Sc Microalloying Research Articles

Effect of quench-controlled precipitation hardening on microstructure and mechanical properties of Al-Zn-Mg-Cu-Zr alloys contain of Sc micro-alloying

The microstructural changes and mechanical properties of Al-Zn-Mg-Cu-Zr alloy samples with 0.05 and 0.1 wt% Sc were investigated after conventional and quench-controlled precipitation hardening treatments. The results showed that by the addition of Sc microcontent, grain-refined microstructures with smooth precipitates were obtained. It was also revealed that aging treatment after the hot rolling process reduced grain size to 20 µm and resulted in the formation of dual-morphology precipitates. Thus, the quench-controlled samples showed a higher precipitate content than the conventionally treated samples, which was related to the increase in the diffusion rate of the dissolved atoms. Furthermore, mechanical tests demonstrated that the quench-controlled samples had the highest yield strength (600 MPa) due to finer microstructure and higher precipitate content.

Effect of Sc Microalloying and Customized Aging Regime on the Properties and Microstructure of Al-Mg-Si-Sn Alloy

Effect of Sc Microalloying and Customized Aging Regime on the Properties and Microstructure of Al-Mg-Si-Sn Alloy

Influence of Sc Micro-Alloying and Low-Frequency Electromagnetic Casting on the Microstructure and Properties of 7a36 Aluminum Alloy

Influence of Sc Micro-Alloying and Low-Frequency Electromagnetic Casting on the Microstructure and Properties of 7a36 Aluminum Alloy

Selective laser melting of crack-free René 104 superalloy by Sc microalloying

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.

Effect of Sc microalloying on microstructure evolution and mechanical properties of extruded Al–Zn–Mg–Cu alloys

The effect of microalloying Sc on the microstructure and mechanical properties of Al–Zn–Mg–Cu alloys was experimentally investigated through X-ray diffraction, optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy characterizations and tensile tests. The presence of Sc caused the formation of the primary Al3Sc phase and behaved a significant modifying effect on the cast microstructure with an equiaxed microstructure attained in the Sc-bearing alloys rather than the dendrites in Sc-free alloys. In the subsequent thermal extrusion processing, a remarkable grain refinement and retardation of recrystallization occurred, because the nano-sized Al3Sc dispersoids contributed to the pinning effect on the dislocation movement and grain boundary migration in the Sc-bearing alloys. The mechanical properties were significantly improved with the addition of Sc. The superior comprehensive mechanical properties in the Sc-bearing alloys were achieved via the optimal extrusion at 350 °C with the yield strength and ultimate tensile strength of 373.7 ± 3 MPa and 490.3 ± 5 MPa, respectively. The strong strengthening behavior of the Sc addition in the extruded alloys is mainly attributed to the grain refinement and the dispersoid strengthening from the Al3Sc particles. The corresponding contributions to the increase in yield strength derived from the grain refinement and the dispersoid strengthening were estimated, which indicated that the highest particle density attained by extrusion at 350 °C resulted in the excellent mechanical properties of the Sc-bearing alloys.

The relation between mechanical properties and microstructural evolution induced by Sc microalloying in Al-20Zn-3Cu alloy

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.

Development of a high-strength Al-Zn-Mg-Cu-based alloy via multi-strengthening mechanisms

We develop high-strength of Al-Zn-Mg-Cu-based alloys (7xxx series) by optimizing the amount of Mg/Cu with a trace addition of Sc. The highest mechanical properties are achieved with σUTS=672 MPa (σy=644 MPa) and ε=8.6%. Based on quantitative symmetry measurements, the improved mechanical strength originates from nanoscopic structural distortion caused by the excessive solutes of Mg and Cu as the amount of Mg/Cu increases. In addition, Sc microalloying induces grain refinement, thus enhancing the mechanical properties.

Influence of Sc Microalloying on the Microstructure of Al5083 Alloy and Its Strengthening Effect

In this study, we investigate the influence of Sc microalloying on the microstructure of the Al5083 alloy. Trace amounts of Sc addition drastically improve the mechanical properties of the Al5083 alloy from 216 MPa to 233 MPa. Macroscopically, the addition of Sc significantly reduces the grain size of Al by approximately 50%. Additionally, a microstructural investigation reveals that the Sc microalloying element induces fine Al3Sc nanoprecipitates in the Al matrix. The formation of Al3Sc nanoprecipitates results in a pinning effect on the dislocations, leading to accumulated dislocations. Compared to a Sc-free Al5083 alloy specimen, the number density of dislocations in the Sc-added Al5083 alloy significantly increases after hot rolling, enhancing the tensile properties. We reveal that the improved mechanical properties of Al5083 with Sc microalloying originate from the grain refinement and the formation of fine Al3Sc nanoprecipitates.

