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Temperature dependence of the flow stress of Ir-based L12 intermetallics
Intermetallics 9 (2001) 423–429 www.elsevier.com/locate/intermet
Temperature dependence of the flow stress of Ir-based L12 intermetallics Y. Yamabe-Mitarai*, Y. Ro, S. Nakazawa High Temperature Materials 21 Project, National Research Institute for Metals (NRIM), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan Received 28 November 2000; received in revised form 20 February 2001; accepted 21 February 2001
Abstract The temperature dependence of compressive strength of Ir3Zr, Ir3Hf, and Ir3Ti with an L12 structure was investigated. Ir3Zr showed weak anomalous behavior; in other words, the strength increased up to a peak strength with increasing temperature at intermediate temperatures but decreased slightly with increasing temperature below room temperature. Ir3Ti and Ir3Hf showed normal strength behavior. Their strength behavior including Ir3Nb is discussed in terms of the phase stability of the L12 structure with respect to other related structures. The lower phase stability of Ir3Ti with respect to the D024 phase suggests stable SISFbounding dislocations, which cause the normal strength behavior to occur. The lower phase stability of Ir3Nb with respect to the D0a causes anomalous behavior at intermediate temperature. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Yield stress; C. Casting; C. Heat treatment; E. Phase stability, prediction; F. Mechanical testing
1. Introduction The temperature dependence of strength has been systematically investigated for Ni- and Pt-based L12 intermetallics. It is well known that the plastic behavior of L12 intermetallics is classified into two groups. The first group, represented by Ni3Al, shows an anomalous increase in hardness or strength with increasing temperature [1]. The second group, represented by Pt3Al, shows normal behavior, that is, the strength decreases with increasing temperature below room temperature with a very weak anomalous temperature dependence of strength above room temperature. Wee et al. indicated that Pt-based L12 intermetallics show two types of strength behavior. For example, Pt3Ga and Pt3In show strongly normal behavior, similarly to Pt3Al, although weak anomalous behavior is also observed. On the other hand, Pt3Ti and Pt3Cr show strong anomalous behavior [2–4]. Among another platinum group metal-based L12 phases, Rh-based L12 intermetallics Rh3Ti, Rh3Nb, and Rh3Ta have been
* Corresponding author. Tel.: +81-298-59-2525; fax: +81-298-592501. E-mail address: [email protected] (Y. Yamabe-Mitarai).
investigated [5]. Both Rh3Nb and Rh3Ta show weak anomalous behavior, while Rh3Ti shows normal behavior. Through our study of Ir-based refractory superalloys with an fcc and L12 two-phase coherent structure as ultra-high temperature materials, we have recognized that it is necessary to know the strength behavior of Irbased L12 intermetallics to understand the strength behavior of fcc and L12 two-phase alloys [6–9]. Although Ir3V, Ir3Cr, and Ir3Ti [2], and Ir3Nb and Ir3Zr [10,11] were investigated, research on Ir-based L12 intermetallics is limited. We have been interested in whether Ir-based L12 intermetallics show anomalous or normal behavior. In a previous study, we found that Ir3Nb shows anomalous strength behavior between room temperature and 1073 K [12]. Observing of the dislocations of Ir3Nb was attempted; however, we determined the dissociation for only a few dislocations. Ir-based intermetallics being very brittle, it is difficulte to prepare thin TEM samples. Ductility improvement of Ir-based L12 intermetallics is under invesitgation. The limitations of dislocation observation in TEM make it difficult to understand the strength behavior mechanism of Ir-based L12 intermetallics by observing dislocation dissociation. In another attempt to understand strength behavior, Suzuki et al. have proposed the application of the concept of the phase stability of L12 phase and
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succeeded in explaining the strength behavior of various L12 phases [2,3,13,14]. The application of the concept of the phase stability seems to be suitable when the observation of dislocations in TEM is difficult. In this study, the compressive strength behavior of Ir3Ti, Ir3Hf, and Ir3Zr was investigated and discussed in terms of the L12 phase stability. The L12 phase stability of Ir3Nb, investigated in previous study, was also discussed together with Ir3Ti, Ir3Hf, and Ir3Zr.
2. Experimental procedure Thirty-gram button ingots of alloys of nominal composition 25 at.% Zr (Ir3Zr), 25 at.% Ti (Ir3Ti), and 24 at.% Hf (Ir3Hf) were prepared by arc melting in an argon atmosphere. For the Ir-Hf alloy, the composition range of L12 was almost a line below 1673 K and expanded to an Ir-rich composition above 1673 K. Thus, we chose 24 at.% Hf for the alloy composition. For compression testing, cylindrical samples (4 mm in diameter and 8 mm in height) were cut from the ingot. These samples were heated at 2273 K for 17 h for Ir3Ti and 72 h for other intermetallics to obtain a homogeneous L12 single-phase microstructure. The microstructure of heated samples was observed with a scanning electron microscope (SEM, Philips XL30). The phase was confirmed by X-ray defractometry. The compression test was carried out in air at room temperature, 873, 1073, 1273, and 1473 K using TENSILON/UTM-1-50000CW, in vacuum at 2073 K using INSTRON 8560, and in liquid N2 at 77 K using Shimazu DSS-10T until a few percentage of plastic strain was reached. The initial compressive strain rate for every compression test was 3.010 4 s 1. The samples were kept at the test temperature for 15 min before loading.
