Research

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14 pages, 26842 KiB

Open AccessArticle

Microstructural Evaluation and Tensile Properties of Al-Mg-Sc-Zr Alloys Prepared by LPBF

by Yuxian Lu, Hao Zhang, Peng Xue, Lihui Wu, Fengchao Liu, Luanluan Jia, Dingrui Ni, Bolv Xiao and Zongyi Ma

Crystals 2023, 13(6), 913; https://doi.org/10.3390/cryst13060913 - 05 Jun 2023

Cited by 2 | Viewed by 1641

Abstract

Laser powder bed fusion (LPBF) is a typical additive manufacturing technology that offers significant advantages in the production of complex components. With the rapid heating and cooling characteristics of LPBF, a large amount of solid solution of alloying elements in the matrix can [...] Read more.

Laser powder bed fusion (LPBF) is a typical additive manufacturing technology that offers significant advantages in the production of complex components. With the rapid heating and cooling characteristics of LPBF, a large amount of solid solution of alloying elements in the matrix can be achieved to form supersaturated solid solutions, thus enhancing the properties of LPBF alloys. For the unique microstructure, the heat treatment process needs to be adjusted accordingly. In this work, a Zr/Sc-modified Al-Mg alloy processed by laser powder bed fusion (LPBF) with relatively low cost and good mechanical properties was investigated. The fine microstructure was obtained under rapid solidification conditions. The nanoscale Al₃(Sc,Zr) particles precipitated at the molten pool boundary during solidification. These particles, as effective heterogeneous nucleators, further refined the α-Al grains and improved the mechanical properties of the alloy. As a result, the alloy exhibited a heterogeneous microstructure consisting of columnar grains in the center of the molten pool and equiaxed grains at the boundaries. The rapid solidification resulted in the supersaturation of solute atoms in the α-Al matrix, which significantly enhanced the solid solution strengthening effect. With the LPBF processing parameters of a combination of a laser power of 250 W, a laser scanning speed of 833 mm/s, and stripe scanning mode, the tensile strength of the alloy reached 401.4 ± 5.7 MPa, which was significantly higher than that of the cast alloys with aging treatment (281.1 ± 1.3 MPa). The heat treatment promoted the formation of secondary Al₃(Sc,Zr), Mn/Mg-rich phases. The ultimate tensile strength and elongation at fracture after aging at 325 °C for 2 h were 536.0 ± 1.7 MPa and 14.8 ± 0.8%, respectively. The results provide insight into the preparation of aluminum alloys with relatively low cost and excellent mechanical properties. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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21 pages, 52467 KiB

Open AccessArticle

New EoL Routes of Al-Li Aircraft Integral LBW and FSW Welded Panels including New Cr-Free Coatings

by Ana Isabel Fernández-Calvo, Clara Delgado, Philippe Dufour, Egoitz Aldanondo, Mario Díaz and María Belén García-Blanco

Crystals 2023, 13(5), 807; https://doi.org/10.3390/cryst13050807 - 11 May 2023

Cited by 2 | Viewed by 1155

Abstract

The end of fife (EoL) of new aircraft panels made of Al-Li alloys in which the stringers and skin were joined, either by laser beam welding (LBW) or by friction stir welding (FSW), was investigated at the lab scale. Different cutting strategies, ranging [...] Read more.

The end of fife (EoL) of new aircraft panels made of Al-Li alloys in which the stringers and skin were joined, either by laser beam welding (LBW) or by friction stir welding (FSW), was investigated at the lab scale. Different cutting strategies, ranging from cutting only for size reduction to full separation of all materials, including the removal of the welded seam, were defined, with the objective of recycling the maximum amount of panel scrap back into high-quality aircraft Al-Li alloys. Those welded aerostructures were coated with two novel Cr-free coating systems. The effect of the coatings on the recyclability of the panels and the need to eliminate the primer and topcoats were researched. Fading/enrichment of the alloying elements during recycling was determined. The chemical compatibility of the recycled alloys with four commercial Al-Li alloys was examined. The EoL route that maximized closed-loop recycling and the conservation of the valuable alloying elements was identified. Nine out of the ten configurations were found to be compatible with joint recycling. Only the LBW structure with ER4047 filler wire required sorting into scrap fractions and removing the weld seam. Decoating by corundum blasting followed by cutting before remelting is the recommended EoL process. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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12 pages, 10875 KiB

