Acta Metallurgica Sinica-English Letters最新文献

The investigation concentrates on friction stir welded (FSW) Al–Cu–Li alloy concerning its local microstructural evolution and mechanical properties. The grain features were characterized by electron back scattered diffraction (EBSD) technology, while precipitate characterization was conducted by using transmission electron microscopy (TEM) aligned along [011]_Al and [001]_Al zone axes. The mechanical properties are evaluated through micro-hardness and tensile testing. It can be found that nugget zones exhibit finely equiaxed grains evolved through complete dynamic recrystallization (DRX), primarily occurring in continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX). In the thermal–mechanically affected zone (TMAZ), numerous sub-structured grains, exhibiting an elongated morphology, were created due to partial DRX, signifying the dominance of CDRX, DDRX, and geometric dynamic recrystallization (GDRX) in this region. T₁ completely dissolves in the nugget zone (NZ) leading to the formation of Guinier–Preston zones and increase of δ′, β′ and S′. Conversely, T₁ partially solubilizes in TMAZ, the lowest hardness zone (LHZ) and heat affected zone (HAZ), and the residual T₁ undergoes marked coarsening, revealing various T₁ variants. The solubilization and coarsening of T₁ are primary contributors to the degradation of hardness and strength. θ′ primarily dissolves and coarsens in NZ and TMAZ, whilst this precipitate largely coarsens in HAZ and LHZ. σ, T_B, grain boundary phases (GBPs) and precipitate-free zone (PFZ) are newly generated during FSW. σ exists in the TMAZ, LHZ and HAZ, whereas T_B nucleates in NZ. GBPs and PFZ mostly develop in LHZ and HAZ, which can cause strain localization during tensile deformation, potentially leading to LHZ joint fracture.

In this study, a high-carbon nano-bainitic GCr15Si1Mo bearing steel was investigated. Specifically, the effects of content and size of undissolved carbides on the microstructure and transformation kinetics of nano-bainite were analyzed. The results demonstrated that after prolonged austempering at low temperatures, the mixed microstructure composed of nano-bainite (NB), undissolved carbides (UC), and retained austenite (RA) was obtained in GCr15SiMo steel. When the experimental steel was austenitized at 900 °C, the undissolved carbides gradually dissolved until reaching a stable state with increasing holding time. Furthermore, at the same austempering temperature, despite different volume fractions of undissolved carbides in the substrate, the volume fractions of nano-bainite in the final microstructures remained essentially the same. Moreover, the higher the content of undissolved carbides in steel, the faster the transformation rate of nano-bainite and the shorter the total transformation time.

Abstract

In this work, the Al–Cu–Mg alloy with different Y (0–0.2 wt%) and Ce (0.5–1.5 wt%) are designed. The effect of mixed addition of Y and Ce on the grain structure and hot tearing for Al–4.4Cu–1.5Mg–0.15Zr alloy was investigated using "cross" hot tearing mould. The results indicate that as rare earth Y and Ce increases, the grain size becomes finer, the grain morphology changes from dendrite to equiaxed grain, and effectively reduce the hot tearing sensitivity coefficient (HTS₁) and crack susceptibility coefficient (CSC) of the alloy. With the increase of Ce element (0.5–1.5 wt%), the hot tearing susceptibility of the alloy decreases first and then increases. With the increase of Y element (0–0.2 wt%), the hot tearing sensitivity of the alloy decreases. When the content of rare earth is 0.2 wt% Y + 1.0 wt% Ce, the minimum HTS₁ value and CSC value of the alloy are 68 and 0.53, respectively. Rare earth Ce refines the alloy microstructure, shortens the feeding channel, and reduces the hot tearing initiation. Meanwhile, the rare earth Y can form Al₆Cu₆Y phase at the grain boundary, improve the feeding capacity of the alloy. Therefore, appropriate addition of rare earth Y and Ce can effectively reduce the hot tearing tendency of the alloy.

