Micropillar compression technique was employed to study the microscale deformation mechanisms of basal slip, twinning and non-basal slips at selected grains in a Mg-2 wt.% Y alloy. The results ...suggest a critical resolved shear stress (τCRSS) for basal slip 12.5 ± 1.7 MPa, and for twin nucleation and twin growth 38.5 ± 1.2 MPa and 33.8 ± 0.7 MPa, respectively. The higher values compared to those in pure Mg suggests a more balanced deformation in Mg alloy with Y addition. The activation of <c+a> dislocations in the twinned orientation is highlighted, which leads to strong work hardening in twinned favorable orientation 101‾0. In addition, at prismatic-slip favorable orientation 112‾0, a twinning-to-prismatic slip transition was observed when elevating temperature from 25 °C to 100 °C and 250 °C. Specially at 250 °C, twinning was completely prohibited, and pure prismatic slip was triggered. The measured τCRSS for prismatic slip at 250 °C was 39.7 ± 0.3 MPa, much higher than that for pure Mg at the same temperature. Finally, at pyramidal-slip favorable orientation 0001, an abnormal strengthening was observed at 100 °C and 250 °C due to activation of pyramidal slips. Decompositions of <c+a> dislocations and Y segregation at stacking faults are the main mechanisms leading to the high-temperature strengthening in Mg–Y alloy.
The slip transmission across an interface is essential for the mechanical properties of dual-phase alloys like Ti-6Al-4 V. However, the correlation between the dislocation-interface interaction and ...the strength and strain hardening anisotropy remains unclear due to the lack of direct experimental evidence. Via in situ scanning electron microscopy micropillar compression, prismatic plane dislocations were preferentially activated and interacted with an individual α/β interface at different angles. Based on transmission electron microscopy characterization, this study suggests that α/β interface shows a more pronounced strengthening effect when the coordinated slip system is more difficult to be activated and the slip deflection angle is larger. Differently, its higher strain hardening rate is initially determined by the larger Burger vector magnitude of interfacial residual dislocation after slip transmission. These results provide a unique basis for understanding the contribution of the interface to the mechanical properties of dual-phase alloys.
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Under mechanical loading, the strain hardening behavior of crystalline face-centered cubic (FCC) metals is of critical importance in determining fracture behavior and overall mechanical performance. ...While strain hardening is typically accompanied by a decrease in ductility, it can also simultaneously enhance the material's resistance to plastic deformation and improve its load bearing capacity. Hence, we conducted a detailed study using copper (Cu) single-crystal micropillars as a model system to investigate and delineate the relationship between strain hardening and defect behavior. We employed in situ compression in a scanning electron microscope (SEM) and dislocation density-based crystal plasticity (DCP) modeling. The strain hardening rate varied with the compression crystallographic orientation, ranging from negligible values (of approximately 80 MPa) to relatively high hardening rates (of approximately 1150 MPa) for nominal strains of up to 15%. Various analysis methods were applied, including slip trace characterization, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). These techniques allowed us to identify the distributions of active slip systems, dislocation structures after compression, and correlated internal lattice rotations. Furthermore, the DCP model was developed to specifically understand how serration events are related to dislocation-density evolution or strain bursts, and this was validated with the micropillar experiments. This integrated experimental and modeling investigation offers valuable insights and predictions regarding the evolution of both total and partial dislocations, including Hirth and Lomer junctions, as well as lattice rotations.
•In situ single crystal pillar compression to explore the relationship between hardening rate and the compression orientation.•The DCP model was developed to understand how serration events are related to dislocation-density evolution or strain bursts.•This integrated experimental and modeling investigation offers valuable insights and predictions of dislocation evolution.
The application of an inverse method for determining the parameters of a crystal plasticity constitutive law of a body-centered-cubic (BCC) single phase material is presented. Nanoindentation is used ...as the primary experimental input. An objective function, based on the deviation between the experimentally measured imprint and the simulated one, is minimized by a differential evolution algorithm to obtain the best fitting crystal plasticity parameters. To aid the identification procedure additional experimental data is used: the upper bounds and the ratios of the critical resolved shear stresses of the three slip plane families in BCC are estimated from micropillar compression experiments and used as a constraint in the optimization. The effect of the imposed constraints and the chosen strategy for mapping experimental to simulated displacements is presented and discussed. The validation of the method is done in the macroscopic regime by comparing an experimental tensile test with a simulated one using the obtained crystal plasticity parameters. Accurate results are achieved from two different indents. Therefore, the method is a promising path for determining crystal plasticity parameters in the case where a direct fitting from a macroscopic stress–strain curve is not possible, i.e. in the case of multi-phase materials.
