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The effect of chromium, oxygen and microstructure on the hardness of zirconium-chromium alloys
Journal of the Less-Common Metals, 39 (1975) 35 - 42 @ Elsevier Sequoia &A., Lausanne - Printed in the Netherlands
35
THE EFFECT OF CHROMIUM, OXYGEN AND MICROSTRUCTURE ON THE HARDNESS OF ZIRCONIUM-CHROMIUM ALLOYS
W. M. RUMBALL Royal Naval Engineering (Received
College, Manadon, Plymouth
(Gt. Britain)
June 19, 1974)
Summary Empirical relationships have been determined between the hardness and the chemical composition of Zr-Cr alloys. The hardness due to chromium in supersaturated solution (P-quenched alloys) is 150 X (wt.% Cr) VPN points, that due to chromium in the form of dispersed ZrCr,, 13 X (wt.% Cr) VPN points, and that due to oxygen in solution 0.05 X (ppm0) VPN points. The effect of oxygen decreases above concentrations of 2500 ppm but oxygen and chromium do not appear to interact in their effects on hardness. p-quenching contributes approximately 30 hardness points due to substructure, whilst the impurities in the base metal contribute about 10 hardness points.
1. Introduction Zirconium-chromium alloys are potential fuel-cladding materials for use in nuclear reactors in the temperature range 300 - 500 “C. This report details hardness measurements on zirconium-chromium alloys variously heattreated during phase-equilibria [ 11 and hardenability [ 21 investigations. The hardnesses are correlated with the chemical compositions and the proportions of phases present in the microstructure. 2. Materials and techniques Table 1 lists the alloys used and Figs. 1 and 2 show the essential features of the phase relationships in the ZrCr0 system [l] . A detailed impurity analysis was available only for the 1.25 wt.% chromium alloys and this is given in Table 2. All heat treatments were carried out in a dynamic vacuum of lO-‘j Ton, and any quenching was completed within two or three seconds of breaking the vacuum. Heat-treated specimens were sectioned and prepared for metallographic examination and hardness testing. Metallographic preparation was carried out in the usual manner, a final chemical polishing being effected with a mixture of 45 parts lactic acid, 45 parts nitric acid and 8 parts hydrofluoric
36 TABLE 1 Analysis of alloys Nominal
Zirconium
Analysis
wt.% Cr
grade
cr (wt.%)
Oxygen
0.2 0.5 1.0 1.0 1.25 1.26 1.25 1.5 2.5
Sponge Sponge Iodide Sponge Sponge Sponge Sponge Sponge Sponge
0.20 0.50 1.00 1.00 1.25 1.25 1.25 1.46 2.25
1050 + 60 1050f 60 #41 1050* 50 1400 f 60 2000 f 200 2400 * 200 1050 k 50 1060 f: 50
Iron (ppm)
( ppm)
N.A. N.A. 80 N.A. 850 850 850 N.A. N.A.
N.A. = Not analysed. TABLE 2 Impurity analysis of the 1.26 wt.% chromium alloys Element
AI
B
C
Cd
Co
Cu
H
Hf
Mg
ppm
<25
<0.2
110
<0.3
<6
<26
6
96
Element
Mn
MO
N
Nb
Ni
Pb
Si
Sn
ppm
26
< 100
<15
<5
50
20
Element
Ta
Ti
Na
V
Zn
ppm
<200<20
<5
<50
I
I 1 '/
/I
850 t
P
B50t
/
P+ZrCrz
2 00 3 c a
BOO-
P
/
-
s P 220
oc+ 8000
I
wt.
ZrCr2 I 1
%
2.
2
000
I 0
oc+ZrCr, I 1
I
wt. % Chromium
Chromium
Fig. 1. Zirconium-chromium Fig.
I
phase diagram.
Vertical section at 1050 ppm oxygen through ZrCr0
phase diagram.
