Creep of tungsten dispersion strengthened by ZrO₂ Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/tm70n059r

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  • Tungsten alloys containing approximately 0.75 and 2.3 vol pct Zr0₂ were prepared by powder metallurgical methods from blends of commercial tungsten powder and ZrW₂ powder. A heat treatment of the extruded rods at 1800°C was found to produce a stable structure with a dispersion of Zr0₂ particles in the tungsten matrix. The mean particle size was about 0.2 μm, the mean particle spacing about 2 pm, and the mean tungsten grain size about 7 μm. There was essentially no difference in microstructure between the two alloys except the 2.3 vol pct Zr0₂ alloy contained more of the coarse Zr0₂ stringers. The tensile strength of the alloys ranged fram 45 ksi at 1500°C to 25 ksi at 2000°C. Stress rupture lives were determined at 1375°C and 1740°C over a stress range of 10 to 60 ksi.Creep measurements were made over the temperature range of 1465° to 1924°C and over a stress range of 3.5 to 20 ksi. The activation energy for creep was found to be a constant 104,200 ± 3,200 cal per g mole °K over the entire range of stress and over the temperature range of 1500° to 1800°C. The activation volume was calculated as 1.3 x 10⁻²¹ cm³. The creep rate was found to vary linearly with stress in the low stress region (σ < 12 ksi), and to vary as stress to the power of 4.6 in the high stress region (σ > 14 ksi), The creep behavior of these tungsten alloys fits the functional relationships of stress and temperature given by the Ansell-Weertman model for dispersion strengthened alloys at both low and high stresses. However, the values of creep rate calculated from the model are about 10³ times greater than the experimental values. The Nabarro-Herring creep model fits the creep behavior of the tungsten alloys at low stresses both with respect to the functional dependence and the values of creep rate. A modification of the Ansell-Weertman model in the high stress region is made in which the area swept out by a dislocation corresponding to a unit of climb is taken as of the order of λ² where λ is the planar interparticle spacing. The density of active Frank -Read sources is taken as 10⁹ per cm³. This modified Ansell-Weertman model is shown to satisfy the stress and temperature relationships for creep of the tungsten alloys and to give values of creep rate which agree very well with experimental results. The modified model is also shown to provide the correction of 10⁴ needed to satisfy earlier experimental results on dispersion strengthened aluminum.
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