Metallic glasses can exhibit a wide range of local atomic arrangements which are determined by their processing history, and which control their mechanical properties. While recent studies show that ...thermally stable thin film metallic glasses with high strength and plasticity can be synthesized using physical vapor deposition, they often differ in terms of their structure and mechanical performance from bulk metallic glasses. In that aspect, two different fabrication methods – physical vapor deposition and arc melting – were utilized to fabricate compositionally similar Zr-based glasses in thin-film and bulk form. The as-fabricated samples were characterized in terms of oxygen concentration, element composition, and atomic structure. The mechanical response of the thin film metallic glass (TFMG) and the bulk metallic glass (BMG) was examined using in situ micropillar compression in a scanning electron microscope over a temperature range of room temperature to 500 °C. While the two glasses show overall similar failure characteristics, the TFMG exhibits higher strength together than the BMG without sacrificing plasticity at all testing temperatures. At elevated temperatures, the deformation mechanism changes to homogenous deformation above glass transition temperature (Tg). The higher strength and thermal stability of the TFMG is associated with a much higher oxygen concentration in the thin film compared to the BMG. The enhanced mechanical performance of the TFMG compared to the BMG highlights the potential of oxygen in the thermal stability characteristics and deformation capacity of Zr-based thin film metallic glasses.
•Investigated mechanical response & thermal stability of thin film & bulk Zr-based metallic glasses via in-situ micro-pillar compression.•Oxygen concentration, degree of crystallization and free volume content were correlated to mechanical properties at various temperatures.•Higher oxygen in TFMG contributes to superior mechanical properties, thermal stability while maintaining plasticity across all temperatures.
MAX phases gain increasing attention as protective coatings due to superior anti-corrosion and oxidation resistance for harsh high-temperature applications. However, the lower strength and ...strength-ductility trade-off in bulk MAX phases still pose an enormous challenge arising from the large ratio of c/a lattice parameters and less activated slips. Here, nanocrystalline Cr2AlC coating with high-purity was fabricated using a hybrid arc/sputtering technique with subsequent thermal annealing. Cylindrical micropillar compression test was conducted by FIB milling to identify the mechanical properties of the coating under deformation. The stress-strain curves demonstrated that an ultrahigh compressive strength of 5.3 GPa with strain beyond 12.5% was surprisingly achieved for Cr2AlC coating, which was 5 times larger than all the reported bulk MAX phases before. Specifically, the extraordinary ductility at RT is ascribed to the synergy of the fundamental basal slip, additional non-basal slip and deformation twinning stimulated by ultra-refinement effect of nano-crystalline Cr2AlC coating.
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MAX phases have exhibited excellent irradiation resistance regarding structure stability, but limited knowledge is available on the irradiation induced change in the mechanical performance of MAX ...phases. In this work, Cr2AlC single crystal samples were irradiated with 6 MeV C ions, and smaples’ mechanical properties were measured by micropillar compression test and nanoindentation technique. After irradiation, new slip traces are activated, which means the deformation behavior changed by irradiation. More important is that both the yield strength and Young’s modulus decrease gradually with the increasing irradiation doses up to 0.098 dpa, suggesting a significant radiation softening effect but not the radiation hardening effect which is very popular in other materials. This radiation softening effect is possible the result of the irradiation-induecd vacancies, which is supported by the DFT calculation. These results indicate that MAX phases like Cr2AlC have an excellent irradiation tolerance regarding mechanical properties and is a new class of promising candidate material for the advanced nuclear systems.
•Micropillar compression test has application on Cr2AlC MAX phase, and show the surprising irradiation softening effects, which is less pronounced in the low dose regime.
We report nanotwinned Al/Ti multilayers have exhibited size-dependent microstructure evolution and high strength. However, their deformation mechanisms are less well understood. In this work, we ...investigated the deformation mechanisms of nanotwinned Al/Ti multilayers with FCC/HCP layer interfaces by using in situ micropillar compression tests. Nanotwinned Al/Ti multilayers exhibit compressive strength up to 2.4 GPa and good work hardening capability. Post-compression TEM analyses reveal high-density stacking faults and the HCP-to-FCC phase transformations in Ti. Molecular dynamics simulations elucidate the mechanisms of deformation induced phase transformation in Ti and the influence of collective movement of partial dislocations on the deformability of Al/Ti multilayers.
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