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Fig. 1—Schematic of (a) a simplified square cornered ECAP die, and (b) the experimental ECAP die with inner and outer corner radii of 5 and 3 mm, respectively.

The orientation of the lamellar phase before and after ECAP (h and h¢ in degrees, respectively) was measured from the SEM images with respect to the exit channel as shown in Figure 1. These inclinations, as well as lamellar spacing and fragment size, were analyzed using the image processing software Image J. The inclination (in degrees) refers to the orientation of a whole lamellar colony (unless otherwise stated) which was measured from the center of the colony. The lamellar spacing was measured at approximately ten points along the length of two lamellae, and this was conducted for all lamellae within a particular colony, resulting in an average lamellar spacing for the colony. Finally, the diameter of fragmented and/or spheroidized lamellae was averaged from a multitude of circle equivalent diameters using carefully selected limits to exclude the coarse jII and the fine jIV phases.

III.

RESULTS

A. Microstructural Evolution During the First Pass The as-cast microstructures of the as-received material are shown in Figure 2 at different magnifications. They are typical of cast NAB, consisting of globular jII, the randomly oriented lamellar jIII, and fine spherical jIV precipitates as labeled in Figure 2(a), although the large (20 to 50 lm), globular jI precipitates are absent because of a higher content of iron.[4] The lamellar spacing was measured to be 0.76 lm. Figures 3 and 4 display typical NAB microstructures following one pass of ECAP at 673 K (400 C). There are four main ways by which the eutectoid lamellae have accommodated the severe shear deformation: 1. Lamellar reorientation The eutectoid lamellae became reoriented following ECAP and adopted a range of new orientations, as illustrated in Figure 3. Only several special orientations were observed after ECAP from an originally random distribution. A particularly common microstructure was that shown in Figure 3(a), whereby in this example, the inclinations of the lamellar colonies were measured to be within the range 13 deg £ h¢ £ 32 deg. The average value of h¢ in this and similarly oriented microstructures VOLUME 44A, DECEMBER 2013—5557

Fig. 2—SEM microstructures of NAB in the as-cast condition. The scale bar in the higher magnification inset image in (b) represents 1 lm.

Fig. 3—SEM microstructures of NAB following 1 pass of ECAP at 673 K (400 C) showing lamellae with h (a) between 13 and 32 deg, (b) between 103 and 150 deg, (c) 45 deg, and (d) 0 or 180 deg to the exit channel. The scale bar inside the higher magnification inset images represents 1 lm. The x axis as shown in Fig. 1 is horizontal.

observed following 1 pass at 673 K (400 C) was ~23 deg. This is the dominant orientation under such conditions and is in contrast to the randomly oriented lamellae in the as-cast material, confirming that the 5558—VOLUME 44A, DECEMBER 2013

lamellae did experience a change in orientation during ECAP. Su