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The effect of Ti-addition on the L12 precipitates of rapidly-solidified AlCrZr alloy
Pergamon
ScriptaMetallurgicaet Materialia,Vol. 31, No. 9, pp. 1259-1264, 1994 Copyright©1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00
THE EFFECT OF T i - A D D I T I O N ON THE L12 P R E C I P I T A T E S O F RAPIDLY-SOLIDIFIED AI-Cr-Zr ALLOY M.S. Chuang and G.C. Tu Institute of Materials Science and Engineering National Chiao Tung University Hsinchu, Taiwan, R.O.C.
(Received May 3, 1994) (Revised June 20, 1994) Introduction The rapidly Solidified (RS) AI-Cr-Zr alloy, possessing excellent strength, ductility and thermal stability in elevated temperatures has received considerable attention in the past decade[I-5]. The age-hardenable A1-Cr-Zr alloys derive their elevated temperature strength and thermal stability from homogeneous precipitation of metastable L12AI3Zr and chromium retained in solid solution[I-5]. For high temperature A1 alloys, the dispersed hardening phase must not undergo a phase transformation to an undesirable phase during long time exposure at the temperature of interest. An additional factor to be considered is the stability of the hardening phase with respect to Ostwald ripening. According to theory, a particle will be resistant to coarsening if the energy of its interface with the matrix is low, and if the diffusivity D and solubility Co of its ratecontrolling element is small. In a review article, Adam[6] indicated that among the transition metals, Zr has the lowest diffusivity flux DCo in AI, followed by Fe, Ti, and Mo. Furthermore, the L 12-AI3Zr is coherent and coplanar with the AI matrix, the lattice misfit, and thus the interfacial energy is small. Thus, aluminum alloys containing L12-A13Zr as the dispersed phase are good candidates for high temperature usage. However, it was suggested that careful selection of additional alloying elements could further reduce the misfit between L12 phase and A1 matrix, thereby reducing interfacial energy and ripening rate of L 12 phase[7-12]. Zedalis et al.[7-1 l] suggested that addition of elements such as V, Ti, or Hf can improve the lattice matching between the stable DO23-structured, as well as metastable L12-structured, Al3Zr and the Al matrix. The alloy AI-Zr-V, in particular, has been studied extensively[7-10]. Metastable L12 phase dispersoids Al3(Zrx, Vi.x), form in RS AI-ZrV alloys. The V-containing L12 particles have lower interfacial energy and higher thermal stability than the Ll2-AI3Zr. However, incoherent All0V forms at the grain boundaries and creates precipitate free zones (PFZs). The PFZs become an increasing problem as the amount of V increased[9,12]. On the other hand, the stable DO23-structured phase matches the A1 lattice rather well by addition of Ti than that of V, and the Ti-containing phases are not expected to form at grain boundaries as readily as All0V does[l 1.12]. Titanium is generally used as grain refiner in aluminum, and several L12 phases were found in AI-Ti alloys with addition of ternary or quaternary elements[ 13,14]. Lieblich et al. [ 15] proposed that Ti solutionized in aluminum could retain the supersaturated vacancies in matrix such that the coarsening rate of dispersoids in AI-Li-Ti alloy was retarded. From these points of view, titanium seems to be a good alloying element to improve the thermal stability of AI-Cr-Zr alloy. In the present work, titanium was added to A1-Cr-Zr alloy to form a quaternary system prepared by rapid solidification. The microstructure of the RS ribbon was studied with particular interest in several L 12 phases present in this alloy. The coarsening kinetics of the L12 dispersoids on aging at 350°C were also presented. 1259
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Experimental Procedure A rapidly solidified A1-Cr-Zr-Ti ribbon prepared by melt-spinning was under examination in the present work. The typical composition determined by inductively coupled plasma-mass spectrometer (ICP) was : AI-4.52Cr-4.18Zr1.46Ti (in wt%), with major impurities Fe (0.052 wt%) and Si (0.03 wt%). A prealloyed ingot was induction-melted in a graphite crucible in Ar atmosphere using pure A1(99.9%), Cr(99.8%), Zr(99.9%), and Ti(99.9%). Rapidly quenched ribbons about 501am thickness and 5mm width, were prepared from the prealloyed ingots under Ar atmosphere using a single roller melt-spinning apparatus with a copper roller rotating at a surface velocity about 33m/s. The microstructures of as-quenched and annealed ribbons were examined by X-ray diffractometer using CuKct radiation and scanning transmission electron microscopy (STEM) affixed with energy dispersive spectrometer (EDS). Thin foils for STEM observation were prepared by jet electropolishing in a standard solution. The STEM and EDS used were JOEL JEM-2000FX, operating at 200KV, and LINK-10000 EDS, respectively. The EDS microanalyses were performed with a 200KV electron beam focused to a ca. 40urn spot on the specimen under examination. More than 20 EDS microanalyses were made for each phase. Intensity correction were made