Divertor is one of the most important components of the magnetic confinement fusion device, which directly sustains the strong particle flow and high heat load during a harsh service circumstance. The heat sink material that accommodates the operation circumstance of the divertor is one of the crucial prerequisites to perform the normal operation of a fusion reactor. The research and engineering experiences over the past three decades indicate that copper alloys are the best and probably the only material group for the heat sink of the water-cooled target of a divertor owing to its high thermal conductivity, strength, thermal stability, and radiation resistance. However, on account of its performance under the typical irradiation scenario of a divertor in the next step fusion reactor, none of the existing commercial copper alloys can satisfy both the harsh working environment and engineering building requirements in the Chinese Fusion Engineering Test Reactor (CFETR). At present, the design and research of CFETR devices have been conducted and is progressing steadily according to the proposal. Therefore, the development of high-performance copper alloys or copper matrix composites for high heat flux components is essential. In this study, the working condition of the heat sink in the next step fusion reactor divertor was first introduced according to the Roadmap of Fusion Energy of China. Thus, the performance requirements for the heat sink and its potential application limitations in the future fusion reactor divertor were reviewed. Finally, certain countermeasures regarding the heat sink materials were proposed for the CFETR divertor.
Hydrogen is widely considered to be one of the future clean energy sources. A large scale of production, storage, transportation, and the use of hydrogen-related energy is likely to be escalated in the next few decades. However, the presence of hydrogen seriously deteriorates the mechanical properties of most structural metals, which is a main threat to the hydrogen economy. Although hydrogen embrittlement has been recognized for more than a century, there is still a lack of effective measures to eliminate this embrittlement in the engineering practices and some aspects of fundamental science; the embrittlement mechanism is still poorly understood. Because hydrogen is the lightest element in the universe, it exhibits several unique characteristics like permeability, fast diffusion, and unstable distribution in different scale defects. These characteristics are seriously affected by external stress, temperature, and other environmental factors. Therefore, the drawback of hydrogen embrittlement is still an extremely complex and attractive scientific topic. In the last two decades, with the development of multi-scale mechanical experimental techniques, various new studies on hydrogen embrittlement have been reported and its mechanism is deeply evaluated. Based on these advances, this review reflects on the complexity and importance of this topic. The macro-mechanical tests such as constant load and slow strain rate tensile tests are presented and their advantage and disadvantage on screening the susceptibility of materials to hydrogen embrittlement are compared. Subsequently, the hydrogen-indentation (hardness) method is introduced at the mesoscale to show the hydrogen-induced cracking and its application. Furthermore, the main focus of this review comprises the modern experimental approaches to fine-scale evaluation of the hydrogen-dislocation interaction, with particular emphasis on the nanoindentation pop-in effect, micro-pillar compression and micro-cantilever bending tests, and environmental transmission electron microscope tests. The fundamental principles of these approaches are overviewed, and their contribution to the elucidation of the hydrogen embrittlement mechanism is discussed. For example, the different effects of hydrogen on the mechanical properties and a mechanism similar to the hydrogen effect are proposed based on the micro-pillar compression and micro-cantilever bending tests. Nevertheless, there are still several discrepancies in the hydrogen-dislocation interaction that needs further investigation.
