Microstructure of Al9(MnFe)xSi intermetallics produced by pressure-assisted reactive sintering of elemental AlMnFeSi powder mixtures

Materials Letters 57 (2003) 2246 – 2252 www.elsevier.com/locate/matlet

Microstructure of Al9(MnFe)xSi intermetallics produced by pressure-assisted reactive sintering of elemental AlMnFeSi powder mixtures J.A.G. Toscano, A.V. Flores, A.R. Salinas*, E.V. Nava Centro de Investigacio´n y de Estudios Avanzados del IPN, Unidad Saltillo, P.O. Box 663, Saltillo, Coahuila, Mexico Received 15 July 2002; accepted 5 September 2002

Abstract Al9(Mn,Fe)xSi intermetallics were prepared by reactive sintering of 56.4 wt.% Al, 14.6 wt.% Fe, 14.3 wt.% Mn and 14.7 wt.% Si powder mixtures at temperature between 873 and 1073 K under applied pressures up to 20 MPa. The microstructures of samples sintered at T < 973 K and pressure up to 20 MPa consisted solely of AlFeSi and AlMnSi phases and unreached Al – Si. At higher temperature, reactive sintering leads to formation of intermetallic mixtures of cubic a-Al9(MnFe)Si and hexagonal hAl9(Mn2Fe)Si. The results showed that the relative amount of the hexagonal intermetallic phase increases as the sintering temperature and applied pressure are increased. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Microstructure; Sintering; Intermetallic compounds

1. Introduction Quaternary Al(Fe,Mn)Si intermetallic coatings can be produced via in situ pressure-assisted reactive sintering of elemental powder mixtures on stainless steel and Co-base alloys [1]. These coatings exhibit excellent strength and hardness and good adherence to the substrate. The adherence of the coating was attributed to formation of Al(Fe, Mn,Cr)Si type compounds at the coating/substrate interface [2,3]. Characterization of coating micro-

* Corresponding author. Tel.: +52-844-438-9649. E-mail address: [email protected] (A.R. Salinas).

structures, by backscattered electron imaging and energy dispersive X-ray spectrometry, has revealed the presence of two distinct intermetallics [4], with chemical compositions within the following ranges: 55– 65 wt.% Al, 8– 16 wt.% Si, 14– 28 wt.% Mn, and 14– 16 wt.% Fe. Preliminary attempts to characterize the structure and stoichiometry of these phases by X-ray diffraction indicated the pressure of the hexagonal Al9FeMn2Si intermetallic reported by Brand [5] and the cubic Al9Mn2Si intermetallic reported by Robinson [6] in the ICDD PDF file 060669. The present paper reports on the results of further research performed to evaluate the effects of temperature and applied pressure on the micro-

0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(02)01204-1

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Table 1 Elemental powders used to manufacture AlFeMnSi intermetallics

3. Results and discussion

Element

Purity (wt.%)

Particles shape

Average size (Am)

Al Si Fe Mn

99.90 99.80 99.50 99.90

Round Round Porous round Acicular, irregular

20.49 F 13.80 4.17 F 2.45 32.70 F 17.05 26.40 F 14.73

Fig. 1 shows a photomicrograph illustrating the microstructure of a sample sintered at 1073 K during 7200 s and an applied pressure of 20 MPa. The microstructure consists of a well-developed assembly of four- or six-sided prismatic grains and small amounts of porosity, mainly at grain junctions. The contrast observed in the microstructure is due to grain orientation differences. Closer examination using backscattered electrons and characteristic X-ray in the SEM revealed the presence of two AlFeMnSi phases with slightly different chemical composition (see Fig. 2). Fig. 3 shows an X-ray diffraction pattern obtained from the same sample. Assuming that the microstructure consisted of a mixture of cubic Al9(Mn,Fe)Si and hexagonal Al9Mn2FeSi, it was possible to identify and quantity by Rietveld analysis all the diffraction peaks appearing in the pattern. The ICDD PDF database does not report any a-AlMnFeSi intermetallic with a cubic crystal structure. However, it has been reported [7] that Fe dissolution via a diffusion-controlled process can lead to formation of iso-structural aAlMnFeSi intermetallic from Al9Mn2Si [ICDD PDF file 06-0669]. The identification and quantification procedure describe above were performed assuming that this mechanism leads to formation of the observed quaternary Al9MnFeSi. Fig. 4 shows the results of the identification of the peaks in the X-ray diffraction pattern obtained from a

structure of the reaction products formed during reactive sintering of Al, Fe, Mn and Si elemental powder mixtures.