with a standard ZAF iterative correction program. The L12 particle sizes were measured directly from TEM negatives which were taken by the central dark field technique from superlattice reflection spots. An eyepiece magnifier with a calibrated scale was used as an aid. More than 200 particles were measured to obtain each value of mean radius, 7. Results and Discussion Chromium-containing phases, in the form of coarse particles with particle sizes ranging from submicrometers to several micrometers[4,5] or "flower-like" precipitate-aggregates[2,3], are generally unsuppressed in most RS AI-Cr-Zr alloys. However, such a form of the chromium-containing phases is not found in the present work; instead, some cuboidal precipitates with a side of 0.1-0.3p.m with L12 superlattice structure are found occasionally in as-spun ribbon, as shown in the arrow-marked region in Fig. 1. Similar cuboidal-shaped precipitates with L12 structure are also observed in several RS AI-Ti-X alloys[16-18]. Hori[16] described this particle as "petal-like structure", and proposed that it is composed of very finely divided lamellar L12-AI3Ti and A1-Ti solid solution. Majumdar[17] and Nie[18], who studied AI-Ti and AI-Ti-Ce RS alloys proposed that this cuboidal particle is a single phase with composition near to AI4Ti. The situation is more complicated in the present work. An enlarged STEM bright field (BF) image of the cuboidal particle in Fig. 1 is shown in Fig. 2a together with the corresponding dark field (DF) image in Fig. 2b. Evidently, the cuboidal particle comprises two parts : a bulk cubic phase in the center (marked with T) enveloped by an outer square dendritic phase (marked with T'). The center bulk assumes a cubic morphology with edge length about 0.1 p.m, suggesting that the T phase is a primary precipitate in solidification. From Fig. 2, the T' phase surrounding T phase is evidently nucleated from the very surface of T phase and grows into a dendritic form. Both T and T' phases are of the L 12 superlattice structure and have a cube-to-cube orientation relationship with the AI matrix, as confirmed by the selected area diffraction patterns (SADPs) shown in Fig. 2c-2e. However, compositions of the two phases are different. Fig. 3a and 3b are EDS spectrums of "T" and "T' + inter-dendritic matrix" phases, respectively. The composition of T phase, as determined by standardless EDS quantitative microanalysis, appears to be Al3(Zrx,Til.x), where x is in the range 0.5-0.6. However, due to the fine scale and the appearance of the dendritic T' phase, the EDS result of Fig. 3b inevitably comprises compositions of both T' phase and the inter-dendritic matrix. For this situation, the EDS spectrum, based on a similar full scale intensity, was taken from bulk matrix and compared with that of Fig. 3b; the two EDS spectrums are superimposed and shown in an inserted in the upper comer of Fig. 3b. Apparently, there is a higher Cr content present in the exact dendritic T' phase than in the bulk matrix. This indicates that the composition of the T' phase comprises A1, Zr, Ti, and Cr. In Fig. 2b, besides the cuboidal particles, there are many other L12 particles precipitated on the grain boundary. The small size (less than 10nm) of the grain boundary precipitates (GBPs) suggests that their formation might be due to a solid-state reaction. After annealing the ribbon at 450°C for 10 minutes, the GBPs coarsened discontinuously as shown in Fig. 4. This observation of discontinuous precipitation of L12 precipitates is in agreement with works of other researchers who studied zirconium-containing aluminum alloys and identified the phase be the metastable
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AI3Zr[1,19]. However, the L12 GBPs of annealed ribbon contain AI, Zr, and Ti in the present work as shown in the EDS spectrum in Fig. 3c. The discontinuous precipitation of GBPs results in coarse AI-Zr-Ti L 12 GBPs accompanied with solute-drained matrix which was swept by the moving grain boundary. Figure 4 shows that, in addition to the cuboidal particles and the GBPs, there are numerous fine L12 particles present uniformly over the whole matrix after annealing. Precipitation of L12-AI3X in annealing is a common feature in RS aluminum alloys containing Zr[8-10,19] and/or Ti[17,18]. In AI-Ti-X alloys[17,18], such precipitations are quite inhomogeneous. "Petal-like" zones of fine-scale L12-AI3Ti extend from comers of the pre-existed coarse cuboidal L12-A13Ti particle, and a "banded structure" containing fine-scale L12-AI3Ti form in those grains apparently free of coarse cuboidal particles during annealing. The former is explained by inhomogeneous distribution of solute resulting from solidification, and the preferred growth of the fine particles in the <111> directions[17]. The "banded structure" reflects a dislocation assisted precipitation of the fine particles[ 17]. On the other hand, precipitations of fine L12 particles are more homogeneous in AI-Zr alloys. In the present work, precipitation of the fine particles is quite homogeneous throughout the matrix irrespective of whether