Work Softening Phenomena in Al-Fe Alloys: the Impurity-Scavenging Effect of the θ-Al₁₃Fe₄ Phase

Abstract

We have investigated the work softening (WS) phenomena during a cold-roll process of an Al-Fe alloy, based on hardness measurements, electron microscopy observations, and thermodynamic calculations. The WS behavior was confirmed in the Al-Fe alloy when the rolling rate was larger than 80％, which contained fine grains with severe deformation. In contrast, the behavior and microstructural features were hardly observed in the A1050 alloy. Composition analyses showed that almost all Fe in the present Al-Fe alloy form the θ-Al₁₃Fe₄ phase, in which a trace impurity element, Si, is found to be significantly segregated. This Si partitioning behavior is confirmed by the thermodynamic calculations and consequently leads to a higher purification of the relevant Al matrix as being almost close to the 4N (99.99 mol％) level, known as a “scavenging effect” of the impurities. It can be concluded that the highly purified aluminum matrix provides an intrinsic origin of the WS of the present Al-Fe alloy. Significant reductions of the impurities may lead to an extended mean-free path of dislocation motions and related grain boundary effects, which promote the occurrence of dynamic recovery and/or recrystallizations at severe deformation ranges even during the cold-roll process.

Fig. 3 (a)-(d) SEM-BSE images and Fe-EDS maps of Al050 alloy and Al-Fe alloy, respectively, each of which was independently obtained from representative areas of the samples. (e) TEM bright field image and selected area diffraction pattern obtained from the Al-Fe compound in Al-Fe alloy. (f) Fe and Si compositions of the θ phase in Al-Fe alloy obtained by SEM-EDS point analyses. A black line represents a regression line of the Si/Fe ratio estimated by the least-squares method. A gray region represents a prediction interval of the regression line with one sigma. (g) HAADF-STEM image of the θ phase in Al-Fe alloy and a line profile of Si k-edge photon counts of the relevant EDS spectra taken across the Al matrix to the θ phase.