1. Introduction
High-strength titanium alloys are used as structural materials in almost all modern load-bearing structures [
1,
2]. In particular, titanium alloys are used in air transportation and aerospace systems for various purposes and in special equipment products [
3]. High strength, low density, high specific strength, and wide temperature range of titanium alloys can significantly reduce the mass of structures and increase the reliability of their work. At present, special titanium alloys with varying strength and, more importantly, high ductility, have been developed. These alloys include two-phase high-strength (α + β) titanium alloys VT23 (σ
us ≥ 1150 MPa, δ = 15%) and VT23M (σ
us ≥ 1100 MPa, δ = 20%) [
4,
5,
6,
7]. In order to attain such enhanced mechanical properties of these two-phase titanium alloys, the developers of semi-finished products realize complex multi-stage modes of thermomechanical treatment (TMT) with different percentage ratios of α and β phases and different amounts of alloying elements added [
8,
9]. Attention should be paid to the fact that most of the stages of complex TMT modes occur under conditions far from thermodynamic equilibrium. Therefore, as early as at the stages of manufacturing, semi-finished titanium alloys and dynamic non-equilibrium processes (DNPs) are realized in melts, which aid in the formation of unique mechanical properties of titanium alloys of the class under consideration [
9,
10].
On the other hand, the search for new methods to improve the mechanical properties of high-strength titanium alloys already on the finished product (rolled metal) is ongoing. Here, we should highlight the promising methods associated with the influence of different energy fields, such as electromagnetic, ultrasonic, and force fields, which realize the chaotic dynamics in materials [
11,
12,
13].
Chausov et al. were the first to propose and test a simple and effective method for realizing DNP by applying additional impulse loading on materials of different classes [
14,
15,
16]. Metallophysical experiments helped to reveal [
17,
18,
19] that under impact-oscillatory loading, due to the pulsed introduction of energy into aluminum alloys, armco-iron, and stainless steel, there occur self-organizing processes in materials with the formation of new spatial dissipative structures, which are connected at different scale levels and look like adiabatic bands of shear. Here, it should be noted that the conditions under which these bands are formed are fundamentally different from the conditions under which adiabatic bands of shear are formed. In particular, the deformation rates of materials are by far less than those at which the classical adiabatic bands of shear are formed. Moreover, as shown by calculations, an increase in temperature of the material, even if all the deformation work is spent on this process, cannot exceed 200 K [
16]. This confirms once again that these processes are not associated with a significant increase in temperature.
In the process of experimenting, it was also found that with an increase in the number of pulses that affect the investigated material, there is an increase in the jump of deformation under DNP [
16,
17]. Accordingly, as experiments have shown, at any given degree of the preliminary static deformation, there is necessarily a critical value of the pulse (
Fimp.cryt.), under which the specimen is practically divided into two parts in the process of DNP [
17]. The revealed structural transformations occur exclusively in the mode of loading (the so-called energetic formation of the structure) and upon cessation of loading, the structure can be subjected to relaxation changes. Despite such relaxation changes in the structure, each new condition of the material acquires new mechanical properties with the subsequent static stretching after the application of the particular pulse that affects the material. In this case, the essential role can be played by the exposure time after the application of such pulse, the number of pulses, as well as the sequence of given specific values of the pulses that affect the material.
In particular, there was a significant increase in the plastic properties of metals after this kind of influence. Previous experiments have also shown that impulse introduction of energy into a material can improve the characteristics of crack resistance. It is also interesting to note that when the DNP is realized due to the impact-oscillatory loading, a significant increase in plasticity does not lead to a significant decrease in strength [
16,
17,
18,
19,
20].
The analysis of the results obtained by the authors with regard to the tests of various class materials under impact-oscillatory loading has shown that in transient modes of loading, which are realized with short pulses of additional force loads, when there occurs a mass transfer while the dissipation of energy, in the classical sense (the transformation of mechanical energy into heat), has not begun yet, the slow diffusion mechanisms of the transfer of motion and energy cannot develop fast enough. Therefore, the energy of the force pulse is transferred from the macroscopic level to a certain intermediate mesoscopic level, at which the energy left in the medium is spent on the formation of new dissipative structures. According to the authors, this process can be realized when the duration of the process of loading and the duration of the process of dissipative structure formation (internal time) are identical or nearly identical.
