Materials Science Forum Vols. 706-709

Abstract: The microstructural changes were investigated in severely deformed 5182 alloy. The as-cast block was cold rolled with different reduction levels up to 98%. The deformation textures appearing after various rolling reductions account for minor qualitative changes (i.e. new texture components emerging with increasing rolling reductions) whereas significant quantitative differences were observed (i.e. changing intensity of rolling components). The presence of large non-deformable constituents affects both deformation and recrystallization textures. During rolling, large particles induce strain path changes in their vicinity, which produces microstructural heterogeneities. In recrystallization, the highly strained field around the particle induces particle stimulated nucleation (PSN). The current contribution tries to clarify the effect of different strain modes involved in severe plastic deformation on the development of recrystallization textures.

Abstract: The aim of the present investigation was to determine and to compare the superplastic behaviour of the AA5083 (Al-Mg-Mn) alloy with Sc and Zr additions. The investigated alloys were processed to form sheets by conventional hot and cold rolling. The superplastic properties were determined with strain rates in the range of 1x10^-4 to 5x10^-2s^-1 and forming temperatures of 350 to 550°C. The results showed that the alloy with about 0.4% Sc exhibited a high superplastic ductility across a wide temperature range and strain rates up to 1x10^-2s^-1. The highest elongations to failure of about 2000% were attained at 550°C and at an initial strain rate of 5x10^-3s^-1. However, the alloy with about 0.15% Zr exhibited elongations up to 600%. The FSP processed Al-4.5Mg alloy with combined addition of about 0.2% Sc and 0.15% Zr exhibited good superplastic properties at higher strain rates (> 1x10^-2s^-1) with elongations up to 1500%.

Abstract: Some progresses of grain refinement in aluminium field in our group are involved in this paper. We invented a new preparation method of Al-Ti-B grain refiner with the application of high-intensity ultrasound in the preparation reaction. Owing to the optimization of TiB₂ particles morphology in the prepared grain refiner, the commercial purity aluminium grains can be refined as small as 45μm, which is superior to the limiting level of the present commercial grain refiner, 120μm. First-principles calculations on the Al/TiB₂ interface prove that the larger Al/TiB₂ interfacial energy than that between α-Al and aluminium melt is responsible for the poor heterogeneous nucleation of α-Al on TiB₂ particles with only TiB₂ particles in melt. When there is redundant solute Ti in the melt, it is theoretically revealed that the spontaneous segregation of solute Ti on TiB₂ particles plays an important role in the heterogeneous nucleation.

Abstract: Effect of liquid hot isostatic pressing (LHIP) on structure and mechanical properties of an A356.0 alloy was examined. Samples from this alloy were produced by gravity die castings. Part of these samples was subjected to homogenization annealing, and the other part was subjected to LHIP following homogenization annealing. All samples were water quenched from the temperature of prior homogenization annealing or LHIP and finally aged. It was shown that the LHIP processing leads to increase in yield stress, ultimate stress and total elongation. A significant increase in fatigue strength and decreased the scattering of fatigue data takes place too. This is caused by the fact that the fatigue crack initiation mostly occurs on lateral surfaces of the samples subjected to LHIP, whereas shrinkage voids in the non-hipped condition play a major role in crack initiation. In addition, crack propagation under fatigue occurs in samples subjected to LHIP in essentially ductile manner. Thus, LHIP eliminating shrinkage porosity enhances significantly mechanical properties and reliability of aluminum casting.

Abstract: The effect of cold deformation on the yield strength and precipitate behavior of Al-1.3Mg-1.2Si-0.5Cu-0.7Mn alloy sheet subjected to T4P temper for automotive body panels before and after paint-bake treatment were investigated by tensile test and differential scanning calorimetry (DSC) analysis. The results show that the T4P alloy sheet has significant work-hardening response (WHR), and the yield strength increment exceeds 190MPa as the tensile deformation strain approaches 15%. The unstrained T4P alloy sheet has obvious paint-baking response (PBR), and the yield strength increment is up to 80MPa, while the PBR of the cold stretched T4P alloy sheets decrease with increasing level of tensile deformation, and the minimal PBR is only 17.8MPa. The prior tensile deformation accelerates the precipitation of β″ and β′ phase from the T4P alloy matrix during paint-bake cycle but weakens the age-hardening response (AHR) and PBR of T4P alloy sheet. Moreover, it is evident that the T4P alloy sheet simulation paint-bake treated at 170°C for 30min can not fully exert the aging potential of the alloy sheet.

