Phase Equilibria in the System MgO-B2O3

Phase Equilibria in the System MgO-B203 T. MUTLUER and M. TIMUCIN Metallurgical Enzineering Department, Middle East Technical University, Ankara, Turkey TEMPERATURE ( * C )

Phase equilibria in the system MgO-B203 were investigated using DTA and quenching techniques. The system contains 4 invariant points. The compounds MgO. 2 B 2 0 3 and 2Mg0. B z 0 3 melt incongruently at 995" and 1312"C, respectively, whereas 3 M g 0 . B,O, melts congruently at 1410°C. A eutectic occurs at 1333°C and 71% MgO. I.

800

56% M g 0 0 X W

Introduction

f

systems containing B 2 0 , arc: of interest because alkalineearth borates are potentially applicable as Fluxing agents in steelmaking. The system MgO-B203 i s of particular interest because MgO is a principal constituent o f the linings of steelmaking furnaces and because part of the furnace slag consists of MgO. Phase relations in the system MgO-B,03 were examined by Toropov and Konovalov' on the basis of cooling curve analyses: they reported the existence of the congruently melting compounds 3 M g 0 . B 2 0 3 (1100"), 2 M g 0 . B , 0 : j (13X1"), and MgO,B,O, (1 193°C). The phase diagram as revised by Davis and Knight2 agreed with the general features otthat ofToropov 2nd Konovalov, except that (1) M g 0 . B , 0 3 was shown to melt incongruently at 988°C and (2) the liquidus temperatures were lower by -40°C. Kuzel:' subsequently synthesized the compound MgO. 2B,O,,; he also reported that MgO. B20:%did not exist as such but rather was a mixture of M g 0 . 2 B 2 0 3 and 2MgO. B 2 0 3 . Kuzel's results were contirmed by Fletcher er al..4 who also detected no solid solutions in the system.

0"

7.5% MgO

e Q

1

0

a

z W

7

8

111.

TemD ("C)

8 .0 8.0 8.0 20.0 20.0 20.0 30.0 30.0 35.5 35.5 35.5 35.5 35.5 35.5 47.5

MIB+L MBr+L 2 liquids MB,+L M,B+L 2 liquids M,B + L L MBP + MZB M2B+ L M,B + L M,B+L M,B + L L M,B+L

I320 I340 I300 I320 I375 I300 I320 1100 1120 1325 1350 1360 1320 I350

1250 1300

14

15

16

17

Results

Phases Present at Equilibration Temperatures

1010 9 85 1110 985 1010 Ill0

1150 1240

I

13

Received Seprember 3 . 1974: revised copy received December 7, 1971. *Model 900, E. 1. DuPont de Nemoun & Co.. Wilmington, DE.

Phases present*

1010

,

12

Endothermic peaks recorded for 6 samples (Fig. I ) were reproducible within? 3°C. In contrast, the high rate ofcrystallimion of the melts caused the exothermic peaks to drift erratically, even at reduced cooling rates: therefore, the cooling data were unreliable.

MgO (wr'l)

1135

11

measured and controlled with an estimated accuracy of S _ 5°C by standardized Pt-Ptl3Rh thermocouples. Equilibration times were >21 h. The MgO contents of DTA samples and those produced in quenching experiments were determined by atomic absorption methods, whereas B,O, was assayed volumetrically" using modified alkali titration in which MgO was removed from the sample solution before it was fortified with sugar. There was noevidence of B 2 0 3 volatilization, within limits of analytical error, in the composition range studied.

Temp ( " C )

9 85

10

Fig. 1. DTA results obtained on heating.

Reagent-grade MgO and H3B03 pswders in the desired proportions were mixed thoroughly, placed in Pt dishes, and heated stepwise to 400°C in 48 h to dehydrate the H:,BO:,. These powders were pressed into pellets 0 . 5 in. in diameter and fired at 950°C until the reactions between the oxides were complete (several days), as ascertained by X-ray diffraction patterns. DTA thennograms of selected samples were obtained by heating the powders in a thermoanalyrer* at a programed rate of-t"C/niin. For quenching experiments, portions ofthe prereacted pellets were placed in small Pt crucibles and heated in a vertical tube furnace at selected constant temperatures. Whert equilibrium was reached, the samples were air-quenched to room temperature, and the phases present were determined by petrographic microscopy and X-ray diffraction analysis using C u K n r,idiation. Temperatures were

113X 1150

9

rnV ( P t - P t 13 Rh; Ref. Junction,O " C )

Experimental Procedure

Table 1.

1000 1100 lZb0 1300 lL00

67X

IDE

11.

