Materials Science and Engineering A304–306 (2001) 520–523

Development of rapidly solidified (RS)magnesium–aluminium–zinc alloy

Govinda,∗, K. Suseelan Naira, M.C. Mittala,Kishori Lalb, R.K. Mahantib, C.S. Sivaramakrishnanb

a Special Materials Division, Materials and Metallurgy Group, Propellants, Polymers,Chemicals and Materials Entity, Vikram Sarabhai Space Centre, Trivandrum 695 022, India

b Materials Processing Division, National Metallurgical Laboratory, Jamshedpur 831 007, India

Abstract

Applications of magnesium alloys in the aerospace industry are limited because of their poor mechanical properties, corrosion resistanceand workability. Refinement of microstructure through rapid solidification processing is one of the highly potential approaches to overcomethese limitations. In the present study, the technology of processing of rapidly solidified (RS) ribbons in Mg-9%Al-1%Zn-0.2%Mn alloyhas been established using melt spinning technique. The effect of wheel speed on thickness and microhardness of the ribbons is presented.Microhardness is found to increase with the wheel speed. It is further observed that microhardness of the ribbons increases with the heattreatment temperature upto 200◦C and thereafter it starts decreasing. Precipitation of the intermetallic phase Mg17Al12, at temperaturesupto 200◦C is found to prevent the grain growth and improve the properties of the ribbons. This in-turn reveals that the temperature forsecondary processing of RS ribbons in Mg-9%Al-1%Zn-0.2%Mn alloy should not exceed 200◦C in order to retain the benefits of rapidsolidification processing. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Development; Rapidly solidified; Magnesium–aluminium–zinc alloy

1. Introduction

Magnesium alloys have great potential for high perfor-mance aerospace and automotive applications primarilyowing to low density. Magnesium is the lightest space struc-tural metal, but its low mechanical properties, poor work-ability and poor corrosion resistance limit its applications.Refinement of microstructure through rapid solidificationprocessing of magnesium alloys can result in stronger, eas-ily workable and more corrosion resistant magnesium alloys[1,2]. However, rapid solidification processing of magne-sium alloys poses critical challenges due to the very highchemical reactivity of magnesium metal itself and a numberof its important alloying elements. The available literatureon rapidly solidified magnesium alloys is very sparse [3,4]and no significant work has been undertaken in the countryso far. In the present study, the technology of processingof rapidly solidified (RS) ribbons in an alloy based onMg–Al–Zn system has been established. The ribbons, soproduced, were characterized through metallographic andX-ray diffraction techniques. The effect of temperature onthe second phase morphology and microhardness was also

∗ Corresponding author.

studied to set the upper limit of temperature for secondaryprocessing of Mg-9%Al-1%Zn-0.2%Mn alloy RS ribbons.

2. Experimental procedure

Alloy for study (Mg-9%Al-1%Zn-0.2%Mn) was preparedin steel crucible using flux melting technique in an electricresistance furnace. Magnesium, aluminium and zinc metalingots of 99.8% purity were used. Manganese was addedas Al-10%Mn hardener. The melt was poured into 30-mmdiameter test bars under the protective cover of SF6 gas.These sand cast test bars were proof machined.

RS ribbon making experiments were carried out usingmelt spinning technique. The 25 mmf specimens, cut fromtest bars, were charged into the cylindrical crucible of themelt spinning unit and the chamber with the crucible wasevacuated to a vacuum level of 1 torr. Subsequently, argonwas purged and vacuum level was brought to 100 torr. Thealloy was melted through induction heating, in the crucibleunder argon atmosphere. The molten alloy was then pressur-ized with argon gas to force the metal through 0.8-mm di-ameter orifice at the bottom of the crucible. The speed of thecopper wheel (300-mm diameter) on to which alloy was RS,

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was varied between 800 (12 m/s) to 2200 rpm (35 m/s) andRS ribbons were produced. The ribbons produced at a wheelspeed of 35 m/s were heated to 115, 200 and 300◦C for timesvarying between 1 and 5 h. The ribbons in as-spun and heattreated conditions were mounted; polished and etched with3% Nital for SEM analysis. Microhardness of ribbons, mea-sured using Vickers pyramid indentor with varying loads viz.10, 15, and 25 g, established that microhardness values wereload independent. Accordingly, a load of 25 g was selectedfor microhardness measurement in present study. X-raydiffraction study was carried out using Co Ka radiation.

3. Results and discussion

3.1. Effect of wheel speed (cooling rate)

Chemical composition of the RS magnesium alloy ribbonsproduced in the present study is listed in Table 1. Coolingrate during liquid to solid transformation increases with theincrease in wheel speed. In general, increased cooling rateleads to refinement of microstructure, enhancement of solu-bility of solutes, etc. and thus results in improved properties[5,6].

The effect of wheel speed on thickness of ribbons, ob-served in the present investigation, is given in Table 2,which shows that as the wheel speed is increased, the rib-bon thickness is inherently decreased. It is also clear fromthe table that an increase in wheel speed results in a sig-nificant increase in microhardness. X-ray diffraction resultsof these ribbons reveal the presence of Mg17Al12 phasealong witha-Mg under all the cooling rates (wheel speeds)studied.

SEM micrograph (Fig. 1a) of as-spun ribbon, processedat wheel speed of 2200 rpm, shows very fine grain size(1–3mm) compared to 250–300mm obtainable through sandcasting of this alloy. This indicates that the increase in themicrohardness is a direct manifestation, resulting from (i)reducing grain size and (ii) enhanced solubility of solutes asexplained later in this paper.