determined by measurement of the sample length before and after the test. Here, X shows that the sample was fractured during the test. All samples were broken during elastic deformation by intergranular fracture at 77 K. Above room temperature, the compression test was stopped during the plastic deformation (arrow in Figs. 1 and 2). Plastic deformation was observed above 298 K for Ir3Hf and Ir3Zr and 873 K for Ir3Ti and Ir3Nb. The temperature dependence of the 0.2% flow stress of Ir-based L12 intermetallics is shown with Ir3Nb represented by shadowed symbols in Fig. 3. The temperature dependences of the compressive strength of Ni3Al and Pt3Al intermetallics with an L12 structure are also plotted as a reference [3]. Ir3Zr showed a similar temperature dependence of strength behavior to Ir3Nb rather than Ni3Al; that is, the strength decreased from 77 K to room temperature, increased up to 1073 K, and
3. Results After homogenization at 2273 K, an L12 single-phase was confirmed in all samples by X-ray analysis. A single phase with a grain size of between 300–400 mm was obtained in Ir3Ti and Ir3Zr, although a weak dendrite contrast remained in Ir3Zr. This shows that as-cast microstructure changed to the L12 single-phase microstructure during annealing at 2273 K in Ir3Ti and Ir3Zr. In Ir3Hf, the L12 phase with 300–400 mm in grain size was observed and a small amount of fcc precipitates were formed inside the L12 phase. We consider a small amount of fcc precipitates does not affect for the strength behavior of L12 phase. The stress–strain curves of Ir3Ti, Ir3Hf, and Ir3Zr tested between 77 and 2073 K are shown in Fig. 1. For reference, the stress–strain curves of Ir3Nb are also shown in Fig. 2. The final plastic strain after the compression test was
Fig. 1. Stress–strain curves of Ir3Ti, Ir3Zr, and Ir3Hf at various temperatures.
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Fig. 2. Stress–strain curves of Ir3Nb at various temperatures.
decreased again up to 2073 K. The anomalous temperature dependence of the strength was observed between room temperature and 1073 K in Ir3Zr. On the other hand, Ni3Al shows the anomalous temperature from 77 K to about 800 K and the strength does not decrease with increasing temperature below room temperature. The peak strength of Ir3Zr was lower than that of Ni3Al. Bruemmer et al. showed that the hardness of Ir3Nb had a weak peak at 1973 K [10]. This is consistent with our result. However, Gyurko et al. indicated that compressive strength of Ir3Nb and Ir3Zr showed normal behavior [11]. We consider this is because they tested at only three temperatures and data were not enough to show temperature dependence of strength behavior. On the other hand, the strength of Ir3Ti and Ir3Hf decreased with increasing temperature. Wee reported that the temperature dependence of the hardness of Ir3Ti decreases with increasing the temperature constantly [2]. This is consistent with the normal strength behavior in our study. However, no drastic decrease of strength obtained in Pt3Al was observed in either Ir3Ti or Ir3Hf. Their strength decrease was gradual compared with that of Pt3Al.
Fig. 3. Temperature dependence of the compression strength of the Irbased L12 phase, Ni3Al and Pt3Al [3].
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The strength behavior was shown by relating it to temperature normalized by melting temperature of the L12 phase, Tm, in Fig. 4. The strength behavior of the Ir-based L12 phase was compared with that of Pt3Ga, Ni3Al, and Co3Ti [3] in Fig. 4a. Ir3Ti and Ir3Hf showed similar behavior to Pt3Ga, that is, the strength decreased gradually although the strength of Ir3Ti and Ir3Hf was smaller than that of Pt3Ga. Ir3Nb and Ir3Zr showed their peak strength around the normalized temperature, 0.4. Ni3Al, Ni3Si, Ni3Ga, and Ni3Ge which are known to be controlled by the Kear–Wilsdorf (K– W) mechanism, showed the peak temperature around 0.5 [3,5]. This suggests that Ir3Nb and Ir3Zr are also controlled by the K–W mechanism in anomalous behavior. The strength behavior of the Ir-based L12 phase against the normalized temperature is also compared with the Rh-based L12 phase [5] in Fig. 4b. Here, Co, Rh, and Ir belong to the 8 subgroup in the periodic table, which suggests that they have similar properties. When minor element B in A3B was the same, that is, Ir3Ti and Rh3Ti or Ir3Nb and Rh3Nb, their strength behavior was similar. Ir3Ti and Rh3Ti showed normal behavior, and Ir3Nb and Rh3Nb showed anomalous behavior. Rh3Ta (Ta belongs to the 5a subgroup, as does Nb, in the periodic table) showed anomalous behavior, as did Ir3Nb and Rh3Nb. The peak temperature of the Rh-based L12 phase was higher (about 0.5) than that of the Ir-based L12 phase. The peak strength of Rh3Nb was equivalent to that of Ir3Zr and lower than that of Ir3Nb. Co3Ti is a
Fig. 4. Normalized temperature dependence of compression strength of the Ir-based L12 phase.
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combination of an 8-group element and a 4b-group element, such as Ir3Ti and Rh3Ti. Strong anomalous behavior with high peak temperature about 0.8 was observed in Co3Ti although Ir3Ti and Rh3Ti show normal behavior. The work hardening rate of Ni3Al, Ni3Fe, and Cu3Au are also known to have similar behavior to strength behavior [15, 16]. Hence, the work hardening rate of the Ir-based L12 phase was investigated, as shown in Fig. 5. The work hardening rate of Ir3Nb and Ir3Zr showed anomalous behavior and decreased after the peak temperature, similarly to that of Ni3Al [15]. This supports the idea that the anomalous strength behavior of these compounds is also controlled by the K–W mechanism. The work hardening rate of Rh3Nb and Rh3Ta, which showed anomalous strength behavior, was also increased anomalously with temperature [5], also supporting our work. On the other hand, Ir3Ti did not show any anomalous behavior of work hardening rate. The work hardening rate decreased with increasing temperature. This suggests that anomalous behavior does not exist in Ir3Ti. In Rh3Ti, which showed normal strength behavior, the work hardening rate decreased with increasing temperature [5]. However, the work hardening rate of Ir3Hf, which showed normal behavior, increased with increasing temperature up to 2073 K. This result is not consistent with strength behavior.
