Glass formation by containerless solidification of metallic droplets in drop tube experiments

Journal of the Less-Common

Metals, 145 (1988) 145 - 152

GLASS FORMATION BY CONTAINERLESS SOLIDIFICATION METALLIC DROPLETS IN DROP TUBE EXPERIMENTS*

145

OF

F. GILLESSEN, D. M. HERLACH and B. FELJERBACHER Institut fiir Raumsimulation,

DFVLR,

D-5000 Kdn

90 (F.R.G.)

(Received May 31.1988)

Summary We have investigated the glass-forming ability of Cu-Zr and Pd-Cu-Si alloys. Container-wall-induced nucleation was completely eliminated by solidification of small droplets during free fall in a drop tube with a total length of 2.5 m. The droplets were analysed in terms of the fraction which solidified as a glass as a function of droplet size D (50 pm < D < 1000 pm). Since the droplet size is directly correlated with the cooling rate during free fall (in purified helium gas), we were able to determine the critical cooling rates for glassy solidification. The results show that glass formation is essentially improved by containerless processing. Fully glassy spheres of C~~~Zrss, The critical cooling rates for Cu5&44 and Pd77.5CU6Si16. 5 are obtained. glassy solidification are up to 20 times smaller than those which are required in splat cooling. The analysis of the microstructure enables the density of nuclei to be determined; this depends essentially on the alloy concentration and the temperature from which the melt is cooled.

1. Introduction In general, glassy solidification by cooling the melt into the amorphous state requires sufficiently high cooling rates to avoid crystallization of the undercooled liquid. For metallic alloys, typical critical cooling rates of the order of lo6 K s-l are needed for glass preparation. Since the crystallization rate depends sensitively on the nucleation rate and subsequent growth, such high critical cooling rates are indicative of correspondingly large nucleation rates. In order to obtain cooling rates of the order of lo6 K s-l, rapid quenching technologies such as melt spinning or splat cooling have been applied. However, these methods imply an enhanced nucleation rate. The cooling substrate with its crystalline structure can act as a heterogeneity to catalyse nucleation. *Paper presented at the Symposium on the Preparation and Properties of Metastable Alloys at the E-MRS Spring Meeting, Strasbourg, May 31 - June 2, 1988. 0022-5088/88/$3.50

0 Elsevier Sequoia/Printed in The Netherlands

146

However, the crystallization rate may be lowered by a reduction in heterogeneous nucleation, thus leading to an equivalent decrease in critical cooling rates for glassy solidification. This has been demonstrated in undercooling experiments on Pd-Ni-P alloys [ 13, where surface-induced heterogeneous nucleation was reduced by embedding the liquid into BsOs as melt fluxing material, Glassy solidification was found to occur at extremely low cooling rates of about 1 K s-i, thus allowing the production of an amorphous alloy in bulk form. Moreover, as demonstrated by differential scanning calorimetry (DSC) measurements, these amorphous bulk samples were found to be much more stable than those prepared by rapid quenching techniques [2,3]. In this work a drop tube was used for containerless solid~ication of small droplets during free fall. In this way, container-wall-induced heterogeneous nucleation is completely eliminated. As demonstrated by investigations on Cu-Zr and Pd-Cu-Si alloys such experimental conditions favour glassy solidification, The analysis shows that the nucleation rate within the volume of the droplets is essentially influenced by the overheating temperature T, > T, ( Tm is the melting temperature) from which the melt is cooled prior to solidification.

2. Expe~e~~

details

The samples were prepared by remelting the constituent elements in an arc furnace under an argon atmosphere. After preevacuating the drop tube to a pressure below lop7 mbar, it was refilled with purified dry helium. Alloy samples of about 1 g were melted inductively in crucibles of quartz or glassy carbon and were subsequently dispersed as small droplets into the drop tube by forcing the melt through a nozzle. Further details on the drop tube are given elsewhere [4). The spheres were sized using assorted wire meshes and were analysed with respect to amorphous or crystalline phases as a function of droplet size by means of differential scanning calorimetry (DSC II, Perkin-Elmer) ,

3. Results 3.1. Cu-Zr alloys The heats of crystallization of fully amorphous Cu,,Zr, and C~,~Zrss splats AH,(splat) were measured by DSC [4]. Using these values, the glassy fraction X for random collections of spheres from each size group with average diameter D was determined by measuring the heats of crystallization AH,(D); thus X = AH,(D)/AH,(splat). Figure l(a) shows the data as obtained for Cu,,Zr,. Initially the melt was heated to a temperature To = T, + 150 K (Tm is the eutectic melting temperat~e). On cooling the droplets in moist helium,

147 18'000

i [K/s1 10'000 5500 r fusedquortz

& TO=Tm glossy

crucible.

