PSI - Issue 82

Juraj Belan et al. / Procedia Structural Integrity 82 (2026) 119–124

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Juraj Belan et al. / Structural Integrity Procedia 00 (2026) 000–000

fact that pure titanium crystallizes in two crystallographic modifications, low-temperature (up to 882 °C) a - phase crystallizing in hcp (a = 0.295 nm; c = 0.468 nm; c/a ratio = 1.587) and high-temperature (882 °C – 1667 °C) b phase crystallizing in bcc (a = 0.332 nm at 900 °C), Lütjering and Williams (2007). Titanium alloys can crystallize in both modifications depending on the content and combination of alloying elements stabilizing the individual a and b phases, Berthaud et al. (2020). Nomenclature HCP Hexagonal Close Packed BCC Body-Centred Cubic SEM Scanning Electron Microscopy OM Optical microscopy R Fatigue cycle asymmetry parameter HRC Rockwell hardness test HV Vickers hardness test UTS Ultimate tensile stress YS Yield stress f Fatigue load frequency N f Number of cycles to failure s o Maximum bending stress s m Fatigue mean stress s a Fatigue stress amplitude Oxygen is an important interstitial element in titanium alloys, significantly influencing the microstructure and mechanical properties of titanium alloys. Based on the Ti-O binary diagram, the maximum solubility of oxygen in the a -hcp phase is up to 13.5 wt.% (for Ti 2 O 10 ÷ 14.4 wt.% at a temperature of approx. 600 °C and Ti 3 O 8 ÷ 13 wt. % at a temperature of approx. 600 °C) and in the b - bcc phase up to 3 wt. % at a peritectic temperature of 1720 °C, ASM Handbook (1998). It follows from the above that the oxygen content has the most significant effect on the a Ti phase. Baillieux et. al (2015) state in their work that oxidation at temperatures of 600 °C ÷ 700 °C significantly deforms the hcp lattice of the a Ti phase and changes the c/a ratio. They state that the Ti50A alloy (ASTM grade 2) has lattice parameters of a = 0.295 nm and c = 0.4683 nm, and a c/a ratio of 1.587 at ambient temperature, and that oxidation in a furnace with a normal atmosphere at a temperature of 700 °C/52 h changed these to a = 0.297 nm, c = 0.479 nm, and a c/a ratio of 1.612. At the same time, it states that the increased oxygen content in the surface layer also affects microhardness. During heat treatment of Ti6Al4V alloy in an oxygen-rich atmosphere, in addition to a brittle oxide layer, a so called a -case layer, Boonchuduang et al. (2020), is also formed on the surface. Several authors have studied the influence of the a -case layer on mechanical properties, with an emphasis on the dynamic properties of Ti6Al4V. The role of the a -case layer in the formation and propagation of fatigue cracks was investigated by Yan et al. (2016) and Gaddam et al. (2015). These studies consistently show that the presence of the a -case layer significantly reduces the fatigue life of the alloy, with cracks usually starting to form from microcracks in the a -case layer. Konda et al. (2023) estimate the fatigue life of additively manufactured Ti6Al4V material under high fatigue cycles based on data analysis and several simulation models. In addition, Afroz et al. (2019) has studied the influence of high temperature and cooling rate on a + b morphology of Ti6Al4V titanium alloy. Changes in the basic microstructure from bimodal and lamellar to equiaxed during fatigue testing led to a shift in the fracture mode, which shifts from ductile to completely brittle failure. The paper deals with the assessment of the influence of oxidation annealing at temperatures above b -transus (1050 °C) with different values of isothermal holding time on the formation of the a -case layer and its influence on the dynamic properties of the Ti6Al4V titanium alloy, initiation and propagation of fatigue cracks in three-point bending fatigue tests with a cycle asymmetry parameter R < 1 and a loading frequency f » 60 ÷ 70 Hz.