Mechanical Property Enhancement and Deformation Mechanism Investigation of Metals Processed by Surface Mechanical Attrition Treatment

表面機械研磨金屬材料力學性能的提高及機理研究

Student thesis: Doctoral Thesis

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Author(s)

  • Xiaowei LIU

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date21 Sept 2016

Abstract

In the past decades, it has been proved that the nanostructured materials can significantly improve the strength of materials. Among the advanced techniques to get nanostructured materials, surface mechanical attrition treatment (SMAT) has been applied to pure metals and alloys to effectively enhance their mechanical properties through the refinement of structures of materials. In this study, we mainly explore the relationship between structures and properties of different materials treated over SMAT with different techniques.

By applying the technique of SMAT to partially treat the copper (Cu) sheet, we can topologically control the planar heterogeneous structures of materials to obtain both high strength and high ductility. The yield strength has been improved twice without any sacrifice of ductility. The hardness measurement, FEM simulation and TEM characterization have clearly indicated the bimodal structures of the partially SMAT Cu of hard phase of nano-twinned structure and soft phase of the coarse-grained matrix. The mechanism-based model was also performed to characterize the mechanical properties of such bimodal structured Cu and the results agreed well with the experiment results. With the combination of hard phase and soft phase in the bimodal structured Cu, we can get the overall enhancement of materials, which provides a further insight into the fundamental investigation of the relationship between structures and properties.

In situ SEM was applied to characterize the mechanical behaviors of bulk materials in micro scale to study the relationship between structures and properties. For magnesium (Mg) alloys are the lightest structural metals. The SMAT AZ31 has different layers of structures of different grain sizes, which can be used to study the relationship between mechanical properties and grain sizes of materials. The results confirmed that for fine-grain sample, the fracture was dominated by the intergranular cracks, while for coarse-grain sample, both trans-granular and intergranular cracks were activated and their nucleation will lead to the final fracture. The advanced in situ SEM technique opened a new way to quantitatively study the relationship between structures and properties in real time.

In recent years, there has been a hot topic of novel structure of hierarchical nanotwins (HNTs) that have been obtained in the twinning-induced plasticity (TWIP) steels with excellent combination of strength and ductility. The pure silver (Ag) has the lowest stacking fault energy (SFE) compared with other metals. Here, by applying SMAT together with pre-stretch, we processed the Ag to get the HNT structures. We have successfully got the third ordered HNTs with much improved mechanical properties. MD simulations were conducted to study the formation mechanism of such HNT Ag. And the MD simulation verified that the interior negative climb and partial dislocation propagation and escape from free surface lead to the thinning/thickening in the HNT Ag, which explained the generation of such HNT structures in Ag.

In summary, we studied the relationship between structures and properties of different materials with different processing techniques and provided clear explanations of such enhancement of properties which resulted from the different structures. The study may provide further insight into the study of the relationship between structures and mechanical properties. And with the further understanding of the underlying mechanism, this study provides people new strategies to better design the structures of materials to get better mechanical properties for applications.