Microstructure‐Dependent Local Fatigue Cracking Resistance of Bimodal Ti–6Al–4V Alloys

Abstract

The fatigue crack growth behavior of the bimodal Ti-6Al-4V alloys with two different volume fractions of the primary α phase (α p ) of 76 and 36% is investigated by the in situ testing technique. The experimental results show that the crack growth rate of the α p ¼ 36% Ti-6Al-4V alloy is lower than that of the α p ¼ 76% one. The local fatigue crack growth rate is evidently decreased by the various boundaries including α p grain boundaries, boundaries between the α p phase and basketweave microstructure, and α/β lamellar interfaces. A criterion associated with the boundary characteristics is obtained to evaluate the grain boundary resistance to the fatigue crack growth in the engineering alloys.

Quantifying lamellar microstructural effect on the fatigue performance of bimodal Ti-6Al-4V with microdefect

“…Conclusions for the microstructural difference is varied even over the change of loading conditions from pure mode I to mixed-mode loading. Conclusively, extensive literatures are primarily focused on the role of microstructure in influencing the fatigue behaviors of titanium alloys [15][16][17][18], with emphasis laid on the arrangement of α and β phases. Considerable effort has been noticeably devoted to highlighting the lamellar microstructural effect in bimodal titanium alloys.…”

“…Recent experimental [17][18][19][20][21][22][23][24][25][26] and numerical [27][28][29] progress on titanium alloys with LM are comprehensively reported. The experimental approach is able to capture the actual influence of LM on the mechanical properties of materials, thus is considered to be reliable for microstructural analysis [30].…”

Investigation on fatigue crack propagation behaviour of novel TiZr-based alloy

“…At interfaces where cracks are easy to traverse without consuming lots of energy, propagation rates are not affected by the grain boundaries or lamella interfaces. Furthermore, Zeng et al [34] investigated the grain boundary effect considering the twist angle (φ) and tilt angle (Ψ) and it is found that the value of 1.63φ +Ψ makes a big difference on FCP. As the value is below 68°, the main crack could grow continuously across grain or lamella interfaces, otherwise it would be deflected or arrested by interfaces.…”

“…The cracks tend to propagate along boundaries of small grains, resulting in tortuous crack paths with deflection range in the level of grain size and transgranular cracks are hard to observe in Figure 8(d). While crack propagation along crystallographic planes of equiaxed grains is always found elsewhere [18,34], and it is even reported the crack easily diverts within equiaxed primary α phase due to the crack growing along prismatic planes in primary α giving the choice of different planes. This may be related to the equiaxed α grain size which is pretty smaller than that in the literature [34] and the crack propagated along boundaries which consume less energy than other modes.…”

“…While crack propagation along crystallographic planes of equiaxed grains is always found elsewhere [18,34], and it is even reported the crack easily diverts within equiaxed primary α phase due to the crack growing along prismatic planes in primary α giving the choice of different planes. This may be related to the equiaxed α grain size which is pretty smaller than that in the literature [34] and the crack propagated along boundaries which consume less energy than other modes. Although the equiaxed microstructure has a longer fatigue lifetime than lamellar microstructure at high stress level, the FCP rate of the former one is higher as compared with the latter one at the same K, which is consistent with the conclusion by Leyens and Peters [35].…”