Effects of Sc,Zr and Ti on the microstructure and properties of Al alloys with high Mg content

The effects of trace Sc, Zr, and Ti on the microstructure and hardness of Al alloys with high Mg content (Al-6Mg, Al-8Mg, and Al-10Mg) were studied by optical microscope, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Brinell hardness. The grain size of the as-cast alloys was refined by the addition of Sc and Zr, and it was further refined by the addition of Ti. With the same contents of Sc, Zr, and Ti, an increase in Mg content was beneficial to the refinement due to the solution of Mg into α-Al. The refined microstructures of the as-cast alloys were favorable for Brinell hardness. Addition of Sc, Zr, and Ti to the Al-10Mg alloy results in the improvement of peak hardness and it is about 45% higher than that of the Al-10Mg alloy, which is due to fine precipitations of Al₃(Sc_1−x Zr _x ), Al₃(Sc_1−x Ti _x ), and Al₃(Sc_1−x−y Zr _x Ti _y ).     

Al3(Sc1-xZrx)纳米粒子对搅拌摩擦焊Al-Mg-Mn合金显微组织和力学性能的影响(英文)

通过拉伸测试和显微分析方法研究搅拌摩擦焊Al-5.50Mg-0.45Mn和Al-5.50Mg-0.45Mn-0.25Sc-0.10Zr(质量分数,%)合金的显微组织和力学性能。结果表明,Al-Mg-Mn接头的屈服强度、抗拉强度和伸长率分别为(191±3) MPa、(315±1) MPa和(4.8±1.9)%,Al-Mg-Mn-Sc-Zr接头的分别为(288±5) MPa、(391±2) MPa和(3.4±1.0)%。相比Al-Mg-Mn接头,Al-Mg-Mn-Sc-Zr接头晶粒更细小、平均取向差角更低、小角度晶界百分数更高。两种接头的断裂位置均位于焊核区(WNZ),在该“最薄弱微区”内,Al₃(Sc_1-xZr_x)纳米粒子的平均尺寸为(9.92±2.69) nm,可提供有效奥罗万和晶界强化,使Al-Mg-Mn接头的屈服强度提高97 MPa。     

Precipitation evolution in Al–0.1Sc,Al–0.1Zr and Al–0.1Sc–0.1Zr (at.%) alloys during isochronal aging

Precipitation strengthening is investigated in binary Al–0.1Sc, Al–0.1Zr and ternary Al–0.1Sc–0.1Zr (at.%) alloys aged isochronally between 200 and 600 °C. Precipitation of Al₃Sc (L1₂) commences between 200 and 250 °C in Al–0.1Sc, reaching a 670 MPa peak microhardness at 325 °C. For Al–0.1Zr, precipitation of Al₃Zr (L1₂) initiates between 350 and 375 °C, resulting in a 420 MPa peak microhardness at 425–450 °C. A pronounced synergistic effect is observed when both Sc and Zr are present. Above 325 °C, Zr additions provide a secondary strength increase from the precipitation of Zr-enriched outer shells onto the Al₃Sc precipitates, leading to a peak microhardness of 780 MPa at 400 °C for Al–0.1Sc–0.1Zr. Compositions, radii, volume fractions and number densities of the Al₃(Sc_1?xZr_x) precipitates are measured directly using atom-probe tomography. This information is used to quantify the observed strengthening increments, attributed to dislocation shearing of the Al₃(Sc_1?xZr_x) precipitates.     

Improved oxidation and hot corrosion resistance of Ta-doped NiAlY alloy at 750°C

The oxidation and hot corrosion behaviors of the NiCrAlY, NiAlY, and Ni–xTa–Al–Y alloys (x = 1, 3, 5, and 10 wt%) were investigated at 750°C. The doped Ta promoted the formation of the protective α-Al₂O₃ scales. The NiTaAlY alloys exhibited an improved oxidation resistance compared with the NiAlY alloy. Under the NaCl-induced hot corrosion test, the addition of Ta reduced the consumption of Al and inhibited the internal corrosion of the alloys. The Ni–xTa–Al–Y alloys (x = 1, 3, 5, and 10 wt%) showed better resistance to the NaCl-induced hot corrosion. Moreover, the hot corrosion mechanism of the tested alloys was also discussed.     

