Metallurgical and Materials Transactions A最新文献

Wide-gap brazing has been widely utilized as one of the go-to alternatives to welding in the repair of turbine components in the aerospace and power generation industries. In this study, differential scanning calorimetry, electron microscopy, and thermodynamic calculations were used to determine the influence of brazing time and temperature on the microstructural evolution for a layered wide-gap brazing process using a MAR-M247/BNi-9 system. Once liquefied, rapid braze infiltration into the MAR-M247 skeleton occurred via capillary action. During infiltration, partial and complete dissolution of the MAR-M247 skeleton occurred, which lead to diffusional solidification at 1068 (^circ )C. Upon further and complete infiltration, it was found that rapid densification was achieved prior to isothermal brazing temperatures. The post-braze microstructure contained (gamma )-Ni matrix grains, precipitated Cr, W, Mo-rich M(_{x})B(_{y}) borides, athermal solidification products along matrix grain boundaries and triple junctions, as well as internal porosity. It was found that brazing temperature dictated the athermal solidification products with binary eutectic (CrB + (gamma )-Ni) at 1150 (^circ )C and ternary eutectic (Cr + (gamma )-Ni + Ni(_{3})B) at 1180 (^circ )C and 1205 (^circ )C. These findings agreed with Scheil–Gulliver predictions. Brazing time influenced the compositional homogeneity of the braze liquid, altering solidification behavior. This resulted in higher and lower solidification ranges for shorter and longer brazing times, respectively. Further, it was found that liquid fraction within the brazement increased with both brazing temperature and time, suggesting a persistent liquid phase. This finding was accompanied by an increase in volume fraction of athermally solidified intermetallics, consistent with an increase in liquid phase with increased brazing time and temperature. Lastly, (gamma )-Ni grain growth occurred, although heterogeneity between the upper and lower regions of the brazement was observed. The upper region displayed larger grains on average when compared to the lower region. This was attributed to boride migration during liquid infiltration, which may have hindered grain growth via a grain boundary pinning mechanism.

The present study investigated a new configuration of friction stir welded joints from two aluminum alloys. Dissimilar welds AA6082/AA1350 were examined, whereas, for AA1350, two states were investigated—coarse-grained (CG) and ultrafine-grained (UFG). Changes in the mechanical and electrochemical properties regarding the microstructure evolution across the welds were discussed. The average grain size in the stir zone (SZ) for all materials equaled 4 to 5 µm with a fraction of high-angle grain boundaries of about 77 pct, indicating the occurrence of continuous dynamic recrystallization. Changes in the microhardness across the welds were connected with differences in grain size (AA1350) and dissolution of β″ precipitates in the SZ of AA6082. As a result, the tensile strength of the welds decreased compared to base materials AA6082 and AA1350 UFG; however, there was an increase when compared to the base material AA1350 CG. Electrochemical experiments revealed that pitting corrosion occurred for AA1350, while for AA6082, it was a combination of pitting and intergranular corrosion. The depth of corrosion attack was higher for AA1350, with a maximum value of ~ 70 µm for base materials, while in the SZ, a depth decreased to 50 µm. For the AA6082, the maximum depth was measured in the SZ and did not exceed 30 µm.

With the push towards sustainable alloy production, using recycled material in casting Al alloys has become essential. However, high recycle content (HRC) aluminum alloys typically have a high iron content, leading to the formation of Fe-bearing intermetallic particles (Fe-IMCs) that affect the mechanical performance and formability of the alloy. Historically, 2D microscopy-based characterization techniques have been used to assess the size and morphology of these Fe-IMCs. While widely used, these 2D techniques are often incapable of capturing the complex 3D interconnected morphologies of the Fe-IMCs. In this work, we present a methodology for the high-throughput compositional and 3D morphological characterization of Fe-IMCs in a primary (AA 5182) and a high recycle content (HRC alloy) in the as-cast and homogenized states, using a combination of 3D X-ray Computed Tomography (XCT) and energy-dispersive X-ray spectroscopy (EDS). To capture the differences in morphology of the Fe-IMCs in the commercial and HRC alloys, we introduce a new 3D morphological descriptor—the particle-to-convex hull volume ratio (p/h). Finally, the effect of homogenization on the Fe-IMCs morphology was tracked using p/h, and a comprehensive analysis of the Fe-IMCs’ compositional and morphological evolution was presented.

