Metallurgical and Materials Transactions A最新文献

The AA7075 high-strength aluminum alloy exhibits a pronounced tendency for hot cracking during fusion welding, which hampers its widespread utilization in lightweight design concepts. This study aims to investigate the effectiveness of friction stir processing (FSP) as a pre-weld treatment to eliminate the occurrence of hot cracking during fusion welding of the AA7075 aluminum alloy. Our findings demonstrate that the FSP pre-weld treatment successfully suppresses liquation cracking in the AA7075 alloy, even when utilizing ER1050 filler metal for welding, which poses a considerable risk of liquation cracking. The FSP treatment induces a transformative effect on the microstructure of the hot-rolled material, leading to the conversion of the typical large elongated pancake-shaped grains into refined recrystallized grains. Electron backscatter diffraction (EBSD) analysis confirms the introduction of finer grains, characterized by an increased grain boundary area, and higher volume fraction of low angle grain boundaries within the partially melted zone (PMZ) of the friction stir processed base metal. This leads to a reduction in the thickness of the liquid film and consequently diminishes the susceptibility to liquation cracking. These findings prove that arc welding with FSP pre-weld treatment is an enabling welding strategy for producing high-performance AA7075 joints.

Graphical Abstract

High-performance press-hardened steels (PHS) are highly desired in automotive body-in-white due to their excellent strength and crashworthiness. PHS alloyed with high Cr and Si show promise in enhancing oxidation resistance as compared with traditional 22MnB5 steel, which may replace the use of expensive Al-Si coatings. Heating temperature, as a critical hot stamping process parameter, manipulates the mechanical properties and oxidation behaviors of PHS. In this study, the mechanical properties and oxidation behaviors of a 2000 MPa grade PHS alloyed with high contents of Cr and Si at heating temperatures ranging from 750 °C to 950 °C are systematically studied, as compared with those of the classical 22MnB5 steel. It shows that fully martensite structures for both steels can be achieved at the lowest heating temperature of 850 °C, and the products of strength and elongation of 32Cr2Si are greater than those of 22MnB5 at all heat-treatment conditions. The 32Cr2Si PHS exhibits much smaller weight gain than 22MnB5 when oxidized at all examining temperatures, due to the presence of thicker Cr- and Si-rich layer between the matrix and oxide scale. This work can provide guidance for material and process optimization for high-performance coating-free press-hardened steels.

Microstructure-aware models are necessary to predict the behavior of material based on process knowledge or to extrapolate mechanical properties of materials to environmental conditions which are not easily reproduced in the laboratory, e.g., nuclear reactor environments. Elemental Ta provides a relatively simple BCC system in which to develop a microstructural understanding of deformation processes which can then be applied to more complicated BCC alloys. In situ neutron diffraction during compressive deformation and subsequent heat treatment have been used to monitor the evolution of microstructural features in Ta throughout simulated processing steps. Crystallographic texture and dislocation density are determined as a function of first plastic strain, then temperature. Lattice strains are determined and attributed to stresses at macroscopic, grain and dislocation length scales. The increase of the dislocation density through deformation and subsequent recovery during heat treatment is monitored through the changing diffraction line profile. Also, randomization of the texture is used as a signature of recrystallization. The recovery of dislocations through annihilation is not observed to depend on the initial dislocation density in the range studied here. In contrast, recrystallization is observed to depend strongly on the initially dislocation density.

In-situ laser-induced breakdown spectroscopy (LIBS) was used for measurements on molten aluminum alloys containing 0.6 wt pct magnesium in the melt temperature range 685 °C to 790 °C. With increasing melt temperature, an exponential growth of magnesium LIBS emission signals was observed, a phenomenon that has previously been attributed to the presence of Mg vapor above the melt surface. Here we show how this temperature dependence of the magnesium signal is affected by the presence of a second alloying element in the melt. For dilute ternary aluminum alloys Al–Mg–M, with M = Si, Zn, or Sn, the change in vapor-phase contribution to the Mg signal was found to be linearly correlated with the concentration of the additional alloying element but differing in sign and magnitude. Ternary alloys containing group-II alloying elements (M = Be, Ca, or Sr), known to inhibit oxidation of the melt, were also studied. The presence of these elements had a strongly reducing effect on the vapor-phase component of the Mg LIBS signal. We attribute this decrease to the formation of Be, Ca, or Sr-containing oxides that effectively inhibit the transport of Mg from the melt to the surface and into the vapor phase.

