Identification of Al20Ti2Gd precipitates in an aged RST Al4Ti4Gd alloy

Scripta METALLURGICA

Vol. 20, pp. 1247-1250, 1986 Printed in the U.S.A.

Pergamon Journals Ltd. All rights reserved

IDENTIFICATION OF Al2oTi2Gd PRECIPITATES IN AN AGEDRSTAI-4Ti-4Gd ALLOY A. G. Jackson*, Y. R. Mahajan** and S. D. Kirchoff** *Systems Research Laboratories, 2800 Indian Ripple Rd., Dayton, Ohio 45440 **AFWAL Materials Laboratory, Wright-Patterson AF Base, Ohio 45433 (Received May 12, 1986) (Revised June 19, 1986) Introduction The effect of adding rare earth elements to aluminum-transition metal alloys is being investigated. These alloys are intended to extend the service temperatures of aluminum alloys by providing microstructures with fine, uniform dispersions of stable intermetallic compounds. Recent efforts on the AI-Ti-Gd system [1,2] have shown that significant gains in hardness up to exposure temperatures of 600% are obtainable as compared with current "elevated temperature" alloys such as AI-Fe-Ce. The identification of the ternary phase present in the AI-4 w/o Ti-4 w/o Gd alloy is the subject of this paper. Experimental The alloy was produced by arc-melting to a button melt, then converted to RS ribbon by melt spinning (1). Bulk x-ray diffraction data were obtained from CuK radiation using a Norelco diffractometer. Electron diffractidn patterns and energy dispersive spectrometer data were obtained using a JEOL 2000FX STEMand Tracor-Northern 5500 x-ray system. Results Bulk X-Ray of Button Melt: The button melt was homogenized at 600°C for X4 h in an attempt to produce equilibrium compounds, particularly the ternary compound. X-ray diffractrometer tracings showed a number of peaks, one set belonging to Al and having a calulated lattice parameter of 0.405 ± 0.0005 nm, which agrees well with the 0.4049 standard value for AI. Peaks were identified with an Al)nTi~Gd compound, FCC, and with an assumed lattice parameter of 1.45 nm. Such a compound is I~l~6pd~ed based on work of Kripyakovich and Zarechnyuk (3) in which compounds of AI~nCr)RE and AI~nV.RE were described, each having lattice parameters of approximately 1.40" tO 1.46 nm a ~ ~ i t h FCC crystal structure (Fd-3m sjnn~try, cF184 type). Intensities from the ternary compound in the x-ray trace were small compared to the AI intensites, implying a small volume fraction. The lattice parameter, calculated assuming a cubic structure, was found to be 1.45 ± 0.05 nm, the large range resulting from inaccuracies in locating the small peaks in the trace. Additional peaks were also present, but were not identified. Most probably they are associated with AI~Gd and AI~Ti compounds and are not of interest in this work. Thus the homogenized button mel~ contains~ small volume fraction of a compound with lattice parameter 1.45 ± 0.05 nm which we associate with the compound AI2oTi2Gd. Energy Dispersive Spectrometry (EDS) Analysis: Analysis of the composition of precipitates from a ribbon of the RS alloy was done using EDS methods. Spectra from 10 precipitates were collected and the composition was calculated using thin film methods with absorption corrections for thickness. The results are shown in Table 1. The precipitate composition is close to the ideal Al)nTi)Gd co~ositlon, being hlgh in A1 and low in Gd content. The uncertainty listed in the t a b l ~ i~ the composition determination is caused by the statistical scatter in the 10 spectra. To this must be added the uncertainty in the values of the Cliff-Lorimer coefficients used to calculate each con~oosition.

Copyright

1247 0036-9748/86 $3.00 + .00 (C) 1986 Pergamon .Journals Ltd.

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I f one allows a ± 10% variation in K~n, then the calculated compositions will display a similar range. Thus the composition range b~omes AI: 89.4 ± 9; Ti: 7.5 ± 1.5; Gd: 3.1 ± 0.5 atomic percent. The Gd value is s t i l l low, suggesting an off-stoichiometric composition in the precipitate. Electron Diffraction Pattern Analyses: Selected area and convergent beam ring diffraction patterns from the precipitates were obtained and analyzed for zone and planes present. Symmetry determination by use of higher order Laue zone (holz) line patterns was not done. Figure I is a selected area diffraction pattern, which when analyzed proved to f i t a [114] FCC zone. The measured values for interplanar distances and angles between planes agree well with values predicted for FCC [114], using a 1.45 nm l a t t i c e parameter (see Table 2). Convergent beam ring pattern from this zone is shown in Fig. 2. Measurement of the ring diameters and comparison with predicted values are shown for this zone and three others in Table 3. The additional ring patterns are presented in Fig. 3 Ca, b, and c). Agreement is very good for these cases, implying that the assumption of FCC with a = 1.45 nm is valid. Table 1 Composition of Precipitates by EDS Precipitate

.'.is(. • " f ize ,

.

.

.

• " "

Figure 1.

