DIFFRACTION STUDIES OF PARTIALLY CRYSTALLIZED Al₈₅Y₁₀Ni₅ AMORPHOUS ALLOY

J. Latuch, H.Matyja

Warsaw University of Technology, Department of Materials Science and Engineering, Narbutta 85, 02-524 Warsaw, Poland, e-mail: jlat@inmat.pw.edu.pl

Keywords: aluminium alloys, crystallization, phase transition

In recent years it has been shown that Al-based alloys with around 15 at.% of a combination of transition metal and lanthanide solutes can be made amorphous by rapid quenching. Process of crystallization of investigated Al₈₅Y₁₀Ni₅ alloy is very complex. At the scan rate of 10 K/min three transformations steps were observed at the differential scanning calorimetry (DSC) curves at the following temperatures: 530 K, 580 K and 621 K. Such heat treatment reveals the presence of multiple phases, mainly metastable, of unknown structure. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies were subjected to identify the structure of these phases.

The crystallization process was studied by continuous heating of the alloy to the temperatures corresponding to the end temperatures of the subsequent transformation. After heating through the first DSC peak (530 K) many small crystals, uniformly distributed in amorphous matrix, have appeared. Diffusive ring with several spotty rings are present at the electron diffraction pattern, confirms the character of observed microstructure. The calculated lattice parameter of this phase (a = 4.076 A) is larger than that of pure aluminium. They fitted very well to the interplanar spaces of solid solution of Y and Ni in a-Al. The spotty rings are not continuous implying very strong crystallographic texture, which may be related to the preferential crystallization.

After heating through the second DSC peak (580 K) some new spotty rings appeared, which were identified to be related to the new b phase. The b phase can be identified from XRD studies as orthorhombic (a = 4.08 A, b = 15.44 A, c = 8.77 A) with structure similar to the ternary Al₄YNi phase. In the electron diffraction pattern, there is still present diffuse ring indicating, that some volume of the alloy have not been transformed yet.

Heating above the third DSC peak (621 K) leads to the decomposition of a-Al solid solution and grow of b phase. Major transformation of the microstructure has been registered after heating at the 700 K for 30 min. The multiphase character of the fully transformed alloy is supported by electron diffraction pattern. The following equilibrium phases have been identified: -

• a-Al (cubic: a = 4.049 A),
• Al₃Y (hexagonal: a = 6.20 A, c = 21.13 A)
• Al₁₆YNi₃ (orthorhombic: a = 4.08 A, b = 16.04 A, c = 27,29 A).

The sequence and composition of phases appearing at heating above the temperature of each DSC peak were recognized. It was found that the structure of metastable phases was different from equilibrium ones formed after final transformation.