Widening the martensitic hysteresis (({T}_{text{Dhys}})) in NiTiNb shape memory alloys (SMAs) holds potential for broadening their working temperature range while enabling the room-temperature storage. In this study, the ({T}_{text{Dhys}}) is divided into two parts: the thermal-induced hysteresis (({T}_{text{hys}})) and the deformation-induced hysteresis (({T}_{text{hys}}{{^{prime}}})). In addition to decreasing the martensitic transformation start temperature (({M}_{text{S}})), it is found that grain refinement is an effective method for widening both ({T}_{text{hys}}) and ({T}_{text{hys}}{{^{prime}}}) of the Ni₄₇Ti₄₄Nb₉ alloy, a commercial SMA widely used for shape memory couplings. According to thermodynamic analysis, grain refinement increases the dissipation energy ((Delta {E}_{text{dis}})) (caused by thermal friction at martensite/austenite interface), thereby widening ({T}_{text{hys}}). Moreover, the Ni₄₇Ti₄₄Nb₉ alloy with finer grains has the potential to release more elastic strain energy ((Delta {E}_{text{el}})) after deformation, thereby exhibiting a wider ({T}_{text{hys}}{{^{prime}}}). When deforming a large strain, the alloy with finer grains generates more dislocations which stabilize the martensitic phase, thus further widening ({T}_{text{hys}}{{^{prime}}}).

Graphical Abstract

This work demonstrates rapid solidification and precision machining to achieve 6061 aluminum alloys with extremely small surface roughness (2.05 nm), good tensile strength (325 MPa), and uniformly distributed coherent nanoprecipitates. The fundamental mechanisms are revealed by means of microstructural and nanomechanical characterization at multi-scales, together with atom probe tomography analysis. This work opens the way to manufacture metallic optics for astronomy, radar, and LED devices with unprecedented low surface roughness and sufficient mechanical strength.

Graphical abstract

As a first step toward developing a downsized and high-performance O₂ separator suitable for large-scale industrial applications, such as steelmaking, we have studied nanocrystalline Ag alloy thin films with high O₂ permeability via rapid diffusion at Ag grain boundaries, operating at plant-waste heat temperatures (200 °C to 500 °C). In the present study, fabrication of nanocrystalline Ag–2at. pctCu–10at. pctAl alloy thin films with Ag grain size below 10 nm was attempted via reactive sputtering in Ar–O₂ using grain boundary pinning force of a large number of alumina particles. Cross-sectional observation of the fabricated thin film showed that the Ag grain size ranged from 4 to 15 nm when the film thickness was less than 200 nm, but when the film thickness exceeded 200 nm, the Ag grains abruptly coarsened, reaching a maximum grain size of 214 nm. Furthermore, large surface irregularities with sizes of up to 500 to 600 nm (equivalent to 2/3 of the film thickness) were also observed. Heat transfer analysis revealed that the Ag film partially melted because of the large amount of heat released by the oxidation of Al during sputtering deposition. The conditions necessary for the fabrication of high-Al nanocrystalline Ag alloy thin films via reactive sputtering in Ar–O₂ gas without film melting were clarified.

