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Precipitation hardening in rapidly quenched TiZrB alloys
Materials Science and Engineering, 98 (1988) 179 183
179
Precipitation Hardening in Rapidly Quenched Ti-Zr-B Alloys* Y. Z. LU and B. C. G1ESSEN
Barnett Institute, Northeastern University, Boston, MA 02115 (U.S.A.)
S. H. WHANG Department c~['Metallurgy and Materials Science, Polytechnic University, Brooklyn, N Y 11201 (U.S.A.)
Abstract
A significant amount o f boron beyond the solid solubill O, limit can be dissolved in titanium matrix by rapid solidification. As a result, it is anticipated that precipitate reactions and age hardening in the quenched titanium alloys will occur upon heat treatment. Bina O, Ti B and ternary Ti Z r B alloys were processed into thin foils and ribbons by rapid solidification techniques. A transmission electron microscopy ( T E M ) stud), showed that t'e o, .fine microstructures without precipitates were present in the as-quenched titanium alloys containing up to 6 at. % B. The precipitation reaction was identified in the annealed Ti~4ZrloB 6 alloys q/ter heat treatment for various durations in the temperature range o[ 500 600 C. The precipitates were indexed to be zirconium borides j r o m the electron diffraction patterns. The effects o f boron and annealing on the age hardening in these alloys were studied by hardness measurements.
!. Introduction
In recent years, research on rapidly solidified titanium alloys has been focused on the stable dispersion of novel additives in the titanium matrix [ 1-5]. In the past, the dispersion strengthening of titanium alloys has been investigated by the addition of silicon using solution treatment [6,7]. In contrast, boron is scarcely soluble in titanium at high temperature and this makes it difficult to utilize solution treatment. Homogeneous solid solutions of Ti B alloys may be prepared through rapid solidification. The succeeding heat treatment will result in the formation of uniform titanium borides (TiB, TiB2), which can be considered to be good strengthening agents. Previously, preliminary studies on microstructure and mechanical properties of Ti 5at.%A12.5at.%Sn lat.%B alloy were reported [8, 9]. In this
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al. August 3- 7. 1987. 0025-5416/88.'$3.50
paper, the effects of boron addition on various properties of Ti B alloys with 10at.% ZrH will be discussed.
2. Experimental details
Small buttons ( l 2 g) of Ti B and T i - Z ~ B alloys were prepared from titanium (purity, 99.9%), zirconium (purity, 99.9%) and boron (purity, 99.5%) in an arc furnace under an argon atmosphere. The buttons were remelted repeatedly to ensure compositional homogeneity. Small pieces were broken from the alloy buttons, and were splat into thin foils (2025 ktm thick) by the hammer and anvil technique. The foil specimens were checked by transmission electron microscopy (TEM) in the as-quenched state as well as in various annealed states. Carbon replicas were also prepared from foil specimens in order to study precipitates. These precipitates in the matrix were first brought into relief in a solution containing 5 vol.% hydrofluoric acid, 5 vol.% nitric acid and 90 vol.% water. The carbon replicas were then detached in a solution containing 10 vol.% hydrofluoric acid, 10 vol.% nitric acid and 80 vol.% water. The examination of these replicas showed that the acid dissolved the matrix, leaving the precipitates behind. The replicas were also examined by TEM. Microhardness of the samples was measured by the Vickers diamond pyramid tester with loads of 50 gf and 100 gf.