Investigation on microstructure, mechanical properties and corrosion behavior of Sc-contained Al-7075 alloys after solution-aging treatment

This work mainly studies on Sc microalloying in Al-7075 alloys. The microstructure characteristics and mechanical properties of the Sc-contained Al-7075 alloys were investigated after homogenization at 460 ℃ for 10 h. The corrosion behavior of the 7075 alloys after aging was examined using a three-electrode system. The results showed that significant reduction in grain sizes from 129.92 μm without Sc alloy to 32.17 μm with 0.6% Sc. The microhardness increased with the increase of Sc content in the alloy from 71.7 ± 1.3 HV without Sc alloy to 89.2 ± 4.7 HV. The tensile strength also increased from 151.82 ± 1.07 MPa to 212.85 ± 7.35 MPa. The results also revealed that the addition of Sc element in 7075 aluminum alloys not only reduced the corrosion current density of the alloy, but also increased the corrosion resistance of the alloy in the corrosive environment.

Tailoring precipitation strategy to optimize microstructural evolution, aging hardening and creep resistance in an Al–Cu–Sc alloy by isochronal aging

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.

Corrosion Behaviour Between Al–Zr Alloy Conductor and Cu Transition Terminal via Sc Addition

The influence of Sc microalloying on corrosion behaviour between Al–0.2Zr alloys conductor and Cu sheet have been studied in 0.1 M Na2SO4 + 0.05 M NaCl weak acid solution. The observations of corroded morphologies of the Al–0.2Zr alloys conductor were obtained by scanning electron microscopy and transmission electron microscopy. After same short-term service, slight local corrosion and pitting pits occur on the surface of Al–0.2Zr alloy, and corrosion products mainly adhere along grain boundaries of Al–0.2Zr–0.1Sc alloy. After same long-term service, there are many large and saggy corrosion pits on the surface of Al–0.2Zr–0.1Sc alloy, while only some staircase corrosion marks appear on the surface of Al–0.2Zr alloy. Al3Sc and Al3(Zr,Sc) may reduce the electrochemical heterogeneity and the electrochemical reaction of precipitated precipitates on subgrain boundaries and inter-subgrains, thus improving corrosion resistance via Sc addition.

Improvement in compressive creep resistance of Al-0.2Zr alloy with L12 structured Sc-enriched precipitates

The influence of Sc microalloying on compressive creep behaviors of Al-0.2Zr and Al-0.2Zr-0.1Sc alloy have been studied. The steady state creep rate of Al-0.2Zr-0.1Sc alloy ranged from 1.124 × 10−9 s−1 to 1.926 × 10−8 s−1 at 90–150 °C /70 MPa. By contrast, the steady state creep rate of Al-0.2Zr alloy ranged from 6.862 × 10−8 s−1 to 8.560 × 10−7 s−1, indicating that Sc addition significantly improve the compressive creep resistance of Al-0.2Zr alloy. According to the TEM micrographs, except for Al3(Zr,Sc) precipitates precipitated during aging, a large number of fine dispersed Al3Sc precipitates precipitated after creep for 70 h, which significantly hinder movements of dislocations and grain boundaries. As a result, the steady-state creep rate of Al-0.2Zr-0.1Sc alloy slightly decreases after 70 h at 150 °C /70 MPa. The critical creep stress value of Al-0.2Zr alloy is 47.2 MPa, and that of Al-0.2Zr-0.1Sc alloy is 68.4 MPa. The higher critical creep stress value indicates the more excellent compressive creep resistance.

Nanostructural Sc-based hierarchy to improve the creep resistance of Al–Cu alloys

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.

Effect of Sc microalloying addition on microstructure and mechanical properties of as-cast Al–12Si alloy

The effect of microalloying 0.1 wt% Sc on microstructure and mechanical properties of as-cast Al–12Si (wt.%) alloys was investigated. Sc refines α-Al grain average size from 400 μm to 135 μm and eutectic Si average size from 4.6 μm to 0.7 μm, and modifies Si from plate-like shape to fibers as well. The microaddition of Sc also leads to the formation of coherent nanosized Al3Sc precipitation and other Sc containing intermetallic phases with a length <10 μm. The tensile strength increases from 183 MPa of the base alloy to 254 MPa of the Sc-modified alloy, and the elongation increases from 3.5% to 8.3%, respectively. Moreover, this Sc-modified alloy has a retention of 85% strength even after thermal exposure at 350 °C for 160 h.

