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High temperature mechanical properties of C15 Laves phase Cr2Nb intermetallics
Materials
High temperature
Science and Engineering
Al 92/l 93 (1995) 805-S 10
mechanical properties of C 15 Laves phase Cr,Nb intermetallics T. Takasugi”, S. Hanadaa, M. Yoshida”
“Institute for Materials Research, Tohoku University Katahira 2-l-1, Aoba-ku, Sendai 980, Japan hMiyagi National College of Technology, Natori, Miyagi Prefecture 981-12, Japan
Abstract High temperature deformation behavior of Cl 5 CrzNb intermetallic compounds was observed by compression tests. These intermetallic compounds are deformable at temperatures above 1473 K. The flow stress decreases with increasing temperature and is little sensitive to small changes in alloy composition. Stress-strain curves are sensitive to temperature and also to the strain rate. At intermediate temperatures (also at intermediate strain rates) an apparent stress peak and a subsequent decrease in flow stress were observed. At high temperatures (also at low strain rates) a steady state flow was observed immediately after yielding. The stress exponent in the constitutive equation is about 3.5 while the apparent activation energy is 470 kJ molt I. The activation of a considerable amount of dislocations was observed at temperatures where compressive plastic deformation is accomplished. Keywords: Laves phases; Chromium;
Niobium; Intermetallics;
Mechanical
1. Introduction
A large pounds been
number
as high carried
of
studies
of
intermetallic
temperature
structural
out on relatively
simple
com-
materials crystal
have
structures
as f.c.c., b.c.c. and h.c.p. structures because they are deformable under various material and experimental conditions. A recent growing demand for intermetallic compounds as structural materials operating at extremely high temperatures and in severe environments means that intermetallic compounds with high melting point, low density, and superior corrosion and oxidation resistance have become very attractive candidates. However, they generally have complex crystal structures, for example the topologically close packed (TCP) materials, the largest group of which is the Laves intermetallic compounds. It is well known that the Laves intermetallic compounds are generally very brittle. However, a recent study showed that a Cl5 Laves Hf-V-Nb alloy can be deformed at room temperature by mechanical twinning [l-4]. Cr,Nb-based intermetallic compounds, which undergo the phase transition from Cl4 (MgZn,, hexagonal crystal structure) to Cl5 (MgCu,, cubic crystal structure) at approximately 1870 K [5], are considered
such
092 I-.5093/9S/$9.500 1995 - Elsevier Science S.A. All rights reserved SSDI 092 l-5093(94)033 19-6
properties
attractive for high temperature structural applications because of the high melting point (about 2003 K), low density (about 7.7 g cm-j), and potential resistance to corrosion and oxidation [6]. A few studies have been reported on the deformation behavior of Cr,Nb-based intermetallic compounds, i.e. the duplex Cr,Nb alloys containing a large volume fraction of Cr solid solution [ 71 and Nb solid solution [6]. However, the deformation behavior of Cr,Nb intermetallic compounds consisting of a single phase of Cl 5 structure has not been reported. In this study, the microstructure of Cr,Nb intermetallic compounds prepared by arc melting was firstly characterized. Next, high temperature compressive tests were performed, and the stress-strain curves and yield strength characterized. These mechanical properties are discussed with respect to the alloy composition, strain rate and testing temperature. Finally, dislocation microstructures observed using transmission electron microscopy (TEM) are shown briefly.
2. Experimental
procedure
Following a recently reported phase diagram [5], three alloy compositions, Cr,,,,Nb,,,, Crh7,0Nb3.1,0and
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/
Materials Science and Engineering A1921193 (1995) 805-810
Cr67.5Nb32.5 1in atomit per cent) which should be single phase or nearly single phase C 15 Laves structure, were prepared in this study. The starting raw materials were 99.99 wt.% Cr and 99.5 wt.% Nb. Small ignots were prepared with a diameter of 30 mm by non-consumable arc melting on a water-sealed hearth in an argon environment. The ingots were remelted several times to ensure chemical homogeneity. The as-solidified ingots were then annealed in dynamic vacuum at 1623 K for 15 h, followed by cooling to room temperature at a rate of 2 K min- ‘. Chemical analysis after heat treatment showed the three alloy compositions to be 65.3 at.% Cr, 66.0 at.% Cr and 67.8 at.% Cr, and therefore deviations from the nominal chemical compositions were about 1.3 at.% Cr poor in Cr,,,Nb,,,, alloy, about 1 at.% Cr poor in Cr,,,ONb,,,O alloy, and about 0.3 at.% Cr rich in Cr67,sNb32,5 alloy. In the following, the three alloys are referred to by the actual chemical compositions, i.e. Cr65,3Nb34,7, Cr,,,ONb,,,O and Cr,,.,Nb,z.z. Compression specimens with dimensions of approximately 2.5 X 2.5 X 5 mm3 were cut by a precision wheel cutter from the central regions of ingots with axes parallel to the solidification direction. The lateral faces of the specimens were abraded on sufficiently fine Sic paper down to no. 1000. Compression tests were carried out mostly at temperatures between 1373 K and 1773 K at initial strain rates between 1.7 X 10m5 ss’ and 1.7 X10W3 s-’ in a vacuum of 3 X 10 - 3 Pa, using an Instron-type approximately machine equipped with a vacuum vessel and a tungsten mesh heater. Microstructures before and after compression tests were examined using optical microscopy (OM), transmission electron microscopy (TEM) equipped with energy dispersive X-ray analysis (EDX) and high resolution electron microscopy (HREM). Metallographic specimens for OM were etched with a solution consisting of one part HF, one part HNO, and three parts H,O. TEM thin foil specimens were mechanically thinned to about 0.1 mm and then jet polished in a solution of 5 vol.% HClO, + 95 vol.% CH,OH at about 200 K. TEM observations were carried out using a JEM-2000EX operating at 200 kV.
