as 1043°C. The full binary and ternary assessment is presented.