Effects of directional solidification on microstructure and mechanical properties of Nb-14Si-22Ti-2Hf-2Al-4Cr alloy

The samples cut from the arc-melted Nb-14Si-22Ti-2Hf-2Al-4Cr alloy were directionally solidified in an optical imaging floating zone furnace under argon atmosphere with different growth rates. The effects of directional solidification (DS) technique on phase composition, microstructure characteristics and the mechanical properties at room and high temperatures were investigated with those of arc-melted alloys for comparison. Results show that compared with the arc-melted alloys, the DS samples are still composed of Nb_SS (Nb solid solution) and Nb₅Si₃, without any change. Nb_ss of DS alloys is dendritic, and the lath-like Nb₅Si₃ distributes along growth direction. The compressive strength of the samples at the DS speeds of 15 and 10 mm/h increase to 442 and 493 MPa at 1250°C respectively, from 290 MPa of arc-melted alloys. After DS at 15 mm/h, the fracture toughness of the samples increases to 15 MPa · m^1/2 from 12 MPa · m^1/2 of the arc-melted alloys. The alloys prepared by the optical floating DS method possess good mechanical properties at room and high temperature.