Co-stabilization of θ′-Al2Cu and Al3Sc precipitates in Sc-microalloyed Al–Cu alloy with enhanced creep resistance

As to the increasing demands for the light materials in weight-sensitive fields, scandium (Sc) is arguably known as one of the most attractive and effective microalloying elements to develop high-performance aluminum-based alloys applied at elevated temperature. This report exemplifies how the Sc microalloying effect can be fully utilized to seek for the synergetic stabilization between dual precipitates, more than simply forming the well-known Al3Sc nanoprecipitates. Two sets of precipitation protocols are introduced to create a coexistence of θ′-Al2Cu and Al3Sc precipitates in an Al–Cu–Sc alloy, and the thermal stabilization of the two precipitates is found to be closely correlated. In the case of θ′-Al2Cu precipitate decomposed because of an invalid protection from weak interfacial Sc segregation, the Al3Sc precipitates will coarsen much quickly, which induces a simultaneous loss in thermal stability and strengthening effect of the dual precipitates. In contrast, a sensible approach is proposed to induce a strong Sc segregation at the θ′-Al2Cu/matrix interface to greatly stabilizes the θ′-Al2Cu precipitates even crept at 300°C and concomitantly suppresses the coarsening of Al3Sc precipitates, leading to extremely high creep resistance. The acceleration effect of the Cu on the Al3Sc coarsening is illustrated and can be alternatively prohibited by modified Sc partitioning to stabilized θ′-Al2Cu precipitates as Cu storage and thus provides a much stronger coupling precipitate strengthening as well as creep resistance at elevated temperatures. This strategy is broadly applicable to other high-temperature alloy containing Sc together with primary elements such as Si, Mg, and Zn via the creation of precipitate co-stabilization.

Grain size-dependent Sc microalloying effect on the yield strength-pitting corrosion correlation in Al-Cu alloys

Coarse-grained (CG), fine-grained (FG), and ultrafine-grained (UFG) Al-2.5 wt% Cu (Al-Cu) alloys were respectively prepared, with and without 0.3 wt% Sc addition, for comparison. The influences of minor Sc addition on the microstructural evolution, tensile mechanical properties and pitting corrosion resistance were systematically studied at different grain scales. A significant Sc microalloying effect on the precipitation was observed that the minor Sc addition promoted the dispersion of finer θ′-Al2Cu precipitates in the CG alloy and favored the intragranular θ′-Al2Cu precipitation in the FG and UFG alloys, with the smaller grain size leading to a stronger Sc microalloying effect. The Sc addition induced convincing increases in the yield strength at all the three grain scales, and improved (in the CG and UFG Al-Cu alloys) or retained (in the FG Al-Cu alloy) the pitting corrosion resistance at the same time. This indicates that the inverse strength-pitting corrosion correlation as usually observed can be broken by minor Sc addition. The strengthening mechanisms were discussed and the grain size-dependent pitting corrosion resistance mediated by the Sc addition was rationalized in terms of a competition between the positive influence derived from the interfacial Sc segregation and the negative influence come from the deformation-induced dislocations. The present findings provide a possible approach to break the inverse strength-pitting resistance correlation in heat-treatable Al alloys by modifying the precipitate/matrix interfaces through the suitable microalloying atom segregation.

The influence of Sc solute partitioning on the microalloying effect and mechanical properties of Al-Cu alloys with minor Sc addition

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.

Length-scale dependent microalloying effects on precipitation behaviors and mechanical properties of Al–Cu alloys with minor Sc addition

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.

The Study on Microstructures and Properties of the Direct Squeeze Casting Al-Zn-Mg-Cu-Zr-Sc Alloy

Trace elements Sc was added to the Al-Zn-Mg-Cu-Zr alloys, the differences of the microstructure and properties using gravity casting and squeeze casting are compared. The result shows that through the addition of trace elements Sc, the Al-Zn-Mg-Cu-Zr alloys casting microstructure was refined effectively, dendrite segregation was alleviated and the mechanical property of alloy was enhanced. Using squeeze casting forming could improve the compactness of the alloys, decrease the probability of occurrence of shrinkage cavities, porosity and segregation, in addition, the grain could be refined by the pressure forced on the alloy melt. Combined the element Sc microalloying and squeeze casting used in Al-Zn-Mg-Cu-Zr alloy forming, we could get the casting with uniform, compact, little defect microstructures. This combination could provide a new way to the Al-Zn-Mg-Cu-Zr alloy forming.