Novel splat-quenching techniques and methods for assessing their performance

Materials Science and Engineering, 23 {1976) 83 - 86

83

© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands

Novel Splat-Quenching Techniques and Methods for Assessing Their Performance*

R. W. CAHN, K. D. KRISHNANAND, M. LARIDJANI**, M. G R E E N H O L Z * * * and R. HILL

Materials Science Laboratory, School of Applied Sciences, University of Sussex, Brigh ton (Gt. Britain)

SUMMARY

INTRODUCTION

A rotary splat-quencher and a two.piston device of novel design will be described. In the first, a levitation-melted alloy drop falls on to a pair of rapidly rotating vanes in vacuo; the vanes atomise the drop and project it against a surrounding copper cylinder. The twopiston device, also used in conjunction with levitation-melting, operates with an efficient magnetic circuit not previously applied to this purpose, and makes use of a condenser discharge; the piston speed is believed to be higher than hitherto achieved. These two devices were compared with a Duwez gun, using a series of aluminiumcopper alloys as test materials. The supersaturation attained, as assessed by X-ray diffraction, was used as a measure of efficiency of quenching. It was found that the D e b y e Scherrer m e t h o d gave misleading results, tracked down to the variable composition of the different parts of a single quenched sample. Diffraction in a Guinier camera with crystal-monochromatised X-radiation gave more reliable results; it was possible to assess efficiency of quenching even with the samples of variable composition. Debye-Scherrer photographs gave a spurious non-linear parameter/composition plot. Efficiencies of the two new instruments were compared with that of the Duwez gun, and found to decrease in the sequence: gun, rotary quencher, two-piston device.

Two new splat-quenching instruments have been developed at Sussex University. The first, a Rotary Splat-Quencher (RSQ), was intended as a possible substitute for the Duwez gun, especially for refractory and/or reactive alloys. The second, in essence a conventional two-piston apparatus, was designed to accelerate more rapidly than previous versions, by means of a novel magnetic circuit, in the hope of attaining more rapid quenching by this intrinsically attractive method. We shall call this the Magnetic Yoke Pistons (MYP) technique.

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*Paper presented at the Second International Conference on Rapidly Quenched Metals, held at the Massachusetts Institute of Technology, Cambridge, Mass., November 17 - 19, 1975. **Now at University of Tehran. ***Now at University of Tel-Aviv.

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THE ROTARY SPLAT-QUENCHER

Since the aim was to quench high-melting alloy, levitation-melting [1, 2] was thought to be the most generally applicable technique. This then excluded any form of gaseous shockwave as a means of atomising the alloy drop. The necessary shock was therefore administered by means of rapidly rotating vanes, in the arrangement shown in Fig. 1. The drop, falling freely in high vacuum, impinged upon one of a pair of vanes attached to a disc rotating at anything up to 25000 r.p.m. (though in our experiments the normal limit was 13000 r.p.m.). The atomised drop was hurled against a surrounding copper cylinder, which could be cooled by liquid nitrogen (a facility not used in the experiments outlined here). The product was a very thin (<10 pm), discontinuous foil, very similar to that produced by a Duwez gun.

contact, a mild steel plate, pinned to the end of each piston, completes a magnetic circuit with the mild steel housing or yoke. Before acceleration begins, when a current passes through the coil, the steel plate is attracted to the yoke, and the attraction increases steeply as the air gap is reduced. The acceleration thus increases during the travel, unlike Harbur's solenoid accelerated piston (see Fig. 4 of ref. 3). The two coils of 300 turns each, in series, are fed with a current rising to 160A from a 1000 volt, 12000 pF condenser bank, which is discharged when the descending hot droplet passes the photocell, situated about 2 mm above the piston axis. (No lamp is used; the photocell, at the end of a long tube fitted with a convex lens, responds to the radiation emitted by the hot drop.) The discharge occupies about 8 msec, whereas the pistons meet after only 4.5 msec; this means that the pistons are very firmly held together during freezing, and there is no rebound. The piston motion was calibrated by attaching a

THE MAGNETIC-YOKE PISTONS

The most carefully designed and calibrated two-piston "dropsmasher" described to date seems to be Harbur's [3]. It depended upon a single steel piston shaft accelerated into a solenoid through which a condenser was discharged, propelling the piston against a stationary anvil. A closing speed of 12.6 ft./ sec was reported. The new apparatus (Fig. 2) depends upon a concept proposed by the authors' colleague, Prof. B. V. Jayawant, an electrical engineer. When the pistons are in

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light-emitting diode pulsed at 1 kHz to one piston and photographing it during accelera. tion; Fig. 3 shows a typical result, together with an oscilloscope plot of the current pulse shape. The pistons start at 30 mm separation and collide at a relative velocity of 13 m/sec ~ 42 ft./sec. By a greater separation this could easily be increased by half. The apparatus was tested with several aluminium alloys, and all produced very uniform foils about 50 p m thick. In our experiments drops 3 - 6 m m in diameter were used; in this range, splat thickness was virtually independent of droplet size or superheat. Drops more than about 3 m m in diameter however did not freeze entirely between the pistons; a jet of liquid was projected on to the glass plate which protected th.e photocell. The thin alloy foil frozen on the glass proved to be just as supersaturated as the foil itself.