Submerged friction stir processing (SFSP) with flowing water was employed to alleviate the porosities and coarse-grained structure introduced by wire-arc manufacturing. As a result, uniform and ultrafine grained (UFG) structure with average grain size of 0.83 μm was achieved with the help of sharply reduced heat input and holding time at elevated temperature. The optimized UFG structure enabled a superior combination of strength and ductility with high ultimate tensile strength and elongation of 273.17 MPa and 15.39%. Specifically, grain refinement strengthening and decentralized θ(Al₂Cu) phase in the sample subjected to SFSP made great contributions to the enhanced strength. In addition, the decrease in residual stresses and removal of pores substantially enhance the ductility. High rates of cooling and low temperature cycling, which are facilitated by the water-cooling environment throughout the machining process, are vital in obtaining superior microstructures. This work provides a new method for developing a uniform and UFG structure with excellent mechanical properties.

The impurity iron in silicon material will seriously affect the photoelectric conversion efficiency of silicon solar cells. However, the traditional silicon purification method has the disadvantages of long cycle, high energy consumption and serious pollution. In this study, an efficient and green pulsed electric current purification technology is proposed. The electromigration effect of iron elements, the current density gradient driving of iron phase, and the gravity of iron phase all affect the migration behavior of iron phase in silicon melt under pulsed electric current. Regardless of the depth of electrode insertion into the silicon melt, the solubility of iron in silicon decreases under the pulsed electric current, which helps to form the iron phase. At the same time, the iron phase tends to sink toward the bottom under the influence of gravity. When the electrode is shallowly inserted, a non-uniform electric field is formed in the silicon melt, and the iron phase is mainly driven by the current density gradient to accelerate sink toward the bottom. When the electrode is fully inserted, an approximately uniform electric field is formed in the silicon melt, and iron elements are preferentially migrated to the cathode by electromigration, forming iron phase sinking at the cathode. The study of impurity iron migration behavior in silicon melt under pulsed electric current provides a new approach for the purification of polycrystalline silicon.

The surface spinning strengthening (3S) mechanism and fatigue life extension mechanism of 316L stainless steel welded joint were systematically elucidated by microstructural analyses and mechanical tests. Results indicate that surface gradient hardening layer of approximately 1 mm is formed in the base material through grain fragmentation and deformation twin strengthening, as well as in the welding zone composed of deformed δ-phases and nanotwins. The fatigue strength of welded joint after 3S significantly rises by 32% (from 190 to 250 MPa), which is attributed to the effective elimination of surface geometric defects, discrete refinement of δ-Fe phases and the appropriate improvement in the surface strength, collectively mitigating strain localization and surface fatigue damage within the gradient strengthening layer. The redistributed fine δ-Fe phases benefited by strong stress transfer of 3S reduce the risk of surface weak phase cracking, causing the fatigue fracture to transition from microstructure defects to crystal defects dominated by slip, further suppressing the initiation and early propagation of fatigue cracks.

Abstract

This paper focuses on the relationship between the microstructure and tensile properties of Fe–Mn–Al–C low-density high-strength steel processes by hot-rolling and air-cooling process. The microstructure analysis reveals that the combination of hot-rolling and air-cooling results in the formation of heterogeneous structures comprising different-sized γ and B2 phases in the low-density steel with the addition of nickel (Ni). The addition of Ni promotes the formation of the B2 phase and induces the pinning of B2 phase particles at the γ grain boundaries. This pinning effect effectively hinders the growth of the γ grains, leading to grain refinement. The tensile test results demonstrate that LDS-5Ni (low-density steel, LDS) exhibits excellent high strength and ductility combination, e.g., a tensile strength of 1535 MPa, yield strength of 1482 MPa, and elongation of 23.3%. These remarkable mechanical properties are primarily attributed to the combined strengthening contributions of grain refinement and duplex nano-sized second-phase precipitation hardening.

