A new model for prediction of dispersoid precipitation in aluminium alloys containing zirconium and scandium

A model has been developed to predict precipitation of ternary Al₃(Sc, Zr) dispersoids in aluminium alloys containing zirconium and scandium. The model is based on the classical numerical method of Kampmann and Wagner, extended to predict precipitation of a ternary phase. The model has been applied to the precipitation of dispersoids in scandium containing AA7050. The dispersoid precipitation kinetics and number density are predicted to be sensitive to the scandium concentration, whilst the dispersoid radius is not. The dispersoids are predicted to enrich in zirconium during precipitation. Coarsening has been investigated in detail and it has been predicted that a steady-state size distribution is only reached once coarsening is well advanced. The addition of scandium is predicted to eliminate the dispersoid free zones observed in scandium free 7050, greatly increasing recrystallization resistance.     

Interactions between zirconium and manganese dispersoid-forming elements on their combined addition in Al–Cu–Li alloys

The present work aims to clarify the interactions that can occur between Zr and Mn in their role as dispersoid-forming elements in Al–Cu–Li alloys when they are jointly added to rolled products. A combination of Zr and Mn are used because their opposite microsegregation patterns can potentially increase the uniformity of the dispersoid pinning pressure, and help prevent recrystallization. It is shown, in the AA2198 base alloy studied, that their combined addition in fact reduced recrystallization resistance. The source of this apparent contradiction has been investigated in detail by examining the behaviour of the dispersoids they form, Al₃Zr and Al₂₀Cu₂Mn₃, from the as-cast ingot to the final rolled product. The Al₃Zr dispersoids were found to be subtly affected by the presence of Mn, which reduced their number density. The main mechanism responsible is the incorporation of low levels of Zr in Mn-rich particles, which slightly reduces the Zr supersaturation in the matrix. Estimates of the level of matrix Zr loss, and modelling of dispersoid precipitation, have been used to demonstrate the high level of sensitivity of this effect on the Al₃Zr dispersoid-free band widths present and resultant recrystallization resistance when the alloy is rolled to sheets.     

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.     

Identification and analysis of intermetallic phases in overaged Zr-containing and Cr-containing Al-Zn-Mg-Cu alloys

This paper investigates the effect of alloying elements on the characteristics of intermetallic phases in Zr-containing and Cr-containing 7xxx Al-Zn-Mg-Cu alloys at overaged conditions. Four Al-Zn-Mg-Cu alloy plates with different alloying element contents were studied by optical microscopy based image analysis, differential scanning calorimetry, scanning electron microscopy combined with energy disperse X-ray spectroscopy and transmission electron microscopy. The grain structures, recrystallisation, intermetallic phases and precipitates in the selected alloys have been analyzed and the presence of coarse intermetallic phases has been interpreted using established phase diagrams. The different effects of Zr or Cr addition to the alloys have been compared. The experimental results showed that the recrystallised area fraction of Zr-containing alloys is less than that of Cr-containing alloys, being attributable to Zr reducing recrystallisation more effectively than Cr. The detected particles are mainly S phase, Al₇Cu₂Fe, as well as dispersoids of Al₃Zr for Zr-containing alloys and Cr-rich E phase for Cr-containing alloys. These coarse particles, especially the S phase which cannot be dissolved during solution treatment, are detrimental to the fracture toughness of the alloys.     

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.     

Stability of nugget zone grain structures in high strength Al-alloy friction stir welds during solution treatment

From studying the solution treatment behaviour of friction stir welds, in a typical high strength Al-alloy (7010), it has been established that the nugget zone grain structure is inherently unstable at high temperatures, despite the presence of Al₃Zr dispersoids that inhibit grain boundary mobility. Good agreement has been found between experimental observations and a unified theory of the stability of cellular microstructures, proposed by Humphreys, which has shown that the condition for instability is highly dependent on the welding parameters. Low heat inputs result in an exceptionally fine nugget grain structure, and abnormal grain growth occurs throughout the nugget zone, encouraged by the dissolution of soluble precipitates. When welds are produced with higher heat inputs, instability is more marginal, as the grain structure after welding is coarser relative to the dispersoid density. However, grains can still grow abnormally into the nugget zone with a planar front and this leads to very large, mm-scale, grains being formed.     

