The effect of cubic-texture on fatigue cracking in a metastable β titanium alloy subjected to high-cycle fatigue

Highlights

• Fatigue crack initiation site shows the contiguous β facets.

• Microstructurally small crack growth behavior was analyzed using FIB-CS method.

• Most faceting occurred in {1 1 0} 〈1 1 1〉 slip system with the highest Schmid factor.

• The fatigued microstructure shows a higher Kernel average misorientation (KAM) value.

Abstract

In this work, stress-controlled high-cycle fatigue tests were performed on Ti-7Mo-3Nb-3Cr-3Al alloy with β-cubic texture to investigate fatigue crack-initiation mechanisms and microstructurally small fatigue crack growth behaviors. The S-N curve shows a general feature of monotonic increase in the mean lifetime with decreasing stress level. Fatigue crack-initiation sites revealed the contiguous β facets. The fracture surface observations and the slip trace analysis show that most faceting occurred in {1 1 0} 〈1 1 1〉 slip system with the highest Schmid factor. Besides, a higher Kernel average misorientation (KAM) was found in fatigued microstructure. This strain localization would contribute to the fatigue crack-initiation process.

Introduction

Due to its high specific strength, good damage tolerant properties, excellent corrosion resistance and high resistance to fatigue cracking, metastable β titanium alloys have been widely used in aerospace industries, especially for high strength applications [1], [2], [3]. Hot-extruded bar is the main semi-finished product of titanium alloy fasteners. These components are inevitably subjected to high-cycle fatigue (HCF) failure associated with high frequency vibration in service. In addition, the hot extrusion process introduces the texture, which play a vital role in damage accumulation during cyclic loading. It is reported that the macrozones (or micro-textured regions), i.e. the regions containing grains with similar crystallographic orientation, in titanium alloys are detrimental to the fatigue properties since the macrozones can be regarded as an individual grain so the dislocation slip length would increase to facilitate crack nucleation [4]. The previous researches of metastable β titanium alloys are mainly focused on the microstructure evolution as a means to optimizing mechanical properties [5], [6], [7], however, little attention was paid to the effect of the texture on the fatigue properties of metastable β titanium alloy and the underlying fatigue damage mechanisms are still lags behind.

Fatigue cracking process in the alloys without the second phase particles involves the irreversible plastic slip within persistent slip bands (PSBs), which often result in either the formation of intrusions/extrusions at specimen surface or their impingement on dominant microstructure obstacles at specimen interior, such as high angle grain boundaries (GBs) or phase boundaries (PBs) [8]. These PSBs/GBs and PSBs/PBs interaction mechanisms lead to lattice/interface disruption and contribute to the fatigue microcracks formation. Zhang and his co-workers revealed that fatigue cracks nucleate along PSBs, deformation bands, high angle GBs and twin boundaries [9], [10], [11]. In the HCF regime, crack formation and subsequent small crack growth stages is life-controlling [12]. The crack-initiation process corresponds to the development of cracks that are approximately 3–10 times of grains and phases, known as microstructurally small fatigue crack (MSFC) [8], [13]. Microstructure substantially influences MSFC growth behavior and facilitate a significant scatter in the overall fatigue lifetime, particularly in the HCF regime. Schmid factor can be used as a metric (but not the sole parameter) to determine whether the slip system is activated in a given grain under the lower cyclic loading [14], [15]. Recent papers revealed that the MSFC growth was along the activated slip system with the highest Schmid factor by analyzing the slip traces and the corresponding Schmid factor values [16], [17]. It is worth pointing out that the small 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 with lowest tilt/twist angle [18], [19], [20].

Fatigue facets are typical features of crack-initiation region. Numerous studies on near α and α + β titanium alloys have shown that crack-initiation was accompanied by crystallographic facet formation across primary α (α_p) particles [14], [21], [22], [23]. Additionally, several Refs. [21], [23] have shown that faceted α_p is favored in certain macrozones that are favorably oriented for either basal < a > or prismatic < a > slip. Moreover, the crystallographic plane of the α_p facet is found to be the basal or near basal plane [21], [22], [24]. The mechanisms of formation of α_p facets involve quasi-cleavage [25], [26], a combination of basal slip and normal stress across the facet plane [21] and pure slip [22]. Nevertheless, the mechanisms for the formation of β facet in the metastable β titanium alloys are relatively scarce. Reportedly, 〈0 0 1〉 directions of bcc metals are most compliant, while 〈1 1 1〉 directions are stiffest, and a rigidity difference by at least 20% between stiff and compliant regions [27].

Subsurface fatigue crack-initiation has been often reported in titanium alloys without any internal defects. The studies on the subsurface fatigue crack-initiation have been mostly conducted on α + β titanium alloys, nevertheless, none of the proposed crack initiation mechanisms give a satisfied explanation, especially in single phase, β titanium alloys. J.A. Ruppen et al. [28] investigated the subsurface crack-initiation in titanium alloys, found that the initiation facets were inclined at 45° and 55° to the loading axis and were probably caused by intense shear. Researchers have proposed various crack-initiation models to explain the fatigue crack-initiation process of alloys [29], [30]. Different to conventional Griffith crack initiation model, Zener crack initiation model, which was first proposed by Zener [31], is formed dislocations piled up along a slip plane and stopped by obstacle where a microcrack is initiated. Later, Stroh provided a modified model and gave the amount of dislocations needed for such nucleation in the absence or presence of a slip plane [32]. Therefore, the cracks arising from dislocation pileup is often referred to as Zener-Stroh crack in literature. Besides Zener-Stroh mechanism of crack initiation, there are some other variants. One was Cottrell crack initiation model [33], where pileup dislocations on two intersecting slip planes can coalesce into a microcrack. Another variant was proposed by Kikuchi [34], in which dislocations of one sign move away from the region, leaving stationary dislocations of the opposite sign behind to form a crack near the particle. Ruppen and McEvily found a Cottrell type of subsurface crack initiation in α + β titanium alloys [35].

In the present research work, the effect of the β-cubic texture on fatigue cracking at HCF regime in Ti-7Mo-3Nb-3Cr-3Al (hereafter abbreviated as Ti-7333) are investigated. The MSFC growth micromechanism is investigated by focus-ion-beam cross-section (FIB-CS) characterization across the contiguous β facets, aiming at understanding of the slip behavior of the faceted grains and non-faceted grains.