PSI - Issue 47

Yurii Sharkeev et al. / Procedia Structural Integrity 47 (2023) 849–854 Yurii Sharkeev et al. / Structural Integrity Procedia 00 (2023) 000 – 000

850

2

1. Introduction One of the prevailing applicable alloys used in medicine are commercially pure titanium (VT1-0, V Т 1-00, Grade 1, 2, 3, 4), titanium alloys (Ti – 6Al – 4V, Ti – 6Al – 4VELi, Ti – 6Al – 7Nb и Ti – 6Al – 2.5Fe), zirconium alloys (Zr – 1.1Nb, Zr – 2.5Nb), and resorbed magnesium alloys (Mg – 0.8Ca, Mg – Y – Nd) (Niinomi et al. 2016, Ozaki et al. 2004, Geetha et al. 2009, Walker et al. 2014). Accordingly, a perspective trend in medical materials technology is the development of biocompatible titanium alloys with a relatively low elastic modulus, for example the Ti-Nb alloy system (Niinomi et al. 2016, Ozaki et al. 2004, Geetha et al. 2009, Walker et al. 2014). The Ti – (40 – 45) wt.% Nb alloy has an elastic modulus in the range of 50-60 GPa (Ozaki et al. 2004). The alloys of this composition are used in medicine as a material for medical implants. The yield strength of the Ti – (40-45) wt.% Nb alloy in the normal coarse-grained state is 400 MPa. This value is insufficient for implants. Moreover, implants under cyclic loading must have a service life of at least 10 9 cycles according to the medical requirements. The formation of nanostructured and/or ultrafine-grained states in metals and alloys by various methods of severe plastic deformation makes it possible to obtain bulk materials with significantly higher mechanical properties, such as yield and strength limits, hardness and microhardness, endurance limit and cyclic durability, etc. (Reck et al. 2018, Bonisch 2014, Sharkeev et al. 2022). Bioinert titanium alloys in the ultrafine-grained state demonstrate higher fatigue properties under gigacycle loading (more than 10 6 loading cycles). Moreover, the ultrafine-grained structure in bioinert titanium alloys is stable under gigacycle loading and provides a service life of up to 10 9 and more cycles with higher fatigue strength. In our work, we tried to focus on two main problems. 1. How could we improve the mechanical properties of the bioinert Ti – (40 – 45) wt.% Nb alloy for medical applications. 2. In what way could we test the Ti – Nb samples in a gigacycle fatigue test within a short period of time, one to three days. We proposed the following solutions. 1. The mechanical properties of the alloy can be improved by extensive milling of the alloy grain structure down to an ultrafine-grained state using the severe plastic deformation techniques. 2. The resonant testing machine with a loading frequency of 20 kHz can be used for the fatigue testing of biomaterials achieving the fatigue life of 10 9 and more cycles in a short period of time, up to 1-3 days. 2. Experimental procedure The study object was the Ti – 45 wt.% Nb alloy. The alloy was produced by electric arc-melting at the Chepetsky Mechanical Plant JSC (Glazov, Russia). The ultrafine-grained structure in the investigated samples was obtained by a two-step combined severe plastic deformation method, which included multi-step abc-pressing in a press-mold and multi-pass rolling in grooved rollers, followed by recrystallization annealing (Sharkeev et al. 2022). Abc pressing included one or three pressing steps at a constant temperature. During the transition from one pressing to the next, the temperature of the samples was gradually decreased by 50 °С in the range of 500 - 400 °С. After each pressing, the sample billet was rotated around its longitudinal axis by 90  С, perpendicular to the previous pressing axis . The use of a press-mold generates space-limited deformation conditions and includes effective grain refinement at minimum increments. The alloy billets with such ultrafine-grained structure have low plasticity under loading, not exceeding 2-3%. Annealing at a temperature of 350 °С increases the plasticity up to 5.5% while maintaining the ultrafine-grained state. The microstructure of the samples was analyzed using transmission electron microscopy (JEM 2100 microscope, JEOL, Japan). The sizes of structural elements were measured in the TEM dark-field images. The average size of structural elements (grains, subgrains, fragments) was estimated using the linear secant method (ASTM E1382-97). An ultrasonic fatigue testing machine (Shimadzu USF-2000, Kyoto, Japan) was used to identify gigacycle fatigue at an amplitude of 100-300 MPa and cyclic oscillation frequency of 20 k Н z with a cycle asymmetry coefficient of R= − 1. Air cooling was applied (Naimark and Palin-Luc 2016, Bannikov et al. 2016). 3. Results and discussion The implementation of severe plastic deformation using a complex two-stage method, including multi-stage pressing and multi-pass rolling in grooved rolls, followed by pre-recrystallization annealing, made it possible to