Open AccessArticle

Microstructure Evolution in a β-γ TiAl Alloy during Hot Deformation under Variable Conditions

by Guoju Li, Zhanxing Chen, Yupeng Wang, Xinzhe Zhang, Qiuwei Xing, Xinfang Zhang, Chunyuan Yuan and Tengfei Ma

Crystals 2023, 13(5), 742; https://doi.org/10.3390/cryst13050742 - 28 Apr 2023

Viewed by 1069

Abstract

In contrast to practical hot compression processes, the testing of the hot workability of TiAl alloys is usually conducted under the conditions of constant strain rates and constant temperatures. This work aims at investigating the microstructural evolution of TiAl alloys on a Gleeble-3800 [...] Read more.

In contrast to practical hot compression processes, the testing of the hot workability of TiAl alloys is usually conducted under the conditions of constant strain rates and constant temperatures. This work aims at investigating the microstructural evolution of TiAl alloys on a Gleeble-3800 thermomechanical simulator under a variable strain rate (0.1, 0.01 and 0.001 s⁻¹) at 1200 °C. The results show that, after a holding time of 30 s, the abrupt change in the strain rate at ε = 0.3 (engineering strain) has a remarkable influence on the flow stress and dynamic recrystallization (DRX) behavior of the β-γ Ti-44Al-6Nb-1Mo-0.3 (B, Y, La, Ce) (at.%) alloy. The flow stress demonstrates a rapid decrease with a sudden reduction in the strain rate. A duplex microstructure of γ + B2/β can be obtained under a high strain rate or continuous medium strain rate. During the two-step deformation, however, both γ→α phase transformation and DRX exist, and the content of the α phase demonstrates a significant increase when the strain rate becomes lower. Finally, a fine-grained structure of γ + B2/β + α₂ phases with low residual stresses can be obtained via the two-step heat treatment processes. This provides a promising approach to significantly improve the hot workability of β-γ TiAl alloys. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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15 pages, 5767 KiB

Open AccessArticle

Crystal Structure and Properties of Zinc Phosphate Layers on Aluminum and Steel Alloy Surfaces

by Beáta Herbáth, Kristóf Kovács, Miklós Jakab and Éva Makó

Crystals 2023, 13(3), 369; https://doi.org/10.3390/cryst13030369 - 21 Feb 2023

Cited by 3 | Viewed by 2582

Abstract

Many studies have been carried out on the phosphating of steel and aluminum alloys used in automotive engineering, but characterization of the properties of the phosphate layers formed by the co-phosphating of these alloys in the presence of different base metals is still [...] Read more.

Many studies have been carried out on the phosphating of steel and aluminum alloys used in automotive engineering, but characterization of the properties of the phosphate layers formed by the co-phosphating of these alloys in the presence of different base metals is still lacking. In this study, the crystal structure and properties of the phosphate conversion layers formed on the surface of the aluminum alloys important in vehicle manufacturing (cast and forged AlSi1MgMn, and AA6014 panel) and the CRS SAE 1008/1010 reference steel plate by co-deposition prior to painting were investigated. On a process line set up for the phosphating of typical iron and steel alloys, the phosphate coating was formed using nitrite and nitroguanidine accelerators under identical technological parameters. The microstructure of the formed phosphate layers was examined using scanning electron microscopy (SEM), its phase composition using X-ray diffraction (XRD), and its elemental composition using energy-dispersive X-ray analysis (EDX). The suggested main crystalline phase (Zn_2.3(Ni_0.1Mn_0.6)(PO₄)₂·4H₂O) in the surface phosphate layer of both aluminum alloys studied was similar to hopeite, whereas in the steel plate, a minor hopeite phase were identified in addition to the main crystalline phosphophyllite phase (~95%). It can be concluded that, during the combined phosphating treatments, the surfaces of different aluminum and steel alloys behaved similarly to the individual treatments and did not impede the coating reactions of the other metal. To obtain an adequate coating of aluminum and steel alloys, fluoride should always be present in the production line. Comparing the effects of accelerators, we found that the use of nitrite accelerator with the same amount of fluoride resulted in a higher coverage and better quality of the surface protective layer of the aluminum alloys. However, for the steel plate, there was no significant difference between the phosphate coatings prepared with the two different accelerators. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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17 pages, 9430 KiB

