Development of high strength and high ductility nanostructured twip steel

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Development of high strength and high ductility nanostructured twip steel

 

Author: Kou, Hongning
Title: Development of high strength and high ductility nanostructured twip steel
Degree: Ph.D.
Year: 2011
Subject: Nanostructured materials
Steel -- Ductility.
Steel -- Testing
Hong Kong Polytechnic University -- Dissertations.
Department: Dept. of Mechanical Engineering
Pages: xix, 182 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2507220
URI: http://theses.lib.polyu.edu.hk/handle/200/6622
Abstract: The current growing awareness about the potential benefits of nanotechnology in the modern engineering industry is pursuing research in the area of nanostructured steels. But the increase of tensile strength of a material is often obtained at the sacrifice of ductility. The main challenge for researchers is to manufacture bulk nanocrystalline steel possessing superior mechanical properties of simultaneous high strength and good ductility at a reasonable cost. To meet this objective, a number of innovative approaches are being developed to produce nanostructured steels. In this study, a preeminent steel candidate with twinning induced plasticity (TWIP) effect is selected for the fabrication of nanostructured steel using surface mechanical attrition treatment (SMAT). After the process, a novel kind of TWIP steel with gradient grain sizes of nano-grains, micro-grains and coarse-grains is produced, which remarkably achieves simultaneous ultrahigh strength and good ductility. We studied the microstructural evolution of nanocrystallined TWIP steel, analyzed their mechanical properties and investigated their behavior after tensile testing. By means of transmission electronic microscope (TEM), high resolution transmission electronic microscope (HRTEM) and scanning electronic microscope (SEM), microstructure of TWIP steel samples subjected to different SMAT processes (some combined with heat treatment) were characterized. Mechanical properties of these nanostructured TWIP steels were tested by uniaxial tensile tests. X-ray diffraction was used to identify the phase transformation. Tensile tests reveal the extremely superior mechanical combination of strength and ductility in these nanostructured SMATed TWIP steels. With the increase of duration of SMAT, both yield strength and tensile strength firstly increased to a maximum value, accompanied by considerable ductility. The most remarkable increase of yield strength even achieves approximately five times higher than conventional TWIP steel samples, achieving 2.4 GPa, accompanied by 18% total elongation. Subsequently, a simultaneous decrease in both tensile strength and elongation happened in the sample that was processed for longest SMAT duration.
It is known that the excellent ductility of traditional TWIP steels is attributed to the instantaneous production of deformation twinning during tensile testing. Based on this, an interesting hierarchical twinning system, which is composed of high density of multi-scale twins (including some stacking faults) in two/three orders, is revealed by TEM/HRTEM observations in these nanostructured TWIP steels. The multiple planer defects consist of multiple twins/stacking faults, which are progressively produced by means of annealing treatment, SMAT and tensile deformation. On one hand, boundaries of these hierarchically multiple twins with different orientations form a three-dimensional networking structure, which restrict the twinning activity by each other and act as strong barriers to dislocation motion, leading to ultrahigh strength. On the other hand, stress concentration is relieved due to deformation transfer from grain to grain caused by twinning, resulting in the extension of plasticity. Therefore, the hierarchical twinning structure is regarded as the most effective element that induces the extraordinary ultrahigh strength and good elongation in the SMATed TWIP steel samples. X-ray diffraction results suggest almost no phase transformation from austenitic matrix to martensite in SMATed TWIP after tension. The stable austenite also contributes to the preservation of good ductility. Nevertheless, an simultaneous drop-off in the volume fraction of hierarchical twins with two/three orders and dislocation density is detected in nanostructured TWIP subjected to the longest SMAT duration, indicating the corresponding decrease in accommodation of sequential twinning after tension and interaction between twins and dislocations. Some ε-martensite, as well as slight amount of α'-martensite, is also observed by TEM/HRTEM. All these factors account for the decrease of strength and early fracture. Another route of fabricating nanostructured TWIP steels with simultaneous high strength and high ductility is performed by combining SMAT and thermomechanical treatment. The interval heat treatment between double SMAT helps to enhance the total elongation to over 50%, while preserving yield strength of about 980 MPa. Microstructural observations indicate that the nanograins occur at the depth of 60μm from the top surface of sample processed by the combined treatment, different from their usual emergence on the top surface layer. Martensitic phase transformation is also discovered. Fractography analyses of the SMATed TWIP steel samples demonstrate typical ductile fractures with large quantities of dimples with different sizes. Sizes of the dimples follow the same trend of grains, from larger to smaller sizes as approaching top surface. Long-lasting plastic deformation with very high strain/strain rate also produces slight brittle crack with some tearing ribs. Microvoids coalescence, with some manganese carbides, leads to the final rupture.

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