Issue 78

A. P. Soldatenkov et alii, Fracture and Structural Integrity, 78 (2026) 248-259; DOI: 10.3221/IGF-ESIS.78.15

I NTRODUCTION

N

owadays, metastable titanium alloys are widely used in the aerospace industry due to their high specific strengths [1–6], ensured by the possibility of processing at temperatures corresponding to the  + β region and subsequent heat treatment (aging) [7-9]. In particular, this class includes the Ti-5Al-5V-5Mo-1Cr-1Fe alloy (the VT22 grade according to the Russian classification), suitable for manufacturing of aircraft landing gears [2]. During operation, the landing gears typically experience alternating high static and variable low-amplitude loads as aircraft taxing. Therefore, it is of importance to simultaneously improve the mechanical and fatigue properties both at high stress amplitudes in the low-cycle fatigue (LCF) regime and at low-amplitude loads in the high-cycle fatigue (HCF) one. For a large number of structural materials, the relationships between their mechanical properties and fatigue limit have been revealed in the LCF and HCF regimes [10]. However, no such dependences have yet been identified for titanium alloys, including those widely used in aviation. Since the same Ti grade with different microstructures but similar mechanical properties can be characterized by various levels of fatigue strength [11,12], the results of each process or treatment technology of material should be tested, subjected to estimation of not only static characteristics but analysis of fatigue behavior. In recent years, radial-shear rolling (RSR) and subsequent aging have been implemented for improving the mechanical properties of metastable titanium alloys [13–16]. They have enabled to form hierarchically organized microstructures in parts made of the Ti-5Al-5V-5Mo-1Cr-1Fe alloy with precipitates of the primary  -phase of ~1 μ m in size at the boundaries of grains/subgrains of the prior  -phase and acicular nanosized inclusions of the secondary  -phase inside subgrains [13,14]. After RSR and subsequent aging at 450  C for 5 hours, the Ti-5Al-5V-5Mo-1Cr-1Fe alloy exhibited an ultimate tensile strength of >1600 MPa and tensile strain of ~3 % at room temperature [13,15]. In addition, it was characterized by great fatigue performance in the very high cycle fatigue (VHCF) regime at stress amplitudes of 550–850 MPa (at ultrasonic vibration frequency, and a stress ratio of R = -1) [13,15]. At stress amplitude of 850 MPa, some tested samples withstood more than 10 7 cycles to failure, including those with oxidized layer [16] which is significantly superior to other titanium alloys [17,18]. At the same time, the Ti-5Al-5V-5Mo-1Cr-1Fe alloy fractured with the dominance of mesoscale failure mechanisms in the range of low durability (less than 10 5 cycles) when tested with a lower frequency of 5 Hz and comparable stress amplitudes of 600–1200 MPa [15]. Such a difference in the fatigue behavior of material at different test frequencies was associated with changes in the fatigue failure mechanisms due to lowering ductility of the Ti-5Al-5V-5Mo-1Cr-1Fe alloy. The same reason was responsible for its brittle-ductile fracture in three-point bending tests at room temperature [19]. Based on the above, the aim of this study was to evaluate the effect of increasing the aging temperature from 450 up to 520 °C on the mechanical and fatigue properties of the Ti-5Al-5V-5Mo-1Cr-1Fe alloy treated by RSR. It was hypothesized that its ductility could be enhanced with a slight decrease in the ultimate tensile strength, improving the fatigue durability in range of both low and high durability through the dominance of the mesoscale failure mechanisms.

M ATERIALS AND METHODS

S

amples from the Ti-5Al-5V-5Mo-1Cr-1Fe alloy (Tab. 1) were treated by the RSR (3 passes in the temperature range of 850–750 °C) and subsequent aging at T =450 °C and T =520 °C for 5 hours (designated as heat treatment 1 and 2, respectively). More detailed information about the treatment procedures is given in [13].

Al

V

Mo

Cr

Fe

Zr

Si

O

C

N

H

Ti

4.74

5.04

5.57

0.81

0.98

0.30

0.15

0.18

0.10

0.05

0.015

Bal.

Table 1: The chemical composition of the Ti-5Al-5V-5Mo-1Cr-1Fe alloy, wt.%.

Tensile tests were carried out using a “MIM 2.50” machine with a loading rate of 5  10 –3 s –1 at room temperature. The rectangular shape tension test specimens were cut out from the treated samples by electric spark machining along the RSR direction. Stress-controlled fatigue tests of the flat coupons (Fig. 1), also cut out from the treated samples, were carried out at room temperature, a stress ratio of 0.1, and cycling frequency of 5 Hz.

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