Compared with the sole Ca or Ce/La element, the Ca-Ce/La synergistic microalloying represents an excellent strengthening effect in extruded Mg–Zn alloy, which is mainly attributed to the grain ...refinement during the extrusion process. Ca-Ce/La addition results in the refinement of the secondary phases and precipitates, promoting the pinning effect for dynamic recrystallized grain growth and dispersion strengthening effect, which also contributes to the increased yield stress. Furthermore, Ca-Ce/La combined addition leads to the more uniform distributions of fine secondary phases, which can improve the ductility and promote the particle stimulated nucleation effect, resulting in a weak basal texture in extruded Mg–Zn alloy.
The segregation behavior of Mg and Ag in the T1 precipitates of an Al-Cu-Li-based alloy has been investigated using high angle annular dark-field scanning transmission electron microscopy ...(HAADF-STEM), energy dispersive X-ray spectroscopy (EDX) and first-principles calculation. For minimal-thickness T1 precipitates, Mg atoms display a preference for occupying Li sites at the precipitate center, while Ag atoms partially replace Al along the broad T1/Al interface. For thicker T1 precipitates, the Ag segregation behavior is correlated with the precipitate structure: standard T1 precipitates are decorated with this element only at the broad interface, whereas Ag is allowed also in interior Al-Li layers for the alternative T1 variant where such layers are paired rather than shared. This “branched” segregation behavior may serve to explain previous experimental disagreements, while further highlighting how microalloying has the potential ability to affect relative stabilities for different variants of the main hardening precipitate in the Al-Cu-Li alloy system.
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The effect of Nb microalloying on microstructure evolution and mechanical properties of 0.02C–7Mn steel was investigated. The results showed that the microstructure was fully quenched martensite ...after subjected to reheating and quenching (RQ) process. The microstructure consisted of tempered α-martensite, reversed austenite (RA), and athermal ε-martensite after tempered at 620 °C, while the microstructure contained tempered α-martensite, RA, athermal ε-martensite, and athermal α-martensite after tempered at 650 °C. The thermal and mechanical stability of both RA and athermal ε-martensite decreased after the 0.05% Nb was added to the 0.02C–7Mn steel, and also decreased as the intercritical tempering (IT) temperature increased, which was mainly because the content of solid solution C in RA decreased. The calculated stacking fault energies (SFEs) were all negative, which spontaneously induced athermal martensitic transformation. Multiphase microstructure resulted in the remarkable mechanical property evolution. After tempered at 620 °C, the optical mechanical properties of 7Mn-0.05Nb steel were obtained: yield strength of 720.0 MPa, tensile strength of 819.0 MPa, total elongation of 26.0% and −40 °C impact energy of 266 J, while the yield strength of 675.0 MPa, tensile strength of 776.5 MPa, the total elongation of 29.5% and the −40 °C impact energy of 274 J appeared in 7Mn–0Nb steel. As the IT temperature increased to 650 °C, the yield strength of both the steels sharply decreased (475.5 MPa for 650 °C–0Nb steel and 460.5 MPa for 650 °C-0.05Nb steel), and the tensile strength of both the steels increased about 56 MPa, while the −40 °C impact energy decreased by 40 and 60 J, respectively, showing that the −40 °C impact energy was not very sensitive to the mechanical stability of RA and athermal ε-martensite. The mean size of NbC particles was mainly about 5–7 nm, which significantly improved the precipitation strengthening, but there was just little damage to low-temperature toughness.
The present work aims to unravel the effect of Ti microalloying on the refinement of co-precipitated nanoscale Cu and M2C carbides, and thereby the improvement of both strength and toughness of an ...ultrahigh-strength Fe-0.05C-1.3Cu steel. The co-precipitated Cu and M2C carbides in Ti-free and Ti-containing steels was characterized after quenching and aging treatment via atom probe tomography (APT), and their contribution to the yield strength was quantified. APT results show that after being aged at 550 °C for 1 h, nanoscale Cu particles and M2C carbides co-precipitated. And, the density of finer Cu was found to be higher in the Ti-containing steel than that in the Ti-free steel which can produce more nucleation sites for M2C carbides precipitation. After being aged at 550 °C for 1 h, the Ti-containing steel had a yield strength as high as 1055 MPa, and an impact toughness of 132 J at −40 °C, which was an improvement of 61 MPa and 27 J, respectively, in comparison to the Ti-free steel. The higher-density M2C carbides were found to contribute to precipitation strengthening enhancement. The co-precipitation strengthening contribution from the Cu-M2C precipitates of the Ti-containing steel was calculated to be 69 MPa higher than that of the Ti-free steel. This is similar to the difference of 61 MPa from the experiment. The high toughness of Ti-containing steel is attributed to a higher density of high angle grain boundaries and a smaller prior austenite grain size by deflecting crack propagation.
