Investigations on the microstructure and room temperature fracture toughness of directionally solidified NiAl-Cr(Mo) eutectic alloy

The microstructures and room temperature fracture toughness of directionally solidified NiAl-xCr-6Mo (x = 28, 32 and 36 at%) alloys were investigated. Fully eutectic microstructure could be obtained in the alloys over a wide composition range. High temperature gradient could increase the planar/cellular transition rate and expand the eutectic coupled growth zone. The volume fraction of Cr(Mo) strengthening phase increased with the increasing content of Cr, accordingly, the fracture toughness of NiAl -Cr(Mo) alloys also gradually increased. The fracture toughness of 26.15 MPa m(1/2) was obtained in the NiAl-36Cr-6Mo hypereutectic alloy solidified at withdrawal rate of 10 mu m/s and temperature gradient of 600 K/cm, which is the highest value in the NiAl Cr Mo alloy system until now. Well-aligned microstructure was beneficial to the enhancement of the fracture toughness, while the existence of primary phase seriously deteriorated the toughness. All the directionally solidified NiAl Cr(Mo) alloy failed as brittle quasi-cleavage fracture. Some toughening mechanisms, such as crack bridging, crack nucleation, crack blunting, crack deflection, interface debonding and shear ligament toughening as well as linkage of microcracks were observed. In addition, mobile dislocation generated from the interface also had significant influence on the toughness. (C) 2014 Elsevier Ltd. All rights reserved.