Effect of orientation on the tensile and creep properties of coarse-grained INCONEL alloy MA754
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NTRODUCTION
INCONEL* alloy MA754 is a nickel-based, oxide*INCONEL, INCOLOY, and NIMONIC are registered trademarks of Special Metals Corporation, Huntington, WV.
dispersion-strengthened (ODS) alloy produced by a powder metallurgy process. A yttrium oxide dispersion (0.6 wt pct) introduced into a Ni-20 wt pct Cr matrix by a mechanical alloying process[1] results in excellent creep strength to temperatures approaching 1200 °C.[2,3] In the annealed condition, the creep strength is enhanced by an elongated grain structure in the direction of prior working;[4,5] a strong [110]100 texture[6] enhances the thermal fatigue resistance.[7] MA754 currently finds application in gas turbine engines and furnace fixturing. This alloy is being considered for use as tubing in the secondary flow loop of advanced gas-cooled pebble-bed reactors, for which supercritical CO2 is being studied as a potential working fluid in the secondary flow loop. Supercritical CO2 will provide increased power generation efficiency over helium due to its higher density and pressure at the critical condition. The operating conditions for tubing are expected to include temperatures from 850 °C to 950 °C and CO2 pressures of 10 MPa. The properties of MA754 are highly anisotropic due to the texture and elongated grain structure. The elevated temperature creep strength in particular is much higher along the long axis of wrought products (longitudinal orientation) than perpendicular to this axis (long and short transverse orientations). The creep behavior in the longitudinal direction has been extensively characterized;[2,5,8–10] as with other ODS alloys, MA754 exhibits so-called “threshold” behavior,
TERRY C. TOTEMEIER, Advisory Engineer, and THOMAS M. LILLO, Staff Engineer, are with the Idaho National Engineering and Environmental Laboratory INEEL, Idaho Falls, ID 83415. Contact e-mail: totetc@ inel.gov Manuscript submitted May 18, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
which is characterized by high apparent stress exponents and activation energies in the typical creep equation:[11] Qapp sa n # min Aa b expa b E RT
[1]
# where emin is the minimum or steady-state creep rate, A a material and structure-dependent constant, a the applied stress, E the elastic modulus, n the apparent stress exponent, Qapp the apparent activation energy, R the gas constant, and T the absolute temperature. The high values of the apparent stress exponent and activation energy are commonly accounted for by substituting an effective stress term, a th, for a in Eq. [1], where th is a “threshold” or “resisting” stress. The origin and best method of calculating the threshold stress are still debated.[11,12] The grain aspect ratio (GAR) has been shown to play a strong role in the creep of ODS alloys in general and the threshold stress behavior in particular, with higher GAR resulting in better creep properties and an increased threshold stress.[4,5] The transverse properties of MA754 have been less well studied. Whittenberger[5] reported on the high-temperature
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