The grain refinement mechanism of cast aluminium by zirconium

The mechanism underlying the grain refinement of cast aluminium by zirconium has been studied through examination of a range of Al alloys with increasing Zr contents. Pro-peritectic Al₃Zr particles are reproducibly identified at or near the grain centres in grain-refined alloy samples based on the observations of optical microscopy, scanning electron microscopy and X-ray diffraction. From the crystallographic study using the edge-to-edge matching model, electron backscatter diffraction and transmission electron microscopy, it is substantiated that the Al₃Zr particles are highly potent nucleants for Al. In addition, the effects of Al₃Zr particle size and distribution on grain refinement has also been investigated. It has been found that the active Al₃Zr particles are bigger than previously reported other types of active particles, such as TiB₂ for heterogeneous nucleation in Al alloys. Considering the low growth restriction effect of Zr in Al (the maximum Q-value of Zr in Al is 1.0 K), it is suggested that the significant grain refinement of Al resulting from the addition of Zr can be mainly attributed to the heterogeneous nucleation facilitated by the in situ formed Al₃Zr particles.     

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.     

Effects of minor Sc and Zr additions on mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys

The effects of minor Sc and Zr additions on the mechanical properties and microstructure evolution of Al−Zn−Mg−Cu alloys were studied using tensile tests, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The ultimate tensile strength of the peak-aged Al−Zn−Mg−Cu alloy is improved by about 105 MPa with the addition of 0.10% Zr. An increase of about 133 MPa is observed with the joint addition of 0.07% Sc and 0.07% Zr. For the alloys modified with the minor addition of Sc and Zr (0.14%), the main strengthening mechanisms of minor addition of Sc and Zr are fine-grain strengthening, sub-structure strengthening and the Orowan strengthening mechanism produced by the Al₃(Sc,Zr) and Al₃Zr dispersoids. The volume of Al₃Zr particles is less than that of Al₃(Sc,Zr) particles, but the distribution of Al₃(Sc,Zr) particles is more dispersed throughout the matrix leading to pinning the dislocations motion and restraining the recrystallization more effectively.     

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 ).     

Microstructures and mechanical properties of in-situ TiB2/Al−xSi−0.3Mg composites

In-situ 2 vol.% TiB₂ particle reinforced Al?xSi?0.3Mg (x=7, 9, 12, 15 wt.%) composites were prepared by the salt–metal reaction, and the microstructures and mechanical properties were investigated. The results show that the TiB₂ particles with a diameter of 20–80 nm and the eutectic Si with a length of 1–10 μm are the main strengthening phases in the TiB₂/Al?xSi?0.3Mg composites. The TiB₂ particles promote grain refinement and modify the eutectic Si from needle-like to short-rod shape. However, the strengthening effect of TiB₂ particles is weakened as the Si content exceeds the eutectic composition, which can be attributed to the formation of large and irregular primary Si. The axial tensile test results and fractography observations indicate that these composites show more brittle fracture characteristics than the corresponding alloy matrixes.     

Coherency loss of Al3(Sc,Zr) precipitates by deformation of an Al–Zn–Mg alloy

Al alloys with additions of Sc and/or Zr exhibit a reasonably stable grain structure due to a uniform distribution of coherent Al₃(Sc,Zr) precipitates that forms at temperatures >300 °C. These precipitates are stable up to the solution treatment temperature and are able to pin subgrain and grain boundaries, inhibiting grain coarsening. The crystallographic structure of these precipitates presents a L1₂ superstructure coherent with the face-centred cubic Al matrix. Changes in the orientation relation between precipitates and the matrix are described in deformed, recovered and partially recrystallized samples of extrusions of AW7010 (AlZn6Mg2Cu2). The coherency of the intracrystalline Al₃(Sc,Zr) precipitates present in the extrusions is lost by severe deformation performed by an equal channel angular pressing process, which produced a fine-grained microstructure. The deformed sample recovers, forming a subgrain structure with restored coherency of the Al₃(Sc,Zr) precipitates. Rapid heating to 470 °C causes partial secondary recrystallization, which transforms the precipitates within the recrystallized grains into incoherent groups of particles that maintain their original orientation with each other.     