Mg-Li alloys have potential applications in the aerospace, military, electronics, and automobile fields due to their superlight density, extremely high specific stiffness, high specific strength, damping, and electromagnetic shielding properties. Due to the limited slip systems of Mg and hcp-structured α(Mg) at room temperature, magnesium and α(Mg)-based alloys are difficult to deform; thus, it is significant to investigate the high-temperature deformation behavior to address this issue. Thus, in this study, an ultralight α(Mg)-based Mg-4.4Li-2.5Zn-0.46Al-0.74Y alloy was fabricated via multi-directional forging and rolling, and its flow stress, microstructural evolution, constitutive modeling, and deformation mechanism at elevated temperatures were investigated by tensile tests, OM, and XRD. The results indicate that the grain refinement mechanism of this alloy processed by multi-directional forging (MDF) exhibits mechanical shearing fragmentation and dynamic recrystallization (DRX). Additionally, the flow stress results demonstrate that strain-hardening occurred at 623 K due to grain coarsening, and microstructural evolution reveals that dynamic recovery and DRX occurred at tensile temperatures of 523-573 K; however, grain coarsening primarily appeared at 573 K (or more). XRD analysis demonstrates that this alloy comprised α(Mg), β(Li), Al2Y, Al12Mg17, LiAl, and Mg2Y phases, and hyperbolic sine constitutive analysis reveals that the stress exponent was 4.4 and the activation energy for deformation was 120.40 kJ/mol. The calculated results for dislocation density, number of dislocations in a grain, and atomic diffusion at 623 K and 1.67 × 10-4 s-1 corresponding to elongation-to-failure of 240% indicate that dislocation creep controlled by lattice diffusion governed the deformation mechanism under this condition. Predictions by grain growth models indicate that the calculated grain sizes were in good agreement with the practical grain sizes at 623 K and 1.67 × 10-4 s-1 when the grain growth factor was equal to 2 and proportional factor was 0.2.
Lightweight and strong lattice materials are suitable for a wide range of applications in aerospace, automotive, biomedical, shipbuilding, and a variety of other significant industries. A class of mathematically defined surfaces that exhibit three-dimensional (3D) periodicity, zero mean curvature, and large surface area is the triply periodic minimal surface (TPMS). Inspired by natural systems, such as biological cubic membranes, sea urchins, and butterfly wing scales, TPMS lattice material is composed of continuous and smooth shells, allowing for decreased stress concentration by comparison with strut-based lattice material. In this study, strut-based lattice materials, namely octet-truss (O) and tetrakaidecahedral (T), shell-based lattice materials, namely Diamond (D); Gyroid (G); and I-WP (I), and Primitive (P) lattice materials, were rationally designed and manufactured using Ti-6Al-4V alloy powder by selective electron beam melting (SEBM) process. The discrepancies between the design and manufactured diameters or thicknesses, optical microstructures, and mechanical properties of these lattice materials have been defined in detail. The results showed that the variations between the design and manufactured diameter or thickness of SEBM manufactured lattice materials were smaller than the value of the electron beam spot diameter, showing good geometric consistency with the original computer-aided design models. Due to the high thermal gradients and rapid cooling rates observed in the SEBM process, the resulting microstructure of lattice materials was columnar prior β grains, which were parallel to the build direction, where inside the columnar β grains were α + β and martensite α' platelets. The key finding is that TPMS lattice materials exhibit superior mechanical properties compared to strut-based lattice materials in compressive strength, elastic modulus, and plasticity, owning to their smooth and continuous surface. Among the SEBM manufactured shell-based lattice materials, the mechanical properties of type D lattice materials perform best. Moreover, the specific compressive strength of SEBM manufactured shell-based lattice materials reached 146.9 MPa/(g·cm-3), which is much higher than that of strut-based lattice materials with 119.6 MPa/(g·cm-3) in the same relative density. These properties make TPMS or shell-based lattice materials potential candidates to be applied as parts in aerospace and/or biomedical industries.
Titanium alloys have the advantages of high specific strength, fatigue resistance, and corrosion resistance. Also, they are widely used in the aviation, aerospace, weapons, petroleum, and chemical industries and other fields. The use of large-scale and integrated aviation forgings, which are an important development in titanium alloy manufacturing technology, can increase the service life, safety and reliability of aircraft structures and engines, and simultaneously reduce their structural weight and shorten their manufacturing cycle. However, problems such as a decline in mechanical properties and the presence of abnormal low-magnification structures due to the strong β phase texture have gradually been revealed. For example, large-size near-β titanium alloy bars often have the problem of coarse and uneven macrostructures, and the center layer of these bars tend to form a strong {100} β phase texture. These defects are easily inherited in the forgings, which adversely affect their performance and threaten their safe use. In this work, 300 mm diameter TC18 titanium alloy bars were used as the research material. The SEM and EBSD techniques were used to study the microstructure and texture characteristics of the β phase after thermal deformation, respectively. This work compared the influence of the thermal deformation parameters (compression/stretching, deformation temperature, reduction, strain rate, and holding time) on the evolution of the β phase microstructure and texture in the TC18 titanium alloy. Also, the deformation, dynamic recovery, dynamic recrystallization, and grain growth behavior of the β phase were investigated. The results showed that when the TC18 titanium alloy was compressed and stretched in the two-phase region, the β phase was mainly dynamic recovery. After thermal compression, the {100} and the {111} textures were mainly formed, while after thermal stretching, the {110} texture was mainly formed. When it was compressed in the β phase region, as the deformation temperature increased, the reduction increased, the strain rate decreased, the strength of the {100} texture increased and the {111} texture decreased. When it was compressed in the two-phase region, as the deformation temperature increased and the reduction increased, the strength of the {100} texture increased and the {111} texture decreased. When it was stretched in the two-phase region, as the reduction increased, the strength of the {110} texture gradually increased.