Fig. 1. Optical microstructure
of Cr,,,,Nb,,,,,
alloy.
observed in Cr,,,,Nb,,,, and Cr,,,Nb3,,0 are Nb solid solution (with 9 at.% Cr) and those observed in Cr,,.,Nb,,., alloys are Cr solid solution. The compositions of these microstructures may be consistent with a recently reported Cr-Nb phase diagram [5]. It has been reported that a Cl 5 phase extends between 6 1.8 and 69.2 at.% Cr at their eutectic temperatures equilibrating with Nb and Cr solid solutions, but between 66.0 and 67.0 at.% Cr at 1273 K, indicating that the solid solution range of the C 15 phase is very narrow at low temperatures. Therefore, note that the three alloy compositions prepared in this study were unfortunately outside the single-phase C 15 region. The large number of twins observed in the three alloy compositions indicates that the stacking fault (SF) energy in the Cl5 Laves phase is low and therefore stable, as has been discussed by Pope and Chu [8]. They are thought to be introduced during the phase transition from Cl4 to Cl5 structures or during long annealing at high temperatures (1623 K). Fig. 2 shows a TEM image of the Cr,,,Nb,,,, alloy. This TEM image along with the HREM image and the selected area diffraction pattern (SADP) showed that the grains consist of C 15 crystal structure and the twin orientations are of the relation { 11 l}( 112) [9]. Also, TEM observations showed that there are no crystal defects except for grain (and twin) boundaries and the second-phase particles [9].
3. Results 3.2. Deformation behavior 3.1. Microstructures Fig. 1 shows a representative microstructure of Cr,,,ONb,,,, alloy. Three alloy compositions used in this study contained small fractions of second particles and also a number of twins in their grains. TEM EDX analysis showed that the second-phase particles
Fig. 3 shows the true stress-strain curves calculated from the measured load vs. cross-head displacement curves for Crh7,XNb32.2alloys. An apparent plastic deformation begins at temperatures above 1473 K. Below this temperature, samples were broken into a large number of pieces at stress levels below the
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/ Materials Science and EngineeringA192/193 (1995) 805-810
807
1200 , 1473K
1000
x fracture
1
0 -0.1
I
0
0.1
I
I
I
I
I
0.2 0.3 True strain
Fig. 4. The effect of strain rate on stress-strain Cr,,.,Nbjz.Z alloys deformed at 1473 K.
Fig. 2. TEM microstructure boundaries are observed.
of Cr,,,Nb,,
1373K
1.7x10-4/s
-
x frachlre
1623K
-0.1
0
0.1
0.2
0.3
0.4
0.5
True strain
Fig. 3. True stress-true deformed
I
0.5 curves
for
, alloy. Note that twin
1200 1000
I
0.4
strain curves for Cr,,,,Nbjz2 at an initial strain rate of 1.7 X 1 O-’ s ‘.
alloys
macroscopic yield stress. At temperatures where visible plastic deformation took place, either rapid work hardening followed by brittle fracture or a yield point was observed. As the temperature increased, an apparent stress peak, and also a subsequent decrease in flow stress and steady flow were observed. This stress peak tended to decrease with increasing temperature. At high temperatures a steady state flow was observed immediately after yielding (i.e. without showing the stress peak), and this steady state flow stress also decreased steadily with increasing temperature. These characteristics were common of the three alloys observed in this study. The effect of strain rate on stress-strain curves is shown in Fig. 4 for Cr67,8Nb32,2 alloys deformed at
1473 K. The effect of strain rate is similar to that of temperature: a decrease in strain rate leads to the same pattern in the stress-strain curves as observed for an increase in temperature. Also, it is interesting to note that an alloy deformed at a moderate strain rate showed an apparent stress peak and a subsequent stress decrease, while an alloy deformed at a low strain rate showed steady state flow immediately after yielding. The 0.2% yield stresses for three alloy compositions were replotted as a function of temperature, as shown in Fig. 5, for an initial strain rate of 1.7 X lOA s-l. As the temperature increases, the 0.2% yield stress decreases rapidly. Here, it is surprising to note that the measured 0.2% yield stresses are primarily independent of alloy composition. This result suggests that second-phase particles contained in the three alloys either did not actually contribute to the flow stresses, or contributed evenly to the flow stresses, regardless of the type of second-phase particles. Assuming that a single thermal activated mechanism operates at high temperatures, the relation between flow stress o and strain rate i is given by $ = A u” exp ( - Q/RT), where A is constant, II is the stress exponent, Q is the activation energy, R is the gas constant, and T is the temperature. The relation between the strain rate i and the 0.2% yield stress on.2 was measured at 1473 K for Cr67,8Nb32,2 alloys and is shown in Fig. 6(a). The stress exponent n is determined to be approximately 3.5. Using the relation between the 0.2% yield stress and temperature at an initial strain rate of 1.7 X 10m4 s-’ (shown in Fig. 6(b)), and the above-determined stress exponent, the apparent activation energy is calculated to be approximately 470 kJ
