A transmission electron microscope and an atom probe tomography were used to quantitatively characterize the microstructural evolution of Al-XCu alloys (X = 1.0, 1.5, and 2.5 wt%) with 0.3 wt% Sc ...addition. A dual solute alloying/microalloying effect on the microstructural evolution was demonstrated. On the one hand, the nucleation and coarsening of Al3Sc dispersoids displayed a Cu alloying effect. By increasing the Cu content, both the Al3Sc disperoid size and the volume fraction decreased after solution treatment. On the other hand, the precipitation of θ′-Al2Cu strengthening particles during aging treatment was promoted by Sc segregation at the θ′/matrix interfaces, showing a notable Sc microalloying effect. The strongest interfacial Sc segregation was generated in the Al-2.5 wt%Cu-Sc alloy, resulting in the most promoted θ′ precipitation. The Sc partitioning between Al3Sc dispersoids and Sc segregation at the θ′/matrix interfaces, tailored by the Cu content, impacted the mechanical properties and deformation behavior at both room temperature and high temperature. The Al-2.5 wt%Cu-Sc alloy had a room temperature yield strength of approximately 2.2 times that in its Sc-free counterpart and approximately 1.8 times that in the Al-1.5 wt%Cu-Sc alloy, which is rationalized by strengthening models. In addition, the improvement in the high-temperature mechanical properties after Sc addition was discussed in terms of the Sc segregation-induced high coarsening resistance of θ′ precipitates.
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The microstructural evolution and precipitation behavior of the warm-rolled (WRed) medium Mn steels with no microalloying addition, single Nb addition and multiple Nb-Mo addition treated at 650 °C ...for various durations were investigated. Different from the lamellar microstructure of hot rolled medium Mn steels and the equiaxed microstructure of cold rolled medium Mn steels after intercritical annealing, the WRed samples of medium Mn steels exhibited the equiaxed and lamellar mixed microstructure, and the equiaxed microstructure ratio became larger with increasing durations. The addition of Nb or Nb-Mo can respectively increase yield strength of ~ 50 MPa or ~ 180 MPa in average and ultimate tensile strength of ~ 70 MPa or ~ 100 MPa in average without sacrificing the tensile ductility. The stability of austenite can be improved for the smaller average austenite grain size due to the addition of Nb or Nb-Mo. The addition of Mo to the Nb-bearing medium Mn steel can increase the density of nano-precipitates by reducing the mismatch degree between the matrix and precipitates and inhibit the coarsening rate of precipitates.
Microstructural stability is a critical factor to consider when designing new alloys for high-temperature applications. An Al-Cu alloy with Mn and Zr additions has recently been developed to ...withstand extended exposures of up to 350 °C. The addition of Mn in combination with Zr and their segregation to precipitate interfaces play a significant role in stabilizing the metastable θ′ precipitates responsible for the alloy's hardness; however, adding Zr and Mn separately only improves the stability to 200 °C and 300 °C, respectively. To this end, the effect of the synergistic additions on interfacial structure and chemistry was studied in detail using atom probe tomography (APT) and scanning transmission electron microscopy (STEM) for Al-Cu-Mn-Zr/Ti-containing alloys subjected to long-term annealing (up to 2,100 h) in the critical temperature range, 300 °C and 350 °C, to investigate the role of Zr/Ti in increasing the θ′-precipitate stability. The APT and STEM results reveal that Mn additions stabilize θ′ long enough for the slower diffusing Zr atoms to segregate to coherent θ′ interfaces that eventually create a θ′/ L12-Al3(Zrx,Ti1-x) co-precipitate structure. The co-precipitate is highly stable, as shown by density functional theory calculations, and is a key factor that governs microstructural stability beyond 300 °C. This study reveals how solute additions with different stabilization mechanisms can work in concert to stabilize a desired microstructure, and the results provide insights that can be applied to other high-temperature alloy systems.
It is one of the most effective methods to manipulate the atomic-scale structures of carbides formed by interphase precipitation to enhance the strengths of the microalloyed steel. In this study, ...conventional TEM, STEM-EDS, and HR-STEM were carried out to understand the origin of fine carbide precipitation in V-Nb multiple microalloyed ferritic steels. TEM characterizations showed that the multiple additions of V and Nb resulted in B1 structured alloy carbides with (V,Nb)C-cores and VC-shells, resulting in denser carbide precipitation compared to single additions of these elements. Thermo-Calc calculations and TEM results suggest that the precipitation kinetics of the (V,Nb)C-cores depends on the balance between its driving force and the misfit strain with the surrounding matrix, while that of the VC-shells is achieved by aging precipitation.
