Sc Zr Alloy Research Articles

Segregation of the major alloying elements to Al3(Sc,Zr) precipitates in an Al–Zn–Mg–Cu–Sc–Zr alloy

Solute segregation of Zn, Mg and Cu in and around Al3(Sc,Zr) precipitates in an Al-Zn-Mg-Cu-Sc-Zr alloy was systematically studied using atom probe microscopy, aberration-corrected scanning transmission electron microscopy and first-principles simulations. Results show that the Al3(Sc,Zr) precipitates occur as a Sc-rich ‘core’ and Zr-rich ‘shell’ structure. Zn segregates to the Zr-rich shell of Al3(Sc,Zr) precipitates and substitute mainly for the Al atoms, and the Zn segregation intensifies with increasing ageing time. Mg and Cu atoms segregate to the Zr-rich shell during the early stages of ageing, while the Mg atoms prefer the Al-matrix with longer ageing time. Density functional theory calculations demonstrate that such segregation is energetically favored and highlight the diverse role of Al3(Sc,Zr) in influencing the distribution of the major alloying elements.

Precipitation sequence in Al–Mg–Si–Sc–Zr alloys during isochronal aging

6xxx-series Al alloys are the most commonly used alloys in the automotive industry as they have an appropriate balance of strength, corrosion resistance and formability. These alloys experience significant strengthening from the precipitation of coherent GP-zones and β'' precipitates. One opportunity to achieve stronger 6xxx-series Al alloys, without affecting corrosion and formability, is the use of Sc. The strengthening benefits from Sc are obtained by the formation of the Al3Sc dispersoids. The addition of Zr, together with Sc, results in the formation of hybrid Al3(Sc,Zr) dispersoids with a core–shell morphology. The microstructural development in 6xxx-series alloys containing Sc is still largely unknown. Here we show, that combining conductivity, hardness and modelling allows for the prediction of the precipitation sequence in an Al–Sc–Zr, an Al–Mg–Si and an Al–Mg–Si–Sc–Zr alloy. The evolution of conductivity and hardness during isochronal aging is discussed in terms of solute depletion and precipitate formation. APT, TEM and SEM are also conducted in key conditions to help interpret the resistivity and hardness fluctuations. Modelling allows for estimation of the precipitation kinetics of the Mg-Si precipitates and Sc–Zr dispersoids during isochronal aging. The precipitation kinetics of Al3(Sc,Zr) is found to be accelerated in the presence of Mg and Si and APT reveals that this is due to the preferred nucleation of the Al3(Sc,Zr) dispersoids on the MgSi precipitates. These results pave the way for the design of suitable heat treatments for 6xxx-series alloys containing Sc that would allow for the formation of both Al3(Sc,Zr) dispersoids and fine MgSi precipitates.

Changes in the Structure and Microhardness of Rapidly Solidified Foils of Aluminum Alloy 1421 during Their Annealing

In this work, the relationship between the microstructure and microhardness of Al–Mg–Li–Zr–Sc alloy (1421 Al) prepared by ultrafast quenching from the melt has been studied. The following methods are used in studying the rapidly solidified (RS) alloy: scanning electron microscopy integrated with energy dispersive X-ray microanalysis, the method of nuclear reaction analysis, and the measurement of microhardness changes during isochronal annealing. The intercept method is applied to determine the size of secondary phases, their volume fraction, and the specific surface area of the interface boundaries in the samples. It is established that the as-quenched rapidly solidified alloy foils are composed of an aluminum-based supersaturated solid solution. It is found that lithium, the content of which reaches 9.0 at %, is unevenly distributed over the subsurface region of foils. After annealing at 300°C, precipitates of (Sc, Zr)-containing phase are detected in the structure of foils in addition to magnesium-containing phases. Nonmonotonic changes in the microhardness are observed during isochronal annealing of the foils in the temperature ranges of 50–100°C, 150–210°C, 230–340°C, which are associated with the precipitation of metastable and stable phases. It is found that heating of the alloy foils to 340°C leads to an increase in the microhardness by 23%, and a sharp decrease in the microhardness takes place at temperatures above 400°C.

