Understanding　the　plastic　behavior　of　crack　tips　is　crucial　for　improving　the　fracture　toughness　of　nanometals.　Although　many　studies　are　carried　out,　most　previous　studies　focus　on　pure　metals,　and　how　the　crack　tip　accommodates　the　plastic　deformation　of　highly　concentrated　solid-solution　alloys　is　unclear　owing　to　a　lack　of　direct　atomic-scale　evidence.　In　this　study,　the　atomic-scale　plastic　behavior　of　the　crack　tip　in　face-centered　cubic　(FCC)　AuCu　alloy　nanocrystals　is　observed　in　situ,　which　provides　direct　evidence　that　plastic　deformation　is　governed　by　the　generation　of　deformation　twins　and　hexagonal　close-packed　(HCP)　2H　and　4H　phases,　recurrence　of　reversible　FCC-HCP　phase　transitions,　and　detwinning,　which　are　rarely　observed　in　pure　metals.　This　unusual　behavior　originates　from　the　inherent　chemical　inhomogeneity　of　the　AuCu　alloy,　which　inhibits　twin　thickening　via　partial　dislocations　on　the　adjacent　plane,　instead　of　random　generation　of　deformation　twins,　phase　transitions,　and　reversible　processes.　This　naturally　implies　a　similar　behavior　at　the　crack　tip　in　other　highly　concentrated　solid-solution　alloys,　including　high-medium-entropy　alloys,　providing　important　insights　that　greatly　improve　the　understanding　of　the　fracture　toughness　of　metallic　materials.　Several　unconventional　and　seldom-reported　deformation　modes　are　observed　at　the　crack　tip　of　AuCu　alloy　nanocrystals,　including　the　random　generation　of　hexagonal　close-packed　(HCP)　2H　and　4H　phases,　recurrence　of　the　reversible　face-centered　cubic-HCP　phase　transition,　and　nanotwins　(T),　which　originates　from　the　chemical　inhomogeneity　induced　by　the　solid-solution　atoms　in　the　AuCu　alloy.image