ADVANCED MATERIALS & PROCESSES | SEPTEMBER 2024 14 severe deformation on the surface. PLM detects c-axis orientation in relation to the incident light direction and vibration plane. According to the generalized Fresnel equations, incident light splits into two refracted (or reflected) waves, each polarized differently and with distinct refractive indices. Upon exiting the sample, these waves interfere destructively, causing color changes. As the Michel-Levy birefringence inter- ference color chart indicates, these colors depend on sample thickness and birefringence value[5]. Rotating crystals on the microscope stage can adjust their color appearance accordingly. Representative PLM images of the deformed surface of WDED cp-Ti tensile test samples after fracture are shown in Fig. 3. Loading direction with respect to the sample is represented in the Fig. 3a inset. Large deformed regions arise from plastic deformation caused by dislocation glide, which leads to strain accumulation and localized deformation within the material, as shown in Figs. 3a and 3b. However, plastic deformation by slipping of WDED cp-Ti is limited due to its low symmetry and reduced number of independent slip systems. Thus, nucleation of twinning deformation occurs to accommodate complete deformation along the c-axis direction when the <a> or <c + a> are limited. Contrary to slip dislocation, twinning dislocations are characterized by shearing and reorientation of one segment of the crystal lattice. As mentioned, PLM interacts with grains with respect to the c-axis angles. Next, the microscope’s analyzer plate angle was rotated to facilitate visualization of the twin planes on the surface and differentiate them from the slip planes (Fig. 3c). PIP indentation of the samples along the buildup shows a large deformation in the vicinity of the indent (Fig. 4). PIP has the ability to provide accurate stress-strain curves that enable calculation of tensile- compressive asymmetry (TCA)[6]. Twin planes are present all around the vicinity of the indent with slip planes nested in between (Fig. 4a). PLM and DIC techniques were combined to characterize the larger twin regions (Fig. 4b). Formation of secondary twins was evidenced as presented in Fig. 4c. In this case, the primary twin acts as a parent grain and allows formation of the secondary twin due to the affinity of its orientation to be deformed again under stress. This phenomenon can be attributed to the complex loading condition that results beneath the indent from the PIP indentation, which is graphically described in the Fig. 4a inset[7]. Stress-strain curves from both the tensile and PIP tests were overlapped and used to compute the mechanical properties of the material under both conditions (Fig. 5). Yield stress (σy) demonstrated comparability with a low TCA percentage, approximately 8%, indicating a more uniform strain during the initial plasticity stage. On the other hand, ultimate tensile strength (UTS) exhibited around 37% TCA, with higher values observed when employing the PIP technique. This suggests that the intricate compressive stresses induced by the indenter interact with the compressive residual stresses of the overlapped WDED layers, alongside the influence of predetermined grain misorientation, which is susceptible to deformation via twinning[8]. The role of twin deformations in the plastic behavior of WDED α-Ti, especially in the work hardening region, is evident and contributes to the observed significant tensile-compressive asymmetry of the UTS. Multiple optical micrographs from both tensile and PIP tests were processed into 16-bit grayscale and edited to highlight twin regions in order to support the current study’s findings. A fixed threshold of 30% was applied to calculate the approximate twinned area fraction of the characterized images (Figs. 6a and 6b). Fig. 4 — Optical micrographs of deformed PIP samples using polarized light and DIC microscopy for characterizing slip and twin planes on the indent’s vicinity. Fig. 3 — Optical micrographs of deformed tensile test samples using polarized light at different angles of the analyzer plate. Different c-axis angles react to various contrasts, allowing identification of individual grains, morphology, and features as slip planes and twins. (a) (b) (c) (a) (b) (c)
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