Materials Science Forum Vols. 794-796

Abstract: A new rate equation developed by Yamamoto was applied to the rate of recovery and recrystallization in pure aluminum. The entire reaction can be expressed by Yamamoto’s equation because it contains the term of particle number. By applying this equation, the entire reaction process was divided into two reactions, the recovery and recrystallization. The obtained values of the time-exponent are 0.5 in the former and 1 in the latter regardless of the soaking conditions and annealing temperatures. Based on the microstructures and time-exponents, it is concluded that the rate of recovery and recrystallization is controlled by the precipitation of silicon on the dislocation cell boundaries during the recovery process and the precipitation of iron on the subgrain boundaries during the recrystallization one.

Abstract: During rapid solidification, interfaces are often driven far from equilibrium and the "solute trapping" phenomenon is usually observed. Very recently, a phase field model with finite interface dissipation, in which separate kinetic equations are assigned to each phase concentration instead of an equilibrium partitioning condition, has been newly developed. By introducing the so-called interface permeability, the phase field model with finite interface dissipation can nicely describe solute trapping during solidification in the length scale of micrometer. This model was then applied to perform a phase field simulation in a Al-Sn alloy (Al-0.2 at.% Sn) during rapid solidification. A simplified linear phase diagram was constructed for providing the reliable driving force and potential information. The other thermophysical parameters, such as interface energy and diffusivities, were directly taken from the literature. As for the interface mobility, it was estimated via a kinetic relationship in the present work. According to the present phase field simulation, the interface velocity increases as temperature decreases, resulting in the enhancement of solute trapping. Moreover, the simulated solute segregation coefficients in Al-0.2 at.% Sn can nicely reproduce the experimental data.

Abstract: To exactly understand the grain refining mechanism of α-Al by the Al-5Ti-1B master alloy, the structural properties of α-Al/solid-TiB₂ (S/S) and liquid-Al/solid-TiB₂ (L/S) interfaces were studied using the first-principles method. Different ordered structures were formed on the interfaces with different terminations of TiB₂ (0001) surface, which determines the nucleant potency of TiB₂. The heterogeneous nucleation of α-Al on the B-terminated surface is frustrated by an AlB₂-like structure formed at the interface. In contrast, a five-layer quasi-solid region with stacking sequence of fcc-Al (111) planes forms on the Ti-terminated TiB₂ (0001) surface, which is the basis of successful heterogeneous nucleation of α-Al. Moreover, when redundant Ti solute being added into the liquid Al region of Ti-terminated liquid-Al/TiB₂ interface, the quasi-solid Al region further extends until entire solidification. The reason for using the Al-5Ti-1B master alloy rather than TiB₂ powders as the commercial refiner in Al industry lies in two aspects: the excessive Ti atoms in the master alloy could guarantee sufficient Ti chemical potential to form Ti-terminated surface of TiB₂, and the redundant Ti solute in inoculated melts could facilitate the growth of quasi-solid Al region at the solid/liquid interface.

Abstract: In order to verify influence of particle velocity non-uniformity on dynamic recrystallization (DRX), shock tests of D16 Al alloy were conducted under uniaxial strain conditions within strain-rate range of 10⁵ ÷ 10⁷ s^-1. The particle velocity non-uniformity arises due to both initial heterogeneity and non-linearity of shock-wave process. Apart from the nature of DRX mechanism, migrational or rotational, the particle velocity non-uniformity facilitates growth of local strain, strain rate and temperature. To adjust a duration of the particle velocity non-uniformity degree, two limiting situations is provided by means of single and double shock loading (reloading). The experimental technique used allows to register both mean particle velocity profile and particle velocity variation which is the quantitative characteristic of velocity non-uniformity. Shock tests of D16 Al alloy in single and reloading regimes [ show that dynamic recrystallization takes place only in the second case. The reloading regime initiates a ten-fold increase in duration of the particle velocity non-uniformity stage, which is sufficient for fulfillment of the well-known DRX conditions: γ 3, dγ/dt 10⁴ s¹, T 0.4T_m. The regions of DRX with equal-axis grains of ~1 μm in diameter are revealed with the metallography and X-ray analysis.

Abstract: Ingots of alloys Al-Cu-Fe were obtained by casting in a graphite mold. Mechanical milling of alloy particles in the as-cast state and after homogenization annealing was carried out in planetary ball mill Retsch PM400 in an argon atmosphere. As a result of mechanical milling granules with an average size of 35-40 μm and fine internal microstructure are formed. The size of coherent scattering regions in quasicrystalline phase after mechanical milling was about 10-15 nm. Mechanical milling after homogenization heat treatment allows much refines the quasicrystalline phase than in the case of mechanical milling of cast alloy.

