Room-temperature deformation behavior of directionally solidified multiphase Ni-Fe-Al alloys

Directionally solidified (DS) beta + (gamma + gamma') Ni-Fe-Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle [001]-oriented beta (B2) phase. The ductile phase in the composite consisted of a fine distribution of-ordered gamma' precipitates in a gamma (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar + proeutectic beta, and discontinuous gamma. The beta matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K-Q) of 30.4 MPa root m were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, [100] slip was seen in the beta phase, whereas [111] slip was seen in the beta phase in the microstructure which showed limited ductility. The high ductility and toughness are explained:in terms of increased nobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer.