# Ultrasonic Surface Rolling Process: Properties, Characterization, and Applications
## 3 Microstructural Development during USRP
### Microstructural Changes in Non-Ferrous Based Materials Subjected to USRP
#### 3.2.4 Mg Alloys  
[PERSON] et al. [40] have explored the effect of USRP on the AZ31B magnesium alloy. The authors revealed that there was no phase transformation due to USRP. However, at the top, extremely fine grain (as small as 339 nm) followed by ultrafine grain (average 2.35 \(\upmu\)m), then unaffected coarse grains were observed. The total affected depth was measured as 573 \(\upmu\)m. They noted that the extent of grain refinement is a direct function of the amount of USRP. The grain refinement is attributed to extensive deformation due to USRP. USRP activated multiple deformation activities, such as a planar slip, twins, and grain boundary movement. With increased plastic deformation, more dislocation tangles, dislocation walls, and grain breakage appear, which lead to observable grain refinement. They noted that since AZ31 is a HCP structure, the number of slip systems is limited, and it is also the cause of limited plastic deformation; however, other deformation mechanisms, namely, twins, were activated, which contributed to grain refinement. [PERSON] et al. [68] have explored USRP on AZ91D Mg alloy to improve the wear resistance. They had used a static load of 240 N at 20 kHz. Their study indicated a deformation depth of 190 \(\upmu\)m, and at the top region, deformation was high, and grain boundaries could not be distinguished. Like the AZ31B alloy, extensive twin deformation was observed along with the slip, and because of the limited-slip systems, only certain properly oriented grains could undergo deformation by slip, while others could deform by twin. The grain refinement mechanism is represented in Figure 8.  
With continued deformation, both twin and slip were activated and were responsible for dislocation tangles and subgrain boundaries. Such observations were noticed even at a depth of 160 \(\upmu\)m. Another essential aspect they reported is the observation of strain-free grains using a TEM at the top surface (within 10 \(\upmu\)m). They attributed this to DRX during USRP. They also proposed that some adiabatic heating takes place during USRP, increasing the possibility of DRX.  
Figure 9 represents the optical microscopic (OM) images of the AZ91D Mg alloy. It can be evident from the figure that the substrate has coarse grains. However, after USRP treatment, a gradient microstructure is observed. The SPD layer has a thickness of 190 \(\upmu\)m. The presence of deformation twins was observed and is due to the plastic flow owing to USRP. At a higher strain rate, the stress concentration in the grain boundaries cannot be released by a slip deformation; hence, slip is hindered. This leads to an increase in twinning and thus high-density twins being formed.  
[PERSON] et al. [69] studied the enhancement in mechanical properties and corrosion resistance of as-cast AZ91 Mg alloy during USRP. The authors revealed that USRP led to gradient microstructure and enhanced the yield strength and ultimate tensile strength by 51% and 50%. In addition, the authors also observed enhanced corrosion resistance, which is attributed to the reduction in surface roughness, formation of nanostructured surface, and introduction of high RCS.  
Microstructural analysis of both ferrous-based and non-ferrous-based material broadly indicates some similarities in microstructure development during USRP, but there are also some noticeable distinctions in the minute details.  
Figure 8: Schematic presentation of grain refinement mechanism in AZ91D subjected to USRP: (**a**) original grain, (**b**) twinning, (**c**) dislocation, (**d**) dislocation rearrangement, (**e**) subgrain boundaries, (**f**) DRX, and (**g**) nanocrystallines. Reprinted with permission from ref. [68]. Copyright 2020 MDPI.