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Zirconium-refractory alloys
415
JOURNAL OF THE LE?S+COMMON METALS Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ZIRCONIUM-REFRACTORY H. M. SKELLY
AND
ALLOYS
C. F. DIXON
Nuclear and Powder Metallurgy Section, Physical Metallurgy Division, Mines Branch, Department of Energy, Mines and Resources, Ottawa (Canada) (Received November 18th, 1970)
SUMMARY
Zirconium alloys containing thorium oxide, yttrium oxide, cerium zirconate or lanthanum oxide were prepared by arc-melting. The refractories apparently~dissolved in the molten zirconium and precipitated as a dispersed phase on solidification. Hightemperature (65O’C) tensile strengths, up to twice that of Zircaloy-2, were observed for arc-melted specimens. The size and distribution of thorium oxide and cerium zirconate particles were shown to vary depending on the heat treatment and, although interparticle spacing, believed to be ideal, was not achieved, particles of a size considered to be fme enough to produce dispersion strengthening were obtained.
INTRODUCTION
This investigation examines the possibility of improving the high-temperature properties of zircoriium through dispersion strengthening by the addition of refractory compounds, particularly oxides, by arc-melting. To dispersion strengthen zirconium by this method it is necessary for the refractory compound to dissolve in molten zirconium and, on cooling, to precipitate as a fine dispersion of the refractory compound itself, or of some stable compound formed by its reaction with zirconium. Because of the high reactivity of zirconium, its high solubility for oxygen, and the stability of the zirconiumoxygen solid solution, only the most stable r&actories would be expected to exist in a zirconium matrix. Thorium oxide (ThO,), because of its high stability, was selected for initial tests. Observations were also -de on the effect of additions to zirconium of yttrium oxide (Y203), lanthanum oxide (Laz03), and cerium zirconate (Cez0,*2ZrOz). The initial part of this work was published earlier in a short communicationi. EXPERIMENTAL PROCEDURE
Preparation of allqys The alloy&ted
in Table I were prepared using reactor-grade
zirconium
Crown Copyrights Reserved. J. Less-Common Metals, 23 (1971) 415425
416 TABLE TENSILE
H. M. SKBLLY,
C. F. DIXON
I PROPERTIES
OF ZIRCONIUM
AND ZIRCONIUM
ALLOYS
AT
650°C UTS (k.s.i.)
Nominal composition
Condition
Zr Zircaloy-2* Zr-0.64%Th Zr-0.34%ZrO, Zr-0.64%Th-0.34%Zr02 Zr-0.7%Th02 Zr-0.7%Th02 Zr-2.5%ThO, Zr-3.5%Th02 Zr-5.0%Th02 Zr-7.0%Th02 Zr-2.5%ThO, Zr-2.5%Th02 Zrl%Ce,O,.2ZrO, Zr-1%Ce,03.2Zr0, Zr-3%Ce,0,.2Zr02 Zr-S%Ce,O,.2ZrO, Zr-7%Ce,O,.2ZrO, Zr-l%Y,O, Zr-2%Y,O, zl-3%YtOs Zr-S%Y,O, Zr-O.S%La,O, Zr-l%La,OB Zr-2%La,O,
As arc-melted As arc-melted Swaged 500°C, H.T. 700°C Swaged 500°C, H.T. 7OO’C Swaged 500°C, H.T. 7OO’C Swaged 500°C, H.T. 7OO’C As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted H.T. 650°C 6 h, W.Q. H.T. 95O’C 6 h, W.Q. Swaged 500°C As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted ,As arc-melted As arc-melted
0.2% ;ksS.i.)
W.Q. = Water quenched. H.T. = Heat treated. UTS = Ultimate tensile strength. YS = Yield strength. * 1.45% Sn, 0.14% Fe, O.lO’% Cr, 0.06% Ni, 0.12%
6 6 6 6
h, h, h, h,
W.Q. W.Q. W.Q. W.Q.
O,, balance
11.2 17.6 9.5 10.0 9.7 23.6 16.2 23.7 27.0 26.6 25.5 26.1 23.7 19.9 15.2 20.6 27.4 36.0 19.5 21.6 24.0 19.2 19.3 19.8 17.1
1.5 11.9 4.1 5.6 4.0 14.7 10.9 16.8 17.3 17.7 16.3 17.5 16.5 11.9 9.3 13.6 18.3 25.7 13.2 14.6 15.3 12.3 12.0 12.5 10.9
o/0 Elongation on 1 in. 44 42 63 59 66 48 36 27 35 26 20 22 27 36 36 8 8 14 20 26 24 22 28 32 27
Zr.