samples of 2.5, 6.0, 12.0 and 17.3 at.% Cu, gave the peculiar lattice parameter plot shown in Fig. 4. In this plot, the "Vegard's Law" line simply joins the lattice parameters of aluminium and copper. The full circles represent a rough correction; the supposed true composition of the supersaturated solid solution differed from the nominal compositions because some CuA12 had been precipitated, and the a m o u n t of CuA12 was estimated from relative intensities of solid solution and CuA12 lines. Much misplaced cogitation was devoted to the meaning of this sharp deviation from Vegard's Law. The true explanation emerged when the Debye-Scherrer camera was replaced by a Guinier camera. This was an extremely well engineered system by Huber, with a m o n o c h r o m a t o r good enough to

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ASSESSMENT OF INSTRUMENTS

Al-12at.',,Cu RSQ

No a t t e m p t was made to assess mean cooling rates. Supersaturation of primary A1-Cu solid solutions has been adopted in our laboratory as a standard criterion for efficacy of quenching; this is, after all, a practical measure of the usefulness of a quenching device. Initially, Debye-Scherrer patterns were used. Using these, with RSQ-quenched

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Fig. 5. Profile of (311) diffraction line from RSQquenched AS-12 at.% Cu foil,using a Guinier diffraction camera.

86 generate CuK~ 1 without a2. The camera was mostly used in oblique transmission. A comparison of Debye-Scherrer and Guinier patterns from the same foils showed that the foils contained solid solutions of a range of compositions. Figure 5 shows the (311) solid solution line, with its densitometric profile, of the Guinier pattern from a RSQquenched Al-12 at.% Cu alloy. This shows that there is present a range of lattice parameters, peaking at the two extremes, which (assuming a perfect Vegard's Law plot to hold), correspond to ~ 2 . 5 at.% Cu (weaker line, All) and ~ 1 7 at.% Cu, the eutectic composition (stronger line, A12), The uncorrected anomalous lattice parameter derived from a Debye-Scherrer pattern of the RSQ-quenched 12 at.% alloy (e in Fig. 4), really corresponds to P', which is the lattice parameter of the concentrated fraction of the heterogeneous alloy, that fraction which produced the stronger subsidiary peak, A12, in Fig. 5. In fact, the broad, unresolved DebyeScherrer peak is unsymmetrical and its position is judged wrongly by the eye. The impressively small error in measurement indicated in Fig. 4 is fallacious! The apparent anomaly in Fig. 4 is thus resolved; Vegard's Law is at least approximately obeyed in fact. Presumably the heterogeneity of the RSQquenched sample is due to the variable thickness (and hence cooling rate) of the foil. A 12 at.% Cu alloy was quenched in the MYP apparatus, and gave a very sharp Guinier pattern which yielded a lattice parameter of 0.40255 ± 0.0001 nm. This corresponds to a supersaturated solid solution containing 5.2 ± 0.3 at.% Cu. Here, therefore, the composition was uniform, and did not reach either extreme of the heterogeneous RSQquenched sample. A1-Cu alloys quenched by the Duwez gun variously show CuA12 or 0' and modest supersaturation, with some lamellar eutectic (gun in air, refs. 4 - 6), more supersaturation

with little 0' if a gun was used in argon [6 - 8]. The RSQ used here gave variable supersaturation with high maximum, always some 6' (observed by electron microscope) and CuA12, but no lamellar eutectic. The RSQ technique is thus only slightly less effective than the gun used in air, as judged by present criteria, b u t considerably less effective than the gun in a proper chamber with protective atmosphere. The piston (MYP) method is less effective than either the gun or the RSQ method in retaining solute, but it gives a sample uniform both in thickness and solute content. There is scope for plenty of improvement in the performance of MYP. The very interesting theoretical study by Bl~try [9] suggests that for the speed of impact of 15 - 20 m/sec, a piston of lower conductivity (giving somewhat slower freezing and thus thinner foils) may actually give faster cooling and better supersaturation. The impact speed can also be fairly easily improved. These possible improvements will next be examined. The prize of excellent uniformity combined with very effective quenching is worth considerable effort. REFERENCES 1 J. N. Baker, C. E. Mighton and W. R. Bitler, Rev. Sci. Instr., 40 {1969) 1065. 2 J. van Audenhove and J. Joyeux, J. Nuci. Mater., 19 (1966) 97. 3 D. R. Harbur, J. W. Anderson and W. J. Maraman, Trans. AIME, 245 (1969) 1055. 4 C. Jansen, D. Sc. Thesis, M. I. T., 1971. 5 M. J. Burden and H. Jones, J. Inst. Metals, 98 (1970) 249. 6 P. Ramachandra Rao, M. Laridjani and R. W. Cahn, Z. Metallk., 63 (1972) 43. 7 M. G. Scott, Ph. D. Thesis, Cambridge, 1973; also M. G. Scott and J. A. Leake, Acta Met., 23 (1975) 503. 8 H. A. Davies and J. B. Hull, J. Mater. Sci., 9 (1974) 707. 9 J. Bl~try, J. Phys. D. (London), 6 (1973) 256.