Microstructural Dependence Of Strength And Ductility Of Two-phase Nanostructured Metals: A Numerical Study

Advances in equipment-manufacturing industrial technology have promoted the development of metallic materials with high strength and good ductility.Since the nanostructured(NS)metals have superior mechanical properties,they have drawn intensive attention.This dissertation is on two kinds of NS metals,namely,nanograined(NG)Cu with nanotwinned(NT)regions and the bimodal NS Cu.This dissertation aims to investigate the dependence of their strength and ductility on microstructures under the tensile loading.First,based on the Johnson–Cook plasticity model and failure criterion,in this dissertation,computer simulations are carried out to clarify the effects of twin spacing together with shape and distribution of NT regions on the strength and ductility of NG Cu with NT regions.The calculations indicate that these attributes can lead to three distinct failure modes,including fracture of matrix,fracture of NT regions,and interface debonding,and thus prominently affect the overall strength and ductility.In particular,it is discovered that a critical twin spacing marks the reversal of the overall ductility,that is,the overall ductility decreases and then increases with the continuous increase of twin spacing.Compared with the circular NT regions,the square and oblique square ones are found to provide higher overall strength and ductility.For the circular and oblique square NT regions,array arrangement tends to perform better in strengthening and toughening,while for the square NT regions,staggered arrangement is advisable.Secondly,a three-dimensional cohesive finite element framework have been built to study the influences of fracture properties of coarse-grained(CG)and NG phases,the distribution of CG regions,and fracture properties of interfaces on the strength and ductility of bimodal NS Cu.The calculations demonstrate that the cohesive strength of NG phase is decisive to the overall performances and it also affects the roles of both the cohesive strength of CG phase and the distribution of CG regions.When the CG regions are array-arranged in the interior of the entire microstructure,the best overall strength and ductility can be obtained.Besides these,both the overall strength and ductility get stabilized when the cohesive strength and the fracture energy of interfaces are larger than those of CG phase.In this dissertation,the influence of the volume fraction of the CG phase on the strength and ductility of two-dimensional bimodal NS Cu specimen have also been investigated by cohesive finite element method.The numerical results indicate that the overall strength and ductility can arrive at the maximum and stabilize.Under different volume fractions of CG phase,the critical cohesive strength of CG phase and NG phase,which makes the overall strength and ductility stabilize,may be distinct.It is believed that this dissertation has offered profound insights into the dependence of strength and ductility of two NS metals on microstructures,which can be stimulating to the metallurgical and manufacturing industries.