Materials Science Forum Vols. 539-543

Abstract: Inconel 718(Ni-19Cr-18Fe-3Mo-5Nb-1Ti-0.5Al) nickel-base superalloy strengthened mainly by Ni3Nb type γ″ and partially by Ni3Al type γ′ precipitation is today’s most widely used superalloy in the world. China has paid special attention on Inconel 718 research and development. Systematic long-term research project has been conducted in close cooperation among our university, research institutes and factories. The goal of this long-term project is in 2 steps. First step is to improve Alloy 718 to get high quality and the second step is for improving the alloy temperature capability from 650oC to 680-700oC. The basic idea for alloy improvement is still to keep the chemical composition in the range of specification by small adjustment or control of minor elements, such as S, P, N, Si and Mg. The main achievements are segregation control by adjustment of S, P, Si, control of N for cleanliness, micro-alloying of Mg for grain boundary strengthening and control of low S and high P for stress rupture life improvement. Modification of Alloy 718 is based on structure stability study and its improvement in adjustment of main second phase strengthening alloy elements Nb, Ti and Al in total amount and the ratios among them. The goal is to achieve more stable second phase strengthening by control of alloy elements. Experimental results show that the future of modified 718 alloys is very attractive to raise the temperature capability improvement from 650oC to 680-700oC.

Abstract: The complete evolution of solute content and second phases during full-scale industrial processing of AA3103 sheets has been measured. During pre-heating, dispersoids, which appear as plates or small polyhedra grow and the Mn solute content decreases. During subsequent breakdown rolling the dispersoid number-density increases significantly. The measured decrease of solute Mn after hot rolling and coil cooling is attributed to constituent particle growth, whereas the solute depletion during the final back-annealing is mainly caused by the growth of the dispersoids. These observations are compared to the predictions obtained by a semi-physical model for precipitation. Although simulations have been performed without any retro-fitting, for hot rolling the results compare quantitatively well with experiment, while for coil cooling and back annealing the modelled Mn solute depletion is underestimated. The precipitation process is found to be very sensitive to the microstructure, which illustrates the importance of coupling precipitation models with work hardening and softening models to obtain reliable predictions.

Abstract: X-ray CT method is a kind of nondestructive inspection, but has strong limitation in sample size due to a small field of view (FOV). The higher the resolution, the smaller FOV is, mainly due to the element number of available detectors commercially. Therefore, sample machining is more or less necessary so that the sample size is fit within the small FOV in the case of the high-resolution observation. Local tomography technique enables a high resolution reconstruction of small region of interests within a sample without the sample machining. In this study, we have evaluated the size effects of aluminum foam samples in terms of the 3D image quality by the local tomography techniques.

Abstract: The knowledge of some mechanical properties of materials and their changes with thermal treatments and/or mechanical treatments are essential to obtain the best results during simulation of processes. In this paper, changes of Young's modulus at room temperature of colddeformed aluminum AA1050 carried out in a tension machine and changes of Young’s modulus and Poisson’s ratio of AA2024 (T6 and T65) have been determined. The elastics constants have been measured by the ultrasound technique in AA2024 alloy and by tensile test in AA1050. In this alloy, the Young's modulus (E) diminishes during the first step of deformation and then increases with the successive cold working. Changes in Young's modulus measured are around 6-8%. In AA2024, the Young's modulus change is about 3% between the annealed and quenched alloy (minimum value); during aging the E parameter increases with respect to quenching. These changes are correlated with the structural changes during thermal treatments. In AA2024, the E parameter remains almost constant during cold-working after the aging treatment. Poisson’s ratio of this alloy remains almost constant in all the treatments. These results are also correlated with the dislocations arrangement in both materials. This behaviour is also compared with cold-deformed pure iron in a tensile test. These results confirm that aluminum AA1050 present similar behaviour than it was observed for pure iron.

