Impression creep behavior of SiC particle-MoSi 2 composites
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Impression creep behavior of SiC particle-MoSi2 composites Darryl P. Butt, David A. Korzekwa, Stuart A. Maloy, H. Kung, and John J. Petrovic Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 8 September 1995; accepted 14 February 1996)
Using a cylindrical indenter (or punch), the impression creep behavior of MoSi2 -SiC composites containing 0–40% SiC by volume, was characterized at 1000 –1200 ±C, 258–362 MPa punch pressure. Through finite element modeling, an equation that depends on the material stress exponent was derived that converts the stress distribution beneath the punch to an effective compressive stress. Using this relationship, direct comparisons were made between impression and compressive creep studies. Under certain conditions, compressive creep and impression creep measurements yield comparable results after correcting for effective stresses and strain rates beneath the punch. However, rate-controlling mechanisms may be quite different under the two stressing conditions, in which case impression creep data should not be used to predict compressive creep behavior. The addition of SiC affects the impression creep behavior of MoSi2 in a complex manner by pinning grain boundaries during pressing, thus leading to smaller MoSi2 grains and by obstructing or altering both dislocation motion and grain boundary sliding.
I. INTRODUCTION
Molybdenum disilicide (MoSi2 ) is a candidate high temperature structural material due, in particular, to its excellent high temperature oxidation and corrosion resistance, good high temperature strength, high melting point, and high thermal conductivity.1,2 However, because of its relatively open, tetragonal (C11b type) crystal structure,3,4 one of the major limitations in the use of MoSi2 at elevated temperatures has been its relatively poor creep resistance.5–8 The purpose of this work was to both investigate the effects of a rigid particulate, silicon carbide (SiC), on the steady-state creep of MoSi2 , and to determine the usefulness of the impression creep method for characterizing the creep behavior of materials. Various investigators have used so-called indentation or impression creep techniques to characterize the creep behavior of materials.9–22 A flat-end, cylindrical indenter or punch, as used in these studies, appears to be the best geometry for impression creep measurements because the stress on the specimen does not change significantly with time or depth of penetration, as occurs with Knoop and Vickers indenters. Since the stress and material hardness remain constant during the impression creep test, steady-state creep can be observed using a cylindrical punch geometry. The general equation governing impression creep for a cylindrical indenter is as follows: i eÙss
∂n ∂ µ µ n 2Q 2m sapp exp Asdd . 2a B RT
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(1)
Contrast Eq. (1) with the equation for compressive creep: µ ∂ ° ¢n 2Q c eÙss Asdd2m sapp exp (2) . RT
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