2
37
acid. Specimens were examined under polarised light and, in some cases, the proportions of phases present were determined by point counting [ 13. Hardness me~urements were always made on chemic~ly-polished specimens using a Vickers hardness testing machine (with either a 10 kg or 30 kg load) and a Tukon microhardness tester, using either a 136’ diamond or a Knoop indentor. 3. Results 3.1 Hardness of p- quenched alloys Figure 3 shows the hardness of the binary alloys containing 1060 ppm oxygen quenched from the P-region. All the structures were martensitic except the 2.25 wt.% Cr alloy which contained the omega phase [ 11. Hardness is almost linearly related to chromium content and may be satisfactorily expressed by : VPN = 150 (wt.% Cr) + 160
(1) over the initial range 0 - 1.0 wt.% Cr. The hardness of the omega containing structure (the 2.26 wt.% Cr alloy) was not si~ifi~~~y different from that expected for the martensitic phase by extrapolation of the line in Fig. 3.
450 -
@ Rumball. Ref.{21 1.25 Wt % Cr, t400 ppm. 0 @ Slattery Ref. (6) 1.1 Wt % Cr, 1175 ppm. 0
Wait&t % Chromium
Fig. 3. Hardness
of quenched
Cooling rate
ZrCr
alloys containing
Fig. 4. Hardness of ZrCr alloys containing cooling rate from the p-region.
deg.&
1050 fr 50 ppm of oxygen.
approximately
0.1 wt.% Fe as a function
of
The hardness of the P-quenched 1.0 wt.% G-41 ppm 0 ahoy was 258 VPN. If a linear dependence of hardness on oxygen concentration is assumed as in other zirconium alloys we may then write: VPNo =mx+C (2)
38 TABLE 3 Slope of VPN us oxygen concentration
1 2 3 4 6 6
curves
Material (wt.%)
Heat-treatment
m (VPN/ppm 0)
Ref.
Zr-1.0 Cr Zr2.6 Nb Zr-2.6 Nb Zircaloy-2 Zirconium (crystal bar) Zirconium (crystal bar)
o-quenched P-quenched a-annealed a-annealed a-annealed
0.060 0.038 0.038 0.052 0.053
This work [31 [31 141
(Y-annealed
0.038
I51
[41
where 111= 0.05, x is the oxygen concentration in ppm and C is a constant. The constant m compares quite well with values of m for other zirconium alloys, Table 3. Studies over wider oxygen limits usually show a decrease in the value of m with oxygen concentrations beyond about 2500 ppm [ 51. The alloys with the lower values of m in Table 3 would, in fact, have values of m more nearly approaching Treco’s [ 51 0.5 if the first 2000 ppm oxygen figures only were considered. We may conclude that the effect of oxygen concentration on the hardness of zirconium alloys is independent of substitutional solute contents* this conclusion is supported by the independence of the oxygen hardness upon whether the substitutional solute is in solution or not (see the data for quenched and annealed Zr-2.6Nb in Table 3). For quenched Zr-1 wt.% Cr alloy the value of C in eqn. (2) is 256 and, assuming little or no chromium/oxygen interaction, from eqn. (1) C = 256 - (1 = 106.
X
150)
We then have the general equation VPN = (0.05 X ppm 0) + (150 X wt.% Cr) + 106
(3) ranges 0 - 1 wt.% chromium and 0 - 200
which applies in the concentration ppm oxygen. It cannot be assumed that the p-quenched 2.25 wt.% Cr alloy consisted wholly of the metastable hexagonal phase, a, nor, indeed, is it likely. However the 2.25 wt.% Cr alloy certainly did contain L? [l],but s1 was absent from all other alloys as far as was ascertained. It can be said that the S2bearing specimens were extremely and consistently hard, approaching 500 VPN. The microstructure and hardness of zirconium-chromium alloys quenched from the p-region is a function of cooling rate [ 2, 61, i.e., section *This is probably not true of solutes having comparable or greater affinity for oxygen, eg., Ti.