Exfoliation corrosion of 7150 Al alloy with various tempers and its electrochemical impedance spectroscopy in EXCO solution

The exfoliation corrosion susceptibility and electrochemical impedance spectroscopy (EIS) of 7150 Al alloys with T6, T73, and RRA (retrogression at 175 °C for 3 h) tempers in EXCO solution were investigated. The anodic equilibrium precipitate η(MgZn₂) is continuous or closely spaced at the grain boundaries in the 7150‐T6 Al alloy, resulting in its greatest susceptibility to exfoliation corrosion. The grain boundary η precipitates in the RRA and T73 treated 7150 Al alloys are coarsened and show a clear discontinuous nature; they possess similar exfoliation corrosion sensitivity and their exfoliation corrosion resistance is greatly increased. At the beginning of immersion in EXCO solution, the EIS plot of the 7150 Al alloys is composed of a capacitive arc in the high to medium frequency range and an inductive component in the medium to low frequency range. As immersion time is increased, exfoliation corrosion with different corrosion ratings occurs on the surface of the 7150 Al alloy with various tempers, two capacitive arcs appear in the high to medium and medium to low frequency ranges, respectively. The fitted medium to low frequency capacitance C₂ of 7150‐T6 Al alloy, corresponding to the new surface caused by the exfoliation corrosion, is much greater than that of the T73 and RRA treated 7150 Al alloy, which is consistent with the greatest exfoliation corrosion susceptibility of the 7150‐T6 Al alloy.     

Nanoscale structural evolution of Al3Sc precipitates in Al(Sc) alloys

Precipitation of the Al₃Sc (L1₂) phase in aluminum alloys, containing 0.1, 0.2 or 0.3 wt% Sc, is studied with conventional transmission and high-resolution (HREM) electron microscopies. The exact morphologies of the Al₃Sc precipitates were determined for the first time by HREM, in Al–0.1 wt% Sc and Al–0.3 wt% Sc alloys. The experimentally determined equilibrium shape of the Al₃Sc precipitates, at 300°C and 0.3 wt% Sc, has 26 facets, which are the 6 {100} (cube), 12 {110} (rhombic dodecahedron), and 8 {111} (octahedron) planes, a Great Rhombicuboctahedron. This equilibrium morphology had been predicted by first principles calculations of the pertinent interfacial energies. The coarsening kinetics obey the (time)^1/3 kinetic law of Lifshitz–Slyozov–Wagner theory and they yield an activation energy for diffusion, 164±9 kJ/mol, that is in agreement with the values obtained from tracer diffusion measurements of Sc in Al and first principles calculations, which implies diffusion-controlled coarsening.     

Effects of Al content on the corrosion properties of Mg–4Y–xAl alloys

The corrosion performances of Mg–4Y–xAl (x = 1, 2, 3, and 4 wt%) alloys in the 3.5% NaCl electrolyte solution are investigated by electrochemical tests, weight loss measurement and corrosion morphology observation. The results indicate that corrosion modes for the alloys are localized corrosion and the filiform type of attack. With Al concentration increasing from 1 to 4 wt%, the corrosion rate of Mg–4Y–xAl alloys decreases firstly and then increases, and WA42 alloy shows the best corrosion resistance. The addition of Al element to Mg–4Y alloys leads to the formation of Al₂Y and Al₁₁Y₃ intermetallic compounds and reduces the proportion of Mg₂₄Y₅ phase. Corrosion resistance of the Mg–4Y–xAl alloys mainly depends on the size and distribution of the second phases. Besides, the addition of excessive Al can greatly consumes the Y element in the matrix, thus leading to a less protective film on the alloys. The effect of the relative Volta potential changes between the second phases and α-Mg on corrosion resistance of Mg–4Y–xAl alloys is insignificant. The main corrosion products of the Mg–4Y–xAl alloys are Mg(OH)₂, Mg₃(OH)₅Cl·4H₂O, Mg_0.72Al_0.28(CO₃)_0.15(OH)_1.98(H₂O)_0.48, and Mg₄Al₂(OH)₁₂CO₃·3H₂O.     

