Evaluation of fatigue properties under four-point bending and fatigue crack propagation in austenitic stainless steel with a bimodal harmonic structure

Austenitic stainless steel (JIS-SUS304L) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. Four-point bending fatigue tests and K-decreasing tests were conducted in air at room temperature under a stress ratio R of 0.1 to investigate fatigue crack propagation in SUS304L. The fatigue limit of this harmonic-structured material is higher than that of the material with a homogeneous coarse-grained structure. This is attributable to the formation of fine grains by mechanical milling and to the suppression of pore formation. In contrast, the threshold stress intensity range, ?Kth, for the harmonic-structured material is lower than that for the homogeneous coarse-grained material, while the crack growth rates, da/dN, are higher at comparable ?K. These results can be attributed to a reduction in the effective threshold stress intensity range, ?Keff,th, due to the presence of fine grains in the harmonic structure

Evaluation of Fatigue Properties under Four-point Bending and Fatigue Crack Propagation in Austenitic Stainless Steel with a Bimodal Harmonic Structure

Austenitic stainless steel (JIS-SUS304L) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. Four-point bending fatigue tests and K-decreasing tests were conducted in air at room temperature under a stress ratio R of 0.1 to investigate fatigue crack propagation in SUS304L. The fatigue limit of this harmonic-structured material was higher than that of the material with a homogeneous coarse-grained structure. This is attributable to the formation of fine grains by mechanical milling and to the suppression of pore formation. In contrast, the threshold stress intensity range, DKth, for the harmonic-structured material was lower than that for the homogeneous coarse-grained material, while the crack growth rates, da/dN, were higher at comparable DK. These results can be attributed to a reduction in the effective threshold stress intensity range, DKeff,th, due to the presence of fine grains in the harmonic structure

Severe plastic deformation for producing superfunctional ultrafine-grained and heterostructured materials: An interdisciplinary review

Ultrafine-grained and heterostructured materials are currently of high interest due to their superior mechanical and functional properties. Severe plastic deformation (SPD) is one of the most effective methods to produce such materials with unique microstructure-property relationships. In this review paper, after summarizing the recent progress in developing various SPD methods for processing bulk, surface and powder of materials, the main structural and microstructural features of SPD-processed materials are explained including lattice defects, grain boundaries and phase transformations. The properties and potential applications of SPD-processed materials are then reviewed in detail including tensile properties, creep, superplasticity, hydrogen embrittlement resistance, electrical conductivity, magnetic properties, optical properties, solar energy harvesting, photocatalysis, elec- trocatalysis, hydrolysis, hydrogen storage, hydrogen production, CO$_ 2$ conversion, corrosion resistance and biocompatibility. It is shown that achieving such properties is not limited to pure metals and conventional metallic alloys, and a wide range of materials are currently processed by SPD, including high-entropy alloys, glasses, semiconductors, ceramics and polymers. It is particularly emphasized that SPD has moved from a simple metal processing tool to a powerful means for the discovery and synthesis of new superfunctional metallic and nonmetallic materials. The article ends by declaring that the borders of SPD have been extended from materials science and it has become an interdisciplinary tool to address scientific questions such as the mechanisms of geological and astronomical phenomena and the origin of life

Advantages of architectured harmonic structure in structural performance

The concept of heterogenous materials with architectured harmonic structure (HS) has emerged recently with the aim of enhancing the structural performance of metals. This work presents critical microstructure characteristics contributing to the superior structural performance of harmonic-structured materials using as an example of commercially-pure nickel. Optical and scanning electron microscopy, along with nano-indentation and tensile testing, as well as digital image correlation analysis, were used for this purpose. A brief overview of the structural performance of several representative systems with HS is provided and interesting directions for future studies are discussed

Ratcheting-Fatigue Behavior of Harmonic-Structure-Designed SUS316L Stainless Steel

Stainless steels with harmonic-structure design have a great balance of high strength and high ductility. Therefore, it is imperative to investigate their fatigue properties for engineering applications. In the present work, the harmonic-structured SUS316L stainless steels were fabricated by mechanical milling (MM) and subsequent hot isostatic pressing (HIP) process. A series of ratcheting-fatigue tests were performed on the harmonic-structured SUS316L steels under stress-control mode at room temperature. Effects of grain structure and stress-loading conditions on ratcheting behavior and fatigue life were investigated. Results showed that grain size and applied mean stress had a significant influence on ratcheting-strain accumulation and fatigue life. Owing to the ultrafine grained structure, tensile strength of the harmonic-structured SUS316L steels could be enhanced, which restrained the ratcheting-strain accumulation, resulting in a prolonged fatigue life. A higher mean stress caused a faster ratcheting-strain accumulation, which led to the deterioration of fatigue life. Moreover, a modified model based on Smith–Watson–Topper (SWT) criterion predicted the ratcheting-fatigue life of the harmonic-structured SUS316L steels well. Most of the fatigue-life points were located in the 5 times error band

Statistical fatigue properties and small fatigue crack propagation in bimodal harmonic structured Ti-6Al-4V alloy under four-point bending

Small fatigue crack propagation in bimodal harmonic structured titanium alloy (Ti-6Al-4V) with high strength and ductility was examined under four-point bending at a stress ratio of 0.1 in the ambient laboratory atmosphere. The crack profiles were observed using optical microscopy and scanning electron microscopy, and analyzed using an electron backscattered diffraction to examine the mechanism of small fatigue crack propagation. Fatigue crack paths were not influenced by the bimodal harmonic structure, and the crack growth rates, da/dN, in the harmonic structured Ti-6Al-4V were almost the same as those in a material with coarse acicular microstructure for comparable values of stress intensity range, ΔK. In contrast, the harmonic structured Ti-6Al-4V had a higher resistance of fatigue crack initiation due to the grain refinement induced by mechanical milling, which resulted in an increase of the fatigue life and fatigue limit. Furthermore, the statistical fatigue properties of Ti-6Al-4V alloy were analyzed using the stress dependence of Weibull parameters to quantitatively examine the effects of the bimodal harmonic structure on its fatigue life

Severe plastic deformation for producing Superfunctional ultrafine-grained and heterostructured materials: An interdisciplinary review

Ultrafine-grained and heterostructured materials are currently of high interest due to their superior mechanical and functional properties. Severe plastic deformation (SPD) is one of the most effective methods to produce such materials with unique microstructure-property relationships. In this review paper, after summarizing the recent progress in developing various SPD methods for processing bulk, surface and powder of materials, the main structural and microstructural features of SPD-processed materials are explained including lattice defects, grain boundaries and phase transformations. The properties and potential applications of SPD-processed materials are then reviewed in detail including tensile properties, creep, superplasticity, hydrogen embrittlement resistance, electrical conductivity, magnetic properties, optical properties, solar energy harvesting, photocatalysis, electrocatalysis, hydrolysis, hydrogen storage, hydrogen production, CO2 conversion, corrosion resistance and biocompatibility. It is shown that achieving such properties is not currently limited to pure metals and conventional metallic alloys, and a wide range of materials are processed by SPD, including high-entropy alloys, glasses, semiconductors, ceramics and polymers. It is particularly emphasized that SPD has moved from a simple metal processing tool to a powerful means for the discovery and synthesis of new superfunctional metallic and nonmetallic materials. The article ends by declaring that the borders of SPD have been extended from materials science and it has become an interdisciplinary tool to address scientific questions such as the mechanism of geological and astronomical phenomena and the origin of life