sponge containing 1100 p.p.m. oxygen. The refractory compounds were in the form of powders, 3-4 pm in particle size. Melting and alloying were conducted in a laboratory-sized tungsten-arc furnace in an argon atmosphere. The zirconium sponge was consolidated by melting into a dense, cigar-shaped ingot in which a cavity was drilled to contain the refractory compound. Each charge weighed approximately 75 g and was remelted at least 4 times to ensure homogeneity. Visual observation of the zirconium and the refractory addition through the dark-glass window of the arc furnace revealed that, during melting, the particles of refractory appeared to dissolve in the molten zirconium. The behaviour of the refractory particles was markedly different in these instances from the behaviour on other occasions in which unsuccessful attempts were made to prepare metal-refractory alloys and the refractory additions obviously remained undissolved during arcmelting. The Zr-O.64’ATh and Zr-O.34%ZrO, alloys were prepared to determine the J. Less-Common
Metals, 23 (1971) 415-425
417
ZIRCONIUM-RBFRACTORY ALLOYS
elect of thorium and oxygen separately in the event of any decomposition of ThO,. The 0.64% Th and the oxygen in 0.34% ZrOa are ~ro~t~y equival,rmt to the thorium and oxygen contained in 0.7% ThOa. The ~~%~~.~%~~ alloy was prepared to determine if thorium and oxygen added together would produce the same effect as 0.7% ThOz. The zirconium-refractory alloys are referred to by their nominal compositions. Sow arc-melted ingots were swaged in air at 500°C to 0.35 in. diam. rod. The heat treatments listed in Tables I and II were conducted under vacuum. Polishing and etching
To investigate dispersed phases present in the alloys, 0.25 in. thick specimens were cut from the am-melted ingots in the rne~~~l conditions listed in Table II. TABLE II Zr-Th cornily
AND
&‘-REFRACTORY
#rn~~tio~
zr-0.64%Th zr-0.7%TbO, Zr-0.64%Th-0.34%ZrO, Zr-2S%TbO, Zr-2.5%TbO, Zr-2.5%Tb02 Zr-2S%ThO, zr-2.5%TbO, Zr-3%Ce,0j.2ZrOz Zr-3%Ce,03*2ZrOl Zr-2%Y,03 Zr-2%YzO3
ALMYS
USED
FOR OBSERVATION
OF DISPERSED
PHASE
Condition
Swaged 500aC, H.T. 700°C 6 h, W.Q. Swaged 500°C, H.T. 700°C 6 h, W.Q. Swaged 5OO”C,H.T. 700% 6 h, W.Q. Arc-melted H.T. 800°C 24 h, W.Q. H.T. 950°C 6 h, W.Q. H.T. 950°C 6 h, quenched in brine at 6’C. H.T. 1400°C 24 h, furnace-cooled. Arc-melted H.T. 950°C 6 h, W.Q. Arc-melted H.T. 850°C 6 h, W.Q.
W.Q. = Water quenched. H.T. = Heat treated.
These specimens were prepared for microexamination by rough polishing on silicon carbide abrasive paper down to 600 grit, followed by polishing with 9 ,WI and 3 pm diamond paste on “Ivlicrocloth”. The specimens were then chemically polish& by swabbing with a solution of 45 ml nitric acid and 9-10 ml hydrofluoric acid in 45 ml of distilled water. For examination by the electron microscope, specimens were polished as above and replicas of the structures were made using a 2% Formvar solution. X-Ray di&action analyses of Zr-‘k.S%ThO,, Zr-3%Cea03*2ZrOz, and Zr2%Y,Q3 alloys were conducted on a GE diflractometer using CM& radiation. The specimens were prepared for X-ray analysis in the same mamier as described under “Polishing and etching”. .I.Less-Comntm
Metais, 23 (1971) 415-425
H. M. SKELLY, C. F. DIXON
418
X-Ray diffraction and fluorescence analyses were also conducted on particles chemically extracted from the Zr-3’ACez0,*2Zr02 alloy by selective solution treatment with bromine-methanol. Tensile testing
Tensile test were conducted on specimens in the metallurgical conditions listed in Table I. Specimens were tested at 65O’C under a vacuum of 5 x lo-’ torr using an Instron universal testing machine. The test specimens were of 1 in. gauge length and 0.16 in. gauge diameter. Each specimen was held at temperature for 1 h before testing ; all tests were in duplicate. Oxygen analysis
Specimens of arc-melted alloys containing 0.7% ThOz ,0.64x Th, 0.34% Zr 02, and 0.64% Th plus 0.34% ZrO, were analysed for oxygen by the vacuum-fusion method. RESULTS
Examination
of microstructure
The results of metallographic examinations of experimental alloys listed in Table II are shown in the photomicrographs (Figs. 1,2,8, and 9) and in the electron micrographs (Figs. 3-7, and 10). Microstructures of the Zr-O.64xTh and Zr-0.7%Th02 alloys, after swaging at 5OO’Cfollowed by heat-treating at 700°C for 6 h, are shown in Figs. 1 and 2, respectively. A dispersed second phase is evident in the Zr-0.7’ATh02 alloy (Fig. 2) but no
Fig. l. Zr-0.64%Th. J. tin-Common
Swaged 5OO”C, H.T. 7OO’C 6 h, W.Q.
Metals, 23 (1971) 415425
(x 150)
ZIRCONIUM-REFRACTORY
419
ALLOYS
Fig. 2.Zr-O.‘I%ThO,.
Swaged 500°C, H.T. 7OO’C 6 h, W.Q.
rig. 3.ZrZ.S”/,ThO,.
Arc-m
second-phase particles are evident at alloy (Fig. 1). There were insufficient ThO, alloy to permit identification phase present in an arc-melted Zr-2.5%ThOz diffraction analysis as ThOz.
(x 150)
50) in the Zr-@A4% Th present in thk.Gr-4,7% but a alloy (Fig. 3) was identifii’by X-ray
J. Less-Common
Metals, 23 (1971) 415-425
420
H.
