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Although the above properties are noteworthy, creep behavior probably will play the determining role in deciding whether MA 754 is suitable for the intended application. Again, due to the variation of microstructure with orientation and the complex stress state expected in various components, creep tests were carried out on samples cut from the longitudinal and transverse directions of the original extrusion. Tables 4-1 & 4-2 present the results of creep testing for the longitudinal and transverse directions, respectively. The data was used to calculate the apparent stress exponent, n, in the Norton law [Dieter, 1986], A n (4-1) min a while the threshold stress was determined by the method of Artz [Artz, 1994]. (For materials exhibiting a threshold stress the minimum strain rate can be expressed as:) The apparent stress exponents, n, and threshold stresses, th, as a function of temperature are shown in Table 4-3. The threshold stress for the transverse direction was not calculated due to the large variation in the apparent stress exponent with temperature which implies a change in the relative contribution of dislocation creep, grain boundary deformation and diffusional creep to the observed creep behavior. Therefore a calculation of the threshold stress under these circumstances is not considered meaningful. The other point to note in Tables 4-1 and 4-2 are the creep elongations at failure. Generally, the longitudinal samples exhibit higher elongation at fracture. However, the values of elongation are quite low, especially in the transverse orientation and at the highest temperature. The fracture is virtually brittle in nature – not a desirable characteristic when employed as a pressure boundary material. Defects will be of a concern in component fabrication. Finally, the apparent activation energy for creep was calculated for the longitudinal orientation using the equation: min data at 900 and 1000°C and a stress level of 150 MPa. The mean of these two values is 630 kJ/mol. This initial value was refined by accounting for the temperature variation in elastic modulus using n a th Q min A E exp RT (4-1a) ln min Qapp R 1/T (4-2) where Qapp is the apparent activation energy. A value of 640 kJ/mol was calculated in this fashion using min data at 800 and 900°C and a stress level of 200 MPa; a value of 610 kJ/mol was calculated using equation (4-3) [Malu, et. al., 1975]: Qc Qapp nRT2 dE E dT (4-3) 58PDF Image | Development Of A Supercritical Carbon Dioxide Brayton Cycle
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