Experimental Study On Yield Strength Of The VB Metals Under Extreme Condition

Recent theoretical studies revealed the anomalous pressure softening of the trigonal shear elastic modulus (C44) in the GroupVB metals (the fifth row in the periodic table of elements) due to an electronic band structure effect. However, there is still no direct experimental proof (such as sound velocity, etc) to support the theoretical prediction. Based on the physical connection between yield strength and shear modulus described by the dislocation theory and the constitutive model, it is feasible to examine the validity of the theoretical prediction by experimental determining the yield strength of the Group VB metals under static loading. However, among the Group VB metals, the experimental results of yield strength and theoretical prediction achieves good consistency for V only, while for Ta and Nb there is discrepancy either among different experimental results of yield strength or between the experimental studies and theoretical studies. It is pointed out that both deficiency/limitation in experiment techniques and/or possible physical reasons (such as preferred orientation, structure transformation, pressure-induced material damage, etc.) are responsible for the discrepancy. Consequently, this study focuses on two issues:(1) the pressure gradient method, by which the yield strength of Ta and V were determined in the previous studies, was studied comprehensively. And then the experimental results of yield strength of Ta and V reported in the previous studies were re-analyzed and re-discussed. By proposing a reasonable principal for experimental design, the pressure gradient method was further developed and improved;(2) the yield strength of Ta and Nb at high pressures was measured with the line width analysis method, which afforded experimental data for examining the theoretical model.The representative conclusions achieved in this work are as following:1. Considering the deficiency/limitation in experiment techniques in the pressure gradient method, a comprehensive study was performed with finite element modeling (FEM) and high-pressure experiments. The main conclusions including that:1) the anomalous decrease in yield strength of Ta reported is connected with incorrect experimental design (for example interference from the gasket and deformation of the diamond anvils). So the anomalous softening of the yield strength of Ta in the previous study can not be only ascribed to pressure. The study on V in the previous study takes a more reasonable experimental design and so the possible interference is excluded or alleviated.2) the pressure gradient method is further developed and improved by putting forward a reasonable principal for experimental design, improving analysis method of experimental data, and revealing the elastic-plastic deformation history of sample at high pressures.3) the SrB4O7:Sm2+pressure scale was synthesized and proposed as a substitute of ruby scale which was proved deficient in the pressure gradient method. The hydrostatic and non-hydrostatic calibrations of SrB4O7:Sm2+optical pressure sensor were calibrated respectively. Comparing to the previous study, the non-hydrostatic calibration is extended up to127GPa. An accurate high-temperature pressure scale was calibrated by combining the coupling effect of pressure and temperature on the wavelength shift of the fluorescence lines of SrB4O7:Sm2+. With the developed optical pressure sensor SrB4O7:Sm2+, the pressure gradient method can be applied to measure the yield strength of materials at much higher pressure and temperature. In summary, the improved pressure gradient method provids a reliable experimental method to determine yield strength under static loading, which has quite importantly scientific signification for establishing accurate constitutive relationship.2. The line width analysis method was studied comprehensively and then a basic principal for experimental design was proposed. Based on that the yield strength of Ta and Nb was determined and the main conclusions are:1) the maximum experimental pressure reaches101GPa and61GPa for Ta and Nb respectively. The anomalous softening of yield strength is found at52-84GPa for Ta and at30～42GPa、42～50GPa for Nb respectively. The possible effect of preferred orientation, structure transformation and pressure-induced material damage on the yield strength is excluded by investigating changes or nonuniform distributions of the intensity around Debye-Scherrer diffraction rings, diffraction peaks broadening and splitting, and the volume discontinuity in isotherm. Therefore, the anomalous softening of yield strength in Ta and Nb is arisen from the pressure softening.2) the yield strength-pressure relationship, and the pressure range in which the yield strength soften in present result both are well consistent with the theoretical model prediction (50-80GPa and20～50GPa for Ta and Nb respectively by theoretical prediction). The good agreement between the experiment results and theoretical prediction strongly supports the theory model proposed by Landa et al., which is also helpful to understand the physical mechanism of pressure softening of yield strength and the unique high-pressure physical properties of the GroupVB metals.