M. SKELLY,
C. F. D:[XON
Figures 3-7 show the effect of heat treatment on the size, distribution and sofubility of ThOl in the Zr-2S”ATh02 alloy. Figure 3 shows ThOz at the a-pla Selet
6h, W.Q. ( x 5000)
Fig. 4. Zr-2.5%Th02.
H.T. 8OVC
Fig. 5. Zr-2.5%Th02.
H.T. 95OL%6 b, W.Q. (X m)
Fig. 6.Zr-2.5%ThO,.
H.T. 950°C 6 h, quenched into brince at 6%
.T. Less-CommonMetals, 23 (1971) 415-425
(XW)
ZIRCONIUM -REFRACTORY
421
ALLOYS
boundaries. Heat-treating at 800°C for 24 h and waterquenching caused the ThOz to coalesce (Fig. 4) ; X-ray diffraction analysis of this specimen confirmed that the particles were ThO,. In the microstructure ofspe&mens beat-treated at 950°C and shown in Fig. 5, the ThOz is at the boundaries of a-platekts that are much finer than those in the arc-melted specimens. Thin specimens quenched from this temperature (9SOOC) into brine at 6°C produced a structure in which ThOz particles were distributed
Fig. 7.Zr-2.5%ThO,.
H.T. 14OO’C 24 h, furnacecooled.
Fig. 8. a-3%Ce,O,.2ZrO,.
Arc-melted.
(x 5000)
(x 1000) J. Less-Common Met&
23 ( 1971) 4 I 5425
H. M. SKELLY, C. F. DIXON
422
Fig. 9.Zr-3%Ce,O,.2ZrO,.
Fig. lO. Zr-3%Ce,O,.2ZrO,.
H.T. 95O’C 6 h, W.Q. (X 1000)
H.T. 95O’C 6 h, W.Q.
(X 8500)
throughout the matrix, as shown in Fig. 6. The ThOz particles in specimens heattreated in a vacuum furnace at 1400°C for 24 h and furnace-cooled were as large as 3 pm (Fig. 7). Metallographic examination of specimens containing 0.64% Th plus 0.34% ZrO, did not reveal any evidence of dispersed particles. The microstructure of arc-melted Zr-3%Ce,0,*2Zr02 alloy is shown in Fig. 8. The particles at the a-platelet boundaries were chemically extracted and identified by X-ray analysis as being Ce,O,. 2Zr02. After a specimen of this alloy had been heated at 950°C for 6 h and then water-quenched, the particles were much finer (- 0.1 pm) J. Less-Common Metals,
23 (1971) 415-425
ZIRCONIUM-RFZRACTORY
423
ALLOYS
aud were distributed as shown in Figs 9 aud 10. The Zr-20/,Yz03 ahoy couta&d partich~ at former /3-grain-boundaries and throughout the matrix. These particles were identified by X-ray difhactiou as being yzo3.
Tensile tests
Table I lists the results of the tensile tests. Values for zirconium and Zircaloy-2 are included for comparisou. All the alloys containing 2.5% or more of ThOz had high-temperature (65O’C) tensile strengths greater than that of Zircaloy2 No significant effect on the strength was observed after the ThOz content had been increased from 3.5% to 7%, or after Zr-2.5%ThOz specimens had been heat-treated at 650°C or at 95O*Cfor 6h. However, after specimens of the Zr-0.7’?Th02 alloy had been swaged at 500°C and heattreated at 700°C for 6 h, the ultimate tensile strength increased to 23.6 k.s.i. from 16.2 k.s.i. for arc-melted specimens. It can be seen that the tensile strengths of Zr-O64%Th, Zr-0.34”/,Zr02 and Zr-O.64%Th-0.34% ZrOz were less than half that of Zr-O.7%ThO, swaged and heattreated, and substantially below that of Zr-0.7’ATh02, as arc-melted. The addition of Cez01*2Zr02, YzOJ, and Laz03 also produced strengthening. The strength of the Zr-Cez03*2Zr02 alloys increased progressively with refractory content. The alloy containing 7% Cez03.2Zr02 had an ultimate strength of 36 k.s.i. Hot-swaging the Zr-l%Ce,0,*2Zr02 alloy also increased the tensile strength. Alloys containing Y,O, showed a maximum ultimate tensile strength of 24 k.s.i. at 3% YzOJ. Addition of ets”/, La,O,, produced as much strengthening as additions of 1% and 2% I.+03. The oxygen analysis results given in Table III show that the addition of 0.34% ZrOz to zirconium and to Zr-O&%Th was effective in increasing the oxygen content to the same level as that in the Zr-0.7%Th02 alloy. TABLE III OXYGEN
CONTENT GF SAMPLB
SWAGW
AT 500
‘c
THEN HGAT-TREATW
AT
700 ‘C
FOR
6
HOURS
Oxyg& content (wt.%) Zr-0.7%ThOl Zr-OM%Th
0.20 0.11
Zr-0,34%ZrO, Z~O.64%T~-O.34%ZrO~
0.20 0.20
DISCUSSION
Dispersed phase