Abstract: The latent heat of solidification of any alloy depends on its chemistry that consequently affects the macro and microstructures for the given solidification conditions. In order to analyze the effects of chemistry on the release of latent heat during solidification of the industrial 3XX series of aluminum alloys, four different levels of silicon (5, 7, 9 and 11wt% Si) and three different levels of copper (1, 2 and 4 wt% of Cu) were taken into consideration. The solidification process was studied at cooling rates of 6 and 10°C/minute. The solidification path of these alloys was determined and the corresponding latent heat released during the solidification process was measured using a Differential Scanning Calorimeter (DSC). The tested hypoeutectic alloy chemical composition was expressed by the novel concept of silicon equivalency. The findings indicate that increases in the cooling rates shift the characteristic temperatures toward lower values without having a significant effect on the amount of released latent heat.

Abstract: The effect of the annealing atmosphere, the annealing temperature etc. on the hydrogen behavior in several Al-4% Mg alloys during heat treatment, was investigated. The results have shown that the hydrogen content in the as-cast slab is uniform, while the hydrogen content in the slab tends to be higher near the surface than in the interior after annealing. Such a tendency becomes more marked when annealing time is prolonged and Si and Fe content is lowered. The condensation of hydrogen near the surface can be seen only when it is annealed in a wet atmosphere. When annealed in a dry atmosphere, the hydrogen content near the surface becomes lower than in the center of the specimen. The hydrogen in Al-Mg alloys tends to be released to outside intrinsically at temperatures around 400°C and above. It is revealed that oxide film formed on the surface prevents the hydrogen from being released to outside.

Abstract: In the present investigation the particle structure in an AA1200 sheet ingot used for litho applications has been studied. Caustic etching of the as-cast material was seen to result in a zone close to the surface with a different etching response. This zone was identified as what is known as a fir-tree zone or an Altenpohl zone [1,2,3,4]. A variation in particle type over the cross section of the as-cast ingot was seen to follow the differences in etching response. After heat treatment of the material, the fir-tree zones were no longer visible, and the accompanying change in particle structure was studied. Samples from the subsurface regions and from a distance of ~20 cm from the surface has been investigated before and after heat treatment. In the as-cast material, the sample from the surface was dominated by featherlike particles with long strings of particles, identified as AlmFe. While closer to the centre Al3Fe and Al6Fe were seen to be the main phases, however, some AlmFe and probably some α-AlFeSi was also found in this sample. After heat treatment, the particle structure was seen to change, and the surface sample contained mainly Al3Fe in addition to a small amount of AlmFe. The change in particle structure during heat treatment is discussed with reference to the change in etching response.

Abstract: A 95% cold-rolled Al-20%Sn-1%Cu alloy was heat-treated in a range of temperatures and times to investigate the evolution of mechanical properties and microstructure. The most interesting combination of properties can be achieved between 300 and 400°C in a process that is simpler than what is used in industry. The physical phenomena which are active during the heat treatment are precipitation, recovery, recrystallisation and change in phase distribution due to surface tension. The former two predominate at low temperatures, while the latter achieve faster kinetics at higher temperatures, where recrystallisation is coupled to the change in morphology of the contiguous Sn-phase.

Abstract: We have developed the new process for refinement of metallic materials during solidification without addition of refiners or without rapid cooling. This process uses electromagnetic body force based on the vibrations caused by simultaneous imposition of direct magnetic field and alternative electric current on the alloy melt during solidification. The vibrations create cavitation in the melt and it breaks out during growth of it. Then explosive force is released toward the surroundings such as the primary solid particles and they are fractured finely. Finally fractured solid particles solidified as very fine grains. This process was applied to hyper- and hypo-eutectic Al-Si alloys, and AZ91D Mg alloys. Primary silicon crystals in Al-17mass%Si alloy decreased the size remarkably by the optimum electromagnetic vibration condition. Primary aluminum dendrites particles in Al-7mass%Si and primary magnesium dendrite particles in AZ91D are also decreased its size markedly. This process has also been applied to create non-equilibrium state metallic materials such as metallic glasses. We have succeeded to obtain metallic glasses in Mg-Y-Cu alloy system and Fe-Co-Si-B-Nb alloy system. These alloys solidified as fully crystals in this cooling rate without electromagnetic vibrations. These glasses are bulky and are used as structural material.