Figure 1

(a) Conventional deformation twinning generates macroscopic strain and changes the grain shape. (b) High-resolution-electron micrograph of a MAP twin in nanocrystalline Cu synthesized by high-pressure torsion of 99.99 wt.% purity coarse-grained copper disk, 0.5 mm thick and 10 mm diameter. Arrows indicate the twin boundaries. Twins and matrix are labeled $T$ and $M$, respectively.Reuse & Permissions

Figure 2

HREM images of typical deformation twins in three nanocrystalline (NC) fcc metals. (a) Al, with 99.9 wt% purity and grain diameters between 0.5 to $1.0\mu \mathrm{m}$, processed by cryomilling (i.e., by ball milling in liquid nitrogen for 8 hours); (b) electrodeposited Ni foils from Goodfellow, Inc. was immersed under liquid nitrogen and deformed by tension at a strain rate of $3\times {10}^{-3}{\mathrm{s}}^{-1}$. The as-received $150\mu \mathrm{m}$ thick foils had an average grain size of 25 nm. Preexisting growth twins were negligible in the $\sim 100$ grains we examined. (c) 99.99 wt.% pure coarse-grained Cu disks, 0.5 mm thick and 10 mm diameter, were processed into NC-Cu via high-pressure torsion (HPT) for 5 revolutions under 7 GPa at room temperature and a very low strain rate of about ${10}^{-2}{\mathrm{s}}^{-1}$. HPT produces fully dense NC materials without introducing measurable impurity into deformed samples. It is evident that the deformation twins did not change the shape of the grains. Arrows indicate the twin boundaries. The broken curves indicate GBs. Twins and matrix are labeled $T$ and $M$, respectively.Reuse & Permissions

Figure 3

The RAP twinning mechanism. (a) $A$, $B$, and $C$ are atom stacking sequences, and ${\mathbf{b}}_{\mathbf{1}}$, ${\mathbf{b}}_{\mathbf{2}}$, and ${\mathbf{b}}_{\mathbf{3}}$ are the Burgers vectors of the partials. (b) Evolution of stacking sequence and formation of a RAP twin via the slip of partials ${\mathbf{b}}_{\mathbf{2}}$, ${\mathbf{b}}_{\mathbf{3}}$, ${\mathbf{b}}_{\mathbf{3}}$, ${\mathbf{b}}_{\mathbf{1}}$, ${\mathbf{b}}_{\mathbf{2}}$, and ${\mathbf{b}}_{\mathbf{1}}$ on layers I through VI, respectively. TB marks the two twin boundaries. Partials with negative Burgers vectors are not possible because they place two atoms on top of each other, increasing the system energy.Reuse & Permissions

Figure 4

Illustrating a possible RAP process. A Shockley dislocation loop with Burgers vector ${\mathbf{b}}_{\mathbf{1}}$ emitted from a GB junction $E$ (shaded blue) grows (shaded gray) and is deposited on other GBs of a hexagonal grain. Part of the dislocation line segments parallel to grain edges $AB$ and $DE$ has pure screw character and can move in the GB to the next slip plane.Reuse & Permissions

Figure 5

Statistics were obtained from 18 to 38 twinned grains. Percentages of RAP and MAP (orange colored bars) deformation twins, as determined by HREM. The samples used were (i) electrodeposited nanocrystalline Ni deformed at LNT under tension at a strain rate of $3\times {10}^{-3}{\mathrm{s}}^{-1}$ (to a strain of 4% and a corresponding maximum stress of about 1500 MPa), (ii) by cold rolling to a thickness reduction of 30% and (iii) under Hopkinson-bar impaction at LNT wherein 10 mm diameter specimens were placed between 10 mm diameter and 2500 mm long input and output bars. A 3 kg projectile, accelerated to a speed of $\sim 30\mathrm{m}\mathrm{s}$, impacts the input bar generating an elastic compressive wave through the input bar. A part of this wave is reflected while the remainder transmits through the specimen, deforming it plastically. The strain rate was about $2.6\times {10}^{+3}{\mathrm{s}}^{-1}$. The flow stress is estimated to be above 2.2 GPa. The changes of grain size distribution and mean grain sizes before and after three tests are negligible. The specimens did not fracture after cryogenic rolling or impaction.Reuse & Permissions