+2M)K

carbon cruclbk?

x.T0.Tm+L50K

(aI

So

150 droplet

200 diamefer

D(pm)

250

* T, .Tm+LOOK

@I

droplet

diameter

LJfpm)

Fig. 1. Glassy fraction X of droplets solidified during free fall as a function diameter D: (a) Cu&5rM; (b) Cu62Zr38.

of droplet

i.e. without purifying the gas using a liquid nitrogen cold trap, only a small amount solidified in the amorphous state. Repeating these measurements in a purified dry helium atmosphere led to a substantial increase in the amorphous fraction. The glass-forming ability was further improved by an increase in T, by an amount AT = 50 K. Under such conditions almost complete amorphous solidification of droplets with diameter D < 95 pm was achieved. These results were reproduced when the quartz crucible was replaced by a glassy carbon container, indicating that the crucible material had no influence on the experimental results. In addition, the fraction X(D) remained unaffected by a further increase in the overheating temperature to T, = T, + 400 K. Obviously there is a saturating behaviour in the X(T,) relation, i.e. while an increase in T, from T, = T, + 150 K to T, = T, + 200 K reveals a remarkable improvement in the glass-forming ability, this effect levels off at overheating temperatures T, > T, + 200 K. Figure l(b) shows the results of equivalent experiments on Cu,zZr,, . In agreement with the behaviour of Cus6ZrM, a strong influence of the overheating temperature T, on the fraction X of amorphous phase was found. Again this effect saturated at temperatures T, > T, + 200 K. Howthe glass-forming ability substantially imever, in contrast with Cu,,Zr, proved as shown by the observation that complete amorphous solidification was attained for droplets up to 240 pm in diameter.

148

In order to estimate the cooling rates of the droplets solidified during the free fall in the drop tube, we used ?(K s-l) = 2.37 X 10’ X (D(E.tm))-1*9

(1)

Equation (1) was calculated on the basis of the heat transfer equations as given in ref. 5. Taking into account a mean value for the specific heat C, of 0.54 J g-l K-l and a mass density p of 9.65.g cme3 for Cu-Zr, and assuming an initial droplet velocity of 1 m s-l, T was found to be slightly larger than the values estimated for other drop tube experiments [ 51. The results of such calculations provide the scale given in the upper ordinate of Fig. 1. From this we derive the critical cooling rates for complete glassy solidification of dfoplets processed in a containerless environment as T, x and Cu,,Zr,s respectively. 4 X lo4 K s-l and TX = 8 X lo3 K s-l for Cu,,Zr, When these values for the critical cooling rates are compared with those determined for fully glassy solidification of Cu-Zr alloys of the same composition by splat cooling [6], it can be seen that those determined in this study are a factor of 3 lower for Cu,,Zr, and a factor of 20 lower for finding supports the assumption that the heterogeneous Cu&3l3 - This nucleation rate is substantially reduced in the containerless solidification technique used here, which can be correlated with a lowering of the crystallization kinetics of the undercooled melt. In order to investigate the origin of the nucleation processes responsible for the crystallization of the undercooled droplets, the microstructures of the droplets containing a small fraction of crystalline phase were analysed. Figure 2 shows the microstructure of a Cu,,Zr, droplet. Small spherical crystalline inclusions of eutectic structure are found embedded within the amorphous matrix. Assuming that a single nucleus leads to the formation of a crystallite, we infer a density of nuclei n of the order of 1015 nuclei mV3. Table 1 summarizes the corresponding values of n for the samples investigated. Obviously, II depends sensitively on both the concentration and the overheating temperature T,, . The higher density of nuclei in Cu,,Zr, can be understood following suggestions proposed by Turnbull [ 71. According to this model the nucleation rate increases with a decrease in the reduced glass transition temperature Trg = T,/T,,, . From DSC measurements on the amorphous droplets we found Trg = 0.61 for Cu,,ZrM and Trg = 0.64 for Cu,,Zr,s. Thus, the alloy with the higher Trg value exhibits a smaller nucleation rate in agreement with the experimental observation. Excluding homogeneous nucleation [ 41, the crystallites must originate by heterogeneous nucleation within the volume. Such nucleation processes can be of different origin. First of all, reactions may occur between the components of the hot liquid and the crucible, resulting in reaction products dissolved as heterogeneities within the melts. However, the observation that the results are independent of the crucible material contradicts this assumption. Further support for this comes from investigation by energy dispersive spectroscopy. No impurities were found in the droplets within a sensitivity of f 1 at.% of the weighed concentration.