同时添加钪、锆对Al-10Mg合金再结晶机制的影响

利用电子背散射衍射(EBSD)和透射电子显微镜(TEM)研究了Al-10Mg及Al-10Mg-0.1Sc-0.1Zr合金在热压缩过程中的组织演变及动态再结晶机制。结果表明：同时添加Sc、Zr能够明显细化Al-10Mg合金的铸态晶粒,热处理后,Sc、Zr能够形成与α-Al基体共格的Al₃(Sc,Zr) 相,这些沉淀相能够提高合金的热变形抗力;在变形过程中,Al₃(Sc,Zr)相能够钉扎位错运动、降低晶界及变形带处的位错密度,使位错在沉淀相周围聚集,因而改变了Al-10Mg合金内部位错增殖与湮灭的过程、进而使Al-10Mg合金动态再结晶方式由不连续动态再结晶(DDRX)转变为连续动态再结晶(CDRX)。     

Effect of minor Sc additions on structure,age hardening and tensile properties of aluminium alloy AA8090 plate

The grain structure, age hardening response, precipitate structure and tensile properties of AA8090 alloy plate without and with minor additions of Sc were investigated. Primary aluminide particles were present in all alloys. Sc additions ⩾0.43% resulted in refinement of the as-cast grain structure. Further investigations on 0.11% and 0.22% Sc containing alloys showed that the subgrain size was finer while the number density of nano-scale Al₃(Sc,Zr)/Al₃Li composite precipitates and the artificially aged hardness were higher for the Sc containing alloys. There was a marginal increase in the yield strength with a concomitant reduction in ductility with increasing Sc content in the alloys.     

Effects of nano-sized TiB2 particles and Al3Zr dispersoids on microstructure and mechanical properties of Al–Zn–Mg–Cu based materials

The effects of TiB₂ and Zr on the microstructure, aging response and mechanical properties of hot-extruded Al–Zn–Mg–Cu based materials were investigated and compared by multi-scale microstructure characterization techniques. The results showed that proper addition of TiB₂ particles could refine grain size during solidification, promote dynamic recrystallization during extrusion, and inhibit grain growth during solution treatment. Meanwhile, Zr addition had minor influence on the grain refinement during solidification, but could effectively suppress recrystallization and grain growth compared with the Zr-free alloy. Furthermore, the TiB₂ addition could simultaneously enhance the aging kinetics and peak-aged hardness of the materials. Comparatively, Zr addition could also improve the peak-aged hardness with minor effect on the aging kinetics of the materials. Finally, the quench sensitivity, elastic modulus and tensile properties of the materials were compared and studied. Specifically, the relationship between the microstructure and mechanical properties, and the strengthening mechanisms were discussed in detail.     

微量Sc和Zr对Al—Mg合金铸态组织的晶粒细化作用

制备了Ａｌ５Ｍｇ、Ａｌ５Ｍｇ０．２Ｓｃ、Ａｌ５Ｍｇ０．１Ｚｒ和Ａｌ５Ｍｇ０．２Ｓｃ０．１Ｚｒ四种铸态合金，采用金相显微镜和扫描电镜观察分析了微量Ｓｃ和Ｚｒ对ＡｌＭｇ合金铸态组织的晶粒细化作用及其机理。结果发现，０２％的Ｓｃ并未使ＡｌＭｇ合金产生晶粒细化作用，而０２％的Ｓｃ与０１％的Ｚｒ复合添加则使ＡｌＭｇ合金产生了极其强烈的晶粒细化作用。这一作用的产生，是由于Ｓｃ、Ｚｒ与Ａｌ在合金熔体中生成了Ａｌ３Ｓｃ和Ａｌ３Ｚｒ复合粒子，这种粒子在合金凝固过程中，起到了非均质晶核细化晶粒的作用     

Precipitation of Al3(Sc,Zr) particles in an Al–Zn–Mg–Cu–Sc–Zr alloy during conventional solution heat treatment and its effect on tensile properties

The effect of heat treatment on precipitation and growth of coherent nanometer-sized Al₃(Sc,Zr) particles and the effect of these particles on tensile properties of a direct chill (DC) cast Al–Zn–Mg–Cu–Sc–Zr alloy were studied. The size distribution, average size, number density and volume fraction of the Al₃(Sc,Zr) particles were determined as a function of the solution treatment temperature and time. An increase in the solution treatment temperature and time resulted in Al₃(Sc,Zr) particles with a larger mean diameter, higher volume fraction and lower number density. The particle size distributions were described well by normal (Gaussian) distributions. The kinetics of the phase transformation followed the Kolmogorov–Johnson–Mehl–Avrami law, with the Avrami exponent m = 0.404. Room temperature tensile properties were evaluated in the as-solution treated and artificially aged conditions. The coherent nanometer-sized Al₃(Sc,Zr) particles provided additional Orowan strengthening, which increased with increasing particle volume fraction and decreasing particle size, and varied from 75 to 118 MPa after different heat treatments.     