Quantitative contributions of solution atoms, precipitates and deformation to microstructures and properties of Al–Sc–Zr alloys

Abstract In order to investigate the effects of solid solution atoms, precipitated particles and cold deformation on the microstructures and properties of Al–Sc–Zr alloys, the Al–Sc–Zr alloys prepared by continuous rheo-extrusion were treated by thermomechanical treatment, analyzed for conductivity and mechanical properties by tensile and microhardness testing, and characterized using optical microscope, TEM and STEM. A mathematical model was established to quantitatively characterize the contribution of solid solution atoms, precipitates and cold deformation to the conductivity of the alloy. The results show that the strength of Al alloy can be significantly improved by solid solution, aging and cold deformation, and the quantitative impacts of solution atoms, precipitates and cold deformation on the conductivity of Al alloy are 10.5% (IACS), 2.3% (IACS) and 0.5% (IACS), respectively. Aging and cold deformation treatments are the keys to obtain high-strength and high-conductivity aluminum alloy wires.

Double-shell structure of Al3(Zr,Sc) precipitate induced by thermomechanical treatment of Al–Zr–Sc alloy cable

A spheroidal Al3(Zr,Sc) precipitate with a double-shell structure, comprising a Sc-enriched core enveloped by a Zr-enriched inner shell and a Sc-enriched outer shell (∼9 nm in thickness), appears in an Al–0.2Zr–0.1Sc alloy cable after thermomechanical treatment. The average diameter of the spheroidal Al3(Zr,Sc) precipitate is approximately 80 nm. The double-shelled Al3(Zr,Sc) precipitate presents three different interfaces and is semi-coherent with the Al matrix. Atom probe tomography (APT) analyses further show that the outer shell of Al3(Zr,Sc) precipitate is Sc element enrichment. The electrical conductivity of Al–0.2Zr–0.1Sc alloy cable increases by 6.5 MS/m within the aging time from 0.2 to 100 h at 350 °C, with double-shelled Al3(Zr,Sc) precipitate.

Influence of dislocations on precipitation processes in hot-extruded Al–Mn–Sc–Zr alloy

Abstract Solute clustering and precipitation processes in hot-extruded Al–Mn–Sc–Zr alloy were studied. Positron annihilation studies were combined with electrical resistometry, microhardness testin...

Effect of precipitates on properties of cold-rolled Al–Mg–Si–Sc–Zr alloy with higher temperature aging

ABSTRACTThe aging hardening behaviours of the cold-rolled Al–Mg–Si–Sc–Zr alloy were investigated. The microstructure, hardness and electrical conductivity (EC) of the Al–Mg–Si–Sc–Zr alloy were measured. The relationship between the microstructure and the properties of the Al–Mg–Si–Sc–Zr alloy with cold-rolling and aging processes was studied. The result shows that the addition of Sc and Zr elements significantly refines the grains of the Al–Mg–Si alloy during casting. The cold rolling promotes the Al–Sc(Zr) precipitation. The precipitate strengthening increases with increasing roll reduction. The EC of the cold rolling + aging Al–Mg–Si–Sc–Zr alloy increases with increasing rolling reductions. The combination effects of the precipitation hardening and DRX softening during the aging process lead to the similar peak hardness values of around 70 HV of the rolled Al–Mg–Si–Sc–Zr alloy with the different reductions.

Effect of platform temperature on the porosity, microstructure and mechanical properties of an Al–Mg–Sc–Zr alloy fabricated by selective laser melting

In this work, an Al–Mg–Sc–Zr alloy was manufactured using selective laser melting (SLM) at platform temperatures of both 35 °C and 200 °C. The effects of platform temperature and applied energy density (E) on porosity characteristics were studied by image analysis. The results show that 60–70 J/mm3 is the minimum applied energy density threshold to build high density parts (> 99.7%), and that the number density and size of pores follow similar trends for both platform temperatures even though the number density of pores is consistently lower at the 200 °C platform temperature. The optimum processing condition of E = 77 J/mm3 was selected for building thin plates to evaluate the tensile properties based on the lowest porosity and highest hardness results. Tensile results indicate that 35 °C fabricated specimens have a low anisotropy, while 200 °C fabricated specimens have higher as-fabricated strengths but inhomogeneous properties from bottom to top. After peak aging, all samples achieve very similar tensile properties with yield strengths of close to 460 MPa, though the elongation for the 200 °C fabricated specimens still presents a gradual decrease from top to bottom of the plate. The microstructure and property evolution was explained in terms of thermal history effects on the different types of grains and precipitates in this alloy.