T. Takasugi et al.
808
0’
1200
I
1300
I
I
1400 1500 Temperature
/
Materials Science and Engineering A1921193 (I 995) 805-810
I
I
1600 (K)
1700
1800
Fig. 5. The 0.2% yield stresses for Cr,,,,Nb,, ,, Cr,,,,,Nb,,,,, and Cr,,,,Nb,2,z alloys as a function of temperature at an initial strain rate of 1.7 X 10m4 s- ‘.
mol- t , being basically identical for three alloy compositions. Quite similar values were calculated for the stress exponent and the apparent activation energy even when the steady stresses were included in the above calculations. The values of the stress exponent and the apparent activation energy measured in this study (n = 3.5 and Q = 470 kJ mol- ‘) are roughly identical with the values measured for duplex Cr,Nb alloys containing the smallest amount of Nb solid solution (n = 2.7 and e = 477 kJ mol- ‘) of three alloy compositions observed in a previous study [6]. Also, the present values are neither close to the values calculated from creep deformation studies ( n = 2.0 and e = 130 kJ mol- ‘) of Cr,Nb alloys prepared by hot pressing [lo], nor close to the values from microindentation creep deformation of Cr,Nb alloys (n = 4.5 and Q = 357 kJ mol- ’) prepared by arc melting [ 111. Since diffusion data for Cr,Mb alloys are not available at the moment, it is impossible to identify the above measured activation energy with a specific diffusional process. However, the calculated stress exponent (n = 3.5) suggests that the glide motion of activated dislocations controls the deformation at temperatures above 1473 K.
3.3. Deformation microstructures
10-s
10-4
10-s
10-z
Strain rate (s-l)
b
1001 6.0 1O-4
6.5 1O-4
7.0 10-4
IfiFig. 6. (a) Relation between the strain rate C and the 0.2% yield stress at 1473 K for Cr,,,,Nb,,,z alloys. (b) Relation between the 0.2% yield stress and temperature T at an initial strain rate of 1.7 X 10mJ s-l. Note that data for three alloy compositions are fitted to one curve.
At relatively low temperatures where plastic deformation takes place, relatively large cavities (probably cracks) were observed. Even at high temperatures where considerable plastic deformation was accomplished, a number of cavities (cracks) were observed. Figs. 7 and 8 show TEM microstructures of alloys plastically deformed to a true Cr,,.sNb,,.z strain of about 0.3 at 1523 K and at 1623 K respectively. At both temperatures the activity of a considerable amount of dislocations was observed. Also, it was shown by a conventional g.b criterion method that the major deformation mode in Cl5 Cr,Nb intermetallic compounds at high temperatures is the activation of b = l/2(0 11) dislocations which are dissociated into a pair of Shockley (i.e. l/6 (112)) partials bound by SFs [9], as has actually been observed in many other Cl5 intermetallic compounds [ 121. At 1523 K dislocations were relatively uniformly distributed, suggesting that the glide motion of dislocations is dominant and therefore may be consistent with the value of the stress exponent of 3.5, while at 1623 K the microstructures consist of a number of junctions, well developed subboundaries (or cells) and dislocation-free areas, thus suggesting that the climb motion of dislocations is active. Details of dislocation structures in plastically deformed Cr,Nb intermetallic compounds will be published in a different paper [9].
T. Takasugi et aI.
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Materials Science and Engineering A1921193 (1995) 805-810
809
200nm
200nm
Fig. 7. TEM microstructures of Cr,,,Nb,? z alloys plastically deformed to a true strain of about 0.3 at 1523 K.
4.
