Effect of Mechanical Alloying on the Dissolution of the Elemental Mn and Al-Mn Compound in Aluminum

Abstract

The grain boundary, solid solution, and precipitation strengthening mechanisms are important for controlling the mechanical properties of Al-based alloys. Due to severe plastic deformation, mechanical alloying refines grain structure to a nanoscale level which leads to a strong increase in solute content and the related strengthening effect of solute atoms and secondary-phase precipitates. This study analyzed the elemental Mn and Al6Mn phase dissolution in Al during high-energy ball milling. For this purpose, XRD data, microstructure, and hardness evolutions were compared for two Al—5.2 at% Mn alloys prepared by mechanical alloying using elemental Al and Mn powders and a pre-melted master alloy. In the two-phase master alloy, containing the Al solid solution and the Al6Mn phase, the strain accumulation, grain refinement, solid solution supersaturation, and milling-induced hardening effects were facilitated. Both elemental Mn and intermetallic compound were dissolved during mechanical alloying, and the maximum solute content was near 3.1 at% Mn. A fine crystalline size of ~25 nm and the maximum Mn solute content were observed after milling of elemental powders and the master alloy for 60 h and 20 h, respectively. The microhardness of ~3 GPa corresponded to a ~3.1% solute Mn content, and the microhardness increased to ~5 GPa after long–term milling due to precipitation strengthening effect of the secondary Al6Mn phase in the master alloy.

The Microstructure and Properties of Al–Mn–Cu–Zr Alloy after High-Energy Ball Milling and Hot-Press Sintering

“…The main processes observed during the mechanical alloying of the studied Al-7.7Mn-4.9Zr-3.2Cu alloy are the grain refinement and dissolution of the alloying elements with further precipitation of the secondary Al6Mn phase after long-term milling. These processes were the same as those for the binary Al-10Mn alloy [69,70] and ternary Al-Mn-Cu alloys, with similar ~8%Mn and ~3.5%Cu [74] and with higher element contents of ~15.3%Mn and ~6.2%Cu [98]. Zirconium was completely dissolved in Al, but the precipitation of the Al3Zr phase was not detected during the mechanical alloying of the studied alloy.…”

“…The milling was processed for 5 min and stopped for 5 min to prevent overheating. The applied milling parameters provided a high efficiency of the milling process for Al-based alloys [69,[71][72][73][74]. Mechanical alloying was carried out in an argon atmosphere to prevent oxidation.…”

“…The increase of the lattice parameter to 0.4042 nm after milling for 12.5 h suggests that the aluminum solid solution contains ~4 wt% (2.1 at%) Mn. The lattice parameter should be ~0.4026 nm, assuming that all 3.2 wt% (1.4 at%) Cu and ~6 wt% (~3 at%) Mn are dissolved (which has been observed for Al-Mn and Al-Mn-Cu alloys [67][68][69]74]), but Zr is not dissolved. For the PCA-free alloy after milling for 5 h, where ZrH2 phase was detected, the lattice parameter was ~0.4033 nm, suggesting the dissolution of ~2.2 wt%Zr (0.7 at%).…”

“…Large concentrations of the lattice defects are conductive to high diffusivity, which increases the solubility of the alloying elements. The abnormally supersaturated solid solutions are formed during MA of Al-Mg [61,62], Al-V [63,64], Al-Fe [65,66], Al-Mn [67][68][69][70], and Al-Zr [71][72][73] alloys. The solubility of Zr and Mn is increased to ~8 at% [71] and ~3.1-3.5 at% [67,74], respectively.…”