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The effects of microalloying with Mg (0–0.23 wt%) on the microstructural evolution and mechanical properties of Al–Cu 224 cast alloys at ambient and elevated temperatures are investigated using ...transmission electron microscopy, differential scanning calorimetry, and tensile/compression testing. The results show that microalloying with Mg significantly enhances the precipitation of the θ′ phase during aging, producing fine, dense, and uniformly distributed θ′ precipitates. These precipitates are much more effective for alloy strengthening than are the θ″ precipitates in the alloy without Mg. During stabilization at 300 °C for 100 h, the dominant process becomes coarsening of the θ′ phase. The Mg-containing alloys have much finer and denser θ′ precipitates and thus considerably higher yield strengths at elevated temperature as compared to those of the alloy without Mg. The improvement is more pronounced at low Mg contents (0.09%–0.13%) than at high contents. The yield strength at 300 °C of the 0.13% Mg alloy is as high as 140 MPa, which is far superior to that of most cast aluminum alloys. Moreover, the enhanced yield strength of this alloy is well preserved during prolonged exposure at 300 °C for 1000 h, indicating that it is a promising lightweight material for high-temperature applications.
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•Effect of Mg on microstructure and room-/high-temperature strength of Al–Cu alloy.•Enhanced precipitation of the θ′ phase by microalloying with Mg.•Upon exposure at 300 °C for 100 h, the best characters of θ′ is obtained at 0.13% Mg addition.•Strength at 300 °C of Al–Cu alloy with 0.13% Mg highly exceeds that of most Al alloys.•Enhanced strength of alloy remains well preserved even after exposure at 300 °C for 1000 h.
This study focuses on improving the strength-toughness properties of low carbon bainitic steel through the optimization of the isothermal quenching process and the adoption of a microalloying ...strategy. The impact of Nb microalloying on the microstructure and mechanical properties was thoroughly investigated. The optimal isothermal quenching process for achieving a balance between strength and toughness is as follows: the steels were austenitized at 900 °C for 30 min and then subjected to an isothermal treatment at 450 °C for 10 min. The resulting microstructure consisted mainly of granular bainite (GB) and lath bainite (LB), with GB accounting for 60 % of the structure. After the optimized isothermal quenching process, the ultimate tensile strength and yield strength of the Nb microalloyed steel increased to 1210 MPa and 693 MPa, respectively. The impact energy, KU2, was measured at 41.8 J. The study revealed that Nb microalloying refines the bainite structure by reducing activation energy and increasing the nucleation rate. This refinement was observed in the reduced length and thickness of the bainitic laths, as well as the increased dislocation density. Additionally, fine composite precipitated particles (Nb, V)C with an average diameter of 8.2 nm were dispersed throughout the microstructure. The addition of Nb significantly enhanced the effects of fine grain strengthening, precipitation strengthening, and dislocation strengthening in low carbon bainitic microalloyed steel. As a result, the strength was greatly improved without sacrificing toughness, thanks to the combined actions of these three strengthening mechanisms.
•The microstructure type with the best strength-toughness matching: 60 % GB+40 %LB.•Nb enhanced the effect of fine grain, dislocation and precipitation strengthening.•Nb refined bainite by reducing activation energy and increasing nucleation rate.•The strength was greatly improved while the toughness was not reduced.
Currently one major barrier for the design of cast high-strength and high-toughness Al alloys is hot tearing. In this work, a strategy of combination of squeeze casting and microalloying (Ca/Ni ...eutectic elements) in Al-Cu-Mn based alloys was employed to inherit the excellent mechanical properties of Al-Cu alloys whilst simultaneously reducing their tendency to hot tearing. The developed alloys exhibit comparable castability (fluidity and hot tear resistance) to A356, which is widely available commercially. However, their comprehensive mechanical properties far exceed those of A356. Trace Ca/Ni additions markedly reduce the grain size of the alloy and simultaneously increase the fraction of intergranular low melting point eutectic liquid phases, which is beneficial to the improvement of liquid feeding. The fine equiaxed grains have excellent thermal shrinkage coordination, while the high volume fraction eutectic liquid phase delays the development of grain cohesive skeleton. Accordingly, the localized shrinkage stress/strain at the hot spot is released timely, thus inhibiting the emergence and propagation of hot cracks in the brittle solidification interval. The diminished hot tearing susceptibility is attributed to the reduced load onset temperature and load values in the hot spot region. A new intergranular bridging mechanism is discovered in low-Ca alloys: based on compositional segregation-induced nucleation of an intergranular bridging skeleton, which strengthens the intergranular adhesive force and effectively reduces the hot tearing tendency of the alloys. The alloys with high-Ca/Ni additions, however, reduce the hot tearing tendency by healing cracks through eutectic liquid phase backfill. The results of the hot tearing experiments of the developed alloys are analyzed with the current hot tearing evaluation criteria. The results demonstrate that the predictions of the Kou's criterion provide guidelines for the design of alloys that are resistant to hot tearing.