3. Results and discussion
3. I. Microstructures The TEM micrograph of rapidly quenched Ti96B4 shows a fine ~" martensite structure without precipitates (Fig. l(a)). The corresponding diffraction ring patterns (Fig. l(b)) clearly identify the h.c.p, structure of the ~-Ti alloy. No extra ring pattern, that might otherwise indicate the presence of a second phase, was found. Therefore it is logical to assume that all the
~ Elsevier Sequoia/Printed in The Netherlands
180
lal
Fig. 1. As-quenched Ti96B4: (a) bright field micrograph; (b) diffraction pattern.
boron dissolved in the titanium matrix and caused the martensitic transformation. Figures 2(a) and 2(b) show the free surface and the substrate side surface of the spun ribbons respectively. The dimpled surface is characteristic in this alloy. Carbon extraction replicas were prepared from the Ti84ZrloB6 alloy, which was rapidly quenched and annealed at 800 °C for 5 h. The bright field micrograph (Fig. 3(a)) of this replica shows relatively spherical particles. The diffraction ring patterns from these particles (Fig. 3(b)) correspond with those of the ZrB2 compound (D6h) as indicated in Table 1. However, a few extra ring patterns are consistent with those of the TiB compound (D~6). From this evidence, it is most likely that the particles contain ZrB2 phase and also possibly TiB phase. If this is so, the composition Tis4ZrloB6 falls into a two-phase region in the Ti-Zr-B phase diagram. When the same alloy was heat treated at 950 °C for 2 h, the precipitates grew into rod-shape structures of high aspect ratio (Fig. 4).
|IDI
Fig. 2. Scanning electron micrographs of surfaces of alloy ribbon: (a) the free surface exposed to argon atmosphere; (b) the substrate side surface contacted with a copper disk.
Fig. 3. Carbon extraction replica of Ti84ZrloB6 alloy aged at 800 °C for 5 h: bright field micrograph.
181 TABLE 1 Electron diffraction interplanar spacings d for dispersoids in Tis4ZrloB 6 alloy
Observed d (nm)
ZrB2 a
TiB b
d (nm)
hkl
0.352 0.271 0.254 0.235 0.214
0.353 0.274
001 100
0.216
101
0.175 0.158 0.150
0.176 0.158 0,148 0.144 0.137
002 110 102 111 200
0.138
d (nm)
hkl
0.254 0.234 0.216 0.214
201,01l 111 210 102
"Hexagonal: a = 0.3169 nm, c = 0.1114 nm. ~Orthogonal: a = 0.612 nm, b = 0.306 nm, c = 0.436 nm.
dified Ti B alloys (cooling rate, 10s-106 K s ' [3]) show precipitation-free matrices up to 6 a t . % B based on a T E M study. The lattice p a r a m e t e r s o f the asq u e n c h e d Ti B alloys measured by X-ray diffraction are shown in Fig. 5. The X-ray patterns exhibit a significant line b r o a d e n i n g with increasing b o r o n concentration, indicative of lattice strain, p r o b a b l y caused by b o r o n solid solution. Both a and c axes increase with increasing b o r o n concentration. In addition, a large increase in the c axis and a small increase in the a axis results in an increase in the c/a ratio. This trend is in a g r e e m e n t with the case of c a r b o n in titanium as shown in Table 2. H o w e v e r , the A In ao value for Ti99Bt is only o n e - h a l f o f that for Ti99C, which raises d o u b t on whether the b o r o n solid solution is a pure interstitial solid solution. T h e r m a l b e h a v i o r o f Ti84ZrmB6 alloy foils has been studied by differential scanning calorimetry~ N o visible exothermic peak was observed up to 1060 K. This indicates that fine boride f o r m a t i o n is not related to a strong exothermic reaction. 3.3. Mechanical properties The increase in m i c r o h a r d n e s s in Tigo xZrloB alloys is linearly p r o p o r t i o n a l to b o r o n c o n c e n t r a t i o n (Fig. 6), which is in a g r e e m e n t with that in Tigo ,ZrmSix alloys, where x = 0-6 a t . % [12]. The
[
-ri
475
B
2 960
Z 470
Fig. 4. Ti84ZrloB6 alloy annealed at 950 °C for 2 h.
c,~,
g 955'
The g r o w t h rates o f the borides are m u c h faster than those o f rare earth dispersoids in a titanium matrix. S o m e boride rods are as long as 1/~m after heat treatm e n t at 950 ~C for 2 h. 3.2. Extended solid solubility The equilibrium solid solubility o f b o r o n in ~-Ti is very small (0.43 a t . % at 886 :C), whereas rapidly soli-
TABLE 2
a,A
/
//
/
/-
~465
2 950<
0
2
4
B,at%
6
Fig. 5. Lattice parameters of Ti B alloys.