Influence of scandium on the pitting behaviour of Al–Zn–Mg–Cu alloys

In this paper the influence of small scandium additions (<0.26 wt.%) on the corrosion properties of the high-strength Al–Zn–Mg–Cu alloy AA7010 is investigated. The addition of scandium (in combination with the grain refiner Zr) leads to the formation of Al₃Sc_xZr_1−x phases. These coarse particles disturb the grain structure near the particle/matrix interface, which facilitates the initiation of localized corrosion in potentio-dynamic scans. Microelectrochemical investigations revealed a slightly cathodic character of these particles and a passive range beyond the breakdown potential of the matrix. Mass loss measurements show that the addition of scandium increases the mass loss during the initial period. The corrosion morphology was investigated with optical and scanning electron microscopy. The composition of the phases was determined with energy-dispersive X-ray analysis. Micro-capillary measurements were performed to investigate the electrochemical properties of single phases surrounded by matrix.     

Structure and electrochemical properties of the Fe substituted Ti–V-based hydrogen storage alloys

To elucidate the effects of Fe on the Ti–V-based hydrogen storage electrode alloys, the Ti_0.8Zr_0.2V_2.7−xMn_0.5Cr_0.8Ni_1.0Fe_x (x = 0.0–0.5) alloys were prepared and their structures and electrochemical properties were systematically investigated. XRD results show that all the alloys consist of a C14 Laves phase with hexagonal structure and a V-based solid solution phase with bcc structure. With increasing Fe content, the abundance of the C14 Laves phase gradually decreases from 43.4 wt% (x = 0.0) to 28.5 wt% (x = 0.5), on the contrary, that of the V-based solid solution phase monotonously increases from 56.6 wt% to 71.5 wt%. In addition, SEM observation finds that the grain size of the V-based solid solution phase is first gradually reduced and then enlarged with increasing x. Electrochemical investigations indicate that the substitution of Fe for V markedly improves the cycling stability and the high rate dischargeability of the alloy electrodes, but decreases the maximum discharge capacity and the activation performance. Further electrochemical impedance spectra, the linear polarization curve and the potentiostatic step discharge measurements reveal that the electrochemical kinetics of the alloy electrodes should be jointly controlled by the charge-transfer reaction rate on the alloy surface and the hydrogen diffusion rate in the bulk of the alloys. For the alloy electrodes with the lower Fe content (x = 0.0–0.2), the hydrogen diffusion in the bulk of the alloys should be the rate-determining step of its discharge process, and while x increases from 0.3 to 0.5, the charge-transfer reaction on the alloy surface becomes to the rate-determining step, which induces that the electrochemical kinetics of the alloy electrodes is firstly improved and then decreased with increasing Fe content.     

Effects of Zr Content on the Yield Strength of an Al-Sc Alloy

The effects of Zr content on the yield strength of an Al-Sc alloy are investigated experimentally. It has been shown that the yield strength increases with time at the beginning of annealing for the investigated one Al-Sc alloy and three Al-Sc-Zr alloys. Such an increase of yield strength results from the high nucleation rate for Al₃Sc particles in Al-Sc alloy and Al₃(Sc_1−x ,Zr _x ) particles in Al-Sc-Zr alloys. Throughout the annealing, the yield strength increases with the Zr content, indicating that the alloy with higher Zr content possesses higher yield strength. The high yield strength of the alloy with high Zr content is due to the higher number density and volume fraction of the particles as well as their smaller size and inter-particle spacing. Such a microstructural feature for the particles exhibits a larger Orowan strengthening effect by inhibiting the dislocation movements.     

Corrosion behavior of Zr–Nb–Cr cladding alloys

Zr-Nb-Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys. The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized. Corrosion tests reveal that the corro- sion resistance of ZrxNb0.1Cr （x = 0.2, 0.5, 0.8, 1.1; wt%） alloys is first improved and then decreased with the increase of the Nb content. The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics. The Cr addition degrades the corrosion resistance of the Zrl.lNb alloy, which is ascribed to Zr（Cr,Fe,Nb）2 precipitates with a much larger size than β-Nb.     

