There is a trade-off relationship between strength and ductility in the case of homogeneous crystalline metals. This work assessed the deformation characteristics of a high-entropy CrMnFeCoNi alloy having a harmonic structure, defined as a coarse-grained structure surrounded by a network of fine grains to solve this problem. These evaluations were performed by in situ tensile tests using micro-digital image correlation (micro-DIC). Mechanically milled CrMnFeCoNi powder was consolidated at 1,273 K by spark plasma sintering to fabricate the harmonic structured alloy. The microstructure of this material was characterized by optical microscopy and electron backscatter diffraction while the micro-hardness distribution was obtained from Vickers hardness tests. Micro-DIC analyses were able to determine strain distributions without dot patterning based on microstructural heterogeneity. Pronounced strain partitioning was observed and high strain peaks were generated near the boundaries between fine- and coarse-grained structures. Fracture surface observations using scanning electron microscopy confirmed that cracks were initiated within the fine-grained structures due to stress partitioning, as verified by finite element analysis. Thus, in contrast to homogeneous course-grained alloys, a harmonic structured CrMnFeCoNi specimen was able to exhibit both high strength and high ductility.

The Harmonic Structure [1] is a novel design concept that facilitates the engineering of metallic materials to achieve enhanced mechanical performance. The Harmonic Structure is composed of soft, coarse-grained regions, designated as the Core, which are surrounded in three dimensions by an interconnected network of hard, ultra-fine grain regions, referred to as the Shell. The interaction in these core/shell regions produces a synergistic effect during plastic deformation, resulting in superior mechanical properties that are of great significance. The distinctive network configuration of the Harmonic Structure enhances the dislocation density within the coarse-grain regions in contact with the interface through stress partitioning, thereby accelerating the work hardening rate and consequently enhancing the strength. This phenomenon is referred to as Hetero Deformation Induced (HDI) strengthening [2]. The fabrication of HS material is achieved through the application of mechanical milling (MM) to the powder, which results in the formation of a deformed layer on the surface of the powder and the creation of bimodal structured particles. However, a notable constraint of the MM process is its extended time requirement to attain the desired bimodal structure. In contrast, the bi-modal milling (BiM) technique involves the controlled mechanical milling of coarse and fine powders in conjunction with each other, with the objective of forming a layer of fine powders of a specified thickness over the coarse particles. The most advantageous aspect of bi-modal milling (BiM) is not only its reduced processing time, but also its superior ability to control the thickness of the surface deformation layer.

A harmonic structure (HS), which is defined as a coarse-grained morphology surrounded by a network of fine grains, can improve both the strength and ductility of various metals, including high-entropy alloys. The present study investigated the influence of a HS, which was fabricated by spark plasma sintering, on near-threshold fatigue crack propagation in a high-entropy CrMnFeCoNi alloy. Stress intensity factor (K) decreasing tests were conducted with a force ratio of 0.1 at room temperature in air. The threshold stress intensity factor range (OKth) for the HS material was slightly higher than those for its homogeneous counterparts. Microstructures near the crack profiles were analyzed using electron backscatter diffraction to elucidate the mechanism of fatigue crack propagation in the HS material. A fatigue crack was arrested within the coarse-grained region of the HS. Thus, the OKth for CrMnFeCoNi alloys with a HS was governed by the coarse-grained structure and can be quantitatively predicted based on the coarse grain size in the HS. Consequently, the results indicate that a HS enhances both strength and ductility without reducing the OKth for CrMnFeCoNi alloys.

The harmonic structure composites with Ti-Ni alloy and Cu were fabricated by mechanical milling (MM) / spark plasma sintering (SPS) process and were investigated mechanical and thermal properties in detail. Fine Ti-Ni alloy powder and coarse Cu powder were mechanically milled using planetary ball mill equipment at cryogenic temperature. The MM powder was sintered by using the SPS apparatus at 1073 to 1273 K. Tensile tests carried out at 383 K as mechanical properties evaluation. Thermal expansion to 1073 K was evaluated using thermomechanical analyzer equipment. Microstructural observation of the MM powders and SPS compacts was achieved using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SPS compacts is the harmonic structure composite having the network structure with Ti-Ni alloy and the dispersive area with Cu. Such a Ti-Ni/Cu harmonic structure composite exhibits unique mechanical properties. T ensile strength and elongation increase with increasing the sintering temperature in the Ti-Ni/Cu harmonic structure composite. The coefficient of linear thermal expansion of the Ti-Ni/Cu harmonic structure composite lies between that of Ti-Ni alloy and Cu, and a sufficient reduction in the coefficient of linear thermal expansion is confirmed.