The semi-solid stir casting method is adopted to prepare 10 wt% SiC_p/Mg–6Zn–0.5Ca–xAl (x = 0, 1, 3 and 5 wt%) composites, and the microstructure evolution and mechanical property of composites with various Al content are investigated. The results show that the addition of 3 wt% Al improves the distribution of SiC_p, whereas the SiC_p cluster occurs again with Al content greater than 3%. An abnormal phenomenon of twinning is observed in the cast composites in this work. The SiC_p/Mg–6Zn–0.5Ca composite possesses the highest twin content of ~ 23%, for which tension twins (TTW) and compression twins (CTW) account for ~ 19% and ~ 3%, respectively. The CTW is only observed in ZXA600 composite. The addition of Al has an inhibiting effect for the generation and growth of twins. The content of twin decreases firstly and then increases with increase of Al content. The lowest twin content is obtained as Al increases to 3 wt%. It is found the existence of twin is detrimental to the mechanical property of composites. As-cast SiC_p/Mg–6Zn–0.5Ca–3Al composite with the lowest twin content exhibits the optimal mechanical property of yield strength, ultimate tensile strength and elongation for 100 MPa, 188 MPa and 4.4%, respectively. The outstanding mechanical property is attributed to the uniform distribution of SiC_p, the low twin content and the well-distributed fine second phases.

In this study, the high-temperature stability and the generation mechanism of the Portevin–Le Chatelier (PLC) effect in solid-solution Mg–1Al–12Y alloy with different heat treatment processes were investigated by adjusting the content of long-period stacking ordered (LPSO) phases. It was found that the content of LPSO phases in the alloys differed the most after heat treatment at 530 °C for 16 h and 24 h, with values of 13.56% and 3.93% respectively. Subsequently, high-temperature tensile experiments were conducted on these two alloys at temperatures of 150 °C, 200 °C, 250 °C, and 300 °C. The results showed that both alloys exhibited the PLC effect at temperatures ranging from 150 to 250 °C. However, at a temperature 300 °C, only the alloy with a greater concentration of LPSO phases exhibited the PLC effect, whereas the alloy with a lower proportion of LPSO phases did not exhibit this phenomenon. Additionally, both alloys exhibited remarkable high-temperature stability, with the alloy containing a greater percentage of LPSO phases also demonstrating superior strength. The underlying mechanism for this phenomenon lies in the exceptional high-temperature stability exhibited by the second phase within the alloy. Furthermore, the LPSO phase effectively obstructs the movement of dislocations, and it also undergoing kinking to facilitate plastic deformation of the alloy. The results indicate that the PLC effect can be suppressed by reducing dislocation pile-up at grain boundaries, which leads to a decrease in alloy plasticity but an increase in strength. The presence of the PLC effect in the WA121 alloy is attributed to the abundant dispersed second phase within the alloy, which initially hinders the movement of dislocations, leading to an increase in stress, and subsequently releases the dislocations, allowing them to continue their movement and thereby reducing in stress.

As a type of austenitic stainless steel, 316L stainless steel has excellent plasticity, corrosion resistance, and biocompatibility, making it widely used in industries, especially in the marine environments. However, its lower yield strength and wear resistance are the obvious disadvantages that restrict its application in more fields. In this work, an Fe-based amorphous alloy (Fe^am) was selected as reinforcement to enhance the 316L stainless steel prepared by selective laser melting (SLM), and microstructure evolution, mechanical properties, tribological and corrosion performance of the SLMed samples were investigated in detail. The relative density values of both 316L stainless steel and Fe^am-reinforced samples are above 99%, which suggests that Fe^am-reinforced samples also have outstanding formability. In the as-etched micrograph, all of the SLMed samples exhibit cellular structure. Fe^am-reinforced samples have thicker sub-grain boundaries, and retained amorphous phase can be observed in the samples reinforced with 10 wt% and 15 wt% Fe^am. As the addition of Fe^am increases, the microhardness and compression strength of the Fe^am-reinforced samples gradually improve and reach 449.2 HV and 2181.9 MPa, respectively. The wear morphologies show that the 316L stainless steel and Fe^am-reinforced samples both experience abrasive wear and corrosion wear in a 3.5 wt% NaCl solution. Meanwhile, as the amount of Fe^am added increases, the coefficient of friction and wear rate of SLMed samples gradually decrease. Compared to the unreinforced sample, Fe^am-reinforced samples have lower corrosion current density and higher pitting potential according to the potentiodynamic polarization curves and also exhibit superior corrosion resistance in the salt spray environment. This work suggests that the addition of Fe-based amorphous alloy can improve the mechanical properties and wear resistance of 316L stainless steel, as well as its ability to withstand salt spray corrosion.