Metallurgical and Materials Transactions A最新文献

Selective laser-melted 316L stainless steel (SLM 316L SS) holds significant potential for application in the energy and chemical sectors owing to its commendable mechanical properties and corrosion resistance. However, the intricate process of microstructure evolution in SLM 316L SS at intermediate temperatures, encompassing the feasible range of service temperatures, needs to be more adequately comprehended. This research endeavors to elucidate the grain size and distribution alterations between 750 °C and 850 °C. Abnormal grain growth before recrystallization and re-refinement phenomena were observed, which deviated from conventional expectations. The texture was found to play a crucial role in former, while recovery-induced dislocation rearrangement and recrystallization nuclei formation contributed to the latter process. These findings provide new insights into the thermodynamic behavior of SLM 316L SS at medium to high temperatures.

Graphical Abstract

The introduction of one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) ultrasounds into solidifying FeCoNiCuAl high-entropy alloy was efficiently optimized, which realized the maximum input of acoustic energy and the effective adjustment of the energy proportion between stable and transient cavitation effects. In addition to the ordinary advantage of grain refinement, the superiority of power ultrasound in modulating such Cu-containing high-entropy alloys with dendritic structures mainly lay in the significant regulation of phase volume fraction and the elimination of severe Cu element microsegregation. As the main energy transmission form under 1D ultrasound, stable cavitation slightly increased the nucleation rate of α and γ₁ phases, which jointly contributed to suppressing the Cu solute enrichment from 41.6 to 36 at pct through the acoustic streaming during the subsequent growth of γ₁ phase. When 2D and 3D ultrasounds were applied, the intensive transient cavitation dominated the solidification process. The induced local high undercooling resulted in the competitive nucleation and growth between α and γ₁ phases, leading to the more than one order of magnitude reduction in their grain sizes and the significant rise of γ₁ phase volume fraction from 13 up to 50 pct. Meanwhile, it strikingly reduced the final Cu content difference between these two phases from over 30 to around 3.8 at pct by decreasing the Cu composition in competitively formed γ₁ nuclei. The above microstructure modification brought in excellent compressive property for 3D ultrasonically solidified alloy, whose strength and ductility were simultaneously enhanced by 27 and 24 pct.

Given the critical role that metastable retained austenite (RA) plays in advanced high-strength steel (AHSS), there is significant interest in obtaining a comprehensive understanding of its stability, to achieve excellent mechanical properties. Despite considerable attention and numerous studies, the significance of individual contributions of various microstructural factors (size, crystallographic orientation, surrounding phases, etc.) on the stability of RA remain unclear, partly due to the difficulty of isolating the direct effects of these factors. In this study, we examined the influence of microstructural factors while minimizing the effect of chemical composition on the mechanical stability of RA. We accomplished this by comparing the austenite (γ) stability in two distinct microstructures: a two-phase RA/martensite microstructure and a one-phase γ microstructure, both with nearly identical γ compositions. We employed in situ high-energy X-ray diffraction during uniaxial tensile testing conducted at both room temperature and 100 °C, facilitating the continuous monitoring of microstructural changes during the deformation process. By establishing a direct correlation between the macroscopic tensile load, phase load partitioning, and the γ/RA transformation, we aimed to understand the significance of the microstructural factors on the mechanical stability of the RA. The results indicate that very fine RA size and the surrounding hard martensitic matrix (aside from contributing to load partitioning) contribute less significantly to RA stability during deformation than expected. The findings of this study emphasize the critical and distinct influence of microstructure on γ/RA stability.

