•Trace amount of Sc (< 0.5 wt%) improves the mechanical properties of the Al-20Zn-3Cu-xSc alloys.•Comprehensive microstructural investigation shows that Sc element induces macro- and micro-scopic ...structural changes.•Al grain size and large grains formed at grain boundaries are greatly reduced.•Formation of nanoprecipitates is facilitated by the addition of Sc, resulting in generation of local strain field.•Quantitative symmetry investigation shows that the formation of nanoprecipitates severely breaks the local symmetry of Al.
We demonstrate the effect of Sc microalloying on the mechanical properties of Al-20Zn-3Cu-xSc (x = 0, 0.1, 0.3, 0.5 wt%) alloys. Trace amounts of Sc addition (<0.5 wt%) simultaneously improve the mechanical strength and the ductility of the Al-20Zn-3Cu-xSc alloys. Among the developed alloys, the Al-20Zn-3Cu-0.3Sc alloy shows the highest tensile strength of 363 MPa with an elongation of 6.8%. Comprehensive microstructural investigation reveals that the Sc microalloying element induces macroscopic (~μm scale) and microscopic (~nm scale) structural changes. Macroscopically, the Al grain size and large particles typically formed in the grain boundaries are significantly reduced. Microscopically, the formation of nanoprecipitates is facilitated by the addition of trace amounts of Sc, resulted in generation of local strain field. Quantitative symmetry investigation then demonstrates that the formation of nanoprecipitates severely break the local symmetry of Al, which affects the mechanical properties of Al-20Zn-3Cu-xSc alloys.
In the present study, we have carried out controlled rolling followed by accelerated cooling to explore the microstructure and mechanical properties of Ti, Ti–Mo, and Ti–B microalloyed steels. The ...objective was to enhance the yield strength of Ti-bearing steel and simultaneously obtain good ductility and toughness. The microstructure of Ti and Ti–Mo steels consisted of polygonal ferrite and the effective grain size was reduced from 5.6μm in Ti-bearing steel to 4.3μm in Ti–Mo microalloyed steel, accompanied by increase in dislocation density. The microstructure of Ti–B steel was acicular ferrite with lath width in the range of ~0.2–0.4μm. The density of precipitates of 3–5nm size was high in all the three steels. Both strength and low temperature toughness were increased on microalloying with 0.09wt% Mo. In steel, containing 0.002wt% B, the yield strength was increased by ~105MPa, and high impact energy of 53.2J at −40°C was obtained. The impact energy was decreased to 14.3J at −60°C because free-B segregated to prior austenite grain boundaries and significantly deteriorated the low temperature toughness. The evolution of fracture surface with temperature was consistent with impact energy.
Nowadays, it is challenging to completely eliminate low angle or high angle grain boundaries (LAGBs or HAGBs) from Nickel-based single crystal (SX) superalloys manufactured using the conventional ...directional solidification technique. The additions of C, B and Hf have been found to be an effective measure in improving the damage resistance of grain boundary (GB) defects, and thus increasing the creep resistance. However, the strengthening mechanism through their additions is still unclear. In this study, a double-seed solidification technique with two misorientation levels, i.e., 5° and 20°, was used to produce a series of bicrystal superalloys with different contents of Hf and B. It is the first report of an alloy with joint Hf and B addition that demonstrates tolerance to GBs with a misorientation as high as ∼20° under all of the creep conditions: 1100 °C/130 MPa, 980 °C/250 MPa and 760 °C/785 MPa. Interestingly, the effect of individual additions of Hf or B was not as pronounced as that of the joint Hf and B addition. To understand the influence of these additions on the creep mechanism in nickel-based superalloys with GB defects, a detailed characterization of the microstructures in the vicinity of the LAGBs or HAGBs was carried out, and the elemental distribution at the HAGBs was analyzed with various techniques. This study will be beneficial for understanding the role of Hf and B additions on improving the GB tolerance, and optimizing the Hf and B additions in nickel-based single crystal superalloys.
