Mechanical behavior of a microsized pillar fabricated from ultrafine-grained ferrite evaluated by a microcompression test
Introduction
In recent years, fabrication of ultrafine-grained (UFG) metals and alloys using severe plastic deformation (SPD) processes such as high-pressure torsion (HPT) [1], [2], equal-channel angular pressing [1], [3] and accumulative roll bonding [4] have been attractive research topics. The unusual and extraordinary mechanical properties of UFG materials have attracted considerable interest from many researchers. These features are related to the grain boundaries, owing to the larger volume fraction of the grain boundary area and the interaction with mobile dislocations in the small grains of UFG materials. UFG materials fabricated by an SPD process have grain boundaries that contain an extensive number of defects, such as dislocations and vacancies, and are termed non-equilibrium grain boundaries [1], [5], [6], [7]. The grain boundary characteristics (e.g. equilibrium, non-equilibrium, coincidence site lattice, misorientation angle) play an important role in deformation behavior [1], [8], [9].
In tensile tests of UFG materials, the yield drop phenomenon, Luder’s band formation and shear banding, which are not observed in their conventional grain size counterparts, are observed [10]. Elongation decreases abruptly as the grain size decreases to 1 μm. In this grain size regime, necking can occur, accompanied by localized shear. Outside of the necked section, surface observation does not show any sign of deformation. This sudden drop in elongation is also observed in UFG interstitial-free steel and aluminum [11]. The lack of dislocation storage due to decreasing grain size is responsible for the notable decrease in work hardening capability. When the grain size becomes smaller than 1 μm, work hardening does not occur to maintain the shape of deformed area, resulting in early failure. Because of this early failure, work hardening behavior cannot be observed in tensile tests. Therefore, compression testing is needed to investigate the deformation behavior across a wide range of strain. Observation of the deformation behavior in UFG materials with a wide range of strain is the main subject of this paper.
In this work, we used HPT to refine the grains of ultralow-carbon steel. The HPT process introduces a large amount of torsional strain to the periphery of disk-shaped materials. Materials subjected to such non-homogeneous deformation with a large strain or a large strain gradient have different crystallographic or structural features within the specimen [12]. Bulk size testing, which is widely used as a mechanical testing method, normalizes the effects of such differences on the mechanical properties. In contrast, the site-specific response of mechanical properties can be obtained using microsized test specimens fabricated from the area of interest using a focused ion beam (FIB). As is widely accepted, the strengths of materials are increased with decreasing dimensions to the micro- or submicroscale. Uchic et al. [13] reported that microcompression of 10 μm diameter samples displayed a mechanical response that matched the behavior of the bulk tension test. The present microcompression test was with 20 μm pillars, and was expected to show comparable mechanical properties to bulk counterparts. Micropillars fabricated from HPT-processed disks were evaluated by the compression test to investigate the site-specific mechanical properties of microstructures observed by electron backscattered diffraction pattern (EBSD) analysis.
Section snippets
Experimental procedures
The starting material of this work was ultralow-carbon steel (11C, <30Si, 20P, <3S <2B 8N, 14O, 300Al, <20Ti, <30Cr, <30Cu, in mass ppm). Homogenization treatment was conducted at 1273 K for 1 h in pure Ar atmosphere. After homogenization, material was cut and polished into disks of 10 mm diameter and 0.85 mm thickness, to subject them to the HPT process. Holding the disk with two anvils, materials were subjected to torsional strain for one or five rotations at a rotation speed of 0.2 rpm under a
EBSD analysis
The crystal orientations of the materials were determined using scanning electron microscopy/EBSD. Fig. 2 shows the orientation map of UFG ferrite obtained by EBSD analysis. The grains are slightly elongated in the shear direction in the upper part of the images, as indicated by the black arrow. Grain size is defined as the diameter of an equivalent circle having the same area surrounded by the grain boundary with a misorientation larger than 15° in the orientation maps. The grain sizes of each
Conclusions
In this work, a microcompression pillar was obtained using two angles of ion irradiation. The obtained pillar was not tapered and had a 20 μm square cross-section. EBSD analysis was performed in specific areas with different amounts of strain. For ultralow-carbon ferrite, the initial grain size of 300 μm was decreased to 300 nm at a shear strain of above 30, and further straining caused a change in the grain boundary misorientation distribution, shifting it to a higher angle. This represents
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