EBSD analysis of fatigue crack initiation behavior in coarse-grained AZ31 magnesium alloy

doi:10.1016/j.ijfatigue.2015.11.010

International Journal of Fatigue

Volume 84, March 2016, Pages 1-8

https://doi.org/10.1016/j.ijfatigue.2015.11.010 Get rights and content

Highlights

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Fatigue crack initiation mechanism in coarse-grained Mg alloy.
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Crystallographic analyses of fatigue crack initiation using EBSD.
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Schmid factors calculation of activated slips and twins.
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Slip and twin lines analyses based on Euler angles.
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Proposal of criteria for both inter- and intergranular fatigue crack initiation.

Abstract

Plane bending fatigue tests had been conducted to investigate fatigue crack initiation mechanism in coarse-grained magnesium alloy, AZ31, with hexagonal close-packed (hcp) crystallographic structure. The initial crystallographic structure was analyzed by an electron backscatter diffraction (EBSD) method. Subsequently, a fatigue test was periodically terminated and time-series EBSD analyses were performed. Basal slip and primary twin operated predominantly. In a twin band, secondary twin operated, and resulted in the fatigue crack initiation. The crack initiation was strongly affected by Schmid factors in the grains and twin bands.

Introduction

Magnesium (Mg) alloys are attractive as structural materials to achieve weight saving and high fuel efficiency because of their light weight and high specific strength. Mg alloy has hcp structure and only basal slip and twining can operate during plastic deformation at room temperature [1], providing the grain size is not too small. And it is known that tensile and compressive plastic deformation behavior is asymmetric [2]. Hence, the plastic deformation behavior had been investigated in detail using transmission electron microscope (TEM) [1], [3], [4], [5], back-scattered electron diffraction (EBSD) technique [6], [7], [8] and so on, from the viewpoints of basal and non-basal slips and twinning. But those studies were basically about plastic deformation under quasi-static loading conditions. To apply Mg alloys for mechanical components, it is very important to understand the fatigue behavior. Therefore some rotating bending [9], [10] and axial loading [11], [12] fatigue tests had been performed to estimate fatigue strengths and understand macroscopic fatigue behavior. Furthermore, King et al. conducted non-destructive and three-dimensional observation of growing small fatigue crack by the combination of X-ray diffraction contrast tomography (DCT) and microtomography [13]. They related stage I crack growth to polycrystalline microstructure. Recently, some studies on fatigue behavior from the viewpoint of crystallographic orientation had been performed based on EBSD analyses [14], [15], [16]. However, those studies were about fatigue damage accumulation during low cycle fatigue (LCF) with relatively large plastic deformation under stress controlled [14] or strain controlled [15], [16] modes. It is known that high cycle fatigue life is mainly dominated by crack initiation life, and fatigue crack initiation behavior is strongly affected by crystallographic orientations. Consequently, understanding of fatigue crack initiation mechanism in Mg alloys with regard to crystallographic orientations is important. Xu and Han had conducted high cycle fatigue test using pure Mg, and investigated fatigue crack initiation mechanism [17]. They concluded that the interaction between {1 0 −1 2} twinning and basal slip operation led to the twin boundary fatigue cracking. But the crystallographic criteria for crack initiation are still unclear.

In the present study, fatigue test was performed using Mg alloy, AZ31, whose crystallographic orientations were analyzed in advance of the test, and subsequently fatigue crack initiation behavior was investigated based on time-series EBSD analyses.

Section snippets

Specimen and fatigue testing procedure

The material used is AZ31 Mg alloy roll plate, whose chemical composition is shown in Table 1. The as-received material has the average grain size of 15 μm. To investigate transgranular crack initiation behavior, grain coarsening heat treatment was applied to the as-received material. It is known that the annealing treatment to the severely-deformed material leads to the significant grain coarsening. Hence, friction stir processing (FSP) was applied to the as-received material to give severe

Macroscopic EBSD analysis

The basic S–N diagram of the as-received and coarse-grained materials is shown in Fig. 3. Compared with the as-received material, the coarse-grained one exhibits lower fatigue strength. The IPF map at the bottom of shallow notch before fatigue test (N/N_f = 0%) is shown in Fig. 4(a), which covers nearly while width of the gauge area. The maximum grain size in this area is about 1055 μm, which is defined from √area of the largest grain, and the grain orientation is nearly random. It is considered

Crack initiation mechanism

The grain with fatigue crack “I” is defined as the grain “A” as shown in Fig. 4(c). Table 2 summarizes SFs and the angles α, β, γ of slip and twin systems. The angle of twin band is about 80° (Fig. 6(b)), which corresponds to the angle of primary twin (1 0 −1 2) plane of 84°. It indicates that this twin band was formed due to the operation of (1 0 −1 2) twin. The angle of the actual twin band was slightly different from the theoretical one. That is because the actual angle is measured from the

Conclusions

Plane bending fatigue tests were conducted using coarse-grained AZ31 magnesium alloy under the stress ratio, R = −1. A fatigue test was periodically terminated and time-series EBSD analyses were performed to investigate the effect of grain orientation on fatigue crack initiation behavior. The conclusions are as follows.

(1)
Coarse-grained AZ31 exhibited lower fatigue strength than the as-received material. When a fatigue test was conducted at the stress amplitude, σ_a = 130 MPa, two fatigue cracks were

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^☆: This paper was submitted for the special issue Fatigue at all Scales (ECF20 Fatigue).

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EBSD analysis of fatigue crack initiation behavior in coarse-grained AZ31 magnesium alloy☆

Highlights

Abstract

Introduction

Section snippets

Specimen and fatigue testing procedure

Macroscopic EBSD analysis

Crack initiation mechanism

Conclusions

Acta Mater

Scripta Mater

Acta Mater

Acta Mater

Acta Mater

Mater Sci Eng A

Mater Sci Eng A

Mater Sci Eng A

Int J Fatigue

Int J Fatigue