大塑性变形制备超细晶/纳米晶Al-Mg铝合金的研究进展

近年来,大塑性变形(SPD)制备具有先进结构和功能的超细晶和纳米晶Al-Mg铝合金的研究取得了很大进展。SPD后,合金的晶粒显著细化、位错密度提高及有非平衡晶界和晶界偏析形成,这些微观结构导致合金的强度、硬度大幅提高。然而,SPD合金的塑性普遍较低。综述了SPD制备的Al-Mg铝合金在结构和性能方面的一些最新研究成果。     

Al-Mg合金的晶界内耗

研究了退火、淬火状态下 Al-Mg 合金的晶界内耗。实验表明,随 Mg 含量的增加,晶界内耗峰的峰宽和弛豫强度单调下降,峰位开始移向高温,后又移向低温;高温淬火处理能压低 Al-2.4wt-%Mg的晶界峰,但对其它低 Mg 含量试样不敏感,一定温度的保温能使压低的晶界峰回升。     

Effect of Interstitial Hydrogen on Cohesive Strength of Al Grain Boundary with Mg Segregation

The effect of interstitial hydrogen on the cohesion of the Al ∑ =11（113） grain boundary （GB） is investigated based on the thermodynamic model of Rice-Wang using the first-principles density tunction calculation. I he results indicate that interstitial H behaves as an embrittler from ＂strengthening energy＂ analysis. The reduced GB cohesion due to the presence of H at the GB is attributed to the low affinity between H and Al, and the weakened bonding of Al atomic pairs perpendicular to GB plane.     

Al-Mg合金的解理与沿晶断裂

研究了化学成分和拉伸条件为Al-Mg合金的断裂行为的影响。结果表明，钠含量对合金的拉伸性能和断裂行为有显著的影响，含钠量较高的合金显示出很低的延伸率，而钠含量较低的合金具有很高的延伸率。断口观察表明，含钠量较高的合金发生脆性断裂，断口为解理和沿晶混合型。而钠含量较高的合金发生韧性断裂，断口为韧窝型。在所研究的范围内，镁对合金断裂行为的影响不大，钠是导致合金发生脆性断裂的原因。     

Kinetics of directed oxidation of Al-Mg alloys in the initial and final stages of synthesis of Al2O3/Al composites

In the directed oxidation of Al-Mg alloys, MgO forms in the initial stage. The mechanism of formation of MgO from the Al-Mg alloy in the initial stage of oxidation was studied. The variables studied were the total pressure in the reaction chamber and partial pressure of oxygen. The oxidation rate in the initial stage was proportional to both the oxygen partial pressure and oxygen diffusivity. These results suggest that MgO forms by reaction-enhanced vaporization of Mg from the alloy followed by oxidation of the Mg vapour in the gas phase. The end of the initial stage corresponds to the arrival of the oxygen front close to the melt surface, when spinel formation occurs.

The kinetics of formation of Al₂O₃ in the growth stage of directed oxidation of the Al-5wt.% Mg alloy was also investigated as a function of time, temperature and oxygen partial pressure. The growth rate decreased as a function of time, was practically independent of oxygen pressure and exhibited an activation energy of 361 kJ mol⁻¹. In the growth stage, the kinetics of oxidation is controlled by the rate of transport of oxygen through the alloy layer near the surface to the alumina-alloy interface.     

Segregation of reactive elements at oxide grain boundaries in FeCrAlRE alloys

Abstract

Extensive previous research has established that the oxidation of FeCrAl alloys at temperatures ≥1000°C results in the formation of α-Al₂O₃ oxide scales, and that minor alloy constituents (particularly Reactive Elements (RE) such as Y, Hf, Zr, etc.) can change the oxide growth mechanism. A knowledge of the segregation behaviour of these REs is thus central to our understanding of the oxidation behaviour of these, technologically important, range of alloys.

The new SuperSTEM microscope at the Daresbury Laboratory offers considerable potential for a detailed study of the segregation to oxide grain boundaries at the atomic level. The microscope has an aberration corrector fitted to the objective lens, allowing the formation of sub-Angstrom probe for simultaneous ultra-high resolution high angle annular dark field (HAADF) imaging and atomic-column electron energy loss spectroscopy (EELS). This paper reports on an initial study of oxide grain boundary segregation in commercial and model FeCrAlRE alloys containing controlled additions of the reactive elements, yttrium, zirconium and hafnium oxidised at 1250°C, in air, for 50 hours. Both yttrium and hafnium are shown to segregate to the grain boundaries while hafnium rich particules form in the outer region of the scale.     