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Al-2.5 wt% Cu alloys with different Sc additions (0, 0.1, 0.3, 0.5 wt%) are studied in comparison to reveal the Sc microalloying effect on precipitation, room temperature mechanical properties, and ...creep resistance. The results show that the Sc addition into the Al–Cu alloys can effectively promote the precipitation of θ′-Al2Cu, reducing the size and narrow the size distribution. However, the Sc-dependences of mechanical properties at room and at high temperatures are much different. Although the 0.3 wt% Sc addition results in the densest homogeneous θ′-Al2Cu precipitation and hence the highest room temperature strength, the 0.5 wt% addition leads to the most improved creep resistance at 300 °C that is derived from a nanostructural Sc-based hierarchy, i.e., Al3Sc dispersoid/heterogeneous θ′-Al2Cu precipitate units, homogeneous θ′-Al2Cu precipitates, strongest Sc segregation at θ′/matrix interfaces, and Sc clusters. The nanostructural hierarchy with thermally resistant nanostructural features at different length scales provides a new way to develop advanced Al alloys with excellent high-temperature stability and mechanical properties. The strengthening mechanisms at room and high temperatures are respectively discussed.
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•The Sc addition have a remarkable microalloying effect on the microstructures and mechanical properties of Al–Cu alloys.•The Sc-dependences of mechanical properties at room and at high temperatures are much different.•A nanostructural Sc-based hierarchy enhanced creep resistance is confirmed and the strengthening mechanisms are unraveled.
•Magnetic properties can be controlled by adjusting annealing temperature.•The precipitated phase of Nb can provide remarkable fine-grain effect.•The {1 1 1} texture weakened and the {1 0 0} and ...{1 1 0} texture enhanced by increasing annealing temperature.•The experimental results can be widely used to control the microstructure and properties of high silicon electrical steel.
Fe-6.5 wt% Si alloy (high silicon electrical steel) is an ideal core material for making motors and transformers with low noise and low power loss. Controlling microstructure and texture is key to improving the magnetic properties of high silicon electrical steel. In order to improve the warm rolling and cold rolling workability of high silicon electrical steel, in this paper, 0.03 wt% Nb is added to the alloy, and a strip sample with a thickness of 0.13 mm was prepared. The effect of annealing temperature on microstructure, texture and magnetic properties of the samples was studied. The results show that annealing at 800 ℃–1300 ℃ for 1 h, with the increase of annealing temperature, the {1 0 0} texture content of the sample decreased from 9.47% to 1.09%, {1 1 0} texture content gradually increased from 1.26% to 33.1%, {1 0 0}+{1 1 0} texture content increased significantly from 10.73% to 34.19%. Nb-rich precipitates are formed in high silicon electrical steel with a size of about 100 nm. When the annealing temperature is lower than 1200 ℃, the Nb-rich precipitates can inhibit the grain growth. When the annealing temperature is higher than 1200℃, the Nb-rich precipitates dissolves and the pinning force to the grain boundary is weakened. The 0.13 mm Nb-microalloyed high silicon electrical steel strip has good magnetic properties at 800–1300 °C for 1 h. At 1200 °C, the grain growth and iron loss decrease due to the dissolution of the Nb-rich precipitates, so that the sample has a high comprehensive magnetic property: B8 is 1.31 T, B50 is 1.61 T, and P10/50 is 0.411 W/kg, P10/400 is 5.416 W/kg.
•A crack-free René 104 superalloy with a refined microstructure was fabricated by SLM with the microalloying of Sc.•Al3Sc nanoparticles promoted the heterogeneous nucleation of Ni to refine the ...grains.•The cracking elimination was ascribed to the alleviation of enriched Mo, Zr and B and accumulated thermal stress.•The heightened tensile performance was due to the cracking elimination, refinement of grains and Al3Sc nanoparticles.
