Metals and Materials International最新文献

Direct aging (DA), a post-heat treatment process that omits the solution treatment step, was applied to study its effects on the microstructure and mechanical properties of an A20X aluminum alloy fabricated by laser powder bed fusion (LPBF). The chemical heterogeneity in the as-built (AB) microstructure, characterized by Ti- and Cu-enriched cell boundaries, facilitates the rapid formation of nanosized precipitates during DA. Precipitation initiated at the cell boundaries and progressed into the matrix, with the size and density of Ω and θ′ phases increasing with aging time. DA slightly enhanced tensile strength compared to the AB condition; however, excessive aging time led to strength reduction owing to excessive precipitate coarsening along the cell boundaries. Additionally, aging mitigated dynamic strain aging, as evidenced by the reduced frequency of serrated flows that results in the ductility recovery in the full-aged sample. These findings emphasize the critical role of DA in tailoring the mechanical properties of LPBF A20X alloys, offering a simplified pathway for optimizing high-strength aluminum additive manufacturing components.

Graphical abstract

This paper systematically investigates the influence of surface pre-deformation on the microstructure evolution and mechanical properties of 7050 aluminum alloy using ultrasonic surface rolling processing (USRP) followed by aging treatment. The results indicate that USRP forms a grain gradient layer with a depth of 160 µm on the surface. After aging, the surface precipitates coarsen significantly, forming η phases. With increasing depth, the precipitate size decreases while density increases, and the precipitates in the sample core transform into finely dispersed η’ phases. This microstructural distribution results in lower surface microhardness (177 HV) compared to the core and unrolled specimens (205 HV), yet without a reduction in strength or ductility. This phenomenon occurs because the grain gradient structure generates a high density of geometrically necessary dislocations (GNDs) during loading, leading to hetero-deformation-induced (HDI) strengthening. Loading and unloading calculations reveal that the grain gradient structure in the USRP sample significantly enhances additional back stress, compensating for the strength loss caused by precipitate coarsening on the surface. Furthermore, the coarse η phase precipitated in the rolled layer due to pre-deformation reduces the electrochemical difference between the grain boundaries and the matrix, improving corrosion resistance. This dual-gradient structure in aluminum alloy is expected to achieve an optimal balance between mechanical strength and corrosion resistance.

The evolution and distribution of cryogenic toughness were investigated within the multi-pass welding heat affected zone (HAZ) of a high Mn austenitic steel. The results reveal that thermal cycles exceeding 1000 °C induce enhancements in cryogenic toughness due to grain growth. Unusually, the evolution of cryogenic toughness is governed by M₂₃C₆ carbide variation within regions below 1000 °C, which act to reduce the cryogenic toughness. The peak temperature (Tp) of final thermal cycle is a key determinant of cryogenic toughness, as it determines whether the M₂₃C₆ carbide precipitate or remelt. The regions exhibiting poor cryogenic toughness spatially overlap with intergranular corrosion grain boundaries (ICGB) zone, because M₂₃C₆ carbide also reduce intergranular corrosion resistance. ICGB zone is cross-ringed in HAZ, and its size and position is governed by welding heat input and bead layout. This study provides valuable insights into controlling and testing performance of welded joint.

Graphical Abstract

This study examined the effect of synergistic alloying of near β-Ti(Al) with niobium (Nb) and tantalum (Ta) on oxidation resistance. Isothermal TGA tests at 1200 °C showed that the (Nb + Ta)-containing alloy exhibited the lowest weight gain and slowest parabolic oxidation rate compared to alloys containing only Nb or only Ta. Isothermal oxidation tests in air at 1000–1200 °C for 9 h revealed that both Nb- and (Nb + Ta)-containing near β-Ti(Al) alloys formed an external oxide zone (EOZ) with rutile (TiO₂) and alumina (α-Al₂O₃) in the outward region, along with sporadic traces of Ta₂O₅ and Nb₂O₅ mixed with (Nb/Ta)-rich TiO₂ in the inward region. Fine lamellae of α-Al₂O₃ within Ti₃Al phase were observed in the internal oxide zone (IOZ). The observed trend in oxidation resistance was attributed to the increased oxygen ion vacancy formation energy in TiO₂ due to the addition of Nb and Ta. Additionally, the higher valence states of Nb and Ta than Ti contributed to reducing the fraction of oxygen vacancies, thereby suppressing the formation of TiO₂.

Graphical abstract

This study investigates the effects of intercritical annealing (IA) on the microstructural evolution and tensile properties of direct-quenched low-carbon steels containing Cr and Mo. The steels were subjected to IA at different temperatures (740, 780, and 820 °C) to examine variations in ferrite-martensite morphology and phase distribution. The microstructural analysis revealed that increasing IA temperature led to an increase in the martensite volume fraction and a transformation from a lath to a fibrous microstructure in all steels, with the CrMo-added steel showing the most significant change, from 17.3% at 740 °C to 54.1% at 820 °C. The addition of Cr and Mo influenced phase transformation kinetics by reducing the mobility of the ferrite/austenite interface at lower IA temperatures due to the solute drag effect. Tensile testing showed an unusual trend where higher martensite fractions resulted in a decrease in strength and an increase in uniform elongation across all compositions, deviating from conventional dual-phase steel behavior. Notably, in the CrMo-added steel, the yield strength decreased from 685 MPa at 740 °C to 572 MPa at 820 °C, while uniform elongation increased from 6.1 to 7.4%. This phenomenon was attributed to changes in mobile dislocation density, hetero-deformation-induced strengthening, and enhanced martensite plasticity at higher IA temperatures. These findings offer valuable insights for optimizing IA conditions and alloy design to develop cost-effective, high-performance low-carbon dual-phase steels with enhanced mechanical properties.

