There are several models describing the inverse H-P effect. Chokshi
et al. [42] firstly suggested grain boundary diffusional creep (Coble
creep) as the dominant deformation mode. This mechanism was
further studied by Masumura et al. [13], who indicated that a critical
grain size exists at which value the H-P mechanism switches
to the Coble creep mechanism. Sun et al. [43] suggested that the
atomic bond order loss might be another factor affecting the H-P
transition; they concluded that the inverse H-P effect originated
from the competition between the bond order loss and the associated
bond strength gain near facet defects, such as the surface,
grain boundary. All these models indicated that the facet defects,
which in our case are GBs, were the origin of inverse H-P effect at
nanoscale.