An investigation into creep-resistant, as-cast magnesium alloys containing yttrium, zinc, neodymium and zirconium
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2 pct Zn 4 pct Zn
2 pct Nd +
4 pct Nd
+0.6pctZr
5 pct Y It is noted, that the purpose of the zirconium addition in these, as in most magnesium alloys, is that of a general grain refiner? EXPERIMENTAL PROCEDURES All the alloys used in this work were prepared in the Metals Development Foundry at Magnesium Elektron Ltd., Manchester, and were made by melting together 99.98 pct magnesium and an antioxidizing flux in a steel crucible. Alloying additions were subsequently made and the melt stirred and then settled before being cast at 780 ~ into square section, horizontal, "finger-bar" molds. The compositions of all the alloys used were spectographically analyzed by Magnesium Electron Ltd., and also by Link Systems Ltd. using their SEM analysis system. Good agreement was obtained between these two methods and alloy additions were found to be within l0 pct of the given nominal values. In order to obtain the average as-cast grain size, finger-bars from each as-cast alloy were polished along their length and then etched in a 1 pct-HNO 3 + H20 solution. Little variation in grain size was observed
along the length of the bars or across their diameters and typical grain sizes and microstructures are shown in Fig.(1). Figure 1(a) shows the microstructure of the as-cast Mg-3 pct Y-Zr alloy and is representative of the three as-cast Mg-Y-Zr alloys discussed later, all of which had a measured ASTM nominal grain size of 6. The dark spots in the microstructures of these alloys have been identified as Zr-rich clusters. Figures l(b) and (c) show the microstructure of the as-cast Mg-3 pct Y-2 pct Zn-2 pct Nd-Zr alloy and are representative of both the as-cast Mg-Y-Zn-Nd-Zr and the as-cast Mg-Y-Zn-Zr alloys. For both these alloy systems the as-cast structures appear to be two-phase, and are comprised of a) a light-colored Mg-Zr matrix containing occasional Zr-rich clusters and b) a eutectic grain boundary second phase which after examination has been seen to contain practically all of the extra alloying elements (i.e. yttrium, zinc and neodymium or yttrium and zinc). For both these two-phase alloy systems an ASTM nominal grain size of 7 has been measured for the Mg-Zr matrix grains. From each cast finger-bar two creep specimens could be machined, these specimens having a 25.4 mm gage length and a 4.54 mm diam. Using these specimens creep measurements were carried out using constant stress "Andrade Cam" type machines,7 each of which had a three-zone furnace which maintained a gradient free temperature along the total length of the specimen to within _+0.5 ~ Specimen extensions were measured with extensometers using capacitive transducers which were capable of measuring extensions of the order of
1/xm. CREEP RESULTS AND DISCUSSION
To obtain the creep results that are shown in Figs. 3, 4 and 5 specimens were initially heated to a constant temperature and then loaded. Subsequent time-strain curves were produced and the specimen was kept in the initial constant temperature-constant stress state until a prolonged steady-state creep regi
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