Understanding　the　deformation　behavior　of　crack　tips　in　metals　is　of　great　significance　for　improving　fracture　toughness.　However,　how　crack　tips　in　nanosized　metallic　alloys　behave　under　loading　is　unclear,　because　most　previous　studies　focused　on　pure　metals.　In　this　study,　the　atomic-scale　deformation　behavior　of　the　crack　tip　in　AuAg　alloy　nanocrystals　was　observed　in　situ.　We　revealed　that　the　deformation　mechanism　near　the　crack　tip　depended　on　the　distance　from　the　tip.　For　the　＇near　region＇　close　to　the　crack　tip,　plastic　deformation　was　governed　by　partial　dislocations,　twinning,　and　their　interactions.　For　the　＇far　region＇,　further　than　-15　nm　from　the　crack　tip,　full　dislocations　dominated,　and　their　interactions　resulted　in　Lomer-dislocation　(LD)　lock　formation　and　destruction.　We　uncovered　that　the　combination　of　blunting　dislocation-twin　interactions,　twin-twin　intersections,　and　formation　and　destruction　of　LD　locks,　as　a　previously　unrecognized　fracture　toughness　improvement　mechanism　in　metals.