Table 1Chemical composition of the RS ribbons

Melt No. Elements (wt.%)

Al Zn Mn Fe Mg

RSM2 8.5 1 0.25 0.015 Balance

Table 2Effect of wheel speed on the ribbon thickness and microhardness

Serial No RS ribbon identification Speed, rpm (m/s) Thickness (mm) Microhardness (kg/mm2)

1 RSM2/32 (Mg-8.5%Al-1%Zn-0.25Mn) 800 (12) 300 552 RSM2/28 (Mg-8.5%Al-1%Zn-0.25Mn) 1800 (29) 65 833 RSM2/8 (Mg-8.5%Al-1%Zn-0.25Mn) 2200 (35) 37 96

Fig. 1. SEM micrographs of RS Mg-alloy ribbons in different conditions.

3.2. Effect of heating

The microhardness of ribbons, processed at 2200 rpm(equivalent linear speed of 35 m/s) and heated to 115, 200and 300◦C for varying times were measured and results aretabulated in Table 3. The standard deviation (S.D.) values,as mentioned in Table 3, are similar to those reported in lit-erature for RS Mg-alloy ribbons [7,8]. Table 4 shows theshift in interplanar spacing (d) of thea-Mg phase in as-spunand heat treated conditions. SEM micrograph of RS ribbonin heat treated condition (200◦C for 5 h) is shown in Fig. 1b.Analysis of these results bring out the following:

1. Microhardness of RS ribbons increase with the increasein heating temperature upto 200◦C. There is a markedreduction in microhardness when heated to 300◦C. This

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Table 3Effect of temperature and time on the microhardness of the RS ribbons

Serial No. Condition RSM2 Mg-alloy

Mean S.D.

1 As-spun 82.8 6.72 115◦C — 1.5 h 90.9 3.13 115◦C — 3 h 91.4 84 115◦C — 5 h 92.6 3.15 200◦C — 1.5 h 109 86 200◦C — 3 h 107.4 16.677 200◦C — 5 h 103.7 8.68 300◦C — 1.5 h 75.7 7.89 300◦C — 3 h 51.6 5

10 300◦C — 5 h 78.9 9.4

can be attributed to age hardening and overaging phe-nomena.

2. However holding at 115 or 200◦C does not appear tochange the microhardness significantly.

3. There is a large shift in interplanar spacing in the as-spunribbons. It has been reported [9] that increase in alu-minium and zinc content of magnesium matrix decreaseslattice parameters, and hence leads to a reduction in in-terplanar spacing. Therefore, it can be inferred that thelarge shift in interplanar spacing is due to the enhancedsolid solubility of aluminium and zinc in the alloy.

4. Heat treatment at 200◦C leads to precipitation ofMg17Al12 phase and takes away the solute from thematrix and therefore the shift in lattice parameter is theleast. The increased shift after heating at 300◦C is dueto the re-dissolution effect, which also leads to reductionin microhardness values.

5. The SEM micrographs of ribbons in as-apun and afterheating to 115◦C appear to be similar and no evidenceof Mg17Al12 is apparent. This may be due to the low

Table 4Shift in the inter planar spacing of thea-Mg phase of the ribbons indifferent conditions

d-Spacing Shift,1d (d0 − dr)

Obtained on ribbons,dr For pure Mg,d0

RSM2/8, as-spun2.7652 2.778 0.01 282.5895 2.605 0.01552.4399 2.452 0.01211.595 1.6047 0.0097

RSM2/B, 3 h at 200◦C2.7696 2.778 0.00842.5973 2.605 0.00772.4437 2.452 0.00831.5979 1.6047 0.0068

RSM2/8, 3 h at 300◦C2.7515 2.778 0.02652.5905 2.605 0.01452.4375 2.452 0.01451.5933 1.6047 0.0114

quantity as well as submicroscopic nature of Mg17Al12present in these conditions. Heating at 200◦C leads tosignificant precipitation along grain boundaries, visibleas ‘necklace’ type structure in Fig. 1b.

6. Such precipitates along the grain boundaries are very ef-fective in preventing grain growth. No significant changein grain size is noticed even after heating to 200◦C. At300◦C, on the other hand, overaging and re-dissolutionof precipitates occur.

7. The results clearly indicate that the secondary processingof this alloy can safely be carried out upto a temperatureof 200◦C.

4. Conclusions

1. Technology of making RS ribbon of highly reactiveMg-9%Al-1%Zn-0.2%Mn alloy has been successfullyestablished. Grain size of 1–3mm could be achieved inas-spun ribbons in contrast to 250–300mm grain sizenormally attained in sand cast structure.

2. Increase in cooling rate, achieved by increasing the cop-per wheel speed, is found to result in decrease of ribbonthickness, and increase in microhardness.

3. Upto a temperature of 200◦C no grain growth was ob-served in RS ribbons of Mg alloy as the precipitates ofintermetallic compound Mg17Al12 pin the grain bound-aries.

4. The secondary processing of this alloy can safely becarried out upto a temperature of 200◦C. Increase intemperature to 300◦C seems to cause overaging andre-dissolution of the precipitates in the RS ribbons whichin turn deteriorates the properties.

Acknowledgements

The authors wish to thank Dr. K. V. Nagarajan, GroupDirector, Materials and Metallurgy Group, and Shri. K.Sitarama Sastri, Dy. Director, Propellants, Polymers, Chem-icals and Materials Entity of Vikram Sarabhai Space Centrefor their constant encouragement and guidance. They arethankful to their colleagues at Materials Characterization Di-vision of Vikram Sarabhai Space Centre and National Met-allurgical Laboratory for their help at various stages of thiswork. The authors are grateful to Dr. S. Srinivasan, Director,VSSC and Dr. P. Ramachandra Rao, Director, NML for theirencouragement and for their permission to present this paper.

References

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