Cracking is the critical issue of “hard-to-weld” nickel-based superalloys, especially in the component fabricated by selective laser melting (SLM). In the work, the cracks of as-printed René 104 superalloy were successfully eliminated by microalloying with Sc. The formation of Al3Sc nanoparticles promoted the heterogeneous nucleation of nickel, resulting in a 74.7% reduction of grain size and a significant decrease in< 001 > texture intensity. Compared with the original René 104 alloy, the modified René 104 alloy exhibits the yield strength of 918 MPa, ultimate tensile strength of 1289 MPa and elongation of 13.9%, which are promoted by 16.8%, 40.6% and 256.4%, respectively. Significant grain refinement provides more grain boundary area, which is beneficial to alleviate the enrichment of low melting point phase forming elements (Mo, Zr and B) and the accumulation of thermal stress at grain boundary, thereby eliminating cracks. The improvement of mechanical properties is basically ascribed to the elimination of cracks, grain refinement and the formation of Al3Sc nanoparticles. These findings provide a new perspective for the research and application of “hard-to-weld” nickel-based superalloy prepared by SLM.
The microstructures and mechanical properties of two new aerospace Al-6.00 Mg-0.40Mn-0.12Zr (wt%) alloy sheets, containing 0.10 wt% Sc (low content) and 0.25 wt% Er(cheap), respectively, were ...investigated by tensile tests and electron microscopy methods. The results showed that microalloying elements were present in the form of core-shell-structured secondary Al3(Sc1−xZrx) and Al3(Er1−xZrx) nanoparticles, whose cores were enriched in Sc and Er, respectively. Stable core-shell structured nanoparticles enabled the annealed sheets to retain a completely non-recrystallized structure and strong β-fiber rolling textures. The ultimate tensile strength (UTS), yield strength (YS) and elongation to failure (Elf) of the annealed sheets reached 422 ± 1 MPa, 312 ± 6 MPa, and 20.7 ± 1.5% in the Al-Mg-Mn-Sc-Zr alloy, and 404 ± 5 MPa, 283 ± 7 MPa, and 24.2 ± 0.9% in the Al-Mg-Mn-Er-Zr alloy, respectively, both exhibiting high strength and superior ductility. The mean diameters of the spheroidal Al3(Sc1−xZrx) and Al3(Er1−xZrx) particles were 12.1 ± 4.2 nm and 20.2 ± 8.4 nm, respectively, meantime, the number densities of the Al3(Sc1−xZrx) and Al3(Er1−xZrx) precipitates were (7.7 ± 3.2) × 1013 m2 and (6.4 ± 1.9) × 1012 m2. The higher number density and the smaller particle sizes of the Al3(Sc1−xZrx) leaded to the higher strength of the Al-Mg-Mn-Sc-Zr alloy. The main strengthening mechanisms from the secondary Al3(Sc1−xZrx)/Al3(Er1−xZrx) nanoparticles were associated with the direct Orowan precipitation strengthening and sub-structure strengthening. Based on the results of this paper, enhanced mechanical properties and a reduced cost can be simultaneously achieved with the new Al-Mg-Mn-Zr alloys with low Sc contents or inexpensive Er addition, offering great potential for the development of new high-strength micro-alloyed AlMg alloys in industrial applications.