4. Discussion In Ir-based L12 intermetallics, we observed two types of strength behavior, namely, anomalous behavior in Ir3Nb and Ir3Zr and normal behavior in Ir3Ti and Ir3Hf. It is generally accepted that the thermally activated cross-slip of screw dislocations from the {111} slip plane to the {100} non-slip plane is the cause of anomalous strength behavior. In the L12 structure, an [Maths: No WMF data] < [Maths: No WMF data]> super-dislocation gliding on a {111} slip plane is dissociated to two dislocations by bounding an anti-phase boundary (APB). Each dissociated dislocation separates into two Shockley
Fig. 5. Work hardening rate against temperature of the Ir-based L12 phase.
partial dislocations with a complex stacking fault (CSF). The cross-slip is enhanced by decreasing the APB energy on {100} compared with {111} and by increasing CSF energy, which is roughly the sum of the APB energy and the stacking-fault (SF) energy on {111}. On the other hand, to understand the normal strength behavior of L12 intermetallics, the SF energy is introduced in the model. If the corresponding APB on {111} either has very high energy or is unstable, the dissociation with APB cannot take place on the {111} planes. Instead, the dislocation is dissociated by bounding a superlattice intrinsic stacking fault (SISF), whose core structure is complex and sessile, resulting in the difficulty of the dislocation motion at low temperature. With increasing temperature, these sessile dislocations start to move by thermal activation, resulting in the decrease of the strength [17–20]. This behavior was observed in Pt3Al [4,21] and L12-modified Al3Ti intermetallics [22] by sliptrace analysis and dislocation observation in TEM. As stated above, the APB energy on {111} and {100} and the SF energy on {111} play important roles in determining the slip mode and the strength behavior. Suzuki and his co-workers applied a concept of phase stability of the L12 structure with respect to other crystal structures related to APB and SF energy on {111} and {100} [2,3,13,14], as shown in Fig. 6, which is slightly modified and redrawn from the figure shown by Suzuki [14]. The D022 structure is obtained from the L12 structure by periodic displacement of two planes of {100} in the a/2 < 100> direction. When the APB energy on {100} is low, the displacement in the [Maths: No WMF data]/2 < 100> direction is favorable. The L12 structure with low stability for the D022 structure will facilitate thermal activated cross-slip from {111} to {100}, thus causing the occurrence of anomalous strength behavior. When the SF energy on {111} is low, the stacking of the L12 structure can be changed to a hexagonal structure such as the D019 structure. Thus, the L12 structure with
Fig. 6. Phase stability of the L12 structure for other structures with APB and SF energies, slightly modified and redrawn from the figure shown by Suzuki [14].
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low stability for the D019 structure will show normal strength behavior. The A1 is a disordered fcc structure, resulting in a smaller APB energy on {111} than that obtained by ordered fcc phases. Concerning APB energy on {100} and {111} and SF energy on {111}, three kinds of phase stability of the L12 structure for the A1 structure (disordered fcc), the D022 structure (stack of two L12 lattices), and the D019 structure (ordered hcp) are especially important. In Fig. 6, the symbols c and h represent the cubic and hexagonal stacking sequences, respectively. The symbols T and R represent the triangular and rectangular ordering layers observed in close-packed planes, respectively. Thus, the L12, D022, and D019 structures are represented as ‘‘cT,’’ ‘‘cR,’’ and ‘‘hT,’’ respectively. Although it is difficult to estimate the APB and SF energy, the phase stability shows relative magnitude of APB energy on {100} and {111} and SF energy on {111} and then the strength behavior can be expected from relative magnitude of APB and SF energies. We discuss the strength behavior in terms of the phase stability of the Ir-based L12 phase using the method of Suzuki and co-workers. The phase stability is governed by the atomic radius ratio and the electron-atom ratio (e/a) [23–25]. Wee et al. showed the change from cubic to hexagonal stacking and the transition from a triangular to a rectangular ordering layer in close-packed planes with increasing atomic radius ratio and electron-atom ratio [2]. The atomic radius ratio and the electron-atom ratio (e/a) of Ir-based L12 and some other compounds are summarized in Table 1. The atomic radius (RB–RA)/RA, where RA and RB are the atomic radii of A3B, was calculated using Goldschmidt’s radius for co-ordination number 12. Fig. 7 shows the phase stability and electron-atom ratio with the change of major element A of A3B around Ir in the periodic table. The atomic radius ratio is also shown for some of the compounds. When the atomic radius ratio increases with the constant electron– atom ratio through Ir3X-> Rh3X->Co3X (X=Zr, Hf, and Nb), the L12 structure (cT) becomes unstable and changes to another structure. This is clearer in Pt3X->
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Pd3X-> Ni3X (X=Ti and Zr). Here, the phase structure changes from the L12 to D024 (chT) or D019 (hT) with increasing the atomic radius ratio; that is, the stacking sequence changes from cubic to hexagonal stacking. This trend is also clearer with increasing the electron-atom ratio. When the electron-atom ratio increases, the sequence of the structure is Ir3X (cT)-> Pt3X (cT, chT, cR, or hR)-> Au3X (hR). The stacking sequence of layers changes from cubic through mixtures of hexagonal and cubic stacking to hexagonal stacking, and the transition from triangular to rectangular layers takes place as well. In Table 2, we summarized the strength behavior of the L12 phase (A3B), consisting of an A element from the 8 subgroup and a B element from the 4a or 5a subgroup in the periodic table. The L12 phase, which consists of the
Table 1 Atomic radius ratio and electron-atom ratio of the L12 phase Atomic radius ratio (RB–RA)/RA Ir3Nb Ir3Zr Ir3Ti Ir3Hf Pt3Al Pt3Ga Pt3In Pt3Ti Ni3Al Co3Ti
0.09 0.19 0.09 0.18 0.05 0.03 0.07 0.06 0.05 0.18
Electron–atom ratio (e/a) 8 7.75 7.75 7.75 10.75 10.75 10.75 8.5 10.75 7.75
Fig. 7. Phase structure for the variation of electron–atom ratio (e/a). The number next to the compounds shows the atomic radius ratio, (RB RA)/RA, where RA and RB are the Goldschmidts’s radii for coordination number 12 of A3B.
