Dislocation processes accompanying the Portevin-Le Chatelier effect in Al–Mg alloys

Aluminum alloys have great potential to replace steels in automotive structures and closure applications. However, formability limitations continue to remain an obstacle in their widespread usage. Solute strengthening in AA5000 Al–Mg alloys can be exploited to increase ductility and strength simultaneously, but dynamic strain ageing and negative strain rate sensitivity in these alloys lead to Portevin-Le Chatelier (PLC) instability and cause premature failure. PLC instability is associated with the diffusion of Mg atoms to dislocations and dislocations moving away from diffusing Mg atoms, resulting in plastic strain occurring in bursts. Macroscopically, the PLC effect has been well characterized. The underlying dislocation structures and atomistic mechanisms responsible for dynamic strain ageing have not been well understood and is the subject of this study. In this investigation, AA5754 sheets have been strained in situ in the scanning electron microscope and the transmission electron microscope (TEM). Combined electron backscatter diffraction and electron dispersive X-ray spectroscopy analyses show high local Mg concentration regions to correlate well with high dislocation densities. In situ TEM straining data show that glissile dislocations contribute to both nucleation and dissolution of small Mg clusters and precipitates. Electron tomography data show the dynamic nature of the dislocation network of glissile and sessile dislocations. The implications of these observations on the mechanism governing the PLC effect in Al–Mg alloys and the current theories of dynamic strain ageing in Al–Mg alloys will be discussed

Antiferroelectric negative capacitance from a structural phase transition in zirconia

Crystalline materials with broken inversion symmetry can exhibit a spontaneous electric polarization, which originates from a microscopic electric dipole moment. Long-range polar or anti-polar order of such permanent dipoles gives rise to ferroelectricity or antiferroelectricity, respectively. However, the recently discovered antiferroelectrics of fluorite structure (HfO$_2$ and ZrO$_2$) are different: A non-polar phase transforms into a polar phase by spontaneous inversion symmetry breaking upon the application of an electric field. Here, we show that this structural transition in antiferroelectric ZrO$_2$ gives rise to a negative capacitance, which is promising for overcoming the fundamental limits of energy efficiency in electronics. Our findings provide insight into the thermodynamically 'forbidden' region of the antiferroelectric transition in ZrO$_2$ and extend the concept of negative capacitance beyond ferroelectricity. This shows that negative capacitance is a more general phenomenon than previously thought and can be expected in a much broader range of materials exhibiting structural phase transitions

Bragg's law diffraction simulations for electron backscatter diffraction analysis, Ultramicroscopy 109

a b s t r a c t In 2006, Angus Wilkinson introduced a cross-correlation-based electron backscatter diffraction (EBSD) texture analysis system capable of measuring lattice rotations and elastic strains to high resolution. A variation of the cross-correlation method is introduced using Bragg's Law-based simulated EBSD patterns as strain free reference patterns that facilitates the use of the cross-correlation method with polycrystalline materials. The lattice state is found by comparing simulated patterns to collected patterns at a number of regions on the pattern using the cross-correlation function and calculating the deformation from the measured shifts of each region. A new pattern can be simulated at the deformed state, and the process can be iterated a number of times to converge on the absolute lattice state. By analyzing an iteratively rotated single crystal silicon sample and recovering the rotation, this method is shown to have an angular resolution of $0.041 and an elastic strain resolution of$7eÀ4. As an example of applications, elastic strain and curvature measurements are used to estimate the dislocation density in a single grain of a compressed polycrystalline Mg-based AZ91 alloy

Multiscale characterization of dislocation processes in Al 5754

<div><p>Multiscale characterization was performed on an Al–Mg alloy, Al 5754 O-temper, including <i>in situ</i> mechanical deformation in both the scanning electron microscope and the transmission electron microscope. Scanning electron microscopy characterization showed corresponding inhomogeneity in the dislocation and Mg distribution, with higher levels of Mg correlating with elevated levels of dislocation density. At the nanoscale, <i>in situ</i> transmission electron microscopy straining experiments showed that dislocation propagation through the Al matrix is characterized by frequent interactions with obstacles smaller than the imaging resolution that resulted in the formation of dislocation debris in the form of dislocation loops. <i>Post</i>-<i>mortem</i> chemical characterization and comparison to dislocation loop behaviour in an Al–Cr alloy suggests that these obstacles are small Mg clusters. Previous theoretical work and indirect experimental evidence have suggested that these Mg nanoclusters are important factors contributing to strain instabilities in Al–Mg alloys. This study provides direct experimental characterization of the interaction of glissile dislocations with these nanoclusters and the stress needed for dislocations to overcome them.</p></div

Multiple Twinning and Stacking Faults in Silver Dendrites

Detailed defect structure of dendrite formation was studied in order to connect the mesoscopic with the atomistic structure. It was demonstrated that twinning and stacking fault formation play a central role in the growth of electrodeposited Ag dendrites. The broad faces of Ag dendrites and the main trunk growth direction were found to be ((1) over bar 11) and [(1) over bar1 (2) over bar], respectively. Dendrite branches also formed and grew from the main trunk parallel to the [12 (1) over bar] and [(211) over bar] crystallographic directions. Twins and stacking faults were found to reside on the {111} crystallographic planes, as expected for a face centered cubic (FCC) Ag crystal. Using electron back scattered diffraction (EBSD) we found two variants of in-plane 60 degrees rotational twin domains in the ((1) over bar 11) broad dendrite surface plane. The intersections of twins and stacking faults with dendrite arm surfaces are perpendicular to the (112) arm growth directions. However, occasionally twins on the {111} planes parallel to the (112) arm growth directions were also observed. Although defect assisted dendrite growth is facilitated by twinning and stacking fault formation on {111} planes, the growth directions of the trunk and branches are not of the (111) type, but rather close to (112). The (112) growth directions are maintained by breaking dendrite facets into thermodynamically stable 111 and 200 steps and structural ledges of different length

Achieving strength-ductility synergy in a laser-powder bed fused near-α titanium alloy through well-crafted heat treatments

The design of a two-step heat treatment (solutionizing and aging) protocol for laser-powder bed fused Ti-6Al-2Sn-4Zr-2Mo (L-PBF-Ti-6242) and its effect on microstructure and mechanical properties are studied. The heat treatment is designed considering the kinetics of α’ to β transformation and nanotwin annihilation with an emphasis on preserving the unique, ultrafine, and hierarchical microstructure. The solutionizing temperature of 900 °C for 10 and 20 min is selected based on the α’ to β transformation percentage. The aging temperatures of 300 °C and 350 °C for 12–72 h are chosen considering the kinetics of nanotwin annihilation. A total of 15 conditions are evaluated from these solutionizing and aging parameters including as-built, only solutionized, solutionized-aged, and direct aging. The as-built microstructure has the highest strength and lowest ductility due to the ultrafine acicular α’ martensite and dense dislocation network. The solutionized microstructure has α/α’and β phases with different dislocation densities. This condition results in the lowest strength and highest ductility, governed by the presence of the bcc phase (β), and high mean effective slip length in the α/α’ phase. The aging process (following the solutionizing step) results in changes in dislocation substructure in the α/α’ phase which leads to an increase in strength, controlled by a reduction in mean effective slip length. The formulated two-step heat treatment leads to the best strength-ductility synergy in this study