A New Ordered Structure of the Off-Stoichiometric Laves Phase Having C15-Type Structure in Mg-Cu-Si System
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A NEW ORDERED STRUCTURE OF THE OFF-STOICHIOMETRIC LAVES PHASE HAVING CI5TYPE STRUCTURE IN Mg-Cu-Si SYSTEM
Y. KOMURA AND T. MATSUNAGA Department of Materials Science, Faculty of Science, Higashi-senda-machi, Naka-ku, Hiroshima 730, Japan
Hiroshima University,
ABSTRACT A new ordered structure has been found in a ternary Mg-Cu-Si Laves phase whose composition is around Mg(Cu0. 8 0 Si 0 . 2 0 ) 2 . 50 . The ordering between Cu and Si of Cl5-type Mg(Cu0.81 Si0.19)2.53
specimen was analysed by a
single crystal X-ray diffraction method.
It
belongs
to a
space group P4 1 32(or P4 3 32) having a=6.9598A contrary to the disordered structure of Fd3m.
The ordered structure
is characterized by the ordered 4-fold screw arrangements of Cu and Si atoms located on four tetrahedra in the unit cell leaving Mg atoms on 8c sites. The order parameter S=,l- 2 was found to be 0.74 in this specimen, where al and a2 are occupancy factors of Cu for 12d and 4a sites of the space group P4 1 32,
respectively.
The order
parameters were measured as a function of annealing temperature by using Warren's method for quenched powder specimens of Mg(Cu 0 . 8 0 Si 0 . 2 0 ) 2 . 4 0 . The maximum value of S reached to 0.75 at 200*C, which is a little lower than the expected value of perfect order, S=0.85. The order parameter then decreases continuously with increasing annealed temperature and become zero at about 600*C, indicating order-disorder transformation temperature.
INTRODUCTION A Mg-base pseudo-binary system of the Laves phase studied thoroughly by Witte [1]. structure (C15-type)
extends from MgCu 2 to 20% MgSi
C14-type (MgZn2 structure) type whose composition is in
the MgZn 2 structure.
MgCu 2 -MgSi
2
has been
Terminal solid solutions of the MgCu
appears in the system.
2,
2
then the hexagonal
The structure of this
Mg2 Cu3 Si has an ordered arrangement of Cu and Si Witte also mentioned that the high temperature
phase of the Mg2 Cu 3 Si compound has a disordered C36-type (MgNi
2
structure).
Ellner and Predel [2] has carried out both X-ray and neutron diffraction investigations on polycrystalline Laves phases of C15-type in the systems of MgCu 2 -MgZn 2 , MgCu 2 -MgNi 2 and MgNi 2 -MgZn 2 . They found no indication of structure change or ordering within the existence range of the solid solutions. In the present study a new kind of ordered structure of C15-type has been found in a ternary Mg-Cu-Si Laves phase, which is off-stoichiometric from a composition of MgX 2 . The purpose of the paper is to describe the new ordered structure with the existence region of the phase and to explain Mat. Res. Soc.Symp. Proc. Vol. 21 (1984) QElsevier science Publishing Co.,. Inc.
326
the effect of annealing temperature on the order parameter.
EXPERIMENTAL Alloy specimens were prepared by melting together pure Mg, Cu and Cu-Si mother alloys in an electric furnace at 1050*C under the presence of a flux (NaCk, KCk and CaC92 mixture), and they were cooled slowly to 500*C then Twenty specimens of Mg-Cu-Si having their held 24 hrs at th
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