This study investigates the deformation behavior of harmonic-structured (HS) low-carbon steel fabricated by mechanical milling (MM) followed by spark plasma sintering (SPS). Mechanical milling generated a nanocrystalline surface layer on the powder particles, which formed an interconnected fine-grained Shell network surrounding coarse-grained Core regions after sintering. The resulting HS compact consisted of a Shell region with an average grain size of 3.1 μm and a Core region with an average grain size of 26.3 μm. Tensile testing revealed that the HS material exhibited an upper yield stress approximately twice that of the homogeneous (Homo) material while maintaining superior tensile toughness. The HS material also showed a pronounced yield drop and a stress-instability region accompanied by serrated flow. Surface observations demonstrated that localized deformation propagated gradually through the specimen, with a characteristic length scale corresponding to that of the Core–Shell units. This deformation mode, termed “pseudo-Lüders deformation,” was distinct from conventional Lüders deformation. Microhardness measurements and transmission electron microscopy revealed preferential hardening and dislocation accumulation in the Shell region, indicating heterogeneous strain partitioning during deformation. The resulting strain incompatibility between the Shell and Core regions is suggested to be responsible for the observed heterogeneous strain hardening. Furthermore, pre-straining eliminated both the yield drop and pseudo-Lüders deformation, resulting in continuous yielding behavior. These findings demonstrate that harmonic structural heterogeneity significantly modifies the yielding behavior of low-carbon steel and provides new insight into the control of strain localization in heterostructured metallic materials.

This study investigates the corrosion behavior of a CoCrFeMnNi high-entropy alloy with a bimodal harmonic microstructure, comprising a continuous fine-grained shell (-2.5 mu m) encasing discrete coarse-grained cores (-25 mu m), in strong acidic 0.5 M HCl and H2SO4. The harmonic microstructure is developed by mechanically milling pre-alloyed CoCrFeMnNi powder, selectively deforming the surface of the powder particles while keeping the inner regions intact, followed by spark plasma sintering. The analyses using electron backscatter diffraction and X-ray diffraction confirm the absence of texture development and secondary phases. The electrochemical studies using open circuit potential, linear polarization, potentiodynamic polarization, and electrochemical impedance spectroscopy demonstrate that the harmonic microstructure exhibits better corrosion resistance, more specifically, significant pitting resistance and passivation compared to its homogeneous coarse-grained counterpart. The post-corrosion characterization attributes this improvement to the excellent passivation provided by the shell region.

Ultrafine-grained (UFG) 2024 aluminum alloys with a heterogeneous lamellar structure were fabricated by powder metallurgy routes, including spark plasma sintering and low temperature hot extrusion. Subsequent deliberate short-duration solution (495 °C/25 min) and aging (150 °C/10 h) treatments preserved the UFGs while inducing dense nano-sized α-AlMnSi precipitates, resulting in superior mechanical properties, i.e., yield strength of 391 MPa, ultimate tensile strength of 522 MPa and uniform elongation of 12.7%. The synergistic strength-ductility enhancement originates from the precipitation strengthening of nano-sized α-AlMnSi precipitates, grain boundary strengthening of UFGs as well as the pinning effects of MgO particles at grain boundaries. The study highlights an effective approach to achieving superior strength and ductility in aluminum alloys through optimized processing and heat treatment, offering valuable insights for the development of high-performance aluminum alloys in diverse industrial applications.

In this paper, microstructure and mechanical properties of equimolar CrFeCoNiMo high entropy alloy (HEA) prepared by mechanical milling (MM) and spark plasma sintering (SPS) process were investigated in detail. The initial and MM powders were sintered 1.8 ks at 1173 to 1373 K by using SPS apparatus. Microstructural observation of the powders and SPS compacts was achieved using scanning electron microscopy (SEM) / electron back scattered diffraction (EBSD). The SPS compact of the initial and MM powders consist of FCC, σ and μ phases. The grain size of the compacts of the MM powders is fine compared with that of the initial powders. The grain sizes of each phase increase with increasing the sintered temperature. The Vickers hardness of the MM powder compacts is higher than that of the initial powder compacts. The Vicker hardness of the compacts of initial and MM powders decreases with increasing the sintered temperature. It is revealed that the hardness of CrFeCoNiMo HEA is attributed to the grain size of each phase composed of the microstructure.