Al-5Ti-B中间合金的热机械处理对铸造铝合金晶粒细化的强化作用(英文)

采用等温液相退火和塑性变形工艺制备含不同尺寸Al3Ti和TiB2粒子的Al-5Ti-B(质量分数,%)中间合金。结果表明:随着中间合金中Al3Ti和TiB2粒子尺寸的减小,合金的晶粒细化作用得到增强,但不适当地减小Al3Ti和TiB2的粒径可能导致这些颗粒作用减弱和结块,从而降低合金的晶粒细化效率。     

Solidification structure of aluminium with trace additions of Sc and Zr

The effect of hypoeutectic Sc addition, by itself or in combination with 0.14% Zr, on the solidification structure of pure Al was investigated. A strong grain refining effect was observed in the alloy containing 0.45% Sc and 0.14% Zr. No grain refinement occurred in binary Al-Sc alloys or in the ternary alloy containing 0.24% Sc. Detailed microstructural investigations showed the presence of coarse cuboidal particles in the grain refined alloy. These particles were found to be enriched in Sc and Zr, had an L1₂ crystal structure, and shared a cube-on-cube orientation relationship with the α-Al matrix. Consequently, the Al-Sc-Zr particles were considered to be responsible for the observed grain refinement.     

高镁低钪Al-Mg-Sc-Zr合金强韧化机理研究

本文通过常规轧制与退火工艺制备了具有高抗拉强度(~502MPa)和高断后延伸率(~22%)的高镁低钪Al-Mg-Sc-Zr合金,退火工艺为673K/1h。通过X射线衍射仪(XRD)、电子背散射衍射仪(EBSD)和透射电子显微镜(TEM)等手段,研究了合金退火后的组织及其强化机制。结果表明：Al-Mg-Sc-Zr合金在退火后获得了具有尺寸为0.42 μm的小晶粒和尺寸为16.2μm的大晶粒的双峰晶粒组织,固溶镁原子与Al₃(Sc,Zr)相的存在与共同作用促进了具有较大晶格畸变、存在大量亚晶及均匀弥散分布析出相的双峰晶粒组织的形成;合金主要强化方式为镁原子的固溶强化、亚晶界阻碍位错引起的亚晶界强化、细晶强化和Al₃(Sc,Zr)相的弥散强化,且合金计算与实测屈服强度相吻合;高镁固溶度、Al₃(Sc,Zr)相、双峰晶粒及再结晶织构的存在为位错增殖提供了空间,提高了合金加工硬化率,进而提高了合金的延伸率。     

Effect of minor Sc and Zr addition on microstructures and mechanical properties of Al-Zn-Mg-Cu alloys

Three kinds of Al-Zn-Mg-Cu based alloys with 0.22%, 0.36%（Sc＋Zr） （mass fraction, %）, and without Sc, Zr addition were prepared by ingot metallurgy. By using optical microscopy, transmission electronic microscopy and scanning electron microscopy, the effects of microalloying elements of Sc, Zr on the microstructure of super-high-strength Al-Zn-Mg-Cu alloys related to mechanical properties were investigated. The tensile properties and microstructures of the studied alloys under different heat treatment conditions were studied. The addition of minor Sc, Zr results in the formation of Ala（Sc,Zr） particles. These particles are highly effective in refining the microstructures, retarding recrystallization, pinning dislocations and subboundaries. The strength of Al-Zn-Mg-Cu alloys was greatly improved by simultaneously adding minor Sc, Zr, meanwhile the ductility of the studied alloys remains at a higher level. The 0.36%（Sc＋Zr） alloys gain the optimal properties after 465 ℃/h solution and 120 ℃/24 h aging. The increment of strength is mainly due to strengthening of fine grain and substructure and precipitation ofAl3（Sc, Zr） particles.     

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。     

Microstructures evolution and mechanical properties disparity in 2070 Al-Li alloy with minor Sc addition

The microstructures and mechanical properties of Al-3.35Cu-1.2Li-0.4Mg-0.4Zn-0.3Mn-0.1Zr (mass fraction, %) alloy with Sc addition or free Sc were investigated through tensile test, SEM, EPMA and TEM. The addition of 0.082% (mass fraction) Sc element leads to the formation of Cu-rich and Sc-contained nano-sized Al₃(ScZr) particles and W phase particles. The Al₃(ScZr) particles can inhibit recrystallization to a certain extent and impede recrystallized grain growth during solution treatment. It is found that W phase cannot dissolve in supersaturated solid solution during the solution heat treatment, and the Cu content in the solutionized matrix is decreased, which causes a decrease in the fraction of Cu-contained strengthening precipitates with T₁ (Al₂CuLi) and θ′ (Al₂Cu) under T8 aging condition. Due to the formation of the W phases, the small Sc addition causes a little reduction in the strength.     