Increase in lattice parameters in Ti99B 1 and Ti99C 1
Alloy
Ti99B1 (as-quenched) Ti99C I (equilibrium)
Lattice expansion per at.% B
Maximum equilibrium .~ solubility
c(h)
~,
a(h)
~,
0.0096 0.0126
0.0020 0.0027
0.0013 0.0027
0.00044 0.00091
0.43 2 [10, 11]
e,,. = (c/'Co)[l at.Vo.= A in co and e.,, = (a/ao)], at % = A In ao, where c and a are lattice parameter increments for 1 at.% B; co = 4.686 A and a o = 2.950 A.
182
g_
ISOCHRONAL
ANNEALING,
2h
~9 6 . 0
AS-QUENCHED
o_ tm
5.0
/
03 (.,3 uJ 5.0 Z ~3
Ti84Zrlo B6 (Foil,20~lm)
cr
< "TO 4.0 ,"r"
o
n
4.5 z 13_ £
T i 9 0 _ x Z r l o B x ----- * R.T.
350
450
550
TEMPERATURE, O3 U.I
650
750
850
*C
Fig. 8. Hardness in isochronally annealed Tis4Zr,oB6.
az 4.0 a:
% Ti90_xZrloSi X
< "10 n"
i/i
.
,,
3.5
/ / /t " //
,-
3.1
Ti9oZrlo
0
J 4 orB),
2 X(Si
(5 at*/.
Fig. 6. Hardness of Tig0_~ZrloBx as a function of boron concentration. degree of hardness increment is more distinct in the boron-containing alloy than in the silicon-containing alloy. The Ti86ZqoB4 foil alloy shows a strong age hardening response at 550 °C, whereas the Ti82Zr10B8 foil alloy exhibits a weak age hardening (Fig. 7). The Ti84ZrloB 6 ribbon alloy has lower hardness than the Ti82ZqoB8 and Tia6ZqoB4 foil alloys and its aging response is relatively weak. Isochronal annealing of
ISOTHERMAL
ANNEALING,
550°C
T i 9 0 - X Zrlo B x
'-96.0 A
- B 8 ( F o i l , 2 0 #m)
1:3 .
.
-- B 4 ( F o i l , 2 0 . u m ~ "
~
5.0 I
at"
< 4.0
~."
- B 6 ( R i bbon, 25/am ) .
~
Tis4ZqoB6 foil alloy (Fig. 8) demonstrates that a significant softening occurs after annealing above 800 °C for 2 h as a result of precipitate coarsening (see Fig. l(a)). Microstructural features such as early precipitation and dendritic structure along a prior fl grain boundary suggest that this boundary is rich in boron. In the isothermal annealing curve, the microhardnesses of the alloy ribbons are systematically lower than those of splat alloys. This is because a low population of fine precipitates (about 100 A in diameter) is present in the ribbon alloys. That is, coarse precipitates already exist in the ribbon materials in the asquenched state. This behavior may be related to the low cooling rate of the ribbon alloy as a result of: (i) the one-sided substrate quench in single wheel melt spinning, and (ii) the fact that the ribbon contact with the substrate is not ideal at the cooling stage as shown by air pockets and wetting patterns. 4. Conclusions (i) Rapid quenching increases the extended solid solubility of boron in titanium by as much as 6 at.%. (ii) The precipitation reaction in Ti-Zr-B alloys is associated with formation of Zr2B and possibly TiB compounds. (iii) Lattice parameter increments ( A l n a o and A In Co) in Ti-B as a function of boron content determined by X-ray diffraction, are 0.0020 and 0.00044 respectively. (iv) A strong solution hardening in the asquenched alloys and a moderate aging response at low temperature (500-600°C) are characteristics of Ti-Zr-B alloys.
