High viscoelastic damping is observed in InZn materials over ranges of composition, frequency, temperature, and annealing time. Microscopy reveals InZn when cast segregates into a heterogeneous micro-structure resembling an in situ composite consisting of a zinc matrix with soft indium platelet inclusions. This morphology is predicted to be advantageous for maximizing the damping figure of merit Etan delta by viscoelastic composite theory. InZn is found to be linearly viscoelastic, unlike other high damping metals. The damping of InZn varies little over a substantial range of temperature, in contrast with polymers. For 5 % In material, the optimal composition, Etan delta is 2.8 GPa at 10 Hz, compared to a peak of 0.6 GPa for high damping rubbers. After annealing for 13 years, Etan delta was still high at 1.9 GPa. InZn demonstrates high damping under a wide range of conditions.