Acta Metallurgica Sinica-English Letters最新文献

This study optimizes the thermomechanical processing to design a heterogeneous layered structure of a tri-phase FeMnCoCrAl high-entropy alloy (HEA), achieving a significant improvement in both strength and ductility compared to the fully recrystallized structure. After annealing at 1023 K for 10 min, the microstructure of the alloy consists of a soft domain of fully recrystallized face-centered cubic (FCC) phase, a hard domain of partially recrystallized FCC phase, and a hard domain of partially recrystallized body-centered cubic phase. The tensile strength and yield strength are 604 MPa and 781 MPa, respectively, with a total elongation of 31.1%. Compared to the fully recrystallized alloy, the tensile strength is enhanced by 25%, and the total elongation increases by 23%. The comprehensive improvement in strength and ductility is attributed to multiple strengthening and toughening mechanisms within the microstructure: grain refinement strengthening from recrystallized grains, dislocation strengthening from partial recrystallization, long-range back-stress effects from the soft-hard domain structure, and deformation mechanisms such as stacking fault nucleation and the transformation-induced plasticity (TRIP)–twinning-induced plasticity (TWIP) effect, which are unique to composite the HEA.

A gradient nanostructured layer was fabricated on the surface of TA15 (Ti-6Al-2Zr-1Mo-1V) alloy (produced by selective laser melting) using severe shot peening (SSP). This study focuses on the evolution of the microstructure and the mechanism of grain refinement in TA15 titanium alloy during SSP treatment. Transmission electron microscopyand Rietveld refinement methods were employed. The residual stress and microhardness variations with depth were also characterized. The results show: (1) At the initial stage of deformation, plastic deformation is primarily accommodated through twinning and dislocation slip. (2) As the strain increases, twinning disappears, and dislocations interact to form tangles. Some dislocations annihilate and rearrange into subgrain boundaries, subdividing the original grains into subgrains. (3) With continued dislocation activity, the subgrain size decreases until nanocrystals are formed through the dynamic rotational recrystallization. SSP introduced compressive residual stress (CRS) in the near-surface layer of the material, with the maximum CRS of approximately −1141 MPa observed in the subsurface layer. It also induced work hardening, increasing the surface hardness to approximately 479 HV. However, the surface roughness increases, leading to a slight deterioration in surface quality.

Mg₃Bi₂-based films are promising near-room-temperature thermoelectric materials for the development of flexible thermoelectric devices. However, the high hole concentration caused by the abundance of intrinsic Mg vacancies easily leads to deterioration of electrical properties, especially for p-type Mg₃Bi₂ film. And the optimization of thermal conductivity of the Mg₃Bi₂-based films is barely investigated. In this work, we demonstrate the improved thermoelectric performances of p-type Mg₃Bi₂ through Ag doping by magnetron sputtering. This doping successfully reduces the hole concentration and broadens the band gap of Mg₃Bi₂, thus resulting in a peak power factor of 442 μW m⁻¹ K⁻² at 525 K. At the same time, Ag doping-induced fluctuations in mass and microscopic strain effectively enhanced the phonon scattering to reduce the lattice thermal conductivity. Consequently, a maximum thermoelectric figure of merit of 0.22 is achieved at 525 K. Its near-room-temperature thermoelectric performances demonstrate superior performance compared to many Mg₃Bi₂-based films. To further evaluate its potential for thermoelectric power generation, we fabricated a thermoelectric device using Ag-doped Mg₃Bi₂ films, which achieved a power density of 864 μW cm⁻² at 35 K temperature difference. This study presents an effective strategy for the advancement of Mg₃Bi₂-based films for application in micro-thermoelectric devices.

Ti-Zr-Cu alloy has garnered significant attention in the field of dental implants due to its excellent biocompatibility, antibacterial properties, and potentially controllable mechanical properties. However, two critical challenges remain in the selective laser melting (SLM) fabrication of Ti-Zr-Cu alloy: First, the high thermal conductivity of the Cu element tends to destabilize the solidification behavior of the molten pool, leading to uncontrollable pore defect evolution; Second, the influence of process parameters on the synergistic effects of zirconium solution strengthening and copper precipitation strengthening is not well understood, hindering precise control over the material's mechanical properties. To address these issues, this study systematically elucidates the quantitative impact of energy input on the defect formation mechanisms and strengthening effects in the SLM processing of Ti15Zr5Cu alloy. By optimizing laser power (120–200 W) and scanning speed (450–1200 mm/s) through a full-factor experimental design, we comprehensively analyze the effects of energy input on defect morphology, microstructure evolution, and mechanical performance. The results demonstrate that as energy density decreases, defect types transition from spherical pores to irregular pores, significantly influencing mechanical properties. Based on the defect evolution trends, three distinct energy density regions are identified: the high-energy region, the low-energy region, and the transition region. Under the optimal processing conditions of a laser power of 180 W and a scanning speed of 1200 mm/s, the Ti15Zr5Cu alloy exhibits a relative density of 99.998%, a tensile strength of 1490 ± 11 MPa, and an elongation at break of 6.0% ± 0.5%. These properties ensure that the material satisfies the stringent requirements for high strength in narrow-diameter implants used in the maxilloanterior region. This study provides theoretical and experimental support for the process-property optimization of Ti-Zr-Cu alloys in additive manufacturing and promotes their application in the fabrication of high-performance, antibacterial dental implants.