Discussion
Plastic deformation of Cr,Nb intermetallic compounds was shown to begin at temperatures above about two-thirds of the melting point. This result is quite similar to results for many Laves alloys as previously reported [ 121. Also, it was found that the l/2(0 11) dislocations were responsible for the considerable plastic deformation at high temperatures in Cr,Nb intermetallic compounds as well as in many other Laves alloys [ 121. However, plastic deformation at temperatures below two-thirds of the melting point, i.e. below 1423 K, was little observed in Cr,Nb intermetallics compounds. This result is in contrast with the results observed for C 15 V,Hf-based intermetallic compounds [3,4]. It has been shown that the Cl5 V,Hfbased intermetallic compound is deformable at room temperature and the major deformation mode is attributed to the { 111}( 112) mechanical twinning,
Fig. 8. TEM microstructures of Crh7,8Nb32,2 alloys plastically deformed to a true strain of about 0.3 at 1623 K.
accompanied by the plastic deformation of a V-rich b.c.c. solid solution [3,4]. To improve the compressive deformability, particularly at low temperatures, in Cr,Nb intermetallic compounds, modification of the alloy stoichiometry or the addition of adequate ternary elements, by which synchronized shear could be promoted [4,13,14], may be required. In this study, the 0.2% yield stresses were primarily independent of alloy composition although the three alloy compositions contained different kinds of second-phase particles. Furthermore, it is surprising to note that the 0.2% yield stresses measured in this study were not very different from those measured in the duplex Cr,b alloys containing a large amount of Nb solid solution (i.e. Cr,,Nb,, and Cr,,Nb,, alloys) although the deformable temperature observed in this study is slightly higher than those observed for the duplex Cr,Nb alloys [6]. However, the values for the constitutive equation (the stress exponent and the
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Materials Science and Engineering A1921193 (1995) 805-810
apparent activation energy) which would be divided into a few sub-regimes, were close to those calculated for the duplex Cr,Nb alloys [6], but not close to those reported in previous studies [ 10,111. To understand further the deformation behavior of Cr,Nb intermetallic compounds, more work, e.g. using singlecrystal Cr,Nb alloy consisting of perfect single-phase Cl 5 structure (if possible), is needed. Detailed TEM observations are also helpful in determining a conclusive deformation mechanism and thus improving the deformability of Cr,Nb intermetallic compounds at low temperatures.
(3) The stress exponent in the constitutive equation is about 3.5, while the apparent activation energy is 470 kJ mol- l and is insensitive to small changes in alloy composition. (4) Many dislocations with b = l/2(0 11) were observed to be activated at temperatures where compressive plastic deformation was accomplished.
References IEEE Trans. Magn., 13 (1977) 840. [21 K. Inoue, T. Kuroda and T. Tachikawa, IEEE Trans. Magn., 15(1979) 635. [31 J.D. Livingston and E.L. Hall, J. Mater. Res., 5( 1990) 5. [41 F. Chu and D. Pope, Mater. Sci. Eng., Al70( 1993) 39. [51 D.J. Thomas and J.H. Perepezko, Mater. Sci. Eng., Al56 (1992) 97. [61 T. Takasugi, S. Hanada and K. Miyamoto, J. Mater. Res., 8 (1993) 3039. [71 M. Takeyama and C.T. Liu, Mater. Sci. Eng., Al32 (1991) 61. D.P. Pope and F. Chu, Philos. Mag. A, 69( 1994) 409. 1;; M. Yoshida, D. Shindo, T. Takasugi and S. Hanada, to be published. 1101 D.L. Anton and D.M. Shah, Mater. Sci. Eng., AI53(1992) 410. [Ill G.E. Vignoul, J.M. Sanchez and J.K. Tien, in L.A. Johnson et al. (eds.), High-Temperature Ordered Intermetallic Alloys IV, MRS Symp. Proc., Vol. 213, Materials Research Society, Pittsburgh, PA, 1991, p. 739. J.D. Livingston, High-Temperature Silicides and Refractory Alloys, MRS Symp. Proc. Vol. 322, Materials Research Society, Pittsburgh, PA, 1994, p. 395. C.W. Allen, P. Delavignette and S. Amelinckx, Phys. Status Solidi A, 9( 1972) 237. P.M. Hazzledine, K.S. Kumar, D.B. Miracle and A.G. Jackson, High Temperature Ordered Intermetallic Alloys V, MRS Symp. Proc., Vol. 288, Materials Research Society, Pittsburgh, PA, 1993, p. 591.
[II K. Inoue and T. Tachikawa,
5. Conclusions The high temperature deformation behaviour of Cl5 Cr,Nb intermetallic compounds, prepared by arc melting and containing small amounts of second-phase particles, was characterized by compression tests. The following results were obtained in this study. (1) These alloys are deformable at temperatures above 1473 K. Their deformability increases considerably with increasing temperature (also decreasing strain rate). The 0.2% yield stress and the flow stress decrease with increasing temperature (and also decreasing strain rate), and are little sensitive to alloy composition (and very sensitive to the strain rate). (2) Stress-strain curves are very sensitive to the temperature and strain rate. At low temperatures (also at high strain rates) either rapid work hardening followed by brittle fracture or a yield point was observed. At intermediate temperatures (also in intermediate strain rates) an apparent stress peak and also a subsequent decrease in flow stress were observed. At high temperatures (also at low strain rates) steady state flow was observed immediately after yielding.