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Table 2 Strength behavior of the L12 compounds (A3B) consisting of A from the 8 subgroup and B from the 4a or 5a subgroupa 4a
5a
Ti Co3Ti [3] A Rh3Ti [5] N Ir3Ti N Zr – Rh3Zr* [2] N Ir3Zr A Hf – Rh3Hf* [2] N Ir3Hf N
V – Rh3V* [2] N Ir3V [2] N Nb – Rh3Nb [5] A Ir3Nb [10, 12] A Ta – Rh3Ta [5] A –
a
–
8 Co
Rh
Ir
A, anomalous behavior; N, normal behavior; *, hardness.
5a subgroup, tends to show anomalous behavior rather than the L12 phase, which consists of the 4a subgroup. The compounds with the element in the upper line in the periodic table, that is, Ti or V, tend to show normal behavior. Next, the strength behavior will be considered in terms of the phase stability. When Ir is replaced by Pt, Ir3Ti changes from the L12(cT) to the D024(chT), as shown in Fig. 7a. This indicates that the L12 phase has low stability with respect to the D024. As clearly shown in Fig. 6, the D024(chT) is the structure whose stacking sequences are a combination of hexagonal and cubic ones, resulting in the decrease of SF energy. This can explain normal behavior in Ir3Ti. In the case of the Rhbased L12 phase, Rh3Ti, which shows normal behavior [5], has low stability with respect to the D024, as shown in Fig. 7a. On the other hand, strong anomalous behavior was obtained in Co3Ti [3,26] even though the phase stability of Co3Ti is low with respect to the D024. The reason is not clear. In the case of Ir3Nb, the L12 phase changes from the L12(cT) to the D0a(hR) when the Ir of Ir3Nb is replaced by Pt. The transition from a triangular to a rectangular ordering layer and the change of the stacking sequence from cubic to hexagonal are expected. This suggests that both the APB energy on {100} and the SF energy on {111} are relatively low in Ir3Nb, as shown in Fig. 6. Low APB energy on {100} suggests anomalous strength behavior, and low SF energy indicates normal strength behavior below room temperature. In the case of Rh3Nb and Rh3Ta, Miura showed that low stability of the L12 phase with respect to the D022 structure suggests the tendency of the stress anomaly [5]. The phase transition to the D022 suggests that the APB energy on {100} is relatively low, resulting in anomalous strength behavior. This indicates that Ir-based and Rh-based L12 phases
have similar relationships for strength behavior and phase stability. In the case of Ir3Zr and Ir3Hf, even though the phase stability of the L12 is low with respect to the D024 (chT) in both compounds in Fig. 7b and c, we found anomalous behavior up to 1073 K in Ir3Zr and normal behavior in Ir3Hf. The strength behavior difference between Ir3Zr and Ir3Hf cannot be explained simply in terms of phase stability. One possibility is that Ir3Hf is located in the energetic boundary of anomalous and normal strength behavior because the work hardening rate of Ir3Hf showed anomalous behavior even though the strength behavior showed normal behavior. If the relative magnitude of APB and SF energies of Ir3Hf is changed by impurity, for example, the strength behavior may change to anomalous behavior. Another point that needs to be explained is that even though the phase stability of Ir3Zr and Ir3Ti is low with respect to the D024 (chT), Ir3Zr showed anomalous behavior, whereas Ir3Ti showed normal behavior. One possibility is that when the electron-atom ratio (e/a) increases, that is, when Ir is replaced by Au, the D0a structure appears in Au3Zr. As shown in the case of Ir3Nb, the D0a structure will decrease both the APB energy on {100} and the SF energy on {111}. The existence of the D0a suggests that the phase stability of Ir3Zr is also close to the boundary where the transition from a triangular to a rectangular ordering layer is preferable.
5. Conclusions The temperature dependence of strength in Ir3Zr, Ir3Hf, and Ir3Ti with an L12 structure was investigated. In Ir3Zr, we observed anomalous strength behavior. Ir3Ti and Ir3Hf showed normal strength behavior. In terms of the phase stability of the L12 structure, the strength behavior was discussed including Ir3Nb in the previous result. The phase stability of Ir3Ti is low with respect to the D024, which includes hexagonal stacking. This suggests that SF energy on {111} in Ir3Ti is relatively low and that the favorable dissociation bounded by SISF causes negative strength behavior. The relationship between the strength behavior and phase stability of Ir3Nb, which had shown anomalous behavior, is clear. The phase stability of Ir3Nb is low with respect to the D0a, which shows that both APB energy on {100} and SF energy are relatively low. As a result, normal strength behavior is caused by SISF bounding dislocation at low temperature, while APB bounding dislocation causes anomalous strength behavior. For Ir3Zr and Ir3Hf, the strength behavior cannot be explained by simply using the concept of the phase stability. However, the phase stability shows that the magnitude of APB and SF energies in Ir3Zr and Ir3Hf may be close each other and then when APB energy is lower than SF energy, anomalous strength behavior is appeared.
