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Register a new userDeformation mechanisms of Mg-Ca-Zn alloys studied by means of micropillar compression tests
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The effect of Ca and Zn in solid solution on the critical resolved shear stress (CRSS) of <a> basal slip, tensile twinning and <c+a> pyramidal slip in Mg alloys has been measured through compression tests on single crystal micropillars with different orientations. The solute atoms increased the CRSS for basal slip to ~ 13.5 MPa, while the CRSS for pyramidal slip was lower than 85 MPa, reducing significantly the plastic anisotropy in comparison with pure Mg. Moreover, the CRSSs for twin nucleation and growth were very similar (~ 37 MPa) and the large value of the CRSS for twin growth hindered the growth of twins during thermo-mechanical processing. Finally, evidence of <a> prismatic slip and cross-slip between basal and prismatic dislocations was found. It is concluded that the reduction of plastic anisotropy, the activation of different slip systems and cross-slip and the weak basal texture promoted by the large CRSS for twin growth are responsible for the improved ductility and formability of Mg-Ca-Zn alloys.
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The Mg-Zn and Al-Zn binary alloys have been investigated theoretically under static isotropic pressure. The stable phases of these binaries on both initially hexagonal-close-packed (HCP) and face-centered-cubic (FCC) lattices have been determined by utilizing an iterative approach that uses a configurational cluster expansion method, Monte Carlo search algorithm, and density functional theory (DFT) calculations. Based on 64-atom models, it is shown that the most stable phases of the Mg-Zn binary alloy under ambient condition are $rm MgZn_3$, $rm Mg_{19}Zn_{45}$, $rm MgZn$, and $rm Mg_{34}Zn_{30}$ for the HCP, and $rm MgZn_3$ and $rm MgZn$ for the FCC lattice, whereas the Al-Zn binary is energetically unfavorable throughout the entire composition range for both the HCP and FCC lattices under all conditions. By applying an isotropic pressure in the HCP lattice, $rm Mg_{19}Zn_{45}$ turns into an unstable phase at P$approx$$10$~GPa, a new stable phase $rm Mg_{3}Zn$ appears at P$gtrsim$$20$~GPa, and $rm Mg_{34}Zn_{30}$ becomes unstable for P$gtrsim$$30$~GPa. For FCC lattice, the $rm Mg_{3}Zn$ phase weakly touches the convex hull at P$gtrsim$$20$~GPa while the other stable phases remain intact up to $approx$$120$~GPa. Furthermore, making use of the obtained DFT results, bulk modulus has been computed for several compositions up to pressure values of the order of $approx$$120$~GPa. The findings suggest that one can switch between $rm Mg$-rich and $rm Zn$-rich early-stage clusters simply by applying external pressure. $rm Zn$-rich alloys and precipitates are more favorable in terms of stiffness and stability against external deformation.
The plastic deformation mechanisms of tungsten carbide at room and elevated temperatures influence the wear and fracture properties of WC-Co hardmetal composite materials. Although the active slip planes and residual defect populations of room-temperature deformed WC have been previously characterised, the relationship between the residual defect structures, including glissile and sessile dislocations and stacking faults, and the active slip modes, which produce slip traces, is not yet clear. Part 1 of this study showed that {10-10} was the primary slip plane at all temperatures and orientations. In the present work, Part 2, crystallographic lattice reorientations of deformed WC micropillar mid-sections were mapped using focused ion beam (FIB) cross-sectioning and electron backscatter diffraction (EBSD). Lattice reorientation axis analysis has been used to discriminate <a> prismatic slip from multiple <c+a> prismatic slip in WC, enabling defect-scale deformation mechanisms to be distinguished, and their contribution to plastic deformation to be assessed, independently of TEM residual defect analysis. In prismatic-oriented pillars, deformation was primarily accommodated by cooperative multiple slip of <c+a> defects at room temperature, and by <a> dislocations at 600 {deg}C. In near-basal oriented pillars, the total slip direction was along <c>. The degree of lattice rotation and plastic buckling in the deformed basal pillar could be explained by prismatic slip constrained by the indenter face and pillar base.
The mechanical properties of Mg-4wt.% Zn alloy single crystals along the [0001] orientation were measured through micropillar compression at 23C and 100C. Basal slip was dominant in the solution treated alloy, while pyramidal slip occurred in the precipitation hardened alloy. Pyramidal dislocations pass the precipitates by forming Orowan loops, leading to homogeneous deformation and to a strong hardening. The predictions of the yield stress based on the Orowan model were in reasonable agreement with the experimental data. The presence of rod-shape precipitates perpendicular to the basal plane leads to a strong reduction in the plastic anisotropy of Mg.
Precipitation in Mg-Zn alloys was analyzed by means of first principles calculations. Formation energies of symmetrically distinct hcp Mg1-xZnx (0 < x < 1) configurations were determined and potential candidates for Guinier-Preston zones coherent with the matrix were identified from the convex hull. The most likely structures were ranked depending on the interface energy along the basal plane. In addition, the formation energy and vibrational entropic contributions of several phases reported experimentally (Mg4Zn7, MgZn2 cubic, MgZn2 hexagonal, Mg21Zn25 and Mg2Zn11) were calculated. The formation energies of Mg4Zn7, MgZn2 cubic, and MgZn2 hexagonal Laves phases were very close because they were formed by different arrangements of rhombohedral and hexagonal lattice units. It was concluded that beta_1^ precipitates were formed by a mixture of all of them. Nevertheless, the differences in the geometrical arrangements led to variations in the entropic energy contributions which determined the high temperature stability. It was found that the MgZn2 hexagonal Laves phase is the most stable phase at high temperature and, thus, beta_1^ precipitates tend to transform into the beta_2^ (MgZn2 hexagonal) precipitates with higher aging temperature or longer aging times. Finally, the equilibrium beta phase (Mg21Zn25) was found to be a long-range order that precipitates the last one on account of the kinetic processes necessary to trigger the transformation from a short-range order phase beta_2^ to beta .
Tungsten carbide cobalt hardmetals are commonly used as cutting tools subject to high operation temperature and pressures, where the mechanical performance of the tungsten carbide phase affects the wear and lifetime of the material. In this study, the mechanical behaviour of the isolated tungsten carbide (WC) phase was investigated using single crystal micropillar compression. Micropillars 1-5 ${mu}$m in diameter, in two crystal orientations, were fabricated using focused ion beam (FIB) machining and subsequently compressed between room temperature and 600 {deg}C. The activated plastic deformation mechanisms were strongly anisotropic and weakly temperature dependent. The flow stresses of basal-oriented pillars were about three times higher than the prismatic pillars, and pillars of both orientations soften slightly with increasing temperature. The basal pillars tended to deform by either unstable cracking or unstable yield, whereas the prismatic pillars deformed by slip-mediated cracking. However, the active deformation mechanisms were also sensitive to pillar size and shape. Slip trace analysis of the deformed pillars showed that {10-10} prismatic planes were the dominant slip plane in WC. Basal slip was also identified as a secondary slip system, activated at high temperatures.
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