The roles of grain orientation and grain boundary characteristics in the enhanced superelasticity of Cu71.8Al17.8Mn10.4 shape memory alloys

Through texture and grain boundary control by continuous unidirectional solidification, the continuous columnar-grained polycrystalline Cu_71.8Al_17.8Mn_10.4 shape memory alloys were prepared and possess a strong 〈0 0 1〉 texture along the solidification direction and straight low-energy grain boundary. The alloys show excellent superelasticity of 10.1% improved from 3% for ordinary polycrystalline counterpart and with a tiny residual strain of less than 0.3% after unloading. There are some reasons for the enhanced superelasticity: (1) The martensitic transformation of all grains with strong 〈0 0 1〉-oriented texture occur at the same time under the tensile loading, which can avoid the significant stress concentration problem and transformation strain incompatibility at the grain boundaries due to the high elastic anisotropy in ordinary polycrystalline alloy. (2) High phase transformation strain can be obtained along 〈0 0 1〉 grain orientation. (3) Straight low-energy grain boundary and the absence of grain boundary triple junctions of continuous columnar-grained polycrystals can significantly reduce the blockage of martensitic transformation at the grain boundaries. These results provide a reference to structure design of high-performance polycrystalline Cu-based shape memory alloys.     

Understanding formation of Mg-depletion zones in Al-Mg alloys under high pressure torsion

Redistribution of elements may take place in alloys during severe plastic deformation, which significantly alters the mechanical properties of the alloys. Therefore, comprehensive knowledge about deformationinduced redistribution of elements has to be established. In the present paper, the distribution of Mg in an Al-Mg alloy processed by high pressure torsion was examined using atom probe tomography(APT).With crystallographic information extracted by APT data analysis, this research reveals that the movement of dislocations plays an important role in the formation of Mg-depletion zones in the deformed microstructure.     

Abnormal grain growth in Al of different purity

The transition from normal to abnormal grain growth has been studied in four Al alloys of various purity (2N, 3N, 4N and 5N). The temperature and time for the onset of abnormal grain growth depend strongly on the deformation and homogenization treatment. Generally, the formation of large grains before cold rolling makes easier the transition to abnormal grain growth during the subsequent annealing. The abnormal grain growth can take place only above a certain temperature which decreases with increasing alloy purity. The onset time of the abnormal grain growth decreases with increasing temperature. It can be qualitatively explained by the dissolution of submicron particles of a second phase.     

纳米晶Al及Al-Mg合金的合成与性能研究进展

主要总结了纳米晶Al及Al-Mg合金的合成与性能研究进展,详尽介绍了用低温球磨法制备纳米晶Al及Al-Mg合金的过程,简要分析了影响低温球磨过程的因素,并探讨了球磨过程对显微组织的影响;介绍和评述了纳米晶Al的热稳定性,并在与粗晶Al及Al-Mg合金对比的基础上,介绍了纳米晶Al及Al-Mg合金的力学性能以及蠕变性能等方面的研究进展.     

Significance of crystallographic grain orientation for oxide scale formation on FeCrAl ODS alloys studied by AFM and MCs+-SIMS

Abstract

Single crystalline specimens were prepared by spark erosion of large grains in an extremely coarse textured bar of an FeCrAl based ODS alloy. These flat single crystalline specimens were studied in respect to oxidation behaviour at 800°C and 1,000°C in air. Initially the oxide scales appeared to grow by outward diffusion of aluminium but subsequently scale growth proceeded by grain boundary oxygen diffusion. The composition, grain size and growth rate of the transient oxide appeared to be dependent on the crystallographic orientation of the alloy. After the inward diffusion of oxygen had become dominant, the oxides on specimens with different crystallographic orientations showed clear differences in their growth rates. Correlation of SIMS results with quantitative grain size analysis performed by AFM showed that the different oxide growth rates, which pertained during longer exposure times, were related to differences in the oxide grain size and therefore the density of oxygen diffusion paths with different crystallographic orientations.     

