Recently, the high ductility of Cu-based bulk metallic glasses (BMGs) has been directly linked to the presence of Y2O3 precipitates, and surrounding crystallized areas. Nevertheless, the formation of the precipitates remains to be defined. In this work, the structure of Y2O3 precipitates and crystallized zones of the BMG are investigated at the nanoscale in three-dimensions, shining light on their spatial distribution and their origin. Two kinds of precipitates were observed: small solid ones and large hollow ones. The mechanism proposed to explain the formation of hollow-precipitates is based on the Kirkendall effect. A model was implemented to assess the feasibility of this mechanism with the experimental results. Moreover, our results suggest that micro and nano-crystallized areas within the BMG are induced by the presence of yttria precipitates, with a correspondence between the size of the precipitate and the size of the crystallized areas. Keywords: Bulk metallic glasses, Microscopy, Kirkendall effect, Yttrium, Y2O3 precipitates