Exfoliation corrosion and electrochemical impedance spectroscopy of an Al‐Li alloy in EXCO solution

The exfoliation corrosion and electrochemical impedance spectroscopy (EIS) of an Al‐2.8%Cu‐1.5%Li‐0.3%Mg‐0.3%Zn‐0.3%Mn‐0.15%Zr alloy with various aging states in EXCO solution were investigated. The equilibrium precipitates at grain boundaries are anodic to the alloy base at their adjacent periphery. With prolonging aging time, the amount and the size of the equilibrium precipitates at grain boundaries are increased, resulting in an enhanced susceptibility to exfoliation corrosion. At the beginning of immersion in EXCO solution, the EIS plot of the alloys is composed of a capacitive arc in the high frequency range and an inductive loop in the low frequency range. As immersion time is increased, two capacitive arcs appear in the high‐mediate and mediate‐low frequency ranges respectively and the appearance time of two capacitive arcs could be an indication of the speed of localized corrosion development in EXCO solution. The longer appearance time of two capacitive arcs of the under‐aged alloy indicates its slower localized corrosion development.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

添加微量Sc、Zr对超高强铝合金微观结构和性能的影响

采用低频电磁铸造技术制备Al-9Zn-2.8Mg-2.5Cu-x Zr-y Sc（x=0,0.15%,0.15%;y=0,0.05%,0.15%）合金,借助金相显微镜、扫描电镜、透射电镜、力学性能测试等手段分别对其均匀化、热挤压态、固溶态和时效态的组织与性能进行对比分析。结果表明：添加微量Sc和Zr,会在凝固过程中形成初生Al3（Sc,Zr）,可显著细化合金铸态晶粒;均匀化时形成的次生Al3（Sc,Zr）粒子可以强烈钉扎位错和亚晶界,有效抑制变形组织的再结晶,显著提高合金的力学性能。与不含Sc、Zr的合金相比,含0.05%Sc和0.15%Zr的合金经固溶处理和峰值时效处理后其抗拉强度和屈服强度分别提高172 MPa和218 MPa,其强化作用主要来自含Sc、Zr化合物对合金起到的亚结构强化、析出强化和细晶强化。     

Effect of different aging processes on the corrosion behavior of new Al–Cu–Li–Zr–Sc alloys

The intergranular corrosion and exfoliation corrosion behaviors of Al–Cu–Li–Zr–Sc alloys under different aging effects, such as single‐stage aging, strain aging, and double‐stage aging, were studied. Among the three aging treatments, single‐stage aging resulted in the best resistance to corrosion, followed by double‐stage aging; strain aging resulted in the worst corrosion resistance. A 3.5% precooling strain could increase the dislocation density, which promoted the precipitation of corrosion‐prone T₁ phase and increased the corrosion driving force of the alloy. Double‐stage aging made the precipitated T₁ phases finer and more uniform and reduced the number of equilibrium phases at grain boundaries, thus improving the corrosion properties of the alloy. The corrosion susceptibility of the alloy was attributed to the T₁ phase and precipitate‐free zone (PFZ), and the underlying corrosion mechanism was revealed as preferential dissolution of the equilibrium phase at grain boundaries and its surrounding distortion zone, followed by expansion of the PFZ along the grain boundaries, resulting in the development of corrosion from the grain boundaries to the intragranular regions.     

Effect of minor Zr and Sc on microstructures and mechanical properties of Al–Mg–Si–Cu–Cr–V alloys

The effects of minor contents of Zr and Sc on the microstructures and mechanical properties of Al–Mg–Si–Cu–Cr–V alloy were studied. The results show that the effects of minor Zr and Sc on the as-cast grain refinement in the ingots, the improvement in the strength of the as-extruded alloys and the restriction of high angle grain boundaries in the aged alloys can be sorted as Al₃Sc>Al₃(Zr,Sc)>Al₃Zr. None of them could stop the nucleation of recrystallization, but Al₃(Zr,Sc) phase is a more effective inhibitor of dislocation movement compared to Al₃Sc in the aged alloys. Compared with the mechanical properties of the aged alloy added only 0.15% Sc, the joint addition of Zr and Sc to the alloy leads to a very slight decrease in strength with even no cost of ductility. Taking both the production cost and the little bad influence on mechanical properties into consideration, an optimal content of Zr and Sc in the Al–Mg–Si–Cu–Cr–V alloy to substitute 0.15% Sc is 0.13% Zr+0.03% Sc.     

