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The effect of heat treatment of the Ti – 45 wt.% Nb alloy in the ultrafine- and coarse-grained states on its structural parameters (lattice parameters, volumetric phase ratio, sizes of coherent scattering regions, residual normal stresses) and their relationship with the heat capacity are studied. It is found out that the differing character of the temperature dependence of heat capacity for the alloy in these states is related to the structural-phase features of the alloy in the UFG state, specifically, to the two-phase structure consisting of β- and α-grains, dispersion hardened by the ω-phase, and the phase transitions in the temperature range of 400−600°C. [ABSTRACT FROM AUTHOR]

The paper presents the results of fatigue-testing ultrafine-grained and coarse-grained Ti-45 wt.% Nb alloy samples under very high cycle fatigue (gigacycle regime), with the stress ratio R = −1. The ultrafine-grained (UFG) structure in the investigated alloy was formed by the two-stage SPD method, which included multidirectional forging (abc–forging) and multipass rolling in grooved rollers, with further recrystallization annealing. The UFG structure of the Ti-45 wt.% Nb alloy samples increased the fatigue limit under the high-cycle fatigue conditions up to 1.5 times compared with that of the coarse-grained (CG) samples. The infrared thermography method was applied to investigate the evolution of temperature fields in the samples under cyclic loading. Based on numerical morphology analysis, the scale invariance (the Hurst exponent) and qualitative differences for UFG and CG structures were determined. The latter resulted from the initiation and propagation of fatigue cracks in both ultra-fine grained and coarse-grained alloy samples under very high-cycle fatigue loading. [ABSTRACT FROM AUTHOR]