Rare earth metals (REM) in aluminium alloys improve their properties. These alloys are promising for the transport industry, aviation, space, military technologies and electronics. At increased temperatures, thanks to their low specific weight, they are perspective of replacing parts from titanium, cast iron, steel, and copper. In such thermal operate conditions, inclusions of the primary intermetallides are the most stable in comparison with strengthened by solid solution disruption and formation of branched eutectic, and this determines tendency increasing of REM concentration in alloys. One of the main reasons for containment REM additives in aluminium alloys is a significant enlargement of primary intermetallides—more than 100 $\mu$m, which acquire a faceting, needle and cruciform shapes. Investigations is carried out on a multicomponent Al–Cu–REM–Mn–Ti–Fe–Zn–Si alloy, with an average Cu content of 3.7% wt. and addition of 13% wt. REM alloy. The aluminium alloy is cooled and solidified at a rate of 10 K/s. In the structure of the alloy, the primary intermetallides are observed with faceting, wave contours, and such, which contain cracks. In the case of flat surfaces, in comparison with the wave surfaces of the intermetallic inclusions, the rise of the concentration range of all components, except for Mn, take place. The concentrations of Cu, REM, and Fe are changed in the most significant way. Under constant magnetic field action with flux density 0.1 T on the cooled and solidified alloy, the change in morphology of the inclusions with faceting to wave contours accompanied by the most insensitive Fe content arising on the intermetallides’ surfaces. In addition to the mathematical planning of the experiment (a second two-factor experiment with equal duplications of the experiments), equations of regressions, which characterized the influence of the concentration of Cu, REM, and Fe, as well as the magnetic field on the Fe concentration on the intermetallic inclusions’ surface are obtained. As found, the most influential factor is the magnetic field, and the content of rare-earth metals and Cu is in 4.7 and 3.8 times weaker, respectively. In the range of variation of the iron content in the alloy treated by a magnetic field no effect of this impurity on the Fe concentration on the intermetallic surface is found. It is revealed that, under the influence of the field, the behaviour of the Fe impurity becomes similar to the modifier of primary intermetallic compounds and its accumulation on the surface of inclusions is accompanied by the loss of faceting by the crystals.