Creep regimes for directionally solidified Al-Al 3 Ni eutectic composite
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I.
INTRODUCTION
A L I G N E D composites produced by unidirectional solidification of eutectic alloys have been shown to display good fiber strengthening even at high temperatures, tl,zm The need for a better understanding of the creep behavior of these materials has come from the anticipated application of these materials for hightemperature service. However, characterizing the creep behavior of these materials in terms of creep parameters used for single-phase materials is not simple, since the reported values of the stress exponent for creep of aligned composites are, in general, substantially larger than the values for constituent components alone, rl,2,3~ Himbeault and Cahoon t4~ described this phenomenon in an article on the creep of directionally solidified Al(rich)-Ni eutectic. Other observers have rationalized the strong dependence of the creep rate on stress and temperature by considering that creep deformation takes place not under the full effect of the applied stress but under an effective stress (0- - tro), where o% is an experimental constant often called the threshold stress. ~5,6~ By including tro, the creep of the composite can be expressed in terms of the creep properties of the matrix phase of the composite. In the present investigation, constant load creep tests over a wide range of stress were carried out on A1-A13Ni eutectic specimens in an effort to characterize more completely the creep behavior of directionally solidified composites and to provide an increased understanding of the creep deformation of fiber composite materials.
II.
EXPERIMENTAL PROCEDURE
The A1-AI3Ni eutectic was prepared by melting approximately 350 g of high-purity aluminum (99.999 pct) in a graphite crucible and adding the appropriate amount D.D. HIMBEAULT, formerly Graduate Student, Department of Mechanical and Industrial Engineering, University of Manitoba, is Research Scientist with Atomic Energy of Canada Limited, Whiteshell Nuclear Establishment, Pinawa, MB ROE 1L0, Canada. J.R. CAHOON, Professor, is with the Metallurgical Sciences Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB R3T 2N2, Canada. Manuscript submitted May 8, 1989. METALLURGICAL TRANSACTIONS A
of nickel (99.99 pct) to give the 6.2 wt pct Ni eutectic composition. Argon was bubbled through the melt for a few minutes prior to casting to remove oxygen. The alloy was cast into rods 160-mm long by 25-mm diameter using graphite molds. Directional solidification was achieved by placing the alloy and graphite mold in a traveling resistance furnace, remelting the alloy while under an argon atmosphere, and raising the furnace from the melt at a constant rate of 2.2 • 10 -2 m m / s , such that solidification occurred in the vertical direction from the bottom to the top of the ingot. The directiona!!y solidified ingots were then cut into blanks with an abrasive saw, and the blanks were milled to approximately 2.4 m m thickness. Flat tensile specimens with gage lengths of 20 m m and 38 m m parallel to the gr
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