Metallographic examination and X-ray analysis of arc-melted zirconiumrefractory alloys show that ThOz, Ce209*2Zr02 and Yz03 are present as discrete particles. J. Less-Common Metals, 23 (1971) 415-425
424
H. M. SKELLY, C. F. DIXON
Comparison of Figs. 3 (Zr-2.5%Th02, arc-melted)and 4 (Zr-2.5%Th02, H.T. 800°C, 6 h, water-quenched) indicates that some coalescence of ThOz particles had occurred on heat-treating at 800°C. R~d~tribution of ThO, on or-platelet boundaries after Zr-2.5°kTh02 specimens were water-quenched from 950°C (cf: Figs. 3 and 5) suggests that the ThOz was soluble in zirconium at this temperature and precipitated on cooling. As shown in Fig. 6, faster cooling of specimens by quenching thin sections into brine at 6°C caused the ThOz to precipitate as a dispersed phase in the matrix. Furnace-cooling of specimens from 1400°C allowed time for the ThO, particles to grow in size (Fig. 7). From the above, it would appear that ThOz is sufficiently soluble in zirconium at 8OO*Cto coalesce and that 2.5% ThOz is completely soluble at 95OOC. The change in size and distribution of the refractory when the Zr-3%Cez0, * 2Zr02 alloy was water-quenched from 950°C suggests that 3% Ce,03*2Zr02 is almost completely dissolved in zirconium at this temperature (c$ Figs. 8 and 9). Tensile properties
The Zr-0.7%Th02 alloy had a high-temperature strength superior to that of the Zr-O.64xTh alloy, the Zr-O.34%ZrO, alloy, or the Zr--0.64%Th-0.34%Zr02 alloy (Table I). The presence of added oxygen would not be expected to influence the tensile strength of these alloys according to Weinstein and Ho&z’, and Miller3, and the results confirm this. The strength superiority of this alloy, therefore, is not due to solution in zirconium of thorium or oxygen but must be due to the presence of the ThOz particles. It is, therefore, reasonable to conclude that the increase in strength of the other Zr-ThOz alloys is likewise due to the presence of the refractory particles. The results in Table I show that Ce,0,.2Zr02, Y,03 and I&O3 additions also improve the tensile strength of zirconium at 65OOC. Values given in the literature for the particle size and interparticle spacing that produce dispersion strengthening in Ni-ThOz and Al-A1,03 systems range from less than 0.1 to 0.25 pm for the particles and from 0.1 to 1.0 m for the spacing4-‘. Metallographic examination of the arc-melted Zr-2.5%ThO,, and Zr-3%Ce,03+2Zr02 alloys (Figs. 3 and 8) showed that the dispersed phases did not match these dimensions. Dispersed particles about 0.1 e diam. were obtained by hot-treat~g these alloys at 950°C for 6 h and quenching (Figs. 6 and lo), but the interparticle spacing remained too large. CONCLUSIONS
Arc-melting zirconium with additions of ThO,, Y,03, or Ce,O,*2ZrO, results in a dispersion of the refractory compound in the alloy. Alloys with high-temperature (650°C) tensile strengths up to twice that of Zircaloy-2 can be produced by arc-melting zirconium with additions of refractory compounds. The high-temperature strength of Zr-ThO, alloys is not due to the solution in zirconium of thorium or oxygen arising from any decom~sition of ThO,. The size and distribution of the ThOz and C&O3 *2Zr0, particles in zirconium can be varied by heat-treatment. However, the interparticle spacing necessary for dispersionstrengthening was not achieved in this work. J. Less-Common Metals, 23 (1971) 415-425
ZIRCONIUM-RBFRA CTORY ALLOYS
425
ACKNOWLEDGEMENTS
The authors wish to thank Miss J. Ng-Yelim for assistance in obtaining the electron micrographs, Dr. C. M. Mitchell for the X-ray ditTraction analysis, and Mr. L. Ripley of the Mineral Science Division for his assistance in isolating and identifying the Ce,03*2ZrOz dispersion. The above-mentioned personnel are all members of the Mines Branch staff. REFERENCES 1 C. F. DIXONAND H. M. SKELLY,Addition’of Common Metals,
refractory compounds to molten zirconium, J. Less-
18 (1969) 440.