Open AccessArticle

Low-Cycle Fatigue Behavior of Hot V-Bent Structural Components Made of AZ31B Wrought Magnesium Alloy

by Florian Mader, Anton Nischler and Otto Huber

Crystals 2023, 13(2), 184; https://doi.org/10.3390/cryst13020184 - 20 Jan 2023

Viewed by 1314

Abstract

Recent studies have shown the change of microstructure during hot-bending in uniaxial specimens made of AZ31B alloy. They also investigated the influence of the changed microstructure on the quasi-static and cyclic material behavior under uniaxial stress states. However, studies on the fatigue behavior [...] Read more.

Recent studies have shown the change of microstructure during hot-bending in uniaxial specimens made of AZ31B alloy. They also investigated the influence of the changed microstructure on the quasi-static and cyclic material behavior under uniaxial stress states. However, studies on the fatigue behavior of hot-bent structural components in which a multiaxial inhomogeneous stress state occurs are still lacking. For this purpose, a novel hot-bent V-shaped specimen was developed, of which three different variants, each with a different bending radius, were fabricated and investigated. Microstructural analyses reveal that band-like accumulations of twinned grains are already formed in the compressively stressed area of the specimen during the bending process. Force-controlled low-cycle fatigue tests were performed to investigate the twinning evolution after cyclic loading. Subsequent microstructure analyses show that bands of twinned grains are no longer visible but also that the occurrence of twins is evenly distributed. Due to the specimen shape, the specimens are subjected to a multiaxial stress state. During LCF tests, the strain was measured using 3D digital image correlation and fatigue life was modeled successfully with the application of the concept of highly strained volume. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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12 pages, 4374 KiB

Open AccessEditor’s ChoiceArticle

The Microstructures and Mechanical Properties of a Welded Ni-Based Hastelloy X Superalloy

by Yuan Liu, Qingqing Ding, Xiao Wei, Yuefei Zhang, Ze Zhang and Hongbin Bei

Crystals 2022, 12(10), 1336; https://doi.org/10.3390/cryst12101336 - 21 Sep 2022

Cited by 2 | Viewed by 1952

Abstract

The Hastelloy X superalloy is a widely used solid-solution Ni-based sheet alloy for gas turbines, aero-engine combustion chambers, and other hot-end components. To investigate the effect of microstructure, especially grain size, on its weldability, Hastelloy X alloy bars are homogenized, cold-rolled to thin [...] Read more.

The Hastelloy X superalloy is a widely used solid-solution Ni-based sheet alloy for gas turbines, aero-engine combustion chambers, and other hot-end components. To investigate the effect of microstructure, especially grain size, on its weldability, Hastelloy X alloy bars are homogenized, cold-rolled to thin sheets, and recrystallized under different conditions to obtain equiaxed grain microstructures with average grain sizes of ~5 μm, ~12 μm, and ~90 μm. The laser welding process is used for joining the alloy sheets, and then the alloy’s weldability is investigated through microstructural and mechanical property characterizations. The microstructures in weld consist of coarse columnar grains with dendrite, and grain sizes of these columnar grains are almost the same when grain size of Hastelloy X base metal increases from ~5 μm to ~90 μm. Moreover, although all welds exhibit lower yield strengths (YS), ultimate tensile strengths (UTS), and elongations to fracture (EF) than the base metal, the degrees of reduction in them become slight when the grain size of base metal increases from ~5 μm to ~90 μm. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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11 pages, 9556 KiB

