Quasicrystals, crystalline phases, and multiple twins in rapidly solidified Al-Cr alloys

Crystal structure of Al2.95Cr0.59, a phase closely related to the η-phase in the binary Al–Cr system

Al_2.95Cr_0.59 was synthesized by high-pressure sinter­ing (HPS). It has a slightly lower Al content than the closely related η-Al₁₁Cr₂ phase.

The monoclinic η-phase in the binary Al–Cr system was initially named by Bradley & Lu [Bradley & Lu (1937 ▸). J. Inst. Met. 60, 319–337] as having the composition Al₁₁Cr₂ (Al:Cr ratio = 5.5:1). Its crystal structure was later refined [Cao & Kuo (2008). J. Alloys Compd.458, 30, 319–337] to have a slightly lower Al content (Al:Cr ratio = 5.16:1). In the present work, a monoclinic phase with composition Al_2.95Cr_0.59 (Al:Cr ratio = 5.04:1) was obtained by high-pressure sinter­ing (HPS) of a stoichiometric Al₁₁Cr₂ mixture. Structure analysis of this phase, hereafter named η′, revealed a close relationship to the previously reported η-Al₁₁Cr₂ structure, but with different mixed-occupied sites. Five fully occupied sites exhibit refined site occupation factors of 0.899 (5), 0.984 (4), 0.977 (5), 0.946 (4) and 0.945 (4) for the corresponding Al atoms. Moreover, there are no split Al sites in the η′-structure as reported for the η-structure. The refined chemical composition of the η′-phase revealed that it comprises two Al atoms fewer and two Cr atoms more than the previously reported η-Al₁₁Cr₂ phase.

Crystal structure of orthorhombic ε-Al4Cr

The crystal structure of the orthorhombic ε-Al4Cr phase in the Al–Cr system was determined by single crystal X-ray structure analysis: space group Cmcm (No. 63), a = 12.521(1)Å, b = 34.705(2) Å, c = 20.223(1) Å, V = 8787.7(9) Å, Pearson symbol oC58420.48, Z = 8, Mr = 2256, composition Al56.23Cr14.21, D x = 3.41 g/cm3; F(000) = 8576, λ (MoKα ) = 0.71069 Å, μ = 4.510 mm−1, room temperature, R = 0.068 for 2839 reflections, goodness of fit S = 1.084. There are 55 (13Cr + 42 Al) independent atoms in the unit cell, in which 12 Cr atoms and 28 Al atoms have icosahedral coordination. Interatomic distances cover the ranges of 2.570 Å ∼ 2.646 Å, 2.421 Å ∼ 2.978 Å, and 2.480 Å ∼ 2.983 Å, for Cr–Cr, Cr–Al and Al–Al atomic pairs, respectively. The crystalline phase appears obviously as a layered structure perpendicular to the a-axis. A typical aggregation of the icosahedral clusters in the structure of the ε-Al4Cr phase has been pointed out.