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There is emerging recognition that crystalline defects such as grain boundaries and dislocations can host structural and chemical environments of their own, which reside in local equilibrium with the bulk material. Targeting these defect phases as objects for materials design would promise new avenues to maximize property gains. Here, we provide experimental proof of a dislocation-templated defect phase using a processing strategy designed to engender defect phase transitions in a nickel-based alloy and demonstrate dramatic effects on strengthening. Following heat treatments designed to encourage solute segregation to dislocations, regions with introduced dislocation populations show evidence of nanoscale ordered domains with a L1 structure, whereas dislocation-free regions remain as a solid solution. Site-specific spherical nanoindentation in regions hosting dislocations and their associated ordered nanodomains exhibit a 40% increase in mean pop-in load compared to similar regions prior to the segregation heat treatment. Strength estimates based on random solute atmospheres around dislocations are not sufficient to predict our measured strengths. Our mechanical measurements, in tandem with detailed electron microscopy and diffraction of the ordered domains, as well as characterization of dislocations in the vicinity of the nanodomains, establish the defect phase framework via direct observations of chemical and structural ordering near dislocations and its potential for offering favorable properties not achievable through conventional materials design.