Er ror_+.2 %
3.0 ~--~ as. quenched
,, TIME,
h'o
z'o 3'o 40
h
Fig. 7. Hardness in isothermally annealed Tigo ~ZrloBx.
Acknowledgment We would like to express our gratitude for the financial support from the Office of Naval Research
183 ( C o n t r a c t O N R N00014-82-K0597). This is c o n t r i b u tion 322 f r o m the B a r n e t t Institute.
References I S. M. L. Sastry, T. C. Peng, P. J. Meschter and J. E. O'Neal, J. Met., 35(1983) 21-28. 2 S. M. L. Sastry, P. J. Meschter and J. E. O'Neal, Metall. Trans. A, 15(1984) 1451-1474. 3 S. H. Whang, J. Met., 36(1984) 34M0. 4 S. H. Whang, J. Mater. Sci., 21 (7) (1986) 2224 2238.
5 H. M. Flower, P. R. Swann and D. R. F. West, Metal1. Trans., 2(1971) 3289. 6 S. M. Tuominen, G. W. Franti and D. A. Koss, Metall. Trans. A, 8 (1977) 457. 7 C. Ramachandra and V. Singh, Metall. Trans. A, 13 (1982) 771. 8 C. S. Chi and S. H. Whang, TMS-AIME F83-14, September 1983. 9 C. S. Chi and S. H. Whang, Proc. Met. Res. Soc. Syrup., 28 (1984) 353 360. 10 1. Cadoff and J. P. Nielsen, Trans. AIME, 197(1953) 248. 11 W. L. Finley and J. A. Snyder, Trans, AIME, 188(1950) 277. 12 S. H. Whang, Y. Z Lu and Y.-W. Kim, Mater. Sci. Lett., 4 (1985) 883.
179
Precipitation Hardening in Rapidly Quenched Ti-Zr-B Alloys* Y. Z. LU and B. C. G1ESSEN
Barnett Institute, Northeastern University, Boston, MA 02115 (U.S.A.)
S. H. WHANG Department c~['Metallurgy and Materials Science, Polytechnic University, Brooklyn, N Y 11201 (U.S.A.)
Abstract
A significant amount o f boron beyond the solid solubill O, limit can be dissolved in titanium matrix by rapid solidification. As a result, it is anticipated that precipitate reactions and age hardening in the quenched titanium alloys will occur upon heat treatment. Bina O, Ti B and ternary Ti Z r B alloys were processed into thin foils and ribbons by rapid solidification techniques. A transmission electron microscopy ( T E M ) stud), showed that t'e o, .fine microstructures without precipitates were present in the as-quenched titanium alloys containing up to 6 at. % B. The precipitation reaction was identified in the annealed Ti~4ZrloB 6 alloys q/ter heat treatment for various durations in the temperature range o[ 500 600 C. The precipitates were indexed to be zirconium borides j r o m the electron diffraction patterns. The effects o f boron and annealing on the age hardening in these alloys were studied by hardness measurements.
!. Introduction
In recent years, research on rapidly solidified titanium alloys has been focused on the stable dispersion of novel additives in the titanium matrix [ 1-5]. In the past, the dispersion strengthening of titanium alloys has been investigated by the addition of silicon using solution treatment [6,7]. In contrast, boron is scarcely soluble in titanium at high temperature and this makes it difficult to utilize solution treatment. Homogeneous solid solutions of Ti B alloys may be prepared through rapid solidification. The succeeding heat treatment will result in the formation of uniform titanium borides (TiB, TiB2), which can be considered to be good strengthening agents. Previously, preliminary studies on microstructure and mechanical properties of Ti 5at.%A12.5at.%Sn lat.%B alloy were reported [8, 9]. In this
*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montr6al. August 3- 7. 1987. 0025-5416/88.'$3.50
paper, the effects of boron addition on various properties of Ti B alloys with 10at.% ZrH will be discussed.
