The　morphology,　composition,　structure　and　formation　mechanisms　of　precipitates　in　three　Ni-base　single　crystal　superalloys　containing　3.2　at.　%　Ru　by　adding　Cr　(0　to　6.5　at.　%)　were　investigated.　The　precipitation　speed　in　the　Low-Cr　alloy　was　low　at　a　temperature　of　1100　°C.　The　ReRu-rich　δ　phase,　with　a　hexagonal　structure,　was　the　primary　phase.　A　high　quantity　of　precipitates　appeared　in　the　high　Cr　alloy　after　50　h　of　heat　treatment.　The　quantity　increased　rapidly　from　that　time.　The　σ　phases　dominated　the　precipitate　after　a　short　period　of　heating　at　1100　°C,　with　the　orientation　relationship　corresponding　to　[111]　γ/γ′　//　[111]　σ　and　(110)　γ/γ′　//　(110)　σ.　The　σ　phase　transformed　into　the　intergrowth　of　the　σ　and　the　P　phases　discontinuously　through　the　σ　needle　as　the　exposure　time　increased.　The　large　elemental　γ/γ′　partitioning　ratio　of　Re　in　6.5　at.　%　Cr　was　the　key　factor　that　promoted　nucleation　in　the　σ　phase.　©　Published　under　licence　by　IOP　Publishing　Ltd.