Metallurgical and Materials Transactions A最新文献

As one of the joints with the greatest load-bearing capacity, the hip joint requires high mechanical strength and plasticity. In this study, Ni-coated Al₂O₃ (Ni@Al₂O₃) particles of different size were employed to enhance the mechanical properties of selective laser melting (SLM)-processed CoCrMo alloys. CoCrMo–Ni@30μmAl₂O₃ composites possessed the best comprehensive mechanical properties and underwent the γ–ε martensite transition with the largest proportion of ε/γ, which contributed to the inhibition of dislocation slip and the improvement of plasticity.

Graphical Abstract

This work investigates the effects of short-time direct aging heat treatments on the mechanical properties and microstructure of additively manufactured Nitinol (NiTi) alloy. Cylindrical samples were produced through laser powder bed fusion (L-PBF), directly aged at different temperatures and compared to the solution annealed and aged conditions. Compression tests were carried out at room temperature both in cyclic mode at constant strain and incremental cyclic mode, to provide a comprehensive analysis on the superelastic features of NiTi after direct aging heat treatments. Furthermore, cyclic compression tests were performed at 37 °C to evaluate the superelastic effect at the body temperature and, therefore, the possibility to use these treatments for biomedical components. The effects of direct aging on the microstructure were characterized using transmission electron microscopy (TEM). High cyclic stability and superelastic recovery up to 10 pct of deformation emerged for the direct aged alloys. The comparable results obtained with and without the solution treatment points out that this step was not necessary in reaching superelasticity, proving the effectiveness of direct aging.

Mo(Si,Al)₂–Al₂O₃ composites were tested in four-point bending at 1300 °C. The addition of Al₂O₃ led to a decrease in grain size, and a subsequent decrease in strength for materials containing up to 15 wt pct Al₂O₃. At higher Al₂O₃ fractions, the decrease in grain size saturates and the strength is recovered as the Al₂O₃ particles prevent grain boundary sliding.

Transient thermoreflectance (TTR) signal generated from pulse laser-induced oscillation of delaminated metal films of Al, W, and In is determined and the acoustic contribution due to strain is evaluated. The decay in the amplitude of the acoustic oscillations is modeled in terms of Granato–Lṻcke theory of vibrating dislocations with mobility restricted under drag exerted by electron scattering. The values of the relative energy loss factor from the experimental observations and from dislocation modeling are shown to correlate well using the expected dislocation density in annealed metal foils.

Prominent δ-ferrite to γ-austenite transition in the coarse-grained heat-affected zone of P92 steel has been documented in situ by confocal scanning laser microscope under different thermal cycles, with emphasis on the quantifying of migration of the γ/δ interface. With reducing cooling rate, the fraction of retained δ-ferrite decreases appreciably from 12.0 to 1.6 pct. It is further quantified that the average migration distance of γ/δ interface is enhanced from 10.74 to 17.81 μm.

This work studied the effect of welding thermal cycling on the microstructures and cryogenic impact toughness of medium-Mn low-temperature steel by single-pass welding thermal simulation test. The microstructures of the heat-affected zone (HAZ) were characterized by optical microscopy, electron backscatter diffraction, X-ray diffraction, and transmission electron microscopy. The results indicated that the microstructures of fine-grain HAZ are fine-lath martensite with film-like retained austenite in the heat input range of 10 ~ 30 kJ/cm. Meanwhile, fine-grain HAZ has excellent low-temperature impact toughness, and the impact energy tested at − 40 °C can reach about 245 J. The coarse martensitic packet and block in the coarse grain HAZ seriously deteriorated the cryogenic impact toughness, and the greater the heat input, the worse the cryogenic impact toughness. The impact energy tested at − 40 °C was 71 J when the heat input was 10 kJ/cm. The cryogenic impact toughness of HAZ gradually deteriorated with the peak temperature increase because of the decrease of retained austenite content and the increase of martensitic lath width.

Laser powder bed fusion (L-PBF) is an innovative technique used to manufacture complex and customized parts. However, fabricating crack-sensitive Al–Cu–Mg alloy L-PBF parts with controlled defect levels remains challenging, as the effects of processing parameters on defect evolution are not well understood. In this study, we systematically investigated the influence of laser power, laser scanning speed, and hatching space on defect formation and evolution in L-PBF printed Al–4.1Cu–1.5Mg–0.6Mn alloy. Our findings indicate that laser power and scanning speed greatly impact the occurrence of lack-of-fusion porosity, hot cracks, and gas-induced pores. Initially, the relative density of L-PBF printed samples increases (from 97 to 99 pct) with increasing volumetric energy density (from 200 to 830 J/mm³), but then decreases with further increases in volumetric energy density (>830 J/mm³). The introduction of alloying elements Cu and Mg leads to increased laser reflectivity and heat dissipation, which in turn affects hot cracking susceptibility (HCS) and reduces the printability of Al–Cu–Mg alloy. This research provides a process map to guide the L-PBF manufacturing of crack-sensitive Al–Cu–Mg alloys.

The work focuses on characterizing the microstructure and selected properties of Al- (1.3 or 2.8 wt pct) and Ta-modified (2 or 3 or 4 wt pct) Alloy 740 Ni-based superalloy after heat treatment. The observations and analyses were conducted using thermodynamic simulations, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, steam oxidation, hardness, tensile, and stress rupture tests. The castings exhibit a typical dendritic structure, with the presence of fine γ′ precipitates, Nb-rich Laves phase precipitates, MC carbides, and MN nitrides are found in all modified castings. However, the presence of the η phase, G phase, and M₂₃C₆ is dependent on the superalloy’s chemical composition. At room temperature, the reference cast superalloy had an ultimate tensile strength and yield strength of 655 MPa and 477 MPa, respectively. In modified castings, the ultimate tensile strength and YS are higher, respectively, 713–1046 MPa and 705–903 MPa. After steam oxidation at 760 °C/1000 h, the mass gain of the modified castings was in the range of 0.339–0.429 mg/cm². The addition of tantalum resulted in decreased oxidation resistance and the formation of the TiTaO₄ oxide at the Cr₂O₃/superalloy interface.

This study examined the effect of high-density pulsed electric current (HDPEC) treatment on the residual stress relief of martensitic stainless steel SCS6. The findings indicate that applying HDPEC at 100 A/mm² for 370 ms, or equivalent current conditions at a maximum temperature of approximately 735 °C, can nearly eliminate surface residual stresses. Moreover, this treatment resulted in a 7 pct increase in strength without deteriorating the ductility. X-ray diffraction and electron backscatter diffraction analysis found that defects (i.e., dislocations) within the grains were remarkably removed, which was the main reason for the residual stress removal. This study demonstrates the feasibility of HDPEC in eliminating residual stresses of martensitic stainless steels, presenting a promising alternative to traditional heat treatment. In addition, this method introduces possibilities for localized non-destructive treatments on substantial components, enabling rapid and straightforward repairs.

Graphical abstract

The influence of stress/strain on the movement/rearrangement of (gamma /epsilon ) interfaces and the coalescence of (epsilon ) plates under a fixed location of a high Mn steel have been investigated through fractal measurement of (epsilon ) plates. Image texture analysis has also been implemented to recognize the stored energy and configurational stability of such plates under (gamma ) matrix as a function of tensile deformation. Fractality of (epsilon ) variants are correlated with their geometrical configurations at different directions with systematic increment of tensile strain.