This study reports the interplay between grain boundary morphology and mechanical behaviour using micropillar compression. Quantitative findings indicate that serrated grain boundaries with multiple curvatures exhibit notably higher yield strengths compared to their straight counterparts. In a bi-crystal system, 18 pct increase in boundary length, achieved through multiple curvature boundaries, results in 21 pct increase in yield strength. The quasi-in-situ electron backscatter diffraction (EBSD) investigations show the concentration of plastic strain within preferentially oriented slip bands, with grain boundaries offering resistance, and slip band leads to changing directions as they traverse from one grain to another, with secondary slips emerging post-yielding during micropillar compression. As compression levels rise, a prominent uniform strain hardening rate emerges in grain boundaries characterized by multiple curvatures. Local resolved shear stress at grain boundaries experiences a pronounced reduction under the applied load, particularly when the serration wavelength exceeds 0.4, and the amplitude ranges from 0.3 to 0.5 times the total grain boundary length. An attempt is made here to shed light upon the underlying microscopic mechanisms that govern grain boundary micromechanics through a comprehensive three-dimensional analysis. It becomes evident that both boundary curvature and inclination of interface plane play critical roles in enhancing material strength, collectively contributing to a 21 pct increase in yield strength in current case of boundary with curvature. Additionally, these morphologies notably reduce the likelihood of heterogeneous plastic deformation compared to straight grain boundaries.

This study focuses on the kinetic analysis of sigma phase formation in filler metal wires on Super Duplex Stainless Steel (SDSS) and Hyper Duplex Stainless Steel (HDSS). Precipitation data reveal that in the solubilized microstructure, sigma phase kinetics are more prominent in SDSS. This increased susceptibility is attributed to the greater number of nucleation sites, which is facilitated by the larger interface area/volume and the higher chromium content in the ferrite. The difference in interface area/volume is significantly more influential in determining kinetics than the composition difference, with nucleation sites playing a central role. The sigma phase transformation in both materials was modeled using the JMAK kinetic law. The JMAK plots exhibit a transition in kinetic mechanisms, evolving from discontinuous precipitation to diffusion-controlled growth. In SDSS, the JMAK values indicate “grain boundary nucleation after saturation,” followed by “thickening of large plates.” In contrast, HDSS values point to “grain edge nucleation after saturation,” followed by “thickening of large needles.” The higher kinetics in SDSS are characterized by a smaller nucleation activation energy of 56.4 kJ/mol, in contrast to HDSS's 490.0 kJ/mol. CALPHAD-based data support the JMAK results, aligning with the maximum kinetics temperature of SDSS (875 °C to 925 °C) and HDSS (900 °C to 925 °C). Therefore, the JMAK sigma phase kinetics effectively describe the experimental data and its dual kinetics behavior, even though CALPHAD-based TTT calculations often overestimate sigma formation.

This study examined the effect of manganese (Mn) contents (3.12 to 8.97 pct) and three-pass deformation distributions on dynamic recrystallization (DRX) and dislocation evolution during hot compression of low-nickel duplex stainless steel (DSS). With a total deformation of 70 pct and a strain rate of 0.1 s⁻¹, the study revealed that elevated Mn content enhances the ferrite phase's DRX, whereas austenite phase's dynamic recovery (DRV) varied with the deformation pass distribution. Changing the pass sequence (30 pct-10 pct-30 pct to 10 pct-30 pct-30 pct, and subsequently to 25 pct-35 pct-10 pct) in the 8.97 pct Mn sample, a reduction in the percentage of deformed grains in the austenite phase was observed. Furthermore, performing two consecutive passes with significant deformations was not favorable for ferrite DRX. Instead, it facilitated the development of subgrains within the austenite phase. The 25 pct-35 pct-10 pct deformation pattern in the 8.97 pct Mn sample led to twin boundaries formation, refining the coarse grains but impeding DRX, resulting in numerous austenite subgrains controlled by DRV. Conversely, a 30 pct-10 pct-30 pct deformation in the 8.97 pct Mn sample encouraged ferrite grains mainly displayed {001} and {111} recrystallization textures, thereby promoting DRX nucleation. The ferrite DRX process was primarily governed by continuous dynamic recrystallization (CDRX) mechanism, leading to the development of sizable, equiaxed grains with fewer dislocations. Kurdjumov-Sachs (K-S) relationship between the two phases enables dislocation slip from austenite to ferrite during compression deformation. With the increased addition of Mn, the ferrite underwent a gradual transition from a deformation texture to a recrystallization texture, whereas the austenite continued to be predominantly characterized by a deformation texture. The dynamic softening effect exhibited a more pronounced behavior in the 30 pct-10 pct-30 pct deformation condition for the sample with 8.97 pct Mn in comparison to the other deformation conditions.