For this study, the researchers aimed to dissimilar weld the Nickel-based superalloy Inconel 718 (IN 718) with austenitic stainless steel 304L (ASS 304L) using the gas tungsten arc welding (GTAW) technique and Nickel-based filler IN 82 (ERNiCr-3). In order to examine the weld microstructures, we utilized optical microscopy (OM) and field emission scanning electron microscopy (FESEM) with energy-dispersive spectroscopy (EDS) to identify any segregation present in various weld zones. Through optical and FESEM analyses, it was revealed that the base metals (BM) exhibit an austenitic character. The IN 718 BM matrix contains dispersed γ′ and γ″ strengthening precipitates within the Nickel matrix. On the other hand, the ASS 304L BM displayed a unique austenitic microstructure characterized by twins features. The weld metal exhibited solidification grain boundaries (SGBs), migrated grain boundaries (MGBs), and distinct dendritic microstructures that had an impact on the properties of the weld. Through extensive analysis and mapping of the IN 82 weld zone, it was discovered that interdendritic regions contain carbides of Nb, Cr, and Ti. In addition, there were Unmixed zone (UZ) areas between the IN 82 filler and the base materials on both sides of the weld zone, appearing as islands and beaches. The texture of the different weld zones was evaluated using electron backscattered diffraction (EBSD) analysis. Additionally, the presence of a notable level of strain within the weld metal grains was observed through Kernel average misorientation (KAM) micrographs. Fractures were observed in the IN 82 weld zone, indicating that it is the weakest area in the IN 718/ASS 304L dissimilar weld at room temperature, according to the outputs of the tensile tests. The micro-hardness profile showed substantial hardness values in the weld zone, which can be attributed to the appearance of a diverse microstructure and additional precipitates. At room temperature, the recorded average tensile strength of the dissimilar weld joint was 626 MPa. In addition, experiments were carried out at high temperatures of 550 °C, 600 °C, and 650 °C to measure the tensile strength. In the high-temperature tensile tests, it was observed that the IN 82 weld zone exhibited higher tensile strength compared to the ASS 304L BM. Interestingly, the high temperatures tensile specimens failed in the 304L BM. The Charpy impact toughness test was performed with notches at ASS 304L HAZ, IN 718 HAZ, and the weld center. Using the deep hole drilling (DHD) technique, we were able to quantify residual stress and identify the location of the highest tensile residual stress, which was found to be 3 mm from the weld surface.

The thermal-mechanical processing (TMP) for twin-induced grain boundary engineering (GBE) generally adopts a small amount of cold deformation and subsequent annealing at solution temperature of austenitic stainless steels. The nucleation mechanism during the TMP of GBE is essential to the understanding of the evolution of grain boundary character distribution (GBCD). The mechanism for recrystallization nucleation is investigated in a 316L austenitic stainless steel which was subjected to short-time annealing at solution-annealing temperature after 5–10 pct tensile deformation. A total of 22 recrystallization nuclei were found, and the analyzing of the orientation relationships between the nuclei and nearby deformed grains revealed that most of the nuclei are formed following the strain-induced boundary migration (SIBM) mechanism. The formation of highly twinned grain-clusters as the typical feature of GBE microstructure is a result of extensive multiple twinning starting from every single nucleus. Low nucleation density is more important than how the nucleus forms during GBE. A portion of the recrystallization front boundaries outside the clusters expanded into the deformation microstructure more extensively than the others. However, the growth advantage does not have an obvious correlation with the misorientation of these recrystallization front boundaries.

The effect of the crystal orientation on freckle formation has been investigated in single crystal Ni-base superalloy heavy-plate castings. Single crystal superalloy heavy-plate castings grown along the <001> , <011> and <111> crystallographic orientations were prepared by the bottom seeding technique and Bridgman method. Optical microscopy (OM) and scanning electron microscopy (SEM) were employed to observe the microstructure, and electron backscatter diffraction (EBSD) was used to characterize the crystallographic orientation of the castings. The morphology of the mushy zone during directional solidification was simulated by ProCAST finite element software. The experimental results show that the space between primary dendrites at the (010) crystal plane of <011> oriented plate casting and the (100) crystal plane of <111> oriented plate casting is wider than that at the same corresponding crystal plane of <001> oriented plate casting. The occurrence of freckles depends not only on orientation but also on dendrite morphology. Compared with orientation, the freckle is more sensitive to dendrite morphology and the space between primary dendrites of the single crystal plates. The freckle formation tendency of the <001> orientation casting was the weakest among the three crystal orientation castings, and the reason for this tendency was discussed.

