4.7 Article

The effects of grain morphology and crystallographic orientation on fatigue crack initiation in a metastable β titanium alloy Ti-7333

Publisher

ELSEVIER SCIENCE SA
DOI: 10.1016/j.msea.2020.140222

Keywords

High cycle fatigue; Metastable beta titanium alloy; Fatigue crack initiation; Short crack propagation; Faceting

Funding

  1. Major State Research Development Program of China [2016YFB0701303]

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Introducing fine alpha particles into the soft beta matrix contributes significantly to the strengthening of the metastable beta titanium alloy microstructures. However, this microstructure design often leads to microcracks initiated at the alpha/beta interface. In this work, fatigue crack-initiation (FCI) modes and FCI mechanisms at high-cycle fatigue regime in a metastable beta titanium alloy, Ti-7Mo-3Nb-3Cr-3Al, are investigated by fracture surface observation and focused-ion-beam cross-section characterizations on a 2-D section through the faceted grains. Based on fracture observation, four categories of FCI modes were summarized from the given microstructures. Fatigue damage mechanism is non-unique and strongly dependent on microstructural constituent combinations although only subtle differences among them. FCI site revealed the compound primary-alpha (alpha(p))/beta facets or isolated alpha(p) facets. Elongated alpha(p) particle or multiple equiaxed alpha(p) particles favorably oriented for basal a slip are preferred crack -initiation sites, resulting in the formation of near basal facets. The beta facet was in close correspondence to the {110} slip plane with high Schmid factor. The occurrence of the elongated alpha(p) facet is usually accompanied by the rugged beta facet. Further, three classes of fatigue-critical microstructural configurations are deduced. This work provides an insight into the fatigue damage process of the alpha precipitate strengthened metastable beta titanium alloys.

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