**4. Conclusions**

The changes of the mechanical properties and crystallographic structures of the TiMoZrTa quaternary alloy with variable doping by Si were investigated using various non-destructive evaluation (NDE) methods. The Si content in the TiMo20Zr7Ta15Six alloy was varied with x = [0.0; 0.5; 0.75; 1.0]. The main NDE investigation methods were RUS (resonant ultrasound spectroscopy), AE (acoustic emission) and X-ray di ffraction. Those methods revealed the importance of the Ta and Zr content in the TiMo binary alloy, that together with NDE methods highlighted the influence of the Si addition on the mechanical properties and crystallographic structure. The changes in crystallographic structure showed that all the samples contained a small foreign phase in the error limit. The ascertained values of the average bond order (*Bo*) and the averaged orbital energy level ( *Md*) indicated that all casted alloys were in the stable β-region. RUS and compression tests showed an influence of the composition on the strength and fracture behaviors with various damage mechanisms.

The test results verified that TiMo20Zr7Ta15Six alloys have good mechanical behavior, with a Young modulus in the range of 69.11 to 89.03 GPa, and shear modulus from 24.70 to 31.87 GPa. All the tested samples were slightly a ffected by the Si content through their moduli with non-linear trends. The compressive strength of the studied Ti alloy, highlighted by AE, turned out to be over 800 MPa. Despite this, the internal cracking, which appeared in AE signals at 300 MPa stress, was followed by rapid failure at 600 MPa. The decrease in AE activity in the region of plastic deformation could be due to the structural modification dynamics. Although Si improves the creep strength of Ti alloys, AE test activity has shown that the plasticity is a ffected at a Si content higher than 0.75 wt.%.

The dependence of crystallographic structure on Si concentration is probably due to modification of the sample's resonance spectrum symmetry, demonstrated by the splitting of the peaks at certain resonance frequencies in RUS.

Structural modifications of samples caused by di fferent Si doping ratios could easily be monitored by RUS, emphasizing the changing of eigen frequencies. This led to the splitting of some spectral peaks, increasing their amplitudes and displacement of the RUS spectra from their initial position. The shape of the RUS spectrum and the resonance frequencies were also modified by the lower content of alloying elements and smaller elastic properties than those prescribed.

The analysis of the alloy microstructure dependence on the TiMo20Zr7Ta15Six components, as well as on the constitutive phases will be reported in detail in another paper.

**Author Contributions:** Conceptualization: A.S., F.N. and A.B.; Data curation: A.S., M.L.C., C.H. and Z.P.; Formal analysis: A.S., S.M., M.L.C., M.C., A.B., R.S. and F.N.; Funding acquisition: A.S.; Investigation: A.S., V.T., C.H., M.L.C., P.V. and S.M.; Methodology: A.B., F.N., M.C., R.S., P.V. and Z.P.; Project administration: A.S.; Resources: A.S., P.V., V.T., F.N. and S.M.; Software: M.L.C., M.C., R.S. and C.H.; Supervision: A.S., S.M., M.L.C. and Z.P.; Validation: A.S., F.N., M.L.C., A.B., C.H. and Z.P.; Visualization A.B., F.N., R.S. and C.H.; Roles/Writing—original draft: A.S., F.N., A.B., Z.P., R.S. and M.L.C.; Writing—review and editing: All authors. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Ministry of Education and Research, Romania under CCCDI-UEFISCDI, project number PNCDI III-P1-1.2-PCCDI-207-03239/60PCCDI 2018—MedicalMetMat.

**Conflicts of Interest:** The authors declare no conflict of interest.