39
size. Below approximately 2000 “C/s the transformation product is Widmanstatten (or “basketweave”) and the hardness depends on cooling rate (Fig. 4). Above 2000 “C/s a twinned martensite forms, the hardness of which is relatively independent of further increases in cooling rate [ 21. This mass effect has been ignored as the specimens used were largely sufficiently thin for the constant maximum hardness to be obtained. This explains some discrepancies, discussed later, between expected and expe~ment~ hardness. 3.2 Hardness of annealed alloys The hardness of the annealed binary alloys is shown in Fig. 5, the change of hardness with chromium concentration agreeing well with the interpolated data of Keeler [7]. The difference between the curve of Keeler and that in this work may be attributed to differences in purity of the two starting materials. The maximum solid solubility of chromium in zirconium is in the order of 0.1 wt.% so that the hardness change of annealed alloys with chromium concentration is due to the dispersion strengthening of ZrCr, precipitates. The linear dependence is adequately described by VPN, = 13 X wt.% Cr where VPN,,, is the hardness contribution
(5) of the ZrCra precipitates.
MO-
d
I 6
I
es
I 1.5
Id
so
040
alloys containing
I
I
I
I
I
010
660
es0
040
060
I
Quenching temp. “C
Chromium awwxmtration, Wt %
Fig. 5. Hardness of annealed
I MO
1050 f 50 ppm oxygen.
Fig. 6. Comparison of calculated and observed &phase hardness in quenched wt.% G-2400 ppm oxygen alloy.
G-l.25
It might be expected that the absolute values for the hardness of annealed alloys could be determined by extrapolation of the effect of chromium in the supersaturated quenched alloys (eqn.( 1)) to an assumed matrix chromium concentration (say 0.1 wt.%), and addition of the contribution due to the precipitate (eqn. (5)), i.e., VPN = VPNcr + VPNo + VPN, = (0.05 X ppm 0) + (150 X wt.% Cr,) + 106 + (13 Xwt.% Cr) (6) where wt.% CrSd represents the concentration
of soluble chromium. However,
40
if this is done the expected values from eqn. (6), lie 81 VPN points higher (Fig. 5). This significant difference can be easily attributed to the complex defect substructure of the quenched ~supe~atura~d) alloys as compared with the low defect-density annealed alloys. The present work cannot establish what contribution, if any, such features as low-angle boundaries, twins, etc., make to the enhanced hardness of the quenched P-phase. X-ray line broadening measurements [ 8] on the 1.25 wt.% G-1400 ppm 0 alloy indicate that lattice distortion in the wa~r-quenched structure contributes some 40 VPN points to the total hardness, and Williams and Gilbert f9] have attributed, by difference, 30 VPN points to such substructural effects in Zr-Nb alloys. Equation (3) is then rewritten for annealed alloys, VPN,,
= (0.05 X ppm 0) + (150 X wt.% Cr) + 75
(7)
where VPN,, distinguishes the hardness in equilibrium alloys from VPN,, that in supersaturated alloys. 4. Discussion The hardness of an alloy quenched from the (a! + p) region of the phase diagram (Figs. 1 and 2) will be given by,
(8) VPN = t (VPN,) + II - 0 (VPNquenohe& + t (VPNticr2) where the volume fraction of a-phase present is t, and VPN, and VPNpuenched_~ refer to the hardness of the two cons~tuents present. The volume fraction of precipitated ZrCra is small above about 900 “C and its contribution (VPNZ,c,2) to the overall room-temperature hardness has been neglected above 900 “C quenching temperatures. VPN, will be given by eqn. (7) and VPN quenched-(l W eW (31, SO VPN = t ((0.05 X ppm 0) + (150 X wt.% Cr) + 75)+ (1 - t) 19) i(O.05 X ppm 0) + jl50 X wt.% Cr) + 106)+ 13 t (X wt.% Cr). The contribution of oxygen to the hardness shouId, of course, be modified when it exceeds 2500 ppm: this is discussed at a later stage. The imprecise knowledge of the effect of oxygen at higher concentrations is likely to limit the accuracy of any hardness predictions, as also will u~ce~ty regarding the actual chemical composition of the phases present. The phase-equilibria in the ZlrCr-0 system has been studied [l] and with these data eqn. (9) may be applied to predict the hardnesses of alloys listed in Table 1 quenched from the a + fl (or ~11 + p + ZrCr,) region. In the present work microhardness measurements were made on (a + 0) quenched alloys and compared with the calculated values, The agreement obtained is reasonable considering the limitations of the calculated hardness values and the difficulty of obtaining meaningful microhardness measurements. A conversion between VPN and Knoop values due to Lustman and Kerze [lo] was employed where necessary. The predicted microhardness of the a-phase in a hypereutectoid alloy,
41
Quenching
Fig.
