Optimazition And Deformation Behavior Of Heterogeneous Structured Materials

The rapid development of modern technologies requires emergence of multi-functional structural materials with high strength,ductility and toughness.Materials fabricated using traditional approaches,such as grain refinement and composites,normally fail to achieve a simultaneous optimization on the basis of strength versus ductility/toughness,which highly restricts their engineering applications.In solving this dilemma,one might resort to heterogenous structural materials,in which microstructures with particular characteristic scales are regulated in specific orders to achieve the "spatial heterogeneity".Such heterogeneity of microstructures is expected to bring strain gradient under applied stress,which promotes the generation of geometric necessary dislocations.The increased dislocation density in such materials would improve the work hardening ability,and hence breaks the inversion correlation between strength versus ductility/toughness,leading to optimized mechanical properties.Controllable preparation is the key issues in obtaining high-quality heterogeneous structural materials.Chemical deposition,an efficient and simple method,can realize the controllable microstructure using layer-by-layer deposition,and therefore is one of the most important method for fabricating the heterogeneous structure.In the first part of this dissertation,how the strength-ductility relationship of materials can be tailored by optimizing the heterogeneous structures are investigated.In particular,we studied the mechanical properties of the Ni-P alloy with symmetric gradient structure,and amorphous Ni-P/coarse grain Ni bi-layered alloys.The main findings are as follow:1.Symmetric gradient-structured(SGS)Ni-P alloys with grain size-gradient and composition-gradient were successfully prepared by electrodeposition,during which the P content could be precisely controlled layer by layer.The grain size of SGS Ni-P alloys could span over the range of 10 nm to 7μm,in continuous gradient structure,which effectively increases the range of Ni grain size.By investigating the correlation among P content,grain size and hardness,we demonstrate that P mainly contribute to grain refinement and solid solution strengthening.Tensile experiment on the symmetric gradient Ni-P alloys reveals that the interaction between gradient structures(in a wide spectrum of grain size)could provide additional enhancement of the overall strength.Besides,the sample exhibits a up-turn of work hardening,and thus shows improved toughness/ductility coordinative property,in comparison with the sample with gradient structure of Ni on one side.Moreover,the surface of SGS Ni-P alloy reaches the highest hardness of the Ni-P system,showing improved wear resistance as compared with coarse grain Ni.2.A bi-layer structure(BLS)consisting of the amorphous Ni-P/coarse grain(CG)Ni(total thickness of 300 μm)were prepared by chemical deposition,where the thickness of amorphous Ni-P/coarse grain were controlled to varied from 0 μm to 13.9μm.Mechanical property test reveals that the thickness of amorphous Ni-P layer not only determines its fracture mode,i.e.shear bands and cracks,but also determines the tensile deformation behavior and deformation mechanism of the whole sample at room temperature.As the thickness of amorphous Ni-P layer increases from 0 to 13.9μm,the overall yield strength of the BLS monotonously increases from 209 to 268 MPa.However,the uniform elongation reaches a maximum value of 32.6%with a Ni-P layer thickness of～6.5μm,signifying an optimum Ni-P layer thickness for simultaneously enhanced strength and ductility which are 1.13 and 1.11 times of those in the CG counterpart,respectively.Owing to the mechanical incompatibility between the amorphous Ni-P layer and CG Ni substrate,a large stress/strain gradient is introduced in this bi-layered structure.In the specimen NP6.5,a widest stress gradient and strain gradient region evolves to the CG Ni substrate,which would promote a maximum back stress,indicating that the highest ductility observed is associated with the highest back stress.Moreover,deformation twinning tends to occur underneath the crack tips of CG Ni substrate in the specimen NP6.5,which could effectively accommodate plastic deformation,also contributing to the superior overall elongation.The present work provides a simple strategy for synthesizing heterogeneous materials with superior mechanical properties.In the second part of the dissertation,the fracture toughness of gradient structure(GS)Ni is studied.Specifically,two setups with different gradient orientations are considered,i.e."Coarse grain" to "Nano-grain"(CG→NG),as well as "Nano-grain" to"Coarse grain"(NG→CG).Main findings are as follow.1.An optimized combination of high strength and high toughness can be achieved in the GS Ni,in comparison with the NG Ni with ultrahigh strength,or CG Ni in uniformed grain-sized structures with low strength and high ductility.2.The CG→NG gradient orientation,where a pre-existing crack initiates from CG zone and propagates into NG zone,displays the best combination of strength and toughness.It shows the largest degree of R-curve toughening behavior,similar to the CG material.At the initial stage of crack propagation,crack blunting occurs in the coarse-grained region,which is characterized by ductile fracture.Once crack extension approaches the end of gradient structure with the grain size of 1-2μm,however,unstable brittle fracture can occur as the crack encounters the nano-sized grains.On the one hand,in the CG→NG gradient structure,there will be a dispersion of stress due to the gradient structure at the crack tip,which makes a lagerer J value of the blunting region and displays an internal toughening mechanism which is the increasing of plastic deformation zone.On the other hand,the crack propagation shows a transform from ductile fracture to brittle fracture,so safety applications needs to be considered.3.The NG→CG gradient orientation,where a pre-existing crack initiates from NG zone and propagates into CG zone,exhibits a degree of R-curve toughening in excess of the NG structure,but less than that of the CG→NG gradient orientation.However,it is less susceptible to outright fracture as the propagation of brittle cracks in the nano-grains of the early part of the gradient region become arrested once they reach the coarser-grained regions due to excessive crack-tip blunting.Such crack blunting,which manifest as a stretch-zone on the fracture surface,indicates that the propagation of cracks undergoes a evolution from brittle fracture to ductile fracture.Therefore,such metallic nickel structures show good potential for safety applications.4.The crack propagation mechanisms of the two kinds of gradient structure,including CG→NG and NG-+CG gradient orientations,do not change.However,the gradient structure with a coarse grain ratio of 60%shows that the both CG→NG and NG→CG gradient orientations exhibited excellent fracture toughness.A longer passivation region was observed for CG→NG gradient orientation,and the position where the brittle-to-ductile transformation occurs earlier.Therefore,the gradient structure with more volume ratio of coarse grain will exhibit more excellent fracture toughness.