The Roles of Mn and Ni Additions to Fe-Contaminated Al in Neutralizing Fe and Stabilizing the Cellular α-Al Microstructure

Abstract

The development of new Al-based alloys has included the addition of transition alloying elements in order to produce phases that are stable at high temperatures. Elements such as Mn, Cr, and V, can lead to supersaturated solid solutions in Al, and elements with low solubility and formers of eutectic systems (e.g., Ni, Fe, Ce) induce mechanical strengthening by the formation of higher fractions of intermetallics. Since Fe is the major impurity in commercial Al and an undesired element in its recycling, another positive effect of both Ni and Mn addition into Al alloys is related to the neutralization of Fe-compounds, known to be harmful to mechanical properties. Both Mn and Ni fulfill the requirements for high-temperature applications, but there is a lack of studies in the literature reporting the relation between microstructure and mechanical properties of Al–Mn–Ni alloys in the as-cast condition. In the present study, Al–1 wt% Mn–(1 wt% Ni) alloys are prepared using Fe-containing Al, which are subjected to directional solidification experiments with a view to producing castings under quite different solidification cooling rates ($${\dot{{T}}}$$) along their lengths. The effects of $${\dot{{T}}}$$ on the morphology of the Al-rich matrix; its characteristic length scale; and composition of the IMCs formed are analyzed. The Al–1 wt% Ni alloy has an Al-rich matrix characterized by a dendritic morphology. However, it is shown that the addition of 1 wt% Mn induces the Al-rich matrix to assume the cellular morphology for $${\dot{{T}}}$$ > 2.3 °C/s. The stabilization of cells for high cooling rates seems to be related to the crystalline structure of the alloying element. Power function experimental equations relating the cellular spacing (λC) of both Al–1 wt% Mn and Al–1 wt% Mn–1 wt% Ni alloys castings are derived, in which the ternary alloy is shown to have λC values about 30% lower than those of the binary alloy. A Hall–Petch-type equation is proposed relating the Vickers microhardness (HV) to λC for the ternary alloy. Moreover, the additions of Mn and Ni to the Fe-containing Al used in the preparation of the alloys seem to be beneficial in neutralizing Fe by forming the Al9(Mn,Fe)Ni IMC, in which Mn is replaced with Fe.