Strengthening in gradient TiAl alloys

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 gra-dient 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 man-ufacturing 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 mul-tiple 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.

Control of dislocation density maximizing precipitation strengthening effect

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.