High temperature
Science and Engineering
Al 92/l 93 (1995) 805-S 10
mechanical properties of C 15 Laves phase Cr,Nb intermetallics T. Takasugi”, S. Hanadaa, M. Yoshida”
“Institute for Materials Research, Tohoku University Katahira 2-l-1, Aoba-ku, Sendai 980, Japan hMiyagi National College of Technology, Natori, Miyagi Prefecture 981-12, Japan
Abstract High temperature deformation behavior of Cl 5 CrzNb intermetallic compounds was observed by compression tests. These intermetallic compounds are deformable at temperatures above 1473 K. The flow stress decreases with increasing temperature and is little sensitive to small changes in alloy composition. Stress-strain curves are sensitive to temperature and also to the strain rate. At intermediate temperatures (also at intermediate strain rates) an apparent stress peak and a subsequent decrease in flow stress were observed. At high temperatures (also at low strain rates) a steady state flow was observed immediately after yielding. The stress exponent in the constitutive equation is about 3.5 while the apparent activation energy is 470 kJ molt I. The activation of a considerable amount of dislocations was observed at temperatures where compressive plastic deformation is accomplished. Keywords: Laves phases; Chromium;
Niobium; Intermetallics;
Mechanical
1. Introduction
A large pounds been
number
as high carried
of
studies
of
intermetallic
temperature
structural
out on relatively
simple
com-
materials crystal
have
structures
as f.c.c., b.c.c. and h.c.p. structures because they are deformable under various material and experimental conditions. A recent growing demand for intermetallic compounds as structural materials operating at extremely high temperatures and in severe environments means that intermetallic compounds with high melting point, low density, and superior corrosion and oxidation resistance have become very attractive candidates. However, they generally have complex crystal structures, for example the topologically close packed (TCP) materials, the largest group of which is the Laves intermetallic compounds. It is well known that the Laves intermetallic compounds are generally very brittle. However, a recent study showed that a Cl5 Laves Hf-V-Nb alloy can be deformed at room temperature by mechanical twinning [l-4]. Cr,Nb-based intermetallic compounds, which undergo the phase transition from Cl4 (MgZn,, hexagonal crystal structure) to Cl5 (MgCu,, cubic crystal structure) at approximately 1870 K [5], are considered
such
092 I-.5093/9S/$9.500 1995 - Elsevier Science S.A. All rights reserved SSDI 092 l-5093(94)033 19-6
properties
attractive for high temperature structural applications because of the high melting point (about 2003 K), low density (about 7.7 g cm-j), and potential resistance to corrosion and oxidation [6]. A few studies have been reported on the deformation behavior of Cr,Nb-based intermetallic compounds, i.e. the duplex Cr,Nb alloys containing a large volume fraction of Cr solid solution [ 71 and Nb solid solution [6]. However, the deformation behavior of Cr,Nb intermetallic compounds consisting of a single phase of Cl 5 structure has not been reported. In this study, the microstructure of Cr,Nb intermetallic compounds prepared by arc melting was firstly characterized. Next, high temperature compressive tests were performed, and the stress-strain curves and yield strength characterized. These mechanical properties are discussed with respect to the alloy composition, strain rate and testing temperature. Finally, dislocation microstructures observed using transmission electron microscopy (TEM) are shown briefly.
2. Experimental
procedure
Following a recently reported phase diagram [5], three alloy compositions, Cr,,,,Nb,,,, Crh7,0Nb3.1,0and
806
T Takasugi et al.
/
Materials Science and Engineering A1921193 (1995) 805-810
Cr67.5Nb32.5 1in atomit per cent) which should be single phase or nearly single phase C 15 Laves structure, were prepared in this study. The starting raw materials were 99.99 wt.% Cr and 99.5 wt.% Nb. Small ignots were prepared with a diameter of 30 mm by non-consumable arc melting on a water-sealed hearth in an argon environment. The ingots were remelted several times to ensure chemical homogeneity. The as-solidified ingots were then annealed in dynamic vacuum at 1623 K for 15 h, followed by cooling to room temperature at a rate of 2 K min- ‘. Chemical analysis after heat treatment showed the three alloy compositions to be 65.3 at.% Cr, 66.0 at.% Cr and 67.8 at.% Cr, and therefore deviations from the nominal chemical compositions were about 1.3 at.% Cr poor in Cr,,,Nb,,,, alloy, about 1 at.% Cr poor in Cr,,,ONb,,,O alloy, and about 0.3 at.% Cr rich in Cr67,sNb32,5 alloy. In the following, the three alloys are referred to by the actual chemical compositions, i.e. Cr65,3Nb34,7, Cr,,,ONb,,,O and Cr,,.,Nb,z.z. Compression specimens with dimensions of approximately 2.5 X 2.5 X 5 mm3 were cut by a precision wheel cutter from the central regions of ingots with axes parallel to the solidification direction. The lateral faces of the specimens were abraded on sufficiently fine Sic paper down to no. 1000. Compression tests were carried out mostly at temperatures between 1373 K and 1773 K at initial strain rates between 1.7 X 10m5 ss’ and 1.7 X10W3 s-’ in a vacuum of 3 X 10 - 3 Pa, using an Instron-type approximately machine equipped with a vacuum vessel and a tungsten mesh heater. Microstructures before and after compression tests were examined using optical microscopy (OM), transmission electron microscopy (TEM) equipped with energy dispersive X-ray analysis (EDX) and high resolution electron microscopy (HREM). Metallographic specimens for OM were etched with a solution consisting of one part HF, one part HNO, and three parts H,O. TEM thin foil specimens were mechanically thinned to about 0.1 mm and then jet polished in a solution of 5 vol.% HClO, + 95 vol.% CH,OH at about 200 K. TEM observations were carried out using a JEM-2000EX operating at 200 kV.
