Rapid solidification of an Al-5Ni alloy processed by spray forming

Abstract

Recently, intermetallic compounds have attracted much attention due to their potential technological applications as high-temperature materials. In particular the intermetallic compounds, associated with the Al-Ni binary system stand out as promising candidates for high-temperature materials for the use in harsh environments. It is expected that a bulk Al-Ni alloy may exceed the strength of many commercial materials. The great challenge in developing these alloys is to manipulate the solidification thermal parameters in order to obtain the desired microstructural features. One of the indicated routes to obtain very refined intermetallic phases dispersed in the microstructure is the spray forming process. The dendritic and eutectic growth dependences on cooling rate are already known for directionally solidified (DS) hypoeutectic Al-Ni alloys. In the case of rapidly solidified (RS) samples, extrapolations of such experimental laws are needed, which can be very helpful to estimate realistic values of high cooling rates imposed during the spray forming process. The present study aims to compare directionally solidified and spray-formed Al-5 wt. (%)Ni alloy samples with a view to providing a basis for understanding how to control solidification parameters and the as-cast microstructure. The Al-5.0 wt. (%)Ni alloy was shown to have a cellular morphology for the overspray powder size range examined (up to 500 μm). The mean cell spacing decreased from 5.0 to 1.1 μm with the decrease in the powder average diameter. It was found that the experimental cooling rates imposed during the atomization step of the overspray powder solidification varied from 10 3 to 2.10 4 K/s. The DSC trace depicted a crystallization peak of an amorphous structure fraction in the smallest Al-5.0 wt. (%)Ni alloy powder size range (<32 μm) estimating a 15 μm critical diameter of amorphous powder in the binary Al 97.5Ni 2.5 (at%) alloy.