E L S E V I E R Materials Science and Engineering A219 (1996) 21-25

M A T E R I A L S SCIENCE &

ENGINEERING

A

Supersaturation and optical properties of metal-rich Zr-O and Z r ' N liquids

Shankar Krishnan a, J.K. Richard Weber a, Collin D. Anderson a, Paul C. Nordine% Craig T. Morton b, William. H. Hofmeister b, R.J. Bayuzick b

aContainerless Research, Inc., Evanston, IL 60201, USA U Vanderbitt University, Nashville, TN 37235, USA

Received 26 February 1996

Abstract

The effects of oxygen and nitrogen additions on the optical properties of pure liquid zirconium were investigated under containerless conditions using rotating analyzer ellipsometry. Liquid zirconium was electromagnetically levitated and melted, and metered flows of oxygen and nitrogen were added to the melt in separate experiments. The optical constants (n and k) and the normal spectral emissivity (eJ of the liquid at 2 = 633 nm were measured over the duration of the experiments. The measured values of n and k decreased and ~). increased with the O and N concentrations for the single-phase liquids. Metastabte Z r -O solutions with 135% of the liquidus O-atom concentration were achieved at a temperature of 2260 K. Precipitation of a solid phase from the Z r - N liquid at 2290 K occurred at a concentration of 7.6 at.% N. Based on these results, a new method using in situ ellipsometric measurements is described for the accurate determination of iiquidus concentrations and temperatures. Dissolved oxygen and nitrogen were found to influence the optical properties of liquid zirconium in a manner proportional to their respective valences.

Keywords: Supersaturation; Optical properties; Zirconium

1. Introduction

There is considerable interest in examining the effects of contaminants on the properties of liquid metals and alloys. Contaminants can influence the resulting mi- crostructure and properties of the solid phase, can influence the solidification kinetics by causing heteroge- nous nucleation, and can alter the intrinsic properties of pure materials. Further, it is also possible to supersatu- rate a liquid with the contaminant species under con- tainerless conditions, so that highly non-equilibrium compositions may be attained.

There are few data in the literature that describe the effects of alloying with gaseous impurities on the optical and electronic properties of high temperature liquids. These effects were investigated in this study of the reactions of liquid zirconium with oxygen and nitrogen. The dissolution of oxygen or nitrogen should lead to increased electrical resistivit Z due to electron scattering by impurities. A corresponding change in the optical properties is also expected, which would decrease the

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reflectivity and therefore increase the emissivity. Be- cause the emissivity can depend on the extent of con- tamination, a constant brightness temperature during the reaction of oxygen or nitrogen with a liquid metal does not insure that the actual specimen temperature is constant.

The absolute specimen temperature was determined throughout the present experiments. This was facilitated by measurements of the optical properties of the liquid during its reactions with oxygen and nitrogen. A rotat- ing analyzer ellipsometer [1-4] was used to measure the optical constants, n and k, of the liquid as a function of time. The optical constants were used to calculate the normal spectral emissivity of the liquid. In combination with the optical pyrometry, accurate specimen tempera- tures were thus obtained. It was hypothesized that the dissolution of oxygen or nitrogen in the liquid would cause a continuous intrinsic change in the optical con- stants; and when a second phase nucleated, the ellipso- metric response would change discontinuously or by larger amounts to unambiguously identify the event.

22 S. Krishnan et al. / Materials Science and Engineering A219 (I996) 21-25

A second hypothesis of this work was that, if accu- rate identification of second phase formation/dissolu- tion could be achieved, it would become possible to obtain accurate phase boundary measurements. High temperature liquid phase boundaries are generally un- certain, particularly for the Z r -N and to some degree the Zr-O systems [5]. We describe and discuss the method by which several points along the liquidus can be measured in a single experiment. The technique can be used to investigate numerous alloy compositions, and is particularly easy to implement if the alloying element is supplied through the gas phase. It can also be used when solid or liquid alloying elements are added to the melts.