Interlayer friction stir processing (FSP) has been proved to be an effective method of enhancing the mechanical properties of wire arc-directed energy deposited (WA-DED) samples. However, the original deposition structure was still retained in the FSP-WA-DED component besides the processed zone (PZ), thus forming a composite structure. Considering the material utilization and practical service process of the deposited component, more attention should be paid on this special composite structure, but the relevant investigation has not been carried out. In this study, an Al–Mg–Sc alloy was prepared by WA-DED with interlayer FSP treatment, and the composite structure was firstly investigated. Almost all of the pores were eliminated under the pressure effect from the tool shoulder. The grains were further refined with an average size of about 1.2 μm in the PZ. Though no severe plastic deformation was involved in the retained WA-DED deposition zone, comparable tensile properties with the PZ sample were obtained in the composite structure. Low ultimate tensile strength (UTS) of 289 MPa and elongation of 3.2% were achieved in the WA-DED sample. After interlayer FSP treatment, the UTS and elongation of the PZ samples were significantly increased to 443 MPa and 16.3%, while those in the composite structure remained at relatively high levels of 410 MPa and 13.5%, respectively. Meanwhile, a high fatigue strength of 180 and 130 MPa was obtained in the PZ and composite structure samples, which was clearly higher than that of the WA-DED sample (100 MPa). It is concluded that the defects in traditional WA-DED process can be eliminated in the composite structure after interlayer FSP treatment, resulting in enhanced tensile and fatigue properties, which provides an effective method of improving the mechanical properties of the WA-DED sample.

In the directed energy deposition (DED) process with high heat input, repeated heating and cooling cycles in the deposited layers have a significant effect on the microstructure. Because of the differences in the cyclic numbers and peak temperatures from the lower layer to the upper layer, inhomogeneous microstructures are formed in the as-built components. In this work, a cyclic heat treatment (CHT) with gradual cooling was used to simulate the thermal process during the DED process of Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe (Ti5321) near-β Ti alloy. The effect of CHT on the microstructural evolution, especially the spheroidization of α phase, was investigated. As the CHT cycle increased, the volume fraction of α phase gradually increased from 35.9% after 1 cycle to 60.9% after 100 cycles, and the length of α phase first increased and then gradually decreased, while the width of α phase increased slowly. The aspect ratio of α phase decreased from 9.90 ± 3.39 after 1 cycle to 2.37 ± 0.87 after 100 cycles, implying that CHT induced α phase spheroidization. This phenomenon resulted from both the boundary splitting mechanism and the termination migration mechanism during CHT. The evolution of microstructure affects its mechanical properties. As the CHT cycles increased, the hardness increased overall, from 342.8 ± 5.3 HV after 1 cycle to 400.3 ± 3.4 HV after 100 cycles. This work provides a potential method to tailor the microstructure of near-β Ti alloys by heat treatment alone, especially for non-deformable additively manufactured metal components.

Ultrasonic vibration treatment (UVT) at varying power was successfully applied to the Cu–TiB₂ composite melt using a SiAlON ceramic sonotrode. The results indicate that TiB₂ particles are more evenly dispersed in the Cu matrix with increasing ultrasonic power, leading to improved mechanical properties of as-cast composites (≤ 1000 W). With 1000 W UVT, the distribution of TiB₂ particles becomes the remarkably uniform and well dispersed, with the size of TiB₂ particle aggregates decreasing from ~ 50 μm without UVT to ~ 5 μm. The ultimate tensile strength, yield strength, and elongation of the as-cast composite are 201 MPa, 85 MPa, and 28.6%, respectively, representing increases of 21.1%, 27.3%, and 43%, respectively, compared to the as-cast composite without UVT. However, when the power is increased to 1500 W, thermal effects are likely to emerge, and the ultrasonic attenuation effect is enhanced, resulting in the re-agglomeration of TiB₂ particles and a deterioration in performance. By quantitatively analyzing the relationships between sound pressure (P_k), sound energy density (I), sound pulse velocity (V), and ultrasonic power, the influence mechanism of ultrasonic power on the composite microstructure has been further elucidated and characterized. This study provides crucial guidance for the industrial application of UVT in the fabrication of Cu matrix composites.