900

*L=liqu*d, M B P = MgO. 26202, M2B = ZMgO. BZO,, M,B = 3Mg0. B,o,, and t r = trace,

196

MrO ( ~ 1 % )

17.5 17.5 53.7 53.7 53.7 56.0 56.0 63.5 63.5 67.0 67.0 67.0

xo.O

80 .0

Phasea preEent*

M,B+L M,B(tr) + L M2B M3B + L M,,B(tr)+ L M2B + M:,B M:,B + L M,B L M,B + MgO(tr) M,B + L

L M:,B + MgO MgO + L

May-June 1975 Table 11. MgO content of sample ( w t 9 )

20.0 8.0 8.0

Strength Degradation of Soda-Lime-Silica Glass During Dynamic Loading

197

Compositions of Immiscible Lisuids Temp. (“C)

I160 1210 1300

1500

MgO content of liquids (wt%) MnO-rich liauid B,O,-rich liauid

0.05

30.0 29.5 29.5

14 00

.ox

.20 ~~

~

1300

L3

’zoo

Y

The phases present at critical. temperatures are summarized in Table I . The presence of glass was determined using a petrographic microscope, and the solid phases were identified by X-ray diffraction. Stratified layers of samples quenched in the 2-liquid region were analyzed (Table 11) to help delineate the boundaries of the immiscibility dome. The phase diagram for the system MgO-B203 (Fig. 2) was constructed from the data given in Tables I and I 1 and Fig. 1. The system contains 4 invariant points, i.e. a peritectic at 995”C, MgO. ZB,O,(s)~liquid (u)+2MgO. B,O,(.s)

c

a

1100 0

I

?

1000 so0

(1)

a monotectic at 1146”C, liquid (hWliquid (c)+2Mg0. B20,(s)

BZ

(2 )

a second peritectic at 1312”C,

(Weight X )

+

2Mg0. B2O3(s)-Iiquid ( d ) 3Mg0. B,O,(s)

(3)

Fig. 2.

and a eutectic at 1333”C, liquid (eIe3MgO. B,O,(s)+ MgO(s)

(4 )

where ( u ) - ( e )designate the equilibrium liquids indicated in Fig. 2. Reactions ( I ) and ( 2 ) and the temperatures at which they occur are similar to those found by Davis and Knight,’ except that the incongruently melting compound in Eq. ( 1 ) is MgO. 2B20:, rather than Mg0.B20, and the monotectic liquid in Eq. ( 2 ) contains 30% MgO rather than 36%. X-ray diffraction patterns for a mixture identical in composition to the “compound” MgO. B,O, consisted of the major peaks of MgO.ZB,O, and 2Mg0.B203 at all temperatures up to 990°C. Further DTA and petrographic evidence suggested that this compound did not exist, in agreement with the observations of other workers .:3,‘ The compound 2Mg0. B20, would melt congruently at 13x1“C, according to Toropov and Konovalov,’ and at 134O”C,according to Davis and Knight.* The present study showed, however, that this compound melts incongruently at 1312°C and is related to 3Mg0.B,03 by a peritectic reaction, as opposed to the eutectic reaction predicted by previous investigators. The melting temperature o f 3 M g 0 . B 2 0 , (1410°C) agrees well with that determined by Toropov and Konovalov. The eutectic reaction isotherm determined in the present work is = 30°C lower than previous estimates.’.’ The composition of the eutectic liquid is the same in all the proposed diagrams. The steep

System MgO-B,03. L = liquid, MB, = Mg0.2B203, M 2 B = 2 M g 0 . B 2 0 3 ,M3B=3Mg0.B203.

slope o f the MgO liquidus, which is inferred above 1450°C, suggests that small additions of B20, to MgO decrease the melting point ofthe latterratherrapidly, thus deteriorating its refractoriness.

Summary

IV.

Phase equilibria in the system MgO-BpO:, were revised as the result of a study using DTA and quenching techniques. It was confirmed that MgO.B,O, does not exist. Because 2Mg0.B203 undergoes a peritectic transformation, the central portion of the diagram is substantially different from those published previously.

Acknowledgments: The writers thank E . Tamer and M. Duzgun. both o f t h e Metallurgical Engineering Depanment of Middle East Technical University, for help in the X-ray and DTA w o k and for perfoming the chemical analyses, respectively. References

’ N . A. Toropov and P. F. Konovalov, “Binary System MagnesiumOxide-Boron Anhydride,“ Zh. Fiz. Khim., 14 [ 3 ] 1103-1109 (1940). H. M. DavisandM. A. Knight, “TheSystemMagnesiumOxide-BoricOxide,”J. Amer. Ceram. Sor., 28 [4] 97-102 (1945). H. J. Kuzel, “Investigation ofthe MgO-BZOt System: Synthesis and X-Ray Study of the Compound Mg0.2B20,,” N e w s Jahrb. M i n e r a l , Monarsh.. 1964, No. 12, pp. 357-60. ‘B. L. Fletcher, 1. R. Stevenson, and A. Whitaker, “PhaseEquilibria in the System Ca0-Mg0-B20, at 900°C.” J . Amer. Ceram. SOC., 53 [2] 95-97 (1970). A. 1. Vogel, Text-Book of Quantitative Inorganic Analysis, 3d ed.; p. 252. Longmans Green & Co. Ltd , London, 1961

’

Strength Degradation of Soda-Lime-Silica Glass During Dynamic Loading HISAO YAMADA* General Electric Company, Nela Park, Cleveland, Ohio 441 12

The strength degradation of soda-lime-silica glass during dynamic loading was studied by using a fracture-mechanics approach. The theoretical and experimental results indicate Presented at the 76th Annual Meeting, The American Ceramic Society, Chicago, IL, April 30. 1974 (Basic Science Division, No. 46-B-74). Received June 17, 1974; revised copy received October 17, 1974. ‘Now with theMaterials Science Division, Argonne National Laboratory, Argonne, IL 60439.

that subcritical flaw growth during loading causes the strength degradation.

I.

C

introduction

progress has been made in understanding the strength of glass by using an approach based on fracture me-

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