2. Experimental

The experimental arrangement used in this work has been described in the literature [1-4]. An electromag- netic (EM) levitation system was used to achieve con- tainerless melting and positioning of liquid zirconium specimens. A 12:1 step-down transformer and a 5-turn levitation coil permitted stable levitation of 0.3-0.4 cm diameter drops of liquid zirconium at temperatures in excess of 2300 K. A calibrated Photomatic II optical pyrometer was employed for measurements of the radi- ance temperature. A rotating analyzer ellipsometer sys- tem was used for emissivity and optical property measurements, as previously described [1-4]. The basic operating principle was to allow a source of monochro- matic light of known, fixed incident polarization to be incident on the specimen, and determine the change in polarization that occurred upon reflection. The latter was accomplished by rotating the analyzer to three fixed azimuthal positions with the aid of stepping mo- tors. The changes in amplitude and phase of the reflected beam relative to the incident beam were mea- sured, from which values of the optical constants, n and k, of the material were calculated. The values of n and k were used to compute the normal spectral emissivity of the material.

In separate experiments starting with pure liquid zirconium, high purity oxygen or nitrogen were deliv- ered to the system with a calibrated flow meter. The oxygen and nitrogen were mixed with a larger flow of pure argon gas immediately downstream of the flow meters to achieve rapid flow in the lines leading to the levitation chamber. The total pressure of argon in the levitation system was approximately 0.3 bar and in- creased slightly during the experiments due to the gas flow into the closed system. Complete reaction of the gases with the specimen was assumed. This is a reason- able assumption since the zirconium is relatively in- volatile at these temperatures and due to the fact that the chamber walls are essentially at room temperature;

the levitated liquid drops were the only significant sink for the reactive gases. In addition, a small quantity of evaporated zirconium would also react with the oxygen or nitrogen. Based on the known vaporization enthalpy of zirconium, and based on experiments with pure liquid Zr, it was estimated that 0.26% of the Zr evapo- rated during the dissolution experiments. Reaction of the evaporated Zr to form ZrO2 or Zr3N4 would give rise to a maximum uncertainty of 0.5 mole% O or 0.35 mole% N dissolved in the liquid drop.

Measurements of the radiance temperature and nor- mal spectral emissivity were obtained on the liquid during its reactions with 02 and Nz. Using Wien's approximation to Planck's law, the true specimen tem- perature was obtained. The specimen temperature was 2260 K for reactions with Oz and 2290 K for N2. The absolute uncertainty in temperature was _+ 9 K, due to emissivity uncertainty (_+ 6 K), pyrometer calibration (_+ 4 K), and temperature fluctuations during the ex- periments ( + 5 K).

Arc melted specimens made from crystal-bar zirco- nium were used for the measurements. Specimen masses of 386.70 mg and 388.78 mg were used for experiments with 02 and N2, respectively. Specimens retrieved after the experiments were re-weighed, and the increased masses were consistent with the amounts of Oz and N2 added to the system.

The absolute uncertainty in the mole fraction of dissolved gas was estimated to be 0.2-0.5 mole%. The overall compositions calculated for the liquid are taken to be bulk compositions, since rapid stirring of the melt occurs during electromagnetic levitation.

3. Results

Figs. 1 and 2 show the effects of oxygen and nitrogen on the derived values of the normal spectral emissivity and the optical constants of liquid Zr at the tempera- tures indicated in the figure captions. Oxygen additions resulted in a small increase in the emissivity with com- position, followed by a large change in the derived emissivity at 14.9 atom% O, after 8.32 cm 3 (STP) 02 were added to the system. This composition exceeded the 11 'atom% O solubility of oxygen in zirconium at the experimental temperature, which is shown by the equilibrium Zr-O phase diagram [5]. The emissivity change was attributed to the precipitation of oxygen rich ~-Zr from the supersaturated liquid. A similar effect was observed (Fig. 2) with N2 additions, except that a nitrogen-rich phase precipitated at about 7.65 atom% N, when 3.96 cm 3 (STP) N2 were added to the system. The optical properties measured on the two- phase mixtures are not true properties but rather serve to illustrate the large instrument response that occurred when the second phase appeared.

S. Krishnan et al. ~ Materials Science and Engineering A219 (1996) 2 I - 2 5 23

>, 0.39 .>_ co

E LU 0.36

O o~

~" 0.33

E O

z 0.3

0.27

4.5

x

z , g

t Nucl. limit

4 8 12 16

Atom % O

4 " O

C o3 t -

3.5 t - 65

t-

O 3 ._o

2 . 5 0

2 20

Fig. i. Normal spectral emissivity and the optical constants for liquid zirconium at 2260 K as a function of the oxygen concentration. The arrow marks the concentration at which the ~-Zr phase nucleated.