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Acknowledgements The authors are grateful to Mr. S. Nishikawa and Mr. T. Maruko of Furuya Metal Co. Ltd. for preparing the ingots. The authors thank Dr. T. Ogata, of the National Research Institute for Metals, for helping with the compression test at 77 K. They also thank Professor S. Miura of Hokkaido University, Dr. H. Hosoda of Tsukuba University, Professor Y. Mishima of Tokyo Institute of Technology, and Professor D.-M. Wee of Korea Advanced Institute of Science and Technology for helpful discussions concerning the phase stability.
References [1] Westbrock JH. Trans Metall Soc A I M E 1957;209:898. [2] Wee D-M, Suzuki T. Trans Japan Inst Metals 1979;20:634. [3] Wee D-M, Noguchi O, Oya Y, Suzuki T. Trans Japan Inst Metals 1980;21:237. [4] Wee D-M, Pope DP, Vitek V. Acta Metall 1984;32(6):829. [5] Miura S, Honma K, Terada Y, Sanchez JM, Mohri T. Intermetallics 2000;8:785. [6] Yamabe Y, Koizumi Y, Murakami H, Ro Y, Maruko T, Harada H. Scripta Metall 1996;35(2):211. [7] Yamabe-Mitarai Y, Koizumi Y, Murakami H, Ro Y, Maruko T, Harada H. Mater Res Soc Symp Proc 1997;460:701. [8] Yamabe-Mitarai Y, Koizumi Y, Murakami H, Ro Y, Maruko T,
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Temperature dependence of the flow stress of Ir-based L12 intermetallics Y. Yamabe-Mitarai*, Y. Ro, S. Nakazawa High Temperature Materials 21 Project, National Research Institute for Metals (NRIM), Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan Received 28 November 2000; received in revised form 20 February 2001; accepted 21 February 2001
Abstract The temperature dependence of compressive strength of Ir3Zr, Ir3Hf, and Ir3Ti with an L12 structure was investigated. Ir3Zr showed weak anomalous behavior; in other words, the strength increased up to a peak strength with increasing temperature at intermediate temperatures but decreased slightly with increasing temperature below room temperature. Ir3Ti and Ir3Hf showed normal strength behavior. Their strength behavior including Ir3Nb is discussed in terms of the phase stability of the L12 structure with respect to other related structures. The lower phase stability of Ir3Ti with respect to the D024 phase suggests stable SISFbounding dislocations, which cause the normal strength behavior to occur. The lower phase stability of Ir3Nb with respect to the D0a causes anomalous behavior at intermediate temperature. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Intermetallics, miscellaneous; B. Yield stress; C. Casting; C. Heat treatment; E. Phase stability, prediction; F. Mechanical testing
1. Introduction The temperature dependence of strength has been systematically investigated for Ni- and Pt-based L12 intermetallics. It is well known that the plastic behavior of L12 intermetallics is classified into two groups. The first group, represented by Ni3Al, shows an anomalous increase in hardness or strength with increasing temperature [1]. The second group, represented by Pt3Al, shows normal behavior, that is, the strength decreases with increasing temperature below room temperature with a very weak anomalous temperature dependence of strength above room temperature. Wee et al. indicated that Pt-based L12 intermetallics show two types of strength behavior. For example, Pt3Ga and Pt3In show strongly normal behavior, similarly to Pt3Al, although weak anomalous behavior is also observed. On the other hand, Pt3Ti and Pt3Cr show strong anomalous behavior [2–4]. Among another platinum group metal-based L12 phases, Rh-based L12 intermetallics Rh3Ti, Rh3Nb, and Rh3Ta have been
* Corresponding author. Tel.: +81-298-59-2525; fax: +81-298-592501. E-mail address: [email protected] (Y. Yamabe-Mitarai).
investigated [5]. Both Rh3Nb and Rh3Ta show weak anomalous behavior, while Rh3Ti shows normal behavior. Through our study of Ir-based refractory superalloys with an fcc and L12 two-phase coherent structure as ultra-high temperature materials, we have recognized that it is necessary to know the strength behavior of Irbased L12 intermetallics to understand the strength behavior of fcc and L12 two-phase alloys [6–9]. Although Ir3V, Ir3Cr, and Ir3Ti [2], and Ir3Nb and Ir3Zr [10,11] were investigated, research on Ir-based L12 intermetallics is limited. We have been interested in whether Ir-based L12 intermetallics show anomalous or normal behavior. In a previous study, we found that Ir3Nb shows anomalous strength behavior between room temperature and 1073 K [12]. Observing of the dislocations of Ir3Nb was attempted; however, we determined the dissociation for only a few dislocations. Ir-based intermetallics being very brittle, it is difficulte to prepare thin TEM samples. Ductility improvement of Ir-based L12 intermetallics is under invesitgation. The limitations of dislocation observation in TEM make it difficult to understand the strength behavior mechanism of Ir-based L12 intermetallics by observing dislocation dissociation. In another attempt to understand strength behavior, Suzuki et al. have proposed the application of the concept of the phase stability of L12 phase and
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succeeded in explaining the strength behavior of various L12 phases [2,3,13,14]. The application of the concept of the phase stability seems to be suitable when the observation of dislocations in TEM is difficult. In this study, the compressive strength behavior of Ir3Ti, Ir3Hf, and Ir3Zr was investigated and discussed in terms of the L12 phase stability. The L12 phase stability of Ir3Nb, investigated in previous study, was also discussed together with Ir3Ti, Ir3Hf, and Ir3Zr.
