Materials Science Forum Vols. 546-549

Abstract: Through-thickness microstructure, texture and mechanical property gradients in the longitudinal section of AA 7055 rolled plate have been investigated using electron back-scattered diffraction (EBSD) technique. Quantitative analysis of the microstructure and texture through the plate thickness was conducted. It was found that the microstructure and texture are obviously non-homogeneous through the plate thickness. From center to surface, more equiaxed grains are observed. As expected, the degree of recrystallization increases with increasing the S value. The grains in the center layer exhibited preferential orientations while the most grains near the surface presented a random one. The strength of each observed layer has also been tested.

Abstract: The effect of minor scandium addition on the microstructures and mechanical properties of Al-Zn-Mg-Cu-Zr alloys has been studied. Three alloys were produced by casting and then extruded to rods. Tensile properties of these alloys in different condition were tested. The relationship between mechanical properties and microstructures of the alloys was discussed in detial. The results showed that the tensile strength of Al-9.0Zn-2.5Mg-1.2Cu-0.15Zr-0.12Sc and Al-9.0Zn-2.5Mg-1.2Cu- 0.15Zr-0.20Sc in solution and T6 reaches 763.7MPa and 774.6MPa, respectively, while the elongation remains 5.7% and 8.3%, respectively.

Abstract: Evolution of flow stress and microstructures of an Al-Zn-Mg series alloy during deformation at elevated temperatures and large ranges of strain rates have been investigated by isothermal compression testing on Gleeble 1500 thermomechanical simulator. The results showed that the flow stress exhibited a characteristic of peak stress followed by smoothing out in the imposed strain rate and temperature ranges. The hot deformation of the alloy is a thermally activated process, which is controlled by rate-controlling mechanisms of dislocation generation and dislocation annihilation. The flow stress decreases with the increase of true strain at higher temperature and lower strain rate, indicating that dynamic softening dominates instead of strain hardening during hot deformation of the alloy. Microstructure observation revealed that typical subgrain cluster structures obviously developed in the original elongated grains of the alloy during hot deformation. The subgrain size of the alloy increased with deformation temperature and downturn of strain rate, where the Zener-Hollomon parameter increased. Fine equiaxed recrystallized grains with large-angle grain boundaries developed significantly in the alloy during hot compression at low Z values, implying activation of dynamic recrystallization.

Abstract: The effect of forward extrusion temperature and rate on the formability and microstructure of 7075 aluminum alloy was investigated by means of scanning electron microscopy (SEM), optical microscopy (OM) and hardometer. The results showed that the outflow rate was in the range of 6~2m/min with the extrusion temperature from 340°C to 420°C. Higher temperature resulted in lower critical extrusion force, decreased permissible extrusion rate and obvious drop in the formability. Lower temperature resulted in improvement in the formability but higher critical extrusion force, which might lead to shuttingdown of the extruder. There were no obvious effects of extrusion temperature and rate on the microstructure and hardness of the as-extruded alloy. It suggests that dynamic recovery or partial recrystallization occurred in the alloy during extrusion.

Abstract: The evolution of microstructure, precipitate size, volume fraction and integrated intensity of Al-8.0Zn-2.05Mg-1.76Cu (7055) and 7055-1.0Li alloys during isothermal ageing has been studied by transmission electron microscopy (TEM) and synchrotron-radiation small angle X-ray scattering (SAXS). According to the TEM results, referring to the thermodynamic phase diagram, it was found that the addition of Li changed the types of the precipitates. T1 phase was observed in the 7055-1.0Li alloy besides the GP zones, η' and η with variant orientations with matrix, η1、η2 and η4. The super-lattice spots of L12 (Cu3Au) structure were probably due to the existence of Al3Li (δ') or Al3(Zr,Li). Furthermore, the precipitation sequence has been modified in Li-containing Al-Zn-Mg-Cu alloys. The precipitate volume fraction derived from the integrated intensity for 7055 alloy reached an plateau except ageing at 120°C and the maximum was about 0.052-0.054 in the temperature range 140-160°C.

