Materials Science Forum Vols. 551-552

Abstract: The compressive deformation behavior of as-cast Zr55.9Cu18.6Ta8Al7.5Ni10 Bulk Metallic Glass (BMG) composite with micro-scale particles of Ta-rich solid solution embedded in an amorphous matrix was investigated in the supercooled liquid region. It was found that the apparent viscosity of the BMG is dependent on temperature and strain rate. A deviation from a Newtonian behavior was observed at high strain rate and low temperature. The experimental results can be described by a master curve based on a stretched exponential function and the free volume theory. The structural state and the thermal ability of the BMG composite after deformation are also discussed in the paper.

Abstract: The deformation behavior of the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass (Vit1) in the supercooled liquid region was investigated. The stress-strain relations for Vit1 at the different specimen sizes were established in a uniaxial compression test using the microforming system. Furthermore, the effects of forming time, temperature and punch velocity on the flow stress of the specimen with a typical size of Φ1×1.5 mm were analyzed. The results indicate that Vit1 has excellent superplastic microformability at minimum forming time, higher temperature and lower punch velocity. Based on the research, Vit1 microgears with reference diameter of Φ1 mm and modulus of 0.1 mm were manufactured using the closed die forging. Optimum processing parameters were obtained. In order to evaluate the quality of Vit1 microgears, scanning electron microscope, atomic force microscope and nanoindentation tests were applied. It is found that the parts with good qualities can be formed successfully in the supercooled liquid region.

Abstract: The deformation behavior of gas pressure forming of amorphous Fe78Si9B13 alloy was investigated under equibiaxial tension. The gas pressure forming was carried out in the temperature range of 430°C~530°C below the crystallization temperature Tx and die apertures of 5mm~10mm. The dome height and amorphous ribbon thickness of deformed specimens at the pole was measured. It was found that amorphous Fe78Si9B13 alloy had exhibited good plasticity in the experimental temperature range. The near-semisphere specimens of the radius 5mm and the height 4.5mm were obtained from the gas-pressure forming at 450°C and 530°C for 30min, which is similar to the superplastic forming.

Abstract: As a new damping material, the authors first developed a Zn-22wt.%-Al eutectoid alloy with ultra-fine grains exhibiting superplasticity at room temperature by means of thermomechanical controlling processes (TMCPs). The Zn-Al alloy has a few advantages such as low work-hardening rate and high ductility over a conventional seismic damping material, for instance, a low-yield-point steel. In addition, Zn-Al alloys are environment-conscious because of no harmful metal like Pb. However, when Zn-Al alloys are subjected to plastic deformation, since its work hardening is small, plastic deformation proceeds locally so that required absorption energy cannot be sufficiently obtained, and local fracture and local deformation instability can take place easily, which is the intrinsic characteristic of superplastic materials. Therefore we attempted to develop a shear panel type, a brace type damper for tall buildings and a bending type damper for Japanese wooden houses using FEM analysis in order to minimize localized strain and local deformation and to determine the optimum shape for this Zn-Al superplastic seismic damper. As a result, an ecological and high-energy absorption seismic dampers, so-called “maintenance-free seismic damper,” was successfully developed.

Abstract: Effects of hydrogen on superplastic deformation behavior were investigated through high temperature tensile experiment in this paper. It is found that reasonable hydrogen contents can improve the superplastic behavior such as lowering flow stress and temperature and increasing m value. While addition of 0.1wt% hydrogen in Ti-6Al-4V alloy, peak flow stress decrease to 53%, deformation temperature decrease 60°C. The influence of hydrogen on microstructure transition by means of optic microscope, SEM, TEM and XRD was also researched. The results show that β phase amounts in the hydrogenated alloy increase with hydrogen contents, while hydrogen contents reach to 0.2wt%, martensite becoming coarser with the increase of hydrogen contents. Moreover, dislocations density of hydrogenated alloy after deformation is lower than that of unhydrogenated alloy because of hydrogen action.

Abstract: Severely rolled sheets of Al-4.5%Mg-0.7%Mn based alloys have been prepared to achieve grain refinement leading to low temperature superplasticity. Rolling is one of straining technique which has a high possibility of industrial application. The alloys used were T1: base one adding 0.08%Zr and 0.05Ti for suppression of grain growth, F1: 0.80%Fe addition and F2: 1.92%Fe addition to increase nucleation sites of recrystallization. For comparison, M1: Al-4.2%Mg was also prepared. All of samples were 99.8% cold rolled to thin sheet 0.1mm thick. The maximum elongation of 150%, which was markedly large in consideration of specimen thickness of 0.1mm, was obtained at 533K and 1.4×10-3s-1 for T1 sample while the elongations for F1 and F2 showed less than 100%. On the other hand, M1 exhibited larger elongation of 190% at 533K and 1.4×10-3s-1. Further, difference in superplastic deformation of these alloys was investigated by using SEM/ EBSP analysis of intragranular misorientation, which reflects strain or stored energy generated during deformation with dislocation glide, to discuss the deformation mechanism.

Abstract: Previous studies of superplasticity using surface markers have tended to use either diamond paste scratches, with which it is difficult to make quantitative measurements, or regular grids with spacing greater than the grain size, which prevents measurements of any deformation occurring within the grains. This paper reports the etching of regular marker grids with submicron line spacings using Focused Ion Beams (FIB), as well as mesoscopic grids with line spacings many grains across. These have enabled us to make quantitative measurements of deformation within the surface grains, and at length scales up to 0.5 millimeters. Further refinements used include the use of Interferometric Surface Profilometry to measure the out-of-surface displacement during deformation. Results on Sn-Pb eutectic alloy tested in tension in Regions I and II showed that there was almost no intragranular deformation anywhere in any of the surface grains. Grain boundary sliding took place, particularly at Sn-Sn grain boundaries, and the accommodation mechanism was separation of interfaces normal to the tensile axis where sliding was blocked, particularly Pb-Pb grain boundaries. It is argued that this is a surface artifact not readily available in the bulk thus the case is made for testing under a different strain condition.

Abstract: Cavitation behavior during superplastic flow of ultra-fine grained (UFG) Ti-6Al-4V alloy was established with the variation of grain size and misorientation. After imposing an effective strainup to 8 via equal-channel angular pressing (ECAP) at 873 K, alpha-phase grains were markedly refined from 11 μm to ≈ 0.3 μm, and misorientation angle was increased. Uniaxial-tension tests were conducted for initial coarse grained (CG) and two UFG alloys (ε = 4 and 8) at temperature of 973 K and strain rate of 10-4 s-1. Quantitative measurements of cavitation evidenced that both the average size and the area fraction of cavities significantly decreased with decreasing grain size and/or increasing misorientation. It was also found that, when compared to CG alloy, cavitation as well as diffused necking was less prevalent in UFG alloys, which was presumably due to the higher value of strain-rate sensitivity. Based on the several theoretical models describing the cavity growth behavior, the cavity growth mechanism in UFG alloys was suggested.