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Superplasticity in nanocrystalline materials
Abstract
High strain rate and/or low temperature superplasticity was observed in 1420 aluminum alloy and in Ni3Al. The enhanced plasticity demonstrated in these materials is attributed to a refinement in grain size to the nano or near-nano range through the use of High Pressure Torsion (HPT) prior to elevated temperature tensile testing. Grain sizes after HPT were approximately 100 nm for the 1420 aluminum alloy, and 50 nm for Ni 3Al. Tensile testing of the alloys was carried out at strain rates up to 10 and temperatures up to 725°C. Tensile curves typically exhibit high flow stresses and a degree of strain hardening that cannot be adequately described by conventional recovery mechanisms such as grain growth. Room temperature TEM in-situ straining experiments reveal some dislocation activity, and no evidence of dislocation pile-ups. It is suggested that the dominant deformation mechanism for elevated temperature plasticity in this study is grain boundary sliding and rotation.
The generated results on superplastic flow (SPF) in materials produced by severe plastic deformation (SePD) have been analyzed. Formation of sliding surfaces with subsequent cooperative grain boundary sliding (CGBS) has been demonstrated to be a main deformation mechanism of superplasticity under conditions of microstructural stability. A substantial role of dislocation glide in accommodation processes is shown. When grain growth is observed during deformation, continuous grain boundary migration prevents formation of slide surfaces and CGBS is restrained. In this case, deformation of the material is more likely to occur by dislocation or diffusion creep than true SPF despite relatively large strains being reached.
A macro–micro scale transition model based on self-consistent approximation is proposed to explore the effect of grain size and grain size distribution, along with the strain rate, on the mechanical response of nanocrystalline (nc) materials. The representative volume element (RVE) is composed of grains supposed to follow an elastic–viscoplastic behavior, and randomly distributed with a grain size distribution following a log-normal statistical function. The grain interior and grain boundary are not taken as two independent phases with different volume fractions, but as an integral object to sustain deformation mechanisms of grain-boundary sliding, grain-boundary diffusion and grain-interior plasticity. Numerical results for the case of nc copper are shown to be in good agreement with available experimental data. The predictions firstly display that not only the mean grain size plays an important role but also the grain size dispersion has a slight impact on the flow stress. A decrease of the yield stress with an increasing of the grain size dispersion occurs. Secondly, the strain rate sensitivity has strong effect on the mechanical behavior of nc materials and the effect increases with decreasing the grain size. Lastly, deviations of the internal stress and local strain from the overall stress and macroscopical strain occur, respectively. Local plastic strains and internal stresses, developing within the RVE, have been recorded and discussed. Using the present model, we have also studied the failure behavior of nc materials.
Severe plastic deformation at temperatures below 0.4...0.5Tm was used to obtain submicrocrystalline (SMC) structure in pure titanium and titanium alloys Ti-6.7Al-4.7Mo and Ti-11Mo-5.5Sn-4.2Zr. The structural studies showed that the density of titanium and its alloys in the SMC condition was less that in the coarse grained (CG) one. This change is connected with the presence of less-density intercrystalline regions. Low temperature superplasticity was studied in the temperature range 450...575°C. A considerable dependence of ductility on alloy composition was shown. The calculated value of apparent activation energy of the Ti-6.7Al-4.7Mo was equal to 315 kJ/mol at n=2.3, that being higher than during superplastic (SP) deformation of the alloy with usual microstructure.
Cooperative grain boundary sliding (CGBS) (sliding of grain groups along surfaces formed by sliding grain boundaries), has been considered as a result of rigid shear of grain rows or as sequential shear of grains. The sequential shear of grains has been analysed in terms of movement of cellular dislocations (imperfections in regular grain structure). Localized and extended cellular dislocations are examined. Coupling of CGBS and cooperative grain boundary migration might take place as the result of movement of grain boundary dislocations associated with ledges.
Tensile testing at constant strain rate and temperature has shown that superplasticity can be observed at lower temperatures in nanocrystalline materials than in the microcrystalline state. However, even at lower superplastic temperatures the onset of superplastic deformation in nanocrystalline materials coincides with microstructural instability. Consequently, ordered intermetallics such as Ni3Al are attractive systems for the study of nanocrystalline superplasticity because grain growth kinetics is inhibited by preferred atomic pairing. Ni3Al was processed by severe plastic deformation to obtain nanocrystalline samples. The strain rate sensitivity, n, was estimated from strain rate jump tests to be 2.6. The grain size dependence of superplastic flow, p, was estimated by testing tensile specimens annealed to produce a range of grain size. The value of p was estimated to be 2.4–1.8. Several interesting features of nanocrystalline superplasticity, such as strain hardening and high flow stresses, were investigated.
Grain boundary (GB) sliding, which involves the relative translation of two adjacent grains by a shear parallel to the boundary plane, is an important mode of deformation in elevated temperature processes such as creep and superplasticity. It is generally accepted that GB sliding occurs by the motion of dislocations rather than the simultaneous shear of the entire boundary. Only limited information has been obtained on the atomic-scale mechanisms by which GB sliding occurs and, in particular, about accommodation mechanisms. High resolution transmission electron microscopy has been able to provide some details on GB structure, but little information on the dynamic processes of GB sliding. This problem is ideal for study via molecular dynamics simulation techniques since atomic-scale processes can be conveniently modeled. Computer simulations were performed to examine the role of GB dislocations in the sliding of tilt boundaries in aluminum. The sliding resistance of nondislocated GBs was determined from a computation of the dependence of the boundary energy on in-plane translation of one grain relative to the other. Low-energy, faulted structures were found that correspond to unstable intermediate states and the presence of stable partial GB dislocations. Extrinsic GB dislocations lowered the sliding stress relative to the nondislocated GBs.