Preparation And Mechanical Properties Of Heterostructured Pure Ti And Cu-30%Zn Alloy With Multimodal Grain Size Distribution

Bulk ultrafine-grained(UFG)metallic materials with high strength are promising in applications in light weight and energy saving.However,numerous studies have shown that,due to the small grain size,the ductility of bulk UFG materials is worse,comparing with traditional coarse-grained materials.Since good ductility is urgently needed in industrial applications,overcoming their low ductility have attracted considerable attention in the field of materials science.In recent years,it has been widely reported that the construction of heterostructure with multimodal grain distribution can significantly improve the tensile ductility of bulk nanostructured and UFG materials and enable unparalleled combination of strength and ductility.Therefore,the fabrication of heterstructured metallic materials with multimodal grain distribution,the microstructure evolution as well as the mechanical property and the relationship between them will be investigated in this work.The commercial pure titanium and brass were processed by equal channel angular pressing(ECAP)method to prepare bulk UFG samples,and then were annealed under different heat treatment conditions to prepare heterostructured materials with multimodal grain distribution.The microstructure evolutions of UFG pure titanium and brass under different annealing conditions were revealed by various means of micro-analysis,and the tensile deformation mechanisms of the heterostructured materials with multimodal grain distribution were critically appraised.The essence of their excellent mechanical properties was revealed based on the framework of hetero-deformation induced(HDI)hardening.The main results of present work are concluded as below.(1)The heterostructured pure Ti with multimodal grain distribution(Multi-Ti),prepared by the four-pass ECAP deformation at room temperature combined with the partial recrystallization annealing at 500 ? for 30 min,possesses excellent combination of strength and ductility.The grain size was mainly distributed in the range of 200 nm-3.5 ?m,and the volume ratio of recrystallized micron-grain zone(soft domain)over ultrafine-grain zone(hard domain)was about 3:7.The Multi-Ti has a high yield strength and ultimate tensile strength of?800 MPa and ?950 MPa,combined with an uniform elongation and elongation-to-failure of?13% and ?28%.The excellent combination of strength and ductility is mainly attributed to the fact that the soft-hard domain interfaces yeild a large number of geometrically necessary dislocations(GNDs)accumulation arrays upon compatible deformation,resulting in a strong long-range internal stress,that is,the back stress,which reacts on the dislocation source and appear to strengthen the soft domains.At the same time,the corresponding forward stress is formed in the hard domains,which helps the hard domains to deform.This changes the local stress state from the uniaxial stress state to the multiaxial stress state,which is beneficial to coordinate the overall plastic deformation.(2)In the process of tensile deformation of Multi-Ti,dislocation multification not only occurs in the micron sized grains,but also accumulates in the ultrafine grains.According to the dislocation invisible criterion,it is found that micron intracrystalline high-density dislocation tangles of basal <a> dislocation and pyramidal <c+a> dislocation and some <c>dislocations under two-beam conditions.The excellent tensile ductility of Multi-Ti is mainly attributed to its high strain hardening ability and relatively high dislocation storage capacity.(3)The heterostructured brass with multimodal grain distribution was prepared by the ECAP deformation combined with partial recrystallization annealing process at 250 ? for200 min.However,as a typical heterostructured material,excellent mechanical properties were not successfully achieved and still followed the traditional law of strength-ductility trade-off.The main reason is that although there are obvious soft and hard domains in ECAP+250 ?-200 min brass,the difference in flow stress between the hard domains and soft domains is too pronounced.After a small amount of deformation,it is difficult for the soft/hard interface to coordinate deformation due to excessive stress concentration,forming microcracks and fracture failure.(4)For heterostructured metal materials with multimodal grain distribution,there are many factors that influence HDI stress.In this study,it is found that when the volume ratio of soft and hard areas is similar,the higher the density of soft/hard domain interface is,the greater the HDI stress is.In order to boost the HDI strengthening effect,the hard domain must hold certain plastic deformation capacity and the difference of the flow stress of plastic deformation with the soft domain may not be too huge.