Factors influencing the creep strength of hot pressed beryllium
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HIS work is part of a program designed to improve the creep resistance of beryllium without degrading its room temperature ductility. Some of the material tested here has been the subject of a previous research program on creep behavior conducted by Pinkerton and Greene (Ref. 1), or has been tested extensively at room temperature by Webster et al (Ref. 2). In addition, the recrystallization and grain growth behavior of high pur ity, hot isostatically pressed (HIP) beryllium has been the subject of a further investigation (Ref. 3) which provides much relevant background information to the present work. 1.0 EXPERIMENTAL TECHNIQUE The materials used in this investigation are listed in Table I. RR243, BTP5, BSP9, and BSP10 are produced by hot isostatic pressing at 1176 K under a pressure of 103.5 MN/m 2 . The other materials are produced by conventional hot pressing at temperatures about 1350 K and pressures about 8 MN/m 2 . Trichlorosilane was added to RR242 powder to increase the silicon level before pressing. Creep testing was performed in compression on cylindrical samples with a diameter of 1.27 cm and a gage length of 2.54 cm. Tests were performed at temperatures between 1033 K and 1422 K. The change in specimen length during the tests was measured by a quartz rod extensometer seated on machined rings at the ends of the gage length. The extensometer and its instrumentation were capable of detecting creep rates down to 0.0002 pet per s. Thermocouples were welded at the top, middle, and bottom of the gage length. The specimens were heated by direct resistance heating which allowed rapid temperature changes to be made. Temperature equalization between different parts of the specimen was accomplished, where necessary, by jets of inert gas placed along the gage length. The specimen was maintained within 5K of the nominal temperature. Testing was performed under an argon atmosphere. Specimens that were to be annealed before creep testing were encapsulated in quartz with an argon atmosphere before annealing. The grain size is reported as the mean linear interD. WEBSTER and D. D. CROOKS are Research Scientists at the Lockheed Palo Alto Research Laboratory, Palo Alto, CA 94304. Manuscript submitted March 27, 1974. METALLURGICAL TRANSACTIONS A
cept of 200 grains. Grain sizes < 10 µm were measured by transmission electron microscopy. 2.0 RESULTS 2.1 Creep Properties Beryllium samples creep tested in compression show a brief region of primary creep followed by an extensive region of constant creep rate that for the materials listed in Table I extends to at least 10 pet strain. Typical creep strain and creep rate vs time plots for commercially pure, hot pressed block are shown in Fig. 1. Creep strength in the data below refers to the stress required to produce a given steady state creep rate (usually 10 -z or 10 -3 pct per s). 2.1.1 The Effect of Composition on Creep Properties. A relationship between creep strength and the concentration of aluminum, magnesium and silicon was first observed by Pinkerton and Green (Ref. 1)
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