Abstract Tensile tests on a FeNi–base austenitic alloy, with different amount of twin boundaries, were conducted at different temperatures and three strain rates, respectively. The results show that serrated flow occurs at temperatures from 300 to 700 ℃. This serrated flow exhibits bulge–like serrations at temperatures from 300 to 600 ℃ and stress–loss serrations at 700 ℃, which manifests the nature of thermal activation, i.e. higher temperatures boost serrations and higher strain rates depress them. Investigations on samples deformed at room temperature (no serrated flow) and 400 ℃ (prominent serrated flow) indicate that twin boundaries are strong enough to block slip deformation at 400 ℃. As a result, stress accumulates on twin boundaries and bulge–like serrations appear on the tensile curves. Effect of twin boundary amount on the morphologies of serrations testified this mechanism.
