Slip band formation in plastic deformation zone at crack tip in fatigue stage II of 2xxx aluminum alloys

Highlights

• The plastic deformation zone at crack tip in fatigue stage II is completed.

• Goss, P and Q grains are hard to plastically deform and form slip bands.

• Cube, S, R, Copper and Brass grains facilitate slip band formation.

• Large tilt angle boundary leads to high FCP rates, suppressing slip band formation.

• T₁ phases retard dislocation motion, suppressing slip band formation.

Abstract

Analysis of the plastic deformation zone at crack tip in fatigue stage II of AA2524-T4 and AA2297-T87 alloys reveals the complex microstructural effects, such as second-phase particles, grain boundaries, Schmid factor and grain orientations, on the formation of slip bands. In AA2524-T4 alloy, slip band formation in plastic deformation zone relies mainly on grain orientation and boundary component. Goss, P and Q grains with low Schmid factor are hard to deform plastically, leading to few slip bands. Cube, S, R, Copper and Brass grains with high Schmid factor tend to form evident slip bands. However, if these grains have pure or high tilt angle component boundaries with adjacent grains, no clear slip bands can be observed because the high FCP rate across these grains fails to allow enough reversible dislocation slipping. Besides, in AA2297-T87 alloy, slip band formation depends heavily on the second-phase particles. From theoretical calculation and quantitative measurement prospective, the large size and density of unshearable T₁ phases can retard dislocation shearing and motion, thereby suppressing the formation of slip bands.

Introduction

Plastic deformation of crystals often occurs by slipping on only a few crystallographically equivalent planes [1]. The relative types of localized banded structures in the deformation zones are called slip bands. They can be described as a pair of closely spaced clusters of slip lines, so these structures are also been called double-walled structures, dipole walls or dipolar structures [2]. They appear in preferentially-oriented single crystals and grains in polycrystals of copper, α-brass, iron, aluminum etc. [3], [4], [5], [6], [7], [8], [9]. In the last century, Heidenreich [10] firstly observed the fine structure of slip bands on the surfaces of plastically deformed crystals. Until now, plenty of researchers [4], [5], [6], [7], [8], [9] have made a substantial contribution to this field, especially the relationship between slip bands and plastic deformation. The present work focuses on slip band formation and its effect on fatigue crack propagation and related properties. In fatigue stage I, it is well known that persistent slip bands (PSBs) can enhance the reversibility of cyclic slipping, thereby reducing damage accumulation during fatigue crack growth. Meanwhile, PSBs may promote crack deflection along a favored slip system which leads to more winding crack path and induces the occurrence of roughness-induced crack closure (RICC) [11]. With crack length increasing, additional slip systems can be activated at the crack tip and the crack propagates along alternating slip systems in fatigue stage II [12]. Apparently, slip bands play a vital role in affecting fatigue crack deflection and properties. It is therefore important to reveal the mechanism of slip band formation during fatigue crack growth. Several researches have investigated single factors, such as second-phase particles, grain boundaries, Schmid factors and grain orientations etc., in governing slip band formation.

Previous findings revealed that the shearable G.P. zones and σ′ (Al₃Li) coherent phase could increase slip reversibility and enhance slip band formation in aluminum alloys [13], [14]. But in Al–Cu–Zr alloy [15], [16], the presence of fine semi-coherent second-phases retarded dislocation rearrangement during annealing and elevated temperature deformation. This impeded slip band formation. In 1970 s, Davis [17] observed that slip bands could get continuously across grain boundaries in aluminum alloys. Later, several researchers [15], [18] further revealed that high-angle boundaries were capable of retarding the occurrence of dislocation motion and dislocation pile-ups and etc., which could impede the formation of slip bands. Besides, Schmid’s laws [19] and other investigations [20], [21] have confirmed that slip bands are liable to form on those grains with high Schmid factor in plenty of FCC metals. As for grain orientations in governing slip band formation, some studies have been done in the past decades. Kramer [21] investigated the evolution of individual slip bands in aluminum single crystals for four different crystallographic tensile axes. Results showed that one slip system tended to be activated near [0 0 1] and [3 1 6] tensile axes, resulting in the formation of some slip bands. In contrast, the near [1 1 3] and [1 1 1] tensile axes, in which multiple slip systems were easy to be opened, produced more evident slip bands. Later, researchers [22], [23], [24] explored the fracture behavior by cyclic deformation in copper bicrystals. Results illustrated that only the primary slip system was activated on [−5 9 13]⊥[−5 7 9] copper bicrystal [22]. The double slip and multiple slip systems were opened on those [−3 4 5]⊥[−1 1 7], [0 0 1]⊥[−1 4 9] and [−1 4 9]⊥[−1 4 9] copper bicrystals [23], [24]. In polycrystalline materials, plenty of oriented grains lead to more complicated plastic deformation. Researches show that ‘soft’ oriented grains with high Schmid factor tend to deform plastically and form slip bands [20], [21], but most of these results are obtained from the theoretical calculation rather than the observation of experiment. Li and Liu [25] recently revealed the fact of texture components in affecting fatigue crack propagation of AA 2524-T4 alloy. Results showed that ‘soft’ Brass and Cube grains tended to be propagated by cracks, while ‘hard’ Goss grain was extremely difficult to be propagated. This suggests that grains with different orientations may possess different deformability and form various numbers of slip bands during crack propagation. Based on our previous result [25], this paper further investigates grain orientation effect on slip band formation during fatigue crack growth.

In addition, all the above researches have made significant contribution to slip band formation, but most of them seldom reveal slip band formation in fatigue stage II (Paris regime). When crack propagates into this stage, plastic deformation at crack tip is quite complicated. Firstly, the size of plastic zone at the crack tip is greatly broadened to tens or hundreds of grains size [26], [27]. Secondly, the plastic deformation mode is converted from a single slip into multiple slip [28], [29]. Besides, the crack plane, which is originally along with the maximum shear stress in stage I, also turns to the plane normal to the maximum principle stress direction in stage II. There is no doubt that all these differences can lead to the complex effects like second-phase particles, grain boundaries and orientations, Schmid factors, FCP rates and path etc. on slip band formation in fatigue stage II. Fatigue crack tends to nucleate and propagate along with slip bands [11], [12], so the investigation on the major factors and the relative relationship among these factors in governing slip band formation contributes to a better understanding the mechanism of fatigue crack propagation. Till present, this complicated problem was not systematically investigated.

Accordingly, this paper focuses mainly on investigating the mechanism of complicated factors, including second-phase particles, grain boundaries and orientations, Schmid factors as well as FCP rates and path, in governing slip band formation via transmission electron microscopy (TEM), scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) techniques in plastic deformation zone at crack tip in fatigue stage II of commercial AA2524-T4 and AA2297-T87 alloys