Metallurgical and Materials Transactions A最新文献

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.

When 304 steel and T91 steel are used as high-temperature structural materials, the presence of Cr-rich M₂₃C₆ carbides significantly impacts their properties. In the case of 304 steel, the inhibition of carbide precipitation is achieved by increasing the proportion of Σ3 grain boundaries (GBs). Conversely, for T91 steel, the proportion of random GBs increases following recrystallization. As a result, carbides precipitate within the ferrite grains instead of at the martensitic boundaries.

Graphical Abstract

Ultra-high nitrogen (N > 3 wt pct) austenitic stainless steel has been prepared by powder metallurgy. However, a large amount of nitride (CrN) precipitation leads to a decrease in corrosion resistance. In order to further improve the comprehensive performance of ultra-high nitrogen austenitic stainless steel, high temperature solution treatment has been carried out in this work. The microstructure, mechanical properties, and corrosion resistance of samples treated by different solution temperatures were investigated through various characterization and testing methods. The results indicate that high temperature solution treatment can promote the decomposition and spheroidization of nitrides, improve the microstructural morphology and distribution uniformity, and significantly enhance corrosion resistance. Especially the solution treatment at 1200 °C achieved the optimal combination of mechanical properties and corrosion resistance. Based on the analysis of TEM and EBSD, this enhancement is attributed to the reduction in both the quantity and size of CrN precipitates following the solution treatment. Additionally, the interface between CrN and austenite becomes less distinct, accompanied by a more ordered atomic arrangement. And, an increase in the density of austenite dislocations and the proportion of small-angle grain boundaries is observed.

The microstructure of the halo ring has been studied in quenched and partitioned (Q&P) steel resistance spot welds. The TEM and EBSD characterizations revealed the presence of an upper bainitic microstructure in the halo ring of the three-sheet stack-up welds. Stalking faults accompanied by nano-twins were identified surrounding the cementite. Diffusion of carbon towards the molten weld pool during solidification led to the formation of bainite at the fusion boundary, triggered the localized softening.

Innovations in electronic devices and their capabilities have driven the demand for improved conductive materials relevant to device fabrication. To gain insights on developing solid solution-type Cu alloy thin wires with a superior balance of strength and conductivity, this study investigated variations in the microstructures and properties of pure Cu wires and Cu–5 at. pct Zn, Cu–5 at. pct Al, and Cu–5 at. pct In alloy wires during intense drawing and analyzed the effects of stacking-fault energy (SFE) of Cu alloys on their microstructural evolution. During the initial drawing stages, lower SFE Cu–5 at. pct Al and Cu–5 at. pct In alloys yielded more high-density deformation twins than pure Cu and Cu–5 at. pct Zn. Deformation twins promoted grain refinement during drawing. Effective grain refinement and dislocation accumulation during drawing in low-SFE Cu alloys substantially strengthened them without adversely impacting electrical conductivity. During intense drawing in the Cu–5 at. pct In alloy wires, ultrafine fibrous grains (diameter ~ 80 nm) and a high-dislocation density yielded excellent tensile strength and conductivity. These results indicate that adjusting the solute element content in Cu matrix to reduce SFE and optimizing deformation strain via wire drawing significantly improve alloy wire performance.