Journal of Materials Research and Technology-Jmr&t最新文献

The single-phase face-centered cubic medium-entropy alloys (MEAs) normally have coarse grains in as-cast state, which exhibit insufficient strength for engineering applications. Here, a superior tensile strength-ductility synergy in a fine grained (CoCrNi)₉₄Al₃Ti₃ MEA hardened by nanoscale L1₂ precipitates was fabricated by spark plasma sintering (SPS) and post-sintering in-situ precipitation treatment. The SPSed MEAs have a fine grain size of ⁓ 5 μm, and a high number density of L1₂ precipitates form after in-situ annealing within the SPS machine. A high tensile yield strength of 1141 MPa with an adequate elongation to fracture of 25.8% was achieved in (CoCrNi)₉₄Al₃Ti₃ MEA after annealing at 700 °C for 4 h. Electron backscattered diffraction and transmission electron microscopy characterizations indicate that the superior mechanical properties mainly originate from fine grains and the coherent spherical L1₂ precipitates. The dislocation slips and stacking faults prevail in all SPSed MEAs during tensile deformation, while extra Lomer-Cottrell locks are observed in annealed MEAs. The deformation twinning is absent in these precipitation-hardened MEAs with a low stacking fault energy, which may be attributed to the fine grains and numerous nanoscale L1₂ precipitates. This study not only confirms the effectiveness of powder metallurgy when sintering and precipitation are combined in-situ during the SPS cycle, but also provide guidance for the microstructure regulation process and practical applications of SPSed HEAs/MEAs.

Duplex stainless steel (DSS) is widely used in the marine, petroleum, chemical, automotive, and other fields owing to its excellent mechanical properties and corrosion resistance. However, the research on duplex stainless steel prepared by additive manufacturing is still limited. In this paper, high-density 2507 DSS was successfully prepared by selective laser melting (SLM) additive manufacturing. The effects of energy density on the formability, phase composition, microstructure and corrosion properties of SLM 2507 DSS were investigated. The results showed that with the decrease of the energy density, the density of the specimen increases first and then decreases, and the density achieves 99.18% with the energy density of 190.5 J/mm³. The types of phases are not affected by the energy density, i.e., all 2507 DSS samples prepared by SLM showed a ferrite phase. The YOZ (parallel to the building direction) plane of the SLM 2507 DSS samples showed predominantly columnar grains attributed to the high temperature gradient and epitaxial growth characteristics. With the increase of energy density, the average grain size decreases slightly from 16.97 μm to 15.78 μm, the KAM value decreases slightly from 1.15 to 1.05, and the low angle grain boundaries (LAGBs) increase significantly from 68.4% to 74.8%. The SLM 2507 DSS sample exhibited excellent corrosion resistance. The self-corrosion potential of the sample is 136 mV and the self-corrosion current density is 2.066 × 10⁻⁸ A/cm² at the maximum density. This investigation provides a new approach for the preparation of super duplex stainless steel, which can provide a theoretical basis and guidance for industrialized application.

In this study, particle-reinforced aluminium matrix composites (PRAMCs) of an Al–Cu–Li alloy were prepared using nano-sized TiB₂+TiC particles. The relationship between TiB₂+TiC nanoparticles and T₁ precipitates during the ageing process, as well as the influence of TiC + TiB₂ particles on the growth process of T₁ precipitates during the ageing process, were investigated. The grain sizes of the A0 and A1 samples were found to be 249.89 μm and 86.42 μm, respectively. The dislocation density is greater in the deformed A1 sample. The coefficient of thermal expansion (CTE) effect generated by TiC and TiB₂ particles stimulates the precipitation process of T₁ precipitates. The A1 alloy reached the peak ageing state in a shorter time and yielded a greater number of T₁ precipitates. The precipitation process of T₁ precipitate is: SSSS→T_1P→T₁. The transition from T_1P (face-centered cubic, FCC) to T₁ (hexagonal close packing, HCP) entails a modification in the order of packing. The recently formed T_1P precipitate is attached to the established T₁ precipitate and extends along the c-axis through the shared copper-rich layer that has formed on both sides of the mature T₁ precipitate. The mechanical properties of sample A1 are optimal at T = 22h, with a yield strength, tensile strength, and elongation of 520 MPa, 553 MPa, and 8.2%, respectively. In comparison to the A0 sample, the yield strength and tensile strength exhibited an increase of 5.05% and 5.13%, respectively.

