Microstructural influences on the high cycle fatigue life dispersion and damage mechanism in a metastable β titanium alloy

Abstract

In this work, the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al (Ti-7333) alloy is investigated. Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1. Results show that the fatigue strength is closely related to the microstructure features, especially the α_p percentage. The Ti-7333 alloy with a lower α_p percentage exhibits a higher scatter in fatigue data. The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lower α_p percentage. Crack initiation region shows the compound α_p/β facets. Faceted α_p particles show crystallographic orientation and morphology dependence characteristics. Crack-initiation was accompanied by faceting process across elongated α_p particles or multiple adjacent α_p particles. These particles generally oriented for basal slip result in near basal facets. Fatigue crack can also initiate at elongated α_p particle well oriented for prismatic <a> slip. The β facet is in close correspondence to {110} or {112} plane with high Schmid factor. Based on the fracture observation and FIB-CS analysis, three classes of fatigue-critical microstructural configurations are deduced. A phenomenological model for the formation of α_p facet in the bimodal microstructure is proposed. This work provides an insight into the fatigue damage process of the α precipitate strengthened metastable β titanium alloys.

Introduction

Metastable β titanium alloys are increasingly used in aerospace industries owing to their high strength to weight ratio, good hardenability and excellent mechanical properties than that of many conventional α + β titanium alloys, and is suitable for making high strength forgings and linkage units, such as helicopter rotors and fastening pieces [[1], [2], [3], [4]]. High-cycle fatigue (HCF) failure associated with high frequency vibration often occurs in these aerospace components. The fine acicular secondary α (α_s) precipitates play a vital role in strengthening of the metastable β titanium alloy microstructures, meanwhile, introducing a certain content of primary α (α_p) particles into the β matrix contributes significantly to the balance of strength and ductility [5,6]. Much of the literature on metastable β titanium alloys are mainly concentrated on phase transformation and microstructure evolution as the means to optimizing mechanical behavior [7,8]. However, the microstructure effects on fatigue behaviors as well as the underlying damage micromechanisms is still lags behind [9,10].

Microstructure attributes, such as grain size, morphology, and crystallographic orientation [[11], [12], [13]], and dominant microstructure obstacles, i.e. slip bands, grain boundaries (GBs) and phase boundaries (PBs) [[14], [15], [16], [17]], have a significantly influence on fatigue crack-initiation of alloys. Fatigue crack-initiation process involves the localization of irreversible plastic strain within slip bands that normally forms intrusions and extrusions at specimen surface or impinges on GBs for grains at specimen interior [18]. These interactions between persistent slip bands (PSBs) and GBs or PBs lead to a significant degree of lattice/interface disruption to initiate fatigue cracks. The early stages of crack formation and growth with length of no greater than just a few grains under HCF regime, known as microstructurally small cracks (MSC) growth stage, is a particularly important point since majority of the total fatigue life is typically spent in this regime [18,19]. The MSC growth behavior shows microstructure-sensitive and contributes significantly to variability of fatigue data and minimum fatigue lives. Recently, the MSC growth behavior were characterized with the slip trace analysis [20,21]. It is reported that fatigue cracks do not necessarily propagate into a neighboring grain along slip planes with the highest apparent Schmid factor, but rather along the most compatible slip plane (lowest tilt/twist angle) [22,23]. These results reinforce the importance of considering effect of crystallographic orientation in MSC formation. Nevertheless, physically-based models for crack nucleation and MSC growth are still in their infancy.

HCF is often an unpredictable failure and usually a significant scatter observed in fatigue lifetime data. Recent research highlighted the importance of correlating scatter in fatigue life with crack-initiation mechanisms in Ti-6Al-2Sn-4Zr-2Mo-0.1Si alloy [24], Ti-6Al-2Sn-4Zr-6Mo alloy [25] and Ti-6Al-4 V alloy [26]. Many publications on near α and α + β titanium alloys have shown that the fatigue crack-initiation with the presence of the α_p facets [[27], [28], [29], [30]]. The crystallographic plane of the α_p facet is found to be the basal or near basal plane [27,28,31]. Besides, facets along the prismatic planes [27] or easy cleavage plane of {10-17} have also been reported [32]. The mechanisms of faceting process including cleavage [32], a combination of shear in (0001) slip bands and tensile stress on the facet plane, i.e. localized slip band cracking [27], and pure slip [28]. The fatigue facets were found on the activated slip system with high Schmid factor in titanium alloys [29] and nickel-based superalloys [33]. It is reported that the plastic slip initiates at α_p particles and the basal slip is activated prior to prismatic slip [34]. Additionally, the slip behavior of the nearest neighbor grains to the faceted grain have a significant influence on facet formation, particularly for low symmetry hcp α grains. Due to a high percentage of β+α_s matrix, the fatigue damage behavior of the metastable β titanium alloys are different from that of the α and α + β titanium alloys. Previous studies have revealed that crack-initiation occurs at α_p particles in metastable β titanium alloys [[35], [36], [37]]. Reportedly, fatigue cracks always initiate either at GB α layers or inner α particles, including α_p particles and coarse α_s platelets [38]. The possible fatigue crack-initiation mechanisms in metastable β titanium alloys including GB cracking, α_p cleavage, α/β interface debonding and α_s shearing [35].

In this work, the α_p characteristics on the fatigue variability of Ti-7Mo-3Nb-3Cr-3Al (hereafter abbreviated as Ti-7333) alloy are investigated. The fracture surfaces are examined, with a focus on crack-initiation site, to determine the crack initiation modes. The early stages of fatigue crack formation and growth mechanisms are investigated by focused-ion-beam cross-section (FIB-CS) characterizations on a 2D section through the faceted grains in order to reveal the microstructural characteristics and crystallographic information by subsequent electron backscatter diffraction (EBSD) measurements. The main focus of the FIB-CS characterization is to differentiate the morphology and crystallographic orientation of faceted grains that produce the difference in crack initiation modes.