Micro Size Effects Of H65 Brass After Cold Rolling And Cryogenic Rolling

In precision instruments,medical equipments and microelectronics industries,there is an increasing demand for micro parts.Micro forming is most promising method to produce micro parts.However,when the one dimension of specimens reduces to a certain extent,the mechanical properties and physical performance will show a distinct size effect.Ultra-fine crystal materials have a tremendous application potential in MENS,and cryogenic rolling technology is the most effective method to manufact it.Therefore,it is of great significance to introduce cryogenic environment into the research of size effect.The microstructure evolutions were characterized by means of Optical Microscope,Transmission Electron Microscope,X-Ray Diffraction after cold rolling and cryogenic rolling.The results show that the equi-axial grains were elongated,the refinement of microstructure and the formation of fiber texture can be observed with the decreasing thickness.After small amount deformation,the main components of deformed microstructure are dislocation multiplication and dislocation cells,a small amount of deformation twins are observed.When the thickness reduced to 70?m,the dislocation density of cold-rolled specimen significantly decreased,whereas the cryo-rolled sample remains a high dislocation density.The XRD profiles of cold rolled and cryo-rolled samples showed that the deformation-induced transformation was not found,it can be noticed that the width of peak of cryo-rolled sample are more broden than cold rolled's.Size effects on the relationship between stress and strain of cold rolled and cryo-rolled H65 brass were investigated by micro tensile test.Statistical analysis revealed that with the decreasing thickness,the tensile strength firstly increase and then decrease for cold rolled specimen,while increase with reducing thickness in cryo-rolled samples,namely smaller is stronger.The influence of specimen thickness and rolling process on mechanical properties by microhardness test.For cold rolled and cryo-rolled samples,the hardness increased first but then dropped with the decreasing thickness.The conductivity and surface roughness were increased with increasing thickness by the measurement of conductivity and surface roughness.In order to acquire different grain size,the rolled sheets of H65 brass were performed at different heat treatment processes.The influence of annealing temperature and specimen size on grain size was studied.In the meantime,the recrystallization behavior of cryo-rolled H65 brass was analyzed by EBSD technique.The results indicated that for cryo-rolled H65 brass,the grain size increased with increasing treatment temperature when the thickness was the same.And the grain size decreased with the decreasing thickness when the treatment temperature was the same.3 different types of zones could clearly be observed from the EBSD micrograph of cryo-rolled H65 brass annealed at 250? for 30min,there are formation deformation bands,fine grains region and sub grains,respectively.The portion of high-angle grain boundaries increased as thickness decreased,and the orientation of grains was of random distribution.Tests of cryo-rolled and annealed H65 brass sheets were performed to investigate the influence of feature size on tensile strength,hardness,elasticity modulus and conductivity,and the relationship between grain size and yield strength were be explored.The conclusion were achieved that the tensile strength showed 'smaller is stronger' in cold rolled and cryo-rolled samples after annealed at 250? for 30min.And cryogenic rolling can simultaneously increase tensile strength and ductility compared with cold rolling.The hardness and conductivity decrease as thickness decrease.Yield strength decreased as grain size increased and meet the relation of Hall-Petch.