# Effect of Laser Treatment on Intrinsic Mechanical Stresses in Titanium and Some of Its Alloys
## 3 Results and Discussion
### Mechanical Properties
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{**BM**} & \multicolumn{2}{c}{**LT**} \\ \cline{2-5}
**Sample** & \begin{tabular}{c} **Hardness** \\ **(GPa)** \\ \end{tabular} & \begin{tabular}{c} **Young’s** \\ **Modulus** \\ **(GPa)** \\ \end{tabular} & \
\begin{tabular}{c} **Hardness** \\ **(GPa)** \\ \end{tabular} &
\begin{tabular}{c} **Young’s** \\ **Modulus** \\ **(GPa)** \\ \end{tabular} \\ \hline
1 & 5.46 \(\pm\) 0.58 & 169.13 \(\pm\) 9.26 & 7.19 \(\pm\) 2.49 & 130.61 \(\pm\) 6.51 \\
2 & 5.17 \(\pm\) 0.65 & 153.34 \(\pm\) 11.44 & 7.45 \(\pm\) 1.40 & 132.01 \(\pm\) 15.32 \\
3 & 5.91 \(\pm\) 1.26 & 128.44 \(\pm\) 16.43 & 7.04 \(\pm\) 0.69 & 97.50 \(\pm\) 13.04 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Hardness and Young’s modulus results after nano-indentation test. The effect of annealing on mechanical behavior is illustrated by the D change in hardness for Ti specimens shown in Figure 6 for native substrates, and in Figure 7 for laser-processed titanium. The hardness distribution maps for the Ti6 Al7 Nb and Ti15 Mo samples are shown in Figures S1-S4 in the Supplementary Materials. The obtained 3D distributions of the tested materials indicated an uneven distribution of mechanical properties (nanohardness) on the tested surface. As the force increased (from 50 to 200 mN), an increase in the uniformity of the distribution of the tested properties was observed for all groups of specimens. This was probably due to a decrease in the influence of the roughness of the tested surface with an increase in the maximum force, which influenced the increase in the maximum depth of the measurement performed. In the case of samples after laser treatment (Figure 7), a more uneven distribution of properties was obtained, which, due to the increase in the surface roughness after the laser treatment process, confirmed the previous hypothesis. The 3D distribution maps presented for Ti  
Figure 6: Three-dimensional hardness maps of the BM and annealed BM of Ti samples. Figure 7: Three-dimensional hardness maps of the LT and annealed LT of Ti samples. specimens for all cases studied showed an increase in hardness after annealing, which was greater for measurements performed with a lower maximum force (50 mN). The increase in hardness after annealing was already observed in a previous study [27] with a simultaneous reduction in residual tensile stresses. Similar results have been previously reported; the higher the annealing temperature, the greater the hardness, the effect disappearing only for annealing over 1000 \({}^{\circ}\)C, and the effect was attributed to the formation of martensite during cooling following annealing [38]. Moreover, it might also be the effect of the rearrangement of the dislocated structure within a thin subsurface layer, mainly its movement towards the surface, stopping and interacting, resulting in the appearance of surface cracks. Based on the obtained results, the differences in the average values of the forces causing the same indentation were calculated for samples before and after stress relief annealing, to determine the stresses  
When discussing the results, it is useful to consider the sign and magnitude of stresses and the effects of loading and alloy grade. Considering the loading value, the higher the load, the higher the resultant stress. Finally, titanium and Ti7 Nb6 Al alloys demonstrated similar values of excessive stress, and the Ti15 Mo alloy showed much higher values. The stresses assessed were always tensile. Such behavior after laser treatment was already observed as, e.g., 402 \(\pm\) 60 MPa in [39]. The residual stresses present in thin layers reached 0.1-3 GPa in compression, and up to 1 GPa in tension [15]. It is known that [14] laser texturing created overall tension. Here, obtained residual stresses are, as a rule, tensile and distinctly higher than those reported in the other research.