Fig. 1. Optical microstructure
of Cr,,,,Nb,,,,,
alloy.
observed in Cr,,,,Nb,,,, and Cr,,,Nb3,,0 are Nb solid solution (with 9 at.% Cr) and those observed in Cr,,.,Nb,,., alloys are Cr solid solution. The compositions of these microstructures may be consistent with a recently reported Cr-Nb phase diagram [5]. It has been reported that a Cl 5 phase extends between 6 1.8 and 69.2 at.% Cr at their eutectic temperatures equilibrating with Nb and Cr solid solutions, but between 66.0 and 67.0 at.% Cr at 1273 K, indicating that the solid solution range of the C 15 phase is very narrow at low temperatures. Therefore, note that the three alloy compositions prepared in this study were unfortunately outside the single-phase C 15 region. The large number of twins observed in the three alloy compositions indicates that the stacking fault (SF) energy in the Cl5 Laves phase is low and therefore stable, as has been discussed by Pope and Chu [8]. They are thought to be introduced during the phase transition from Cl4 to Cl5 structures or during long annealing at high temperatures (1623 K). Fig. 2 shows a TEM image of the Cr,,,Nb,,,, alloy. This TEM image along with the HREM image and the selected area diffraction pattern (SADP) showed that the grains consist of C 15 crystal structure and the twin orientations are of the relation { 11 l}( 112) [9]. Also, TEM observations showed that there are no crystal defects except for grain (and twin) boundaries and the second-phase particles [9].
3. Results 3.2. Deformation behavior 3.1. Microstructures Fig. 1 shows a representative microstructure of Cr,,,ONb,,,, alloy. Three alloy compositions used in this study contained small fractions of second particles and also a number of twins in their grains. TEM EDX analysis showed that the second-phase particles
Fig. 3 shows the true stress-strain curves calculated from the measured load vs. cross-head displacement curves for Crh7,XNb32.2alloys. An apparent plastic deformation begins at temperatures above 1473 K. Below this temperature, samples were broken into a large number of pieces at stress levels below the
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/ Materials Science and EngineeringA192/193 (1995) 805-810
807
1200 , 1473K
1000
x fracture
1
0 -0.1
I
0
0.1
I
I
I
I
I
0.2 0.3 True strain
Fig. 4. The effect of strain rate on stress-strain Cr,,.,Nbjz.Z alloys deformed at 1473 K.
Fig. 2. TEM microstructure boundaries are observed.
of Cr,,,Nb,,
1373K
1.7x10-4/s
-
x frachlre
1623K
-0.1
0
0.1
0.2
0.3
0.4
0.5
True strain
Fig. 3. True stress-true deformed
I
0.5 curves
for
, alloy. Note that twin
1200 1000
I
0.4
strain curves for Cr,,,,Nbjz2 at an initial strain rate of 1.7 X 1 O-’ s ‘.
alloys
macroscopic yield stress. At temperatures where visible plastic deformation took place, either rapid work hardening followed by brittle fracture or a yield point was observed. As the temperature increased, an apparent stress peak, and also a subsequent decrease in flow stress and steady flow were observed. This stress peak tended to decrease with increasing temperature. At high temperatures a steady state flow was observed immediately after yielding (i.e. without showing the stress peak), and this steady state flow stress also decreased steadily with increasing temperature. These characteristics were common of the three alloys observed in this study. The effect of strain rate on stress-strain curves is shown in Fig. 4 for Cr67,8Nb32,2 alloys deformed at
1473 K. The effect of strain rate is similar to that of temperature: a decrease in strain rate leads to the same pattern in the stress-strain curves as observed for an increase in temperature. Also, it is interesting to note that an alloy deformed at a moderate strain rate showed an apparent stress peak and a subsequent stress decrease, while an alloy deformed at a low strain rate showed steady state flow immediately after yielding. The 0.2% yield stresses for three alloy compositions were replotted as a function of temperature, as shown in Fig. 5, for an initial strain rate of 1.7 X lOA s-l. As the temperature increases, the 0.2% yield stress decreases rapidly. Here, it is surprising to note that the measured 0.2% yield stresses are primarily independent of alloy composition. This result suggests that second-phase particles contained in the three alloys either did not actually contribute to the flow stresses, or contributed evenly to the flow stresses, regardless of the type of second-phase particles. Assuming that a single thermal activated mechanism operates at high temperatures, the relation between flow stress o and strain rate i is given by $ = A u” exp ( - Q/RT), where A is constant, II is the stress exponent, Q is the activation energy, R is the gas constant, and T is the temperature. The relation between the strain rate i and the 0.2% yield stress on.2 was measured at 1473 K for Cr67,8Nb32,2 alloys and is shown in Fig. 6(a). The stress exponent n is determined to be approximately 3.5. Using the relation between the 0.2% yield stress and temperature at an initial strain rate of 1.7 X 10m4 s-’ (shown in Fig. 6(b)), and the above-determined stress exponent, the apparent activation energy is calculated to be approximately 470 kJ
