Observations of unique plastic behavior in micro-pillars of an ultrafine-grained alloy

Abstract

When ultrafine-grained (UFG) samples are deformed plastically, it is necessary to consider the role of grain boundaries even at the micrometer scale in sample size. We report here the occurrence of intensive grain boundary sliding (GBS) at room temperature in micro-pillars of a UFG aluminum alloy having an unusually high strain rate sensitivity. A consequence of this GBS is that the intermittent flow with detrimental strain avalanches characterizing micro-sized conventional crystals is not present in UFG materials, thereby illustrating a potential for effectively applying these UFG materials in micro-devices.Recently, the size-scale effects in plastic deformation have been studied both experimentally [1][2][3] and theoretically [4,5] for several different materials. In the case of conventional crystalline materials, as with metals, it is well established that the decreasing sample size may significantly affect the fundamental mechanisms of plasticity: for example, in dislocation storage, multiplication, motion, pinning, etc. Therefore, although the plasticity of macroscopic samples appears as a smooth process, in the plastic deformation of microcrystals specific dislocation avalanches are formed leading to an uncontrolled deformation process including also the possibility of catastrophic failure. Accordingly, micrometer-sized samples of coarse-grained metals are not suitable for use in the fabrication of micro-devices.It is well established that bulk ultrafine-grained (UFG) materials may be achieved by using severe plastic deformation (SPD) techniques, [6,7] decreasing the grain sizes into the submicrometer and nanometer ranges. Typically, the materials produced by SPD exhibit tensile ductilities below 10% at ambient temperature due to exhaustion in their work hardening capacity. Furthermore, this low ductility is correlated with extremely low strain rate sensitivity (SRS) of~0.01-0.03 characterizing the mechanical behavior of these materials. [8] It was suggested that the mechanical properties of UFG metals may be improved if grain boundary sliding (GBS) is achieved at relatively low temperatures. [9][10][11] It was recently shown that SPD is capable of improving the room-temperature ductility of an Al-30 wt% Zn UFG alloy processed by high-pressure torsion (HPT) [11] since this UFG material became super-ductile with unusually high elongations up to >150% and an unusually high SRS of~0.22. In this work, we applied a focused ion beam (FIB), depth-sensing indentations and scanning electron microscopy (SEM) to reveal experimentally the basic deformation mechanism in this UFG alloy at the micrometer scale. We report here the plastic deformation of UFG micro-pillars, demonstrate the occurrence of intensive GBS at room temperature and note the potential for effectively applying these UFG materials in micro-devices.The experiments were conducted using an aluminum-based alloy containing 30 wt% Zn prepared from high-purity components by vacuum induction melting. Disks having diameter of 20 mm and thicknesses of 0.8 mm wer...

Evidence for exceptional low temperature ductility in polycrystalline magnesium processed by severe plastic deformation

“…A similar effect was observed in an Al-30% Zn alloy in which coarse-grained samples exhibited avalanches in the load-displacement curves during micro-pillar compression while nanocrystalline samples exhibited stable deformation [43]. Thus, the compressed micro-pillars showed evidence of extreme slip in the coarsegrained samples while the nanocrystalline samples exhibited no slip traces but significant grain boundary sliding [43]. These earlier results are consistent with the present investigation where the appearance of the surface of a sample of magnesium processed by HPT and tested in tension, as shown in Fig.…”

“…Furthermore, the micro-pillar surfaces showed evidence of bulges which might suggest an inhomogeneous strain distribution and a contribution to the deformation from grain boundary sliding. A similar effect was observed in an Al-30% Zn alloy in which coarse-grained samples exhibited avalanches in the load-displacement curves during micro-pillar compression while nanocrystalline samples exhibited stable deformation [43]. Thus, the compressed micro-pillars showed evidence of extreme slip in the coarsegrained samples while the nanocrystalline samples exhibited no slip traces but significant grain boundary sliding [43].…”

The Strength–Grain Size Relationship in Ultrafine-Grained Metals

“…[127] Micropillars of an Al-30% Zn alloy were prepared by HPT and FIB milling and tested in compression at room temperature within a depth-sensing ultra-hardness testing machine and this gave a high strain rate sensitivity and direct evidence for the occurrence of GBS. [128] In addition, these experiments showed that the strain avalanches that conventionally occur in the coarse-grained alloy during flow are absent in the UFG material.…”

Influence of Zn content on the microstructure and mechanical performance of ultrafine-grained Al–Zn alloys processed by high-pressure torsion

“…Details of the pillar preparation were given earlier [3]. 3 The coherent domain size and the dislocation density in the HPT-processed disks were determined from analyses of X-ray diffraction line profiles measured by a high-resolution rotating anode diffractometer (type: RA-MultiMax9, manufacturer: Rigaku) using CuK 1 (wavelength  = 0.15406 nm) radiation.…”

“…The best-known Al-Zn alloys, such as eutectic Al-95 wt.% Zn, eutectoid Al-78 wt.% Zn and solid solutions with Zn contents lower than 31.6 wt%, have been extensively studied [1][2][3][4][5][6][7][8][9].…”