Graphical abstract

Ultrasonic-assisted abrasive peening (UAP) is proposed in this study which offers an efficient approach to pre-treat metallic components. This process employs a peening medium comprising of a fluid mixed with a stabilizer and abrasives. The properties of the peening medium, such as density, surface tension, and viscosity influence the dynamics of cavitation bubble and, hence, the resultant outcomes of the peening process. In this study, the effect of peening medium was investigated by mixing 2 wt% surfactant and 4 wt% YSZ abrasives into water, soybean oil, corn oil, and olive oil as peening media. The cavitation bubble dynamics and the induced abrasive velocity was numerically determined by the Keller–Miksis and Newtonian mechanics equations. It was observed that the abrasive velocity in water was 2.3-fold higher than in oils. Specifically, water imparted maximum abrasive velocity of 283 m/s induced by microjets and 231 m/s induced by shockwaves, whereas olive oil resulted in lower abrasive velocity of 126 m/s and 134 m/s, respectively. Consistent with the numerical analysis, the water-peened Ti–6Al–4V sample exhibited highest residual stress of (−)575.1 ± 10 MPa, while no residual stresses were observed with oil-based peening. Viscosity was identified as the key medium property influencing the cavitation dynamics, with oils demonstrating 75 times higher viscosity than water. Hence, selection of low viscous peening medium is essential for enhanced UAP outcomes.

Graphical abstract

To study the influence of different deformation parameters of the hot working process on the microstructure of 20Cr2Ni4A gear steel during the plastic forming process, the heat deformation experiment of 20Cr2Ni4A gear steel was carried out using the Gleeble-3500 Thermal Machanical Simulator, and the heat deformation behavior of hot 20Cr2Ni4A gear steel under different deformation conditions was investigated. Through the dislocation density model, recrystallization nucleation and grain growth model, we innovatively wrote the cellular automaton program to simulate the dynamic recrystallization behavior during the heat deformation process. The simulation results were verified through EBSD and metallographic diagrams.

Investigations on the impact of fabrication path planning in directed energy deposition via interlayer dwell time have revealed significant improvements in the mechanical properties of the solidified material. However, the correlation between these improvements and microstructural phenomena during melt-pool solidification remains largely unexplored. As the size of the component increases, interlayer dwell time becomes more critical, as it can alter the cooling rate and help mitigate issues related to excessive heat buildup in the part. In this work, we characterize the impact of increasing interlayer dwell times on the resulting microstructure features, solute segregation, and solidification cracking. We use various modeling techniques in cooperation, including thermo-mechanical, thermo-fluid, finite difference Monte Carlo, phase-field, and analytical models to simulate the grain and dendritic growth, solute segregation, Laves phase formation, and cracking behavior during melt-pool solidification and examine their role in properties improvement in an Inconel 718 alloy. Our findings indicate that reducing interlayer dwell time can result in coarse dendritic structures that solidify primarily in the build direction due to low cooling rates and thermal gradients. In contrast, longer dwell times produce tilted dendrites driven by higher cooling rates and thermal gradients. The coarse dendritic structures create wider and longer liquid channels between the dendrite arms, which increases Nb segregation and favors the formation of detrimental Laves phase during terminal solidification, subsequently increasing the susceptibility to solidification cracking in the mushy zone. The numerical results are directly compared against experimental measurements with reasonable agreement.

Graphical Abstract

Ni content has a significant effect on martensitic transformation behaviour and tensile mechanical properties of Ni-rich NiTi shape memory alloys. In this study, the effect of grain size on sensitivity of tensile mechanical properties to Ni content of nanocrystalline NiTi alloys was investigated. Two NiTi alloys with nominal compositions of Ni-49.6 at %Ti and Ni-48.8 at %Ti were cold drawn into 21%−72% deformations, followed by annealing at 350 − 450 ºC. Nanocrystalline NiTi alloys were acquired when cold drawing strain was larger than 62% after annealing and average grain size that smaller than 20 nm was acquired when annealed at 350 ºC. The results of tensile mechanical properties suggest that sensitivity of upper plateau stress to Ni content decreased with the decreasing grain size. When grain size was smaller than 20 nm, the upper plateau stress of samples with different Ni content were close, which may be attributed to the Ni concentration at grain boundaries. When cold drawing strain was smaller than 52%, the sensitivity of stress hysteresis to Ni content of 350 ºC annealed samples decreased, which is attributed to the high-density dislocations in grains.

Graphical Abstract