D. WEINSTEIN AND F. C. HOLTZ, On dispersion strengthening of zirconium, Trans. AZME, 227 (1963) 1463. 3 G. L. Mu.=, Zirconium, Butterworth’s, London, 2nd edn., 1957, p. 253. 4 F. J. ANDBRS,JR., G. B. ALEXANDWANDW. S. WARIEL, A dispersion strengthened nickel alloy, Metal -Progress, Dec. (1962) 88. 5 N. HANSBN,Dispersion strengthened aluminum products, Danish At. Energy Comm., Res. Estab. Ris& Risii Rept. No. 113, May, 1966. 6 R. W. FRASBR,B. MEDDINGS,D. J. I. EVANSANDV. N. MACKIW,Dispersion-strengthened nickel by compaction and rolling of powder by pressure hydrometallurgy. In H. H. HAUSNER(ed.), Modern Lkvelopments in Powder Metallwgy, Vol. 2, Applications, Plenum, New York, 1966, p. 87. 7 P. GUYOT,On the mechanisms of plastic deformation of SAP-type alloys, in H. H. HAU~N~ (ed.), Modern Developments in Powder Metalhwgy, Vol. 2, Applications, Plenum, New York, 1966, p. 112. 8 S. M. WOLF, Properties and applications of dispersion strengthened metals, J. Metals, 19 (1967) 22. 9 D. W. ASHALL.ANDP. E. EVANS,Yielding and work-hardening in Ni-ThO, alloys, Metal Science J., 2 (1968) 96. 2
J. Less-Common Metals, 23 (1971) 415-425
JOURNAL OF THE LE?S+COMMON METALS Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
ZIRCONIUM-REFRACTORY H. M. SKELLY
AND
ALLOYS
C. F. DIXON
Nuclear and Powder Metallurgy Section, Physical Metallurgy Division, Mines Branch, Department of Energy, Mines and Resources, Ottawa (Canada) (Received November 18th, 1970)
SUMMARY
Zirconium alloys containing thorium oxide, yttrium oxide, cerium zirconate or lanthanum oxide were prepared by arc-melting. The refractories apparently~dissolved in the molten zirconium and precipitated as a dispersed phase on solidification. Hightemperature (65O’C) tensile strengths, up to twice that of Zircaloy-2, were observed for arc-melted specimens. The size and distribution of thorium oxide and cerium zirconate particles were shown to vary depending on the heat treatment and, although interparticle spacing, believed to be ideal, was not achieved, particles of a size considered to be fme enough to produce dispersion strengthening were obtained.
INTRODUCTION
This investigation examines the possibility of improving the high-temperature properties of zircoriium through dispersion strengthening by the addition of refractory compounds, particularly oxides, by arc-melting. To dispersion strengthen zirconium by this method it is necessary for the refractory compound to dissolve in molten zirconium and, on cooling, to precipitate as a fine dispersion of the refractory compound itself, or of some stable compound formed by its reaction with zirconium. Because of the high reactivity of zirconium, its high solubility for oxygen, and the stability of the zirconiumoxygen solid solution, only the most stable r&actories would be expected to exist in a zirconium matrix. Thorium oxide (ThO,), because of its high stability, was selected for initial tests. Observations were also -de on the effect of additions to zirconium of yttrium oxide (Y203), lanthanum oxide (Laz03), and cerium zirconate (Cez0,*2ZrOz). The initial part of this work was published earlier in a short communicationi. EXPERIMENTAL PROCEDURE
Preparation of allqys The alloy&ted
in Table I were prepared using reactor-grade
zirconium
Crown Copyrights Reserved. J. Less-Common Metals, 23 (1971) 415425
416 TABLE TENSILE
H. M. SKBLLY,
C. F. DIXON
I PROPERTIES
OF ZIRCONIUM
AND ZIRCONIUM
ALLOYS
AT
650°C UTS (k.s.i.)
Nominal composition
Condition
Zr Zircaloy-2* Zr-0.64%Th Zr-0.34%ZrO, Zr-0.64%Th-0.34%Zr02 Zr-0.7%Th02 Zr-0.7%Th02 Zr-2.5%ThO, Zr-3.5%Th02 Zr-5.0%Th02 Zr-7.0%Th02 Zr-2.5%ThO, Zr-2.5%Th02 Zrl%Ce,O,.2ZrO, Zr-1%Ce,03.2Zr0, Zr-3%Ce,0,.2Zr02 Zr-S%Ce,O,.2ZrO, Zr-7%Ce,O,.2ZrO, Zr-l%Y,O, Zr-2%Y,O, zl-3%YtOs Zr-S%Y,O, Zr-O.S%La,O, Zr-l%La,OB Zr-2%La,O,
As arc-melted As arc-melted Swaged 500°C, H.T. 700°C Swaged 500°C, H.T. 7OO’C Swaged 500°C, H.T. 7OO’C Swaged 500°C, H.T. 7OO’C As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted H.T. 650°C 6 h, W.Q. H.T. 95O’C 6 h, W.Q. Swaged 500°C As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted As arc-melted ,As arc-melted As arc-melted
0.2% ;ksS.i.)
W.Q. = Water quenched. H.T. = Heat treated. UTS = Ultimate tensile strength. YS = Yield strength. * 1.45% Sn, 0.14% Fe, O.lO’% Cr, 0.06% Ni, 0.12%
6 6 6 6
h, h, h, h,
W.Q. W.Q. W.Q. W.Q.
O,, balance
11.2 17.6 9.5 10.0 9.7 23.6 16.2 23.7 27.0 26.6 25.5 26.1 23.7 19.9 15.2 20.6 27.4 36.0 19.5 21.6 24.0 19.2 19.3 19.8 17.1
1.5 11.9 4.1 5.6 4.0 14.7 10.9 16.8 17.3 17.7 16.3 17.5 16.5 11.9 9.3 13.6 18.3 25.7 13.2 14.6 15.3 12.3 12.0 12.5 10.9
o/0 Elongation on 1 in. 44 42 63 59 66 48 36 27 35 26 20 22 27 36 36 8 8 14 20 26 24 22 28 32 27
Zr.