Graphical Abstract

The substantial development that the additive manufacturing technique of powder bed fusion using a laser beam (PBF-LB) underwent in the past decades, though expressive, has been restricted to particular materials and applications. When coming to Ni-based superalloys, the technology has been mostly developed regarding a few polycrystalline Ni–Cr–Fe and Ni–Cr alloys, particularly Inconel 718 and 625. However, when produced using PBF-LB, these materials should undergo tailored heat treatment sequences to adjust its microstructure to industrial standards, which must be developed according to the behavior of each particular alloy. In view of such restrictiveness, this study assessed 77 experimental heat treatments for PBF-LB Inconel X-750, an alloy with comparatively limited research volume when considering additive manufacturing, aiming at providing guidelines for its post-processing after PBF-LB manufacturing. These heat treatments were based on the standard ASM 5668 sequence for maximization of creep resistance, and, contradicting the known precipitation behavior of the conventional material, often resulted in coarse precipitation of detrimental bulk η-Ni₃Ti intermetallic phases. This indicates insufficient chemical homogenization after heat treatment, evidencing a different microstructural response of the material when processed using PBF-LB and the importance of optimizing the post-processing of such materials.

We examined the hydrogen diffusion behavior in the Al–Si-coating layer which underwent the press-hardening simulation. The microstructure evolution, in particular, the type of intermetallic compounds, was revealed to have a remarkable influence on the hydrogen absorption. It was found that the formation of AlFe in the coating layer was advantageous regarding to the suppression of hydrogen penetration across the coating layer, which was originating from the lower hydrogen diffusivity compared to the Al₅Fe₂.

NiTi smart alloys are known for their characteristic shape memory behavior. The current work focuses on the physical and mechanical characterization of Ni_(50−X)Ti₅₀Fe_X shape memory alloys prepared by the spark plasma sintering (SPS) process and their dependence on the concentration of Fe. The physical characterization of the samples confirmed the presence of the FeNiTi phase along with the Ti- and Ni-rich phases. Enhanced mechanical properties were observed in 8 at. pct Fe samples, which contained secondary intermetallic phases such as Ti₂Ni, Ni₃Ti, Fe₂Ti, and Ni₄Ti₃. Higher fraction of NiTi phase in the 8 at. pct Fe sample resulted in better shape memory properties while showing a higher friction coefficient. Ball on disk wear tests were done to identify the mechanisms contributing to the wear in the sintered sample. It is observed that the abrasive wear as well as the adhesive wear are the most prominent contributors for the surface material removal, and the dependence of characterization is observed with the variation of Fe content in NiTiFe alloy.

Multiprincipal Element Alloys (MPEAs) represent a new category of metallic alloys that stand out for exclusively containing solute elements in equiatomic/nearly equiatomic proportions in their composition. Due to their remarkable mechanical properties, these alloys have garnered significant interest within the scientific community. However, one of the major challenges associated with these alloys is their industrial-scale production. Therefore, this study aims to evaluate production and processing routes for obtaining MPEAs on an upscale, i.e., with masses on the order of several kilograms. To achieve this goal, we produced the Cr₄₀Co₃₀Ni₃₀ alloy (at. pct) using a vacuum induction furnace (VIM), resulting in ~ 50-kg ingots. Subsequently, the samples underwent hot forging and rolling processes, followed by analyses of composition and inclusion formation. The presence of Cr and Al oxide inclusions in the samples was observed in both samples. The composition remained homogeneous throughout the ingot’s cross-section. However, the forging process proved ineffective and resulted in several cracks during the procedure. On the other hand, hot rolling proved a more viable process, also promoting dynamic recrystallization, although crack formations also occurred. In both processes, as well as in the casting, the formation of the sigma phase was not observed.