Simultaneously Increasing the Ductility and Strength of Nanostructured Alloys

Strength and ductility are two of the most important mechanical properties of structural materials. However, they are often mutually exclusive, i.e., a material may be strong or ductile, but rarely both at the same time. This is also true for bulk nanostructured materials, which usually have high strength, but disappointingly low ductility. Bulk nanostructured materials are usually synthesized by either a two-step approach such as nanopowder consolidation, or a one-step approach such as severe plastic deformation. The latter approach can synthesize flaw-free nanostructured materials with higher ductility than those synthesized by nanopowder consolidation. However, even these nanostructured materials often exhibit a very low uniform elongation (strain before necking). Uniform elongation should be used as a measure of the ductility of nanostructured materials because it is much less affected by the gauge length than the elongation to failure. The latter gives a false impression of high ductility in samples with very short gauge length (e.g., less than 5 mm, as used in many studies) due to large post-necking strain. A high work-hardening rate is essential for good uniform elongation because it can help delay localized deformation (necking) under tensile stress. Bulk nanostructured materials often have a very low or no work-hardening rate because of their low dislocation accumulation capability. Indeed, there has been considerable effort to address the pressing need of increasing the ductility of nanostructured materials at room temperature, but all the previous attempts to this end have sacrificed some of their yield strengths gained from nanostructuring. In this paper we report a strategy to simultaneously increase the ductility and strength of bulk nanostructured materials. By engineering very small second-phase particles into a nanostructured Al alloy matrix, we were able to more than double its uniform elongation, while further gaining rather than sacrificing its yield strength. The simultaneous enhancement of ductility and strength is due to the increased dislocation accumulation and resistance to dislocation-slip by second-phase particles, respectively. Our strategy is applicable to many nanostructured alloys and composites, and paves a way for their large-scale industrial applications. The material used in this model study is 7075 Al alloy. The alloy was solution-treated to obtain a coarse-grained (CG) solid solution. The CG sample was immediately cryogenically rolled to produce nanostructures with an average grain size of ca. 100 nm (designated as NS sample). The NS sample was then aged at low temperature to introduce very small secondphase particles (designated as NS+P sample). The engineering stress–strain curves of these samples are compared in Figure 1a. The 0.2% yield strengths (marked by circles) of the CG, NS, and NS+P samples are 145 MPa, 550 MPa, and 615 MPa, respectively. Therefore, the low-temperature aging enhanced the yield strength of the NS sample by 12%. The uniform elongation (marked by the symbol on the curves in Fig. 1a) was determined by the Considère criterion (Eq. 1) governing the onset of localized deformation