Effect of annealing treatment on microstructure and fatigue crack growth behavior of Al–Zn–Mg–Sc–Zr alloy

Al–Zn–Mg–Sc–Zr alloy samples were annealed to four different states (under-aging, peak-aging, over-aging and double-aging) and then thoroughly investigated by means of electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), tensile and fatigue crack growth rate tests to explore the influence of annealing treatment on microstructure and fatigue crack growth behavior. The results indicate that Al3(Sc,Zr) particles can effectively refine grains and enhance tensile properties and fatigue properties. After annealing treatment, the under-aged sample and double-aged sample obtained average grain sizes of 4.9473 and 4.1257 μm, and the maximum value of yield/tensile strength (561 MPa/581 MPa) was obtained in peak-aged state. In the Paris region, fatigue crack growth rate, crack deflection and bifurcation, crack blunting and inter/trans-granular propagation were discussed based on data fitting and Laird model and Griffith theory. And the results show that the under-aged sample possesses the best resistance to fatigue crack propagation and the most tortuous and bifurcated crack path. For all samples, the fatigue crack growth rate in the rupture region was inversely proportional to yield strength.

Effect of Sc and Zr on Microstructure and Mechanical Properties of As‐Cast Al–Li–Cu Alloys

Effect of Sc and Zr addition on microstructure and mechanical properties of as‐cast Al–Li–Cu alloys are investigated. The results show that, a significant grain refinement can be caused by Sc and Zr. The coarse dendritic microstructure is transformed into equiaxed grains after adding Sc and Zr. The refinement can be attributed to the primary particles, Al3(Sc,Zr), of which the orientation is close to that of α‐Al matrix. The primary particles prevent the unsuitable diffusion and act as heterogeneous nucleation sites for α‐Al in the process of solidification. The model for the multi‐layer structure evolution of primary particle is established. Moreover, the mechanical properties of as‐cast Al–Li–Cu–Sc–Zr alloy are affected markedly because of the addition of Sc and Zr. The solid solution strengthening and Hall–Petch strengthening are primary strengthening mechanisms, YS = ΔσHPS + Δσsss + 104 (MPa).

Annealing Effects in Conventionally Cast and Homogenized Al-Zn-Mg(-Sc-Zr) Alloy

Al-based alloys are very preferred for automotive manufacture to produce lightweight vehicles. The positive effect of Sc,Zr-addition on the mechanical properties in Al-based alloys is generally known. Microstructure, mechanical, electrical and thermal properties of the conventionally cast and homogenized Al–Zn–Mg alloy with and without Sc,Zr-addition during isochronal annealing were studied. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared to microstructure development that was observed by optical microscopy and transmission and scanning electron microscopy. It was observed that the Sc,Zr-content in the alloy after casting is not homogeneously distributed but concentrated in randomly localized matrix regions and together with Zn and Mg in the particles at grain boundaries. However, the hardening effect after annealing above 280 °C lightly reflects the Sc,Zr-addition. The distinct changes in resistivity and microhardness as well as in heat flow of the alloys studied are mainly caused by formation/dissolution of the Guinier-Preston zones and subsequent precipitation of the metastable particles from the Al–Zn–Mg system. The eutectic Zn,Mg-containing phase partly disappeared during the annealing above ~ 390 °C. Melting of the Zn,Mg-containing phase was observed at ~ 475 °C. The decomposition sequence of the supersaturated solid solution of the studied alloys is compatible with the decomposition sequence of the Al–Zn–Mg system minimally up to ~ 380 °C.