Abstract: Both rapid and stable semi-solid slurry preparation of hypereutectic Al-14.8%Si-4.5%Cu-1.1%Mg alloy (in mass%) with fine solid granules as well as finely dispersed primary silicon particles was tried using a simple method newly developed, where the low-superheat melt passes through a gap between a rotating regular octagonal rotor and a circumscribed chill block. By examining effects of melt temperature running out from tundish, gap distance and rotational speed of rotor on solidification structure of the semi-solid slurry manufactured, the optimum making condition of the targeted semi-solid slurry was found out. Mean sizes of solid granules and primary silicon particles within the slurry, which had been made quickly with the good reproducibility, were 45μm and 20μm, respectively. Subsequently the obtained semi-solid slurries were rheocast into a cylindrical shape cavity by the use of a high pressure casting machine. It has been shown that the rheocast samples have peak hardness value between the gravity casting and the hot-extrusion, but a good age-hardenability somewhat higher than that of gravity casting.

Abstract: Polycrystalline Al-1wt%Mg-0.27wt%Sc alloys bearing Al₃Sc particles with different average sizes of 4 and 11nm in diameter have been cyclically deformed at 423K under various constant stress amplitudes, and the relationship between fatigue characteristics and microstructure of the alloy has been investigated. The specimen bearing 11 nm particles exhibited a cyclic hardening to saturation, while in specimens with the small particles a cyclic softening was observed after initial hardening. In the specimen with large particles, dislocations were uniformly distributed under all applied stress amplitudes, whereas the specimens bearing small particles, in which cyclic softening occurred exhibited clearly developed slip bands. The cyclic softening for the latter specimen was explained by particle shearing within the strongly strained slip bands. The width of precipitate free zones (PFZs) has been found to be one of the factors affecting the fatigue life of the specimens at 423K. The two-step aging decreases the width of PFZs, resulting in increase in the fatigue life.

Abstract: In the present work, quench sensitivity of an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy is investigated by temperature-time-property (TTP) curve and TEM analysis, comparing with traditional AA 7B04 and AA 7150. The results indicate that the nose temperature of TTP curve and the corresponding incubation period of the alloy, AA 7150 and AA 7B04 are about 290°C/4.5s, 320°C/2.6s and 335°C/0.1s, respectively, The nose temperature of the alloy is the lowest among three alloys and the critical time at the nose temperature is the longest for the alloy, which is obvious that the alloy has lower quench sensitivity. Further TEM analysis shows that, with the prolongation of keeping time at the nose temperature, quench-induced precipitation phenomenon becomes obvious.

Abstract: The deformability and the microstructures of Al-12.2Si-0.6Mg alloy during hot-rolling were investigated by means of rolling the specimens of wedge bars with length of 180mm and width of 30mm, which had front thickness of 5mm and back thickness of 44mm.The wedge bars were cut from the ingots of the Al-12.2Si-0.6Mg alloy by the semi-continuously direct chill (DC in short) casting. The specimens of wedge bars were hot-rolled following holding between 410°Cand 480°C for different time. The results show that the size, morphology, distribution characters of eutectic Si particles in the Al-12.2Si-0.6Mg alloy can be remarkably modified by semi-continuously DC casting, which consists of coarse ribbon-like Si-particles with less than 5μm in length and 1μm in width and quite a lot eutectic phases of less than 0.4μm in size and space. The results also show that the ingots of the Al-12.2Si-0.6Mg alloy by the semi-continuously DC casting can possess excellent deformability during hot-rolling if the extent of heating is provided over 440°C for 60min and 410°C for 120min, and they cannot emerge cracked edges with the compression ratio of 85% by single-pass rolling. Their hot-plasticity depends on the size and space between eutectic phases in the ingots. Hot-rolling deformation makes ribbon-like Si phases in them crack and spheroidize, and then results in the sizes of coarse Si particles tending to be consistent.

Abstract: High rate film deposition via Gas Flow Sputtering was used to develop both metallic titanium films and ceramic titania coatings for implants. The structure formation of the coatings can be described by a modified model of structure zones according to Thornton. The potential of Gas Flow Sputtering for designing implant surfaces with controlled morphology and roughness is demonstrated.