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The tensile properties and fracture behavior of a cast nickel-base superalloy K445 in the temperature range of 25-1 000 ℃ were investigated.The microstructure and fracture surfaces of the alloy were investigated by OM,SEM and TEM.The results revealed that an anomalous yield strength phenomenon exists in the alloy at medium high temperature.The yield strength decreases gradually with the increase of temperature,reaches the minimum value at 650 ℃,and then increases again to obtain 940 MPa,which is almost the same as that at 400 ℃.While the ultimate fracture strength attains the maximum value at 750 ℃,the yield strength and fracture strength decrease entirely.The elongation and fracture shortage almost keep increasing trend with the increase of testing temperature,except the little drop at 400 ℃ and 900 ℃.The fracture surface of K445 alloy below 400 ℃ exhibits most cleavage characteristic with some dimples.With the increase of temperature,the ductile feature increases,but the cleavage still dominates the fracture till 650 ℃.Therefore,it can be concluded that the secondary hardening effect results in the high yield strength and fracture strength with better ductility,which makes it more appropriate to be used at medium high temperature.