# Effect of Laser Treatment on Intrinsic Mechanical Stresses in Titanium and Some of Its Alloys
## 3 Results and Discussion
### Mechanical Properties  
For each of the materials, base, laser treatment, and annealed base and laser treatment, a load-strain hysteresis diagram was plotted during the nano-indentation measurement. The shape of the unloading curve directly affected the mechanical properties of the material determined by the Oliver-Pharr method. The unloading curve started from the value of the maximum indenter depth. Its end occurred at the maximum depth for which the indenter was in contact with the surface of the sample. An example of such a curve is shown in Figure 5. The graphs illustrate the shape of the curve for the 13 th nano-indentation measurement for all groups of samples. A decrease in the depth of maximum indentation after annealing compared to samples without annealing was observed for all samples tested. The decrease was observed for both bare materials (BMs) and laser-treated (LT) samples. The slope angle of the unloading curve indicates the plastic-elastic nature of the tested material [37]. In the case of the measurement with a maximum force of 50 mN, it was observed that the angle of the unloading curve (the tangent relative to the top 60% of the obtained force-displacement values) concerning the \(x\)-axis was smaller than for the measurement with a maximum force of 200 mN, which may indicate a change similar to the tested material depending on the maximum depth at which the measurement was performed. As the force and maximum depth increased, the material showed a decreasing plastic-elastic character. The greatest changes in the hysteresis shape for the samples before and after annealing was observed for sample number 3 (Ti15 Mo). \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **Mean Thickness \(\pm\) SD** \\  & **(\(\upmu\)m)** \\ \hline LT1 & 65.87 \(\pm\) 8.82 \\ LT2 & 126.42 \(\pm\) 11.90 \\ LT3 & 72.50 \(\pm\) 1.56 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Thickness values with standard deviations. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**Roughness Ra**} \\  & \multicolumn{2}{c}{**[\(\upmu\)m]**} \\ \cline{2-3}  & **BM** & **LT** \\ \hline
1 & 0.17 \(\pm\) 0.03 & 0.22 \(\pm\) 0.05 \\
2 & 0.16 \(\pm\) 0.04 & 1.05 \(\pm\) 0.09 \\
3 & 0.38 \(\pm\) 0.03 & 1.68 \(\pm\) 0.06 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Roughness results with standard deviations. The results of the nano-indentation tests are summarized in Table 8. The chemical composition of the material influences the hardness and Young's modulus. The laser treatment significantly toughened the surface layer, as demonstrated by the hardness increasing by over 30% and Young's modulus decreasing by over 15%. The effect resulted from the formation of a small-grain recrystallization zone within the subsurface layer. Compared to the other results, laser irradiation with a femtosecond laser resulted in an increased hardness to 408 HV [34]; from 300 to 440 HV [35]; from 217 HV to 348 HV; Ra from 50 to 230 um; hardness from 300 to 770 HV; [18] Ra from 15 to 65 nm; hardness from 330 to 440 HV [12]; and hardness of 8 GPa [19; 33]. These values correspond to the present results and the decreasing Young's modulus values with increasing hardness. The most considerable increase in hardness was obtained for the Ti7 Nb6 Al alloy. As regards Young's modulus, its increase corresponded with the previous result [23], showing a change in the E value from 40 to 60 GPa at 3 um. Figure 5: Indentation curves of BM and LT samples and annealed BM and LT samples ((1)—Ti grade IV; (2)—Ti7 Nb6 Al; (3)—Ti15 Mo) loaded with a maximum force of 50 and 200 mN.