Random　access　memory　devices　that　rely　on　phase　changes　are　primarily　limited　by　the　speed　of　crystallization.　However,　imaging　structural　dynamics　in　working　devices　with　nanoscale　resolution　remains　elusive.　Using　an　ultrafast　liquid-quenching　system　in　transmission　electron　microscope,　we　obtain　a　melt-quenched　amorphous　phase-change　material　Ge2Sb2Te5　nanosheet.　We　find　that　the　applied　incubation　field　(pretreatment)　leads　to　nonuniform　nucleus　formation　and　growth　in　the　amorphous　Ge2Sb2Te5,　accompanied　by　a　slight　change　in　electric　properties.　Moreover,　the　phase-change　mechanism　changes　from　the　material　inherent　crystallization　mechanism　(nucleation-dominated)　to　the　heterocrystallization　mechanism,　indicating　the　incubation　period　of　nucleation　might　be　bypassed.