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.
•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.
Fe83(Cox,Niy)(B11Si2P3C1)1-x,y/17 (x, y=1–3) amorphous alloys with high saturation magnetic flux density (Bs) and excellent soft-magnetic properties were developed and then the microalloying and ...clustering effects were explored. The microalloying of Co and Ni improves the Bs from 1.65T to 1.67–1.72T and 1.66–1.68T, respectively. The Ni-doped alloys exhibit better soft-magnetic properties, containing a low coercivity (Hc) of about 5.0A/m and a high Effective permeability (μe) of (8–10)×103, whereas the microalloying of Co leads to a deteriorative Hc of 5.0–13.0A/m and a μe of (5–8)×103. Moreover, microalloying of Ni can increase the ductile-brittle transition (DBT) temperature of the ribbons, while a totally opposite effect is found in the Co-doped alloys. The formation of dense α-Fe(Co,Ni) clusters during annealing process is used to explain the distinct effects of Co and Ni microalloying on the magnetic properties and bending toughness.
As a kind of important light alloys, the Al alloys exhibit mechanical properties that are closely related to the microstructures. Changing the main alloying elements and adjusting heat treatments are ...usually approaches to tune the microstructure and hence artificially control the mechanical properties. However, the windows for the two approaches have become quite narrow, after extensive studies in the last half of century. Microalloying has become the most promising strategy to further modify the microstructure and improve the mechanical properties of Al alloys, among which the element of scandium (Sc) is especially powerful. In this paper, the recent progresses in Al alloys microalloyed with Sc are briefly reviewed, focusing on the microstructural characterization, strengthening response, and underlying mechanisms. The possible key research points are also proposed for the further development of Al alloys microalloyed with Sc and other rare earth elements.
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.
Al-XCu (X = 1.0, 1.5, and 2.5 wt%) alloys with and without 0.3 wt% Sc addition were prepared respectively. The effects of composition and heat treatment processes on the microstructural evolution ...were systematically investigated by using transmission electron microscope and atom probe tomography (APT). Both Al3Sc dispersoids and θ′-Al2Cu precipitates coexisted after artificial aging, and a strong Sc segregation at θ′-Al2Cu/matrix interfaces was detected and quantified through APT examinations. A Sc solute partitioning was demonstrated between in the Al3Sc dispersoids and at the θ′-Al2Cu/matrix interfaces, mediated by the Cu content. Increasing the Cu content, both the size and volume fraction of the Al3Sc decreased after solid solution treatment. As a result, the interfacial Sc segregation was accordingly intensified during subsequent aging treatment. A parameter of reduction in interfacial energy (Δγ), derived from APT analyses, was proposed to characterize the degree of interfacial Sc segregation. The coupling effect of Al3Sc dispersoids and θ′-Al2Cu precipitates on ductile fracture was discussed, where a micromechanical model was developed to describe a quantitative relationship between the fracture strain with Δγ as well as parameters of the Al3Sc dispersoids.
Refractory metals (W, Nb, or Mo) microalloying Pt-based alloys with unprecedented performance may serve as advanced electrocatalysts for proton exchange membrane fuel cells (PEMFCs). These alloys are ...endowed with unique stabilizing substructures or lattice defects through the microalloying effect. Herein, trace W microalloying PtCuCo medium entropy alloys (W-PtCuCo) are reported via a stepwise synthesis strategy, starting with home-made Cu nanowires as sacrificial templates by anhydrous solid-phase milling route, and then followed by galvanic replacement-assisted solvothermal in ethylene glycol (EG). In PEMFC tests, the obtained W-PtCuCo exhibits an ultrahigh peak power density and mass power density (relative to cathode) reaching 2.09 W cm
and 20.9 W mg
, respectively. During the accelerated degradation test (ADT), the mass activity just lost only 3% after 30 k cycles, much better than the above benchmark catalyst. The microalloying-dependent performances shall be attributed to the presence of abundant stepped surfaces, twisted edges, and other lattice defects terminated by W via substructure reconstruction that indeed alters the electronic structure and strain level of the alloys. This work first provides an atomic-level insight into the microalloying-dependent electrocatalytic performance of Pt-based alloys, which is of great significance for developing next-generation efficient catalysts for PEMFC.
