A Review on Heterogeneous Nanostructures: A Strategy for Superior Mechanical Properties in Metals

Abstract

Generally, strength and ductility are mutually exclusive in homogeneous metals. Nanostructured metals can have much higher strength when compared to their coarse-grained counterparts, while simple microstructure refinement to nanoscale generally results in poor strain hardening and limited ductility. In recent years, heterogeneous nanostructures in metals have been proven to be a new strategy to achieve unprecedented mechanical properties that are not accessible to their homogeneous counterparts. Here, we review recent advances in overcoming this strength–ductility trade-off by the designs of several heterogeneous nanostructures in metals: heterogeneous grain/lamellar/phase structures, gradient structure, nanotwinned structure and structure with nanoprecipitates. These structural heterogeneities can induce stress/strain partitioning between domains with dramatically different strengths, strain gradients and geometrically necessary dislocations near domain interfaces, and back-stress strengthening/hardening for high strength and large ductility. This review also provides the guideline for optimizing the mechanical properties in heterogeneous nanostructures by highlighting future challenges and opportunities.

Simultaneous Improvement of Yield Strength and Ductility at Cryogenic Temperature by Gradient Structure in 304 Stainless Steel

“…Moreover, a simultaneously improvement in the strength and the uniform elongation is found at cryogenic temperature for the GS2 sample, as compared to the CG sample. According to the previous papers [30][31][32]59], excellent tensile properties can be achieved by heterogeneous structures due to the HDI hardening, which can be attributed to the back stress that arises from plastic deformation incompatibility between hard and soft domains. In the gradient structures, the various layers at different depths have various hardness and strain hardening ability; thus, these layers can be considered as hard and soft domains, and HDI hardening should play an important role during tensile loading for the gradient structures [31,60].…”

Realistic hardening-to-softening transition effects of metals over the finite strain range up to failure

“…In particular, the case for metals with pronounced Bauschinger effects needs to be studied in association with the back stress (c.f., e.g. Ma et al , 2019; Zhu and Wu, 2019). In this case, anisotropic hardening and softening need be taken into account toward properly simulating finite rotation effects involved in shearing and twisting problems (cf., e.g.…”

Perspectives of Microstructure Refinement of Aluminum and Its Alloys by the Reciprocating Extrusion (Cyclic Extrusion Compression—CEC)

“…Therefore, it has been postulated that only certain alloys, in which the development of recovery and recrystallization processes is hindered, may potentially offer the possibility of producing nanometric structure in the deformed material [ 21 ]. Examples include Mg-AZ31 alloys [ 21 ], austenitic steels [ 22 ], steel, Cu, Ni [ 23 , 24 ], Al7.5%Zn-2.7%Mg-2.3%Cu-0.15%Zr alloy [ 25 ], (Ni 1.5 FeCoCr 0.5 ) 87.5 Al 7.5 Ti 5.0 [ 26 ], Cu-Fe-P, Cu-Ni-Si, Cu-Cr-Zr [ 27 ]. Studies have shown that the formation of nanostructure is associated with an increase in the level of hardening (Ni 1.5 FeCoCr 0.5 ) 87.5 Al 7.5 Ti 5.0 .…”