Plastic Flow Behavior And Constitutive Relation Of BCC Metals

Since the second World War, especially since 1990's, because the region warshave ever broken out, a variety of high performance attacking weapons andcorresponding shield structures are urgently needed. The material property research ofthe projectile bodies, battle ships, aircrafts, and beneath shield structures have beenattracting the material scientist, metallurgist and mechanical researcher interests. Nowhuman being is increasingly exploring outer space, these structural materials of thespace application, and other materials using in the defence, aviation and astronautictechnique, and civil fields are in extreme work environment, e.g., the impact in highvelocity, the deformation in high temperature, and the larger plastic flow. Thesestructural materials are required to have good performance. Such, while new materials isbeing explore, the material behavior in extreme work environment must also be highlyunderstood in order to construct the base for practice engineering application. BCC(body-centered cubic) polycrystalline metals and those alloys mainly possessing BCCstructure often exhibit higher melting point, favorable heat conductivity, well fracturetoughness, considerably better corrosion resistance and weldability, these metals alwaysget scientist comprehensive interests. But the properties of the BCC metals stronglydepend on the temperatures, the deforming rate and the material microstructures.In the present paper, through systematically large amounts of the experimentalstudies and analyses, main purpose is to detect the plastic flow behavior of the BCCmetals and alloys over a wide range of temperatures and strain rates, and largerdeformation. Main content includes: the dynamic measuring technique for the materialproperty; the plastic flow characteristic and deformation mechanism of the materialsunder different temperatures and strain rates; and the constitutive relation study. Thestudy methods and contents are briefly described as following:(1) The dynamic measuring technique for the material property. Based on a lot ofexperimental property studies of the metal materials in the foretime years, especially inrecent three years, the present issue mainly emphasizes the split Hopkinson compressive bar technique, and how to achieve the couple methods of testing temperatures and strainrates. These metals include three kinds of tantalum (Ta), vessel steel Nitronic-50, andthree kinds of tungsten (W). The material property under low strain-rate tests arecarried out through electronic-mechanical driving testing machine, and conventionalservohydraulic machine. Combing the spilt Hopkinson bar with quasi-staticservohydraulic machine, Strains over 70% are achieved in these tests over a temperaturerange of 77-1,100K and strain rate of 10^-4-10⁴/s, these material plastic flow propertyare studied.(2) The plastic flow characteristic and deformation mechanism of the materialsunder different temperatures and strain rates. Based on the section (1) techniques, theuniaxial compression mechanical properties of three kinds of tantalum (Ta), vessel steelNitronic-50, and three kinds of tungsten (W) are measured, then relatinmg these resultswith accumulating other materials data of foretime years, the plastic flow characteristicand plastic flow mechanism of BCC metals are systematically analysed over atemperature range of 77-1,100K and strain rate of 10^-4-10⁴/s.(3) The constitutive relation study. In terms of plastic flow characteristic of theseBCC metals under different temperatures and strain rates, and their plastic flowprinciple and deformation behavior, based on the thermal activation glide mechanismthat dislocation overcome various obstacles and potential barriers, a unified constitutivemodel with physical concept are developed. The parameter of the model are defined andexplained. A practice constitutive model describing plastic flow stresses for BCC metalsare presented, the parameters of BCC model are obtained with the help of experiments,To verify the model, the experimental results are compared with the model predictions.Based on these systematically studies, main conclusions and achievement are asfollowing:1) In the present paper, a special split Hopkinson compressive bar withsynchronization mechanism are developed, this equipment have a trap function ofsecond stress loading pulse, with the help of this Hopkinson technique, the strain rateover 30,000/s and temperature over 1,100K can be obtained to measure plastic flow stress of materials at strain exceeding 70%. A couple testing of temperatures and strainrates can be achieved in this Hopkinson bar technique.2) Through a lot of analyses, it is found that, the plastic flow stresses of BCCmetels are very sensitive on strain rates and temperatures. But the loading history hasweak effect on plastic flow stress. The dynamic strain aging occurs at lower strain rates,the peaks in the region of dynamic strain aging will shift higher temperature, ordisappear with increasing strain rates.3) It is found that, the third dynamic strain aging happens (DSA) in all BCC(body-centered cubic) polycrystalline metals and these alloys mainly possessing BCCstructure. BCC metals do not have an initial pre-straining strain as the onset of the thirddynamic strain aging. The third DSA occurs in a matching coincidence of thetemperature and strain rate, its temperature region will shift to higher region withincreasing strain rates. Such, the mechanism of third DSA is thought as therapid/continuous formation of the solute atmospheres at the mobile dislocation core bythe pipe diffusion along vast collective forest dislocations to result in a continuous risecurve of flow stress.4) The plastic flow characteristic and mechanism of these BCC metals include that,the shear deformation at lower strain rates, adiabatic shear band occurs in higher strainrates, deformation twinning producing at low temperatures. There is the plasticviscous-drag mechanism in higher temperatures and higher strain rates.5) A unified constitutive model with physical concept are developed, this model issuitable to predict the plastic flow of metals. During deducing this model, theknowledge of solid thermal mechanics, material science, metallurgy and mechanics areused, such that, this model has remarkable physical concepts. Through comparing thetesting results with model predictions, the good agreement is obtained. These haveshown that the present model is reasonable and reliable.