In this work, the high performance as-cast low RE (rare earths) Mg96.5Zn1Y1.5Mn1 alloys with LPSO (long-periodic stacking ordered) phase were fabricated by Ni and Bi microalloying, and the influences ...of Ni and Bi additions on microstructural evolution and mechanical properties in as-cast Mg96.5Zn1Y1.5Mn1 alloy were investigated. Results show that the Ni and Bi co-alloying can greatly refine the microstructure and facilitate the precipitation of 18R-LPSO phase. Meanwhile, in the Mg95.9Zn1Y1.5Mn1Ni0.5Bi0.1 alloy, the nucleation and growth ability of 18R-LPSO phase were higher than those of W phase, and the growth pattern of 18R-LPSO and W phase was optimized from coupled to divorced growth. Besides, the Mg95.9Zn1Y1.5Mn1Ni0.5Bi0.1 alloy exhibited superior mechanical properties with yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of 167±1 MPa, 239±1.6 MPa and 12±0.45%. Grain refinement, continuous 18R-LPSO phase with optimized growth pattern and the LPSO fibers precipitated in W phase were responsible for the outstanding mechanical properties.
•Effect of Ni/Bi on low RE Mg96.5Zn1Y1.5Mn1 alloy containing LPSO was studied.•The growth pattern transformation of 18R-LPSO and W phase was clarified.•The 18R-LPSO phase exhibited higher nucleation and growth ability than W phase.•The excellent mechanical properties were achieved via Ni and Bi co-microalloying.
Effect of Si addition on the high temperature tribological properties of TiZrV0.5Nb0.5 refractory high entropy alloy was systematically studied. The results demonstrate that Si-microalloying affects ...the tribological properties by modulating the evolution of tribo-layer. Si-microalloying produces local tribo-layer with strong wear resistance by increasing the sintering rate of wear debris below 600 ℃, ultimately significantly reducing the coefficient of friction and wear rate. At 600 °C, Si-microalloying deteriorates the wear resistance due to the stress concentration at hard particles leading to accelerated fracture of the tribo-layer which hinders the formation of continuous protective tribo-layer. The enhanced tribological properties at 800 °C are attributed to the synergistic effect of suppressed oxide volatilization and rapid oxidation behavior ensuring continuous dense tribo-layer formation.
•The high temperature tribological properties of TiZrV0.5Nb0.5Six RHEA was studied.•The evolution of the tribo-layer plays a crucial role in the wear process.•The sintering rate of debris determines the formation of tribo-layer below 600 ℃.•A continuous and dense tribo-layer was formed for the Si0.3 alloy at 800 °C.
•La microalloying method is proposed for grain refinement and texture weakness for copper tube annealed at high temperature.•Growth of recrystallized grain was effectively inhibited by second phase ...particles pinning and twin junction boundary drag.•Copper tubes with superior surface quality are successively fabricated with La microalloying method.
The effect of La microalloying on microstructure evolution of oxygen-free copper tube annealed at elevated temperature was investigated. Different with the significant grain growth in La-free copper tube, growth behavior of La microalloyed tube is effectively inhibited. Meanwhile, Cu6La particles are detected with mainly segregating near annealing twin boundaries but less at grain boundaries. With such experimental observations, a new mechanism for constrained growth of recrystallized grain is proposed with pinning of Cu6La particle coupled with annealing twin dragging. Moreover, refined microstructure is maintained for La microalloyed tube after elevated temperature, which greatly improves the surface quality during secondary bending and flattening processes.