149

Fig. 2. Optical micrograph of a Cu56Zr44 droplet of size D phous matrix with crystalline inclusions of eutectic structure TABLE

'2120 pm showing an amor(magnification,

270x).

1

Density of nuclei n for Cus6Zr44 and Cue2Zr3s as a function of the overheating temperature Z’, from which the melt is cooled. (The reduced glass transition temperatures Z’r, = and critical cooling rates T, for glassy Ts/Tnl (Trill is the eutectic melting temperature) solidification are also given) Alloy

To (K)

f’x (K

T,+

T,+ 200

150

T,+ 150 T,+ 200

n(nuclei

mp3)

1.5 x 10s 4 x 104

Primary 10’5

tryst.

1.2 x 104 8 x lo3

10’6 10’3

s-‘)

T rg 0.61 0.64

However, the influence of the overheating temperature on the number of quenched-in nuclei points to a progressive destruction of heterogeneities with increasing temperature up to 200 K above the eutectic melting temperature. Similar effects observed for other glass-forming alloys [3, 8, 91 have been explained by the formation of cluster associates with intermetallic compound symmetry which can act as nucleation centres. Increasing the temperature above the liquidus temperature of the corresponding intermetallit phase will destroy these associates with the consequent reduction or even elimination of the corresponding nuclei. According to the phase diagram of Cu,Zr, --c [lo], we would expect Cu,Zr associates to be present in the melt for both samples in the concentration range 0.5 < c < 0.7, since the liquidus

150

temperature Z’r = 1380 K of this CusZr compound coincides approximately with the temperature T, = T, + 200 K at which the effect of the overheating temperature on the fraction X saturates. 3.2. The Pd-Cu-Si alloy Figure 3 shows the results of the drop tube experiments as performed on the easy glass-forming alloy I’d,,. @@ii,. s . The fraction X of amorphous phase is plotted us. the droplet diameter D. On cooling the liquid from a temperature T, = T, + 50 K, fully glassy spheres of diameters up to D = 900 pm are obtained. At diameters larger than 900 pm, the fraction X decreases rapidly, in agreement with previous results of drop tube experiments on the same alloy [ 111. However, when the overheating temperature is increased from T, = 1080 K to T, = 1480 K all droplets which solidify during free fall are completely amorphous. Similar experiments have been performed on the same system using the drop tower of the NASA-Marshall Space Flight Center (total length of 32 m). With the increased time of free fall (2.6 s) amorphous solidification was observed for droplet diameters up to 1500 E.trn[12]. 2150

WI

1.0

1 X

8

i

I

L \

droplets-

.\ \

0.8 . To=1080K q T;=lLBOK

Tx =690K

700

11w

900

droplet Fig. 3. Glassy fraction

drop tower

INASA-MSCI

\ \ \ \ \ \ \ \

TP = 709K

500

- splats /

\.

T, = 6L5K

300

L

32m

\ \ \ \

AH,=38.0J/g?0.5Jlg

o-

510 i [K/s] UO

660

890

X of Pd7&&Si16.5

IJW

15w

diameter droplets

D+ml

-

as a function

of droplet

diameter

D.