7.
temp. “C
Hardness of quenched Zr-0.5
Fig. 8. Hardness
of quenched
Zr-1.25
Quenching
wt.%
temp. “C
Cr-1050 ppm oxygen alloy.
wt.% G-2400
ppm oxygen
alloy.
Fig. 6, agrees well with experimental findings considering the extreme sensitivity of the hardness to oxygen concentration which varies dramatically in the temperature range 835 - 960 “C [ 11. As the contribution of oxygen to the hardness is not linear above about 2500 ppm, for concentrations of 2500 - 7500 ppm the contribution used was (0.0375 X ppm 0) whilst above 7500 ppm a contribution of (0.017 X ppm 0) was used [4]. For hypoeutectoid alloys the variation of oxygen concentration in the a-phase is assumed to be the same as for hypereutectoid alloys, but as this is not necessarily so the qualitative agreement between calculated and observed hardness noted, for example, in the Zr-0.5 wt.% Cr alloy, Fig. 7, is good. The predicted hardness of the Zr-1.25 wt.% Cr-2400 ppm oxygen alloy compares very well with observed hardness up to 920 “C or so (Fig. S), above which temperature the effect of specimen mass clearly.contributes to the low measured hardness values. 4.2 Hardaess of un-alloyed zirconium The constant, 75, in eqn. (7) represents the hardness of the original unalloyed zirconium, having eliminated the solutes, oxygen and chromium, the dispersed precipitate ZrCrs, and substructural effects. Treco (41 has reported iodide zirconium to have a hardness of 65 VPN. The additional 10 hardness points observed in this work must therefore be due to the greater impurity content of commercial zirconium. From the impurity analysis, Table 2, we may estimate the total impurity content by arbitrarily taking half the quoted maximum of “less than” concentrations. We obtain, I: substitutional .-E. lntentitial
impurities impurities
=
522 ppm,
=
31 ppm.
Assuming the substitutional impurities to have the same effect as chromium on the hardness of zirconium and, in the case of the interstitials, the same effect as oxygen, we find from eqn. (7).
42
VPN = (0.05 X 31) + (150 X 0.0522) = 9.4 VPN points, i.e., in close agreement with the observed difference of 10 VPN points. 5. Conclusions The effect of chromium and oxygen in solution on the hardness of zirconium alloys may be represented by, VPN = (0.05
X
ppm 0) + (150 X wt.% Cr) f Ahi + Ah, + 65
where Ahi is the contribution of impurity solutes (in this work about 10 hardness points) and Ah, is a hardness contribution of substructure. Ah, is zero for annealed alloys and 31 VPN points for p-quenched material. The above relationship is considered to be valid for oxygen concentrations below about 2600 ppm but deviations from linearity at higher concen~ations require a more complex [ 53, and approximate, relationship VPNo = 605 X ppm 0 + 0.038 X (ppm 0 - 2500) for 0 > 2500 and < 7600 ppm, + 0.017 X (ppm 0 - 7500) for 0 > 7500 ppm. Chromium as ZrCrs precipitates co~t~butes VPN, = 13 X wt.% Cr to the total hardness. Applications of these basic equations enable reasonably accurate assessments of hardness to be made if the necessary quantitative metallographic data are available. 6. References 1 2 3 4 6 6 7 8 9
W. M. Rumba11 and F. G. Elder, J. Less-Common Metals, 19 (1969) 345. W. M. Rumball, At. Energy Canada Rept. No AECL-3050, (1968). J. Winton and R. A. Murgatroyd, Electrochem. Tech., 4 (1966) 358. R. M. Treco, Trans. Amer. Sot. Metals, 46 (1953) 872. J. J. Kearns and J. N. Chirigos, unpublished work. G. F. Slattery, J. Less-Common Metals, 16 (1968) 91. J. H. Keeler, Trans. Amer. Sot. Metals, 48 (1956) 825. W. M. Rumball, J. Less-Common Metals, 22 (1970) 287. C. D. Williams and R. W. Gilbert, Trans. Japan Inst. Metals, 9 (1968) Supplement 626. 10 B. Lustman and F. Kerze, Met~lu~y of Zirconium, McGraw-Hill, New York, 1965.