The lines drawn through the data in Figs. 1 and 2 are 1east squares fits to the equations:

n,k, o r e ~ = A + B ' X (1)

where X is the atom% O or N in the liquid. The coefficients of Eq. (1) are given in Table 1. Alt

data up to the last point prior to the discontinuity in optical property were used in calculating the results. The table includes results for a fit of the imaginary part of the dielectric constant, ez = 2nk, to Eq. (1). Standard errors of the slope, given for the 2nk fits, will be discussed.

Table 1 Fits of the optical property versus composition data for dissoiution of O/N in liquid Zr

Solution and optical property A B

Zr-O liquid (T = 2260 K) n 2.651 -0.00522 k 4.213 -0.01430 e~ 0.341 +0.00110 2nk 22.34 -0.118 _+ 0.008 Zr-N liquid (T = 2290 K) n 2.682 -0.0227 k 4.192 -0,0221 e~. 0.345 +0.000984 2nk 22.31 -0.149 _+ 0.029

4. Discussion

Zirconium is nominally a 4-valent metal; the effect of dissolution of oxygen or nitrogen is to decrease the number of fl'ee electrons that are available in the con- duction band. The number of free electrons can be taken equal to four minus the oxygen or nitrogen valence (2 and 3, respectively) multiplied by the O:Zr or N:Zr concentration ratios. The imaginary part of the dielectric constant, e2 = 2nk, in metals is directly related to the density of the conduction electrons. Fig. 3 shows the values of 2nk as a function of the number of conduction electrons available per Zr-atom in the liq- uid. In making Fig. 3 and in calculating the coefficients for 2nk given in Table 1, we have eliminated the data points obtained beyond the nucleation limits. The last three points before this limit were also removed from the Z r - N data, because these points showed excessive deviations. It can be seen that oxygen and nitrogen

0.39 ._> co

E uJ 0.36

O3

~- 0.33

E O

z 0.3

I

O O •

k

T V ~

• ~ . " , , " , ' ,

E ,k

0.27 2 10

r3

Nucl. limit i ~ P P

0 2 4 6 8

Atom % N

4.5

3.5

2.5

"13 ¢-

r -

E

o3 t - O O

._o

O

Fig. 2. Normal spectral emissivity and the optical constants for liquid zirconium at 2290 K as a function of the nitrogen concentration. The arrow marks the concentration at which a solid phase nucleated.

24

~- 2 2 OJ

II e4

20

Pure Liquid Zr • O atoms • N a t o m s

18 ' P ' ~ P '

4.1 4.0 3.9 3.8 3.7 3.6

Free Electrons/Zr

Fig, 3. Plot of the optical property, E z = 2nk, versus the free-electron concentration per Zr atom for liquid Zr-O (filled circles) and Zr-N (filled triangles).

24 S. Krishnan et al. / Materials Science and Engineering A219 (1996) 21-25

22001

= 2000 ,.

4600 , , , , I , , , , , . . . . . . . . , , , , , i , , , , m ,

0 10 20 30 Atomic Percent Oxygen

Fig. 4. Zr-rich section of the Zr-O phase diagram, reproduced from Ref. [5]. The path of the dissolution experiment is indicated by the straight dashed line at T = 2260 K (1987 °C). The symbol + marks the point where precipitation of ¢~-Zr was observed.

have a practically identical effect when their respective valences are taken into account. This presentation ne- glects any differences in the effect of added oxygen and nitrogen on the number of zirconium atoms per unit volume. The results in Fig. 3 are confirmed in Table 1, where it is seen the plots of 2 n k versus concentration have slopes of approximately 1:1.5, i.e., in proportion to the O:N valence ratio.

The emissivity values were insensitive to concentra- tion and changed by only 0.01 over the full range o f concentrations. Therefore, the derived slopes for emis- sivity vs. composition (see Table 1) do not exhibit the 1:1.5 ratio expected for the O:N effects. Nevertheless, a plot of emissivity vs. concentration (not given) shows excellent agreement for both sets of data. The intercepts for e;~ given in Table 1 also agree within 0.005 with the published values of the spectral emissivity of pure liquid Zr [a,3].

