B. Dubiel (Sp), A. Czyrska-Filemonowicz, AGH University of Science and Technology, Krakow (Poland); D. Bale, ALSTOM Power Turbo-Systems Technology Centre, Whetstone, Leicester (UK); M. Blackler, Howmet Ltd., Exeter (UK)
Increases in the operating temperature of industrial gas turbines are driving the requirement for high temperature single crystal nickel alloys for turbine blade and vane applications. CM186LC superalloy is the candidate for vanes application. The nominal chemical composition of the CM186LC alloy is as follows: Ni- 9.3Co-8.4W- 6.1Cr- 5.7Al- 3.4Ta- 2.9Re- 1.4Hf- 0.73Ti- 0.51Mo- 0.062C- 0.016B- 0.004Zr (wt %). CM186LC investigated was supplied in the form of solid cast bars of <001> orientation. Detailed microstructural investigations were conducted using LM, SEM and TEM methods. Creep tests were performed at 750°C at stresses 560 and 675 MPa. Both fractured and terminated creep specimens were selected for detailed microstructural investigations.
The microstructure of not deformed CM186LC consists of pronounced dendrites containing about 70% volume fraction of cuboidal gamma prime particles precipitated inside gamma matrix and 20-25% of eutectic gamma-gamma prime areas within interdendritic regions. TEM microstructural analysis of creep tested specimens revealed the differences of the microstructure in the dendritic and eutectic regions.
In specimens deformed at 560 MPa and terminated during primary creep stage, the high density of dislocations in the gamma channels, as well as shearing of gamma prime precipitates by stacking faults are observed in the dendritic regions. TEM analysis showed that dislocations in gamma channels have 1/2 <112> Burgers vector. All stacking faults lie in the same {111} plane and they are either intrinsic or extrinsic. After creep rupture at higher stress of 675 MPa besides stacking faults, large deformation twins are present.
In the eutectic regions of the specimens creep ruptured at 750°C, independent on stress applied, single dislocations, stacking faults and dislocation dipoles are observed within large gamma prime precipitates, besides dislocations in gamma channels. It can be postulated that heterogeneities in the CM186LC microstructure are detrimental to its creep properties.
