T. Pelachová

Microstructure and mechanical properties of a directionally solidified and aged intermetallic Ni–Al–Cr–Ti alloy with β-γ′-γ-α structure

Abstract Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′( L 1 2 )-particles (Ni 3 Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates.

Microstructure and Mechanical Properties of a Cast Intermetallic Ti-46Al-8Ta Alloy

Microstructure and mechanical properties of a new cast air-hardenable intermetallic Ti-46Al-8Ta (at.%) alloy are studied. Primary solidification phase, solidification path and high temperature phase equilibria are determined using quenching during directional solidification (QDS) experiments combined with microstructural analysis. Vickers hardness, tensile, compression and creep properties are presented. The effect of short-term high temperature exposure on room temperature (RT) ductility of uncoated specimens is evaluated and discussed. Creep properties such as minimum creep rate, time to 1 % creep deformation, fracture time and creep fracture mechanisms are presented and compared to other TiAl base alloys developed for industrial applications. Long-term high temperature microstructural stability and the effect of ageing on Vickers hardness and RT tensile properties are presented. Based on the achieved experimental results, the potential of this new alloy to fulfill industrial specifications is discussed.