Creep damage in a single crystal turbine blade alloy

摘要

The use of single crystal (SX) nickel-base superalloys will increase in the future with the introduction of SX blades into gas turbines for electricity production. Prolonged periods of use at high temperatures cause creep deformation and the assessment of creep damage can give large financial savings. At 950 deg C, it was found that the strengthening gamma-prime particles, initially cuboidal in shape, coalesced to form large plates or rafts to the applied stress. The cavity density also increased markedly during creep at 950 deg C. However, after creep at 750 deg C, no changes could be seen in the number of pores or in the size or shape of the particles in spite of a large amount of strain. The fact that no changes in the microstructure could be seen after creep deformation to failure at 750 deg C poses some serious problems. It was not possible to know that the material is damaged by looking at the microstructure. The results of this project show that CMSX-4 is a very stable alloy, but creep strain at 750 deg C followed by a suitable heat treatment (336 h at 1000 deg C) resulted in the formation of rafts. Apart from a heat treatment of 24 h at 950 deg C, the levels of porosity in CMSX-4 were largely unaffected by heat treatment, but a large increase in porosity was seen after creep at 950 deg C. Porosity cannot be used as a tool for assessing low temperature creep damage in SX alloys, but a suitable heat treatment produces large changes in the gamma-prime particle shape, causing them to form rafts. In CMSX-4, the heat treatment of creep deformed material to produce rafts is a possible method for the assessment of accumulated strain 19 refs, 35 figs, 3 tabs