Microstructural interpretation of negligible strain-hardening behavior of submicrometer-grained low-carbon steel during
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at 200 kV. Room-temperature tensile properties of the steel with the different ferrite grain sizes, i.e., one before ECAP, the as-ECAPed, and the annealed under two different conditions, were measured on an Instron** machine **Instron is a trademark of Instron Co., Canton, MA.
KYUNG-TAE PARK, Professor, is with the Division of Advanced Materials and Engineering, Hanbat National University, Taejon 305-719, South Korea. DONG HYUK SHIN, Professor, is with the Department of Metallurgical and Materials Science, Hanyang University, Ansan, Kyunggi-Do 425791, South Korea. Manuscript submitted June 12, 2001.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Fig. 1—Room-temperature nominal stress-strain curves of the present steel with various ferrite grain sizes.
with an initial strain rate of 1.33 ⫻ 10⫺3 s⫺1. The tensiledeformed microstructures of the steel were also examined by TEM. The nominal stress-strain curves of the steel with the different ferrite grain sizes are presented in Figure 1. As expected, the two coarse-grained steels, i.e., one with a ferrite grain size of 30 m (before ECAP) and the other with a ferrite grain size of 10 m (873 K annealed for 1 hour after ECAP), exhibited extensive strain hardening. On the contrary, very little strain hardening occurred in the two SMG steels. Figures 2(a) and (b) show the cell structure formed at a nominal strain (e) of 15 pct for the steels with 30 and 10 m ferrite grains, respectively: a tensile deformation of 15 pct is close to the ultimate tensile strength (UTS) in both steels. Regardless of the initial ferrite grain size, the cell sizes of both steels were about 0.35 m. The tensiledeformed (e ⬵ 10 pct) microstructure of the steel annealed at 753 K for 72 hours after ECAP (the ferrite grain size of 0.45 m) is shown in Figure 3. In this case, the UTS appeared at e ⬵ 10 pct as seen in Figure 1. Most grains of the deformed sample were elongated (Figure 3(a)), indicating that considerable intragranular strain was induced during deformation of e ⬵ 10 pct. In addition, inspection with a higher magnification (Figure 3(b)) revealed that dislocations were not distributed uniformly inside the grains but were localized in the vicinity of grain boundaries. At the initial stage of plastic deformation, the dislocation density increases and its distribution is relatively uniform in the grain interior, causing strain hardening. As plastic deformation proceeds, a dislocation cell structure is formed due to dislocation tangling. In such a condition, the cell size (␦ ) is equivalent to the mean free dislocation length (L).[6] The mean free dislocation length is inversely proportional to the shear stress ( ), i.e.,
⬀
Gb Gb ⫽ L ␦
[1]
VOLUME 33A, MARCH 2002—705
Fig. 3—(a) TEM micrograph showing the tensile-deformed (e ⬵ 10 pct) microstructure of the steel annealed at 753 K for 72 h after ECAP (ferrite grain size of 0.45 m). (b) TEM micrograph showing the localized dislocation distribution in the vicinity of grain boundaries of the sample of (a).
Fig. 2—TEM micrographs showing the
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