2. Experimental procedure Thirty-gram button ingots of alloys of nominal composition 25 at.% Zr (Ir3Zr), 25 at.% Ti (Ir3Ti), and 24 at.% Hf (Ir3Hf) were prepared by arc melting in an argon atmosphere. For the Ir-Hf alloy, the composition range of L12 was almost a line below 1673 K and expanded to an Ir-rich composition above 1673 K. Thus, we chose 24 at.% Hf for the alloy composition. For compression testing, cylindrical samples (4 mm in diameter and 8 mm in height) were cut from the ingot. These samples were heated at 2273 K for 17 h for Ir3Ti and 72 h for other intermetallics to obtain a homogeneous L12 single-phase microstructure. The microstructure of heated samples was observed with a scanning electron microscope (SEM, Philips XL30). The phase was confirmed by X-ray defractometry. The compression test was carried out in air at room temperature, 873, 1073, 1273, and 1473 K using TENSILON/UTM-1-50000CW, in vacuum at 2073 K using INSTRON 8560, and in liquid N2 at 77 K using Shimazu DSS-10T until a few percentage of plastic strain was reached. The initial compressive strain rate for every compression test was 3.010 4 s 1. The samples were kept at the test temperature for 15 min before loading.
determined by measurement of the sample length before and after the test. Here, X shows that the sample was fractured during the test. All samples were broken during elastic deformation by intergranular fracture at 77 K. Above room temperature, the compression test was stopped during the plastic deformation (arrow in Figs. 1 and 2). Plastic deformation was observed above 298 K for Ir3Hf and Ir3Zr and 873 K for Ir3Ti and Ir3Nb. The temperature dependence of the 0.2% flow stress of Ir-based L12 intermetallics is shown with Ir3Nb represented by shadowed symbols in Fig. 3. The temperature dependences of the compressive strength of Ni3Al and Pt3Al intermetallics with an L12 structure are also plotted as a reference [3]. Ir3Zr showed a similar temperature dependence of strength behavior to Ir3Nb rather than Ni3Al; that is, the strength decreased from 77 K to room temperature, increased up to 1073 K, and
3. Results After homogenization at 2273 K, an L12 single-phase was confirmed in all samples by X-ray analysis. A single phase with a grain size of between 300–400 mm was obtained in Ir3Ti and Ir3Zr, although a weak dendrite contrast remained in Ir3Zr. This shows that as-cast microstructure changed to the L12 single-phase microstructure during annealing at 2273 K in Ir3Ti and Ir3Zr. In Ir3Hf, the L12 phase with 300–400 mm in grain size was observed and a small amount of fcc precipitates were formed inside the L12 phase. We consider a small amount of fcc precipitates does not affect for the strength behavior of L12 phase. The stress–strain curves of Ir3Ti, Ir3Hf, and Ir3Zr tested between 77 and 2073 K are shown in Fig. 1. For reference, the stress–strain curves of Ir3Nb are also shown in Fig. 2. The final plastic strain after the compression test was
Fig. 1. Stress–strain curves of Ir3Ti, Ir3Zr, and Ir3Hf at various temperatures.
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Fig. 2. Stress–strain curves of Ir3Nb at various temperatures.
decreased again up to 2073 K. The anomalous temperature dependence of the strength was observed between room temperature and 1073 K in Ir3Zr. On the other hand, Ni3Al shows the anomalous temperature from 77 K to about 800 K and the strength does not decrease with increasing temperature below room temperature. The peak strength of Ir3Zr was lower than that of Ni3Al. Bruemmer et al. showed that the hardness of Ir3Nb had a weak peak at 1973 K [10]. This is consistent with our result. However, Gyurko et al. indicated that compressive strength of Ir3Nb and Ir3Zr showed normal behavior [11]. We consider this is because they tested at only three temperatures and data were not enough to show temperature dependence of strength behavior. On the other hand, the strength of Ir3Ti and Ir3Hf decreased with increasing temperature. Wee reported that the temperature dependence of the hardness of Ir3Ti decreases with increasing the temperature constantly [2]. This is consistent with the normal strength behavior in our study. However, no drastic decrease of strength obtained in Pt3Al was observed in either Ir3Ti or Ir3Hf. Their strength decrease was gradual compared with that of Pt3Al.
Fig. 3. Temperature dependence of the compression strength of the Irbased L12 phase, Ni3Al and Pt3Al [3].
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The strength behavior was shown by relating it to temperature normalized by melting temperature of the L12 phase, Tm, in Fig. 4. The strength behavior of the Ir-based L12 phase was compared with that of Pt3Ga, Ni3Al, and Co3Ti [3] in Fig. 4a. Ir3Ti and Ir3Hf showed similar behavior to Pt3Ga, that is, the strength decreased gradually although the strength of Ir3Ti and Ir3Hf was smaller than that of Pt3Ga. Ir3Nb and Ir3Zr showed their peak strength around the normalized temperature, 0.4. Ni3Al, Ni3Si, Ni3Ga, and Ni3Ge which are known to be controlled by the Kear–Wilsdorf (K– W) mechanism, showed the peak temperature around 0.5 [3,5]. This suggests that Ir3Nb and Ir3Zr are also controlled by the K–W mechanism in anomalous behavior. The strength behavior of the Ir-based L12 phase against the normalized temperature is also compared with the Rh-based L12 phase [5] in Fig. 4b. Here, Co, Rh, and Ir belong to the 8 subgroup in the periodic table, which suggests that they have similar properties. When minor element B in A3B was the same, that is, Ir3Ti and Rh3Ti or Ir3Nb and Rh3Nb, their strength behavior was similar. Ir3Ti and Rh3Ti showed normal behavior, and Ir3Nb and Rh3Nb showed anomalous behavior. Rh3Ta (Ta belongs to the 5a subgroup, as does Nb, in the periodic table) showed anomalous behavior, as did Ir3Nb and Rh3Nb. The peak temperature of the Rh-based L12 phase was higher (about 0.5) than that of the Ir-based L12 phase. The peak strength of Rh3Nb was equivalent to that of Ir3Zr and lower than that of Ir3Nb. Co3Ti is a
Fig. 4. Normalized temperature dependence of compression strength of the Ir-based L12 phase.