Open AccessArticle

Investigating the Microstructure, Crystallographic Texture and Mechanical Behavior of Hot-Rolled Pure Mg and Mg-2Al-1Zn-1Ca Alloy

by Umer Masood Chaudry, Kotiba Hamad and Tea-Sung Jun

Crystals 2022, 12(10), 1330; https://doi.org/10.3390/cryst12101330 - 21 Sep 2022

Cited by 7 | Viewed by 1314

Abstract

In this study, the microstructure, crystallographic texture and the mechanical performance of hot-rolled pure Mg and Mg-2Al-1Zn-1Ca (herein inferred as AZX211) were thoroughly investigated. The results showed that the designed AZX211 alloy exhibited an exceptional strength/ductility synergy where an almost 40% increase in [...] Read more.

In this study, the microstructure, crystallographic texture and the mechanical performance of hot-rolled pure Mg and Mg-2Al-1Zn-1Ca (herein inferred as AZX211) were thoroughly investigated. The results showed that the designed AZX211 alloy exhibited an exceptional strength/ductility synergy where an almost 40% increase in ductility was received for AZX211. The microstructural characterization revealed the grain refinement in the AZX211, where the grain size was reduced by more than 50% (24.5 µm, 10 µm for the pure Mg and the AZX211, respectively). Moreover, a discernible number of precipitates were dispersed in the AZX211, which were confirmed to be (Mg, Al)₂Ca. The pure Mg showed a conventional strong basal texture while a significantly weakened split basal texture was received for the AZX211. The fraction of basal-oriented grains was 21% for the pure Mg and 5% for the AZX211. The significant texture weakening for the AZX211 can be attributed to the precipitation and co-segregation that triggered the preferential evolution of the non-basal grains while impeding the growth of the basal grains. This was also confirmed by the crystal orientation and the pseudo-rocking curves. The higher ductility of the AZX211 was explained based on the texture softening and Schmid factor for the basal and non-basal slip systems. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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16 pages, 5303 KiB

Open AccessArticle

Interpretable Machine Learning Analysis of Stress Concentration in Magnesium: An Insight beyond the Black Box of Predictive Modeling

by Russlan Jaafreh, Jung-Gu Kim and Kotiba Hamad

Crystals 2022, 12(9), 1247; https://doi.org/10.3390/cryst12091247 - 02 Sep 2022

Cited by 2 | Viewed by 1663

Abstract

In the present work, machine learning (ML) was employed to build a model, and through it, the microstructural features (parameters) affecting the stress concentration (SC) during plastic deformation of magnesium (Mg)-based materials are determined. As a descriptor for the SC, the kernel average [...] Read more.

In the present work, machine learning (ML) was employed to build a model, and through it, the microstructural features (parameters) affecting the stress concentration (SC) during plastic deformation of magnesium (Mg)-based materials are determined. As a descriptor for the SC, the kernel average misorientation (KAM) was used, and starting from the microstructural features of pure Mg and AZ31 Mg alloy, as recorded using electron backscattered diffraction (EBSD), the ML model was trained and constructed using various types of ML algorithms, including Logistic Regression (LR), Decision Trees (DT), Random Forest (RF), Naive Bayes Classifier (NBC), K-Nearest Neighbor (KNN), Multilayer Perceptron (MLP), and Extremely Randomized Trees (ERT). The results show that the accuracy of the ERT-based model was higher compared to other models, and accordingly, the nine most-important features in the ERT-based model, those with a Gini impurity higher than 0.025, were extracted. The feature importance showed that the grain size is the most effective microstructural parameter for controlling the SC in Mg-based materials, and according to the relative Accumulated Local Effects (ALE) plot, calculated to show the relationship between KAM and grain size, it was found that SC occurs with a lower probability in the fine range of grain size. All findings from the ML-based model built in the present work were experimentally confirmed through EBSD observations. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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10 pages, 8432 KiB

