Influence of precipitate morphology on intermediate temperature creep properties of a nickel-base superalloy single crys
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I. I N T R O D U C T I O N
A lamellar 3,-3/' structure naturally develops during high temperature creep in many single crystal superalloys. These lamellae, or rafts, are considered to be beneficial for high temperature creep resistance, provided that they are continuous, finely spaced, and relatively perfect, t1~1 Although some initial results showed that "prerafting" could also improve creep resistance at intermediate temperatures, t2] a few experiments have indicated that the rafted structure is not beneficial under all testing conditions. For example, it has been found that specimens which had a pre-rafted microstructure formed by creep testing above 1000 ~ had reduced creep resistance at lower temperatures (850 to 925 ~ compared to as-heat treated specimens with cuboidal 3,,j5,6] Additionally, specimens with a rafted structure exhibited lower yield strengths at 982 ~ than specimens with cuboidal precipitates. ]7~ The purpose of this work is to examine in more detail the relative creep behavior of rafted (pre-crept at 1000 ~ and cuboidal (as-heat treated) 3" microstructures at 760 ~ This temperature was chosen on the belief t4,9] that directional coarsening would be suppressed, thus permitting an uncomplicated comparison of the two different microstructures.
were solution treated at 1300 ~ for 4 hours and air cooled. Tensile specimens with 3.8 mm diameter and 20 mm gage length were machined by grinding. The specimens were then re-solutioned at 1300 ~ for 10 minutes and air cooled in order to refine the as-heat treated 3" size. Machined specimens with longitudinal axes within 5 pct of [001] were used for creep testing. Creep tests were performed at 760 ~ using constant load lever arms, and temperature was measured with Type R thermocouples attached to the specimen gage length. Creep strain was measured using extensometers attached to grooves in the specimen shoulders. Specimens were tested in either the as-heat treated or in the pre-rafted condition, The pre-rafting treatment consisted of tensile creep at 1000 ~ and 148 MPa for 50 hours, followed b y cooling under load. The pre-rafted samples were cleaned and remeasured before further testing at 760 ~ Whereas some tests were run to failure, others were interrupted at various stages of creep and cooled under load for examination by scanning (SEM) and transmission (TEM) electron microscopy. Specimens for optical or SEM observation were prepared metallographically and etched by swabbing with 33 pct H 2 0 , 33 pct HNO3, 33 pct CH3COOH, and 1 pct HF. TEM foils were electropolished with a solution of 5 pct perchloric acid, 45 pct acetic acid, and 45 pctbutylcellosolve at 4 ~ and 10 volts.
II. M A T E R I A L S A N D P R O C E D U R E S
Directionally solidified single crystals of NASAIR 100 were procured from a commercial vendor. NASAIR 100 (analyzed composition in weight percent: 9.5 Cr, 5.5 A1, 1.2 Ti, 3.2 Ta, 10.0 W, 1.0 Mo, balance Ni) is a first generation single crystal alloy which has been characterized in some detail previously, t6,81 The as-received bars M.
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