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combination of an 8-group element and a 4b-group element, such as Ir3Ti and Rh3Ti. Strong anomalous behavior with high peak temperature about 0.8 was observed in Co3Ti although Ir3Ti and Rh3Ti show normal behavior. The work hardening rate of Ni3Al, Ni3Fe, and Cu3Au are also known to have similar behavior to strength behavior [15, 16]. Hence, the work hardening rate of the Ir-based L12 phase was investigated, as shown in Fig. 5. The work hardening rate of Ir3Nb and Ir3Zr showed anomalous behavior and decreased after the peak temperature, similarly to that of Ni3Al [15]. This supports the idea that the anomalous strength behavior of these compounds is also controlled by the K–W mechanism. The work hardening rate of Rh3Nb and Rh3Ta, which showed anomalous strength behavior, was also increased anomalously with temperature [5], also supporting our work. On the other hand, Ir3Ti did not show any anomalous behavior of work hardening rate. The work hardening rate decreased with increasing temperature. This suggests that anomalous behavior does not exist in Ir3Ti. In Rh3Ti, which showed normal strength behavior, the work hardening rate decreased with increasing temperature [5]. However, the work hardening rate of Ir3Hf, which showed normal behavior, increased with increasing temperature up to 2073 K. This result is not consistent with strength behavior.
4. Discussion In Ir-based L12 intermetallics, we observed two types of strength behavior, namely, anomalous behavior in Ir3Nb and Ir3Zr and normal behavior in Ir3Ti and Ir3Hf. It is generally accepted that the thermally activated cross-slip of screw dislocations from the {111} slip plane to the {100} non-slip plane is the cause of anomalous strength behavior. In the L12 structure, an [Maths: No WMF data] < [Maths: No WMF data]> super-dislocation gliding on a {111} slip plane is dissociated to two dislocations by bounding an anti-phase boundary (APB). Each dissociated dislocation separates into two Shockley
Fig. 5. Work hardening rate against temperature of the Ir-based L12 phase.
partial dislocations with a complex stacking fault (CSF). The cross-slip is enhanced by decreasing the APB energy on {100} compared with {111} and by increasing CSF energy, which is roughly the sum of the APB energy and the stacking-fault (SF) energy on {111}. On the other hand, to understand the normal strength behavior of L12 intermetallics, the SF energy is introduced in the model. If the corresponding APB on {111} either has very high energy or is unstable, the dissociation with APB cannot take place on the {111} planes. Instead, the dislocation is dissociated by bounding a superlattice intrinsic stacking fault (SISF), whose core structure is complex and sessile, resulting in the difficulty of the dislocation motion at low temperature. With increasing temperature, these sessile dislocations start to move by thermal activation, resulting in the decrease of the strength [17–20]. This behavior was observed in Pt3Al [4,21] and L12-modified Al3Ti intermetallics [22] by sliptrace analysis and dislocation observation in TEM. As stated above, the APB energy on {111} and {100} and the SF energy on {111} play important roles in determining the slip mode and the strength behavior. Suzuki and his co-workers applied a concept of phase stability of the L12 structure with respect to other crystal structures related to APB and SF energy on {111} and {100} [2,3,13,14], as shown in Fig. 6, which is slightly modified and redrawn from the figure shown by Suzuki [14]. The D022 structure is obtained from the L12 structure by periodic displacement of two planes of {100} in the a/2 < 100> direction. When the APB energy on {100} is low, the displacement in the [Maths: No WMF data]/2 < 100> direction is favorable. The L12 structure with low stability for the D022 structure will facilitate thermal activated cross-slip from {111} to {100}, thus causing the occurrence of anomalous strength behavior. When the SF energy on {111} is low, the stacking of the L12 structure can be changed to a hexagonal structure such as the D019 structure. Thus, the L12 structure with
Fig. 6. Phase stability of the L12 structure for other structures with APB and SF energies, slightly modified and redrawn from the figure shown by Suzuki [14].