the cuboidal particle existing or not. Furthermore, the particle sizes are very small, with mean particle size about 1.Snm. However, fine particles were absent in the solutedrained matrix evolving simultaneously with the growth of discontinuous L 12 GBPs. In spite of this localized feature, the fine particle size and the homogeneous distribution of L12 particles in bulk matrix would theoretically result in a good precipitation hardening property to the alloy. To investigate the thermal stability of the precipitated hardening particles, samples ageing at 450°C for 10 min followed by soaking at 350°C for 100 h and 240 h were examined, and the microstmctures are shown in Fig. 5a and 5b, respectively, which are TEM DF micrographs taking from the L12 diffraction spot. Figure 5a shows that homogeneous distribution of the L12 fine particles remains unchanged. Fig. 5b, with a higher magnification, reveals the rounded shape of the particles. Even after aging at 350°C for 240 h, SADPs shown in Fig. 5c to 5e reveal that the precipitates remain L12 structure, and no other structure phases precipitate. The mean particle sizes of samples annealed for 100 h and 240 h are 2.5nm and 3.1nm, respectively. The mean particle spacing measured from sample shown in Fig. 5b is about 12nm, which is smaller than the figure of 16nm, suggested by Adam[6] for thermally stable AI alloys. The volume fraction of the dispersed particles is about 13%, as determined by a point-counting technique[20]. In view of the composition of the present alloy, the high volume fraction of the precipitated phase excludes the possibility that the phase is simply AlaZr. This results because the zirconium content (4.18 wt%) can only form 5.76 vol% of A13Zr at the most, which is far beneath the experimental value; instead, the precipitated phase is more likely to be AI3(Zr,Ti), as 10.2 vol% of A13(Zr,Ti) can be produced according to a theoretical calculation based on the solute contents. However, a discrepancy still exists between the calculated and experimental value. This may be due to either the experimental error, or to the possibility that Cr is incorporated with Zr, Ti to form LI2A13(Zr,Ti,Cr) precipitate particles. Further work is required to clarify this point. For simplicity, L12-AI3(Zr,Ti) will be used to represent the fine precipitate particles in the following discussion. According to the modified LSW theory of volume controlled coarsening[21], the average particle radius should increase in accordance with the relationship F 3 _Fo3 = k ( t _ t o )
(1)
where F is the average particle radius at time t, r0 is the value corresponding to t 0, and k represents the volumetric coarsening rate constant given by k - 8aDC°V"~
(2)
9vRT
where a is the interracial energy of the particle, D and Co are the diffusivity and equilibrium concentration of the ratecontrolling solute, v is the stoichiometric factor, and I'm is the molar volume of the particle, R and T have their usual meanings. A volumetric coarsening rate content k of 1.25 × 10.29 m3/h is calculated from eq. (1) for the L12-A13(Zr,Ti) particles at 350°C. This value is quite small compared with those of some other high temperature A1 alloys[10]. To
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evaluate the interfacial energy of the L12-A13(Zr,Ti) fine particles, eq. (2) was used, and Zr is assumed to be the rate controlling component since Zr has the lowest diffusivity flux DCo. In the present calculation, if 7.63 × 10-2t m2/s for the diffusivity D of Zr at 350°C [22], 43 mole Zr/m 3 of C018], 4.13 × 10-5 m3/mole of V,[8] and Wagner's stoichiometric factor v=l [8] are adopted, an interfacial energy of 0.036 J/m 2 results. This value agrees with the reported values for a coherent interface[8,10]. The agreement between the calculated and measured coarsening rates indicates that the coarsening kinetics of the L12-A13(Zr,Ti) particles in the present alloy are indeed controlled by volume diffusion of Zr.
Summary 1. The addition of titanium to AI-Cr-Zr alloy enhances precipitation of an L12 metastable phase. Three LI2 type phases were found in the as-spun ribbon and numerous tiny L12-AI3(Zr,Ti) precipitates was also found in annealed and aged ribbon. 2. A T phase, with an L12 structure and Al3(Zrx,Tit.x) composition, precipitates during solidification with a cube appearance and is enveloped by dendritic T' phase, which is also of L12 st.ructure but contains Cr besides Zr and Ti. These two phases combine to form a cuboidal shape precipitate sited in the grain interior. 3. LI2 structured precipitates composed of A1, Zr and Ti are found on the grain boundaries. The grain boundary precipitates (GBPs) grow in a discontinuous precipitation manner during annealing and ageing. 4. A high volume fraction of metastable L12-AI3(Zr,Ti) precipitates homogeneously within grains after annealing and ageing. The coarsening rate constant and interracial energy were measured and calculated; the result suggest that the coarsening of the fine L12 precipitates is controlled by volume diffusion of Zr and that these precipitates are coherent with A1 matrix.