35
THE EFFECT OF CHROMIUM, OXYGEN AND MICROSTRUCTURE ON THE HARDNESS OF ZIRCONIUM-CHROMIUM ALLOYS
W. M. RUMBALL Royal Naval Engineering (Received
College, Manadon, Plymouth
(Gt. Britain)
June 19, 1974)
Summary Empirical relationships have been determined between the hardness and the chemical composition of Zr-Cr alloys. The hardness due to chromium in supersaturated solution (P-quenched alloys) is 150 X (wt.% Cr) VPN points, that due to chromium in the form of dispersed ZrCr,, 13 X (wt.% Cr) VPN points, and that due to oxygen in solution 0.05 X (ppm0) VPN points. The effect of oxygen decreases above concentrations of 2500 ppm but oxygen and chromium do not appear to interact in their effects on hardness. p-quenching contributes approximately 30 hardness points due to substructure, whilst the impurities in the base metal contribute about 10 hardness points.
1. Introduction Zirconium-chromium alloys are potential fuel-cladding materials for use in nuclear reactors in the temperature range 300 - 500 “C. This report details hardness measurements on zirconium-chromium alloys variously heattreated during phase-equilibria [ 11 and hardenability [ 21 investigations. The hardnesses are correlated with the chemical compositions and the proportions of phases present in the microstructure. 2. Materials and techniques Table 1 lists the alloys used and Figs. 1 and 2 show the essential features of the phase relationships in the ZrCr0 system [l] . A detailed impurity analysis was available only for the 1.25 wt.% chromium alloys and this is given in Table 2. All heat treatments were carried out in a dynamic vacuum of lO-‘j Ton, and any quenching was completed within two or three seconds of breaking the vacuum. Heat-treated specimens were sectioned and prepared for metallographic examination and hardness testing. Metallographic preparation was carried out in the usual manner, a final chemical polishing being effected with a mixture of 45 parts lactic acid, 45 parts nitric acid and 8 parts hydrofluoric
36 TABLE 1 Analysis of alloys Nominal
Zirconium
Analysis
wt.% Cr
grade
cr (wt.%)
Oxygen
0.2 0.5 1.0 1.0 1.25 1.26 1.25 1.5 2.5
Sponge Sponge Iodide Sponge Sponge Sponge Sponge Sponge Sponge
0.20 0.50 1.00 1.00 1.25 1.25 1.25 1.46 2.25
1050 + 60 1050f 60 #41 1050* 50 1400 f 60 2000 f 200 2400 * 200 1050 k 50 1060 f: 50
Iron (ppm)
( ppm)
N.A. N.A. 80 N.A. 850 850 850 N.A. N.A.
N.A. = Not analysed. TABLE 2 Impurity analysis of the 1.26 wt.% chromium alloys Element
AI
B
C
Cd
Co
Cu
H
Hf
Mg
ppm
<25
<0.2
110
<0.3
<6
<26
6
96
Element
Mn
MO
N
Nb
Ni
Pb
Si
Sn
ppm
26
< 100
<15
<5
50
20
Element
Ta
Ti
Na
V
Zn
ppm
<200<20
<5
<50
I
I 1 '/
/I
850 t
P
B50t
/
P+ZrCrz
2 00 3 c a
BOO-
P
/
-
s P 220
oc+ 8000
I
wt.
ZrCr2 I 1
%
2.
2
000
I 0
oc+ZrCr, I 1
I
wt. % Chromium
Chromium
Fig. 1. Zirconium-chromium Fig.
I
phase diagram.
Vertical section at 1050 ppm oxygen through ZrCr0
phase diagram.