改善Al-Mg合金流动性及力学性能的新方法

目的改善Al-Mg合金的流动性和组织性能。方法向Al-Mg合金中加入含有掺杂型Ti Cx的Al-Ti-C-B晶种合金(简称Al-Ti-C-B),借助其对Al-Mg合金枝晶大小及形貌调控,研究其对Al-Mg合金流动性和组织性能的影响,并与传统的Al-Ti-B中间合金细化剂进行对比。结果加入Al-Ti-C-B后Al-Mg合金的流动性和力学性能均高于Al-Ti-B,螺旋流动性试样长度由692 mm提高到937 mm,提高了约35.4%;Al-Mg合金的拉伸强度分别由192 MPa提高至216 MPa,伸长率由2.1%提高至4.1%,分别提高了12.5%和95.2%。结论借助Al-Ti-C-B及其晶种技术能够实现对Al-Mg合金流动性和力学性能的同步提升。     

Texture design for microwave dielectric (Ca0.7Nd0.3)0.87TiO3 ceramics through reactive-templated grain growth

Abstract

Plate-like Ca₃Ti₂O₇ (CT) and Nd₂Ti₂O₇ (NT) particles were synthesized in molten salts and used as reactive templates for the preparation of highly textured (Ca_0.7Nd_0.3)_0.87TiO₃ bulk ceramics (CNT) with preferred pseudocubic 〈100〉 and 〈110〉 orientations, respectively. During flux growth CT and NT particles developed facets parallel to the pseudocubic {100} and {110} planes, respectively, in a perovskite unit cell, since those planes correspond to the interlayers of the layered perovskite-type crystal structures. Complementary reactants for the CNT stoichiometry were wet-mixed with the reactive templates and the slurries were tape-cast. Then stacked tapes were heat-treated for dense single-phase CNT ceramics with a distorted and A-site deficient regular perovskite-type structure. The CNT ceramics prepared with CT and NT reactive templates exhibited strong pseudocubic 100- and 110-family x-ray diffraction peaks, respectively, with other peaks drastically suppressed when non-perovskite sources were used as complementary reactants. The textured ceramics possess unique microstructures; as either parallel or obliquely stacked block structures with a pseudocubic {100} plane faceted. The pseudocubic {100}-and {110}-textured CNT ceramics exhibited ～10 and ～20% higher products of the dielectric quality factor and frequency, Q · f, respectively, than conventional ceramic sintered at the same temperature. When Q · f is compared based on the same grain size, the {100}-textured CNT exhibited 27% higher values than non-textured while relative permittivity and temperature coefficient of resonant frequency were of similar values. Simple geometrical relationships between electric field and penetrated pseudocubic {hk0}-type grain boundaries must lead to the reduced scattering and dielectric loss.     

离子注N对6061铝合金表层晶粒取向的影响

通过掠入射X射线衍射及面探扫描实验研究了经氮离子注入后，6061铝合金表层的晶粒取向。结果表明离子注入对铝合金表层的晶粒取向有较大影响，并且随着注入时间的增加，其晶粒取向也发生变化。同时对离子注入层中晶粒取向的形成进行了探讨。     

Abnormal grain growth via the migration of planar growth interfaces

Nanocrystalline nickel electrodeposits undergo a sequence of morphologically distinct grain growth stages during annealing. The nanostructure initially undergoes a rapid sequence of abnormal grain growth, followed by a much slower normal grain growth stage. Out of this uniformly growing structure comes a second stage of abnormal grain growth which not only accelerates the overall growth rate, but the transformation also occurs by the migration of planar reaction fronts. These planar growth interfaces are composed of many individual grain boundary segments, migrating together at essentially the same velocity. Grain shape was studied from intergranular fracture surfaces; it was found that the abnormally growing grains were cuboidal in shape and were present either as individually growing grains or as cuboid clusters. Electron backscatter diffraction showed numerous twin-related cuboidal grain clusters having complex compound planes.     

