An official website of the United States government
Textured Mg alloy sheet samples were tensile tested parallel to the transverse direction, at Zener–Hollomon parameter values ranging from Z ~ 50 at room temperature and 10−3 s−1 down to Z ~ 18 at 350 °C and 10−5 s−1. At high Z, the samples exhibit strong texture evolution indicative of significant prismatic slip of dislocations with Burgers vectors. Correspondingly, the plastic anisotropy is high, r ~ 4. At low Z, the texture evolution is minimal and the response is nearly isotropic, r ~ 1. Previously, it has been asserted that the high ductility and low plastic anisotropy observed at low Z conditions is due to enhanced activity of non-basal slip modes, including prismatic slip of dislocations and pyramidal slip of and dislocations. The present results call this understanding into question and suggest that the enhanced ductility is more closely associated with the climb of dislocations.
more »
« less
(2021) Accounting for the Effects of Dislocation Climb Mediated Flow in Mg Alloy ZK10 Sheet. In: Miller V.M., Maier P., Jordon J.B., Neelameggham N.R. (eds) . The Minerals, Metals & Materials Series. Springer, Cham. https://doi.org/10.1007/978-3-030-65528-0_6
Tensile samples of an Mg alloy ZK10 sheet were tested at a range of temperatures and strain rates designed to rather evenly probe a range of Zener Hollomon parameter values, from ln(Z) ≈ 15 (10–4 s−1 and 623 K) up to ln(Z) ≈ 50 (10–3 s−1 and 300 K). In contrast with more commonly examined Mg alloy AZ31B sheet material, ZK10 sheet material shows modest strain anisotropy (r-value) at low temperatures for both 45° (r45 ≈ 1.2) and TD (rTD ≈ 1.4) sample orientations, despite showing evidence of significant prismatic slip of dislocations, which often leads to high r-values at low temperatures. These low r-values become even lower (r45 ≈ 0.84 and rTD ≈ 0.89) at high temperatures. These behaviors are hypothesized to occur due to a distinct initial texture and deformation mechanism activity, which includes a modest level of tensile twinning and slip at both room and elevated temperature. A version of the viscoplastic self-consistent (VPSC) code, which accounts for the kinematics of dislocation climb, is used to simulate the behavior of a textured Mg alloy ZK10 sheet reveals that both the glide of pyramidal dislocations and the climb of basal < a > dislocations are required to describe the behavior at elevated temperatures.
more »
« less
- Award ID(s):
- 1810197
- PAR ID:
- 10315293
- Editor(s):
- Miller, Victoria M.; Maier, Petra; Jordon, J. Brian; Neelameggham, Neel R.
- Date Published:
- Journal Name:
- Magnesium Technology 2021
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy’s behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m1/2, which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip.more » « less
-
Schuh, Christopher A (Ed.)The {-1012} tensile twins terminating inside the grains of a deformed Mg-Y alloy were investigated by transmission electron microscopy. The crystallographic features of terminating twins and associated slip structures were quantified and correlated. The local stresses developed at a terminating {-1012} twin were computed using crystal plasticity simulations in order to interpret the observed slip patterns. Results indicate that both basal and matrix glide were involved in accommodating the plastic stresses developed in the vicinity of terminating twins. Along the twin boundary, the defect contrast consistent with that of lattice dislocations and twinning partials was observed. Based on these observations, a dislocation reaction is proposed that establishes an interrelationship between the observed matrix glide and {-1012} twinning in Mg-Y alloys.more » « less
-
Abstract Refractory high‐entropy alloys (RHEAs) show promising applications at high temperatures. However, achieving high strengths at elevated temperatures above 1173K is still challenging due to heat softening. Using intrinsic material characteristics as the alloy‐design principles, a single‐phase body‐centered‐cubic (BCC) CrMoNbV RHEA with high‐temperature strengths (beyond 1000 MPa at 1273 K) is designed, superior to other reported RHEAs as well as conventional superalloys. The origin of the high‐temperature strength is revealed by in situ neutron scattering, transmission‐electron microscopy, and first‐principles calculations. The CrMoNbV's elevated‐temperature strength retention up to 1273 K arises from its large atomic‐size and elastic‐modulus mismatches, the insensitive temperature dependence of elastic constants, and the dominance of non‐screw character dislocations caused by the strong solute pinning, which makes the solid‐solution strengthening pronounced. The alloy‐design principles and the insights in this study pave the way to design RHEAs with outstanding high‐temperature strength.more » « less
-
Abstract The magnesium alloy AZ31, which has undergone high-pressure torsion processing, was subjected to in situ annealing microbeam synchrotron high-energy X-ray diffraction and compared to the as-received rolled sheet material that was investigated through in situ neutron diffraction. While the latter only exhibits thermal expansion and minor recovery, the nanostructured specimen displays a complex evolution, including recovery, strong recrystallization, phase transformations, and various regimes of grain growth. Nanometer-scale grain sizes, determined using Williamson–Hall analysis, exhibit seamless growth, aligning with the transition to larger grains, as assessed through the occupancy of single-grain reflections on the diffraction rings. The study uncovers strain anomalies resulting from thermal expansion, segregation of Al atoms, and the kinetics of vacancy creation and annihilation. Notably, a substantial number of excess vacancies were generated through high-pressure torsion and maintained for driving the recrystallization and forming highly activated volumes for diffusion and phase precipitation during heating. The unsystematic scatter observed in the Williamson–Hall plot indicates high dislocation densities following severe plastic deformation, which significantly decrease during recrystallization. Subsequently, dislocations reappear during grain growth, likely in response to torque gradients in larger grains. It is worth noting that the characteristics of unsystematic scatter differ for dislocations created at high and low temperatures, underscoring the strong temperature dependence of slip system activation. Graphical Abstractmore » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1007/978-3-030-65528-0_6