Open AccessArticle

Features of the Process Obtaining of Mg-Zn-Y Master Alloy by the Metallothermic Recovery Method of Yttrium Fluoride Melt

by Sergey Savchenkov and Ilia Beloglazov

Crystals 2022, 12(6), 771; https://doi.org/10.3390/cryst12060771 - 26 May 2022

Cited by 6 | Viewed by 2010

Abstract

At present, magnesium master alloys with such rare earth metals (REM) as yttrium are used in the production of alloys of magnesium and aluminum. These alloys especially the system Mg-6Zn-1Y-0,5Zr are commonly used in the aircraft and automotive industries. The article is devoted [...] Read more.

At present, magnesium master alloys with such rare earth metals (REM) as yttrium are used in the production of alloys of magnesium and aluminum. These alloys especially the system Mg-6Zn-1Y-0,5Zr are commonly used in the aircraft and automotive industries. The article is devoted to the exploration of the synthesis process features for ternary magnesium master alloys with yttrium and zinc. The authors used X-ray fluorescence analysis (XRF), differential thermal analysis (DTA), and X-ray spectral analysis (XRD). Optical microscopy was used to conduct microstructural studies. The thermal effects that occur during metallothermic reactions of yttrium reduction from the YF₃-NaCl-KCl-CaCl₂ salt mixture with a melt of magnesium and zinc were investigated, and the temperatures of these effects were determined. It has been confirmed that the metallothermic reaction of yttrium reduction proceeds from the precursors of the composition: Na_1.5Y_2.5F₉, NaYF₄, Na₅Y₉F₃₂, and KY₇F₂₂, and starts at a temperature of 471 °C. The results of experimental studies of the process of metallothermic reduction of yttrium from the salt mixture YF₃-NaCl-KCl-CaCl₂ are presented in detail. These experiments were carried out in a pit furnace at temperatures ranging from 650 to 700 °C, and it was found that, at a synthesis temperature of 700 °C, the yttrium yield is up to 99.1–99.8%. The paper establishes rational technological regimes for the synthesis (temperature 700 °C, exposure for 25 min, the ratio of chlorides to yttrium fluoride 6:1, periodic stirring of the molten metal) at which the yttrium yield reaches up to 99.8%. The structure of the master alloy samples obtained during the experiments was studied. That structure can be distinguished by a uniform distribution of ternary intermetallic compounds (Mg₃YZn₆) in the bulk of the double magnesium–zinc eutectic. Studies have been carried out on testing the obtained ternary master alloy as an alloying material in the production of alloys of the Mg-6Zn-1Y-0.5Zr system, while the digestibility of yttrium ranged from 91 to 95%. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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24 pages, 4198 KiB

Open AccessArticle

Reaction Sintering of MgAlON at 1500 °C from Al₂O₃, MgO and AlN and Its Wettability by AlSi7Mg

by Alina Schramm, Martin Thümmler, Olga Fabrichnaya, Simon Brehm, Jakob Kraus, Jens Kortus, David Rafaja, Christiane Scharf and Christos G. Aneziris

Crystals 2022, 12(5), 654; https://doi.org/10.3390/cryst12050654 - 03 May 2022

Cited by 2 | Viewed by 1977

Abstract

The aim of this study is the investigation of a technological synthesis of MgAlON, which is a prospective coating material on ceramic foam filters for the filtration of magnesium, aluminum, and other metal melts. Thermodynamic calculations are performed, and the synthesis is carried [...] Read more.

