Enhanced thermal stability of nanograined metals below a critical grain size

Abstract

The limitation of nanograined materials is their strong tendency to coarsen at elevated temperatures. As grain size decreases into the nanoscale, grain coarsening occurs at much lower temperatures, as low as ambient temperatures for some metals. We discovered that nanometer-sized grains in pure copper and nickel produced from plastic deformation at low temperatures exhibit notable thermal stability below a critical grain size. The instability temperature rises substantially at smaller grain sizes, and the nanograins remain stable even above the recrystallization temperatures of coarse grains. The inherent thermal stability of nanograins originates from an autonomous grain boundary evolution to low-energy states due to activation of partial dislocations in plastic deformation.

Rapid heating induced ultrahigh stability of nanograined copper

“…Adopting low energy boundaries such as twin boundaries and low-angle GBs, which are more reluctant to recrystallize (8)(9)(10), is an alternative stabilization strategy without changing material chemistry. Our recent study revealed that nanograined metals with random GBs can be stabilized through GB relaxation (11)(12)(13). Both thermal and mechanical stability of very fine nanograins in a number of facecentered cubic (fcc) metals are enhanced considerably, originating from a deformation-induced autonomous GB relaxation to lower energy states through emission of stacking faults or twins from the boundaries.…”

“…Literature showed that in Cu with a purity of 99.9 to 99.99% and a deformation strain below 100%, annealing twins are usually formed upon annealing at a temperature ranging from 473 to 523 K (19,20). Unfortunately, this temperature window is well above that for nanograin coarsening in Cu, typically 393 to 450 K for grain sizes of 50 to 200 nm (11). In other words, as nanograins are heated, nanograin coarsening through GB migration occurs before formation of annealing twins.…”

Crystal Strengths at Micro- and Nano-Scale Dimensions

“…These same authors provided confirmation of H-P grain size strengthening via separately reported hardness measurements. In related research on nano-grained copper and nickel materials, Zhou et al have given emphasis to the importance of establishing thermal stability of the grain boundary structures in such materials [36]. Otherwise, Figure 4 shows v* to be an effective monitor for detecting the onset of grain size weakening, for example, in line with the determination of material creep behavior as described in a previous report by Armstrong et al [37].…”

Room-temperature excitonic emission with a phonon replica from graphene nanosheets deposited on Ni-nanocrystallites/Si-nanoporous pillar array

“…Compared with Ni/Si-NPA, the pillars of sample A maintained their shapes and became slightly shorter after CVD growth, as shown in figure 2 c . The change of the texture and morphology might be due to the recrystallization and size growth of nc -Ni at 1000°C during the CVD process, which was a considerably higher temperature than the highest reported stability temperature of Ni nanograins at 900°C [22,23]. The recrystallization and growth size of nc -Ni also corresponded to the narrower full width at half maximum of the Ni XRD peaks in sample A.…”