Micromechanical Properties of Beryllium and Other Instrument Materials.

Abstract : A program has been underway at the National Bureau of Standards since October 1977 to measure and understand the microcreep of materials used in gyroscopes so that dimensional instability can be improved. A methodology for microcreep testing has been developed and is described herein. Tensile microcreep test results between room temperature and 62 C on I-400 beryllium indicate that the microcreep strain does not exceed 1/100,000 in three months for applied stresses up to 20,000 psi. The microcreep rate of I-400 beryllium does not appear to be strongly temperature dependent in the range, 45 to 90 C. The phenomenon of microcreep exhaustion has been demonstrated experimentally and interpreted in terms of dislocation deformation processes. It is suggested that suitable micromechanical processing to promote microcreep exhaustion might serve as a method to improve dimensional stability of I-400 beryllium. The possible existence of a correlation between the microcreep stress required to produce a given strain rate and the 0.2% offset yield stress has been explored. This report includes descriptions of efforts to develop instrumentation to measure uniaxial strain at 62 C in the range, 1/10 to the 8th power to 1/10 to the 6th power and biaxial strains at room temperature in the range, 1/1000,000 to 1/1,000,000. (Author)