The aim of this study is the investigation of a technological synthesis of MgAlON, which is a prospective coating material on ceramic foam filters for the filtration of magnesium, aluminum, and other metal melts. Thermodynamic calculations are performed, and the synthesis is carried out at 1500 °C in nitrogen atmosphere using samples consisting of different fractions of Al₂O₃, MgO, and AlN as starting materials. The effect of the quantity of these components on the conversion degree of the educts is evaluated. Furthermore, the effect of the holding time at the synthesis temperature, as well as the composition points or regions showing the highest conversion degree, are determined. XRD analysis is performed to evaluate the phase fractions and lattice parameters of the spinel after the respective reaction, and the nitrogen content of selected samples is evaluated. Sessile drop tests using AlSi7Mg are performed at 950 °C on selected sintered samples, determining their wettability, and therefore, applicability of the material in light metal melt filtration. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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14 pages, 7474 KiB

Open AccessArticle

Effects of Orientations, Roughnesses, and Cavities on Stress-Corrosion Coupled Damage in Magnesium

by Jianhui Ye, Shijie Bai, Xinke He, Jianbin Chen and Chao Xie

Crystals 2022, 12(5), 635; https://doi.org/10.3390/cryst12050635 - 28 Apr 2022

Cited by 1 | Viewed by 1422

Abstract

Orientations, roughnesses, and cavities of crystals are typical factors influencing the servicing reliability of metals in corrosive environments. A phase-field scheme for modeling stress-corrosion coupled damage (SCCD) is developed. The effects of the crystal factors on SCCD are numerically simulated using the incremental-iterative [...] Read more.

Orientations, roughnesses, and cavities of crystals are typical factors influencing the servicing reliability of metals in corrosive environments. A phase-field scheme for modeling stress-corrosion coupled damage (SCCD) is developed. The effects of the crystal factors on SCCD are numerically simulated using the incremental-iterative scheme of the user-defined finite elements. The impacts of orientations, roughnesses, and cavities on the corrosion rate of magnesium (Mg) in corrosive environments are discussed quantitatively. It is found that crystal textures and surface roughnesses can significantly influence the diffusion-controlled corrosion rate. Strong basal texture and a smooth surface of the crystal can significantly enhance the corrosion resistance of Mg. The cavity, as a typical crystal defect, is capable of inducing the damage path and modulating the corrosion rate. The design of crystal-scale features, such as orientations, roughnesses, and cavities, is promising for the enhancement of the resistance to SCCD. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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Review

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30 pages, 5076 KiB

Open AccessReview

Platinum Group Metals: Green Recovery from Spent Auto-Catalysts and Reuse in New Catalysts—A Review

by Maria Luisa Grilli, Anca Elena Slobozeanu, Claudio Larosa, Daniela Paneva, Iakovos Yakoumis and Zara Cherkezova-Zheleva

Crystals 2023, 13(4), 550; https://doi.org/10.3390/cryst13040550 - 23 Mar 2023

Cited by 10 | Viewed by 4897

Abstract

This manuscript reviews the current trends in the recovery of Platinum Group Metals (PGMs) from end-of-life autocatalysts and the aims of the recently funded Marie Sklodowska-Curie Project “Chemistry of Platinum Group Metals-CHemPGM” towards the greening of PGMs recovery processes and the reusing of [...] Read more.

This manuscript reviews the current trends in the recovery of Platinum Group Metals (PGMs) from end-of-life autocatalysts and the aims of the recently funded Marie Sklodowska-Curie Project “Chemistry of Platinum Group Metals-CHemPGM” towards the greening of PGMs recovery processes and the reusing of recovered PGMs for preparation of new catalysts. Together with the analysis of the state of the art recovery of PGMs from spent autocatalysts through pyrometallurgical and hydrometallurgical routes and the recent trends in reducing their environmental impact, also emerging sustainable and green technologies are analyzed. Particular focus is given on the mechanochemical processing as a promising sustainable route not only for the pretreatment of waste materials but also for direct PGMs leaching. The present review identifies also the trends in catalysts for carbon neutrality and the few recent efforts in developing PGM-based catalysts starting directly from the liquor of the leach solutions of spent catalysts envisaging therefore a possible key to close PGMs loop in a more efficient and sustainable way. Full article

(This article belongs to the Special Issue High-Tech Metals - Overview, Microstructure, Properties and Recyclability)

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