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By examining a wide region of the Al-Cu-Ru phase diagram, a thorough analysis of the compositional and thermal stability of the icosahedral phase has been completed. The primary solidification product of rapid solidification was a topologically and chemically disordered icosahedral phase with an extensive compositional region. Crystallization through exothermic events of the as-solidified materials produced crystalline phases, without the formation of the face-centered-icosahedral (FCI) phase. However, the FCI phase does form at higher temperatures through an endothermic reaction, indicating that it is a stable phase of the system, but only at elevated temperatures. Of the alloys studied, the FCI phase field was found to encompass Al65Cu23Ru12, Al65Cu20Ru10, and Al70Cu15Ru15. The transformation to the FCI phase involves an intermediate approximant phase that is very similar to the FCI structure. Also, a cubic approximant containing atomic arrangements with local icosahedral symmetry similar to α -A1 Mn Si was determined to exist near the FCI phase field.