Lack of experimental data in this area of research makes it impossible to unambiguously answer the question of which parameters of the initial structure of the two-phase titanium alloys are most sensitive to dynamic nonequilibrium processes. Previous studies on titanium alloys of various classes have shown that, for instance, for a two-phase titanium alloy VT22 subjected to impact-oscillatory loading, the plastic deformation increases significantly—by 2.5 times as compared to the initial state, and the strength remains virtually unchanged, i.e., it does not decrease [
21,
22]. Metallophysical research has established a new mechanism for the formation of dissipative structures in the titanium alloy VT22 after the realization of DNP, a significant fragmentation of the structure [
21,
22]. Similar tests on submicrocrystalline titanium alloy VT1-0, on the contrary, have established a significant decrease both in plastic deformation and strength after the realization of DNP due to impact-oscillatory loading [
23]. Thus, we can draw a preliminary conclusion that the positive or negative effect of DNP on changes in the mechanical properties of two-phase titanium alloys due to impact-oscillatory loading depends primarily on the initial parameters of the structure.
The purpose of this research was to evaluate the effect of impact-oscillatory loading on changes in the mechanical properties of sheet two-phase titanium alloys VT23 and VT23M, the chemical composition of which is virtually the same, but the percentage ratio of α and β phases is different.
Author Contributions
Conceptualization, M.C.; Formal analysis, J.B. and A.G.; Investigation, M.C., J.B., A.P., P.M., L.T., and A.G.; Methodology, M.C. and A.P.; Project administration, P.M.; Validation, P.M, M.C., A.P., and J.B.; Writing—original draft, P.M., M.C., J.B., A.P., and A.G.; Writing—review and editing, L.T.
Funding
This research was funded by Ministry of Education of the Slovak Republic VEGA No. 1/0424/17 and of the Slovak Research and Development Agency APVV-16-0359 and the APC was funded by Slovak Research and Development Agency.
Acknowledgments
This work was supported by scientific grant agency of the Ministry of Education of the Slovak Republic VEGA No. 1/0424/17 and of the Slovak Research and Development Agency APVV-16-0359.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Specimens: for mechanical tests at static loading and DNP (a) and for crack resistance investigation (b); A—hole with a diameter of 0.8 mm.
Figure 2.
Results of diffractometric study on the evaluation of percentage of α and β phases in the studied titanium alloys: (a) VT23M alloy (β-phase occupied 22% by weight, α-phase was 78% by weight); (b) VT23 alloy (β-phase occupied 43% by weight, α-phase was 57% by weight).
Figure 3.
Stress-strain diagrams of the tested specimens from alloy VT23 (loading modes are given in
Table 2).
Figure 4.
Dependence of plastic deformation of specimens from titanium alloy VT23 under static stretching (εplast) on impulse deformation (εimp) obtained under dynamic non-equilibrium processes (DNP).
Figure 5.
Fracture surface of the specimen from the VT23 alloy under static stretching (curve 19 in
Figure 3) obtained at macro-(
a,
b) and micro-(
c,
d) levels.
Figure 6.
Fracture surface of the specimen from the VT23 alloy previously subjected to impulse loading (curve 27 in
Figure 3) obtained at macro-(
a,
b) and micro-(
c,
d) levels.
Figure 7.
Stress-strain diagrams of the tested specimens from the VT23 alloy (loading schemes are described in
Table 5).
Figure 8.
Dependence of plastic deformation of specimens from titanium alloy VT23 under static stretching (εplast) on impulse deformation (εimp), obtained under DNP.
Figure 9.
Fractograms of the fracture surface of the specimen from the VT23M alloy subjected to static stretching, 33y (a–c) and to DNP, 5y (d–f).
Figure 10.
Shape of the cut on a specimen.
Figure 11.
Impact toughness of the VT23M alloy for impact testing in the initial state and after DNP.
Figure 12.