A1

AI2oTi2Gd

Precipitate

AI2oTi2Gd

w/o

sd

w/o

a/o

sd

a/o

74.1

1.4

58.1

89.4

0.8

87.0

7.5 3.1

0.7 0.2

8.7 4.3

TI 1 1 . 1 1 . 0 12.1 Gd 14.8 0.7 19.8 (average of 10 spectra) (sd = std dev)

SAD Pattern from Precipitate. Zone = [114] FCC. Table 2 Selected Area Diffraction Data

Figure 2.

Convergent Beam Ring Pattern from [I14] zone.

d(nm) d(nm) plane meas theor

meas theor meas theor Zone angle angle ratio ratio

A 0.513 0.5127 {220} B 0.437 0.4371 {113} C 0.437 0.4371 {113}

AB 55 64.75 1.17 1.173 [I14] BC 50 50.48 FCC a = 1.45 nm

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Table 3 Convergent Beam Ring Data Zone Ring Number Meas Diam (mm) G meas (nm-1) G theor H meas H theor Ratio of r a d i i :

[114]

2/i: Zone Ring Number Meas Diam (mm) G meas (nm-1) G theor H meas H theor Ratio of r a d i i : 2/1: 3/1: 4/i:

2 61.5 23.3 22.5 0.695 0.650 theor 1.41

1 24.5 13.03 13.05 0.217 0.218 meas 1.41 1.74 1.98

2 34.5 18.35 18.46 0.470 0.436 theor 1.41 1.73 2.00

[123] 1 2 3 22.5 38.5 46.5 12.0 20,5 24.7 12.0 20.8 24.0 0.184 0.538 0.781 0.184 0.552 0.736 meas theor 2/1: 1.71 1.73 3/1: 2.07 2.00 [013]

(a) Figure 3.

1 44.0 16.7 16.1 0.357 0.325 meas 1.40

(b)

3 42.5 22.61 22.60 0.654 0.654

[002] 4 1 2 48.5 31.0 43.5 25.80 1 6 . 4 9 23.14 26.10 1 6 . 4 1 23.21 0 . 8 5 2 0 . 3 4 8 0.685 0 . 8 7 2 0 . 3 4 5 0.690 meas theor 1.40 1.41

(c)

Convergent beam ring Patterns from (a) [i23], (b) [013], (c) [002] Zones.

Since holz rings are frequently shifted with respect to the zero order Laue zone (zolz), an analysis of the s h i f t for the [114] zone was done. In the FCC system, the f i r s t order Laue zone (folz) to appear is associated with indices satisfying the N - 2 zone equation because of the intervening plane in FCC non-primitive c e i l . Hence indices of the f o l z are those associated with N = 2 in the zone law equation. The s h i f t vector of the folz with respect to the zolz is given by !=~-

uHN u

where ~ is the f o l z vector, u is a u n i t vector in the [uvw] d i r e c t i o n , H is the f o l z i n t e r planar spacing, and N is zone number. As a f r a c t i o n , f , of the zolz vector to which ~ is p a r a l l e l , we have f = 1/lu) 2

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Thus for [114] = - Jul = ~ and f = 1/18. For~= (020), t = (2,34,~), i . e . , (020) in the folz is parallel Uto (2,34,B) in the zolz and is located 1/18 the distance along a line drawn from the center to the zolz plane. The entire folz pattern is shifted in this direction by the amount calculated. The combined pattern is drawn in Fig. 4, in which the zolz pattern is shown together with the folz ring and the direction of the shift. Measurement of the shift agrees well with the predicted shift.

•

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e

~

J

+

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/ •

•

I~I

dt•

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.

•

~44o

I

•

Figure 4.

•

t= utel o C U ¢ ~ l

•

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Composite of Zolz [114] Zone Pattern with Folz Ring and Shift of Pattern along (2,34,~) Line.

Discussion X-ray and diffraction results indicate that the ternary phase present as a precipitate in the RS ribbon material, aged at GO0°C for 24 hours, is consistent with an FCC crystal type with lattice parameter of 1.45 ± 0.05 nm. Results reported in the literature for rare earth ternaries of Al-Cr also are FCC with lattice parameters of approximately 1.45 rim, and crystal type Fd~m, cF184. The substitution of Ti for the transition metal in the compound AI~nM~RE appears to be the case for the precipitates examined. Hence the precipitates are isostru~uPal to the AlpnMpRE compounds. While symmetry determinations are required to explicitly identify the crysta+F type, the data from bulk x-ray, selected area diffraction, convergent beam ring patterns, and EDS composition determinations are all consistent with the conclusion that the ternary compound is Al2oTi2Gd. Acknowledgements The support of AF contract F33615-C-83-5073 is gratefully acknowledged. We also thank R. Omlor for preparation of the extraction replicas, J. Paine for conversion of the alloy to RS ribbon and R. Sweeny for preparation of the button melts. References

1. 2. 3.

Y. R. Mahajan, S. D. Kirchoff, and F. H. Froes, Scrip. Met., 20(5) May, 1986. S. D. Kirchoff, A. G. Jackson, and Y. R. Mahajan, presented at Annual meeting TMS-AIME, March, 1986, New Orleans, LA. P. I. Kripyakevich and O. S. Zarechnyuk, Doklady Akad. Nauk Ukranian S.S.R., 4, 368 (1968).