Tetragonal compact bodies obtained by quenching from the melt a ZrO2-2,8 mol% Y2O3 commercial powder have been investigated between room temperature and 1150 degreesC, using mainly perturbed angular correlations spectroscopy. The resulting nontransformable t' phase observed by optical micrography is characterized at nanoscopic level by a hyperfine interaction describing defective and disordered Zr4+ neighborhoods very different from those of the regular tetragonal phase. A small amount of remaining Zr4+ sites corresponds to a scarcely distorted tetragonal structure, As the compacts are heated, two processes activate: the movement of vacancies and the gradual and irreversible conversion of t' defective sites into less defective ones, probably resulting from oxygen absorption.