SA508-3 steel is the key structural material extensively used in large components of third-generation nuclear power plants. For increasing the process efficiency of nuclear power plants, extremely thick cross-sectional heavy forgings are necessary for constructing large components for these plants. Owing to thick cross sections, the as-quenched microstructure of the center of heavy forgings is typically granular bainite, composed of bainitic ferrite and martensite (M) and retained austenite (AR) (M-A) islands. An M-A island is an undesired microstructure that results in the SA508-3 steel having a poor low-temperature impact toughness after conventional tempering at 650oC. However, it is difficult to tailor the as-quenched microstructure owing to the limited cooling rate during the quenching process. Therefore, the modification of the tempering process is a more feasible method to adjust the microstructure and improve the mechanical properties of heavy forgings. Herein, the decomposition of AR within M-A islands during different transformation paths and its effect on the mechanical properties of SA508-3 steel have been investigated. The results show that clusters of ferrite and agglomerated M3C carbides are formed during conventional tempering at 650oC. These coarse M3C carbides decorate the boundary of the cluster, reducing the impact toughness of the SA508-3 steel. Accordingly, the size and distribution of these M3C carbides are tentatively modified by introducing pretreatments at different temperatures before conventional tempering at 650oC. This modification is because, during pretreatments, AR first decomposes into various transitional microstructures such as martensite, bainite, or pearlite, which further transform into clusters of ferrite and M3C carbides during tempering at 650oC. The results show that 400oC is the optimal pretempering temperature to improve the impact toughness of SA508-3 steel. Microstructural observations reveal that during tempering at 400oC, AR completely decomposes into fine bainite comprising bainitic packets and high-density cementite particles. This provides additional nucleation sites for M3C carbides inside the clusters during the subsequent tempering at 650°C, avoiding the formation of coarse M3C carbides distributed along these cluster boundaries.
In this work, a constrained groove pressing experiment was carried out to investigate the influence of constrained groove pressing on precipitated phase dissolution and mechanical properties of China low activation martensitic (CLAM) steels. The aim of this study is to improve the comprehensive service performances of CLAM steels used in the first wall of fusion reactor cladding. The influence of the dissolution and precipitation of precipitates on the mechanical properties of CLAM steel subjected to multi-pass groove pressing was investigated via tensile tests at room temperature and 500oC, microhardness tests, SEM, and TEM. The results show that the grains and precipitated phases are effectively refined after three passes of groove pressing, the volume fraction of grains above 5 μm is reduced to 0.49%, and the average size of M23C6 and MX phases is reduced from 107.32 and 17.12 nm to 93.97 and 13.59 nm, respectively. When the cumulative strain of the billets reaches a value of 2.32 (pass two), the tensile strength and microhardness are found to be 720 MPa and 2.46 GPa, respectively. When the cumulative strain increases to 3.48 (pass three), the strength of the CLAM steel decreases by 4.31%, whereas the microhardness and elongation increase by 2.03% and 6.27%, respectively. These trends are related to the evident dissolution of the precipitates during the deformation process.
