On the origin of ultrahigh cryogenic strength of nanocrystalline metals

Abstract

At liquid nitrogen temperature, the yield strength of nanocrystalline Ni and Co increases by as much as 50%–80% over the already-impressive (∼1GPa) room-temperature values. This unusual strength ratio as well as the remarkable magnitude of flow stress reached (as high as 2.5GPa) are unexpected for conventional close-packed pure metals. The strong temperature dependence is attributed to the unusually small activation volume measured in strain rate change tests. Grain boundary dislocation nucleation is proposed as the thermally activated deformation mechanism in nanocrystalline grains.

Elastic and Plastic Deformation Behavior Studied by <i>In-Situ</i> Synchrotron X-ray Diffraction in Nanocrystalline Nickel

“…However, the strength of NC materials is highly dependent on the strain rate, such that a decrease in the strain rate causes a large decrease in strength. [10][11][12][13][14] For this reason, the use of the intermittent step-by-step method to acquire a diffraction pro le during interruption of the tensile deformation with a xed crosshead stroke 28,29) does not yield accurate real-time data during deformation, especially in the case of NC materials.…”

“…In addition, NC materials are reported to possess unique mechanical properties compared with those of conventional grained (CG) materials. Examples include the extra-hardening phenomenon, with reported yield strengths greater than those predicted by the Hall-Petch curve, 8,9) the yield drop of face-centered cubic (FCC) metals that are not seen for CG materials, 9) greater variations in the strain rate with temperature in comparison to CG materials, [10][11][12][13][14] a hardening by annealing/softening by deformation effect 15) and an abnormal temperature dependence of the activation volume. 16) In order to nd practical applications for NC materials, the appearance of each of these phenomena must be explained, even though there is only a limited understanding of these causes of appearance at present.…”

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

“…The dominant role of dislocations in mediating deformation is consistent with the indications given by the recently measured values of strain-rate sensitivity and activation volume in NC Ni. 55,56 The steady-state microstructure is found to be, as expected, dependent on deformation conditions (temperature, in our case, and the corresponding flow stress levels 66 ). While our results are for LNT deformation, the same behavior is expected for RT deformation for metals with high melting temperature (such that RT is a low homologous temperature, as LNT is for Ni) or for alloys with added elements that suppress recovery.…”

“…33,34,[55][56][57][58][59] Our TEM studies here and elsewhere 34,36 have in fact identified the Burgers vectors and the character of dislocations in NC Ni. As such, we can rationalize the subdivision of the NC grain into volume elements by continuing to adopt the concept of slip deformation.…”

Inherent tensile strength of molybdenum nanocrystals

“…Nanocrystalline metals are predicted to have extremely high breaking strengths, similar to those of perfect crystals; thus, they might have potential applications in nanoindentation, antireflection coatings, scanning probe microscopy and field emission [13][14][15]. The technological relevance of these materials has led to increased experimental and theoretical research on the mechanical behavior of nanocrystals.…”