The effect of dispersoids on the grain refinement mechanisms during deformation of aluminium alloys to ultra-high strains

The effect of fine dispersoids on the mechanisms and rate of grain refinement has been investigated during the severe deformation of a model aluminium alloy. A binary Al–0.2Sc alloy, containing coherent Al₃Sc dispersoids, of ∼20 nm in diameter and ∼100 nm spacing, has been deformed by equal channel angular extrusion to an effective strain of ten. The resulting deformation structures were quantitatively analysed using high-resolution electron backscattered diffraction orientation mapping, and the results have been compared to those obtained from a single-phase Al–0.13Mg alloy, deformed under identical conditions. The presence of fine, non-shearable, dispersoids has been found to homogenise slip, retard the formation of a cellular substructure and inhibit the formation of microshear bands during deformation. These factors combine to reduce the rate of high-angle grain boundary generation at low to medium strains and, hence, retard the formation of a submicron grain structure to higher strains during severe deformation.     

Precipitation kinetic of Al3(Sc,Zr) dispersoids in aluminium

The non-isothermal formation of Sc- and Zr-containing dispersoids in Al–Zr–Sc ternary alloys has been investigated by atom probe tomography (APT). In the early stages of precipitation, a high number density of Sc-rich clusters form. These clusters subsequently transform into Al₃Sc particles with a L1₂ structure. When Zr diffusion becomes significant, Zr atoms are found to segregate to Al₃Sc/α-Al matrix interfaces. Further annealing at 748 K gives rise to a duplex core/shell structure.     

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)。     

用球差校正扫描透射电镜研究人工时效7075铝合金的耐蚀性（英文）

使用先进的球差校正扫描透射电子显微镜研究人工时效7075铝合金耐腐蚀性增强机制。通过阻抗谱、等效电路分析、极化测量和浸泡试验,研究人工时效7075铝合金在3.5%(质量分数) NaCl溶液中的腐蚀行为。结果表明,人工时效时间越长,7075铝合金的耐腐蚀性越好。这可能归因于扫描透射电镜技术揭示的以下显微组织特征：在过时效条件下,晶界处的铜偏析有助于延缓晶间腐蚀,在Al₁₈Mg₃(Cr,Mn)₂分散体表面形成的Mg(Zn,Cu)₂沉淀有效地将作为腐蚀阴极的分散体与铝基体隔离。这项研究证明,可以通过适当的合金化和人工时效来设计耐腐蚀合金的可能性。     

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.     

Effect of Ca addition on microstructure of twin-roll cast AZ31 Mg alloy

A twin-roll cast (TRC) AZ31-0.7Ca alloy sheet has been subjected to thermo-mechanical treatment (TMT) followed by annealing and its microstructure was evaluated. The as-cast microstructure of TRC AZ31-0.7Ca alloy is essentially similar to that of TRC AZ31 alloy, except for the presence of Al₂Ca dispersoid particles. The presence of Al₂Ca particles imparts significant changes to the microstructure of TMT-ed TRC alloy such as the refinement of grain size and evolution of textures different from those of ingot cast and TRC AZ31 alloys.     

The high-temperature oxidation of Ni-20%Cr alloys containing various oxide dispersions

The oxidation behavior of Ni-20%Cr alloys containing approximately 3 vol.% Y₂O₃, ThO₂, and A1₂O₃ as dispersed particles has been examined in the temperature range 900 to 1200° C in slowly flowing oxygen at 100 Torr. The results show that the oxidation behavior of the Y₂O₃-, ThO₂-, Al₂O₃-, and Ce0₂-containing alloys is very similar and that some anomalies in the behavior of the ThO₂-containing alloy might be explained by the slower rate of chromium diffusion in this coarse-grained alloy. Two Al₂O₃-containing alloys were studied. One with a relatively coarse dispersoid size behaved in a manner analogous to a dispersion-free Ni-30% Cr alloy at 1100°C. The other alloy contained a dispersion of fine Al₂O₃ particles and behaved exactly like the Y₂O₃-containing alloy at 1000 and 1100°C, but at 1200° C oxidized at a faster rate. It has been shown that the adherent scales on dispersion-containing alloys have a stabilized fine grain size, whereas the nonadherent scales on dispersion-free alloys undergo grain growth.This work has been supported by the Naval Air Systems Command under Contract No. N00019-72-C-0190.     