The formation of hydrogen-induced defects in 316 stainless steel and the interaction between hydrogen and defects are crucial aspects to understand the failure law of the hydrogen-induced mechanical properties. Introducing various types of hydrogen sinks, such as interfaces and dislocations, is a popular method for reducing the concentration of residual hydrogen and curbing the mobility of hydrogen atoms in materials. In this work, positron annihilation spectroscopy and thermal desorption spectroscopy (TDS) were used to measure the distribution of hydrogen-induced defects and hydrogen content in deformed 316 stainless steel with hydrogen charging. In particular, the influence of dislocations on the formation of hydrogen-induced defects and the hydrogen retention behavior in the specimens were experimentally investigated. The results show that the S-parameter increases upon hydrogen charging, and the W-parameter is negatively correlated with the S-parameter. The S-parameter value of the deformed sample was found to be larger than that of the annealed sample, indicating that the introduction of hydrogen results in the formation of vacancy defects in the sample. Additionally, hydrogen atoms may gather together to form a large number of volume defects near dislocations. The S-W curves show that the (S, W) point for the sample containing dislocations aggregates towards the surface after hydrogen charging, due to the hindered dislocation motion. In the deformed samples with low hydrogen charge current density, the vacancy formation rate was found to be slow, and the combination of excess hydrogen and vacancies was observed to give rise to hydrogen-vacancy clusters (HmVn), where n > m. The TDS results show that both the activation energy for hydrogen desorption and the amount of hydrogen retention increase due to the presence of dislocations.
A previous study proposed a novel Nb nanowire-reinforced NiTi shape memory alloy composite possessing high yield strength (> 1.6 GPa), low apparent Young's modulus (< 30 GPa), and large quasilinear elastic strain (> 6%). This composite occupies a unique spot on the chart of the mechanical properties of conventional bulk metals, ceramics, and polymer materials. It can be used in dental braces, cardiac pacemakers, implantable devices, and flexible medical instruments. Furthermore, this study suggested that when the NiTi shape memory alloy was adopted as a matrix, the stress-induced martensitic transformation of NiTi would help the embedded nanowire reinforcement to exhibit inherent high strength. Ultralarge elastic strain (4%-7%) of Nb nanowires has been observed in these NiTi-Nb composites. Tailoring superior structural-functional properties by combining a shape memory alloy with other nanoreinforcements have recently gained research attention in materials science research focus. However, in most previous works, the volume fractions of the embedded Nb nanowires were not > 25%. It is reasonable to assume that an increase in the volume fraction of Nb nanowire would further improve the strength of the composite, and make the mechanical performance of the bulk composite much closer to that of a single nano reinforcement. As a result, a study on the high volume fraction of an Nb nanowire-reinforced NiTi shape memory alloy composite is crucial. Herein, an in situ NiTi-NbTi shape memory alloy composite with a high Nb volume fraction was prepared through arc melting, forging, and wire drawing. The microscopic analysis showed that NbTi and NiTi nanofibers were alternatively distributed in the composite along the wire axial direction. In situ synchrotron X-ray diffraction measurements were carried out to study the deformation mechanism of the composite. Results revealed that although the volume fraction of NiTi was only about 30%, the deformation of the composite was mainly controlled by the martensitic transformation of NiTi. The prepared composite showed a homogenous deformation and homogenous martensitic phase transformation before the yielding. It then exhibited Lüders-like deformation that originated from the Lüders-like stress-induced martensitic phase transformation in the region of yielding. Stress transfer was observed in the Lüders band front from the transforming B2-NiTi phase to the NbTi phase and simutaneously to the previously existing B19'-NiTi martensite phase generated during the homogenous martensitic phase transformation process.