Acknowledgement We would like to acknowledge the support of the work by the National Science Council of Taiwan, R.O.C. under contract No. NSC 83-0405-E-009-007.
Reference 1. S. P. Midson, R. A. Buckley and H. Jones, in "Rapidly Quenched Metal V" vol. I, (ed. S. Steeb and H. Warlimont), p. 923, North-Holland, Amsterdam (1985). 2. K. Ioannidis and T. Sheppard, J. Mat. Sci., 26, 4795(199l). 3. E. K. Ioannidis and T. Sheppard, Mat. Sci. and Tech., 6, 528(1990). 4. G. J. Marshall and E. K. Ioannidis, J. Mat. Sci., 27, 3552(1992). 5. G. J. Marshall, I. R. Hughes and W. S. Miller, Mat. Sci. and Tech., 2, 394(1986). 6. C.M. Adam, in "Rapidly Solidified Amorphous and crystalline Alloys", (ed. B.H. Kear, B.C. Giessen, and M. Cohen), p.411, North-Holland, Oxford (1982). 7. M. Zedalis and M.E. Fine, Scripta Metall., 17, 1247(1983). 8. M.S. Zedalis and M.E. Fine, Met. Trans., 17A, 2187(1986). 9. Y.C. Chen, M.E. Fine and J.R. Weertman, Acta. Met., 38, No.5, 771(1990). 10. Y.C. Chen, M.E. Fine, J.R. Weertman and R.E. Lewis, Scripta Metall., 21, 1003(1987). 11. V.R. Parameswaran, J.R. Weertman and M.E. Fine, Scripta Metall., 23, 147(1989). 12. H.M. Lee, J. Lee and Z.H. Lee, Scripta Metall., 25, 517(1991). 13. S. Zhang, J.P. Nic and D.E. Mikkola, Scripta Metall., 24, 57(1990). 14. Y. Ma, T. Amesen, J. Gjflnnes and J. Taftfl, J. Mater. Res., 7, No.7, 1722(1992). 15. M. Lieblich and M. Torralba, J. Mat. Sci., 27, 3474(1992). 16. S. Hori, H. Tai and Y. Narita, in "Rapidly Quenched Metal V", Vol.1, (ed. S. Steeb and H. Warlimont), p. 911, North-Holland, Amsterdam (1985). 17. A. Majumdar, R.H. Mair and B.C. Muddle, in "Science and Technology of Rapidly Quenched Alloys", (ed. M. Tenhover, W.L. Johnson and L.E. Tanner), p. 253, Materials Research Society, Pittsburgh (1987). 18. J. F. Nie, S. Sridhara and B. C. Muddle, Met. Trans., 23A, 3193(1992). 19. Z.A. Chaudhury and C. Suryanarayana, Metallography, 17, 231(1984).
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20. E.E. Underwood, 'Applications of Quantitative Metallography' in "Metals Handbook", vol. 8, 8th Ed., American Society for Metals(1973). 21. A.D. Brailsford and P. Wynblatt, Acta Met.,27, 489(1979). 22. H. Jones, in "Rapid Solidification Processing : Principles and Technologies II" (ed. R. Mehrabian, B.H. Kear and M. Cohen), p. 306, Claitor's Pub. Div., Baton Rouge (1980).
FIG. 1. TEM micrograph (BF) of as-spun AI-Cr-Zr-Ti ribbon. The arrow-marked cuboidal particle is the only type of coarser dispersoid found in the as-spun ribbon.
FIG. 2. TEM mierographs of the euboidal particle. (a) BF, (b) DF, using a g<010>Llz reflection. Zone axis near < 001 > AI. (c)-(e) Three selected-area diffraction patterns taken from the whole particle. The zone axes are (c) [001], (d) [011], (e) [111], respectively. ( hkl=Al matrix, hkl--L12 phase).
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AL R
J "
I
.
.
.
i AL
It
.T I :'.
J
) CR
t
..... T'+ inter-dendritic matrix ::::i::::::i::::~ bulk matrix
AL
]<
.o
5 . IqO
i,:eu
10.3
Energy ( keY )
Fig. 3. EDS spectrum from (a) T phase, (b) T' + interdendritic matrix, (c) GBPs. For the inserted picture, see context.
Fig. 5. TEM DF micrograph of ribbon.annealed at 450°C for 10 min, then soaked at 350°C for (a) 100 hr (b) 240hr, using a g < 110> L12 reflection, zone axis near [001]AI. (c)-(e) Three SADPs taken from sample shown in (b). The zone axes are (c) [001], (d) [011], (e) [111], respectively. ( hkl=Al matrix, hkl=L12 phase ).
Fig. 4. TEM DF micrograph, using a g < 011> Lh reflection from ribbon annealed at 450 °C for 10 min. Zone axis near <211 >AI.
