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.
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.
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.
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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Microalloyed Mo5SiB2 alloys were fabricated by spark plasma sintering (SPS). The effects of different sintering temperatures and yttrium additions on the microstructure and high-temperature oxidation ...behavior of the alloys were systematically investigated. For the optimum Mo-13Si-25B-0.2Y alloy sintered at 1750 °C, adding small amounts of rare-earth yttrium refined the grains to an average size of ~400 nm without reducing the Mo5SiB2 content. During oxidation at 1300 °C, the “fishlike” Y2Si2O7 and Y2O3 phases were formed and anchored on the surface of the oxide layers, promoting the oxidation of silicon and accelerating the formation and coverage of the oxide protective film. Moreover, yttrium promoted the in situ deformation of a self-generated protective borosilicate glass film under high-temperature oxidation by accelerating Si oxidation. With prolonged oxidation time, viscous flow occurred to repair the internal cracks and tiny defects caused by the volatilization of MoO3 and prevented the inward diffusion of oxygen, as evidenced by a low oxidation rate constant of 0.95 mg2 cm−4 h−1 at 1300 °C for 50 h in the Mo-13Si-25B-0.2Y alloy.
•Yttrium doped Mo-13Si-25B alloy was prepared by SPS.•The yttrium-containing borosilicate film was formed in situ at 1300 °C.•Yttrium addition promoting the oxidation of silicon and accelerating the formation and coverage of the oxide protective film.•This oxide film own the ability of self-recovery at high temperatures.•This alloy displays excellence high-temperature oxidation resistance.
In this study, cobalt (Co) particles were incorporated as reinforcing agents into the Sn58Bi solder. The solderability of the solder and the strength of the solder joints were investigated. The ...results indicated that adding Co particles did not alter the low melting point characteristics of the Sn58Bi solder, yet its undercooling was reduced, and the wettability was enhanced. In the Sn58Bi/Cu solder joint, adding Co led to forming free (Cu, Co)6Sn5 intermetallic compound (IMC) and generating the CoSn2 phase through microalloying. The interfacial IMC was gradually transformed from a fan-shaped Cu6Sn5 to a flatter layer-like (Cu, Co)6Sn5 IMC, eventually exhibiting a coral-like structure with increased Co content. Adding Co particles increased the thickness of the IMC layer, but at low levels, the Co could refine the IMC grains. Simultaneously, implementing grain refinement and Orowan strengthening resulted in heightened microhardness and shear strength. In the shear test, the fracture of solder joints predominantly transpired within the solder matrix. Excessive Co particles were prone to aggregating within the solder, forming irregular block-shaped (Cu, Co)6Sn5. Concurrently, the coral-like IMC at the interface led to a decline in the mechanical performance of the solder joints, with some interfacial IMC being exposed in the fracture region.
•Co particles boosted Sn58Bi solder wettability, cut undercooling; induced (Cu, Co)6Sn5 and CoSn2 phases, transformed IMCs.•Interfacial IMC shifted from Cu6Sn5 to (Cu, Co)6Sn5, coral-like; Co addition thickened IMC, refined grains, enhancing mechanical properties.•This study showed Co's strengthening effect on solder joints via microalloying and particle reinforcement.
In the present work, martensitic stainless steel (MSS) coatings with niobium (Nb) contents of 0–1.0 wt%, combining high strength and ductility with improved corrosion resistance, have been developed ...and fabricated by a laser cladding technique. The effects of Nb-microalloying on the microstructure and properties of the laser-cladded MSS coatings have been carefully investigated. The results show that the original Nb-free MSS coating is mainly composed of martensite and austenite, along with nano-M2N and M23C6. With the addition of Nb, the lath-shaped martensite is refined, and an increasing occurrence of nano-sized Nb-enriched precipitates (NbC and Nb(C,N)) is observed in the coatings with increasing Nb content. With the successive addition of Nb, the microhardness of the specimens is found to increase from the 437 HV0.2 of the Nb-free coating to the 502 HV0.2 of the 1.0 wt% Nb-alloyed coating. At the same time, the tensile mechanical properties and corrosion resistance are found to increase with increasing Nb content of up to 0.5 wt% and subsequently decrease with further addition of Nb. Thus, the addition of 0.5 wt% Nb provides a laser-cladded MSS coating that is superior to those reported in the literature, combining the optimum tensile properties of high strength (1780 MPa) and high ductility (EL = 10.5%) along with excellent corrosion resistance. The remarkable overall properties of the as-prepared MSS coatings can be ascribed to the combined effects of solid solution, microstructural refinement, and nano-precipitation.
•Fabricate novel Fe-based laser-cladded MSS coatings by microalloying Nb (0–1.0 wt%).•Develop a 0.5 wt% Nb microalloyed MSS coating with striking comprehensive properties.•Explore the effects of Nb contents on microstructure and properties of MSS coatings.•Elucidate microstructural mechanism related to significant enhancement of property.
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