Microstructure And Mechanical Properties Of Multilayered Cu/Ti Composites Fabricated By Accumulative Roll Bonding

Metallic multilayered composites(MMCs)can combine the unique advantages of the constituent metals and improve a diversity of properties such as mechanics,electricity and magnetics,which are becoming the hotspot in materials science today.However,these materials often undergo deformation with large strain and cyclic load during the process of preparation and usage,which puts forward a very high request of the ability to resist plastic deformation.Therefore,it’s necessary to develop such kind of material and make research in its mechanical properties.In this study,we choosed the multilayered composites composed of pure copper and pure titanium as the studied materials.The composites were fabricated by accumulative roll bonding(ARB).The macroscopic lamellar structures,microstructure,distribution of misorientation and mechanical stability of the composites were investigated systematically by using scanning electron microscope(SEM),X-ray diffraction(XRD),energy dispersive spectrum analysis(EDS),transmission electron microscope(TEM)and electron back scattered diffraction(EBSD).Based on the observation of lamellar structures by SEM,Cu and Ti layers were well bonded during the co-deformation of the Cu/Ti multilayers.With the increasing number of ARB cycles,Ti layers with relatively large strength and work hardening ability started to neck,fracture and even segregate within the Cu matrix,which resulted in the most prominent feature of some lenticular nodules produced in the microstructure.This is attributed to the activation of shear bands across the multiple metal layers.At larger ARB cycles,the distribution of small fragmentations of Ti inside the Cu matrix is homogeneous.The results of EDS and XRD show that at the phase boundaries only diffusion between the constituent metals occurs and no metallic phases are formed during the repeated rolling processing.Through the technique of TEM,we found that there is a big difference of grain size in two phases of multilayered Cu/Ti composites after four ARB cycles.Meanwhile,in the Ti layers of the composites,there exist two characteristic morphologies:the intense dislocation tangles/networks and the sub-grain cells with a mean size less than 200 nm.For the Cu layers,a considerable amount of annealing twins together with significantly low dislocation density was the prominent feature.It can be found from the EBSD results that the Cu layers are mainly Goss texture,which is different from that of Cu rolled in monolithic form.The phenomenon is attributed to the effect of shear bands(SBs)in laminated structure.The orientation in Ti layers is concentrated on {0 0 0 1} basal fiber.At the same time,a few grains with other orientation are observed.So it can be inferred that in multilayered composites,the hetero-interfaces also have an effect on the evolution of texture in metals with hexagonal close-packed(HCP)structure.For the Ti layers within the composites,the hardness increases with increasing ARB numbers from 1 to 3 cycles and a small decrease of hardness is found after the fourth cycle.For the Cu layers,however,during the whole period of ARB the hardness only shows a minor increase.The fracture mechanism of the Cu/Ti composites is ductile fracture in each metal.Compared with the composites without annealing or with annealing at 300℃,the Cu-Ti interfaces in the composites annealed at 500℃ and 800℃ are much flatter after cold rolling,indicating that annealing suppresses the formation of shear bands.Although the composites experience annealing at high temperature,high ductility and considerable strength are retained,indicating the excellent mechanical stability of the composites after annealing.