Display omitted •Face-centered-cubic (FCC) Cr, L12 Cr3Cu, C14 Cr2Nb, C15 Cr2Nb, and (Cr, Cu)2Nb were detected in the alloy. A time–temperature dependent phase transition map was established.•The competition of nucleation and growth of L12 Cr3Cu and C14 Cr2Nb during aged at 600 °C was discussed.•It is discussed why C15 Cr2Nb is precipitated during the solidification process of the alloy, but C14 Cr2Nb is precipitated during the aging process. Cu-Cr-Nb alloys have attracted many researchers’ interest for decades while their structure and second phases inside were unclear. We have characterized the second phase particles formed during solidification, hot rolling, and aging treatments in a Cu-4.06Cr-1.25Nb alloy. Face-centered-cubic (FCC) Cr, L12 Cr3Cu, C14 Cr2Nb, C15 Cr2Nb, and Nb-rich particles were detected in the alloy. The crystal structure, precipitation, phase transformation behavior and orientation relationship (OR) were uncovered. C15 Cr2Nb particles formed in the solidification process while C14 Cr2Nb particles precipitated as aged at 600 °C. The competition of nucleation between nano-scale Cr3Cu and Cr2Nb particles aged at 600 °C was discussed. The nucleation of L12 Cr3Cu was prior to that of C14 Cr2Nb, and the precipitation of Cr2Nb particles would limit the coarsening of Cr particles. A phase transformation map during age treatment was established on the basis of transmission electron microscope (TEM) images and X-ray diffraction (XRD) patterns. These findings are helpful in developing high-performance Cu-Cr-Nb alloys.