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 three-step homogenization on microstructure and properties of 7N01 aluminum alloys

The effect of different homogenization treatments on the microstructure and properties of the 7N01 aluminum alloy was investigated using hardness measurements, electrical conductivity measurements, tensile and slow strain rate tests, electron probe microanalysis, optical microscopy, scanning electron microscopy, and transmission electron microscopy. The results revealed that three-step homogenization improved the uniformity of Zr distribution by eliminating segregation of the main alloying elements. During the second homogenization step at 350 °C for 10 h, coarse and strip-like equilibrium η phases formed which assisted the nucleation of Al₃Zr dispersoids and reduced the width of the precipitate-free zone of A1₃Zr dispersoids. As a result, coarse recrystallization was greatly reduced after homogenization at 200 °C, 2 h + 350 °C, 10 h + 470 °C, 12 h, which contributed to improving the overall properties of the 7N01 aluminum alloys.     

单级及双级均匀化处理工艺对AA7150合金Al3Zr弥散相析出的影响（英文）

用半定量电子探针、扫描电镜、透射电镜方法,分析了单、双级均匀化对AA7150合金中Al3Zr分布的影响。结果表明,在铸态条件下Zr在枝晶心部浓度高,枝晶边缘处浓度低。在均匀化温度460°C下处理20h并不足以改变其浓度梯度。当均匀化温度达到460～480°C时,在枝晶心部可获得大量的Al3Zr弥散相,且Al3Zr弥散相的尺寸随着单双级均匀化处理温度的升高而增大。     

Discontinuities in the porous anodic film formed on AA2099-T8 aluminium alloy

The anodizing behaviour of constituent particles (Al–Fe–Mn–Cu) and dispersoids (Al–Cu–Mn–Li and β′(Al₃Zr)) in AA2099-T8 has been investigated. Low-copper-containing Al–Fe–Mn–Cu particles anodized more slowly than the alloy matrix, forming a highly porous anodic oxide film. Medium- and high-copper-containing Al–Fe–Mn–Cu particles were rapidly dissolved, resulting in defects in the anodic film. The anodizing of Al–Cu–Mn–Li dispersoids is slightly slower than the alloy matrix, forming a less regular anodic oxide film. β′(Al₃Zr) dispersoids anodized at a similar rate to the alloy matrix. Further, the potential impact of the discontinuities in the resultant anodic films on the performance of the filmed alloy is discussed.     

Effect of the Size of Al<Subscript>3</Subscript>(Sc,Zr) Precipitates on the Structure of Multi-Directionally Isothermally Forged Al-Mg-Sc-Zr Alloy

The effect of Al₃(Sc,Zr) dispersoids on the evolution of the cast Al-Mg-Sc-Zr alloy structure under multi-directional isothermal forging (MIF) has been investigated. The alloy, which has an equiaxed grain structure with a grain size of ~25 μm and contains dispersoids 5–10 and 20–50 nm in size after onestage (at 360°C for 6 h) and two-stage (360°C for 6 h + 520°C for 1 h) annealing, respectively, was deformed at 325°C (~0.65 T_m) up to cumulative strains of e = 8.4. In the initial stages of MIF, new fine (sub)grains surrounded by low-angle and high-angle boundaries (HABs) were formed near the initial grain boundaries. With increasing strain, the volume fraction and misorientation of these crystallites increased, which led to the replacement of a coarse-grained structure with a fine-grained one with a grain size of ~1.5-2.0 μm. Dynamic recrystallization occurred in accordance to a continuous mechanism and was controlled by the interaction of lattice dislocations and/or (sub)grain boundaries with dispersoids that effectively inhibited the migration of boundaries, as well as the rearrangement of lattice dislocations and their annihilation. The particle size and the density of their distribution significantly affected the parameters of the evolved structure; in an alloy with smaller particles, a structure with a smaller grain size and a larger HAB fraction developed.     

Effect of magnesium on dispersoid strengthening of Al—Mn—Mg—Si (3xxx) alloys

The effects of magnesium addition on the dispersoid precipitation as well as mechanical properties of 3xxx alloys were investigated. The microstructures in as-cast and heat-treated conditions were evaluated by optical microscopy and transmission electron microscopy. The results reveal that Mg has a strong influence on the distribution and volume fraction of dispersoids during precipitation heat treatment. The microhardness and yield strength at ambient temperature increase with increasing Mg content. The solid solution and dispersoid strengthening mechanisms of materials after heat treatment are quantitatively analyzed. Dispersoid strengthening for the alloys is the predominant strengthening mechanism after precipitation heat treatment. An analytical model is introduced to predict the evolution of ambient-temperature yield 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.