•Trace amounts of Ca and Ni elements dramatically reduce the hot tearing tendency of the matrix alloy.•The Ca/Ni rich eutectic phase retards the formation of brittle dendritic skeleton and inhibits the initiation and extension of hot cracks.•Eutectic colonies induce the development of bridges between grains and enhance the cohesion of grain boundaries.
Fe-based amorphous materials with the superiorities of high efficiency and low Fe sludge secondary pollution can be seen as a potential alternative to Fenton catalysts. However, catalyst deactivation ...on the catalyst surface hinders the sustainability of high degradation activity. In this study, the degradation capabilities of FeBCCr amorphous alloys in methylene blue (MB) solution by Fenton-like method were explored. The optimum (Fe81B10C9)99Cr1 ribbon with a more uniform Fe3B-like amorphous structure, which exhibits a thin and easily-peeled oxide layer, provides unique dynamic self-renewing to obtain excellent efficiency and recyclability. The degradation rate with 0.05 g/L (Fe81B10C9)99Cr1 ribbon reached 0.248 min−1 and the recyclability reached a charming record of over 90 times with a dosage of 0.1 g ribbon. The activation energy of (Fe81B10C9)99Cr1 ribbon (12.97 kJ mol−1) was found to be superior to that of most metallic catalysts. More importantly, the boron-rich film in the interfacial region can both retard further oxidation of iron and provides a channel for electron transmission, which can contribute to the persistence of a highly active microenvironment with more Fe(Ⅱ) sites and highly utilization of H2O2. The limit segmentation method has verified the good performance in both heterogeneous and homogeneous reactions. This study introduces a strategy for designing high-performance non-noble metallic catalysts through cluster structure regulation, which provides a novel Fenton-like amorphous material with long recyclability of high degradation activity for water remediation.
•B and C atoms coordination with Cr atoms to promote the formation of a more homogeneous structure.•The more homogeneous structure resulting in a thinner oxide layer to expose more active sites.•An Fe-based amorphous catalyst with excellent recycling performance and stable reaction rate.
The effects of Nb on the microstructure and mechanical properties of Ti-Mo microalloyed high strength ferritic steel were systematically investigated. It was found that Nb addition inhibited the ...bainite transformation and was beneficial for obtaining ferritic steel. Further, Nb accelerated the precipitation in austenite and finer ferrite grains were obtained in Ti-Nb-Mo steel. The strengthening mechanism analysis results of experimental steels showed that the precipitation hardening effect of the (Ti, Mo, Nb)C particles was remarkable due to their larger volume fraction, although their average particle size was greater than that of (Ti, Mo)C in Ti-Mo steel. With the decrease of coiling temperature from 650 °C to 550 °C, the grain refinement strengthening and precipitation hardening of experimental steels increased. When the coiling temperature was 550 °C, the ferritic steel with superior mechanical properties was obtained by means of Ti-Nb-Mo complex microalloying. And the tensile strength, yield strength, and elongation of the Ti-Nb-Mo ferritic steel were 755 MPa, 712 MPa, and 22%, respectively.
•Co-segregation of dopant al (Cr or Ni) and impurity O in UN Σ5(210) grain boundary is analyzed by first-principles.•Grain boundary decorated with Ni atom exhibits powerful antioxidant activity but ...promotes the embrittling sensitivity.•Dopant Cr not only effectively reduces o segregation but also positively enhances the cohesive capability of oxidized grain boundary.•Ni has the strongest capacity to reduce the O concentration of grain boundary, followed by Cr and Al.
The oxidation susceptibility of UN Σ5(210) grain boundary (GB) decorated with Al, Cr or Ni is assessed by first-principles modelling. The predictions show that the low segregation energy of O at the Al-doped GB leads to increasing O enrichment, which explains the high O concentration observed in experiments. GB with Ni exhibits formidable antioxidant capability but enhances sensitivity to embrittlement. Cr dopant not only effectively reduces O segregation but also improves the cohesive capability of oxidized GB. Our work clarifies the mechanisms of (un)-doped UN GB oxidation from an atomic perspective, in which analysis so far rests on experiments only.
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