Deformation of Single Crystal Molybdenum at High Pressure

Bonner, B P ; Aracne, C ; Farber, D L ; Boro, C O ; Lassila, D H

Lawrence Livermore National Laboratory

关键词: Compression;    Pneumatics;    75 Condensed Matter Physics, Superconductivity And Superfluidity;    Fluorescence;    36 Materials Science;   

DOI  :  10.2172/15009799
RP-ID  :  UCRL-TR-202579
RP-ID  :  W-7405-ENG-48
RP-ID  :  15009799

美国|英语

来源: UNT Digital Library

【 摘 要 】

Single crystal samples of micron dimensions oriented in the [001] direction were shortened 10 to 40% in uniaxial compression with superposed hydrostatic pressure to begin investigation of how the onset of yielding evolves with pressure. A testing machine based on opposed anvil geometry with precision pneumatic control of the applied force and capability to measure sub micron displacements was developed to produce shape changing deformation at pressure. The experiments extend observations of pressure dependent deformation to {approx}5Gpa at shortening rates of {approx}2*10{sup -4}. Samples have been recovered for post run characterization and analysis to determine if deformation mechanisms are altered by pressure. Experiments under hydrostatic pressure provide insight into the nature of materials under extreme conditions, and also provide a means for altering deformation behavior in a controlled fashion. The approach has a long history demonstrating that pressure enhances ductility in general, and produces enhanced hardening relative to that expected from normal cold work in the BCC metals Mo, Ta and Nb{sup 2}. The pressure hardening is in excess of that predicted from the measured increase in shear modulus at pressure, and therefore is likely due to a dislocation mechanism, such as suppression of kink pair formation or the interaction of forest dislocation cores, and not from lattice resistance. The effect has not been observed in FCC metals, suggesting a fundamental difference between deformation mechanisms at pressure for the two classes. The purpose of this letter is to investigate the origin of pressure hardening with new experiments that extend the pressure range beyond 3 GPa, the upper limit of conventional large sample (1cm{sup 3}) testing methods. Most previous high pressure deformation studies have been on poly crystals, relying on model dependent analysis to infer the maximum deviatoric stress that a deformed sample can support. In one experiment, a single crystal of the silicate olivine was compressed at 16 GPa with a sapphire uniaxial stress gage in the sample chamber. Splitting of the ruby fluorescence line increases with deviatoric stress, enabling direct measurement of the sample stress. Unfortunately, this method is not sensitive enough to determine first yielding of [001] single crystal Mo, and was not used in this study.