A new approach to designing a grain refiner for Mg casting alloys and its use in Mg–Y-based alloys

A totally new grain refiner, Al₂Y, for cast Mg alloys has been predicted using the recently developed edge-to-edge matching (E2EM) crystallographic model. An addition of 0.6–1.0 wt.% Al into the Mg–10 wt.% Y melt promotes the in situ formation of Al₂Y, which reduces the average grain size from about 180 to 36 μm. Active nucleation Al₂Y particles were reproducibly observed at the centres of many refined grains. The excellent grain-refining efficiency is comparable to that of Zr for the same alloy, but the thermal stability of the grains refined by Al₂Y is much higher than those refined by Zr up to a temperature of 550 °C for 48 h. The mechanisms of grain refinement and the superior thermal stability of the refined grains due to Al addition in the Mg–Y alloy are discussed based on the current experimental results.     

Effect of ultrasonic vibration on microstructural evolution of the reinforcements and degassing of in situ TiB2p/Al-12Si-4Cu composites

Some issues such as clustering of TiB₂ particles, formation of long rod-like Al₃Ti particles, as well as high porosity level usually associate with the fabrication of in situ TiB_2p/Al-alloy composites via conventional stir casting technique using Ti and B as reactants. High-intensity ultrasonic vibration was introduced in our research to solve the above issues. The process involved that the original in situ TiB_2p/Al-12Si-4Cu sample with large clusters of TiB₂ particles, large long rod-like Al₃Ti particles and high porosity was remelted at 850 °C, and then ultrasonic vibration was applied to the melt with an ultrasonic probe. The microstructural evolution of the samples treated by ultrasonic vibration with different time was examined by using SEM. After treated by ultrasonic vibration for 12 min, large clusters of TiB₂ particles were broken up effectively and TiB₂ particles were dispersed uniformly in the matrix, and long rod-like Al₃Ti particles were turned into blocky ones with the size of 10 μm due to the effect of ultrasonic stirring. In the meantime, the porosity in the composites decreased from about 6.5% to 0.86% due to the effect of ultrasonic degassing. Microhardness test suggested that a homogeneous microstructure of the composite was achieved after ultrasonic treatment. An effective approach using high-intensity ultrasonic vibration to optimize the microstructures of the particulate reinforced Al-alloy composites was proposed, and the mechanism of the effect of high-intensity ultrasonic vibration on the microstructural evolution of the reinforcements and degassing of composites was also discussed in our research.     

Contributions of phase and structural transformations in multicomponent Al-Mg alloys to the linear and nonlinear mechanisms of anelasticity

The effects of the processes of severe plastic deformation (SPD), recrystallization, and precipitation of the β phase in multicomponent alloys of the Al-5Mg-Mn-Cr and Al-(4–5%)Mg-Mn-Zn-Sc systems on the mechanisms of grain-boundary relaxation and dislocation-induced microplasticity have been studied in some detail. To stabilize the ultrafine-grained structure and prevent grain growth, dispersed Al-transition-metal particles, such as Al₃Zr, Al₆Mn, Al₇Cr, Al₆(Mn,Cr), Al₁₈Cr₂Mg₃ have been used. We have special interest in alloys with additions of scandium, which forms compounds of the Al₃Sc type and favors the precipitation of finer particles compared to the aluminides of other transition metals. After SPD, Al-(4–5%)Mg-Mn-Zr-Sc alloys exhibit an enhanced recrystallization temperature. The general features of the dislocation and grain-boundary anelasticity that have been established for the binary Al-Mg alloys are retained; i.e., (1) the decrease in the dislocation density in the process of recrystallization of cold-worked alloys leads to the formation of a pseudo-peak in the curves of the temperature dependences of internal friction (TDIF) and to a decrease in the critical amplitude of deformation corresponding to the onset of dislocation motion in a stress field; (2) the precipitation of the β phase suppresses the grain-boundary relaxation; (3) the dissolution of the β phase, the passage of the magnesium atoms into the solid solution, and the precipitation of the β’ phase upon heating hinder the motion of dislocations; (4) the coarsening of the highly dispersed particles containing Zr and Sc increases the dislocation mobility. The grain-boundary relaxation and dislocation-impurity interaction and their temperature dependences, as well as processes of the additional alloying of the binary alloys by Mn, Cr, Zr, and Sc, have been estimated quantitatively.