The high temperature deformation behavior of SUS316L austenitic stainless steel with a homogeneous and a harmonic structure (HS) was investigated by a compression test. The HS is a heterostructure having a coarse grain structure (Core) embedded in a fine grain network structure (Shell). In the tensile test at room temperature, the HS materials exhibit high strength with high ductility due to the hetero- deformation-induced (HDI) strengthening, i.e., enhanced strain hardening. However, the high temperature deformation behavior of HS and homogeneous specimens is not clear, especially under different strain rates. In the present study, the high temperature deformation behavior of the HS and homogeneous structure SUS316L stainless steel specimens was investigated from the activation energy point of view of the high temperature deformation. As a result, the activation energy mainly depends on the grain size of the specimens under high temperature deformation.

A high-pressure torsion ( HPT ) processed Fe-21Cr-5Ni-2Mo ( mass% ) two-phase stainless steel was used to study the morphology and crystallographic features of austenite ( gamma ) precipitated from ferrite ( alpha ) during aging in the ( alpha + gamma ) two-phase region. The starting material was a gas-atomized powder with a completely ferritic structure. The HPT process was carried out to produce a fully dense compact under 6 GPa for 5 revolutions. The compact was given an equivalent strain of about 130. After the HPT process, the matrix ferrite formed a pancake-like nanograined structure with a strong texture, i.e. ND ( Normal Direction) // {110} alpha . By annealing at 1173 K for 3.6 ks, an ultrafine ( alpha + gamma ) microduplex structure with high-angle grain boundaries was formed. In addition, the strong texture formation of {110} alpha / {111} gamma / ND plane was formed in the alpha and the gamma grain duplex structure. The alpha and gamma phases had average grain sizes of 2.1 mu m and 1.6 mu m, respectively. The area fraction of the gamma phase was 37.2%, which exceeded that of a cold-pressed compact, 6.7%. Both ultrafine grain refinement and gamma precipitation were accelerated by the HPT process. In other words, the application of the HPT process to the two-phase alloys enables the formation of the ultrafine microduplex structure. The Kurdjumov-Sachs ( K-S) orientation relationship between alpha and gamma phases is usually observed in the alloy, however, the K-S orientation relationship was not dominant except for the close packing plane parallel orientation relationship, {110} alpha / {111} gamma , in the HPT-processed material.

The fabrication of a series of CrMnFeCoNi alloy compacts with homogeneous (Homo) and harmonic structure (HS) via powder metallurgy routes was followed by the subjection of these specimens to high-temperature compression tests at initial strain rates of 1.0×10-2 s-1, 1.0×10-3 s-1, and 1.0×10-4 s-1 at 1073 K and 1173 K. The HS compacts exhibited a well-developed ultra-fine grain (UFG) network structure, designated as the "Shell." Consequently, the HS-designed specimen exhibited a pronounced reduction in flow stress in comparison to the homogeneous structure counterpart. Moreover, the homogeneous specimens exhibited a strain rate sensitivity value of less than 0.3, while the HS specimens demonstrated a strain rate sensitivity value greater than 0.3, with a value of 0.61 observed at 1173 K. This indicates that the HS specimens displayed pseudo-superplastic deformation behavior. This softening behavior can be attributed to the accumulation of dislocations in the shell region, which leads to dynamic recrystallization. This phenomenon can be understood as hetero-deformation-induced (HDI) softening.

The Harmonic Structure (HS) design was applied to a high entropy CrMnFeCoNi alloy with two different ultrafine-grained (UFG) fractions. The high-temperature deformation behavior of the alloy with those kinds of HS compacts, as well as a homogeneous (Homo) compact, was studied by compression tests at a strain rate of 0.01/s at 1073 K. The stress-strain curves of the Homo and HS compacts exhibited strikingly disparate characteristics, rendering them quite distinctive. The Homo compact exhibited a steady state deformation following the onset of yielding, whereas the two types of HS compacts demonstrated a softening behavior with increasing strain. It is notable that the HS compacts exhibited the formation of a smaller misoriented grain concentration at the UFG network structure, designated as the "Shell," at the initial stage of deformation. Conversely, the Homo compact exhibited a comparable microstructural alteration only following the occurrence of a substantial strain region. It can thus be posited that the process of dynamic recrystallization occurred at an earlier stage in the HS compacts in comparison to the Homo compacts. In other words, a selective dynamic crystallization occurs at the Shell in the HS compacts at high temperature deformation.

