Abstract
The effect of heat treatments on the microstructure and mechanical properties of directionally solidified (DS) multiphase intermetallic Ni-21.9Al-8.1Cr-4.2Ta-0.9Mo-0.3Zr (in at.%) alloy was studied. Two-step solution annealing of DS alloy at 1373 K for 100 h and 1483 K for 2 h leads to dissolution of spherical α-Cr and needle-like γ´ (Ni₃Al) precipitates within the β (NiAl) dendrites. In the interdendritic region, dissolution of Cr-based particles and formation of irregular shaped γ areas in the vicinity of dendritic-interdendritic interfaces were observed. During ageing at temperatures from 1223 to 1323 K the growth of spherical α-Cr and lath-shaped γ´ precipitates in the β dendrites was observed. In the interdendritic region, the ageing leads to intensive precipitation of lath-shaped α-Cr precipitates, formation of irregular shaped Cr-based particles and transformation of γ matrix (Ni-based solid solution) with γ´ precipitates to the γ´ matrix with needle-like γ precipitates. The studied heat treatments can be successfully applied to soften the DS alloy. All Vickers hardness values measured for specimens aged at 1273 and 1323 K are lower than those for DS samples. A linear relationship between the Vickers hardness and compressive yield strength is determined for DS and heat-treated (HT) specimens, which allows the mechanical properties of the alloy to be predicted from simple hardness measurements. Optimal combination of the yield strength, ultimate compressive strength and plastic deformation to fracture is achieved after ageing at 1323 K for 30 h.

Key words
nickel aluminides, Ni₃Al, NiAl, microstructure, heat treatment, mechanical properties

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