T. Takasugi et al.
808
0’
1200
I
1300
I
I
1400 1500 Temperature
/
Materials Science and Engineering A1921193 (I 995) 805-810
I
I
1600 (K)
1700
1800
Fig. 5. The 0.2% yield stresses for Cr,,,,Nb,, ,, Cr,,,,,Nb,,,,, and Cr,,,,Nb,2,z alloys as a function of temperature at an initial strain rate of 1.7 X 10m4 s- ‘.
mol- t , being basically identical for three alloy compositions. Quite similar values were calculated for the stress exponent and the apparent activation energy even when the steady stresses were included in the above calculations. The values of the stress exponent and the apparent activation energy measured in this study (n = 3.5 and Q = 470 kJ mol- ‘) are roughly identical with the values measured for duplex Cr,Nb alloys containing the smallest amount of Nb solid solution (n = 2.7 and e = 477 kJ mol- ‘) of three alloy compositions observed in a previous study [6]. Also, the present values are neither close to the values calculated from creep deformation studies ( n = 2.0 and e = 130 kJ mol- ‘) of Cr,Nb alloys prepared by hot pressing [lo], nor close to the values from microindentation creep deformation of Cr,Nb alloys (n = 4.5 and Q = 357 kJ mol- ’) prepared by arc melting [ 111. Since diffusion data for Cr,Mb alloys are not available at the moment, it is impossible to identify the above measured activation energy with a specific diffusional process. However, the calculated stress exponent (n = 3.5) suggests that the glide motion of activated dislocations controls the deformation at temperatures above 1473 K.
3.3. Deformation microstructures
10-s
10-4
10-s
10-z
Strain rate (s-l)
b
1001 6.0 1O-4
6.5 1O-4
7.0 10-4
IfiFig. 6. (a) Relation between the strain rate C and the 0.2% yield stress at 1473 K for Cr,,,,Nb,,,z alloys. (b) Relation between the 0.2% yield stress and temperature T at an initial strain rate of 1.7 X 10mJ s-l. Note that data for three alloy compositions are fitted to one curve.
At relatively low temperatures where plastic deformation takes place, relatively large cavities (probably cracks) were observed. Even at high temperatures where considerable plastic deformation was accomplished, a number of cavities (cracks) were observed. Figs. 7 and 8 show TEM microstructures of alloys plastically deformed to a true Cr,,.sNb,,.z strain of about 0.3 at 1523 K and at 1623 K respectively. At both temperatures the activity of a considerable amount of dislocations was observed. Also, it was shown by a conventional g.b criterion method that the major deformation mode in Cl5 Cr,Nb intermetallic compounds at high temperatures is the activation of b = l/2(0 11) dislocations which are dissociated into a pair of Shockley (i.e. l/6 (112)) partials bound by SFs [9], as has actually been observed in many other Cl5 intermetallic compounds [ 121. At 1523 K dislocations were relatively uniformly distributed, suggesting that the glide motion of dislocations is dominant and therefore may be consistent with the value of the stress exponent of 3.5, while at 1623 K the microstructures consist of a number of junctions, well developed subboundaries (or cells) and dislocation-free areas, thus suggesting that the climb motion of dislocations is active. Details of dislocation structures in plastically deformed Cr,Nb intermetallic compounds will be published in a different paper [9].
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809
200nm
200nm
Fig. 7. TEM microstructures of Cr,,,Nb,? z alloys plastically deformed to a true strain of about 0.3 at 1523 K.
4.
