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ation Mapping Characteristics for the TEM Foils Depth from the Surface

Step size (nm) Electron beam size (nm)

0 (lm)

27 (lm)

52 (lm)

98 (lm)

4 13

12 16

20 25

28 25

Fig. 3—(a) Image quality (IQ) maps of the cross section under the ultrasonic impact-treated zone. (b) Image quality (IQ) maps with confidence index >0.1 of the cross section under the ultrasonic impact-treated zone. (c) Inverse pole figure (IPF) map with indexation. 816—VOLUME 46A, FEBRUARY 2015

METALLURGICAL AND MATERIALS TRANSACTIONS A

III.

prior to the ultrasonic impact treatment. The IPF maps show the detailed morphology and crystallographic features of the lath martensite structure. The colors correspond to the crystallographic orientation normal to the observed plane, as indicated by the stereographic triangle. The boundaries are drawn for misorientations between adjacent points greater than 10 deg, since the misorientation calculations imply that all the packets and block boundaries should have misorientations larger than 10 deg.[43] The high-angle boundaries are defined as the boundaries whose misorientations are larger than 15 deg. The total length of the high-angle boundaries in measured region (236:6  184:64 lm2 ) is 55.4 mm. Assuming that the measured section is the representative of the three-dimensional morphology of the martensitic structure, the area of the high-angle boundaries per unit volume, Sv is 1.268 lm1. Sv is related to the mean intercept length (L) for any threedimensional shape of masses by[44]:

RESULTS

A. EBSD Observations of the Heat-Affected Zone Prior to Ultrasonic Impact Treatment Figure 1 shows a micrograph of the heat-affected zone consisting of a lath martensitic microstructure

Sv ¼

2 : L

½2

From the above equation, L is calculated to be 1.58 lm. This can be considered as the mean grain size of the lath martensite.[45] B. Optical Microscope and EBSD Observations PostUltrasonic Impact Treatment The cross-sectional optical microstructure and EBSD maps of the ultrasonic impact-treated zone at the weld toe are shown in Figures 2b and 3, respectively. It is observed that the submicronic structures that appear on the surface and down to a depth of 75 lm is outside the resolution of optical and scanning electron microscope (SEM). Figure 3(a) shows the image quality (IQ) map of the first 8.45 lm from the surface. It is evident that the ultrasonic impact treatment resulted in surface nanocrystallization. However, when the same image was produced with a confidence index of greater that 0.1 (Figure 3(b)), only 12 pct of the total pixels remained. It is also evident from Figure 3(c), that the characterization of the microstructure close to the surface under the ultrasonic impact-treated zone is beyond the resolution of SEM and EBSD. C. TEM Investigations of Post-weld Ultrasonic Impact Treatment Fig. 4—TEM micrograph of the welded zone after the ultrasonic impact treatment at a depth of (a) 98 lm from the surface; (b) 52 lm from the surface; (c) 0 lm from the surface.

Table III.

To investigate the grain refinemen