Experimental investigation of the Mg–Mn–Nd isothermal section at 450 °C

Abstract

The Mg–Mn–Nd isothermal section at 450°C was constructed experimentally using solid–solid diffusion couples and key alloys. Diffusion couples and key alloys were annealed at the same temperature for different annealing time periods to understand the diffusion behavior of the system components, and to achieve the phase equilibrium in the system. The phase relations and solubility limits were determined using scanning electron microscope coupled with wave/energy dispersive X-ray spectrometers, X-ray diffraction and metallography. No ternary compounds could be observed in this system at 450°C. The microstructures of the diffusion zones showed Mn either as a pure element or as a solute atom. However, microstructures of the annealed key alloys near the Mn-rich side showed the formation of Mn17Nd2 at 450°C. Other Mn binary compounds, Mn2Nd and Mn23Nd6, were observed only in the as-cast alloys, because they are not stable at 450°C. The experiments showed that Mn remained stationary in some diffusion zones forming Mn+(MgNd), Mn+(Mg3Nd) and Mn+Mg41Nd5 two-phase regions. However, some of Mn atoms were diffusing in the system forming a continuous thin layer of pure Mn at the interface of pure Mg end-member, or to form (α-Nd)Mg,Mn, (MgNd) and (Mg3Nd) ternary solid solutions. The solubility of Mn in these solutions was estimated as 3.0, 13.0 and 3.5at.% Mn, respectively. Since Mn is in equilibrium with most of the phases in the system, the phase boundary lines are pointing toward Mn-rich corner except that of (α-Nd)Mg,Mn+(MgNd)+Mn17Nd2 three-phase field. This was confirmed based on the consistency of the results obtained from diffusion couples and key alloys.