**4. Discussion**

#### **4.1. Structural multilayering**

The present experimental studies show the plastic deformed area for the MHSed material was markedly greater than that for monolithic NG (**Figure 4**) and this is consistent with the results obtained from FEA microindentation simulations, where the indenter increasingly induces the more compliant UFG core as the load increases (**Figure 6c** and **d**). Further FEA simulations showed that the degree of energy dissipation (occurring by the inelastic deformation) of multilayered cases (discrete and gradient) increased with increasing load (**Figure 6e**). These results indicate that the macroscopic indentation behavior was directly governed by the underlying micromechanics of the multilayered structure. The load–depth FEA simulations (**Figure 6**) revealed that there was negligible difference in the load–depth response for the discrete and gradient models, suggesting that it is the overall structural multilayering that provides the effective macroscopic mechanical loading resistance rather than the grading.

Recent experimental nanoindentation studies, supported by cross-sectional electron microscopy observations, revealed that the multilayered structures provide a higher resistance to deformation than monolithic counterparts [25–27]. FEA simulations indicated that the structural layering modified the stress distribution and reduced the overall strain values, suppressing crack formation [28]. In our present study, the multilayered models (discrete and gradient) showed a considerable redistribution of the overall equivalent plastic strain field and a significant reduction in the maximum strain levels (**Figure 7**). The plastic equivalent strain contours revealed an increased depth and strain area of plastic deformation for the multilayered systems compared with the monolithic NG material. This is a direct result of transferring the plastic strain to the underlying UFG core with a lower *σY* than the NG layer, thereby diffusing the total plastic deformation energy. The FEA results coincide with the experimental results presented in **Figure 4** where the MHSed Ti diffuses plastic deformation over a greater region relative to the monolithic NG Ti.