2. Experimental details
Small buttons ( l 2 g) of Ti B and T i - Z ~ B alloys were prepared from titanium (purity, 99.9%), zirconium (purity, 99.9%) and boron (purity, 99.5%) in an arc furnace under an argon atmosphere. The buttons were remelted repeatedly to ensure compositional homogeneity. Small pieces were broken from the alloy buttons, and were splat into thin foils (2025 ktm thick) by the hammer and anvil technique. The foil specimens were checked by transmission electron microscopy (TEM) in the as-quenched state as well as in various annealed states. Carbon replicas were also prepared from foil specimens in order to study precipitates. These precipitates in the matrix were first brought into relief in a solution containing 5 vol.% hydrofluoric acid, 5 vol.% nitric acid and 90 vol.% water. The carbon replicas were then detached in a solution containing 10 vol.% hydrofluoric acid, 10 vol.% nitric acid and 80 vol.% water. The examination of these replicas showed that the acid dissolved the matrix, leaving the precipitates behind. The replicas were also examined by TEM. Microhardness of the samples was measured by the Vickers diamond pyramid tester with loads of 50 gf and 100 gf.
3. Results and discussion
3. I. Microstructures The TEM micrograph of rapidly quenched Ti96B4 shows a fine ~" martensite structure without precipitates (Fig. l(a)). The corresponding diffraction ring patterns (Fig. l(b)) clearly identify the h.c.p, structure of the ~-Ti alloy. No extra ring pattern, that might otherwise indicate the presence of a second phase, was found. Therefore it is logical to assume that all the
~ Elsevier Sequoia/Printed in The Netherlands
180
lal
Fig. 1. As-quenched Ti96B4: (a) bright field micrograph; (b) diffraction pattern.
boron dissolved in the titanium matrix and caused the martensitic transformation. Figures 2(a) and 2(b) show the free surface and the substrate side surface of the spun ribbons respectively. The dimpled surface is characteristic in this alloy. Carbon extraction replicas were prepared from the Ti84ZrloB6 alloy, which was rapidly quenched and annealed at 800 °C for 5 h. The bright field micrograph (Fig. 3(a)) of this replica shows relatively spherical particles. The diffraction ring patterns from these particles (Fig. 3(b)) correspond with those of the ZrB2 compound (D6h) as indicated in Table 1. However, a few extra ring patterns are consistent with those of the TiB compound (D~6). From this evidence, it is most likely that the particles contain ZrB2 phase and also possibly TiB phase. If this is so, the composition Tis4ZrloB6 falls into a two-phase region in the Ti-Zr-B phase diagram. When the same alloy was heat treated at 950 °C for 2 h, the precipitates grew into rod-shape structures of high aspect ratio (Fig. 4).
|IDI
Fig. 2. Scanning electron micrographs of surfaces of alloy ribbon: (a) the free surface exposed to argon atmosphere; (b) the substrate side surface contacted with a copper disk.
Fig. 3. Carbon extraction replica of Ti84ZrloB6 alloy aged at 800 °C for 5 h: bright field micrograph.
181 TABLE 1 Electron diffraction interplanar spacings d for dispersoids in Tis4ZrloB 6 alloy
Observed d (nm)
ZrB2 a
TiB b
d (nm)
hkl
0.352 0.271 0.254 0.235 0.214
0.353 0.274
001 100
0.216
101
0.175 0.158 0.150
0.176 0.158 0,148 0.144 0.137
002 110 102 111 200
0.138
d (nm)
hkl
0.254 0.234 0.216 0.214
201,01l 111 210 102
"Hexagonal: a = 0.3169 nm, c = 0.1114 nm. ~Orthogonal: a = 0.612 nm, b = 0.306 nm, c = 0.436 nm.