ScriptaMetallurgicaet Materialia,Vol. 31, No. 9, pp. 1259-1264, 1994 Copyright©1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00
THE EFFECT OF T i - A D D I T I O N ON THE L12 P R E C I P I T A T E S O F RAPIDLY-SOLIDIFIED AI-Cr-Zr ALLOY M.S. Chuang and G.C. Tu Institute of Materials Science and Engineering National Chiao Tung University Hsinchu, Taiwan, R.O.C.
(Received May 3, 1994) (Revised June 20, 1994) Introduction The rapidly Solidified (RS) AI-Cr-Zr alloy, possessing excellent strength, ductility and thermal stability in elevated temperatures has received considerable attention in the past decade[I-5]. The age-hardenable A1-Cr-Zr alloys derive their elevated temperature strength and thermal stability from homogeneous precipitation of metastable L12AI3Zr and chromium retained in solid solution[I-5]. For high temperature A1 alloys, the dispersed hardening phase must not undergo a phase transformation to an undesirable phase during long time exposure at the temperature of interest. An additional factor to be considered is the stability of the hardening phase with respect to Ostwald ripening. According to theory, a particle will be resistant to coarsening if the energy of its interface with the matrix is low, and if the diffusivity D and solubility Co of its ratecontrolling element is small. In a review article, Adam[6] indicated that among the transition metals, Zr has the lowest diffusivity flux DCo in AI, followed by Fe, Ti, and Mo. Furthermore, the L 12-AI3Zr is coherent and coplanar with the AI matrix, the lattice misfit, and thus the interfacial energy is small. Thus, aluminum alloys containing L12-A13Zr as the dispersed phase are good candidates for high temperature usage. However, it was suggested that careful selection of additional alloying elements could further reduce the misfit between L12 phase and A1 matrix, thereby reducing interfacial energy and ripening rate of L 12 phase[7-12]. Zedalis et al.[7-1 l] suggested that addition of elements such as V, Ti, or Hf can improve the lattice matching between the stable DO23-structured, as well as metastable L12-structured, Al3Zr and the Al matrix. The alloy AI-Zr-V, in particular, has been studied extensively[7-10]. Metastable L12 phase dispersoids Al3(Zrx, Vi.x), form in RS AI-ZrV alloys. The V-containing L12 particles have lower interfacial energy and higher thermal stability than the Ll2-AI3Zr. However, incoherent All0V forms at the grain boundaries and creates precipitate free zones (PFZs). The PFZs become an increasing problem as the amount of V increased[9,12]. On the other hand, the stable DO23-structured phase matches the A1 lattice rather well by addition of Ti than that of V, and the Ti-containing phases are not expected to form at grain boundaries as readily as All0V does[l 1.12]. Titanium is generally used as grain refiner in aluminum, and several L12 phases were found in AI-Ti alloys with addition of ternary or quaternary elements[ 13,14]. Lieblich et al. [ 15] proposed that Ti solutionized in aluminum could retain the supersaturated vacancies in matrix such that the coarsening rate of dispersoids in AI-Li-Ti alloy was retarded. From these points of view, titanium seems to be a good alloying element to improve the thermal stability of AI-Cr-Zr alloy. In the present work, titanium was added to A1-Cr-Zr alloy to form a quaternary system prepared by rapid solidification. The microstructure of the RS ribbon was studied with particular interest in several L 12 phases present in this alloy. The coarsening kinetics of the L12 dispersoids on aging at 350°C were also presented. 1259
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Experimental Procedure A rapidly solidified A1-Cr-Zr-Ti ribbon prepared by melt-spinning was under examination in the present work. The typical composition determined by inductively coupled plasma-mass spectrometer (ICP) was : AI-4.52Cr-4.18Zr1.46Ti (in wt%), with major impurities Fe (0.052 wt%) and Si (0.03 wt%). A prealloyed ingot was induction-melted in a graphite crucible in Ar atmosphere using pure A1(99.9%), Cr(99.8%), Zr(99.9%), and Ti(99.9%). Rapidly quenched ribbons about 501am thickness and 5mm width, were prepared from the prealloyed ingots under Ar atmosphere using a single roller melt-spinning apparatus with a copper roller rotating at a surface velocity about 33m/s. The microstructures of as-quenched and annealed ribbons were examined by X-ray diffractometer using CuKct radiation and scanning transmission electron microscopy (STEM) affixed with energy dispersive spectrometer (EDS). Thin foils for STEM observation were prepared by jet electropolishing in a standard solution. The STEM and EDS used were JOEL JEM-2000FX, operating at 200KV, and LINK-10000 EDS, respectively. The EDS microanalyses were performed with a 200KV electron beam focused to a ca. 40urn spot on the specimen under examination. More than 20 EDS microanalyses were made for each phase. Intensity correction were made with a standard ZAF iterative correction program. The L12 particle sizes were measured directly