sponge containing 1100 p.p.m. oxygen. The refractory compounds were in the form of powders, 3-4 pm in particle size. Melting and alloying were conducted in a laboratory-sized tungsten-arc furnace in an argon atmosphere. The zirconium sponge was consolidated by melting into a dense, cigar-shaped ingot in which a cavity was drilled to contain the refractory compound. Each charge weighed approximately 75 g and was remelted at least 4 times to ensure homogeneity. Visual observation of the zirconium and the refractory addition through the dark-glass window of the arc furnace revealed that, during melting, the particles of refractory appeared to dissolve in the molten zirconium. The behaviour of the refractory particles was markedly different in these instances from the behaviour on other occasions in which unsuccessful attempts were made to prepare metal-refractory alloys and the refractory additions obviously remained undissolved during arcmelting. The Zr-O.64’ATh and Zr-O.34%ZrO, alloys were prepared to determine the J. Less-Common
Metals, 23 (1971) 415-425
417
ZIRCONIUM-RBFRACTORY ALLOYS
elect of thorium and oxygen separately in the event of any decomposition of ThO,. The 0.64% Th and the oxygen in 0.34% ZrOa are ~ro~t~y equival,rmt to the thorium and oxygen contained in 0.7% ThOa. The ~~%~~.~%~~ alloy was prepared to determine if thorium and oxygen added together would produce the same effect as 0.7% ThOz. The zirconium-refractory alloys are referred to by their nominal compositions. Sow arc-melted ingots were swaged in air at 500°C to 0.35 in. diam. rod. The heat treatments listed in Tables I and II were conducted under vacuum. Polishing and etching
To investigate dispersed phases present in the alloys, 0.25 in. thick specimens were cut from the am-melted ingots in the rne~~~l conditions listed in Table II. TABLE II Zr-Th cornily
AND
&‘-REFRACTORY
#rn~~tio~
zr-0.64%Th zr-0.7%TbO, Zr-0.64%Th-0.34%ZrO, Zr-2S%TbO, Zr-2.5%TbO, Zr-2.5%Tb02 Zr-2S%ThO, zr-2.5%TbO, Zr-3%Ce,0j.2ZrOz Zr-3%Ce,03*2ZrOl Zr-2%Y,03 Zr-2%YzO3
ALMYS
USED
FOR OBSERVATION
OF DISPERSED
PHASE
Condition
Swaged 500aC, H.T. 700°C 6 h, W.Q. Swaged 500°C, H.T. 700°C 6 h, W.Q. Swaged 5OO”C,H.T. 700% 6 h, W.Q. Arc-melted H.T. 800°C 24 h, W.Q. H.T. 950°C 6 h, W.Q. H.T. 950°C 6 h, quenched in brine at 6’C. H.T. 1400°C 24 h, furnace-cooled. Arc-melted H.T. 950°C 6 h, W.Q. Arc-melted H.T. 850°C 6 h, W.Q.
W.Q. = Water quenched. H.T. = Heat treated.
These specimens were prepared for microexamination by rough polishing on silicon carbide abrasive paper down to 600 grit, followed by polishing with 9 ,WI and 3 pm diamond paste on “Ivlicrocloth”. The specimens were then chemically polish& by swabbing with a solution of 45 ml nitric acid and 9-10 ml hydrofluoric acid in 45 ml of distilled water. For examination by the electron microscope, specimens were polished as above and replicas of the structures were made using a 2% Formvar solution. X-Ray di&action analyses of Zr-‘k.S%ThO,, Zr-3%Cea03*2ZrOz, and Zr2%Y,Q3 alloys were conducted on a GE diflractometer using CM& radiation. The specimens were prepared for X-ray analysis in the same mamier as described under “Polishing and etching”. .I.Less-Comntm
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X-Ray diffraction and fluorescence analyses were also conducted on particles chemically extracted from the Zr-3’ACez0,*2Zr02 alloy by selective solution treatment with bromine-methanol. Tensile testing
Tensile test were conducted on specimens in the metallurgical conditions listed in Table I. Specimens were tested at 65O’C under a vacuum of 5 x lo-’ torr using an Instron universal testing machine. The test specimens were of 1 in. gauge length and 0.16 in. gauge diameter. Each specimen was held at temperature for 1 h before testing ; all tests were in duplicate. Oxygen analysis
Specimens of arc-melted alloys containing 0.7% ThOz ,0.64x Th, 0.34% Zr 02, and 0.64% Th plus 0.34% ZrO, were analysed for oxygen by the vacuum-fusion method. RESULTS
Examination
of microstructure
The results of metallographic examinations of experimental alloys listed in Table II are shown in the photomicrographs (Figs. 1,2,8, and 9) and in the electron micrographs (Figs. 3-7, and 10). Microstructures of the Zr-O.64xTh and Zr-0.7%Th02 alloys, after swaging at 5OO’Cfollowed by heat-treating at 700°C for 6 h, are shown in Figs. 1 and 2, respectively. A dispersed second phase is evident in the Zr-0.7’ATh02 alloy (Fig. 2) but no
Fig. l. Zr-0.64%Th. J. tin-Common
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ALLOYS
Fig. 2.Zr-O.‘I%ThO,.
Swaged 500°C, H.T. 7OO’C 6 h, W.Q.
rig. 3.ZrZ.S”/,ThO,.
Arc-m
second-phase particles are evident at alloy (Fig. 1). There were insufficient ThO, alloy to permit identification phase present in an arc-melted Zr-2.5%ThOz diffraction analysis as ThOz.
(x 150)
50) in the Zr-@A4% Th present in thk.Gr-4,7% but a alloy (Fig. 3) was identifii’by X-ray
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H.
