Microstructure and mechanical properties of a directionally solidified andaged intermetallic Ni-Al-Cr-Ti alloy with beta-gamma '-gamma-alpha structure

Microstructure and mechanical properties were investigated in a directional ly solidified (DS) Ni-21.7Al-7.5Cr-5.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of beta (B2)-matrix (NiAl), coarse gamma'(L1( 2))-particles (Ni3Al), fine gamma'-needles and spherical alpha (A2)-precipi tates (Cr-based solid solution). The majority of fine gamma'-precipitates w as found to be twinned. The interdendritic region contained gamma (Al)-matr ix (Ni-based solid solution) separating ordered domains of gamma'-phase and fine lath-shaped a precipitates. Ageing in the temperature range 973-1373 K decreased the volume fraction of dendrites from about 50 vol.% measured i n the as-grown material to about 38 vol.% in the material aged at 1373 K fo r 300 h. During ageing in the temperature range 973-1273 K the gamma -phase transformed to the gamma'-phase in the interdendritic region. This transfo rmation was connected with precipitation of lath-shaped alpha -precipitates . Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisatio n of the gamma -phase and precipitation of spherical gamma'-particles in th e interdendritic region. Ageing at 973 K significantly increased the microh ardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in in creasing the ductility of the alloy. The measured tensile ductility up to 1 .1% can be attributed to the effect of extrinsic toughening mechanisms oper ating in the P-phase such as blunting and bridging of cracks by the alpha- and gamma'-precipitates. (C) 2000 Elsevier Science Ltd. All rights reserved .