from TEM negatives which were taken by the central dark field technique from superlattice reflection spots. An eyepiece magnifier with a calibrated scale was used as an aid. More than 200 particles were measured to obtain each value of mean radius, 7. Results and Discussion Chromium-containing phases, in the form of coarse particles with particle sizes ranging from submicrometers to several micrometers[4,5] or "flower-like" precipitate-aggregates[2,3], are generally unsuppressed in most RS AI-Cr-Zr alloys. However, such a form of the chromium-containing phases is not found in the present work; instead, some cuboidal precipitates with a side of 0.1-0.3p.m with L12 superlattice structure are found occasionally in as-spun ribbon, as shown in the arrow-marked region in Fig. 1. Similar cuboidal-shaped precipitates with L12 structure are also observed in several RS AI-Ti-X alloys[16-18]. Hori[16] described this particle as "petal-like structure", and proposed that it is composed of very finely divided lamellar L12-AI3Ti and A1-Ti solid solution. Majumdar[17] and Nie[18], who studied AI-Ti and AI-Ti-Ce RS alloys proposed that this cuboidal particle is a single phase with composition near to AI4Ti. The situation is more complicated in the present work. An enlarged STEM bright field (BF) image of the cuboidal particle in Fig. 1 is shown in Fig. 2a together with the corresponding dark field (DF) image in Fig. 2b. Evidently, the cuboidal particle comprises two parts : a bulk cubic phase in the center (marked with T) enveloped by an outer square dendritic phase (marked with T'). The center bulk assumes a cubic morphology with edge length about 0.1 p.m, suggesting that the T phase is a primary precipitate in solidification. From Fig. 2, the T' phase surrounding T phase is evidently nucleated from the very surface of T phase and grows into a dendritic form. Both T and T' phases are of the L 12 superlattice structure and have a cube-to-cube orientation relationship with the AI matrix, as confirmed by the selected area diffraction patterns (SADPs) shown in Fig. 2c-2e. However, compositions of the two phases are different. Fig. 3a and 3b are EDS spectrums of "T" and "T' + inter-dendritic matrix" phases, respectively. The composition of T phase, as determined by standardless EDS quantitative microanalysis, appears to be Al3(Zrx,Til.x), where x is in the range 0.5-0.6. However, due to the fine scale and the appearance of the dendritic T' phase, the EDS result of Fig. 3b inevitably comprises compositions of both T' phase and the inter-dendritic matrix. For this situation, the EDS spectrum, based on a similar full scale intensity, was taken from bulk matrix and compared with that of Fig. 3b; the two EDS spectrums are superimposed and shown in an inserted in the upper comer of Fig. 3b. Apparently, there is a higher Cr content present in the exact dendritic T' phase than in the bulk matrix. This indicates that the composition of the T' phase comprises A1, Zr, Ti, and Cr. In Fig. 2b, besides the cuboidal particles, there are many other L12 particles precipitated on the grain boundary. The small size (less than 10nm) of the grain boundary precipitates (GBPs) suggests that their formation might be due to a solid-state reaction. After annealing the ribbon at 450°C for 10 minutes, the GBPs coarsened discontinuously as shown in Fig. 4. This observation of discontinuous precipitation of L12 precipitates is in agreement with works of other researchers who studied zirconium-containing aluminum alloys and identified the phase be the metastable
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AI3Zr[1,19]. However, the L12 GBPs of annealed ribbon contain AI, Zr, and Ti in the present work as shown in the EDS spectrum in Fig. 3c. The discontinuous precipitation of GBPs results in coarse AI-Zr-Ti L 12 GBPs accompanied with solute-drained matrix which was swept by the moving grain boundary. Figure 4 shows that, in addition to the cuboidal particles and the GBPs, there are numerous fine L12 particles present uniformly over the whole matrix after annealing. Precipitation of L12-AI3X in annealing is a common feature in RS aluminum alloys containing Zr[8-10,19] and/or Ti[17,18]. In AI-Ti-X alloys[17,18], such precipitations are quite inhomogeneous. "Petal-like" zones of fine-scale L12-AI3Ti extend from comers of the pre-existed coarse cuboidal L12-A13Ti particle, and a "banded structure" containing fine-scale L12-AI3Ti form in those grains apparently free of coarse cuboidal particles during annealing. The former is explained by inhomogeneous distribution of solute resulting from solidification, and the preferred growth of the fine particles in the <111> directions[17]. The "banded structure" reflects a dislocation assisted precipitation of the fine particles[ 17]. On the other hand, precipitations of fine L12 particles are more homogeneous in AI-Zr alloys. In the present work, precipitation of the fine particles is quite homogeneous throughout the matrix irrespective of whether the cuboidal particle existing or not. Furthermore, the