Most Al-Mg-Si-Cu alloys and their composites are affected by natural aging processes. When natural aging precedes artificial aging, it impairs the hardening during artificial aging. This study investigates the influence of Cu content on the negative effects of natural aging in silicon carbide (SiC) (17% volume fraction) reinforced Al-1.2Mg-0.6Si-xCu (x = 0, 0.2, 0.6, 1.0, and 1.2, mass fraction, %) composites. The samples were investigated by hardness analysis, DSC, and TEM. For comparison, Al-1.2Mg-0.6Si-xCu alloys were examined by the same methods. The difference in hardness (ΔH) between samples in the direct artificial aging state and those that were naturally aged for 14 d before artificial aging were compared with Cu-containing and Cu-free samples. The values of ΔH were lower in the Cu-containing samples, indicating that Cu mitigated the negative effects of natural aging. However, the values of ΔH fluctuated as the Cu content increased. DSC and TEM results revealed the addition of Cu promoted the precipitation of β" phases (the primary strengthening phases in Al-Mg-Si alloys) and the formation of stable L phases during artificial aging. This morphological behavior explained why Cu inhibited the negative effects of natural aging. On the downside, Cu aggravated the formation of clusters during natural aging, which resisted precipitation and negatively affected the hardening during artificial aging. The contrasting beneficial and adverse influence on the effects of natural aging caused the fluctuations in ΔH. The mitigating effect of Cu differed between the 17%SiC/Al-1.2Mg-0.6Si-xCu composites and Al-1.2Mg-0.6Si-xCu alloys. A small amount of Cu (0.2%, mass fraction) significantly reduced the ΔHof the composite, but the Al alloy with 0.2%Cu failed to elicit this effect. This result can be explained by two observations. First, the DSC results showed that in the Al-1.2Mg-0.6Si-0.2Cu alloys, Cu significantly aggravated the clustering of solute atoms during natural aging, whereas in the 17%SiC/Al-1.2Mg-0.6Si-0.2Cu composites, formation of clusters was low because the vacancies were annihilated by interfaces and dislocations. Second, the TEM results revealed the presence of L phases in the 17%SiC/Al-1.2Mg-0.6Si-0.2Cu composites, which were absent in the Al-1.2Mg-0.6Si-0.2Cu alloys.
Rapid development of high-power electronic equipment for 5G and other advanced communication devices leads to a highly compact component size with increased heat flux density in integrated circuits, which requires electronic packaging materials to meet excellent heat dissipation performance. In this work, a novel liquid-solid separation technology was used to prepare a 40% (volume fraction) diamond/Al composite for electronic packaging substrates. By SEM, EPMA, and XRD techniques to investigate the fracture morphology and interface structure of the composite, the influence of diamond particle sizes (90, 106, 124, and 210 μm) on the thermophysical properties of diamond/Al composite was studied. The results showed that with the increase of a diamond particle size, the density of a composite material increased first and then sharply decreased and attained the optimal value when the diamond particle size was 106 μm. No harmful intermetallic Al4C3 was generated at the interface of the composite material. Coefficient of thermal expansion (αc) of the composite material increases slightly with increase in diamond particle sizes and remains relatively stable. The Kerner model can accurately simulate the αc of the diamond/Al composite. Thermal conductivity (λ) is also affected by diamond particle sizes and interface behaviors, and the trend of change in λ with the size of the diamond particles follows a similar trend to that between the density of composites and the particle size. The diamond/Al composite with diamond particle size of 106 μm has the best overall performance, relative density and αc attained 97.12% and 12.4 × 10-6 K-1, respectively. λ is 153.1 W/(m·K), meeting 69.31% of the Maxwell-Eucken model prediction value. The airtightness value meets the military standard for this type of an electronic packaging material.
Among the rare-earth permanent magnetic materials, Sm-Co-based alloys exhibit superior magnetic properties at high temperatures. However, their high-temperature applications are limited by their relatively low saturation magnetization and structural stability. The stability and magnetic properties of these alloys can be enhanced by adding proper alloying elements. In the search for new permanent high-performance magnets, researchers have systematically investigated the structures and magnetic properties of variously doped Sm-Co phases. The present research investigates the structural stability and magnetic properties of Sm-Co based alloys using a crystal structure model of SmCo7 with an accurate atomic ratio (Sm∶Co = 1∶7). To explain the coexisting multiphase phenomenon observed in experiments, Mn- and In-doped Sm-Co alloys were modeled by first-principles calculations. Systematic calculations were conducted on these models, and were combined with thermodynamics calculations to determine the preferred occupation sites of the doping elements and their change rule with temperature. Based on the calculated energy and electronic structures, the structural stabilities of the alloys were studied. The Mn and In dopants influenced the interactions among the Co atoms in the studied alloys. A microscopic mechanism of stability improvement in the SmCo7-based alloys was then proposed. The magnetic-moment calculation showed that the Mn additive enhanced the total magnetic moment of the SmCo7 alloys. This finding explains the effects of doping elements on the saturation magnetization of the SmCo7 alloys.
