Mechanical Propepties of Rapidly Solidified MG-ZN Alloys
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MECHANICAL PROPERTIES OF RAPIDLY SOLIDIFIED MG-ZN ALLOYS D. THEMINES*, W. RIEHEMANN, W. HENNING AND B.L. MORDIKE Institut f~r Werkstoffkunde und Werkstofftechnik, Technische Universitat, Agricolastr. 2, 3392 Clausthal-Zellerfeld, FRG L.A. 251, ISMRa Universit6 * Equipe Mat~riaux-Mikrostructure, Caen, Rue d' Edimbourg, 14032 Caen, France INTRODUCTION Rapidly quenched alloys differ from those that are conventionally solidified in the degree of supersaturation, the microstructure and the grain size. The quenching rate can thus be used to modify the mechanical properties. A fine grain size increases the hardness and ductility. The precipitation behaviour is modified by the high density of heterogeneous nuclei or high density of quenched in defects in that the rate of precipitation is increased. Discontinuous precipitation should make a significant contribution to the increase in strength as a result of the fine grain size. The aim of this work was to investigate the effect of rapid quenching on the mechanical properties of Mg-Zn alloys. Fig. 1 shows the equilibrium diagram. The concentration of the alloys investigated are shown as vertical lines. The Mg-Zn system is particularly suited for an investigation of the effects of rapid quenching for the following reasons: (1) There is a very narrow range of solid solution which should be expanded by rapid solidification leading to increased solid solution hardening. (2) The deep eutectic indicates the possibility of glass formation, as was pointed out by Calka [1,2]. (3) Microcrystalline microstructures can be prepared by rapidly quenching from the melt and also by crystallization of amorphous ribbons.
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Fig.l1 according to Elliott [3]. The vertical lines indicate the compositions of the metallic tapes
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SPECIMEN PREPARATION The alloys were melted under argon in a high frequency furnace using 99.5% pure Mg and 99.98% pure Zn. They were cast in the form of 6 mm diameter cylindrical rods. Metal ribbons were prepared by the melt spinning technique.
Mat. Re$. Soc: Symp. Proc:. Vol. 58. •1986 Materials Research Society
276
The alloys were melted under vacuum (0.4 Pa) in a boron nitride crucible and forced through a 0.5 mm diameter nozzle onto the rotating copper wheel. The peripheral speed of the copper wheel was 16 m/s and the nozzle inclined at 160 to the wheel normal. The compositions of the ribbons were determined by atomic absorption spectroscopy. EXPERIMENTAL RESULTS Metallographic and X-ray investigations showed that alloyF in the concentration range up to 2.2 at% Zn solidify as a sirco.crystalline phase.
Fig. 2 Scanning electron microscope picture of surface crystals in a pure Mg ribbon
Fig. 3 Scanning electron microscope picture of fracture surface of amorphous ribbon with a few quenched in crystals
Fig. 2 shows the microstructure of a rapidly quenched pure Mg ribbon. The ribbon was microcrystalline as was the case for other single phase specimens. Fig. 3 shows a scanning electron microscope picture of a predominantly amorpho
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