dified Ti B alloys (cooling rate, 10s-106 K s ' [3]) show precipitation-free matrices up to 6 a t . % B based on a T E M study. The lattice p a r a m e t e r s o f the asq u e n c h e d Ti B alloys measured by X-ray diffraction are shown in Fig. 5. The X-ray patterns exhibit a significant line b r o a d e n i n g with increasing b o r o n concentration, indicative of lattice strain, p r o b a b l y caused by b o r o n solid solution. Both a and c axes increase with increasing b o r o n concentration. In addition, a large increase in the c axis and a small increase in the a axis results in an increase in the c/a ratio. This trend is in a g r e e m e n t with the case of c a r b o n in titanium as shown in Table 2. H o w e v e r , the A In ao value for Ti99Bt is only o n e - h a l f o f that for Ti99C, which raises d o u b t on whether the b o r o n solid solution is a pure interstitial solid solution. T h e r m a l b e h a v i o r o f Ti84ZrmB6 alloy foils has been studied by differential scanning calorimetry~ N o visible exothermic peak was observed up to 1060 K. This indicates that fine boride f o r m a t i o n is not related to a strong exothermic reaction. 3.3. Mechanical properties The increase in m i c r o h a r d n e s s in Tigo xZrloB alloys is linearly p r o p o r t i o n a l to b o r o n c o n c e n t r a t i o n (Fig. 6), which is in a g r e e m e n t with that in Tigo ,ZrmSix alloys, where x = 0-6 a t . % [12]. The
[
-ri
475
B
2 960
Z 470
Fig. 4. Ti84ZrloB6 alloy annealed at 950 °C for 2 h.
c,~,
g 955'
The g r o w t h rates o f the borides are m u c h faster than those o f rare earth dispersoids in a titanium matrix. S o m e boride rods are as long as 1/~m after heat treatm e n t at 950 ~C for 2 h. 3.2. Extended solid solubility The equilibrium solid solubility o f b o r o n in ~-Ti is very small (0.43 a t . % at 886 :C), whereas rapidly soli-
TABLE 2
a,A
/
//
/
/-
~465
2 950<
0
2
4
B,at%
6
Fig. 5. Lattice parameters of Ti B alloys.
Increase in lattice parameters in Ti99B 1 and Ti99C 1
Alloy
Ti99B1 (as-quenched) Ti99C I (equilibrium)
Lattice expansion per at.% B
Maximum equilibrium .~ solubility
c(h)
~,
a(h)
~,
0.0096 0.0126
0.0020 0.0027
0.0013 0.0027
0.00044 0.00091
0.43 2 [10, 11]
e,,. = (c/'Co)[l at.Vo.= A in co and e.,, = (a/ao)], at % = A In ao, where c and a are lattice parameter increments for 1 at.% B; co = 4.686 A and a o = 2.950 A.
182
g_
ISOCHRONAL
ANNEALING,
2h
~9 6 . 0
AS-QUENCHED
o_ tm
5.0
/
03 (.,3 uJ 5.0 Z ~3
Ti84Zrlo B6 (Foil,20~lm)
cr
< "TO 4.0 ,"r"
o
n
4.5 z 13_ £
T i 9 0 _ x Z r l o B x ----- * R.T.
350
450
550
TEMPERATURE, O3 U.I
650
750
850
*C
Fig. 8. Hardness in isochronally annealed Tis4Zr,oB6.
az 4.0 a:
% Ti90_xZrloSi X
< "10 n"
i/i
.
,,
3.5
/ / /t " //
,-
3.1
Ti9oZrlo
0
J 4 orB),
2 X(Si
(5 at*/.
