Mechanical Properties And Thermostability Of Nano-laminated Structured Pure Tantalum

In this work nano-laminated structured pure tantalum was processed by equal channel angular press(ECAP)followed by unidirectional rolling.Nano-laminated structured tantalum with thickness of 40-200 nm can be achieved by controlling ECAP passes,rolling reduction and rolling temperature.The microstructure evolution of nano-laminated structure processed in different deformation conditions was characterized by electron backscatter diffraction(EBSD)and transmission electron microscope(TEM).Mechanical properties were characterized by microhardness,tensile and nanoindentation tests.The effect of deoformation procedure and microstructure on the thermostability was also researched.The result shows that the misorientation of low angle grainboundary increased continually during rolling of pure tantalum processed by 4 passes ECAP,inducing the increase of the fraction of high angle grainboundary;due to the fraction of high angle grainboundary becoming saturation after 8 passes ECAP processing,the remarkable enhancement of texture leads to the decrease of high angle grainboundary after subsequent rolling.The investigation indicated that nano-laminated structure with minimum thickness was achieved by 8 passes ECAP via route A followed by unidirectional rolling in liquid nitrogen temperature.With the increase of rolling reduction,the thickness of nano-lamella reduced constantly and decreased to even about 43 nm when the rolling reduction reached 90%.However,the lamellar thickness shows certain deviation from the theoretical calculating value of pure tantalum in rolling.When the rolling reduction was less,the propagation,piling up and tangling of dislocations formed cellular structure,contirbuting extra refining effect,inducing the less lamellar thickness than the theoretical value;When more rolling reduction was introduced,recovery effect was dominant in the formation of nano-lamella,leading to a higher lamellar thickness than the theoretical value.The tensile strength and microhardness increased with refining of the thickness of nano-lamella.However,the elongation and strain rate sensitivity reduced at the same time.When the thickness of nano-lamella reduced below 100 nm,the relation between microhardness and thickness of nano-lamella deviated from the Hall-Petch relationship.The fracture model changed from ductile fracture to brittle fracture and the elongation decreased obviously.In addition,when the rolling reduction increased,the texture intensity and the fraction of low angle grain boundary in nano-laminated structured tantalum processed by 8 passes ECAP followed by unidirectional rolling increased,leading to the increase of thermostability.The recrystallization temperature of tantalum with 90%rolling reduction was higher than that processed with 20%reduction for about 100 ?.Also the recrystallized grain size decreased with the increase of accumulated deformation before annealing.