Fractograms of fractures of specimens from the VT23M alloy subjected to impact toughness tests under schemes 33y (a–c) and 48y, εimp = 0.6% (d–f).
Figure 13.
Complete stress-strain diagrams of specimens from the VT23M alloy in different states: 14y, in the initial state; 46, after DNP (εimp = 2.2%).
Table 1.
Mechanical properties of titanium alloys VT23 and VT23M.
Titanium Alloys | Mechanical Properties |
---|
σys, MPa | σus, MPa | δ, % |
---|
VT23 | 980–1180 | 1080–1280 | 15 |
VT23M | 1000–1150 | 1080–1180 | 20 |
Table 2.
Testing modes for specimens from high-strength titanium alloys VT22 and VT22M.
Titanium Alloys | Scheme of Loading | Preliminary Static Deformation, εstat, % | Impact Loading, Fimp, kN |
---|
VT23 | Static tension (No. 19) | - | - |
Static tension–DNP-Static stretching (No. 28) | 0.367 | 122 |
Static tension–DNP-Static stretching (No. 31) | 0.322 | 125 |
Static tension–DNP-Static stretching (No. 27) | 0.363 | 125 |
VT23M | Static tension (No. 33y) | - | - |
Static tension–DNP-Static stretching (No. 42y) | 0.031 | 93 |
Static tension–DNP-Static stretching (No. 37y) | 0.025 | 76 |
Static tension–DNP-Static stretching (No. 40y) | 0.046 | 103 |
Static tension–DNP-Static stretching (No. 41y) | 0.034 | 106 |
Static tension–DNP-Static stretching (No. 2y) | 0.093 | 105 |
Static tension–DNP-Static stretching (No. 7y) | 0.288 | 108 |
Static tension–DNP-Static stretching (No. 5y) | 0.585 | 85 |
Static tension–DNP-Static stretching (No. 4y) | 0.754 | 98 |
Static tension–DNP-Static stretching (No. 3y) | 0.852 | 92 |
Static tension–DNP-Static stretching (No. 39y) | 1.488 | 77 |
Static tension–DNP-Static stretching (No. 32y) | 3.75 | 91 |
Table 3.
Chemical composition of titanium alloys VT23 and VT23M.
Titanium Alloys | Fe | Cr | Mo | V | Ti | Al |
---|
VT23 | 0.6 | 1.2 | 2.0 | 4.3 | 86.9 | 5.0 |
VT23M | 0.7 | 1.1 | 2.2 | 4.5 | 86.7 | 4.8 |
Table 4.
Parameters of deformation and fracture of specimens from alloy VT23.
Alloy | Scheme of Loading | σu, MPa | Failure Strain, εf, % | λ = (εfDNP − εfst)/εfst* × 100% |
---|
VT23 | 19 | 1129* | 15.7 | 0 |
28 | 1116 | 16.4 | 4.39 |
31 | 1170 | 19.8 | 26.17 |
27 | 1107 | 21.1 | 34.33 |
Table 5.
Influence of impulse loading modes for testing specimens from the VT23M alloy on changes in the mechanical properties of the alloy.
Alloy | Scheme of Loading | σu, MPa | Failure Strain, εf, % | λ = (εfDNP − εfst)/εfst* × 100% |
---|
VT23M | 33y | 1086 | 21.1 | 0 |
42y | 1092 | 15.9 | −25.02 |
37y | 1102 | 16.8 | −20.67 |
40y | 1114 | 17.2 | −18.64 |
41y | 1095 | 18.3 | −13.29 |
2y | 1094 | 21.7 | 2.60 |
7y | 1093 | 22.3 | 5.39 |
5y | 1101 | 21.5 | 1.70 |
4y | 1104 | 19.3 | -8.65 |
3y | 1115 | 18.9 | -10.79 |
39y | 1131 | 12.8 | -39.36 |
32y | 1125 | 3.0 | −86.05 |
Table 6.
Value of crack resistance parameter Kλ for the VT23M alloy in different states.
Scheme of Loading | mm
| σk, MPa | E, GPa | |
---|
46y | 1.24 | 1102.7 | 120 | 405.7 |
14y | 0.90 | 1089.5 | 112 | 331.8 |
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