Influence of grain size on tensile properties of Al-Mg alloys

Abstract

Grain size refinement is an important strengthening mechanism in Al-Mg 5000 series alloys, which have a relatively large Hall-Petch slope compared with other Al alloys. In addition, the high work hardening rate exhibited by Al-Mg alloys provides excellent formability. This paper investigates the influence of grain size on the flow stress over a range of strains, and in several different Al-Mg alloys. It is found that the Hall-Petch slope decreases after yield, indicating that the large grain size effect is primarily associated with initiating plasticity in these alloys. Beyond yield the slope decreases to a value equivalent to other, non-Mg containing alloys, and shows no clear dependence on strain. The intercept stress from the Hall-Petch plots at different strains is non-linear with ? 1/2 for alloys containing up to 3 wt-%Mg, which indicates that the free slip distance is strain dependent in these alloys. In an Al-5 wt-%Mg alloy the intercept stress is linear with ? 1/2, indicating that solute atoms are controlling the free slip distance. If Mn is added to the Al-5 wt-%Mg alloy, as it is in commercial alloys, it has little influence on the grain size dependence, but it does increase the frictional stress at the highest Mn level of 0.7 wt-%.     

Beta grain growth behaviour of TG6 and Ti17 titanium alloys

The grain growth kinetics of TG6 and Ti17 alloys in the β phase have been studied. The grain size parameters have been obtained by means of the image analysis software for different heat treatment temperatures and times. The growth time exponents and activation energies have also been determined. The activation energies for grain growth of both alloys increase with the increase in soaking time. This trend might be due to the different solute drag effect. The uniformity of grain size under each heat treatment has also been studied and the variations resulted from the combined action of nucleation rate of β phase and β grain growth velocity.     

Effects of solute content on microstructure of nano precipitate-fine grain synergistically reinforced copper alloys

ABSTRACT

Alloying of Fe, Co was reported to tailor microstructure of copper alloys into a nanoprecipitate-fine grain (NPFG) structure, i.e. nano-sized iron-rich precipitates dispersed inside refined grains. Here, we investigate the solute effect of Sn, Zn on NPFG structure in as-cast copper samples. Mechanisms are proposed to account for the solute effect on precipitate and grain features. Solutes restrict coarsening but facilitate undesirable morphology transition from spherical to angular of iron-rich precipitates. Meantime, solutes allow more precipitates to be active in the nucleation of copper and consequently induce finer grains. Minor Sn is added to optimise NPFG structure and leads to an excellent strength–ductility combination in Cu–1.5Fe–0.1Sn (wt-%) alloy. This work provides a solute-alloying strategy to achieve desired mechanical properties in metals.     

Grain size, grain boundary sliding, and grain boundary interaction effects on nanocrystalline behavior

A dislocation–density grain boundary (GB) interaction scheme, a GB misorientation dependent dislocation–density relation, and a grain boundary sliding (GBS) model are presented to account for the behavior of nanocrystalline aggregates with grain sizes ranging from 25 nm to 200 nm. These schemes are coupled to a dislocation–density multiple slip crystalline plasticity formulation and specialized finite element algorithms to predict the response of nanocrystalline aggregates. These schemes are based on slip system compatibility, local resolved shear stresses, and immobile and mobile dislocation–density evolution. A conservation law for dislocation–densities is used to balance dislocation–density absorption, transmission and emission from the GB. The relation between yield stresses and grain sizes is consistent with the Hall–Petch relation. The results also indicate that GB sliding and grain-size effects affect crack behavior by local dislocation–density and slip evolution at critical GBs. Furthermore, the predictions indicate that GBS increases with decreasing grain sizes, and results in lower normal stresses in critical locations. Hence, GBS may offset strength increases associated with decreases in grain size.     

Grain boundary influence during short fatigue crack growth using a discrete dislocation technique

We have studied the effect of a grain boundary in front of a short edge crack on its propagation under cyclic loading conditions in bcc iron. The used model is a combination of a discrete dislocation formulation and a boundary element approach where the boundary is described by dislocation dipole elements, while the local plasticity is modeled by discrete dislocations. The grain boundary is considered impenetrable, but dislocations positioned in the vicinity of a grain boundary give raise to high stresses in neighboring grains which, eventually, results in nucleation of dislocations and a spread of the plastic zone into the next grain. __________ Translated from Problemy Prochnosti, No. 1, pp. 163–166, January–February, 2008.