2
37
acid. Specimens were examined under polarised light and, in some cases, the proportions of phases present were determined by point counting [ 13. Hardness me~urements were always made on chemic~ly-polished specimens using a Vickers hardness testing machine (with either a 10 kg or 30 kg load) and a Tukon microhardness tester, using either a 136’ diamond or a Knoop indentor. 3. Results 3.1 Hardness of p- quenched alloys Figure 3 shows the hardness of the binary alloys containing 1060 ppm oxygen quenched from the P-region. All the structures were martensitic except the 2.25 wt.% Cr alloy which contained the omega phase [ 11. Hardness is almost linearly related to chromium content and may be satisfactorily expressed by : VPN = 150 (wt.% Cr) + 160
(1) over the initial range 0 - 1.0 wt.% Cr. The hardness of the omega containing structure (the 2.26 wt.% Cr alloy) was not si~ifi~~~y different from that expected for the martensitic phase by extrapolation of the line in Fig. 3.
450 -
@ Rumball. Ref.{21 1.25 Wt % Cr, t400 ppm. 0 @ Slattery Ref. (6) 1.1 Wt % Cr, 1175 ppm. 0
Wait&t % Chromium
Fig. 3. Hardness
of quenched
Cooling rate
ZrCr
alloys containing
Fig. 4. Hardness of ZrCr alloys containing cooling rate from the p-region.
deg.&
1050 fr 50 ppm of oxygen.
approximately
0.1 wt.% Fe as a function
of
The hardness of the P-quenched 1.0 wt.% G-41 ppm 0 ahoy was 258 VPN. If a linear dependence of hardness on oxygen concentration is assumed as in other zirconium alloys we may then write: VPNo =mx+C (2)
38 TABLE 3 Slope of VPN us oxygen concentration
1 2 3 4 6 6
curves
Material (wt.%)
Heat-treatment
m (VPN/ppm 0)
Ref.
Zr-1.0 Cr Zr2.6 Nb Zr-2.6 Nb Zircaloy-2 Zirconium (crystal bar) Zirconium (crystal bar)
o-quenched P-quenched a-annealed a-annealed a-annealed
0.060 0.038 0.038 0.052 0.053
This work [31 [31 141
(Y-annealed
0.038
I51
[41
where 111= 0.05, x is the oxygen concentration in ppm and C is a constant. The constant m compares quite well with values of m for other zirconium alloys, Table 3. Studies over wider oxygen limits usually show a decrease in the value of m with oxygen concentrations beyond about 2500 ppm [ 51. The alloys with the lower values of m in Table 3 would, in fact, have values of m more nearly approaching Treco’s [ 51 0.5 if the first 2000 ppm oxygen figures only were considered. We may conclude that the effect of oxygen concentration on the hardness of zirconium alloys is independent of substitutional solute contents* this conclusion is supported by the independence of the oxygen hardness upon whether the substitutional solute is in solution or not (see the data for quenched and annealed Zr-2.6Nb in Table 3). For quenched Zr-1 wt.% Cr alloy the value of C in eqn. (2) is 256 and, assuming little or no chromium/oxygen interaction, from eqn. (1) C = 256 - (1 = 106.
X
150)
We then have the general equation VPN = (0.05 X ppm 0) + (150 X wt.% Cr) + 106
(3) ranges 0 - 1 wt.% chromium and 0 - 200
which applies in the concentration ppm oxygen. It cannot be assumed that the p-quenched 2.25 wt.% Cr alloy consisted wholly of the metastable hexagonal phase, a, nor, indeed, is it likely. However the 2.25 wt.% Cr alloy certainly did contain L? [l],but s1 was absent from all other alloys as far as was ascertained. It can be said that the S2bearing specimens were extremely and consistently hard, approaching 500 VPN. The microstructure and hardness of zirconium-chromium alloys quenched from the p-region is a function of cooling rate [ 2, 61, i.e., section *This is probably not true of solutes having comparable or greater affinity for oxygen, eg., Ti.
39
size. Below approximately 2000 “C/s the transformation product is Widmanstatten (or “basketweave”) and the hardness depends on cooling rate (Fig. 4). Above 2000 “C/s a twinned martensite forms, the hardness of which is relatively independent of further increases in cooling rate [ 21. This mass effect has been ignored as the specimens used were largely sufficiently thin for the constant maximum hardness to be obtained. This explains some discrepancies, discussed later, between expected and expe~ment~ hardness. 3.2 Hardness of annealed alloys The hardness of the annealed binary alloys is shown in Fig. 5, the change of hardness with chromium concentration agreeing well with the interpolated data of Keeler [7]. The difference between the curve of Keeler and that in this work may be attributed to differences in purity of the two starting materials. The maximum solid solubility of chromium in zirconium is in the order of 0.1 wt.% so that the hardness change of annealed alloys with chromium concentration is due to the dispersion strengthening of ZrCr, precipitates. The linear dependence is adequately described by VPN, = 13 X wt.% Cr where VPN,,, is the hardness contribution
(5) of the ZrCra precipitates.
