A. Baldan, COMBINED EFFECTS OF THIN-SECTION SIZE, GRAIN-SIZE AND CAVITIES ON THEHIGH-TEMPERATURE CREEP FRACTURE PROPERTIES OF A NICKEL-BASE SUPERALLOY, Journal of Materials Science, 32(1), 1997, pp. 35-45
The creep fracture characteristics of a conventionally cast (CC) MAR-M
002 superalloy, controlled by the grain-boundary diffusion mechanism,
have been investigated at various specimen section-sizes D, and grain
sizes, d. It is observed that the creep rupture strain (or ductility)
, epsilon(R), is controlled by the D-2/(n(G)/) ratio, where n(G) is th
e number of grains per cross-section of specimen and / is the half-cav
ity spacing, at the creep conditions (900 degrees C/300 MPa). A rapid
improvement in creep rupture life can be made by reducing the (d(C)/d)
/D ratio [or, equivalently, the (d(C)n(G))/D-2 ratio] below a critical
value (similar to 100 x 10(-8) mu m(-1)), where d(C) is the cavity si
ze. The thin-section size dependent creep rupture life, t(R)/D, and cr
eep rupture strain, epsilon(R)/D, are explained on the basis of grain
boundary sliding (GBS) and creep crack growth (CCG) behaviour of the a
lloy. epsilon(R)/D and t(R)/D can be improved by reducing the GBS rate
. A large improvement in t(R)/D can be achieved by reducing the GBS an
d CCG rates below the critical values of these rates by reducing the c
rack size through increasing the grain size above a critical value. (A
bove a critical grain size value the crack size becomes so small that,
as a result, a large increment of t(R) is achieved.)