Two-Step SPD Processing of a Trimodal Al-Based Nano-Composite
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INTRODUCTION
ULTRAFINE-GRAINED (UFG) materials, characterized by average grain sizes of 50 C).[34–36]
VOLUME 46A, DECEMBER 2015—5883
Fig. 10—(a) Tensile stress-strain curves of the conventional AA 5083, ‘‘0T,’’ ‘‘1/4T’’ [annealed at 673K (400C)], ‘‘1T’’ [annealed at 673 K (400 C)], and ‘‘5T’’ [annealed at 673 K (400 C)], (b) partial enlargement of the stress–strain curves near fracture.
Table II. Sample ID Conventional AA 5083 0T 1/4T annealed at 673 K (400 C) 1T annealed at 673 K (400 C) 5T annealed at 673 K (400 C)
List of the Tensile Properties Measured from the Stress–Strain Curves Yield Strength (MPa)
Ultimate Tensile Stress (MPa)
Fracture Elongation (pct)
271 603 — 677 746
324 607 611 678 781
16.9 1.0 1.0 1.2 1.9
Fig. 11—Fracture surfaces of (a) ‘‘0T’’ and (b) ‘‘5T’’ (annealed at 673 K (400 C)) with artificially tinted CG regions. (c) SEM Micrograph showing intergranular fracture in the UFG matrix and dimple-like ductile fracture in the CG region of sample ‘‘5T’’ [annealed at 673 K (400 C)].
D. Tension Room temperature tensile tests were performed at a strain rate of ~4.8 9 10 4 s 1 using a micro-tensile stage. The tensile stress–strain curves for the ‘‘0T,’’ ‘‘1/ 4T’’ annealed at 673 K (400 C), ‘‘1T’’ 673 K (400 C), ‘‘5T’’ 673 K (400 C), and a conventional work-hardened AA 5083 are plotted in Figure 10. The yield strength, ultimate tensile strength, and fracture elongation are listed in Table II. To eliminate the size effect of 5884—VOLUME 46A, DECEMBER 2015
the tensile specimens, the properties listed in Table II were measured using identical specimen geometry and testing conditions. As shown in Figure 10(a), all trimodal AA 5083 samples exhibited high strength (>600 MPa), at least twice that of conventional AA 5083 (~271 MPa). In particular, the annealed ‘‘5T’’ sample showed the highest yield strength (~746 MPa), 24 pct greater than for sample ‘‘0T.’’ In addition, the ‘‘5T’’ sample annealed at 673 K (400 C) showed the METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 12—Schematic illustration of the failure mechanisms in sample ‘‘5T’’ [annealed at 673 K (400 C)].
respectively. The CG regions of sample ‘‘0T’’ were elongated perpendicular to the forging direction. In contrast, the CG regions of sample ‘‘5T’’ (annealed at 673 K (400 C)) had smaller spacings and no preferred direction of elongation. Figure 11(c) shows brittle intergranular fracture observed in the UFG matrix. The CG region in Figure 11(c) shows a dimple and cup fracture surface, an indication of ductile failure. Figure 12 schematically shows the toughening process of the CG regions in sample ‘‘5T’’ (annealed at 673 K (400 C)). First, the majority of micro-cracks initiated at the brittle intermetallic dispersoid sites under tensile loading. Because the spacing of the CG regions was intentionally reduced to the plastic zone size of the UFG matrix,[27] the crack growth was effectively retarded due to a toughening mechanism in adjacent CG regions, as shown in Figure 13, where a surface micro-crack was arrested at a CG reg
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