2. Experimental procedure Disk-shaped samples, 10 mm in diameter and 3 mm in thickness, consisting of 56.4 wt.% Al, 14.6 wt.% Fe, 14.3 wt.% Si high purity elemental powders were prepared by dry mixing during 600 s with the aid of ultrasonic stirring and cold compaction. Purity, morphology and size of the powders are given in Table 1. The samples were sintered during 3600 and 7200 s at 873, 973, and 1073 K under applied pressures of 0, 5, 10 and 20 MPa. Heating and pressure were simultaneously applied to the samples within an infrared radiation unit fitted with a quartz tube placed coaxially with the loading axis of an universal Instron testing machine. During sintering, a constant flow of high purity Ar was passed through the quartz tube to avoid oxidation. The specimen temperature was controlled and measured continuously by means of a K-type thermocouple. The heating and cooling rates were kept constant at 0.83 K/s. Details of the testing apparatus and procedure are described elsewhere [4]. The microstructures of the sintered samples were characterized using X-ray diffraction, optical microscopy, energy dispersive X-ray spectrometry and scanning electron microscopy. The X-ray diffraction patterns were measured using Cu-Ka radiation. Phase identification and quantification was performed using Rietveld line-profile fitting with the aid of the Fillprof software package. Chemical etching of the samples to reveal the microstructures for optical and SEM metallography was carried out using a fresh solution of 2 vol% H2SeO4, 13 vol% HCl in ethanol.

Fig. 1. Microstructure of a sample sintered during 7200 s at 1073 K and 20 MPa.

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Fig. 2. Morphology and composition of (1) Al9FeMnSi and (2) Al9FeMn2S intermetallics observed in a sample sintered during 7200 s at 1073 K and 20 MPa.

Fig. 3. X-ray diffraction pattern of a sample sintered during 7200 s at 1073 K and 20 MPa (c = peaks corresponding to Al9FeMnSi, h = peaks corresponding to Al9FeMn2Si).

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Fig. 4. X-ray diffraction pattern of a sample sintered during 7200 s at 873 K and 10 MPa (c = peaks corresponding to Al9Mn2Si, h = peaks corresponding to Al9FeMn2Si, t = peaks corresponding to Al4FeSi2, Al = aluminum peaks, Si = silicon peaks).

sample sintered during 7200 s at 873 K under an applied pressure of 10 MPa. In this case, no quaternary intermetallic was identified in the diffraction pattern and only cubic a-AlMnSi (Al9Mn2Si, PDF file 060669), monoclinic h-AlFeSi (Al5FeSi, PDF file 200031) and tetragonal y-AlFeSi (Al4FeSi2, PDF file 200031) phases were positively identified. The results presented in Tables 2 and 3 may be summarized as follows: a compacted, disk-shaped sample of a mixture of pure Al, Mn, Fe and Si particles sintered at elevated temperatures under applied pressure, is transformed to a monolith with a phase constitution which strongly depends on sintering temperature. Samples processed at 873 K and applied pressures as high as 20 MPa only exhibit the presence of a a-AlMnSi, h-AlFeSi and y-AlFeSi phases and significant amounts of unreacted Al and Si particles. When sintering is performed at 973 K, the phase distribution changes significantly and only y-AlFeSi, a-AlFeSi and hAlFeMnSi intermetallics are observed. A further increase in temperature to 1073 K results in micro-

structures consisting of a a-AlFeMnSi and hAlFeMnSi intermetallics. During heating and the initial stages of sintering, solid state diffusion of Si to Al/Si particle interfaces causes formation of an Al –Si solid solution. Once the Si solubility limit in Al is reached (f 1.65% Si at 850 K), incipient eutectic melting at the interface takes place. This process leads to formation of an Al – Si liquid film around solid particles. The amount of liquid formed depends on the sintering temperature and time. Differential thermal analysis of AlFeMnSi powder samples showed evidence of melting at 851 K [4]. When the liquid film comes into contact with Fe and Mn solid particles, rapid liquid metal attack causes dissolution of these elements. However, Fe and Mn solubility in liquid Al – Si alloys is very low, 0.05 wt.% Fe at 928 K and 1.82 wt.% Mn at 931 K, respectively. Therefore, solid AlFeSi and AlMnSi intermetallics will form almost spontaneously at the liquid/solid interface. Continued growth of the intermetallic layer surrounding the Fe and Mn particles will depend on

