After　solid　solution　at　850　degrees　C,　the　Custom　455　was　aged　at　420　degrees　C　for　different　times.　The　hardness　changes　were　measured　using　a　Vickers　hardness　tester,　and　the　microstructure　and　composition　of　the　second　phase　were　studied　using　atom　probe　tomography　(APT)　and　high-resolution　transmission　electron　microscopy　(HRTEM).　The　APT　results　indicate　that　in　the　early　stage　of　aging,　Cu　atoms　first　segregate　and　attract　Ti　to　form　CuTi　clusters;　As　the　aging　time　increases,　Ti　atoms　in　CuTi　clusters　attract　Ni　and　form　CuTiNi　clusters,　resulting　in　a　rapid　increase　in　sample　hardness;　Subsequently,　CuNiTi　clusters　grew　and　gradually　separated　into　Cu-rich　and　NiTi-rich　particles;　As　the　aging　time　further　increases,　some　NiTi-rich　particles　grow　into　rod-shaped　Ni3Ti　precipitates,　while　others　have　Si　atoms　and　grow　into　spherical　G　phase　particles.　The　orientation　relationship　between　Cu-rich　phase,　Ni3Ti　phase,　and　G　phase　is:　[100]M//[0-11]Cu//[-12-10]Ni3Ti//[100]G,　(011)M//(111)Cu//(0004)Ni3Ti//(022)G.　The　formation　of　the　Cu-rich,　Ni3Ti,　and　G　phase　resulted　in　a　hardness　peak　of　538　HV　after　16　h　of　aging.　Subsequently,　the　decrease　in　hardness　caused　by　the　coarsening　of　the　Cu-rich　and　Ni3Ti　particles　was　offset　by　the　newly　formed　G　phase,　resulting　in　a　slow　decrease　in　hardness.