In order to elucidate the effect of cooling rate on the carbide formation during the solidification process of high manganese steel Mn13, the solidification process is artificially divided into two stages, namely the liquid-solid phase transition stage and solid-state cooling stage. The final Mn13 samples with different cooling rates are used to the analysis of solidification structure and carbides by high-temperature confocal microscopy (HTCLSM), optical microscopy (OM), electron backscatter diffraction (EBSD), and scanning electron microscopy (SEM). The result shows that the change of cooling rate at the liquid-solid phase transition stage has a great influence on the solidification structure and carbide precipitation. At this stage, with the increase of cooling rate from 0.2 °C/s to 5.0 °C/s, the dendrite structure is obviously refined, and the secondary dendrite arm spacing and cooling rate are satisfied with the relationship: ${\lambda }_{\Pi }=54.14\times v^{-0.33}$. Also, the average grain size in the Mn13 sample decreases from 549 μm to 346 μm, and the aspect ratio of gains in the Mn13 sample increases from 1.7 to 2.0. Moreover, the distribution of carbide in the interdendritic regions and grain boundaries increases, and the morphology of carbide at the grain boundary change from block to dendrite. But it seems that the change of cooling rate from 1 °C/s to 5.0 °C/s at the solid-state cooling stage has a slight effect on the secondary dendrite arm spacing, the average size and aspect ratio of the grain structure and the amount and morphology of carbide precipitation at the grain boundary.

To simultaneously improve the mechanical strength and plasticity of the CoCu_0.5FeNiTa_0.1 high entropy alloy, we adopted the heat treatment and investigated the evolution of different phases containing γ′, δ and γ phases. Two different γ′ phases, the Cu-rich phase and the Ta-rich phase, were detected in the as-cast alloy. During heat treatment, Cu outwards diffused from γ′(Cu-rich) into γ matrix, γ′(Ta-rich) phase and δ phase. The accumulation of Cu and Ta into the δ phase strongly promoted its growth. One significant increase was measured in the compressive yield strength from 1040 MPa to 1370 MPa, ultimate compressive strength from 2020 MPa to 2735 MPa, compressive strain from 34% to 50.2% and hardness from 3.854 GPa to 4.315 GPa. The substantial enhancement in ductility mainly resulted from the distance increase among γ′ particles in γ matrix, the reduction of lattice distortion and the decrease in aspect ratio of δ phases. The yield strengthening mechanisms were mainly from solid strengthening and Orowan strengthening.

In this study, the process of multi-particle cold spraying has been numerically simulated and compared with the actual cold spraying results. The results show that in the continuous multi-particle models, the maximum depths of compressive residual stress reached 4.90 μm, 3.99 μm, 4.78 μm, and 5.19 μm for each increment in particle number. The penetration depth of residual stress increases then decreases, due to two opposite factors effects on the penetration depth of residual stress. the maximum compressive residual stresses on the substrate are 438.14 MPa, 293.57 MPa, 286.19 MPa, and 279.30 MPa respectively, declining as the number of impacting particles grows. With the subsequent deposition of particles, the further deformation of the substrate causes the stress on the side to gradually homogenize, and the peak value decreases. Surface stress of the workpiece alleviates after multiple Al particles impact Cu substrate. In all random multi-particle models of different gas pressure, the residual stress begins to disappear at about 100 μm from the surface, and basically disappear at about 300 μm. As the collision speed of particles increases, the substrate deformation increases, but the growth rate of deformation decreases. The influence of coating thickness on the substrate deformation gradually decreases with the increase of coating thickness.

