69 巻 (2005) 2 号 p. 272-275
Advanced high-temperature strengths in Ni-base single crystal (SC) superalloys have been achieved through the addition of Re in small amounts. However, Re has a strong tendency to form topologically close-packed (TCP) phases that decrease the creep strength, thus, increasing the Re content has undesirable consequences. This effect of Re addition is controlled in 4th generation superalloys where the formation of TCP phases is avoided by the addition of platinum-group metals (PGMs) such as Ru and Ir, and increased creep strengths have been achieved. Furthermore, PGM elements and solution strengthening elements including Re have been added in 5th generation superalloys to achieve superior high temperature strength. However, because Re is an expensive element and longer homogenization time is required for high-Re alloys, the use of this alloying element results in higher material and production cost, and lower productivity.
In this paper, a new type of Ni-base single crystal superalloy is proposed. This alloy does not contain Re but still reaches the high temperature capabilities of 4th and 5th generation superalloys, and has the advantages of low material cost and easy heat treatment. 1st generation single crystal superalloys do not contain Re, therefore, these are suitable material to be used as the master alloy for this alloy development. TMS-26, which was developed through High Temperature Materials 21 Project, NIMS, was used as the master alloy and, by the addition of Mo and Ru, a new single crystal superalloy TMS-174 was developed. Mo has the effect of controlling the lattice misfit and solid solution strengthening. Mo also promotes TCP formation, however this is avoided by the addition of Ru. The creep rupture life of TMS-174 at 900°C and 392 MPa was slightly shorter than that of TMS-138, which is a conventional 4th generation single-crystal superalloy with 5 mass%Ru. However, at 1100°C and 137 MPa, creep rupture life of TMS-174 was found to be 18% longer compared to TMS-138. This new alloy development strategy of adding solid solution strengthening elements and PGM elements to form non-Re containing superalloys was demonstrated to be successful. This strategy has the potential for superior alloy developments in the future because of the benefits of cost reduction and attainable material properties that are comparable to 4th and 5th generation superalloys.