PSI - Issue 23

Yoshikazu Nakai et al. / Procedia Structural Integrity 23 (2019) 83–88 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

84

2

1. Introduction

To reduce global warming problem, the improvement of energy-saving systems is required, particularly for transportation, and the utilization of high-specific-strength materials is one of the most promising approaches for accomplishing such improvement. Since magnesium alloys have the highest specific strength among the engineering metallic materials, they have received considerable attention. However, it is necessary to guarantee their long-term strength by clarifying their fatigue fracture mechanisms before these alloys can be employed for structural use. Because the slip system is limited for magnesium alloys with a hexagonal close-packed (hcp) crystal structure, twinning is considered to be an important mechanism of plastic deformation, especially under the compression stress. Under cyclic loading, detwinning occurs tensile loading after twinning under compression stress as reported by Wu et al. (2008). However, the role of twinning and detwining for fatigue crack initiation mechanism has not been well understood. In the high-cycle fatigue of metallic materials, plastic deformation usually occurs in part of the grains, and slip bands cannot be observed in all the grains as reported by Tanaka et al. (1982) and Nakai et al. (1997, 2002). Thus, the twinning may not be essential for crack initiation. Regardless of the involvement of twinning, Nakai et al. (2019) reported that fatigue cracks initiate from grains where the Schmid factor of the basal plane is sufficiently high for slipping to occur. In the present study, twinning and detwinning behavior under cyclic lading was observed by electron back scatter analysis (EBSD) and diffraction contrast tomography (DCT) using ultra-bright synchrotron radiation X-ray proposed by Ludwig et al. (2008) to elucidate the fatigue crack initiation mechanism. Total misorientation of each crystallographic plane was measured by DCT using a technique developed by Shiozawa et al. (2016) and Nakai et al. (2017).

Nomenclature β

total misorientation

ω

rotation angle of a sample from a reference position Δω diff spread of rotation angle for a grain that satisfies the Bragg’s diffraction condition ϕ angle between the normal of the diffraction plane and the rotation axes θ diffraction angle ψ angle between the diffraction plane and the rotation axis

2. Diffraction contrast tomography

Principle

The apparatus used for the DCT is identical to the conventional microtomographic imaging setup. During a tomographic scan, the grains embedded in the bulk of a polycrystalline sample produce a bright diffraction spot for a grain that satisfies Bragg’s con dition of diffraction as shown in Fig. 1. At the same time, a variant extinction spot can be observed in the transmitted beam, which is set behind the sample. Since each grain produces diffraction and extinction spot, they must be classified into groups belonging to individual grains. For grains with no misorientation, the shapes of diffraction/extinction spots are corresponding to the projection of grain from incident X-ray direction, then, the three-dimensional grain shapes can be reconstructed by algebraic reconstruction techniques based on parallel beam geometry (Gordon et al., 1970). Once the diffraction/extinction spot pairs have been assigned to a grain, its crystallographic orientation can be identified.