Abstract: The as-cast microstructure feature of Al sheet with high forming properties used for pressure can manufacturing pre-treated by different melt-treatment technique was observed and analyzed by SEM, EDAX, TEM and selected area electron diffraction (SAED), etc. It was found that the oxide inclusions in Al sheet were mainly γ-Al2O3, and the Fe-rich impurity phases in the sheet mainly Al3Fe, T1(Al12Fe3Si) or (AlFeSiRE). The crystal structure of (AlFeSiRE) phase was too complicated to be determined. The effect of melt-treatment technique on the as-cast microstructure of the sheet was remarkable. There were a number of oxide inclusions in the Al sheet before or after conventional melt-treatment, which distributed non-uniformly and gathered together in larger block shape with the size of about 10-40μm. The Fe-rich impurity phases in the sheet were mainly Al3Fe and T1, and a few of the meta-stable phase Al6Fe existed in the sheet. These Fe-rich impurity phases appeared in a coarse, long needle/flake or skeletal form, and were easily to gather together with oxide inclusions. When the Al sheet was treated by high-efficient melt-treatment, the amout of oxide inclusions was decreased obviously, and these inclusions distributed more uniformly in the tiny granular form (about or less than 4μm). The Fe-rich impurity phases were mainly (AlFeSiRE), and a small amount of Al3Fe phases also existed in the material. The (AlFeSiRE) phases distributed uniformly along grain boundary in tiny group/sphere or short stick form. No congregation of Fe-rich phases with oxide inclusions was found in the material.

Abstract: Microporosity in both HIPped and non-HIPped unmodified aluminum cast alloy A356-T6 was quantified metallographically in terms of its area, area percentage, length, sphericity and perimeter. In the studied materials, the secondary dendrite arm spacing (SDAS) values vary from 82μm to 96μm for both the HIPped and the non-HIPped castings. HIPping process significantly reduces porosity area fraction and pore sizes. The maximum area fraction of porosity and maximum pore area of the non-HIPped specimens are increased with increasing SDAS.

Abstract: An Al-3.43Cu-1.28Li-0.49Mg-0.12Zr containing 0.62Zn and 0.29Mn was designed and the microstructures and mechanical properties of the alloy with various heat treatments were investigated. The precipitates of the alloy consist of T1 (Al2CuLi), θ′ (Al2Cu)
σ (Al5Cu6Mg2) and δ′ (Al3Li). As solution temperature is changed from 485°C to 530°C, the solution degree of alloying elements in alloy increased, the amount of T1 in the alloy aged at 160°C for 18 h increased and that of θ′ is decreased, resulting in an increase of strength. After solution treatment at 530°C, the alloy aged for 18 h at 145°C is mainly strengthened by G P zones, and a little amount of T1 precipitates. As aging temperature is increased to 160°C and 175°C, the strength increased, due to the sufficient precipitation of σ and T1. The smaller amount of T1 in the alloy aged at 190°C is consistent with its lower strength. Meanwhile, it is found that the σ precipitate does not coarsen as aging temperature increases in the range from 160°C to 190°C.

Abstract: In the present study, the influencing factors such as the intensity and the direction of gradient magnetic fields, the magnetic susceptibilities of non-magnetic metals on the structures are studied theoretically and experimentally. In the theoretical analyses, the influences of high gradient magnetic fields on nucleation and structures are investigated. In the experimental research, high gradient magnetic fields are imposed on paramagnetic material Al and diamagnetic one Sn during their solidification processes. Then the macro- and microstructures of these samples are examined and the influences of magnetic susceptibilities of metals, the intensity and the direction of high magnetic fields are analyzed in details. It is found that solidified structures could be refined when the magnetization force and gravity were in the same directions, while the solidified structures could be coarsened and the coarse dendrites grew along the direction of the imposed magnetic fields when the directions of these two forces were opposite. Those phenomena could be explained from the views of reduced gravity and elevated gravity effects caused by magnetization force and the convection suppression effect caused by high magnetic fields. The results indicate that high magnetic fields can be applied to control the solidified structures of metals and then improve the quality and the properties of materials for different purposes.

Abstract: The particle swarm optimization (PSO) algorithm is introduced into ghost correction, which is also compared with the NMS algorithm. With linear regression correlation factor as evaluation parameter, it is found that both algorithms have the same quality for model ODF, but when it comes to complicated textures, the PSO algorithm shows high ODF fitting quality. It is also demonstrates that the ghost peaks in the reduced ODF can be excluded out in the true ODF from PSO components with both even and odd terms in the series expansion method.