Formation of a quasicrystalline phase in Al–Mn base alloys cast at intermediate cooling rates

Abstract

Al-rich 94Al-6Mn and 94Al-4Mn-2Fe alloys were suction-cast to evaluate the feasibility of obtaining bulk quasicrystal-strengthened Al-alloys at intermediate cooling rates alloyed with non-toxic, easily accessible and affordable additions. The influence of different cooling rates on the potential formation of a quasicrystalline phase was examined by means of scanning and transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Increased cooling rates in the thinnest castings entailed a change in sample phase composition. The highest cooling rates turned out to be insufficient to form an icosahedral quasicrystalline phase (I-phase) in the binary alloy. Instead, an orthorhombic approximant phase occurred (L-phase). The addition of Fe to the 94Al-6Mn binary alloy enhanced the formation of a quasicrystalline phase. At intermediate cooling rates of 10 2 -10 3 K/s, various metastable phases were formed, including decagonal and icosahedral quasicrystals and their approximants. Rods (1 mm in diameter) composed of I-phase particles embedded in Al matrix exhibited a hardness of 1.5 GPa, much higher than the 1.1 GPa of 94Al-6Mn.

Formation of Quasicrystalline Phases and Their Close Approximants in Cast Al-Mn Base Alloys Modified by Transition Metals

“…The Cr-containing alloy exhibits a more homogeneous microstructure ( Figure 2a The microstructure of the 94Al-4Mn-2Cr sample observed in the zone A consists of a mixture of different crystalline phases. The dominant phase observed was identified as an orthorhombic L-phase with lattice parameters a = 12.4, b = 12.6, c = 30.5 Å, which has also been found in the binary 94Al-6Mn sample in the same zone studied in the previous works [26,27]. This phase occurs in the form of plate-like particles (Figure 3a (Figure 3c).…”

“…The dominant phase observed was identified as an orthorhombic L-phase with lattice parameters a = 12.4, b = 12.6, c = 30.5 Å, which has also been found in the binary 94Al-6Mn sample in the same zone studied in the previous works [26,27]. This phase occurs in the form of plate-like particles (Figure 3a).…”

“…In the thicker zones (Figure 11b To compare properties of the selected alloys, hardness tests on the 1-mm rods were performed. The microstructure of 1-mm rods is essentially similar to that observed in the first zone of the wedge-shape castings [26]. This zone contains the finest and most homogenously-distributed particles.…”

“…Based on the microstructure analysis, Cu addition also promotes nucleation of a quasicrystalline decagonal phase at low cooling rates. Based on the previous studies, Fe enhances nucleation of icosahedral quasicrystalline particles in Al-Mn alloys at intermediate cooling rates [26,27].…”

“…Some other works show that in aluminum alloys alloyed with Mn, Cu, Mg, and Si, primary I-phase particles with petal morphology can be formed at low cooling rates [25]. Recently we have shown enhanced I-phase formability in the 94Al-6Mn alloy modified by various Fe additions obtained by different casting techniques [26,27]. In Fe-modified alloys, microstructures containing fine eutectic structures or larger (a few microns in size) petal icosahedral particles were developed when cooling rates of the order of 10 3 K/s were applied.…”