Accommodation of large plastic strains and defect accumulation in nanocrystalline Ni grains

Abstract

A transmission electron microscopy (TEM) study has been carried out to uncover how dislocations and twins accommodate large plastic strains and accumulate in very small nanocrystalline Ni grains during low-temperature deformation. We illustrate dislocation patterns that suggest preferential deformation and nonuniform defect storage inside the nanocrystalline grain. Dislocations are present in individual and dipole configurations. Most dislocations are of the 60°type and pile up on (111) slip planes. Various deformation responses, in the forms of dislocations and twinning, may simultaneously occur inside a nanocrystalline grain. Evidence for twin boundary migration has been obtained. The rearrangement and organization of dislocations, sometimes interacting with the twins, lead to the formation of subgrain boundaries, subdividing the nanograin into mosaic domain structures. The observation of strain (deformation)-induced refinement contrasts with the recently reported stress-assisted grain growth in nanocrystalline metals and has implications for understanding the stability and deformation behavior of these highly nonequilibrium materials.

Deformation-Induced Grain Refinement and Amorphization in Ti-10V-2Fe-3Al Alloy

“…Besides severe lattice distortion, edge dislocations marked with a ''T'' were observed from the extra half-planes of crystal lattices. The dislocation density was calculated to be around 2 9 10 13 /cm 2 according to the method proposed by Wu et al [22,23] Figure 8(c) is a HRTEM image of area 2 in Figure 7(a), which shows the transition from nanocrystalline to amorphous. The proportion of the crystalline structure enclosed by the dashed line was smaller than that in Figure 8(a).…”

Mechanical Behavior of Cryomilled Ni Superalloy by Spark Plasma Sintering

“…[24] A high population of twins was also observed in nanostructured Ni prepared via cold rolling. [25] Following SPS, the XRD technique estimated that there existed two to four twins in each grain. Compared to the TEM micrograph in Figure 3, the average twin population by XRD is slightly overestimated and the XRD result appears to correspond to the twin population of the twinned grains in the SPS samples.…”

Growth of deformation twins in room-temperature rolled nanocrystalline nickel

“…If plastic strain increases continually, the TB will evolve into a distorted GB with higher energy. 9,10 In summary, in this letter, deformation twinning has been observed in room-temperature cold-rolled nc Ni. Growth of twins by the emission of partial dislocations from GB has been illustrated in detail.…”

“…11 However, deformation twinning has been observed in deformed nc Ni rolled to different strains at low temperature ͑in liquid nitrogen͒. 6,7,9 In this letter, we report that deformation twinning can also take place during room-temperature cold-rolling of nc Ni. The growth of twins by the emission of partial dislocations from GBs is discussed in detail for the cases of three different types of twin boundaries ͑TBs͒.…”

“…[6][7][8][9] However, our samples were deformed at room temperature, whereas the samples analyzed in Refs. 6-9 were deformed at low temperature ͑in liquid nitrogen͒.…”