Metallurgical and Materials Transactions A最新文献

Grain refining is of practical interest in welding, casting, and additive manufacturing. Despite extensive studies, it is still not very clear why grain refining can decrease the susceptibility to cracking during solidification and why over grain refining can increase it. To help understand why, the present study included the continuity of the interdendritic liquid and solute poisoning into consideration. The present study also attempted to estimate the decrease in the cracking susceptibility caused by grain refining. To do this, a simple index for the cracking susceptibility was modified by the extent of grain refining. The validity of the estimation was checked against cracking susceptibility tests in welding and casting.

Microstructural evolutions and cryogenic mechanical properties of AISI 904L superaustenitic stainless steel were studied for the first time and compared with those of metastable AISI 304L and 316L austenitic stainless steels. The temperature-dependencies of these properties were critically discussed and correlated to the microstructures, focusing on the solid solution strengthening, transformation-induced plasticity (TRIP), and twinning-induced plasticity (TWIP) effects. The 904L alloy showed an extraordinary cryogenic strength-ductility synergy with excellent work-hardening behavior.

The microstructure of electronic solder joints is generated by the solidification of a small volume of bulk undercooled liquid. Here, we study β-Sn dendrite growth in Sn–3Ag–0.5Cu in the specific geometry and nucleation conditions of ball grid array (BGA) solder joints by combining electron backscatter diffraction and imaging of microstructures. It is shown that, while (langle 110rangle) is the preferred dendrite growth direction, out-of-plane branching and growth with (langle 11Wrangle) directions are important for allowing dendrites to fan out into the spheroidal volume of BGA joints due to the low symmetry of β-Sn. We find that the crystallographic orientation of β-Sn at the nucleation point plays a strong role in subsequent dendrite growth. In single-grain joints, dendrites are often unfavorably oriented for growth, resulting in different types of zig-zag dendrite growth. In cyclic-twinned joints, it is shown how competitive out-of-plane trunk growth between three dendrite orientations produces {101} boundaries and the characteristic beachball microstructure.

We investigated the structural, elastic, vibrational, and electronic properties of the Nb₅Si₂B compound combining density functional theory (DFT) calculations and experimental methods. We compared our results with the parent compound Nb₅Si₃ with two non-equivalent Si sites namely Si(4a) and Si(8h). The analysis of elastic constants and phonon spectra indicate that Nb₅Si₂B is respectively mechanically and dynamically stable. Based on the phonon calculation we evaluate the theoretical constant volume lattice specific heat (C_V) for different site occupancies and compare it with experimental specific heat (C_p) measurements. We found an excellent agreement between theoretical and experimental results for Nb₅Si₂B with the B at the Si(8h) site, which agrees with the calculated formation energy. In addition, we also performed DFT calculations aimed at showing and comparing the total DOS near the Fermi level (E_F) for Nb₅Si₃ and Nb₅Si₂B. The XPS valence band (VB) of the Nb₅Si₂B is largely dominated by two characteristic peaks at − 8.7 and − 1.7 eV, respectively. Based on DFT calculations, it follows that the main sharp peak at − 1.7 eV comes as a contribution from Nb 4d states, while the smaller and broader one located at − 8.7 eV results mainly from Si 3s states weakly hybridized with Nb 4d states. In this connection, the majority contribution in the binding energy range from − 12 eV to the E_F comes from Nb 4d states, while the contribution from Si and B atoms is very small in this region. The core levels of Nb 3d, Si 2s, 2p, and B 1s were also identified using the XPS technique.

The study brings original data on the effect of linear velocity during melt-spinning process on magneto-mechanical properties of Heusler Ni-Mn-Ga-based melt-spun ribbons. The research revealed that different linear velocity of the copper wheel had a significant impact on the ribbon's geometry resulting in distinct changes in magneto-mechanical properties. X-ray diffraction measurements were used to examine the phase composition, confirming the presence of L2₁ austenite phase. To assess the mechanical properties of the Ni-Mn-Ga-based melt-spun ribbons, cyclic bending experiments were conducted at a strain rate of 0.1 mm/s. Additionally, experiments involving magnetic field-induced bending were carried out in an external magnetic field ranging from 0 to 0.28 T. Finally, it was observed that there was a proportional relationship between the linear velocity of the copper wheel and magnetic field-induced ribbons deflection. Conversely, the dependence between linear velocity and mechanical bending load was found to be inversely proportional. Electron backscattered diffraction measurements revealed that melt-spun ribbons produced at high linear velocity of 18.5 m/s exhibited fine-grained microstructure in contrast to low linear velocity of 3 m/s. Based on these results it seems feasible to optimize the functional properties of the studied ribbons by varying the linear velocity of the melt-spinning process.

The effect of rapid solidification upon hypereutectic Sn–Ag solder alloy has been investigated using a 6.5 m drop tube. Powder sizes ranging from > 850 to < 38 μm were produced, with equivalent cooling rates of 250 to 14,800 K s⁻¹ for 850 and 38 μm droplets, respectively. At all cooling rates investigated, dendritic β-Sn was observed as the primary solidification phase, not proeutectic Ag₃Sn as predicted by the phase diagram. The volume fraction of interdendritic eutectic was observed to decrease with increasing cooling rate, with the Ag concentration in the residual interdendritic liquid estimated at 12.5–15 wt pct Ag, far in excess of the eutectic concentration of 3.8 wt pct. Much of the Ag₃Sn observed within the eutectic had a blocky, divorced eutectic appearance. A model is proposed which can explain these observations in terms of sluggish nucleation of the Ag₃Sn intermetallic, coupled with a metastable phase diagram that permits significant supersaturation of Ag at modest undercooling.

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.