The role of boron and carbon on the defect tolerability of a nickel base single crystal superalloy was investigated. In order to elucidate the effect of boron and carbon on the alloy, an experimental nickel base single crystal superalloy called RR2072 was selected as the BASE alloy. Three new alloys were designed by modifying the composition of the BASE alloy and were regarded as the MOD alloys. Single crystal bars and bicrystal slabs of each alloy were fabricated by the modified Bridgeman method. Bicrystal slabs were classified into three categories having low, medium, and high misorientation angles which were about 6, 12, and 18 degree, respectively.
The solutionizing temperature of the MOD E alloy containing 0.010 wt% boron and 0.05 wt% carbon was determined to be 1295℃ which was 25℃ lower than that of the BASE alloy, to prevent local incipient melting. Relatively low temperature solutionizing of the MOD E single crystal resulted in inhomogeneous microstructure incorporating undissolved coarse eutectic γ / γ in the interdendritic region. Two kinds of MC carbide, blocky type and script type, were observed in the interdendritic region of MOD E.
In order to evaluate the effect of carbon and boron on the grain boundary strengthening, stress rupture tests of the bicrystal specimens were undertaken at 950℃ and 1050℃. The rupture lives of the BASE bicrystal specimens decreased sharply with increasing misorientation angle. On the other hands, the MOD alloys showed relatively long lives even at high misorientation angle implying significant grain boundary strengthening effect of carbon and boron. The rupture life of the MOD E alloy was superior to those of the other alloys for all test conditions.
Thick bands of γ´ phase appeared along the grain boundaries of the BASE alloy isothermally exposed at 950℃ and 1050℃. Within the γ´ bands, the grain boundary precipitates which have more than 40 wt% of Re contents were found. The phase was identified to be as P phase. In the MOD alloys carried out similar heat treatments, the γ´ bands were relatively thin compared with that of the BASE alloy. Furthermore, $M_{23}C_6$ carbides instead of P phase precipitated along the boundaries of the MOD alloy bicrystal. The grain boundary strengthening of the MOD alloys might be attributed to the thin γ´ bands formation as well as $M_{23}C_6$ carbides precipitation on the boundary of the MOD alloys. Especially, the relatively thin γ´ bands in the MOD E alloy bicrystal was attributed to the slow diffusion of alloying elements around grain boundary due to the presence of the excess boron.
To evaluate the effect of carbon and boron on the bulk mechanical properties of single crystal, creep tests of the BASE and the MOD E alloy single crystals were performed at 950℃ and $1050℃. At 1050℃, the creep life of the MOD E single crystal was a little shorter than that of the BASE single crystal. At 950℃, the life of the MOD E single crystal was nearly same as that of the BASE single crystal which was fully solutionized at 1320℃. Activation energy and stress exponents of the MOD E and the BASE single crystal were also nearly same to be 510 ± 30 kJ/mol and 5 at 950℃ and 8 - 9 at 1050℃, respectively. It means that both alloy have same deformation mechanism during creep which is considered as dislocation climb motion.
In order to find out whether the difference of creep lives was originated from the difference of composition or originated from the difference of solutionizing temperature, creep tests were carried out on the BASE single crystal solutionized at the temperature for solutionizing of the MOD E alloy. The results showed that the creep properties of the BASE alloy were inferior to those of the MOD E alloy when they were both solutionized at 1295℃. This result implies that the microstructural inhomogeneity caused by insufficient solution treatment decreased significantly the creep life.
There was high amount of σ phase precipitation in the BASE alloy single crystal during exposure at high temperature at either stressed or unstressed condition. It is noted that the formation of σ phase in the MOD E was retarded considerably during early stage of precipitation. Microstructural observation of the 0.5% interrupted specimen crept at 950℃ with 290 MPa condition suggested that the diffusion of alloying element for the formation of sigma phase was considerably inhibited due to boron in the MOD E alloy single crystal.
