EBSD Analysis

For better clarification of the interfacial structure, EBSD analysis observations on the strongest joint of type B are done and taken in Figure 12. Figure 12a–c shows the interface area in different magnifications prepared by EBSD phase map (12a) and inverse pole figure map (12b,c).

**Figure 12.** EBSD analysis on joint type B (1100 J), for (**a**) low magnification phase map with grain boundaries, (**b**), and (**c**) IPF colored map of interface in different magnifications.

Figure 12a illustrates a low magnification phase map of the interfacial region. A chain of base metal relatively fine grains (in comparison to large grains of base metal after HP-USW), adjacent to the interface centerline, is distinguishable. For better understanding, this region was graphically covered with a pink mask. Other researchers observed this type of fine-grained structure and they called it a "necklace-like" grain structure. The main effective parameter to produce this chain of smaller grains is occurrence of DRX, because, among interfacial area, this region suffers higher strain rates and plastic deformation that were caused due to friction between two faying surfaces [9,27,29]. Other regions far from the interface, as described in 3.3.1, shows an equiaxed grain structure, where the size of grains is larger than the base metal before welding. The IMC containing layer in Figure 12a has a dark feature at the interface centerline. Since that IMC grains are ultrafine, the grain boundaries covered the face of grains to be darkened. With looking overall at the interface in the metallography and EBSD photos, it can be found that some regions lost the Cu coating because of high frequency vibrations and friction. Moreover, the deposited layer thickness is not uniform in various parts of interface; thereupon, the width of interfacial reaction layer differs part by part. There is a direct bare contact between top plate and bottom plate without the presence of an interlayer in some regions of the interface.

In Figure 12b, the IMC reaction products are visible as fine grains that are distributed along the interface and neighborhood areas. The noticed region inside an oval in Figure 12b is an IMC-containing area at the interface, which is shown in more detail in Figure 12c. Figure 12c shows that a portion of ultrafine particles that are probably molten during HP-USW diffused through the grain boundaries of base metal to farther distances, in addition to the presence at the interface centerline. Some of diffused particles were typically pointed with black arrows to have better perception. With use of EBSD computer software, it was found that the diffused particles are Cu-containing reaction products. More accurate identification of particles was made possible through EPMA and TEM analysis.

Consequently, according to EBSD observations, good evidences were obtained for grain size strengthening based on well-known "Hall-Petch" equation, as supported by fine grains of base metal around the interface and ultrafine grains of hard IMC particles distributed in interfacial region.
