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Physical Methods for Characterizing Solids
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
The presence of translational symmetry elements in a crystal structure can be detected because they each lead to a set of systematic absences in the hkl reflections. Figures 1.10 and 1.11 showed how a two-fold screw (21) along z introduces a plane of atoms exactly halfway between the 001 planes: reflections from these planes will destructively interfere with reflections from the 001 planes and the 001 reflection will be absent, as will any reflection for which l is odd. The effect of a glide plane (Figure 1.9) is to introduce a plane of atoms halfway along the unit cell in the direction of the glide. For an a glide perpendicular to b, the 10l reflection will be absent, and in general the h0l reflections will only be present when h is even. Systematic absences are summarized in Table 2.4.
Nonplanar core structure of the screw dislocations in tantalum from the improved Peierls–Nabarro theory
Published in Philosophical Magazine, 2018
The nonplanar dislocation can be hardly driven to move if the multi-fold structure keep invariant. The movement mechanism of a nonplanar dislocation is a difficult problem. In the following, a possible way of movement of a nonplanar dislocation is proposed. Because the distribution of infinitesimal dislocations is always confined to the glide planes, the motion path of infinitesimal dislocations should be also confined to the glide planes. When an external field is applied, the glide planes are nearly not affected (neglecting the deformation under the external stress), but each infinitesimal dislocation may move along one of the glide planes. For a sixfold structure as shown in Figure 2, each infinitesimal dislocation, represents by circle with a point centre in, moves along planes only. When the shear stress acts on the plane composed of and directions, the dislocation flux converge from the other planes to the dislocation centre. Eventually, infinitesimal dislocations gather to one glide plane and transforms into a planar dislocation due to the expel interaction between the infinitesimal dislocations. The resulted planar dislocation will be driven to move by Peierls stress predicted in Ref. [47].
Microstructure evolution and mechanical properties of wire arc additively manufactured Mg-Gd-Y-Zr alloy by post heat treatments
Published in Virtual and Physical Prototyping, 2023
Dong Ma, Chunjie Xu, Shang Sui, Jun Tian, Can Guo, Xiangquan Wu, Zhongming Zhang, Sergei Remennik, Dan Shechtman
A tremendous YS increment of 46 MPa was realised for the WAAM-T6 GW63 alloys following peak ageing heat treatment. The yield strength of the T6 state sample is expressed using the following formula: The contribution of is the same as that of the WAAM-T4 sample. Following ageing treatment, the Gd and Y were decreased in the Mg matrix and the EDS measured the composition of ∼1.01 at.% Gd and ∼0.78 at.% Y due to the precipitation of Gd and Y. In comparison to the as-built sample, the value of was evaluated as 15.2 MPa. However, the contribution of decreased by 10.9 MPa compared to the WAAM-T4 sample. Intragranular precipitates play an essential role in the precipitation strengthening effect for Mg-Gd-Y alloys due to the nano-sized β′ precipitates being uniformly and dispersedly precipitated from the supersaturated ɑ-Mg solid solution (Yang et al. 2015). In this experiment, the length of the β′ precipitate on the projection plane was approximately 20–40 nm and the width was approximately 5–20 nm. For simplicity, the nano-precipitates can impart a strengthening effect via the Orowan mechanism and the contribution to the YS is calculated using the Orowan equation (Ji et al. 2014): where represents the shear modulus, represents the Poisson's ratio, represents the magnitude of the Burgers vector, is the average diameter of prismatic plates, is the planar thickness of prismatic plates and is the volume fraction of β′ precipitates. The values of , and were taken as 16,600 MPa, 0.29 and 0.32 nm (Lei et al. 2022) and the and of β′ precipitates in the GW63 alloy was estimated as 32.3 and 14.3 nm. Accurately determining the volume fraction of the β′ precipitates is incredibly difficult. The decision was made to roughly estimate the content of the β′ precipitates based on the TEM images and the content of the β′ precipitates was approximately 4.0%. Therefore, the was calculated as 64.1 MPa. It should be noted that there a large number of stacking faults still remain in the WAAM-T4 and WAAM-T6 samples. As a two-dimensional lattice defect, it remains unclear whether stacking faults impact the yield properties of materials. However, the introduction of stacking faults can result in the glide plane softening effect, thereby promoting the plane slip of dislocations and helping improve plasticity (Sandlöbes et al. 2012).