particle sizes are very small, with mean particle size about 1.Snm. However, fine particles were absent in the solutedrained matrix evolving simultaneously with the growth of discontinuous L 12 GBPs. In spite of this localized feature, the fine particle size and the homogeneous distribution of L12 particles in bulk matrix would theoretically result in a good precipitation hardening property to the alloy. To investigate the thermal stability of the precipitated hardening particles, samples ageing at 450°C for 10 min followed by soaking at 350°C for 100 h and 240 h were examined, and the microstmctures are shown in Fig. 5a and 5b, respectively, which are TEM DF micrographs taking from the L12 diffraction spot. Figure 5a shows that homogeneous distribution of the L12 fine particles remains unchanged. Fig. 5b, with a higher magnification, reveals the rounded shape of the particles. Even after aging at 350°C for 240 h, SADPs shown in Fig. 5c to 5e reveal that the precipitates remain L12 structure, and no other structure phases precipitate. The mean particle sizes of samples annealed for 100 h and 240 h are 2.5nm and 3.1nm, respectively. The mean particle spacing measured from sample shown in Fig. 5b is about 12nm, which is smaller than the figure of 16nm, suggested by Adam[6] for thermally stable AI alloys. The volume fraction of the dispersed particles is about 13%, as determined by a point-counting technique[20]. In view of the composition of the present alloy, the high volume fraction of the precipitated phase excludes the possibility that the phase is simply AlaZr. This results because the zirconium content (4.18 wt%) can only form 5.76 vol% of A13Zr at the most, which is far beneath the experimental value; instead, the precipitated phase is more likely to be AI3(Zr,Ti), as 10.2 vol% of A13(Zr,Ti) can be produced according to a theoretical calculation based on the solute contents. However, a discrepancy still exists between the calculated and experimental value. This may be due to either the experimental error, or to the possibility that Cr is incorporated with Zr, Ti to form LI2A13(Zr,Ti,Cr) precipitate particles. Further work is required to clarify this point. For simplicity, L12-AI3(Zr,Ti) will be used to represent the fine precipitate particles in the following discussion. According to the modified LSW theory of volume controlled coarsening[21], the average particle radius should increase in accordance with the relationship F 3 _Fo3 = k ( t _ t o )
(1)
where F is the average particle radius at time t, r0 is the value corresponding to t 0, and k represents the volumetric coarsening rate constant given by k - 8aDC°V"~
(2)
9vRT
where a is the interracial energy of the particle, D and Co are the diffusivity and equilibrium concentration of the ratecontrolling solute, v is the stoichiometric factor, and I'm is the molar volume of the particle, R and T have their usual meanings. A volumetric coarsening rate content k of 1.25 × 10.29 m3/h is calculated from eq. (1) for the L12-A13(Zr,Ti) particles at 350°C. This value is quite small compared with those of some other high temperature A1 alloys[10]. To
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evaluate the interfacial energy of the L12-A13(Zr,Ti) fine particles, eq. (2) was used, and Zr is assumed to be the rate controlling component since Zr has the lowest diffusivity flux DCo. In the present calculation, if 7.63 × 10-2t m2/s for the diffusivity D of Zr at 350°C [22], 43 mole Zr/m 3 of C018], 4.13 × 10-5 m3/mole of V,[8] and Wagner's stoichiometric factor v=l [8] are adopted, an interfacial energy of 0.036 J/m 2 results. This value agrees with the reported values for a coherent interface[8,10]. The agreement between the calculated and measured coarsening rates indicates that the coarsening kinetics of the L12-A13(Zr,Ti) particles in the present alloy are indeed controlled by volume diffusion of Zr.
Summary 1. The addition of titanium to AI-Cr-Zr alloy enhances precipitation of an L12 metastable phase. Three LI2 type phases were found in the as-spun ribbon and numerous tiny L12-AI3(Zr,Ti) precipitates was also found in annealed and aged ribbon. 2. A T phase, with an L12 structure and Al3(Zrx,Tit.x) composition, precipitates during solidification with a cube appearance and is enveloped by dendritic T' phase, which is also of L12 st.ructure but contains Cr besides Zr and Ti. These two phases combine to form a cuboidal shape precipitate sited in the grain interior. 3. LI2 structured precipitates composed of A1, Zr and Ti are found on the grain boundaries. The grain boundary precipitates (GBPs) grow in a discontinuous precipitation manner during annealing and ageing. 4. A high volume fraction of metastable L12-AI3(Zr,Ti) precipitates homogeneously within grains after annealing and ageing. The coarsening rate constant and interracial energy were measured and calculated; the result suggest that the coarsening of the fine L12 precipitates is controlled by volume diffusion of Zr and that these precipitates are coherent with A1 matrix.