The effects of lanthanum on microstructure and electrochemical properties of Al–Zn–In based sacrificial anode alloys

In order to improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloys, Al–5Zn–0.03In–1Mg–0.05Ti–xLa (x = 0.3, 0.5 and 0.7 wt.%) alloys were developed. Microstructures and electrochemical properties of the alloys were investigated. The results show that the optimal microstructures and electrochemical properties are obtained in Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy. The main precipitate phase is Al₂LaZn₂ particles. The excellent electrochemical properties of Al–5Zn–0.03In–1Mg–0.05Ti–0.5La alloy is mainly attributed to fine grains and grain boundaries containing fine Al₂LaZn₂ precipitates. At the same time the fine grains can improve the non-uniform corrosion of Al–0.5Zn–0.03In–1Mg–0.05Ti alloy.     

Core–shell nanoscale precipitates in Al–0.06 at.% Sc microalloyed with Tb,Ho, Tm or Lu

The age-hardening response at 300 °C of Al–0.06Sc–0.02RE (at.%, with RE = Tb, Ho, Tm or Lu) is found to be similar to that of binary Al–0.08Sc (at.%), except that a shorter incubation period for hardening is observed, which is associated with nanoscale RE-rich Al₃(RE_1?xSc_x) precipitates. In addition, Al–0.06Sc–0.02Tb (at.%) has a much lower peak microhardness than that of Al–0.08Sc (at.%) due to the small solubility of Tb in α-Al(Sc). Peak-age hardening occurs after 24 h, and is associated with a high number density of nanoscale Sc-rich Al₃(Sc_1?xRE_x) precipitates. Analysis by three-dimensional local-electrode atom-probe tomography shows that x increases with increasing atomic number, and that the REs partition to the core of the precipitates.     

Microstructural factors affecting hardness property of dual two-phase intermetallic alloys based on Ni3Al–Ni3V pseudo-binary alloy system

Dual two-phase intermetallic alloys that have alloy compositions of Ni₇₅Al_xNb_2.5V_22.5−x and are composed of geometrically close packed (GCP) Ni₃Al (L1₂) and Ni₃V (D0₂₂) phases containing Nb were studied, focusing on the relationship between microstructural parameter and high-temperature hardness property. The two-phase microstructures defined by primary Ni₃Al precipitates and eutectoid (i.e., channel) region (consisting of Ni₃Al and Ni₃V phases) were characterized in terms of size, volume fraction and number density of primary Ni₃Al precipitates. The high-temperature hardness was evaluated as a function of temperature. The volume fraction of primary Ni₃Al phase precipitates, and interfacial area between primary Ni₃Al precipitates and channel region were found to be important factors affecting the hardness of the dual two-phase intermetallic alloys. Possible mechanisms responsible for the observed extra hardening were discussed, taking the role of interfaces among the constituent phases into consideration.     

Influence of oxygen on the crystallization behavior of Zr65Cu27.5Al7.5 and Zr66.7Cu33.3 metallic glasses

The influence of oxygen on the crystallization behavior of Zr_65−xCu_27.5Al_7.5O_x (x=0.14, 0.43 and 0.82) and Zr_66.7−xCu_33.3O_x (x=0.14 and 0.82) metallic glasses has been studied. The supercooled liquid regime (ΔT_x) decreases with increase in oxygen content for the Zr–Cu–Al alloy, while it increases for the Zr–Cu metallic glass. In the case of the Zr–Cu metallic glass, the crystallization product (Zr₂Cu) is not influenced by the oxygen content, while in Zr–Cu–Al alloys the oxygen level has a strong influence on the crystallization sequence. At low oxygen level (x=0.14), the ternary glass crystallizes polymorphously to Zr₂(Cu,Al). At higher oxygen content, the ternary amorphous alloy crystallizes in two stages by primary crystallization into an icosahedral phase and subsequently to the stable Zr₂(Cu,Al) phase. Three-dimensional atom probe results have shown that the composition of the icosahedral and amorphous phases is close to Zr₇₅Cu₁₅Al₅O₅ and Zr₆₂Cu₂₄Al₁₄, respectively.