In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of twophase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study. [1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.

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c¢¢ diffraction peaks are hard to discern in neutron/X-ray diffraction patterns, hindering studies on the c¢¢-strengthened superalloys using in-situ diffraction. In this study, we propose a variant selection method to increase the intensity of c¢¢ peaks and to facilitate accurate fitting. The specific variants of c¢¢ are controlled by applying a 300 MPa tensile stress during aging at 790°C for 5 hours. The interaction energy between the applied stress and the transformation strain of each c¢¢ variant differs, leading to an increase in the amount of the variants with a greater energy reduction at the expense of other variants. The enhanced variants result in greater c¢¢ peak intensities in neutron diffraction patterns, allowing both the Pawley refinement and single peak fitting to be performed. Lattice parameters of c¢¢ and c phases, and lattice misfit between the two phases and volume fraction of c¢¢ are acquired. The uncertainties associated with the fitting maintain an acceptable level corresponding to 150 microstrains. The proposed variant selection method shows potential for studying the role of c¢¢ phase in Ni-base superalloys.

Misfit stresses in the c¢¢-strengthened Ni-base superalloy Inconel 718 were calculated from the measured constrained misfit strain using Eshelby's inclusion method. The constrained misfit strains of the c¢¢ precipitates were measured using neutron diffraction at various temperatures with the aid of the stress-induced variant selection method. Eshelby tensor was calculated using the expressions for the case of anisotropic matrix given by Mura. Results show the presence of significant compressive misfit stresses in the c¢¢ precipitates with an anisotropic distribution, namely 3.0 GPa along the habit plane and 1.7 GPa along the plane normal direction at room temperature, and 2.0 and 1.2 GPa at 664°C. The decrease in misfit stresses was due to the decrease in stiffness and the different coefficients of thermal expansion of the c and c¢¢ phases. The average internal stresses in the c matrix due to lattice misfit were found to be~329 MPa at room temperature and~186 MPa at 664°C in tension. The possibility of relieving such high levels of misfit stresses in precipitates by loss of coherency during continued growth of precipitates is also discussed.

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