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• A gradient structure TiAl alloys was manufactured by additive manufacturing. • The room-temperature compressive strength of gradient TiAl alloys is 2.6 GPa. • The compressive strength is 1.28 GPa at 750 °C. • The strengthening effect is attributed to more intersection slips. Gradient structure is emerging as an effective strategy to fabricate metals with remarkable mechanical performance, but have not been verified in intermetallic compounds for high-temperature applications. Through experiments and atomic simulations, we show that a typical intermetallic TiAl alloy with gradient structure has a significant strengthening effect both at room temperature and high temperatures. The room-temperature compressive strength of TiAl alloys with gradient grain obtained by additive manufacturing is 2.57 GPa, which is ∼2.7 times as strong as that with equiaxed grain. The strengthening effect is attributed to more sessile dislocations in gradient structure caused by the intersections of multiple slip systems in gradient grain. More importantly, the strengthening effect is still effective at high temperatures and the compressive strength is 1.28 GPa at 750 °C. The simulation results show that this strengthening effect is due to the increased Hirth dislocation at high temperatures. This study expands the applications of TiAl alloys for load-bearing structures and provides a new strategy for improving the strength of intermetallic compounds at both room temperature and high temperatures. [Display omitted] [ABSTRACT FROM AUTHOR]

• The effects of dislocations on the formation and structural transformation of coherent nanoprecipitates were studied. • Dislocations are harmful for the formation of small and abundant coherent nanoprecipitates in a certain density range. The traditional solution treatment of low carbon steels can produce martensite with dislocation density that fall within the negative effect range. • The design of composition and processing routes should either diminish dislocations to guarantee homogeneous nucleation, or increase the dislocations to a certain amount to promote heterogeneous nucleation. The strength-ductility trade-off has been the most challenging problem for structural metals for centuries. Nanoprecipitation strengthening is an ideal approach to enhance the strength without significantly sacrificing the ductility. Stable nanoprecipitates have been successfully acquired by nanostructural design, but the number density of nanoprecipitates cannot be further increased. Researchers attempted to enhance number density by introducing highly potent nucleation sites (e.g., dislocations). However, there remains controversy over the influence of dislocations on the nucleation and growth of coherent nanoprecipitates with minimized nucleation barrier. Here, Cu-rich nanoprecipitates in an HSLA steel, as a typical type of coherent nanoprecipitates, are investigated. By combining analytical calculation and experiments, we show that dislocations are harmful for the formation of large numbered Cu-rich nanoprecipitates in a certain density range. Insufficient dislocations deprive solute atoms which decrease homogenous precipitation that cannot be compensated by the increase in heterogeneous precipitation. Under such circumstance, Cu-rich nanoprecipitates have smaller number density but larger size and higher fraction of incoherent structures due to rapid Ostwald ripening. As a result, by controlling dislocation density, the yield strength is increased by 24% without obvious loss in ductility as compared with traditional solution-quench-age process. Our work would help to optimize composition and processing routes that fully exploit the nanoprecipitation strengthening effect. [ABSTRACT FROM AUTHOR]

The creep strength enhanced martensitic steels are key material for the main power generating units in ultra-supercritical plants. Studies on the evaluation of their creep rupture life show there is an overestimation of rupture life after long-term creep, which is known as premature failure. However, the microstructural origin of the premature failure remains unclear. Here in this study, we have carefully investigated the microstructural transformations and their influences on creep rupture behavior, showing that the evolution of martensite and M 23 C 6 carbides as well as Laves phase are responsible for the premature failure. By using multi-step TTP-LMP method, we confirmed a three-stage creep rupture behavior under different stress regions. Further quantitative analysis showed that the coarsening of M 23 C 6 carbides and recovery of martensite exert equal and dominant effects on the premature failure in the medium stress region, while precipitation and coarsening of Laves phase are responsible for the premature failure in the low stress region. [ABSTRACT FROM AUTHOR]