Based on the present results, we present a hypothesis and a suggestion of practical value. The first is that solutions of electronegative elements (such as oxygen and nitrogen) in a liquid metal will produce the same effects on emissivity if the concentrations are normal- ized by the valence of the electronegative element. The suggestion concerns the effect of a given impurity on different metals. It is that the effect of a dissolved electronegative impurity element on the emissivity of liquid metals will decrease as the metal valence in- creases. Some further investigation with liquid metals with different valences will be required to confirm these ideas.

Figs. 4 and 5 show implications of the present work relative to the Z r - O and Z r - N phase diagrams [5].

Straight dashed lines are drawn in the diagrams to intersect at the temperatures and compositions at which nucleation was observed. These nucleation points are marked with the + symbol. The horizontal dashed lines show the paths of the two dissolution experiments. It can be seen that the path for the Z r - O experiment extends into the well-established two-phase region be- tween liquid and solid ~-Zr. Nucleation of the ~-Zr occurred when the liquid supersaturation with oxygen was 135% of the equilibrium liquidus composition. The Z r - N phase diagram is rather uncertain, as indicated by the use of dashed lines for the liquidus curves shown in Fig. 5. The present work demonstrated that a solid phase precipitated where the published phase diagram shows the liquid to be stable. The change in optical properties was sudden and large in the Z r - N experi- ment,~ showing that a significant supersaturation had occurred. We conclude that the Z r - N phase diagram may need to be revised to show a liquidus line at < 7.5 atom% N and 2290 K.

The methods of this work may be readily extended to obtain accurate liquidus measurements at very high temperatures. The procedure would be to (i) leviate and melt the pure metal, (ii) add oxygen until the solid phase precipitates, and (iii) heat the melt until the solid phase is completely dissolved. The ellipsometric results will identify this point by exhibiting a discontinuity in the value or slope of a plot of optical properties vs. apparent temperature or time. The temperature at this

2200= /' L l v /

" - 2 0 0 0 . . . . . . . . . . . + l I @l~ ~

E 1800

( Zr 1600 ,, , , , ,

0

( Zr)

10 20

Fig. 5. Zr-rich section of the Zr-N phase diagram reproduced from Ref. [5]. The path of the dissolution experiment is indicated by the straight dashed line at T= 2290 K (2017 °C). The symbol + marks the point where precipitation of the solid phase was observed. This point does not fall in a two-phase region, showing that this region of the phase diagram may need to be revised.

Atomic Percent Nitrogen

S. Krishnan et al. / Materials Science and Engineering A219 (1996) 21-25 25

point is the liqnidus temperature which would be accu- rately determined f rom the optical properties and radia- tion pyrometry . The process could be cont inued with further oxygen additions to measure the liquidus tem- peratures at increasing O-a tom concentrat ions. The method is no t restricted to O or N atoms, and could be applied to other gaseous or solid elemental addit ions to the melt.

Acknowledgements

Suppor t for this work was provided by grants f rom the Nat ional Aeronaut ics and Space Adminis t ra t ion, Microgravi ty Sciences and Applicat ions Division, grant numbers NASW-4687 and NAG-8978.

References

[1] S. Krishnan and P.C. Nordine, The Optical Properties of Liquid Aluminum in the t.2-3.5 eV Energy Range, Phys. Rev. B, 47 (1993) 11780.

[2] S. Krishnan and P.C. Nordine, The Optical Properties of Liquid and Solid Zirconium, Phys. Rev BI, 49 (1993) 3161.

[3] S. Krishnan, J.K.R. Weber, C.D. Anderson, P.C. Nordine and R.I. Sheldon, Spectral Emissivity and Optical Properties at 2 = 632.8 nm for Liquid Uranium and Zirconium at High Tempera- ture, J. Nuct. Materials, 203 (1993) 112.

[4] S. Krishnan, C.D. Anderson, J.K.R. Weber, P.C. Nordine, W.H. Hofmeister and R.J. Bayuzick, Optical Properties and Spectral Emissivities at 632.8 nm in the Titanium-Aluminum System, Met. Trans., 24A (1993) 67.

[5] J.L. Murray, L.H. Bennett and H. Baker, (eds.), Binary Alloy Phase Diagrams, American Society for Metals, Metals Park, Ohio, 1986.