The rapid expansion of marine industries has created an urgent demand for advanced engineering materials with superior multifunctional performance. While Cu–Ni alloys demonstrate favorable stability and tribological characteristics, their practical applications are constrained by compromised thermal conductivity and insufficient mechanical strength due to the solid solution of a high amount of Ni in the Cu matrix. Cu–Ni matrix composites reinforced with hexagonal boron nitride (h-BN) have garnered significant attention due to their potential for tailored mechanical and thermal properties. However, challenges such as BN agglomerations in Cu–Ni matrix and poor interfacial bonding hinder their practical applications. To address these limitations, this study proposes an innovative fabrication strategy for boron nitride nanosheets (BNNSs) reinforced Cu–Ni composites by integrating the in situ synthesis of BNNSs on Cu powders via chemical vapor deposition with powder metallurgy. Benefited by the in situ strategy, BNNSs with high crystallinity distribute uniformly within the Cu matrix and have an intimate interfacial bonding without voids or other types of defects. Remarkably, the BNNSs/Cu-30%Ni composite achieves simultaneous enhancement in strength and ductility, exhibiting an ultimate tensile strength of 417 MPa and fracture elongation of 17.5%, representing 30% and 118% improvements over pure Cu–Ni alloys, respectively. This exceptional mechanical synergy originates from threefold strengthening mechanisms: grain refinement, mobile dislocation pinning, and efficient stress transfer via robust interfaces. The microstructural analysis confirms that homogenous distribution of BNNSs optimized stress distribution, mitigating strain localization in the composites. Fractographic examination demonstrates uniformly distributed dimples containing embedded BNNSs, indicative of effective crack bridging and deflection during failure. Furthermore, the composite possesses excellent corrosion resistance comparable to matrix alloys, while achieving 21.23% enhancement in thermal conductivity and 20% reduction in coefficient of friction. The scalable fabrication protocol successfully resolves longstanding challenges in BNNSs dispersion and interfacial bonding, offering a viable pathway for designing high-performance CMCs for marine applications.

In this work, Ti_p/Mg-7Gd-2Y-3Zn (Ti_p/GWZ723) composites with various Ti_p sizes (~ 10 μm, ~ 20 μm and ~ 35 μm) were fabricated using semi-solid stirring casting method, the composites were subjected to hot extrusion, and the influence of Ti_p size on long-period stacking ordered (LPSO) phase, dynamic recrystallization (DRX), mechanical properties, and work hardening behavior of the Ti_p/GWZ723 composites was investigated. The results indicate that with the increase in Ti_p size, the grain size of the as-cast Ti_p/GWZ723 composites increases, and the lamellar 14H LPSO phase precipitates within the matrix after homogenization treatment. With the increase in Ti_p size, the reduction in the Ti_p surface area leads to a decrease in surface energy. Consequently, the enrichment of RE element is reduced, which facilitates the formation of the 14H LPSO phase. Moreover, the layer spacing of the 14H LPSO phase decreases. Particle deformation zone (PDZ) is formed around the Ti_p after extrusion, promoting the nucleation of DRX. The PDZ size increases with the increase in the Ti_p size. Nevertheless, the elongation of the Ti_p releases stress and reduces the PDZ size. Simultaneously, the 14H LPSO phase with a small interlayer spacing inhibits the non-basal slip, and the volume fraction of DRX (V_DRX) decreases with the increase in the Ti_p size. With the increase in Ti_p size, the refined grain size and the 14H LPSO phase with smaller interlayer spacing contribute to enhancing the work hardening rate and dynamic recovery rate of the Ti_p/GWZ723 composites. The Ti_p/Mg laminar-like interface formed in the Ti_p/GWZ723 composites can alleviate local stress concentration and inhibit the initiation and propagation of cracks.

Additive manufacturing (AM) has emerged as one of the most utilized processes in manufacturing due to its ability to produce complex geometries with minimal material waste and greater design freedom. Laser-based AM (LAM) technologies use high-power lasers to melt metallic materials, which then solidify to form parts. However, it inherently induces self-equilibrating residual stress during fabrication due to thermal loads and plastic deformation. These residual stresses can cause defects such as delamination, cracking, and distortion, as well as premature failure under service conditions, necessitating mitigation. While post-treatment methods can reduce residual stresses, they are often costly and time-consuming. Therefore, tuning the fabrication process parameters presents a more feasible approach. Accordingly, in addition to providing a comprehensive view of residual stress by their classification, formation mechanisms, measurement methods, and common post-treatment, this paper reviews and compares the studies conducted on the effect of key parameters of the LAM process on the resulting residual stresses. This review focuses on proactively adjusting LAM process parameters as a strategic approach to mitigate residual stress formation. It provides a result of the various parameters influencing residual stress outcomes, such as laser power, scanning speed, beam diameter, hatch spacing, and scanning strategies. Finally, the paper identifies existing research gaps and proposes future studies needed to deepen understanding of the relationship between process parameters and residual stress mitigation in LAM.