Abstract: The aging behavior of a cast Al-2 wt.% Fe alloy processed by High-Pressure Torsion (HPT) at room temperature was studied by subsequent aging treatments at 200 °C. Observations by Transmission Electron Microscopy (TEM) revealed that the microstructure after HPT processing reached an ultrafine-grained level with an average grain size in the Al matrix of ~120 nm. The initial eutectic structures were fragmented into particles with sizes of less than 400 nm and partially dissolved in the matrix up to a supersaturated Fe content of ~1% as confirmed by X-Ray Diffraction (XRD) analysis. The peak-age condition was achieved within 0.25 h of aging, which provides the maximum hardness of ~200 HV. Analyses by high-resolution S/TEM show that round particles of Al₆Fe with sizes of ~5-10 nm and semi-coherent with the matrix are the dominant precipitates in the peak-aged condition. The hardness increases by aging for 12 h above the as-HPT-processed level of 185 HV. The dominant precipitate phase transforms to Al₃Fe in the over-aged condition with a loss of coherency during growth. Enhanced precipitation kinetics was observed because of high density of lattice defects induced by the HPT processing, which were also confirmed by significant recovery in the electrical conductivity of the samples after aging.

Abstract: Clad strip consisting of 5182 aluminum alloy and other aluminum alloys could be cast using a twin roll caster equipped with a scraper. This twin roll caster could carry out the strip casting and the bonding of the strips. The equipment, that was developed to prevent the contact between the bonding surface of the strip and oxidizing environment, was adopted. The developed equipment was a scraper. The 5182 strip could be bonded to other aluminum alloy strips by the effect of the scraper. Aluminum alloys for casting has poor formability, especially, bending ability is poor. The clad strip consisting of A356 casting aluminum alloy and 3003 wrought aluminum alloy was cast. 180 degree bending test was carried out on this clad strip. In the condition that the 3003 strip was outer side and A356 strip was inner side, the crack did not occur at the outer 3003 strip. In the deep drawing test or the clad strip, LDR (Limiting Drawing Ratio) was 1.8. These results mean that the casting aluminum alloy has ability to be used for the sheet forming, if the casting aluminum alloy is cladded with the wrought aluminum alloy.

Abstract: Effects of high-speed deformation on age hardening and microstructural evolution behavior of 6061 aluminum alloys were studied. By affecting the high-speed impact compression (about 5 GPa) to the 6061 aluminum alloy plate in the state of quenching after the solution heat treatment, the maximum hardness became twice as high as the original hardness. Even after the impact compression, age-hardening was clearly identified both at 175 °C and 100 °C. TEM observation revealed that point defect clusters were distributed densely inside grains after the impact compression, possibly due to the effect of high-speed deformation. The point defect clusters observed were assumed to be stacking fault tetrahedra on the basis of high resolution TEM analysis. The point defect clusters and precipitates were both visible even after the peak-aged condition at 175 °C. The 6061 aluminum alloy specimen after the solution heat treatment, followed by the impact compression (8.0 GPa) and the peak-aged condition showed the highest hardness value (154 Hv) among the testing conditions selected in the present study.

Abstract: Mechanisms of dynamic recrystallization operating at severe plastic deformation in a wide temperature range are reviewed for aluminum alloys. The main mechanism of grain refinement in all aluminum alloys is continuous dynamic recrystallization (CDRX). Temperature, deformation process and distribution of secondary phases strongly affect the CDRX mechanism. Initial formation of geometrically necessary boundaries (GNBs) and a dispersion of nanoscale particles accelerate CDRX facilitating the formation of a 3D network of low-angle boundaries (LAB) followed by their gradual transformation to high-angle boundaries (HAB). At high and intermediate temperatures, 3D networks of LABs may evolve due to rearrangement of lattice dislocations by climb, and mutual intersection of GNB, respectively. At high temperatures, in aluminum alloys containing no nanoscale dispersoids the CDRX occurs through the impingement of initial boundaries forced by deformation-induced LABs. This recrystallization process is termed as geometric dynamic recrystallization (GDRX). At low temperatures, the extensive grain refinement occurs through a continuous reaction which is distinguished from CDRX by restricted rearrangement of lattice dislocation. Introduction of large misorientation may occur through the formation of 3D networks of GNBs, only.

Abstract: In this paper, a novel processing method (reactive precursor method) to manufacture high-melting point porous Al-Ti intermetallics is investigated. Especially, morphological control of porous structure is focused. In the reactive precursor process, precursors are made by blending aluminum and titanium powders. The precursor is heated to ignite an exothermic reaction (so called “combustion reaction”) between the elemental powders. Pore formation is a well-known intrinsic feature of the combustion reaction, and we tried to control the pore morphology. Fundamentally, the closed-cell structure can be obtained when the maximum temperature during the reaction exceeds the melting point of the reaction product. By blending the exothermic agent powder in the precursor, the maximum temperature is increased and the reaction products are melted. The porosity is controlled by the maximum temperature. In contrast, an open-cell porous structure can be obtained when the maximum temperature is below the melting point of the reaction product. Microwave heating turned out to be an effective method to create an open cell structure. A powdery substance that does not react with other elemental powders (heat-absorbing agent powder) decreases the temperature during the reaction. Closed, open and bimodal-sized open pores have been achieved by the reactive precursor process so far.