Discussion
Plastic deformation of Cr,Nb intermetallic compounds was shown to begin at temperatures above about two-thirds of the melting point. This result is quite similar to results for many Laves alloys as previously reported [ 121. Also, it was found that the l/2(0 11) dislocations were responsible for the considerable plastic deformation at high temperatures in Cr,Nb intermetallic compounds as well as in many other Laves alloys [ 121. However, plastic deformation at temperatures below two-thirds of the melting point, i.e. below 1423 K, was little observed in Cr,Nb intermetallics compounds. This result is in contrast with the results observed for C 15 V,Hf-based intermetallic compounds [3,4]. It has been shown that the Cl5 V,Hfbased intermetallic compound is deformable at room temperature and the major deformation mode is attributed to the { 111}( 112) mechanical twinning,
Fig. 8. TEM microstructures of Crh7,8Nb32,2 alloys plastically deformed to a true strain of about 0.3 at 1623 K.
accompanied by the plastic deformation of a V-rich b.c.c. solid solution [3,4]. To improve the compressive deformability, particularly at low temperatures, in Cr,Nb intermetallic compounds, modification of the alloy stoichiometry or the addition of adequate ternary elements, by which synchronized shear could be promoted [4,13,14], may be required. In this study, the 0.2% yield stresses were primarily independent of alloy composition although the three alloy compositions contained different kinds of second-phase particles. Furthermore, it is surprising to note that the 0.2% yield stresses measured in this study were not very different from those measured in the duplex Cr,b alloys containing a large amount of Nb solid solution (i.e. Cr,,Nb,, and Cr,,Nb,, alloys) although the deformable temperature observed in this study is slightly higher than those observed for the duplex Cr,Nb alloys [6]. However, the values for the constitutive equation (the stress exponent and the
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apparent activation energy) which would be divided into a few sub-regimes, were close to those calculated for the duplex Cr,Nb alloys [6], but not close to those reported in previous studies [ 10,111. To understand further the deformation behavior of Cr,Nb intermetallic compounds, more work, e.g. using singlecrystal Cr,Nb alloy consisting of perfect single-phase Cl 5 structure (if possible), is needed. Detailed TEM observations are also helpful in determining a conclusive deformation mechanism and thus improving the deformability of Cr,Nb intermetallic compounds at low temperatures.
(3) The stress exponent in the constitutive equation is about 3.5, while the apparent activation energy is 470 kJ mol- l and is insensitive to small changes in alloy composition. (4) Many dislocations with b = l/2(0 11) were observed to be activated at temperatures where compressive plastic deformation was accomplished.
References IEEE Trans. Magn., 13 (1977) 840. [21 K. Inoue, T. Kuroda and T. Tachikawa, IEEE Trans. Magn., 15(1979) 635. [31 J.D. Livingston and E.L. Hall, J. Mater. Res., 5( 1990) 5. [41 F. Chu and D. Pope, Mater. Sci. Eng., Al70( 1993) 39. [51 D.J. Thomas and J.H. Perepezko, Mater. Sci. Eng., Al56 (1992) 97. [61 T. Takasugi, S. Hanada and K. Miyamoto, J. Mater. Res., 8 (1993) 3039. [71 M. Takeyama and C.T. Liu, Mater. Sci. Eng., Al32 (1991) 61. D.P. Pope and F. Chu, Philos. Mag. A, 69( 1994) 409. 1;; M. Yoshida, D. Shindo, T. Takasugi and S. Hanada, to be published. 1101 D.L. Anton and D.M. Shah, Mater. Sci. Eng., AI53(1992) 410. [Ill G.E. Vignoul, J.M. Sanchez and J.K. Tien, in L.A. Johnson et al. (eds.), High-Temperature Ordered Intermetallic Alloys IV, MRS Symp. Proc., Vol. 213, Materials Research Society, Pittsburgh, PA, 1991, p. 739. J.D. Livingston, High-Temperature Silicides and Refractory Alloys, MRS Symp. Proc. Vol. 322, Materials Research Society, Pittsburgh, PA, 1994, p. 395. C.W. Allen, P. Delavignette and S. Amelinckx, Phys. Status Solidi A, 9( 1972) 237. P.M. Hazzledine, K.S. Kumar, D.B. Miracle and A.G. Jackson, High Temperature Ordered Intermetallic Alloys V, MRS Symp. Proc., Vol. 288, Materials Research Society, Pittsburgh, PA, 1993, p. 591.
[II K. Inoue and T. Tachikawa,
5. Conclusions The high temperature deformation behaviour of Cl5 Cr,Nb intermetallic compounds, prepared by arc melting and containing small amounts of second-phase particles, was characterized by compression tests. The following results were obtained in this study. (1) These alloys are deformable at temperatures above 1473 K. Their deformability increases considerably with increasing temperature (also decreasing strain rate). The 0.2% yield stress and the flow stress decrease with increasing temperature (and also decreasing strain rate), and are little sensitive to alloy composition (and very sensitive to the strain rate). (2) Stress-strain curves are very sensitive to the temperature and strain rate. At low temperatures (also at high strain rates) either rapid work hardening followed by brittle fracture or a yield point was observed. At intermediate temperatures (also in intermediate strain rates) an apparent stress peak and also a subsequent decrease in flow stress were observed. At high temperatures (also at low strain rates) steady state flow was observed immediately after yielding.