For multicomponent Al-based alloys, one of the most valuable approaches of microstructural design is to find a path to maximize the multiple microalloying effects while overcoming their negative ...counterparts. In this paper, triple Sc-Fe-Si microalloying was performed in an Al–Cu alloy to assemble the co-existence of θ′-Al2Cu and Al3Sc precipitates. Besides, the mutual interactions among triple microalloying elements were utilized to maximize the positive effects on tailoring the dual precipitates, as illustrated in two goals of microstructural design: (i). As to θ′-Al2Cu plates, the multiple Sc-Fe-Si segregation at θ′-Al2Cu/matrix interface was preferentially created in the alloy before creep (as-aged condition). During subsequent high-temperature creep, such interfacial warden will be rapidly reinforced by solute repositioning, as a process of accumulating solutes diffusing from both inner θ′-Al2Cu precipitate and outer matrix to limit the interfacial migration. (ii). For Al3Sc precipitates, the beneficial Si microalloying effect on encouraging the nucleation of Sc-rich entities (precursor of Al3Sc) was successfully acquired during aging, while the detrimental Si effect on accelerating Al3Sc coarsening is generally prohibited by tuning Fe–Si synergy within Al3Sc interior. The establishments of (i) and (ii) enable the satisfactory dispersion as well as the outstanding thermal stability of dual precipitates in the current Al-Cu-Sc-Fe-Si system, leading to a good creep resistance at a high homologous temperature of 0.61Tm ~ 300 °C (where Tm is the melting temperature of the α-Al matrix).
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•Triple Sc-Fe-Si microalloying was exploited in an Al–Cu alloy to co-stabilize θ′-Al2Cu and Al3Sc precipitates during 300oC-creeping.•The Sc-Fe-Si co-segregation pattern at θ′-Al2Cu/matrix interface was produced by tuning solute repositioning to greatly suppressing the coarsening.•The Fe–Si synergy maximizes Si microalloying effect by preserving Si-promoted Sc precipitation while avoiding Si-accelerated Al3Sc coarsening.
Sc microalloying has been regarded as one of the most effective methods to improve the high-temperature resistance of Al-based alloys via additional Al3Sc precipitation. However, synergetic ...precipitation of Al3Sc and some conventional precipitates (i.e. θ′-Al2Cu) is usually difficult in traditional series Al alloys due to the huge temperature gaps between their formation, which limits the Sc microalloying effect in multicomponent systems. Here, an optimized isochronal aging strategy was illustrated in an Al–Cu-Sc alloy to achieve better ambient- and high-temperature properties in comparison to the artificially aged counterparts. A series of microstructural characterizations reveal that, upon isochronal aging, the Sc-rich entities preferentially form in regions adjacent to θ′-Al2Cu precipitates at relatively low temperature stage (~250 °C) and are responsible for refining the θ′-Al2Cu precipitation in Al–Cu-Sc alloy. At higher aging temperature (~300 °C), the preferentially formed Sc-rich entities will be collected by adjacent pre-existing θ′-Al2Cu precipitates, accompanying with additional Al3Sc formation in matrix. The coexistence of θ′-Al2Cu and Al3Sc precipitates realized by isochronal aging contributes to the improved aging hardening behavior as well as better creep resistance at 300 °C in Al–Cu-Sc alloy compared with its Sc-free or isothermally aged counterparts.
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Multi-microalloying technique has been widely used to improve strength and ductility in steels, however, the effect of multi-microalloying on the hydrogen diffusion and hydrogen embrittlement (HE) ...behavior of steels is still not completely understood. This study evaluates the effect of Nb–Ti multi-microalloying on the hydrogen trapping efficiency and HE of hot-stamped steel by a combination of hydrogen permeation test, slow strain rate tensile (SSRT) test and quantitative analysis. The microstructural examination and hydrogen permeation tests showed that with increasing Nb + Ti content, more nano-sized (Nb, Ti) C precipitates were formed, and the martensite grain boundaries areas increased, thus increasing the irreversible and reversible hydrogen trap sites and decreasing the hydrogen diffusion coefficient in the hot-stamped steel. A quantitative analysis of the hydrogen traps demonstrated that the number of hydrogen traps induced by the (Nb, Ti) C precipitates was larger than that of the grain boundaries and dislocations in the Nb–Ti bearing steels. In addition, the SSRT tests showed that the HE susceptibility of the test steel decreased with increasing Nb + Ti content. This was because the additional hydrogen traps generated by the Nb–Ti addition hindered localized hydrogen accumulation at prior austenite grain boundaries, the cracking resistance was improved by a lower Σ3 boundary fraction, and the pinning role of the (Nb, Ti) C precipitates on the movable dislocation could inhibit H-dislocation interaction.
