# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 3 Microstructural Development during USRP
### Microstructural Changes in Non-Ferrous Based Materials Subjected to USRP
#### 3.2.1 Ti Alloys  
Amongst non-ferrous alloys, titanium and its alloys were explored in intense detail. [PERSON] et al. [55] analyzed the effect of USRP on the pure Ti using a static load of 350 N and vibration frequency of 20 kHz. They reported extensive grain refinement and the formation of nanocrystals of 38 nm at the specimen's top surface. [PERSON] et al. [56] explored the plastic deformation associated with multi-pass USRP on hot isostatic processed Ti6 Al4V alloy. The authors observed plastic deformation in the direction of the feed. Due to plastic deformation, the shape was curved. The extent of deformation was dependent on the applied load. The total deformation layer thickness was about 35 \(\upmu\)m. At the top surface, refined, equiaxed, and random orientated grains were observed. There was an increase in dislocations, dislocation tangles, and twins inside the grains. Some of the refined grains had subgrains within themselves. [PERSON] et al. [34] conducted USRP experiments on pure titanium and studied the gradient nanostructure and phase transformation behavior. The gradient microstructure along the depth direction from the surface is further classified as a severe deformation layer (region 1), deformation twinning profuse layer (region 2), and low-strain coarse-grained layer (region 3). The corresponding SEM and EBSD images of these three layers are shown in Figure 6. The SEM of USRP'ed pure titanium is shown in Figure 6a. It is evident that significant grain refinement was observed in region 1. In addition, an increase in grain size is observed along the depth direction. Figure 6b shows the grain boundary orientation map. It shows many blue lines on the severe deformation layer (region 1). This represents low angle grain boundaries (misorientation less than 15\({}^{\circ}\)). Substructures are visible in region 1 and region 2. However, in region 3 the red lines are dominant, indicating high angle grain boundaries (misorientation more than 15\({}^{\circ}\)). Figure 6c represents the inverse pole figure analysis of the gradient microstructure. It indicates the formation of many lamellar/ lens structures in region 1 due to SPD during USRP. The orientation of lamellar/lens structures is different from the parent grain. This indicates that lamellar/lens structures represent deformation twinning. There is no grain refinement observed in region 3. [PERSON] et al. [57] performed USRP on Ti6 Al4V using 10 mm WC-Co balls. The authors reported grain refinement up to a depth of 100 \(\upmu\)m-110 \(\upmu\)m. The authors observed that more than 89 % of the grains on the surface were in the size of 1.46 \(\upmu\)m. [PERSON] et al. [58] have explored the effect of multiple USRP processing stages on the microstructural evolution and surface integrity of Ti6 Al4V alloy. The authors adopted an ultrasonic vibration frequency of 28 kHz, an amplitude of 5 \(\upmu\)m, a static load of 800 N, and they varied processing stages as 1,  
Figure 6: Gradient microstructure after USRP on pure titanium: (**a**) SEM, (**b**) grain boundary orientation map, and (**c**) inverse pole figure map. Reprinted with permission from ref. [34]. Copyright 2022. Elsevier. 2, 4, 8, 12, and 16. They noted that the deformation layer thickness increased from 16.5 \(\upmu\)m to 304 \(\upmu\)m when the processing stage varied from 1 to 16. The matrix microstructure was equiaxed \(\alpha\) and lamellar \(\alpha+\beta\) mixture. On USRP, the sample showed elongated \(\alpha\) and \(\alpha+\beta\). Both \(\alpha\) and \(\beta\) phases contain a high density of dislocations due to SPD generated during USRP. Figure 7 represents the TEM images of the USRP'ed Ti6 Al4V processed with 16 rolling stages. The microstructure and corresponding selected area diffraction (SAED) pattern of circle A in the inset at a depth of 5 \(\upmu\)m from the surface is shown in Figure 7a. The microstructure indicates the presence of nanocrystals.