In Situ Uniaxial Compression of Textured Magnesium AZ31B

Abstract

Strain-controlled uniaxial compression tests on textured magnesium AZ31B sheet samples were carried out using a 5 kN Kammrath & Weiss tension–compression in situ stage using a scanning electron microscope in combination with real-time electron backscatter diffraction lattice orientation mapping. The distribution of deformation twins in the samples was studied and correlated with the results of finite element simulation of the elastic strain to show that bands of twinned grains formed in areas where the principal compressive stress (σ3) was a maximum, and they formed normal to the trajectory of the principal direction of σ3. This was correlated with maps of lattice disorientation within the grains, which showed the inclination for twins to grow in alignment with local and larger-scale distributions of elastic strain. Mappings of the same area at different values of strain were made to examine the formation and growth of individual twins within the macroscopic bands of twinned grains. All the twins observed were consistent with the extension-type twin, with 86.3° disorientation with respect to the parent grain. Mappings of the grain internal disorientation were related to the elastic strain, and it was found that twin formation and growth followed the contours of the highest elastic strain within and across grains. The maximum angular disorientation found within the grains was approximately 10°, suggesting that this might correspond to a threshold of elastic strain required to initiate twinning.

In Situ Pure Shear Tests on Textured Magnesium AZ31B Sheets

“…Figure 3b shows the polished sample fixed using hardened steel gripping plates in the in situ stage. Figure 4c to the minor principal stress σ 3 * as in the uniaxial experiments in [15], although a pure shear stress state prevails. Figure 3d shows the distribution of the minor principal stress σ 3…”

“…These bands formed normal to the maximal principal compressive stress σ * 3 and individual twins inside the bands were aligned approximately parallel to those bands. Recent in situ studies of magnesium subjected to uniaxial compressive stress confirmed this and investigated in more detail the initiation and growth of individual twins [15] under this stress state. The current study applies this approach of in situ testing with pure shear stress, an area that has not yet been explored in the literature.…”

“…The experiment used the in situ stage to apply a tensile force (F 1 ) to the sample in the x 1 direction. Dimensions of the sample were chosen so that it would fit into the in situ stage and have the same length (60 mm) and thickness (3 mm) as the earlier uniaxial compression samples [15]. The geometry, based on [28], was optimized with the help of numerical simulations to produce an almost pure shear stress state in the area between the 2 notches.…”

“…In addition to the triaxiality factor η and the maximum shear stress τ max , the principal stresses σ * 1 and σ * 3 and the orientation of the corresponding principal x * 1 -x * 3 axis system were analyzed for the central gauge area. This is of crucial importance, as one of the main objectives of this study is to investigate whether the twins align perpendicular to the minor principal stress σ * 3 as in the uniaxial experiments in [15], although a pure shear stress state prevails. Figure 3d shows the distribution of the minor principal stress σ * load of F 1 = 725 N. Additionally, Figure 3d shows the orientation of the principal axis system and the associated principal stresses.…”

“…Figure 1 shows the basic unit cell of magnesium and their planes for dislocation slip and twinning [15]. Because of the hexagonal crystal system, basal slip and extension twinning are the main modes of plastic deformation in magnesium at room temperature, both with a critical resolved shear stress (CRSS RT ) of approximately 0.5 MPa [16][17][18][19][20][21].…”