Present study deals with the oxidation properties of the harmonic structured high-entropy Cantor (CoCrFeMnNi) alloy at 600 degrees C, 700 degrees C, and 800 degrees C in zero-grade air. The microstructure of the Cantor alloy is modified to a harmonic distribution with grain sizes at shell of -2 mu m and core of -20 mu m. The oxidation behavior of the harmonic one is compared with its non-harmonic counterpart and as-received 304L stainless steel. Both the highentropy Cantor alloys show inferior oxidation resistance than the 304L stainless steel in this temperature range. Studies show that an Mn-rich oxide scale forms on the surface of the Cantor alloys, whereas a Cr-rich oxide scale forms on the 304L stainless steel surface. The high Mn content in the Cantor alloys deteriorates their oxidation resistance. However, the harmonic structure improves the oxidation resistance in the Cantor alloy due to the preferential oxidation of the shell region and the formation of a more compact oxide scale than its non-harmonic counterpart.

This paper presents the deformation behavior of CoCrFeMnNi high-entropy alloy (HEA) produced from plastically undeformed and deformed powders. The investigation commenced with the preparation of ball-milled HEA powder at 90ks and 180ks, with a ball-to-powder ratio of 2:1, to induce severe plastic deformation on the outer layer of the HEA powder. Subsequently, the plastically undeformed and deformed HEA powders were successfully consolidated in a spark plasma sintering (SPS) furnace. The SEM observations revealed that the sintered plastically undeformed HEA powder exhibited conventional homogeneous microstructures (Homo), while sintered deformed HEA powders exhibited a regulated bimodal grain distribution known as harmonic structure (HS). The mechanical properties of the materials were characterized by performing tensile tests at room temperature (RT) and a low temperature of 193 K. The HS HEA exhibited an increase in proof strength of 16%-40%, while the ultimate tensile strength (UTS) increased by 8%-20% compared to the Homo HEA. Moreover, the strain hardening rate (SHR) curve indicates that both Homo and HS HEA exhibited oscillatory behavior at low temperature. However, Homo HEA exhibited more pronounced oscillation than HS HEA. The current results indicate that oscillatory behavior in the SHR curve is indicative of twin deformation. At low temperatures, the high rate of twinning deformation in Homo HEA was attributed to its higher ductility, while a good combination of high strength and high ductility produced by HS material was found to be influenced by HS design in addition to twinning deformation.

In this study, the microstructure of the beta-type Ti-25Nb-25Zr alloy is modified to a bimodal harmonic design, exhibiting fine and coarse grains of similar to 3 mu m and similar to 70 mu m, respectively. The high-angle grain boundary density of the harmonic Ti-25Nb-25Zr alloy increases nearly fourfold over its coarse-grained counterpart. Both the harmonic and course-grained Ti-25Nb-25Zr alloys show excellent passivation and corrosion resistance when studied electrochemically in Fusayama-Meyer's simulated saliva solution. The harmonic structure in the Ti-25Nb-25Zr further improves corrosion resistance, almost six times the coarse-grained counterpart. The improved corrosion resistance is attributed to the excessive high-angle grain boundary, which binds the corrosion product to the substrate, restricting further corrosion.

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.

Materials with heterogeneous microstructures have been reported to have an attractive combination of strength and ductility. This is attributed to synergistic strengthening effects from the difference in strength of fine- and coarse-grained regions. Understanding the interaction of the regions is crucial for further optimization of the microstructures. In this work, we fabricated nickel of harmonic structure (HS) and a reference with homogenous coarse grains. The HS constitutes of an interconnected fine-grained network that surrounds regions of coarse grains. The interplay of the regions was studied by monitoring Bragg reflections from individual grains in situ during tensile deformation until approximately 2 % strain through synchrotron X-ray diffraction. The technique allows grain-level assessment of the degree of plastic deformation. Two grains were followed in the reference and two small grains (fine-grained region) and two large grains (coarse-grained region) in the HS. Three deformation regimes were identified: elastic deformation, onset of plastic deformation and significant plastic deformation. Our results reveal that the large grains in the harmonic structure onset plastic deformation during the macroscopic elastic stage. With increasing applied stress, the small grains yield plastically also and once a large fraction of the fine-grained network deforms plastically the large grains undergo significant plastic deformation. Notably, the onset of significant plastic deformation of large grains in the HS occurs at approximately 100 MPa higher applied stress than in the grains in the reference. This shows that fine grains constrain the large grains from deforming plastically in the HS.