In order to estimate the cooling rates for the Pd-Cu-Si droplets a relation given by Drehman and Turnbull [ 131 for Pd-Si droplets falling through the atmosphere was used +(K s-l) = 7.6 X lo* X D(E.tm)-‘.5

(2) From eqn. (2) we calculate ?(D = 900 pm) = 890 K s-i and ‘?(O = 1500 pm) = 410 K s-l. The results obtained on the Pd77.5CugSi16. 5 alloy system are qualitatively similar to those found for the Cu-Zr alloys; however, the critical cooling rates for glassy solidification are one to two orders of magnitude smaller. This was expected since Pd,,.5CugSi16.5 belongs to the easy glass-forming alloys.

151

In this metal-metalloid system a strong influence of T, on the amorphous fraction X was found. This observation is in agreement with previous investigations on the undercooling behaviour of bulk Pd,,.,Cu,Si16_s melts [9]. These experiments revealed a critical overheating temperature T,, = 1400 K which coincides approximately with the liquidus temperature of the intermetallic phase PdsSi. Overheating temperatures lower than 1400 K lead to moderate undercooling levels with solidified droplets showing microstructures with a dendritic Pd,Si phase embedded in a eutectic matrix. Overheating temperatures higher than 1400 K give rise to an extension in achievable undercooling up to AT = T, -. T,, = 300 K (T, is the nucleation temperature) with a simultaneous disappearance of PdsSi dendrites. The microstructures of the highly undercooled samples consist exclusively of a metastable fine-grained eutectic. These results are consistent with the observations on the small I’d-Cu-Si droplets. When cooling the melt from a low overheating temperature T, = 1080 K, the droplets of diameter D > 900 I.trn contain PdsSi dendrites within an amorphous matrix. This amorphous matrix changes into a fine-grained eutectic structure when the droplet diameter increases further, i.e. with decreasing cooling rate.

4. Conclusions Drop tube experiments on Cu-Zr and Pd-Cu-Si alloys demonstrate that containerless solidification under pure environmental conditions improves the glass-forming ability of these alloys. This has been explained by the elimination of heterogeneous container-wall-induced nucleation and a reduction in heterogeneous surface-induced nucleation. Both alloy systems reveal a significant influence of the overheating temperature on the fraction X of amorphous phase formed during solidification. This observation is attributed to the formation of nuclei of intermetallic compound symmetry within the volume of the undercooled droplets. Increasing the overheating temperature above the liquidus temperatures of the corresponding intermetallic phases leads to a progressive destruction of such heterogeneities with a consequent extension of the glass-forming ability.

References 1 H. W. Kui, A. L. Greer and D. Turnbull, Appl. Phys. Lett., 45 (1984) 615 - 616. 2 H. W. Kui and D. Turnbull, Appl. Phys. Lett., 47 (1985) 796 - 797. 3 F. Gillessen, D. M. Herlach and B. Feuerbacher, 2. Phys. Chem., 156 (1988) 129 133. 4 F. Gillessen and D. M. Herlach, Mater. Sci. Eng., 97 (1988) 147 - 151. 5 A. J. Drehman and D. Turnbull, Ser. Metall., 15 (1981) 543 - 548. 6 Y. Nishi, T. Morohoshi, M. Kawakami, K. Suzuki and T. Masumoto, in T. Masumoto and K. Suzuki (eds.), Proc. 4th Znt. Conf. on Rapidly Quenched Metals, Sendai, Japan, Japan Institute of Metals, 1982, pp. 111 - 114.

152 7 D. Turnbull, Contemp. Phys., 10 (1969) 473 - 488. 8 A. Gladys, M. Brohl, F. Schlawne and H. Alexander, 2. Metallhd., 76 (1985) 254 256. 9 D. M. Herlach and F. Gillessen, J. Phys. F, 17 (1987) 1635 - 1644. 10 M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 2nd edn., 1958, pp. 655 - 657. 11 C. S. Kiminami and P. R. Sahm, Acta Metall., 34 (1986) 2129 - 2137. 12 J. Steinberg, A. E. Lord, Jr., L. L. Lacy and J. Johnson,Appl. Phys. Lett., 38 (1981) 135 - 137. 13 A. J. Drehman and D. Turnbull, in G. E. Rindone (ed.), Material Processing in the Reduced Gravity Environment ofSpace, Elsevier, Amsterdam, 1982, pp. 81 - 85.