This study addresses challenges in elucidating the mechanism of phase transformations occurring in steel of near-peritectic composition. The importance of using and integrating, complementary experimental techniques is emphasized. While thermal analysis tools such as Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA) are vital, they offer limited insight on events occurring during cooling. Employing standard thermal analysis (DSC) alongside high-temperature microscopy, incorporating simultaneous thermal analysis within a high-temperature microscope, and concentric solidification, two of steels of near-peritectic composition were investigated. Key findings include the correlation between heating rates and completion temperatures of phase transformation in the DSC heating experiments; absence of a peritectic transition inferred from DSC cooling curves supported by visual observation, and insights into restricted austenite phase nucleation attributed to diffusional constraint and limited nucleation sites. This investigation not only contributes to understanding phase transformation behaviour in peritectic steels, but more generally provides a framework for utilizing different techniques synergistically to address complexities in the interpretation of the mechanism of phase development.

Triple-layer stainless-steel clad plate having 317L stainless steel (SS317L) as cladding layer and ASTM SA516 GR60 (GR60) as backing layer was successfully fabricated through vacuum hot roll bonding (VHRB) at 1373 K (1100 °C) temperature and strain rate regime of 1–5 s⁻¹, which were identified through process efficiency maps of the base materials (SS317L and GR60). The process efficiency maps were constructed by conducting isothermal compression tests within the temperature range of 1173 K (900 °C)–1473 K (1200 °C) and 0.1–50 s⁻¹strain rate regime. Effect of post-rolling heat treatments on the mechanical properties of clad plate was studied after solutionization at 1173 K (900 °C) for 1 h followed by cooling at different rates, i.e., water quenching, air cooling, and furnace cooling. As compared to other post-rolling heat treatments, the ultimate tensile strength, uniform plastic elongation, and maximum shear strength showed a significant change from 524 MPa, 0.46 and 519 MPa to 652 MPa, 0.36 and 410 MPa, when the normalized clad plate was solutionized at 1173 K (900 °C) and water quenched. A drastic change in shear fracture mode from gradual failure in normalized condition to catastrophic failure was also noticed after water quenching. These changes are essentially manifestation of the microstructural change in GR60 layer which led to the change in mechanical properties.

Circumferential near-neutral pH corrosion fatigue (C-NNpH-CF) is the result of the simultaneous impact of axial residual and applied stresses along with the near-neutral pH corrosive environment established on the external surface of the buried pipeline because of leakage through the protective coating. (This mechanism has previously been referred to as near-neutral pH stress corrosion cracking.) Since integrity management measures should be implemented before Stage III (rapid crack propagation to rupture), this study aims to evaluate the effect of bending residual stress (a suitable source of axial residual stress) and cyclic loading (simulated pipeline pressure fluctuations) on crack growth at Stage II. Based on the digital image correlation (DIC) method, the final stress distribution in length and thickness direction was used to analyze crack growth in various test parameters, including applied cyclic loading, initial notch depth/position, and bending angle/direction. As a result of stress gradients in the depth direction of bent pipelines, a new method was developed to obtain the stress intensity factor. A comparison of crack growth rates between circumferentially oriented and longitudinally oriented NNpH-CF was performed to reveal the growth mechanism. Crack growth was maximum at 1 mm depth initial notch, 20 deg bend (inward), and 50 pct cycling load.

In the present study, 3D non-woven needle-punched preform (NPP) C_f–SiC_m composite with 7.5 pct volume fraction of carbon fiber is prepared via liquid silicon infiltration technique and characterized for microstructure, phase formation and mechanical behaviors. Additionally, the composite is subjected to plasma arc jet tests for evaluation of ablation resistance under ultra-high temperature oxidation environment. The dense composite (density ~ 2.5 to 2.6 g/cm³) contains β-SiC phase due to the reaction between infiltrated molten silicon and carbon matrix surrounding the carbon fibers. The resultant C_f–SiC_m composite shows high hardness and high abrasion resistance due to a higher proportion of hard SiC matrix as well as exhibits various toughening mechanisms from the carbon fiber reinforcement causing a delay in fracture. It also contains excellent resistance to thermal shock and thermo-oxidative erosion resistance during plasma arc jet ablation test without any visible crack or damage on the exposed surface.

