Severe Plastic Flow Behavior And Constitutive Theory Of Ultra-fine Grained Copper

At present, compared with all kinds of methods to improve strength of metals, grainrefinement, which could improve the strength of materials and not lost the toughness of manyalloys, can be considered as the best choice. Among all kinds of severe plastic deformationprocesses to obtain grain refinement, equal channel angle extrusion (ECAE), which is atypical and effective process, has been applied to realize the grain refinement of metals.Because of the widely application in many fields, grain refinement of commercially purecopper has been focusing on by researchers.In this paper, to demonstrate the severe plastic flow behavior and constitutive relationsof ultra-fine grained copper (UFG Cu) processed by ECAE, the author tested the mechanicalproperties and hardness of UFG Cu with different number of passes at different strain rates,calculated the strain rate sensitivity (SRS) and thermal activation volume (V*) with differentnumber of passes, and then analyzed the deformation mechanism of UFG Cu. In addition,based on the establishment method of constitutive relations for typical metal materials, suchas DH-36steel and other metals and alloys, the author learned and obtained the constitutivetheory of UFG Cu. The main contents of this paper are as following:(1) To test the severe plastic deformation behavior of UFG Cu at different strain rates, theMTS test machine and split Hopkinson pressure bar (SHPB) have been employed to test themechanical properties and the strain rates are10-3/s and~4,000/s. The results show that, atquasi-static strain rates (10-3/s), with pass numbers (or equivalent strain) increasing, the flowstress increases to a peak value and then decreases to a equilibrium state; while at high strainrates (~4,000/s), with strain increasing, the flow stress increases sharply and then saturates.(2) To demonstrate the relation among hardness, number of passes and strain rates of UFG Cu,the author obtained the hardness value of this material with different strain rates andequivalent strains, based on nanoindentation technique, and then calculated strain ratesensitivity (SRS) and thermal activation volume (V*). The results show that with equivalentstrain increasing strain rate sensitivity of ECAE Cu increases, while the thermal activationvolume decreases firstly and then saturates.(3) To further learn grain boundary misorientation effect on the flow stress, based on a three-parameter constitutive model on the basis of dynamic recrystallization and grain boundarymigration theory proposed by Gourdet, the relation between flow stress and grain boundaryorientation, grain size and dislocation density inside sub-grains has been discussed, the results show that, the flow stress decreases with the fraction of high angle grain boundary (HAGB)increasing, and so, increasing trend of the strain rate sensitivity can be explained.(4) To learn the microstructure evolution of UFG Cu with different number of passes, theauthor tested microstructure of four typical specimens by Electron Back Scattered Diffraction(EBSD). It shows that, the fraction of HAGB increases to41%and the grain size decreases to341nm when the pass numbers in crease to32; and for the specimens with the same passnumbers (equivalent strain), the microstructure, such as grain size and fraction of HAGB, willaffect the mechanical properties of ECAE Cu.(5) To obtain the constitutive model of UFG Cu under severe plastic deformation, combinedwith experimental results and empirical constitutive method, the author systematically learnedthe physically-based constitutive model based on several typical metals and alloys, such asDH-36steel and other metals. However, the author found that these constitutive models arenot suitable for the severe plastic deformation materials. Therefore, borrowed the researchideas and methods of these constitutive models, based on the nanoindentation results of UFGCu, a new constitutive model coupled with strain rates has been developed, and goodagreement between the model predictions and the experimental results has been reached.