Acknowledgement We would like to acknowledge the support of the work by the National Science Council of Taiwan, R.O.C. under contract No. NSC 83-0405-E-009-007.
Reference 1. S. P. Midson, R. A. Buckley and H. Jones, in "Rapidly Quenched Metal V" vol. I, (ed. S. Steeb and H. Warlimont), p. 923, North-Holland, Amsterdam (1985). 2. K. Ioannidis and T. Sheppard, J. Mat. Sci., 26, 4795(199l). 3. E. K. Ioannidis and T. Sheppard, Mat. Sci. and Tech., 6, 528(1990). 4. G. J. Marshall and E. K. Ioannidis, J. Mat. Sci., 27, 3552(1992). 5. G. J. Marshall, I. R. Hughes and W. S. Miller, Mat. Sci. and Tech., 2, 394(1986). 6. C.M. Adam, in "Rapidly Solidified Amorphous and crystalline Alloys", (ed. B.H. Kear, B.C. Giessen, and M. Cohen), p.411, North-Holland, Oxford (1982). 7. M. Zedalis and M.E. Fine, Scripta Metall., 17, 1247(1983). 8. M.S. Zedalis and M.E. Fine, Met. Trans., 17A, 2187(1986). 9. Y.C. Chen, M.E. Fine and J.R. Weertman, Acta. Met., 38, No.5, 771(1990). 10. Y.C. Chen, M.E. Fine, J.R. Weertman and R.E. Lewis, Scripta Metall., 21, 1003(1987). 11. V.R. Parameswaran, J.R. Weertman and M.E. Fine, Scripta Metall., 23, 147(1989). 12. H.M. Lee, J. Lee and Z.H. Lee, Scripta Metall., 25, 517(1991). 13. S. Zhang, J.P. Nic and D.E. Mikkola, Scripta Metall., 24, 57(1990). 14. Y. Ma, T. Amesen, J. Gjflnnes and J. Taftfl, J. Mater. Res., 7, No.7, 1722(1992). 15. M. Lieblich and M. Torralba, J. Mat. Sci., 27, 3474(1992). 16. S. Hori, H. Tai and Y. Narita, in "Rapidly Quenched Metal V", Vol.1, (ed. S. Steeb and H. Warlimont), p. 911, North-Holland, Amsterdam (1985). 17. A. Majumdar, R.H. Mair and B.C. Muddle, in "Science and Technology of Rapidly Quenched Alloys", (ed. M. Tenhover, W.L. Johnson and L.E. Tanner), p. 253, Materials Research Society, Pittsburgh (1987). 18. J. F. Nie, S. Sridhara and B. C. Muddle, Met. Trans., 23A, 3193(1992). 19. Z.A. Chaudhury and C. Suryanarayana, Metallography, 17, 231(1984).
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20. E.E. Underwood, 'Applications of Quantitative Metallography' in "Metals Handbook", vol. 8, 8th Ed., American Society for Metals(1973). 21. A.D. Brailsford and P. Wynblatt, Acta Met.,27, 489(1979). 22. H. Jones, in "Rapid Solidification Processing : Principles and Technologies II" (ed. R. Mehrabian, B.H. Kear and M. Cohen), p. 306, Claitor's Pub. Div., Baton Rouge (1980).
FIG. 1. TEM micrograph (BF) of as-spun AI-Cr-Zr-Ti ribbon. The arrow-marked cuboidal particle is the only type of coarser dispersoid found in the as-spun ribbon.
FIG. 2. TEM mierographs of the euboidal particle. (a) BF, (b) DF, using a g<010>Llz reflection. Zone axis near < 001 > AI. (c)-(e) Three selected-area diffraction patterns taken from the whole particle. The zone axes are (c) [001], (d) [011], (e) [111], respectively. ( hkl=Al matrix, hkl--L12 phase).
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AL R
J "
I
.
.
.
i AL
It
.T I :'.
J
) CR
t
..... T'+ inter-dendritic matrix ::::i::::::i::::~ bulk matrix
AL
]<
.o
5 . IqO
i,:eu
10.3
Energy ( keY )
Fig. 3. EDS spectrum from (a) T phase, (b) T' + interdendritic matrix, (c) GBPs. For the inserted picture, see context.
Fig. 5. TEM DF micrograph of ribbon.annealed at 450°C for 10 min, then soaked at 350°C for (a) 100 hr (b) 240hr, using a g < 110> L12 reflection, zone axis near [001]AI. (c)-(e) Three SADPs taken from sample shown in (b). The zone axes are (c) [001], (d) [011], (e) [111], respectively. ( hkl=Al matrix, hkl=L12 phase ).
Fig. 4. TEM DF micrograph, using a g < 011> Lh reflection from ribbon annealed at 450 °C for 10 min. Zone axis near <211 >AI.