EFFECT OF THE GROWTH RATE ON THE MICROSTRUCTURE AND ROOM-TEMPERATURE TENSILE PROPERTIES OF DIRECTIONALLY SOLIDIFIED Ni_3Al-BASED IN-SITU COMPOSITES

摘要

Effect of the growth rate on the microstructure and room-temperature tensile properties of two directionally solidified (DS) Ni_3Al-based in-situ composites with the chemical compositions Ni-20.2Mo-14.1Al-3.6Cr and Ni-20.4Mo-14.7Al-1.8V [at. percent] was studied. Directional solidification was performed in a Bridgman apparatus at constant growth rates R ranging from 1.39 X 10~(-6) to 11.1 X 10~(-6) m centre dot s~(-1) and a constant temperature gradient in liquid at the solid liquid of G_L = 15 X 10~3 K centre dot m~(-1). Increase of the growth rate leads to a decrease of interfibre spacing A according to a relationship A = K_1R~(-0.5), where a material constant K_1 for both alloys is determined to be 4.44 X 10~(-9) m~(1.5) s~(-0.5). The growth rates of 6.94 X 10~(-6) and 8.33 X 10~(-6) m centre dot 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.