The effects of microadditions of Mg on the mechanical behavior and fracture mechanism of MAR-M247 superalloy were investigated in this study. The microstructural observations and image analysis showed that a Mg microaddition ranging from 30 to 80 ppm significantly changed the primary MC carbide characteristics and inhibited the scriptlike carbide formation. After a 80 ppm Mg addition, the elongation measured at 1172 K increased over 3 times found that for the Mg-free MAR-M247 superalloy. The creep life and rupture elongation of the MAR-M247 superalloy with 80 ppm Mg was also improved up to 3 to 5 times that of the alloy without Mg during a 1033 K/724 MPa creep test. The fracture analyses demonstrated that cracks were mainly initiated and propagated at the interface of scriptlike MC carbides in the Mg-free MAR-M247 superalloy at elevated temperatures. The Mg microaddition effectively refined and spheroidized these coarse carbides so that a change in the crack initiation occurred from the carbide/matrix interface to that along the ␥-␥' eutectic. Interfacial analysis using Auger electron spectroscopy illustrated that Mg segregated to the interface of the MC carbide/matrix, causing a change in the morphology and interfacial behavior of the carbides. This improvement contributed to a prolonged rupture life and upgraded the moderate temperature ductility of the MAR-M247 superalloy.
Owing to the presence of a large amount of fine (Fe,Mn) 3 AlC carbides within austenite () matrix, the tensile property of the Fe-30%Mn-8.5Al%-2.0%C (in mass%) alloy in the as-quenched condition was clearly superior to that of the as-quenched FeMnAlC (C 5 1:3%) alloys investigated by previous workers. After being aged at 823 K for 3 h, the present alloy could possess high yield strength up to 1262 MPa with an excellent 32.5% elongation. With almost equivalent ductility, the yield strength obtained was about 16% higher than that of the FeMnAlC (C 5 1:3%) alloys after solution heat-treatment or controlled-rolling followed by an optimal aging at 823 K. Additionally, due to the pre-existing fine (Fe,Mn) 3 AlC carbides within the matrix in the as-quenched alloy, the aging time required for attaining the optimal combination of strength and ductility was much less than that of the FeMnAlC (C 5 1:3%) alloys aged at 823 K. When the present alloy was aged at 823 K for a time period longer than 4 h, both the strength and ductility were drastically dropped due to the occurrence of o = ( o : carbon-deficient austenite) lamellar structure on the = grain boundaries.
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