Abstract
Effect of the growth rate on the microstructure and room-temperature tensile properties of two directionally solidified (DS) Ni3Al-based in-situ composites with the chemical compositions Ni-20.2Mo-14.1Al-3.6Cr and Ni-20.4Mo-14.7Al-1.8V [at.%] was studied. Directional solidification was performed in a Bridgman apparatus at constant growth rates R ranging from 1.39 × 10^-6 to 11.1 × 10^-6 m.s^-1 and a constant temperature gradient in liquid at the solid liquid of G_L = 15 × 103 K.m^-1. Increase of the growth rate leads to a decrease of interfibre spacing λ according to a relationship λ = K₁R^-0.5, where a material constant K₁ for both alloys is determined to be 4.44 × 10^-9 m^1.5.s^-0.5. The growth rates of 6.94 × 10^-6 and 8.33 × 10^-6 m.s^-1 are found to be critical for a planar front solidification for the alloy modified by chromium and vanadium, respectively. At higher growth rates, a breakdown of planar front solidification is observed. For the composites with a perfect aligned fibre structure, the yield stress of the composites increases with increasing growth rate. Formation of cellular structure results in a decrease of the yield stress and ultimate tensile strength. A model predicting the offset yield stress of the composites in relation to their microstructural variables is proposed.

Key words
Ni₃Al, in-situ composites, microstructure, directional solidification, tensile properties

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