Understanding the mechanism of oxygen on toughness of titanium alloys is crucial for popularizing powder metallurgy. In this work, the effect of oxygen content (1500, 3000 and 5000 ppm) on the impact toughness of powder metallurgically modified titanium alloys with fine equiaxed microstructures (∼1.5 μm) was systematically investigated. With increasing oxygen content, the c/a value of the α-phase lattice parameter increases to a maximum of 1.593 Å, and the hardness increases from 4.03 GPa to 5.05 GPa, resulting in an increase in strength of ∼150 MPa and a considerable decrease in the ability of the two phases to coordinate. The crack initiation energy is similar for different oxygen contents, whereas the crack propagation energy increases considerably with decreasing oxygen content; the impact energy of stable crack extension increases from 4 J to 13 J and 35 J, and the impact energy of unstable crack extension and crack collapse stages increases from zero to 14 J and 25 J, respectively, indicating that decreasing oxygen content can substantially improve the crack extension resistance. The activation of numerous dislocations within the two phases and the formation of subgranular boundaries at low oxygen contents promote the release of internal stresses in the grains, and simultaneously, the interfacial resistance to dislocation migration and the concentration of interfacial stresses are also reduced, which improves the coordination of the two-phase plastic deformation of the equiaxed microstructures; the crack extension paths in the impact process become more tortuous and finally achieve impact energies as high as 85–100 J.

Hydroxyapatite-reduced graphene oxide (HA/rGO) nanocomposite coatings were developed on Ti6Al4V alloy using plasma electrolytic oxidation (PEO). The PEO electrolyte comprised calcium acetate (C₄H₆CaO₄) and calcium glycerophosphate (C₃H₇O₅CaP). Prior to integrating reduced graphene oxide into the coating solution, parameters such as voltage and current density were optimized using scanning electron microscopy (SEM). The influence of various current densities and graphene concentrations on coating properties was analyzed. Coating phase structures, surface morphologies, functional groups, and chemical compositions were characterized by X-ray diffraction (XRD), SEM, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and energy dispersive spectroscopy (EDS). Raman spectroscopy confirmed the presence of graphene in the coatings. Surface morphology examinations revealed that surface cracks appeared at voltages above 350 V due to thermal stresses. Increasing current density reduced the number of porosities but increased pore size. Adding graphene to the solution to form HA/rGO coatings further decreased both the number and size of porosities. XRD analysis identified phases of titanium, anatase, rutile, titanium phosphide, tri-calcium phosphate (TCP), and hydroxyapatite in the coatings. Corrosion properties were assessed via potentiodynamic polarization tests in simulated body fluid (SBF) solution. Tribological and mechanical properties were evaluated by pin-on-disk and microhardness, respectively. The in vitro apatite-formation ability of the coatings was assessed by immersion in SBF at 37 °C, with ion concentration changes measured by inductively coupled plasma spectrometry (ICP). Results indicated that increasing current density reduced porosities and increased the Ca/P ratio.

This study established a circulating system to control the concentration of substances and temperature in the aqueous solution. Simultaneously, sensors were used to continuously monitor the corrosion of three pipe materials: ductile iron (DI), surface-treated ductile iron (SDI), and carbon steel (CS). A corrosion decision model based on a machine learning framework was developed for data mining. The results show that the developed model provides accurate corrosion prediction strategies. Analysis revealed that high temperature is the primary factor accelerating corrosion in water systems. SDI accelerates at 60 °C, reaching its peak at 90 °C, while DI and CS peak at 80 °C. The superior corrosion resistance of SDI is attributed to its ability to withstand accelerated corrosion under high temperatures and environmental coupling, making it more stable when immersed in water.

At cryogenic temperatures, 316L austenitic stainless steel (ASS) exhibits remarkable strength while retaining high ductility, defying the conventional stress-strain trade-off. Despite extensive studies documenting the cryo-tensile properties of ASSs, the underlying mechanisms behind this phenomenon remain largely unexplored. This study systematically re-examines the tensile properties of 316L stainless steel and the associated mechanisms across a range of low temperatures (293 K, 223 K, 123 K, and 77 K). The reasons for the superior stress-strain balance (∼80 % GPa) are discussed using results from electron backscatter diffraction (EBSD) microstructure characteristics. The results undoubtedly suggest that the transformation mechanisms, specifically the shift from deformation twinning to martensitic transformation (γ → ε → α′), play a crucial role in enhancing elongation at cryogenic temperatures. At these temperatures, the Gibbs free energy difference between ε-martensite and γ-austenite approaches zero, resulting in slow martensite growth. The stress-strain curves at low temperatures satisfy the Considère criterion, indicating delayed necking under these conditions. This behavior is ascribed to the presence of various hierarchical microstructures, including ε, α′, γ-twins, ε-twins and their intersections, which act as sources of work hardening. This study provides new insights into deformation behavior of ASSs under cryogenic conditions.