M. SKELLY,
C. F. D:[XON
Figures 3-7 show the effect of heat treatment on the size, distribution and sofubility of ThOl in the Zr-2S”ATh02 alloy. Figure 3 shows ThOz at the a-pla Selet
6h, W.Q. ( x 5000)
Fig. 4. Zr-2.5%Th02.
H.T. 8OVC
Fig. 5. Zr-2.5%Th02.
H.T. 95OL%6 b, W.Q. (X m)
Fig. 6.Zr-2.5%ThO,.
H.T. 950°C 6 h, quenched into brince at 6%
.T. Less-CommonMetals, 23 (1971) 415-425
(XW)
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ALLOYS
boundaries. Heat-treating at 800°C for 24 h and waterquenching caused the ThOz to coalesce (Fig. 4) ; X-ray diffraction analysis of this specimen confirmed that the particles were ThO,. In the microstructure ofspe&mens beat-treated at 950°C and shown in Fig. 5, the ThOz is at the boundaries of a-platekts that are much finer than those in the arc-melted specimens. Thin specimens quenched from this temperature (9SOOC) into brine at 6°C produced a structure in which ThOz particles were distributed
Fig. 7.Zr-2.5%ThO,.
H.T. 14OO’C 24 h, furnacecooled.
Fig. 8. a-3%Ce,O,.2ZrO,.
Arc-melted.
(x 5000)
(x 1000) J. Less-Common Met&
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H. M. SKELLY, C. F. DIXON
422
Fig. 9.Zr-3%Ce,O,.2ZrO,.
Fig. lO. Zr-3%Ce,O,.2ZrO,.
H.T. 95O’C 6 h, W.Q. (X 1000)
H.T. 95O’C 6 h, W.Q.
(X 8500)
throughout the matrix, as shown in Fig. 6. The ThOz particles in specimens heattreated in a vacuum furnace at 1400°C for 24 h and furnace-cooled were as large as 3 pm (Fig. 7). Metallographic examination of specimens containing 0.64% Th plus 0.34% ZrO, did not reveal any evidence of dispersed particles. The microstructure of arc-melted Zr-3%Ce,0,*2Zr02 alloy is shown in Fig. 8. The particles at the a-platelet boundaries were chemically extracted and identified by X-ray analysis as being Ce,O,. 2Zr02. After a specimen of this alloy had been heated at 950°C for 6 h and then water-quenched, the particles were much finer (- 0.1 pm) J. Less-Common Metals,
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ALLOYS
aud were distributed as shown in Figs 9 aud 10. The Zr-20/,Yz03 ahoy couta&d partich~ at former /3-grain-boundaries and throughout the matrix. These particles were identified by X-ray difhactiou as being yzo3.
Tensile tests
Table I lists the results of the tensile tests. Values for zirconium and Zircaloy-2 are included for comparisou. All the alloys containing 2.5% or more of ThOz had high-temperature (65O’C) tensile strengths greater than that of Zircaloy2 No significant effect on the strength was observed after the ThOz content had been increased from 3.5% to 7%, or after Zr-2.5%ThOz specimens had been heat-treated at 650°C or at 95O*Cfor 6h. However, after specimens of the Zr-0.7’?Th02 alloy had been swaged at 500°C and heattreated at 700°C for 6 h, the ultimate tensile strength increased to 23.6 k.s.i. from 16.2 k.s.i. for arc-melted specimens. It can be seen that the tensile strengths of Zr-O64%Th, Zr-0.34”/,Zr02 and Zr-O.64%Th-0.34% ZrOz were less than half that of Zr-O.7%ThO, swaged and heattreated, and substantially below that of Zr-0.7’ATh02, as arc-melted. The addition of Cez01*2Zr02, YzOJ, and Laz03 also produced strengthening. The strength of the Zr-Cez03*2Zr02 alloys increased progressively with refractory content. The alloy containing 7% Cez03.2Zr02 had an ultimate strength of 36 k.s.i. Hot-swaging the Zr-l%Ce,0,*2Zr02 alloy also increased the tensile strength. Alloys containing Y,O, showed a maximum ultimate tensile strength of 24 k.s.i. at 3% YzOJ. Addition of ets”/, La,O,, produced as much strengthening as additions of 1% and 2% I.+03. The oxygen analysis results given in Table III show that the addition of 0.34% ZrOz to zirconium and to Zr-O&%Th was effective in increasing the oxygen content to the same level as that in the Zr-0.7%Th02 alloy. TABLE III OXYGEN
CONTENT GF SAMPLB
SWAGW
AT 500
‘c
THEN HGAT-TREATW
AT
700 ‘C
FOR
6
HOURS
Oxyg& content (wt.%) Zr-0.7%ThOl Zr-OM%Th
0.20 0.11
Zr-0,34%ZrO, Z~O.64%T~-O.34%ZrO~
0.20 0.20
DISCUSSION
Dispersed phase
Metallographic examination and X-ray analysis of arc-melted zirconiumrefractory alloys show that ThOz, Ce209*2Zr02 and Yz03 are present as discrete particles. J. Less-Common Metals, 23 (1971) 415-425
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H. M. SKELLY, C. F. DIXON