Phase Transformations and Recrystallization in Cold-Rolled Al-Mg-Sc-Zr Alloy Prepared by Powder Metallurgy

The mechanical, thermal and electrical properties and recrystallization behaviour of the cold-rolled AlMgScZr alloy prepared by powder metallurgy were studied. The materials were investigated during isothermal annealing (400 and 550 °C) and during step-by-step linear annealing from room temperature up to 570 °C. The observed results were compared with microstructure observation by transmission electron microscopy and electron diffraction from a previous study of the Al–Mg-based alloys with Sc and Zr. The precipitation sequence of the Al–Mg system and coarsening of the Sc,Zr-containing particles caused electrical and heat flow changes during the annealing. The presence of the Al3(Sc,Zr) particles has an anti-recrystallization effect that prevents recrystallization at temperature minimally up to ~ 400 °C. A partial recrystallization of the alloy was registered after annealing at 550 °C already for 30 min. The cause of the anti-recrystallization effect is precipitation of the Mg-containing particles as follows from a comparison to the alloy without Mg. But the Mg-addition to the Al–Sc–Zr alloy prepared by powder metallurgy has a poorer anti-recrystallization effect than a Mn-addition.

Effect of heat treatments on the microstructure and formability of Al–Mg–Mn–Sc–Zr alloy

Microstructures and formability of scandium and zirconium added Al-Mg-Mn alloy sheets with various heating conditions were examined to improve their mechanical properties. Formability of these samples were judged by the Lankford value, r-value. It was possible to fabricate mechanically balanced Al-Mg-Mn-Sc-Zr alloy with high hardness 76.2 Hv and with high formability with r=1.2, by not only adding scandium and zirconium but also optimizing the heat treatment conditions.

Viscosity of the liquid Al–6Mg–1Mn–0.2Sc–0.1Zr alloy

The microstructure and the phase composition of as-cast Al–Mg–Mn–Sc–Zr alloy samples are studied by electron microscopy and electron-probe microanalysis. The processes of solidification and melting of this alloy are described. The temperature dependence of the kinematic viscosity of the Al–Mg–Mn–Sc–Zr melts is studied during heating and subsequent cooling of the samples. The measurement results are used to determine the temperature at which inherited microheterogeneities in the melts are destroyed irreversibly.

Heat treatment and age hardening of Al–Si–Mg–Mn commercial alloy with addition of Sc and Zr

Microstructure and precipitation processes in an Al–Si–Mg–Mn–Sc–Zr alloy were studied. The as-cast alloy contains fine Al3(Sc,Zr) particles with L12 structure, rods and/or needles of the Mn,Fe-containing phase and Si,Mg co-clusters. These co-clusters have a detrimental effect on precipitation hardening which is caused exclusively by fine needle-shaped β″ phase particles. The pronounced precipitation of the Al6(Mn,Fe)-phase was observed during isochronal annealing in the as-cast alloy. Nevertheless, the precipitation has a negligible effect on hardness. The apparent activation energy for the Al6(Mn,Fe)-phase precipitation was calculated as (170±20) kJ·mol−1. Heat treatment at 530°C for 45min dissolves Si,Mg co-clusters but causes a precipitation of the Al12(Mn,Fe)3Si phase particles while the Al3(Sc,Zr) particles remain unaffected. The Sc and Zr addition to the Al–Si–Mg–Mn alloy influences precipitation hardening in the course of isochronal annealing neither in the as-cast nor in the solution-treated alloy. The age hardening is very poor in the as-cast alloy (~4%) and it is more pronounced in the solution-treated alloy (~60%) due to more convenient size, aspect ratio and number density of the β″ and β′ phases. The apparent activation energy values of precipitation of the transient β″ and β′ phase were determined as ~90kJ·mol−1 and ~130kJ·mol−1, respectively.

Effects of Sc(Zr) on the microstructure and mechanical properties of as-cast Al–Mg alloys

Both the addition of 0.6% Sc and simultaneous addition of 0.2% Sc and 0.1% Zr exerted a remarkable effect on grain refinement of as-cast Al–Mg alloys, changing typical dendritic microstructure into fine equiaxed grains. Such effect was found to be related to the formation of primary particles, which acted as heterogeneous nucleation sites for α-Al matrix during solidification. Primary particles formed in Al–Mg–Sc–Zr alloy could be identified as the eutectic structure consisting of multilayer of ‘Al3(Sc,Zr) + α-Al + Al3(Sc,Zr)’, with a ‘cellular-dendritic’ mode of growth. In addition, an attractive comprehensive property of as-cast Al–5Mg alloy due to the addition of 0.2% Sc and 0.1% Zr was obtained.

