Traditional structure materials made from metals and alloys, such as iron, steel and copper, are one of the comer stone of modem society. However, general strengthening approaches invariably compromise the ductility of the materials, which is believed to be an intrinsic "Achilles' heel" of strengthening structure metals and alloys. It has been found that nanostructured (ns) materials, when evolves deformation induced martensitic transformation (DIMT), nano-twins (nt) or the gradient grain size/phase structure, could simultaneously strengthen and toughen. In this thesis, I mainly focused on the characterization of microstructure, phase transition in gradient structural AISI 304 stainless steels and the plastic deformation mechanisms of nt-Cu under stress relaxation and creep test...[ Read more ]

Traditional structure materials made from metals and alloys, such as iron, steel and copper, are one of the comer stone of modem society. However, general strengthening approaches invariably compromise the ductility of the materials, which is believed to be an intrinsic "Achilles' heel" of strengthening structure metals and alloys. It has been found that nanostructured (ns) materials, when evolves deformation induced martensitic transformation (DIMT), nano-twins (nt) or the gradient grain size/phase structure, could simultaneously strengthen and toughen. In this thesis, I mainly focused on the characterization of microstructure, phase transition in gradient structural AISI 304 stainless steels and the plastic deformation mechanisms of nt-Cu under stress relaxation and creep tests at different temperatures.

Firstly, the gradient structure in AISI 304 stainless steels was introduced by surface mechanical attrition treatment (SMAT). The transmission electron microscopy (TEM) was mainly used to characterize the microstructure change after SMAT and understand the grain refinement mechanisms along the cross-sectional direction. It is found that three polymorphic DIMT in nano-scale, including γ(fcc) → Twin → α' (bcc), γ(fcc) → FBs(faulted bands)→ α'(bcc) and γ(fcc) → ε (hcp) → α' (bcc), mainly contribute to the gradient grain refinement. The formation of deformation twins and hcp-ε were distinguished at atomic-scale. The morphologies and the crystallographic orientation relationships (ORs) between γ matrix, twins, FBs, ε and α' of three polymorphic DIMT were also characterized.

A persistently unsolved bottleneck problem hampers our understanding the martensitic transformation is the microscopic mechanism, with crucial crux lying in the underlying atomistic processes. Therefore, secondly the atomic arrangements and crystalline defects associated with deformation induced γ → ε → α' phase transformation were typically dissected by high-resolution transmission electron microscopy (HRTEM). Specifically, it showed misfit dislocations at the γ, ε and α' interfaces to release misfit strains between these phases, and the diffuse tilt GBs that cause the change in crystal orientation of the α' inclusions. Furthermore, the atomic scale observations demonstrate the γ'_3T/8 and γ'_T/3 transition lattices within the diffuse ε/α' interfaces, catering the "3T/8-T/3" shear. The plastic deformation induced ε plates reduce the "3T/8-T/3" shear to the reverse shuffle/shear of excess "T/8-T/6" transformation dislocations, which result in the SF and γ strips inside the ε plates and the necks of the ε plates. The unprecedented level of details we have uncovered experimentally provides systematic evidence that verifies the 50-years-old BBOC model for the first time, and enriches our understanding of the DIMT mechanism.

At last, the repeated stress relaxation with a relative long-term and/or tensile creep tests were performed on the coarse-grained (cg)-Cu, twin-free nanograined (ng)-Cu and nt-Cu with relative same grain size at different temperatures, respectively. It is found that the deformation mechanisms are stress/strain rate-dependent, indicating the mechanism transition. The mobile dislocations exhaustion rate in stress relaxation, activation energy and volume in both of stress relaxation and creep, and the stress exponent in creep were quantitatively calculated at the different transition regions. Compared to those deformation mechanism parameters, it concluded that nano-twin boundaries (TBs) in nt-Cu can significantly promote the plasticity and resist the creep deformation. Furthermore, the scanning electron microscopy (SEM) observations on failure fracture surface and HRTEM observations of deformed samples showed that the partial dislocations/stacking faults are nucleated and parallel to the TBs at low stress level, while that transfers to the perfect dislocations pile-up and cutting through the TBs at high stress level. In brief, the stress relaxation and creep experiments in our work indicate that, as stress and strain rate decreases, the deformation mechanisms in nt-Cu transition from inclined perfect dislocations to TBs migration and finally diffusion Coble creep. The findings in this thesis have implications for improved microstructural control in metals and alloys.