Acta Metallurgica Sinica-English Letters最新文献

The coarsening behavior and strengthening effect of L1₂-Ni₃(Ti,Al) precipitates in a face-centered-cubic (FCC) (FeCoNi)₉₂Al_2.5Ti_5.5 high entropy alloy have been systematically investigated. The coherent L1₂ precipitates, uniformly distributed throughout the FCC matrix, consistently retain a spherical shape. The coarsening rate coefficient of precipitate is determined by employing the Philippe-Voorhees (PV) model, suggesting excellent thermal stability. Furthermore, the elemental partitioning and compositional evolution of the L1₂ precipitates is analyzed by atom probe tomography, which identify aluminum (Al) as the slowest diffusion species during the coarsening process. In addition, the precipitation strengthening effect is quantified to ascertain the optimal size of the precipitates. Our study enhances the understanding of precipitate coarsening in high entropy alloys, presenting valuable insights into their thermal stability and mechanical properties.

Medical bone implant magnesium (Mg) alloys are subjected to both corrosive environments and complex loads in the human body. The increasing number of hyperglycemic and diabetic patients in recent years has brought new challenges to the fatigue performance of Mg alloys. Therefore, it is significant to study the corrosion fatigue (CF) behavior of medical Mg alloys in glucose-containing simulated body fluids for their clinical applications. Herein, the corrosion and fatigue properties of extruded Mg-Zn-Zr-Nd alloy in Hank’s balanced salt solution (HBSS) containing different concentrations (1 g/L and 3 g/L) of glucose were investigated. The average grain size of the alloy is about 5 μm, which provides excellent overall mechanical properties. The conditional fatigue strength of the alloy was 127 MPa in air and 88 MPa and 70 MPa in HBSS containing 1 g/L glucose and 3 g/L glucose, respectively. Fatigue crack initiation points for alloys in air are oxide inclusions and in solution are corrosion pits. The corrosion rate of the alloy is high at the beginning, and decreases as the surface corrosion product layer thickens with the increase of immersion time. The corrosion products of the alloy are mainly Mg(OH)₂, MgO and a small amount of Ca-P compounds. The electrochemical results indicated that the corrosion rate of the alloys gradually decreased with increasing immersion time, but the corrosion tendency of the alloy was greater in HBSS containing 3 g/L glucose. On the one hand, glucose accelerates the corrosion process by adsorbing large amounts of aggressive Cl⁻ ions. On the other hand, glucose will be oxidized to form gluconic acid, and then reacts with Mg(OH)₂ and MgO to form Mg gluconate, which destroys the corrosion product film and leads to the aggravation of corrosion and the accumulation of fatigue damage.

Obtaining an appropriate grain size is crucial for Al alloys or Al matrix composites prior to processing, as it significantly influences the mechanical properties of components and workability during the manufacturing process. TiB₂ particles are exceptional grain refiners in Al and serve as excellent reinforcement particles for particulate-reinforced aluminum matrix composites. However, the optimal particle content for achieving excellent refinement and strengthening effects depends on the matrix composition and requires further investigation. Additionally, homogenization is essential for mitigating the element segregation in the ingot. Although it is anticipated that adding suitable particles can effectively inhibit undesired grain growth during homogenization, comprehensive investigations on this aspect are currently lacking. Therefore, TiB₂/2219Al matrix composites with varying reinforcement contents (0, 1, 3, 5 wt%) were fabricated through traditional casting followed by homogenization treatment to address these research gaps. The effects of reinforcement content and homogenization treatment on the microstructure and mechanical properties of in-situ TiB₂/2219Al composites were investigated. The results demonstrate a gradual strengthening of the refining effect with increasing particle concentration. Moreover, composites containing 3 wt% TiB₂ particles exhibit superior comprehensive mechanical properties in both as-cast and homogenized state. Additionally, potential orientation relationships are observed and calculated between undissolved Al₂Cu eutectic phase and submicron or nanometer-sized TiB₂ particles, resulting in a mixture structure with enhanced bonding strength. This mixture structure is continuously distributed along grain boundaries during solidification, forming a three-dimensional cellular network that acts as primary retarding forces for grain growth during homogenization. Furthermore, the established homogenization kinetic equations were further utilized to analyze the correlation between homogenization time and grain size, as well as the influence of homogenization temperature.