Characterisation of a novel Sc and Zr modified Al–Mg alloy fabricated by selective laser melting

Correlations between densification, hardness and electrical conductivity were investigated over a wide range of applied volumetric energy densities (E) for an Al–Mg–Sc–Zr alloy fabricated by selective laser melting. It is shown that porosity dominates electrical conductivity in the low E region up to 77J/mm3, while the contribution of solute in solution is more significant in the medium–high E region. Reasons for these differences are discussed and a linear relationship between electrical conductivity and densification in the absence of solute effects is presented.

High strain rate superplasticity in an Al–Mg–Sc–Zr alloy processed via simple rolling

The superplastic behavior of Al–Mg–Sc–Zr samples with standard gauge size (18mm by 6mm) were prepared using simple rolling and were tested in the temperature range from 450°C to 525°C at strain rates ranging from 1.67×10–3s−1 to 1×10–1s−1. With proper deformation parameters, the Al–Mg–Sc–Zr alloy has an elongation to failure much higher than 300% and the maximum elongation is 740%. The Microstructure and dislocation substructure investigation using optical microscopy (OM) and transmission electron microscopy (TEM) revealed a dynamic recrystallization in it. The grain size and activation energy on the deformation mechanisms of superplastic is discussed. Results also show that these nano-scale Al3(Sc1−xZrx) particles play an important role in the superplastic process. Al6FeMn particles were found to induce the formation and growth of cavities, which can lead to the fracture of specimens.

Effect of aging time on precipitation behavior, mechanical and corrosion properties of a novel Al-Zn-Mg-Sc-Zr alloy

The precipitation behavior, mechanical properties and corrosion resistance of a novel Al—Zn—Mg—Sc—Zr alloy aged at different time were studied by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), tensile tests, potentiodynamic polarization and electrochemical impedance spectroscopy. The results revealed that with increasing aging time at 120 °C, the hardness and tensile strength of the alloy increased rapidly at first and then slightly decreased. The resistance of exfoliation corrosion (EXCO) and intergranular corrosion (IGC) increased gradually with increasing aging time. The same trend of corrosion properties was demonstrated by electrochemical polarization curves and EIS test. The characteristics of grain boundary precipitates and precipitate free zone (PFZ) had a significant influence on the mechanical and corrosion behaviors of the studied alloy. On the basis of TEM observations, the size of grain boundary precipitates and the width of PFZ became larger, and the distributed spacing of grain boundary precipitates was enhanced with increasing aging time.

Superplasticity of ultrafine-grained Al–Mg–Sc–Zr alloy

The superplastic behavior of an Al–Mg–Sc–Zr alloy with a grain size of ~0.7µm produced by equal channel angular pressing (ECAP) was examined in the temperature interval 150–500°C at strain rates ranging from 10−5 to 10−1s−1. No significant grain coarsening occurs under static annealing up to 450°C because of the strong pinning effect of Al3(Sc, Zr) dispersoids. The alloy showed superior ductility of 365 pct at 175°C, 1200 pct at 275°C and ~3300 pct at 450°C and strain rates of 1.4×10−4s−1, 5.6×10−3s−1 and 5.6×10−1, with corresponding strain rate sensitivities of 0.3, 0.49 and 0.2, respectively. Analysis of the superplastic behavior in terms of threshold stress and surface observations showed that grain boundary sliding (GBS) controlled by grain boundary diffusion is the dominant deformation mechanism under all of the conditions. The strong pinning effect of coherent Al3(Sc, Zr) particles leads to a high dislocation density within grains after superplastic deformation that leads to initial strain hardening at low temperatures and high threshold stress. Analysis of the superplastic behavior showed that the strong temperature dependence of the threshold stress is most likely attributable to the interaction between dislocations and the coherent dispersoids, and the effect of temperature on the optimal strain rate of superplastic deformation associated with the highest values of elongation-to-failure is attributable to absorption of a lattice dislocation by a grain boundary rather than the climb of this dislocation along the boundary.