Materials Science Forum Vols. 551-552

Abstract: After superplastic deformation of Al-7475 and some other aluminium alloys, straininduced cavities are seen to be associated with long fibres parallel to the tensile direction. These fibres, whiskers or filaments are also observed on the fracture surface. This effect has become known as microsuperplasticity. The whisker characteristics are affected by the deformation conditions, particularly temperature and strain rate. To study the effect of these variables more fully, tensile samples of Al 7475 have been strained to failure at temperatures ranging from 480OC to 530OC and strain rates from 1.0E- 04s-1 to 5.0E-03s-1. Additional samples were deformed at 450OC and 420OC and a single strain rate. Some whiskering was observed under all testing conditions. The longest whiskers were generally seen at high temperatures and low strain rates. A TEM study of macroscopic whiskers produced under conditions of around 540OC and 1.0E-04 /s showed an amorphous structure. Annealing prior to deformation was shown to have little effect on whisker formation. EDX analysis showed the whole surface of the alloy to be enriched in alloying elements compared with the bulk alloy. The high levels of Mg detected were connected with the formation of magnesia as the surface oxide verified using Cr3+ fluorescence microscopy. Use of the differential scanning calorimeter (DSC) showed no conclusive evidence of partial melting below the testing temperatures. Considerations of capillarity and the DSC analysis suggest whiskering did not occur by a mechanism of viscous flow.

Abstract: In connection with ancient and recent view on cosmology, it is interesting to note that our universe could be a spherical crystal and it moves as a crystal in a relative position with others spherical universes, where the Burgers vector for cellular dislocations dynamics is the Hubble length: λH=1.32x1026m. The expansion process of this polycrystalline spatially extended system obey the hyperbolic granular flow, which it is due to an accelerated motion manifested during the deformation process of super plastic advanced structural universes in a similar behaviour of super plastic advanced structural materials. Consequently, in this work the phenomenology and mechanics of super plastic flow are analyzed in the context of the unified interpretation of Hubble flow, plastic flow and super plastic flow, where the combination of fundamentals constants with the natural Planck length, allows obtain in a closed agreement with the Orowan equation the magnitude of the nature Burgers vector of dislocation in the cosmic structure for the universe as follow: 1.62 10 . 35 3 0 x m c H G b P − ⊥ ⊥ ⊥ = = = = h λ ρ ν Where, b⊥ = magnitude of the nature Burgers vector for the universe (b⊥ = 1.62x10-35m), λP = Planck length (λP = 1.62x10-35m), H0 = the Hubble parameter (H0=70 (km/sec)/Mpc = 2.26854593 x10-18s-1), ρ⊥ = dislocation density (ρ⊥ = 1.273x1011 dislocations/m2) in the universe. ν⊥ = the recession velocity of galaxies related with dislocations dynamics in the cosmic structure (ν⊥ = 1100x103 m/s, it is the recession velocity of the Virgo super cluster at 16 Mpc distance). h = h / 2π . Here h = the Planck constant (h = 6.6262x10-34 Joule-s), G = the Newtonian constant (G = 6.67259x10-11 m3/kg s2) and c = the speed of light (c = 299 792 458 m/s) [1-3].

Abstract: The deformation mechanism in the nanometer grain size range has been basically investigated from the results of microstructural observation after superplastic deformation in a Zn-Al eutectoid alloy in which a reverse grain size dependence of superplasticity was previously reported: flow stress increases and elongation decreases with decreasing grain size when grain size is markedly reduced to nanometer range. By controlling the aging condition after solution treatment and subsequent quenching, two specimens are prepared: the as-quenched specimen with ultrafine grains of 83nm in diameter and aged specimen with normally fine grains of 2.6μm. The elongation is confirmed to be smaller in the as-quenched specimen than in the aged specimen, although the flow stress is lower. As a result of TEM observation on the interior of the grains, dislocations are rare in the as-quenched specimen, while a significant density of dislocations are observed in the aged specimen. This result strongly supports the mechanism previously proposed by Mishra et al. that the accommodation process, i.e., the dislocation glide inside the grains, becomes more difficult with decreasing grain size in the nanometer grain size range, even though the grain boundary sliding as the major process becomes facilitated. Roughly assessed m-value was in accord with this mechanism.

Abstract: Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 μm) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

Abstract: The superplastic forming and diffusion bonding (SPF/DB) is applied in aviation and space flight field. The SPF/DB process with gas pressure control for dissimilar superalloy structure was studied. Diffusion bonding parameters, including bonding temperature T, pressure P, time t, affect the joining mechanism. When the bonded specimen with 50&m thick nickel foil interlayer was tensile at room temperature, shear fracture of the joints with nickel foil interlayer takes place at the GH4141 superalloy part. The SPF/DB of four-layer sheets structure was investigated. The optimum parameters for the SPF/DB process are: forming temperature T=1243K, forming pressure P=1MPa, forming time t=35min. The microstructure of the bonded samples was characterized. The microstructure shows an excellent bonding at the interfaces. The distribution of thickness after SPF/DB was investigated.

Abstract: Influence of surface treatment to diffusion bonding before joining was analyzed during the process of magnesium alloys diffusion bonding. We processed diffusion bonding by using ZK60 rolling superplasticity magnesium alloy sheets with thickness 1.5mm and grain size 8.9$m. Joint was carried out ageing strengthening by experiment study to increase its shearing strength. Ageing strengthening performance was researched under ageing temperature 150-190 °C and the ageing time 6-48h. The results showed: diffusion bonding joints’ shearing strength had enhanced in some degrees and reached 86% of basal strength when the ageing temperature is 160 °C and the ageing time is 24h. It showed that the magnesium alloy joint’s strength could be greatly improved by ageing strengthening.

Abstract: Mechanical behavior and microstructure evolution of Ti and Ti-64 titanium alloy during warm “abc” deformation has been studied. The “abc” deformation was consisted of successive compression of a sample along three orthogonal directions. Mechanical behavior of each material was described by set of successive σ-ε curves combined into cumulative σ-Σε curve. Microstructure of Ti was found to be refined to a grain size of about 0.4 μm due to formation of deformation-induced boundaries within initial grains. Although a stage like steady state flow was observed at the cumulative σ-Σε curve such mechanical behavior was hardly associated with superplastic flow. In two-phase Ti-64 alloy the structure was found to be refined to a grain size of about 0.4 μm after warm “abc” deformation due to globularization α- and β-particles following breaking down of α-lamellar and β- layers. Microstructure refinement of the alloy was associated with softening and superplastic flow.

Abstract: Cold and hot Equal-Channel Angular Pressing (ECAP) is an effective method to refine metallic grains. In this paper, superplastic properties of 1933 aluminium alloy were evaluated and the effect of hot ECAP on grain refinement and superplasticity was investigated. The testing results show that the refinement of grains can not be infinitely increased with the increasing of ECAP passes (or total strain). Under the isothermal ECAP conditions of the present study, optimum ECAP passes for 1933 alloy are 4 passes. The grain size of 1933 alloy was refined from 20~50μm to 7~12μm by means of ECAP for 4 passes at 300°C (route Bc), so its superplasticity was improved. Compared with original samples annealed at 400°C, the superplastic elongation of samples processed by ECAP for 4 passes increases by a factor of 130% about, and the range of superplastic temperature varies from 140°C to 210°C. The optimal superplastic temperature and initial strain rate is 510°C and 3.3×10-4s-1 individually, at which the elongation reaches 262% and the flow stress is 7.8MPa only. In a word, 1933 aluminium alloy can present more excellent superplasticity in wide range of superplastic temperature and strain rate.