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Table 2 Effect of applied pressure and temperature on phase constitution of AlMnFeSi intermetallics produced by a 3600-s reactive sintering process Unreacted particles

Quaternary intermetallic phases

Cryst. structure Stoichiometry ICDD PDF File

Cubic Al9Mn2Si 06-0669

Ternary intermetallic phases Monoclinic Al5FeSi 20-0031

Tetragonal Al4FeSi2 20-0033

FCC Al 85-1327

Cubic Si 77-2107

Cubic Al9FeMnSi 06-0669

Hexagonal Al9FeMn2Si 42-1206

T (K)

P (MPa)

a-AlMnSi (wt.%)

h-AlFeSi (wt.%)

y-AlFeSi (wt.%)

Al (wt.%)

Si (wt.%)

a-AlFeMnSi (wt.%)

h-AlFeMnSi (wt.%)

873

0 5 20 0 5 20 0 5 20

50.16 61.13 68.56 – – – – – –

20.14 18.54 16.73 1.53 – – – – –

18.64 14.67 10.39 12.30 14.44 14.29 1.21 – –

7.98 4.08 3.11 1.86 – – – – –

3.08 1.58 1.21 0.72 – – – – –

– – – 75.04 75.98 76.57 77.89 53.74 50.68

– – – 8.36 9.58 9.14 20.90 46.26 49.32

973

1073

the rates of solid diffusion of Fe and Mn through the intermetallics, the availability of liquid Al – Si at the intermetallic/liquid interface and the rate of the metallurgical reaction leading to formation of the intermetallic. The rates of solid state diffusion and metallurgical reaction at the solid/liquid interface may be enhanced by the local temperature increases resulting from the exothermic nature of the reaction leading to formation of the intermetallics. Whittenberger [8] suggested that the interfacial liquid/solid reactions do not limit the process and that the ratecontrolling step is the time required for solid state diffusion of Mn and Fe to the reaction front.

The results of the present investigation show that, apart from the temperature effect, the applied pressure affects significantly the constitution of the reaction products. The effects of temperature and applied pressure on the relative amounts of cubic and hexagonal intermetallics are shown in Figs. 5 and 6, respectively. In Fig. 5, the points at 873 K for the various applied pressures correspond to cubic aAlMnSi which, as mentioned earlier, is iso-structural with cubic a-AlMnFeSi. When sintering is carried out under no applied pressure, the amount of cubic aAlMnFeSi increases linearly for temperatures up to 973 K and then remains content (at about 70 wt.%) up

Table 3 Effect of applied pressure and temperature on phase constitution of AlMnFeSi intermetallics produced by a 7020-s reactive sintering process Ternary intermetallic phases

Unreacted powder particles

Quaternary intermetallic phases

Cryst. structure Stoichiometry ICDD PDF File

Cubic Al9Mn2Si 06-0669

Monoclinic Al5FeSi 20-0031

Tetragonal Al4FeSi2 20-0033

FCC Al 85-1327

Cubic Si 77-2107

Cubic Al9FeMnSi 06-0669

Hexagonal Al9FeMn2Si 42-1206

T (K)

P (MPa)

a-AlMnSi (wt.%)

h-(AlFeSi) (wt.%)

y-AlFeSi (wt.%)

Al (wt.%)

Si (wt.%)

a-AlFeMnSi (wt.%)

h-AlFeMnSi (wt.%)

873

0 5 20 0 5 20 0 5 20

55.19 62.51 68.86 – – – – – –

17.06 13.20 9.09 1.24 – – – – –

14.3 11.92 9.75 13.54 9.32 5.50 – – –

9.15 8.66 8.85 – – – – – –

4.30 3.71 3.45 – – – – – –

– – – 73.94 78.68 80.21 74.81 49.16 36.06

– – – 11.28 14.00 16.29 27.19 50.84 63.94

973

1073

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Fig. 5. Effect of temperature and applied pressure on the relative amount of cubic a-AlFeMnSi intermetallic.

to 1073 K. In contrast, the amount of hexagonal hAlMnFeSi increases linearly to about 30 wt.% at 1073 K (Fig. 6). A different behavior is observed when pressure is applied during sintering. In this case, the amount of cubic a-AlMnFeSi rapidly decreases for temperatures higher that 973 K. This effect occurs concurrently with a rapid increase in the amount of hexagonal h-AlMnFeSi. Nishimura and Liu [9] have

suggested that compressive forces applied externally during reactive sintering of intermetallics improve contact between powder particles of the species involved and favor formation of a more compacted grain microstructure. This effect is probably responsible for the relatively porous-free microstructures observed in samples sintered at the highest temperatures and pressures (Fig. 1).