MO-
d
I 6
I
es
I 1.5
Id
so
040
alloys containing
I
I
I
I
I
010
660
es0
040
060
I
Quenching temp. “C
Chromium awwxmtration, Wt %
Fig. 5. Hardness of annealed
I MO
1050 f 50 ppm oxygen.
Fig. 6. Comparison of calculated and observed &phase hardness in quenched wt.% G-2400 ppm oxygen alloy.
G-l.25
It might be expected that the absolute values for the hardness of annealed alloys could be determined by extrapolation of the effect of chromium in the supersaturated quenched alloys (eqn.( 1)) to an assumed matrix chromium concentration (say 0.1 wt.%), and addition of the contribution due to the precipitate (eqn. (5)), i.e., VPN = VPNcr + VPNo + VPN, = (0.05 X ppm 0) + (150 X wt.% Cr,) + 106 + (13 Xwt.% Cr) (6) where wt.% CrSd represents the concentration
of soluble chromium. However,
40
if this is done the expected values from eqn. (6), lie 81 VPN points higher (Fig. 5). This significant difference can be easily attributed to the complex defect substructure of the quenched ~supe~atura~d) alloys as compared with the low defect-density annealed alloys. The present work cannot establish what contribution, if any, such features as low-angle boundaries, twins, etc., make to the enhanced hardness of the quenched P-phase. X-ray line broadening measurements [ 8] on the 1.25 wt.% G-1400 ppm 0 alloy indicate that lattice distortion in the wa~r-quenched structure contributes some 40 VPN points to the total hardness, and Williams and Gilbert f9] have attributed, by difference, 30 VPN points to such substructural effects in Zr-Nb alloys. Equation (3) is then rewritten for annealed alloys, VPN,,
= (0.05 X ppm 0) + (150 X wt.% Cr) + 75
(7)
where VPN,, distinguishes the hardness in equilibrium alloys from VPN,, that in supersaturated alloys. 4. Discussion The hardness of an alloy quenched from the (a! + p) region of the phase diagram (Figs. 1 and 2) will be given by,
(8) VPN = t (VPN,) + II - 0 (VPNquenohe& + t (VPNticr2) where the volume fraction of a-phase present is t, and VPN, and VPNpuenched_~ refer to the hardness of the two cons~tuents present. The volume fraction of precipitated ZrCra is small above about 900 “C and its contribution (VPNZ,c,2) to the overall room-temperature hardness has been neglected above 900 “C quenching temperatures. VPN, will be given by eqn. (7) and VPN quenched-(l W eW (31, SO VPN = t ((0.05 X ppm 0) + (150 X wt.% Cr) + 75)+ (1 - t) 19) i(O.05 X ppm 0) + jl50 X wt.% Cr) + 106)+ 13 t (X wt.% Cr). The contribution of oxygen to the hardness shouId, of course, be modified when it exceeds 2500 ppm: this is discussed at a later stage. The imprecise knowledge of the effect of oxygen at higher concentrations is likely to limit the accuracy of any hardness predictions, as also will u~ce~ty regarding the actual chemical composition of the phases present. The phase-equilibria in the ZlrCr-0 system has been studied [l] and with these data eqn. (9) may be applied to predict the hardnesses of alloys listed in Table 1 quenched from the a + fl (or ~11 + p + ZrCr,) region. In the present work microhardness measurements were made on (a + 0) quenched alloys and compared with the calculated values, The agreement obtained is reasonable considering the limitations of the calculated hardness values and the difficulty of obtaining meaningful microhardness measurements. A conversion between VPN and Knoop values due to Lustman and Kerze [lo] was employed where necessary. The predicted microhardness of the a-phase in a hypereutectoid alloy,
41
Quenching
Fig.
