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Creep and creep rupture behavior of an advanced silicon nitride ceramic were systematically characterized in the temperature range 1150" to 1300°C using uniaxial tensile creep tests. Absence of tertiary creep and the order-ofmagnitude breaks in both creep rate and rupture lifetime at certain threshold combinations of stress and temperature were two characteristic features of the creep behavior observed. Thermal annealing was found to have enhanced both subsequent creep resistance and creep rupture life. The stress exponent ( n ) and the activation energy ( Q ) defined in the Norton relation were found to be 12.6 and 1645 kJ/mol for the material investigated. Both values appear to fall in the general range of those reported for other but similar types of Si,N, ceramic materials. The stress exponent, m, equivalent to the slope of the LarsonMiller equation was found to be in the range 13 to 14.4, and that defined asp in the Monkman-Grant relation to be 0.91, based on the available experimental data. The values of m, n, and p obtained above approximately support the interrelationship of the three exponents given byp = mln.

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Data obtained from creep and creep-rupture tests conducted on 18 heats of Alloy 718 were used to formulate models for predicting high temperature time dependent behavior of this alloy. Creep tests were conducted on specimens taken from a number of commercial product forms including plate, bar, and forging material that had been procured and heat treated in accordance with ASTM specifications B-670 or B-637. Data were obtained over the temperature range of 427 to 760°C and at test times to about 87,000 h. Comparisons are given between experimental data and the analytical models. The analytical models for creep-rupture included one based on lot-centering regression analysis, which uses a single heat constant to distinguish between behavior of individual heats, and one based on the Minimum Commitment Method, which used two heat constants. A 'master" curve approach was used to develop an equation for estimating creep deformation up to the onset of tertiary creep.

Results from elevated temperature-strain controlled fatigue and constant-strain-rate tensile tests conducted on specimens of stainless steel Types 304, 304L (titanium modified), 316, as well as Incoloy 800 are reported. Specimens were irradiated to fluences of 0.4 to 5 × 1022 n/cm2, E>0.1 MeV at 700 to 750 C (1292 to 1382 F), while the postirradiation test temperature was maintained at 700 C. Reductions in tensile ductility and fatigue life occurred, with reductions in fatigue life ranging from factors of approximately 1.5 to 2.5 for the stainless steels and up to 35 for Incoloy 800 in comparison with the thermal controls. Comparisons are made between actual irradiated fatigue behavior and predictions based on several semi-empirical methods using irradiated tensile data. These methods generally provided good estimates of the irradiated fatigue behavior of these materials. Introducing tensile hold times into the fatigue cycles of irradiated and unirradiated Type 316 stainless steel resulted in substantial reductions in the fatigue life of this material. However, for tensile hold times in excess of 0.1 h a tendency towards saturation of the hold-time effect was found in both the irradiated and unirradiated material. Creep and fatigue damage for Type 316 stainless was determined and summed linearly. This total damage was found to be a function of strain range, duration of tensile hold time, and irradiation condition for Type 316 stainless steel.

This paper summarizes recent experimental results, obtained at Oak Ridge National Laboratory (ORNL), on creep behavior and creep rupture of a commercial grade of Si3N4 ceramic in the temperature range of 1150°C to 1300°C. A uniaxial model capable of describing the behavior under general thermomechanical loading is introduced and compared with existing models. An exploratory extension of the new model to a multiaxial form is then discussed. Issues are also discussed concerning the standardization of data analysis methodology and future research needs in the area related to development of creep database and life prediction methodology for high temperature structural ceramics.

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