Ductility of nanocrystalline metals: Intrinsic or extrinsic

While the super-high strength of various nanocrystalline metals (NC metals, grain size d<100 nm) has drawn great attention in the past few decades, the poor ductility has become a seemingly insurmountable obstacle for the wide application of such metals. Other limiting factors include, in particular, the high cost associated with delivering such materials. Recent efforts from various groups have brought forth strong hope that the ductility of NC metals can be greatly improved by the application of "microstructure engineering". However, the testing methods used to evaluate the mechanical properties, especially at small scales, have invited concerns. A natural question arises as to whether the reported allegedly good ductility of many NC metals is intrinsic or extrinsic. This article attempts to critically evaluate such issues. We will examine the experimental results in conjunction with some finite element modeling on the stress state and strain measurement of specimens during mechanical testing. Factors such as sample geometry and loading mode will be critically assessed. We point out that while intrinsic ductility may be possessed by some NC metals, careful attention should be paid to how strain measurement is made.