7.
temp. “C
Hardness of quenched Zr-0.5
Fig. 8. Hardness
of quenched
Zr-1.25
Quenching
wt.%
temp. “C
Cr-1050 ppm oxygen alloy.
wt.% G-2400
ppm oxygen
alloy.
Fig. 6, agrees well with experimental findings considering the extreme sensitivity of the hardness to oxygen concentration which varies dramatically in the temperature range 835 - 960 “C [ 11. As the contribution of oxygen to the hardness is not linear above about 2500 ppm, for concentrations of 2500 - 7500 ppm the contribution used was (0.0375 X ppm 0) whilst above 7500 ppm a contribution of (0.017 X ppm 0) was used [4]. For hypoeutectoid alloys the variation of oxygen concentration in the a-phase is assumed to be the same as for hypereutectoid alloys, but as this is not necessarily so the qualitative agreement between calculated and observed hardness noted, for example, in the Zr-0.5 wt.% Cr alloy, Fig. 7, is good. The predicted hardness of the Zr-1.25 wt.% Cr-2400 ppm oxygen alloy compares very well with observed hardness up to 920 “C or so (Fig. S), above which temperature the effect of specimen mass clearly.contributes to the low measured hardness values. 4.2 Hardaess of un-alloyed zirconium The constant, 75, in eqn. (7) represents the hardness of the original unalloyed zirconium, having eliminated the solutes, oxygen and chromium, the dispersed precipitate ZrCrs, and substructural effects. Treco (41 has reported iodide zirconium to have a hardness of 65 VPN. The additional 10 hardness points observed in this work must therefore be due to the greater impurity content of commercial zirconium. From the impurity analysis, Table 2, we may estimate the total impurity content by arbitrarily taking half the quoted maximum of “less than” concentrations. We obtain, I: substitutional .-E. lntentitial
impurities impurities
=
522 ppm,
=
31 ppm.
Assuming the substitutional impurities to have the same effect as chromium on the hardness of zirconium and, in the case of the interstitials, the same effect as oxygen, we find from eqn. (7).
42
VPN = (0.05 X 31) + (150 X 0.0522) = 9.4 VPN points, i.e., in close agreement with the observed difference of 10 VPN points. 5. Conclusions The effect of chromium and oxygen in solution on the hardness of zirconium alloys may be represented by, VPN = (0.05
X
ppm 0) + (150 X wt.% Cr) f Ahi + Ah, + 65
where Ahi is the contribution of impurity solutes (in this work about 10 hardness points) and Ah, is a hardness contribution of substructure. Ah, is zero for annealed alloys and 31 VPN points for p-quenched material. The above relationship is considered to be valid for oxygen concentrations below about 2600 ppm but deviations from linearity at higher concen~ations require a more complex [ 53, and approximate, relationship VPNo = 605 X ppm 0 + 0.038 X (ppm 0 - 2500) for 0 > 2500 and < 7600 ppm, + 0.017 X (ppm 0 - 7500) for 0 > 7500 ppm. Chromium as ZrCrs precipitates co~t~butes VPN, = 13 X wt.% Cr to the total hardness. Applications of these basic equations enable reasonably accurate assessments of hardness to be made if the necessary quantitative metallographic data are available. 6. References 1 2 3 4 6 6 7 8 9
W. M. Rumba11 and F. G. Elder, J. Less-Common Metals, 19 (1969) 345. W. M. Rumball, At. Energy Canada Rept. No AECL-3050, (1968). J. Winton and R. A. Murgatroyd, Electrochem. Tech., 4 (1966) 358. R. M. Treco, Trans. Amer. Sot. Metals, 46 (1953) 872. J. J. Kearns and J. N. Chirigos, unpublished work. G. F. Slattery, J. Less-Common Metals, 16 (1968) 91. J. H. Keeler, Trans. Amer. Sot. Metals, 48 (1956) 825. W. M. Rumball, J. Less-Common Metals, 22 (1970) 287. C. D. Williams and R. W. Gilbert, Trans. Japan Inst. Metals, 9 (1968) Supplement 626. 10 B. Lustman and F. Kerze, Met~lu~y of Zirconium, McGraw-Hill, New York, 1965.