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•Successfully prepare two new Al-Mg-Mn alloys with secondary Al3(Sc1−xZrx) and Al3(Er1−xZrx) nanoparticles•Characterize the microstructure evolution of new alloy sheets during their preparations•Clarify the existing forms of the combined additions of low-content Sc and Zr or Er and Zr•Discuss the potential for developing new cost competitive Al-Mg-Mn-Sc-Zr/ Al-Mg-Mn-Er-Zr alloys
The application of conventional Al–Zn–Mg–Cu alloys for selective laser melting (SLM) of additive manufacturing is hindered by hot cracking during the SLM process. In the present work, a novel Si- and ...Zr-modified Al–Zn–Mg–Cu (Al7075) prealloyed powder was developed for SLM, and the printability, microstructure, and mechanical properties of the as-built samples were evaluated at various laser powers. The as-built samples are completely crack-free at various laser powers, showing that Si and Zr effectively prevent hot cracking for SLM printed Al–Zn–Mg–Cu. The mechanisms of tensile property enhancement are as follows: first, the low melting point Si-rich eutectics backfill the cracks; second, the Al3Zr significantly decreases the grain sizes. The microstructure has a bipolar grain distribution with fine grains at the bottom of the melt pool (2–5 μm) and coarse columnar grains (10–30 μm) at the top of the melt pool. The synergistic effect of the Si and Zr contribute to outstanding tensile properties (446 MPa tensile strength and 6.5 % elongation), which are higher than those for SLM printed Al–Zn–Mg–Cu alloys without the addition of Si and Zr.
Heat-treatable Al alloys containing Al–2.5wt% Cu (Al–Cu) and Al–2.5wt% Cu–0.3wt% Sc (Al–Cu–Sc) with different grain length scales, i.e., average grain size >10μm ( defined coarse grained, CG), 1–2μm ...(fine grained, FG), and <1μm (ultrafine grained, UFG), were prepared by equal-channel angular pressing (ECAP). The length scale and Sc microalloying effects and their interplay on the precipitation behavior and mechanical properties of the Al–Cu alloys were systematically investigated. In the Al–Cu alloys, intergranular θ-Al2Cu precipitation gradually dominated by sacrificing the intragranular θ′-Al2Cu precipitation with reducing the length scale. Especially in the UFG regime, only intergranular θ-Al2Cu particles were precipitated and intragranular θ′-Al2Cu precipitation was completely disappeared. This led to a remarkable reduction in yield strength and ductility due to insufficient dislocation storage capacity. The minor Sc addition resulted in a microalloying effect in the Al–Cu alloy, which, however, is strongly dependent on the length scale. The smaller is the grain size, the more active is the microalloying effect that promotes the intragranular precipitation while reduces the intergranular precipitation. Correspondingly, compared with their Sc-free counterparts, the yield strength of post-aged CG, FG, and UFG Al–Cu alloys with Sc addition increased by ~36MPa, ~56MPa, and ~150MPa, simultaneously in tensile elongation by ~20%, ~30%, and 280%, respectively. The grain size-induced evolutions in vacancy concentration/distribution and number density of vacancy-solute/solute–solute clusters and their influences on precipitation nucleation and kinetics have been comprehensively considered to rationalize the length scale-dependent Sc microalloying mechanisms using positron annihilation lifetime spectrum and three dimension atom probe. The increase in ductility was analyzed in the light of Sc microalloying effect and the strength contributions by different strengthening mechanisms was quantified as well.
We elucidate here the effect of microalloying with niobium (Nb) on very high cycle fatigue (VHCF) behavior in high-strength C–Mn–Si–Cr bainite/martensite (B/M) multiphase steels studied through ...ultrasonic fatigue testing. The tensile strength (Rm) and fatigue limit strength after 109 cycles (σw9) and in the non-failure condition of the steel microalloyed with Nb were 1640MPa and 900MPa, respectively. Thus, the value of σw9/Rm exceeded in comparison to conventional steels and was approximate 0.55. Three types of failure modes were observed in Nb-bearing steels depending on the surface condition, inclusion, and the matrix microstructure, i.e., surface defect-induced failure mode (S-mode), inclusion-induced failure mode (I-mode), and non-inclusion induced failure mode (N-mode). Only two failure modes were observed in Nb-free steels, the S-mode and the N-mode. The study clearly suggests that Nb had a distinct effect on the VHCF properties of B/M steels. The VHCF limit of Nb-bearing steel was enhanced by 200MPa because of refinement of the microstructure and pinning of dislocations by randomly distributed nanometer-sized Nb(C, N) precipitates. It is underscored that microalloying with Nb is a potential approach to enhance VHCF properties in advanced high-strength steels.
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