Strengthening/Toughening Mechanisms Of Face-centered Cubic Nanocrystalline Metallic Materials And Their Deformation Behaviors Over A Wide Range Of Strain Rate

Nanocrystalline(NC)metals and alloys,which are by definition metallic polycrystalline structures with mean grain size less than 100 nm,have attracted a great deal of attention since the original nanocrystalline concept was first introduced in early 1990s.The nanoscale grains as well as exceptionally large amounts of grain boundaries in NC metallic materials boost their strength to a record-high level.More than that,some unusual deformation mechanisms that accommodate plasticity at the nano scale provide useful insights into the essence of plastic deformation.Despite intensive studies have been performed on NC materials in terms of processing technology,intrinsic deformation behaviors and structure-properties relationship,some critical issues remain controversial and need to be further explored.First,mounting evidence have pointed out that the mechanical properties of NC metals and alloys are highly sensitive to the imposed rate of deformation.But what is behind such rate-dependent mechanical behaviors remains largely unknown.Second,although the quasi-static mechanical responses of NC materials have been extensively studied,their mechanical and structural behaviors at high strain rate have not.More important,the severe strength-ductility trade-off in NC metals substantially restrict their further development and potential applications.In the present thesis,we performed the following investigations to address upon these fundamental issues:1)Tensile tests over a wide range of strain rates were performed on NCNi and NiCo alloys with different grain sizes to systematically investigate the coupling effects of strain rate and grain size on the mechanical behaviors.It was found that the grain size significantly affected the dependency of mechanical responses to the imposed strain rate.By carefully analyzing the changes in strain rate sensitivity exponent and apparent activation volume, such unique phenomenon was rationalized in terms of the synergy of grain size and strain rate on the transition of deformation mechanisms.Building on classifying the characteristics of plastic deformation on the nano scale,a two- dimensional deformation mechanism map was proposed to comprehensively elucidate the interactive effects of grain size and strain rate on the deformation mechanisms and the related mechanical behaviors in face- centered-cubic NCNi and Ni-based alloys.2)The dynamic mechanical responses and evolution of microstructure and texture in NC face-centered cubic materials subjected to high-strain-rate (HSR)compression are investigated by means of split-Hopkinson pressure bar technique,the transmission Kikuchi diffraction technique and discrete- crystal plasticity finite element(D-CPFE)simulations.The results show that, the microstructure evolution of NC metals during dynamic deformation is substantially affected by the synergy effects of the initial grain size of materials and its dynamic variation,together with the ultra-high strain rate imposed.The dynamic variation of grain size upon shock loading also result in the transition of predominant deformation mechanisms in NC metals and alloys,and thus their HSR mechanical responses.3)A novel strengthening-toughening mechanism,which exploits the NC microstructure and nano-scale compositional heterogeneities,was proposed for NC metallic materials.The nanostructuring endows concentrated solid solutions ultra-high strength.Meanwhile,the composition undulation was escalated on top of the statistical concentration fluctuation that is intrinsic for such heavily concentrated solutions.As a result,multi-scale compositional undulations were introduced into the nano-grains.Such compositional undulations render stacking fault energy and lattice strains spatially varying over multiple length scales,to effectively interact with moving dislocations.The motion of dislocations becomes sluggish,promoting their interaction,interlocking and accumulation inside the tiny NC grains.The flow stress is elevated as a result,simultaneously with extraordinary dislocation storage to elevate strain hardening and hence ductility.Meanwhile,the segment detrapping along the dislocation line entails a small activation volume and elevated strain-rate sensitivity,also stabilizing the tensile flow.The technical feasibility of this approach was demonstrated in an electrodeposited concentrated Ni₅₀Co₅₀ NC alloy:by introducing multi-scale intra-granular compositional undulations,the alloy exhibits a yield strength as high as?1.6GPa and an ultimate tensile strength up to?2.3 GPa while having a good ductility with tensile elongation to failure of?16%,outperforming all previous NC metals.