This study focuses on developing a novel multiphase stainless steel with enhanced ductility and an ultralow yield ratio achieved through solid-solution treatment. The steel exhibits remarkable mechanical properties: a tensile strength of approximately 1114 MPa, an ultralow yield ratio of 0.36, exceptional uniform elongation of approximately 17.48%, and total elongation of approximately 21.73%. The remarkable ductility of the steel can be attributed to the transformation-induced plasticity (TRIP) effect observed in the retained austenite, while its exceptional strength results from the combined effects of TRIP and the martensite phase.

The Al₂O₃ laminated preforms with different layers thickness were prepared by freezing casting in present work. Then, the Al₂O_3p/AZ91 magnesium matrix laminated materials were obtained by infiltrating the AZ91 alloy melt into the Al₂O₃ laminated preform based on pressure infiltration process. Subsequently, the influence of freezing temperature on the microstructure, mechanical properties and fracture behavior of magnesium-based laminates was investigated. The results indicated that with the decrease of freezing temperature, the thickness of Al₂O₃ layers decreases gradually, the number of layers increases obviously, and the interlayers spacing decreases. Accompanied with the decrease of interlayers spacing, the size of Mg₁₇Al₁₂ phase precipitated in the AZ91 alloy layers was refined, and the compression strength and strain were both improved obviously. The micro-cracks initiated in Al₂O₃ layers during loading process, while the AZ91 layers could effectively suppress the initiation and propagation of micro-cracks. Furthermore, the changing layers structure influenced by the decrease of freezing temperature had significant inhibiting effect on the initiation and propagation of micro-cracks, which endowed the Al₂O_3p/AZ91 magnesium matrix laminated materials with better strength and toughness. Notably, the best compression properties of Al₂O_3p/AZ91 magnesium matrix laminated materials could be obtained at the freezing temperature of − 50 °C, the compression strength and elastic modulus of which were the 160% and 250% of monolithic AZ91 alloy, respectively.

The corrosion behavior of the pre-oxidized GH4169 alloy was studied after 20 h of exposure under a solid NaCl deposit film in a wet O₂ environment at 600 °C by mass-gain measurements, X-ray diffraction, scanning electron microscopy, transmission electron microscopy methods. The results indicate that the pre-oxidized GH4169 alloy undergoes serious corrosion in the corrosive condition. The preformed Cr₂O₃ layer is gradually destroyed by deposit NaCl, and the inner Nb₂O₅ layer beneath the complete outer Cr₂O₃ layer also reacts with NaCl to form NaNbO₃. The electrochemical test results testified the existence of electrochemical reactions during this corrosion process. The corrosion behavior of pre-oxidized GH4169 under a solid NaCl deposit film in a wet O₂ environment at 600 °C has been discussed in detail.

Here, the composition of TaMoNbZrTiAl refractory high entropy alloy (RHEA) is optimized by increasing Ti content to improve its mechanical property especially the ductility, through comparing two RHEAs with different Ti content. The RHEAs contain two body-centered-cubic (BCC) phases. The BCC phase in the dendritic region is rich in Ta, Mo and Nb, and the BCC phase in the interdendritic region is enriched in Zr, Ti and Al. The as-cast RHEA with a higher Ti content remains dendritic microstructures, and Ti is mainly enriched in the interdendritic region. After annealing treatment at 1300 °C for 48 h, the dendritic microstructures change into equiaxed-grain morphology, accompanied by needle-like micron precipitates at grain boundaries in the RHEA with higher Ti content. For the as-cast RHEAs, the fracture strain increases by ~ 6.6% and the uniform plastic strain increases by ~ 5.9% at the compression test due to the increase of Ti content. Our work offers a reference for the composition design of RHEAs and makes a preliminary exploration of the optimization of the microstructures and mechanical properties.

