The intergranular carbide precipitation behavior and its effect on the tensile properties were investigated in alloy 690. The precipitation of intergranular carbides, identified as Crrich M 23 C 6 , was retarded on the low-angle grain boundaries and the coincidence-site lattice boundaries. The M 23 C 6 carbides have a cube-cube orientation relationship with the matrix. The ultimate tensile strength, yield strength, and elongation of the solution annealed alloy 690 are 648.2±8.2 MPa, 242.8±10.5 MPa and 44.9±2.3 %, respectively. The ultimate tensile strength and the yield strength increased to 764.8±7.8 MPa and 364.8±10.2 MPa until the aging time reached 16 hours. This increase is ascribed to the M 23 C 6 carbide acting as reinforcements. However, when the aging time exceed 16 hours, these properties gradually decreased with increasing aging time. The decrease in ultimate tensile strength, yield strength, and elongation were mainly caused by the intergranular cracking due to the low bond strength between the carbide and the matrix.
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