Papers by Keyword: High Strain Rate Superplasticity

Abstract: Zn–22Al alloy was processed using a well-designed two-step equal channel angular extrusion/pressing (ECAE/P), and ultrafine-grained (UFG) microstructure with 200 nm grain size was achieved. UFG Zn-22Al was subjected to long-term (up to 60 days) aging at room temperature (RT) and it was seen that natural aging caused limited grain growth in the microstructure. Grain sizes of about 300 nm, 350 nm and 350 nm were measured after 15, 30 and 60 days aging, which mean that UFG Zn-22Al alloy has a good microstructural stability at RT up to 60 days. ECAPed Zn-22Al alloy showed a maximum elongation of about 400% at a high strain rate of 5x10^-2 s^-1 and maximum elongation decreased with increasing grain size. Elongation to failures of ~375% and ~350% were obtained with the samples having 300 nm and 350 nm grain sizes, respectively. In addition, natural aging slightly decreased the strain rate at which superplastic region formed. While the maximum elongation occurred at the strain rate of 5x10^-2 s^-1 in ECAPed UFG alloy, it took place at lower strain rate of 1x10^-2 s^-1 after aging for all time periods. Also, flow stress of the alloy increased with increasing grain size during natural aging.

Abstract: Abstract: Spray forming UHCS-1.6Al material has a refined equiaxed pearlitic structure, which was necessary for high-train-rate superplasticity. During deformation, the cementite particles pin the grain boundaries and prevent the grains growing up. As a result it exhibited excellent high-strain-rate superplasticity, with a peak ductility of 187% occurring at a strain rate of 5*10-3s-1.

Abstract: 1.5 mm, 0.7 mm and 0.3 mm thicknesses TiNP/2014Al composite sheets were obtained by hot rolling deformation carried out on as-extruded TiNP/2014Al composite rod. The effect of hot rolling deformation on high strain rate superplastic deformation behavior of the composite was researched by tensile experiment, OM, and SEM. Results show that 0.7mm thickness TiNP/2014Al composite sheet can gain the maximum elongation of 351% at 818 K and 3.3×10-1 s-1, and the m value is 0.43. The optimum strain rate increases with decreasing thickness of the TiNP/2014Al composite sheets. Flow stress and work hardening ability show contrary change tendency to optimum strain rate. The 0.7 mm thickness TiNP/2014Al composite sheet has medium flow resistance stress and shows excellent stability of plastic flow. Fracture surfaces show that the main superplastic deformation mechanism of the TiNP/2014Al composite includes in grain boundary sliding. Subgrain boundary sliding maybe another superplastic deformation mechanism.

Abstract: High-strain-rate superplasticity (HSRS) can be attained in tetragonal ZrO2-30vol% MgAl2O4 spinel composite. In order to examine the flow behavior of the two-phase composite, the standard rule of the mixture model was employed. The strain rate of the composite can be explained by the isostrain model that is predicted from the data set of Al2O3 doped ZrO2 and spinel polycrystals. For the isostrain model, since the strain and strain rate are the same for ZrO2 and spinel phases, the harder ZrO2 phase carries more of the stress in the composite. In order to preserve homogeneous deformation and material continuity, a concomitant accommodation process within the harder ZrO2 grains is also necessary. For HSRS in the ZrO2-spinel composite, therefore, the rate of deformation may be controlled by the slower dislocation recovery process limited by the lattice diffusion within harder ZrO2 grains rather than within spinel grains.

Abstract: Equal-channel angular pressing (ECAP) at 443 K was used to introduce an ultra-fine grained (UFG) microstructure to a Zr and Sc modified 7075 aluminum alloy. Using the methods of TEM and EBSD, an average grain size of 0.6 1m was recorded after the pressing. The UFG microstructure remained very stable up to the temperature of 723 K, where the material exhibited high strain rate superplasticity (HSRSP) with elongations to failure of 610 % and 410 % at initial strain rates of 6.4 x 10-2 s-1 and 1 x 10-1 s-1, respectively. A strain rate sensitivity parameter m in the vicinity of 0.45 was observed at temperatures as high as 773 K. At this temperature, the material still reached an elongation to failure of 430 % at 2 x 10-2 s-1. These results confirm the stabilizing effect of the Zr and Sc additions on the UFG microstructure in a 7XXX series aluminum alloy produced by severe plastic deformation.

Abstract: The stress-strain curves at high strain rate superplasticity were analyzed for Ti-6Al-4V and aluminum 1420 industrial alloys in ultra-fine grained state produced by severe plastic deformation. For both alloys the observed strengthening effect can be caused by grain growth under the above mentioned conditions resulting to increase of grain boundary sliding resistance. In the case of aluminum alloy the grain recovery can be accompanied by significant change in phase composition which is also stimulates the recrystallization process.

Abstract: There are presented original experimental results of studying rheological behavior of Vitralloy 1 (Zr41Ti14Cu12.5Ni10Be22.5), which shows in the certain strain rate range stable linear viscous (Newtonian) flow. Comparative analysis of these results and the results related to other superplastic alloys as well as to crystallizing metallic melts has been carried out.

Abstract: An average particle size of d ≈ 300 nm can be reduced to a nanocrystalline size of about 10 nm or less after 400 h HEBM process. Amorphous-like phase is also observed among the nanocrystalline particles. Using SPS, the nanocrystalline powder can successfully be consolidated into a dense nanocrystalline ZrO2-spinel composite of d ≈ 90 nm. As compared with the data for submicro-grain composite with d = 350 nm, nano-crystalization increased the strain rate by one order of magnitude or lowered the deforming temperature by about 100 K.

Abstract: The recent investigations on spray formed ultrahigh-carbon steels (UHCSs) are reviewed. A satisfactory combination of strength and ductility in spray formed UHCSs can be obtained by hot rolling and annealing. The composition and hot rolling have a marked effect on the formation of graphite in UHCSs. The possibility of achieving high strain rate superplasticity in the spray formed UHCS was first revealed by very recent investigations in Shanghai Baosteel Research Institute. The UHCS processed by a combination of spray forming and hot rolling exhibited high strain rate superplasticity.