Fig. 6. Effect of temperature and applied pressure on the relative amount of hexagonal h-AlFeMnSi intermetallic.

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The results of the present investigations show additionally that applied pressure during reactive sintering of Al – Mn – Fe –Si powder mixtures at temperatures higher than 973 K makes the cubic structure of a-AlMnFeSi intermetallic unstable. The origin of this effect cannot, however, be explained in terms of the present experimental results. Taking into account that hexagonal h-AlMnFeSi is slightly richer in Mn than cubic a-AlMnFeSi and that the diffusivity of Mn is slower than that of Fe, at least in Al – Si alloys; 11 Mn Fe D Al cm 2 /s and D Al – Si = 1.58  10 – Si = 3.74  10 2 10 cm /s [10], it can be argued that the applied pressure facilitates diffusion of Mn through the intermetallic layers and, therefore, favors the formation of the hexagonal phase at high temperatures and pressures. This suggestion, nevertheless, requires further investigation.

4. Conclusions Reactive sintering of a mixture consisting of 56.4 wt.% Al, 14.6 wt.% Fe, 14.3 wt.% Mn and 14.7 wt.% Si pure powders leads to formation of monolithics intermetallics. The phase constitution of the samples depends on the temperature and applied pressure during reactive sintering. The results of the present investigations showed that, at temperatures lower that 973 K, the microstructure consists solely of ternary phases (AlFeSi and AlMnSi) and unreacted Al – Si. At higher temperatures, reactive sintering results in microstructures consisting of mixtures of cubic a-Al9(MnFe)Si and hexagonal h-Al9(Mn2Fe)Si intermetallics. The relative amount of the hexagonal intermetallic increases as the sintering temperature and applied pressure are increased.

Acknowledgements A.T.G. is indebted to the National Science and Technology Council of Mexico (CONACYT) for the financial support granted during his graduate studies. A.S.R would also like to express his gratitude for the financial funding of this work through projects DAIC26320A and DIO Materials Corridor Network 700-1-1. Technical assistance of M.C. Sergio Rodrı´guez Arias is greatly appreciated.

References [1] A. Flores, A. Toscano, H. Castillejos, A. Acosta, J. Escobedo, Development of surface coatings for Co – Cr – Mo alloys, based on the Al8FeMnSi2 intermetallic phase, in: J.A. Disegi, R.L. Kennedy, R. Pilliar (Eds.), Cobalt-Base Alloys for Biomedical Applications, ASTM STP, vol. 1365, 1999, p. 179. [2] S. Yaneva, N. Stoichev, Z. Kamenova, S. Budurov, Quaternary iron-containing phases in Al – Si cast alloy, Z. Met.kd. 75 (5) (1984) 395. [3] A.L. Dons, Superstructures in a-Al(Mn,Fe,Cr)Si, Z. Met.kd. 76 (2) (1985) 151. [4] J.A. Toscano-Giles, PhD thesis, CINVESTAV-Saltillo, March, 2002. [5] R. Brand, Crystallographic data of Al9FeMn2Si hexagonal intermetallic phase, J. Phys., Condens. Matter 2 (1990) 3855. [6] K. Robinson, The structure of h(AlMnSi)-Mn3SiAl9, Acta Crystallogr. 5 (1952) 397. [7] C.Y. Sun, L.F. Mondolfo, A clarification of the phases occurring in Al-rich Al – Fe – Si alloys, J. Inst. Met. 95 (1967) 384. [8] J.D. Whittenbergen, in: A.H. Claver, J.J. Barbadillo (Eds.), Proceedings of the Symposium on Solid State Powder Processing, 1989, October 1 – 5, pp. 137 – 155. [9] C. Nishimura, C.T. Liu, Reactive sintering of Ni3Al under compression, Acta Metall. Mater. 41 (1) (1993) 113. [10] A. Brandes, G.B. Brook (Eds.), Smithells Metals Reference Book, 7th ed., Butterworth Heinemann, 1992, pp. 13 – 1 – 13 – 116.