Creep rupture in a nickel-based superalloy
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I.
INTRODUCTION
IN high-temperature alloys for use under small loads at intermediate temperatures (typically under 10 -3 to 10 -4/x at 0.4 to 0.6 Tin, where/x and Tm refer to shear modulus and melting temperature, respectively), damage accumulation on grain boundaries due to the nucleation and growth of creep cavities is the primary source of creep rupture, t~-Sl In superalloys, cavities are generally observed to nucleate on grain boundary carbides/matrix interfaces t6'7,Sj and suggested to grow by the constrained diffusive growth mechanism due to the rigid matrix surrounding cavitating facets, t9,1~ However, it is still not clear by which mechanisms cavity nucleation and growth occur in superalloys and how they are related to rupture time (ty). In the present work, creep rupture tests were conducted with RENI~* 80, and development of creep *RENI~ is a trademark of General Electric Company, Fairfield, CT.
cavities and surface microcracks was successively investigated from the specimen surfaces. Then, through the rupture time analysis based on the continual nucleation of cavities by Dyson, tll~ predictions by the constrained and unconstrained growth mechanisms are compared with experimental data. The present work also shows that it is possible to measure creep crack growth rate (v) from smooth bar creep tests under certain conditions. II.
EXPERIMENTAL PROCEDURE
A. Creep Tests The heats of RENI~ 80 used in the present work were prepared at the Korea Institute of Machinery and Materials (KIMM), and the chemical composition is given in Table I. The materials were received as cast rods with a diameter of 120 mm and grain size of ASTM No. 3 and heat-treated in 10 -2 to 10 -3 Pascal (about 10 -4 to 10 -5 tort) according to the standard heat treatment shown in Figure 1. Modified creep specimens with flat side surfaces were prepared for the easy observation of surface
cavities or microcracks as in Figure 2(a) and creep tested in air under constant load at 1033 and 1144 K; these tests were frequently interrupted for the subsequent observation of specimen surfaces by optical microscopy or scanning electron microscopy. Even though cavities were observed on fracture surfaces of ruptured specimens or polished surfaces of interrupted specimens, interrupted specimens generally could not be broken along grain boundaries even at 77 K, presumably due to zig-zag grain boundaries and the soft matrix. In order to quantify the level of cavitation (or microcracking), the total length of cavities (or microcracks) in a strip which covered the most necked region was measured as in Figure 2(b), and its relative ratio to the specimen width (w) was defined as parameter L. Height of the strip was taken as 1 grain diameter which was the repeating distance of cavitating facets in the load direction. The parameter is dimensionless and corresponds to the linear fraction of grain boundaries covered by cavities (or microcracks) when all the cavities (or microcracks) in the strip are assumed to lie in a flat grain boundary normal to the load axis
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