Comparison of Figs. 3 (Zr-2.5%Th02, arc-melted)and 4 (Zr-2.5%Th02, H.T. 800°C, 6 h, water-quenched) indicates that some coalescence of ThOz particles had occurred on heat-treating at 800°C. R~d~tribution of ThO, on or-platelet boundaries after Zr-2.5°kTh02 specimens were water-quenched from 950°C (cf: Figs. 3 and 5) suggests that the ThOz was soluble in zirconium at this temperature and precipitated on cooling. As shown in Fig. 6, faster cooling of specimens by quenching thin sections into brine at 6°C caused the ThOz to precipitate as a dispersed phase in the matrix. Furnace-cooling of specimens from 1400°C allowed time for the ThO, particles to grow in size (Fig. 7). From the above, it would appear that ThOz is sufficiently soluble in zirconium at 8OO*Cto coalesce and that 2.5% ThOz is completely soluble at 95OOC. The change in size and distribution of the refractory when the Zr-3%Cez0, * 2Zr02 alloy was water-quenched from 950°C suggests that 3% Ce,03*2Zr02 is almost completely dissolved in zirconium at this temperature (c$ Figs. 8 and 9). Tensile properties
The Zr-0.7%Th02 alloy had a high-temperature strength superior to that of the Zr-O.64xTh alloy, the Zr-O.34%ZrO, alloy, or the Zr--0.64%Th-0.34%Zr02 alloy (Table I). The presence of added oxygen would not be expected to influence the tensile strength of these alloys according to Weinstein and Ho&z’, and Miller3, and the results confirm this. The strength superiority of this alloy, therefore, is not due to solution in zirconium of thorium or oxygen but must be due to the presence of the ThOz particles. It is, therefore, reasonable to conclude that the increase in strength of the other Zr-ThOz alloys is likewise due to the presence of the refractory particles. The results in Table I show that Ce,0,.2Zr02, Y,03 and I&O3 additions also improve the tensile strength of zirconium at 65OOC. Values given in the literature for the particle size and interparticle spacing that produce dispersion strengthening in Ni-ThOz and Al-A1,03 systems range from less than 0.1 to 0.25 pm for the particles and from 0.1 to 1.0 m for the spacing4-‘. Metallographic examination of the arc-melted Zr-2.5%ThO,, and Zr-3%Ce,03+2Zr02 alloys (Figs. 3 and 8) showed that the dispersed phases did not match these dimensions. Dispersed particles about 0.1 e diam. were obtained by hot-treat~g these alloys at 950°C for 6 h and quenching (Figs. 6 and lo), but the interparticle spacing remained too large. CONCLUSIONS
Arc-melting zirconium with additions of ThO,, Y,03, or Ce,O,*2ZrO, results in a dispersion of the refractory compound in the alloy. Alloys with high-temperature (650°C) tensile strengths up to twice that of Zircaloy-2 can be produced by arc-melting zirconium with additions of refractory compounds. The high-temperature strength of Zr-ThO, alloys is not due to the solution in zirconium of thorium or oxygen arising from any decom~sition of ThO,. The size and distribution of the ThOz and C&O3 *2Zr0, particles in zirconium can be varied by heat-treatment. However, the interparticle spacing necessary for dispersionstrengthening was not achieved in this work. J. Less-Common Metals, 23 (1971) 415-425
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425
ACKNOWLEDGEMENTS
The authors wish to thank Miss J. Ng-Yelim for assistance in obtaining the electron micrographs, Dr. C. M. Mitchell for the X-ray ditTraction analysis, and Mr. L. Ripley of the Mineral Science Division for his assistance in isolating and identifying the Ce,03*2ZrOz dispersion. The above-mentioned personnel are all members of the Mines Branch staff. REFERENCES 1 C. F. DIXONAND H. M. SKELLY,Addition’of Common Metals,
refractory compounds to molten zirconium, J. Less-
18 (1969) 440.
D. WEINSTEIN AND F. C. HOLTZ, On dispersion strengthening of zirconium, Trans. AZME, 227 (1963) 1463. 3 G. L. Mu.=, Zirconium, Butterworth’s, London, 2nd edn., 1957, p. 253. 4 F. J. ANDBRS,JR., G. B. ALEXANDWANDW. S. WARIEL, A dispersion strengthened nickel alloy, Metal -Progress, Dec. (1962) 88. 5 N. HANSBN,Dispersion strengthened aluminum products, Danish At. Energy Comm., Res. Estab. Ris& Risii Rept. No. 113, May, 1966. 6 R. W. FRASBR,B. MEDDINGS,D. J. I. EVANSANDV. N. MACKIW,Dispersion-strengthened nickel by compaction and rolling of powder by pressure hydrometallurgy. In H. H. HAUSNER(ed.), Modern Lkvelopments in Powder Metallwgy, Vol. 2, Applications, Plenum, New York, 1966, p. 87. 7 P. GUYOT,On the mechanisms of plastic deformation of SAP-type alloys, in H. H. HAU~N~ (ed.), Modern Developments in Powder Metalhwgy, Vol. 2, Applications, Plenum, New York, 1966, p. 112. 8 S. M. WOLF, Properties and applications of dispersion strengthened metals, J. Metals, 19 (1967) 22. 9 D. W. ASHALL.ANDP. E. EVANS,Yielding and work-hardening in Ni-ThO, alloys, Metal Science J., 2 (1968) 96. 2
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