Exfoliation corrosion (EFC) has rarely been reported for wrought magnesium alloys. Enhanced EFC of the as-extruded Mg–1Li–1Ca alloy was observed after the removal of outermost fine-grained skins. The microstructure and corrosion products were investigated using optical microscopy, scanning electron microscopy, electron backscattered diffraction, X-ray diffraction, and Fourier transform infrared spectrometry. The corrosion behavior was analyzed using electrochemical polarization, electrochemical impedance spectroscopy, and scanning Kelvin probe techniques. The results indicated that the surface of Mg–1Li–1Ca, with the superficial layer removed, was more susceptible to EFC. A linear correlation between grain boundary density and corrosion resistance was established. Additionally, the influence of factors such as grain size, intermetallic compounds, different alloy surfaces, and corrosion products on EFC were discussed, and the corresponding EFC mechanism was clarified.

Through carrying out the high-temperature tensile experiments on an as-extruded Mg–11wt%Y alloy at 350 °C, 400 °C, 450 °C, 500 °C and 550 °C, the mechanical behavior and fracture mechanisms at elevated temperatures are investigated and compared. Tensile results show that with the increase of temperature, the yield strength and ultimate tensile strength of the alloy increase at first and then decrease, while that the elongation ratio decreases firstly and then increases. For the sample being tested at 350 °C, the values of yield strength, ultimate tensile strength and the elongation ratio are 188 MPa, 266 MPa and 11%, respectively. At 400 °C, the yield strength and ultimate tensile strength reach the maximum values of, respectively, 198 MPa and 277 MPa, but the elongation ratio is the lowest and its value is only 8%. When the applied temperature is increased to 550 °C, the values of yield strength and ultimate tensile strength, respectively, decrease to 140 MPa and 192 MPa and the elongation ratio increases to 38%. Failure analysis demonstrates that the fracture surfaces of different samples are mainly composed of plastic dimples and exhibit the typical characteristic of ductile fracture. The observation to the fracture side surfaces indicates that at the temperatures of 350 °C and 400 °C, microcracks mainly initiate in the interior of Mg₂₄Y₅ particles. When the temperatures are 450 °C, 500 °C and 550 °C, the cracks preferentially initiate at the Mg₂₄Y₅/α-Mg interfaces.

Mg alloy seamless tubes (MASTs) were prepared through three-high rotary piercing process, effect of billet temperature, feed angle and plug advance on microstructure, texture and mechanical properties of tubes were investigated. The effect on the deformation mechanism and improving mechanical properties mechanism of this process for MASTs were studied. The results show that the grain size could be refined to 11.3–31.1% of the initial grain size and the microstructure was more uniform due to the accumulation of strain. The formation of high strain gradient at the grain boundary activated the non-basal slip. This piercing process could change the grain orientation of as-extruded billet and eliminate the initial basal texture to produce new favorable texture. And the process could accelerate the continuous dynamic recrystallization process. After piercing, yield strength of pierced tubes decreased by 6.7%, ultimate tensile strength (UTS) and elongation increased by 32.4 and 45%, respectively, at optimal parameters. The plate-shaped β₁-Mg₁₇Al₁₂ orientation transformed from basal plates to prismatic plates, facilitating the increase in UTS and ductility. The decrease size of nanoscale precipitates could reduce the cracking possibility. The critical resolved shear stress ratios of pyramidal (10−11) slip and (11−22) slip to basal slip for the sample including prismatic plates both decreased compared to that including basal plates. This could enhance the ductility of tube sample. Moreover, grain boundary sliding could contribute to a better ductility via coordinating deformation and reducing stress concentration during piercing process.