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Microstructures in the as-cast and BP-ECAPed materials were observed using optical
microscopy (OM) and scanning electron microscopy (SEM). The samples for OM and SEM were cut
perpendicular to the longitudinal direction and prepared by standard metallographic procedures
followed by chemical etching using a solution of 10 ml nitric acid, 30 ml acetic acid, 40 ml distilled
water and 120 ml ethanol. Average grain sizes of the as-cast and BP-ECAPed materials were obtained
following the linear method.
Compression specimens of 3 mm x 3 mm x 6 mm were cut along the longitudinal direction of
the BP-ECAPed samples. Compression tests were performed at room temperature with an initial strain
rate of 1×10 -3 s -1 . For each material at least three compression tests were performed. Microstructures on
the samples after the compression tests were also observed along cross section.
Results and discussion
Fig. 1 showed that the majority grains in the as-cast Mg were equiaxed and very coarse with
irregular morphology. The average grain size was larger than 800 µm. Fig. 2 (a) displayed the
appearances of a sample after 1 pass through the die at 250 °C without back pressure. Fracture
segments distributed uniformly along the sample and the cracks went though the bottom to top along
shearing direction which was about 45° to the extruded direction. In contrast, all the samples after BP-
ECAP at temperatures ranging from 200 °C down to room temperature with 23 °C interval showed no
cracks at all. Fig. 2 (b) exhibited the sample after 4 passes at room temperature. It demonstrated that
applying back pressure was significantly helpful in ECAP processing of hard-to-deform metals at a
relatively low temperature or even at room temperature as an appropriate back pressure can effectively
prevent the material from cracking during ECAP processing.
Fig.1 Optical microstructure in pure cast ingot
Fig. 2 Appearance of pure Mg billets: (a) 250 ºC
without back pressure; (b) 23 ºC withbackpressure
Fig. 3 (a) through (c) presented the typical microstructures under OM for the materials after 4
passes at temperatures of 200, 150 and 100 °C, respectively, and Fig. 3 (d) revealed the representative
microstructure under TEM for the material after 4 passes at room temperature. The grain refinement
was significant. Especially, the grain size was refined into submicron level when the BP-ECAP
process was carried out at room temperature. The average grain sizes of the materials were listed in
Table 1.
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