Fig. 6. Hardness of Tig0_~ZrloBx as a function of boron concentration. degree of hardness increment is more distinct in the boron-containing alloy than in the silicon-containing alloy. The Ti86ZqoB4 foil alloy shows a strong age hardening response at 550 °C, whereas the Ti82Zr10B8 foil alloy exhibits a weak age hardening (Fig. 7). The Ti84ZrloB 6 ribbon alloy has lower hardness than the Ti82ZqoB8 and Tia6ZqoB4 foil alloys and its aging response is relatively weak. Isochronal annealing of
ISOTHERMAL
ANNEALING,
550°C
T i 9 0 - X Zrlo B x
'-96.0 A
- B 8 ( F o i l , 2 0 #m)
1:3 .
.
-- B 4 ( F o i l , 2 0 . u m ~ "
~
5.0 I
at"
< 4.0
~."
- B 6 ( R i bbon, 25/am ) .
~
Tis4ZqoB6 foil alloy (Fig. 8) demonstrates that a significant softening occurs after annealing above 800 °C for 2 h as a result of precipitate coarsening (see Fig. l(a)). Microstructural features such as early precipitation and dendritic structure along a prior fl grain boundary suggest that this boundary is rich in boron. In the isothermal annealing curve, the microhardnesses of the alloy ribbons are systematically lower than those of splat alloys. This is because a low population of fine precipitates (about 100 A in diameter) is present in the ribbon alloys. That is, coarse precipitates already exist in the ribbon materials in the asquenched state. This behavior may be related to the low cooling rate of the ribbon alloy as a result of: (i) the one-sided substrate quench in single wheel melt spinning, and (ii) the fact that the ribbon contact with the substrate is not ideal at the cooling stage as shown by air pockets and wetting patterns. 4. Conclusions (i) Rapid quenching increases the extended solid solubility of boron in titanium by as much as 6 at.%. (ii) The precipitation reaction in Ti-Zr-B alloys is associated with formation of Zr2B and possibly TiB compounds. (iii) Lattice parameter increments ( A l n a o and A In Co) in Ti-B as a function of boron content determined by X-ray diffraction, are 0.0020 and 0.00044 respectively. (iv) A strong solution hardening in the asquenched alloys and a moderate aging response at low temperature (500-600°C) are characteristics of Ti-Zr-B alloys.
Er ror_+.2 %
3.0 ~--~ as. quenched
,, TIME,
h'o
z'o 3'o 40
h
Fig. 7. Hardness in isothermally annealed Tigo ~ZrloBx.
Acknowledgment We would like to express our gratitude for the financial support from the Office of Naval Research
183 ( C o n t r a c t O N R N00014-82-K0597). This is c o n t r i b u tion 322 f r o m the B a r n e t t Institute.
References I S. M. L. Sastry, T. C. Peng, P. J. Meschter and J. E. O'Neal, J. Met., 35(1983) 21-28. 2 S. M. L. Sastry, P. J. Meschter and J. E. O'Neal, Metall. Trans. A, 15(1984) 1451-1474. 3 S. H. Whang, J. Met., 36(1984) 34M0. 4 S. H. Whang, J. Mater. Sci., 21 (7) (1986) 2224 2238.
5 H. M. Flower, P. R. Swann and D. R. F. West, Metal1. Trans., 2(1971) 3289. 6 S. M. Tuominen, G. W. Franti and D. A. Koss, Metall. Trans. A, 8 (1977) 457. 7 C. Ramachandra and V. Singh, Metall. Trans. A, 13 (1982) 771. 8 C. S. Chi and S. H. Whang, TMS-AIME F83-14, September 1983. 9 C. S. Chi and S. H. Whang, Proc. Met. Res. Soc. Syrup., 28 (1984) 353 360. 10 1. Cadoff and J. P. Nielsen, Trans. AIME, 197(1953) 248. 11 W. L. Finley and J. A. Snyder, Trans, AIME, 188(1950) 277. 12 S. H. Whang, Y. Z Lu and Y.-W. Kim, Mater. Sci. Lett., 4 (1985) 883.