Graphical Abstract

The performance of modern dual hardening steels strongly relies on a well-controlled precipitation processes during manufacturing and heat treatment. Here, the precipitation of intermetallic β-NiAl in recently developed dual hardening steels has been investigated during aging using combined high-energy synchrotron X-ray diffraction and small-angle scattering. The effects of heating rate and aging temperature on the precipitation kinetics and lattice mismatch in two alloys (Hybrid 55 and Hybrid 60) were studied. Precipitation starts already during heating, typically in the temperature range 450 °C to 500 °C. The precipitation process is significantly faster at 570 °C compared to 545 °C for both steel grades, and the number density reaches its maximum already within 1 hours during aging at 545 °C and within 15 minutes during aging at 570 °C. The effect of heating rate is limited, but the precipitation during heating increases in Hybrid 60 when slower heating rate is used. This led to slightly higher volume fractions during subsequent aging, but did not affect the particle size. The lattice mismatch between β-NiAl and the matrix initially develops rapidly with time during aging, presumably due to a developing chemistry of the β phase, until a particle size of around 1.5 nm is reached, whereafter it saturates. After saturation, the lattice mismatch is small, but positive, and independent of temperature during cooling.

Emulating the Ni-base superalloy (gamma ) + (gamma ^{prime }) microstructure in BCC–B2 refractory alloys is a promising design strategy to achieve high temperature strength and ductility. Ru-base B2 precipitates have shown exceptional thermal stability but can be difficult to solutionize, making high cooling rate solidification pathways like additive manufacturing (AM) a promising approach for synthesis of more homogeneous microstructures. Using single track laser experiments on aged bulk substrates, five representative refractory alloys with varying Ru-base B2 precipitates (AlRu, HfRu, TiRu) and matrix constituents (Mo, Nb) were investigated for their solidification behavior and defect susceptibility under laser melting conditions. Susceptibility to solidification cracking, solid-state cracking, and keyhole formation was found to be highly dependent on the matrix composition. Characterization of the melt pools by scanning and transmission electron microscopy shows evidence for disordered BCC upon solidification, enabling tailoring of the B2 precipitates that are thermodynamically stable above 1300 °C. The B2 precipitate morphologies in the melt tracks after aging treatments are influenced by the partitioning behavior of Ru from laser melting. Results from these single track experiments provide guidance toward design strategies for fabricable refractory BCC–B2 alloys.

In this study, the influence of various mechanical and chemical surface treatments on the adhesion strength and surface properties of sodium alginate coatings electrophoretically deposited (EPD) on 316L stainless steel substrates was investigated. XPS and TEM results revealed the presence of oxide layers containing elements from the substrates, with thicknesses varying from 1 to 45 nm, depending on the treatment used. Most substrates exhibited high roughness and hydrophilic properties (CA with water 62.8–82.6 deg). Sodium alginate coatings with uniform morphology were deposited with the same process parameters, i.e., 5 V and 300 s. The surface topography of the coatings was closely related to that of the substrate on which they were deposited. All coatings exhibited higher hydrophilicity (CA with water 29.5–49.7 deg) compared to the substrates (CA with water 62.8–82.6 deg). The coatings on the etched and anodized substrates demonstrated the highest adhesion strength (class 4B), attributed to the very low oxide layer thickness and the specific substrate surface topography. Mechanical interlocking was identified as the primary adhesion mechanism for these coatings. This work provides insight into optimizing surface treatments for improved adhesion of sodium alginate coatings to stainless steel substrates widely used for temporary bone implants. The results obtained will also be helpful in providing high adhesion of sodium alginate-based composite coatings to steel substrates.