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low stability for the D019 structure will show normal strength behavior. The A1 is a disordered fcc structure, resulting in a smaller APB energy on {111} than that obtained by ordered fcc phases. Concerning APB energy on {100} and {111} and SF energy on {111}, three kinds of phase stability of the L12 structure for the A1 structure (disordered fcc), the D022 structure (stack of two L12 lattices), and the D019 structure (ordered hcp) are especially important. In Fig. 6, the symbols c and h represent the cubic and hexagonal stacking sequences, respectively. The symbols T and R represent the triangular and rectangular ordering layers observed in close-packed planes, respectively. Thus, the L12, D022, and D019 structures are represented as ‘‘cT,’’ ‘‘cR,’’ and ‘‘hT,’’ respectively. Although it is difficult to estimate the APB and SF energy, the phase stability shows relative magnitude of APB energy on {100} and {111} and SF energy on {111} and then the strength behavior can be expected from relative magnitude of APB and SF energies. We discuss the strength behavior in terms of the phase stability of the Ir-based L12 phase using the method of Suzuki and co-workers. The phase stability is governed by the atomic radius ratio and the electron-atom ratio (e/a) [23–25]. Wee et al. showed the change from cubic to hexagonal stacking and the transition from a triangular to a rectangular ordering layer in close-packed planes with increasing atomic radius ratio and electron-atom ratio [2]. The atomic radius ratio and the electron-atom ratio (e/a) of Ir-based L12 and some other compounds are summarized in Table 1. The atomic radius (RB–RA)/RA, where RA and RB are the atomic radii of A3B, was calculated using Goldschmidt’s radius for co-ordination number 12. Fig. 7 shows the phase stability and electron-atom ratio with the change of major element A of A3B around Ir in the periodic table. The atomic radius ratio is also shown for some of the compounds. When the atomic radius ratio increases with the constant electron– atom ratio through Ir3X-> Rh3X->Co3X (X=Zr, Hf, and Nb), the L12 structure (cT) becomes unstable and changes to another structure. This is clearer in Pt3X->
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Pd3X-> Ni3X (X=Ti and Zr). Here, the phase structure changes from the L12 to D024 (chT) or D019 (hT) with increasing the atomic radius ratio; that is, the stacking sequence changes from cubic to hexagonal stacking. This trend is also clearer with increasing the electron-atom ratio. When the electron-atom ratio increases, the sequence of the structure is Ir3X (cT)-> Pt3X (cT, chT, cR, or hR)-> Au3X (hR). The stacking sequence of layers changes from cubic through mixtures of hexagonal and cubic stacking to hexagonal stacking, and the transition from triangular to rectangular layers takes place as well. In Table 2, we summarized the strength behavior of the L12 phase (A3B), consisting of an A element from the 8 subgroup and a B element from the 4a or 5a subgroup in the periodic table. The L12 phase, which consists of the
Table 1 Atomic radius ratio and electron-atom ratio of the L12 phase Atomic radius ratio (RB–RA)/RA Ir3Nb Ir3Zr Ir3Ti Ir3Hf Pt3Al Pt3Ga Pt3In Pt3Ti Ni3Al Co3Ti
0.09 0.19 0.09 0.18 0.05 0.03 0.07 0.06 0.05 0.18
Electron–atom ratio (e/a) 8 7.75 7.75 7.75 10.75 10.75 10.75 8.5 10.75 7.75
Fig. 7. Phase structure for the variation of electron–atom ratio (e/a). The number next to the compounds shows the atomic radius ratio, (RB RA)/RA, where RA and RB are the Goldschmidts’s radii for coordination number 12 of A3B.
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Table 2 Strength behavior of the L12 compounds (A3B) consisting of A from the 8 subgroup and B from the 4a or 5a subgroupa 4a
5a
Ti Co3Ti [3] A Rh3Ti [5] N Ir3Ti N Zr – Rh3Zr* [2] N Ir3Zr A Hf – Rh3Hf* [2] N Ir3Hf N
V – Rh3V* [2] N Ir3V [2] N Nb – Rh3Nb [5] A Ir3Nb [10, 12] A Ta – Rh3Ta [5] A –
a
–
8 Co
Rh
Ir
A, anomalous behavior; N, normal behavior; *, hardness.
5a subgroup, tends to show anomalous behavior rather than the L12 phase, which consists of the 4a subgroup. The compounds with the element in the upper line in the periodic table, that is, Ti or V, tend to show normal behavior. Next, the strength behavior will be considered in terms of the phase stability. When Ir is replaced by Pt, Ir3Ti changes from the L12(cT) to the D024(chT), as shown in Fig. 7a. This indicates that the L12 phase has low stability with respect to the D024. As clearly shown in Fig. 6, the D024(chT) is the structure whose stacking sequences are a combination of hexagonal and cubic ones, resulting in the decrease of SF energy. This can explain normal behavior in Ir3Ti. In the case of the Rhbased L12 phase, Rh3Ti, which shows normal behavior [5], has low stability with respect to the D024, as shown in Fig. 7a. On the other hand, strong anomalous behavior was obtained in Co3Ti [3,26] even though the phase stability of Co3Ti is low with respect to the D024. The reason is not clear. In the case of Ir3Nb, the L12 phase changes from the L12(cT) to the D0a(hR) when the Ir of Ir3Nb is replaced by Pt. The transition from a triangular to a rectangular ordering layer and the change of the stacking sequence from cubic to hexagonal are expected. This suggests that both the APB energy on {100} and the SF energy on {111} are relatively low in Ir3Nb, as shown in Fig. 6. Low APB energy on {100} suggests anomalous strength behavior, and low SF energy indicates normal strength behavior below room temperature. In the case of Rh3Nb and Rh3Ta, Miura showed that low stability of the L12 phase with respect to the D022 structure suggests the tendency of the stress anomaly [5]. The phase transition to the D022 suggests that the APB energy on {100} is relatively low, resulting in anomalous strength behavior. This indicates that Ir-based and Rh-based L12 phases
have similar relationships for strength behavior and phase stability. In the case of Ir3Zr and Ir3Hf, even though the phase stability of the L12 is low with respect to the D024 (chT) in both compounds in Fig. 7b and c, we found anomalous behavior up to 1073 K in Ir3Zr and normal behavior in Ir3Hf. The strength behavior difference between Ir3Zr and Ir3Hf cannot be explained simply in terms of phase stability. One possibility is that Ir3Hf is located in the energetic boundary of anomalous and normal strength behavior because the work hardening rate of Ir3Hf showed anomalous behavior even though the strength behavior showed normal behavior. If the relative magnitude of APB and SF energies of Ir3Hf is changed by impurity, for example, the strength behavior may change to anomalous behavior. Another point that needs to be explained is that even though the phase stability of Ir3Zr and Ir3Ti is low with respect to the D024 (chT), Ir3Zr showed anomalous behavior, whereas Ir3Ti showed normal behavior. One possibility is that when the electron-atom ratio (e/a) increases, that is, when Ir is replaced by Au, the D0a structure appears in Au3Zr. As shown in the case of Ir3Nb, the D0a structure will decrease both the APB energy on {100} and the SF energy on {111}. The existence of the D0a suggests that the phase stability of Ir3Zr is also close to the boundary where the transition from a triangular to a rectangular ordering layer is preferable.
5. Conclusions The temperature dependence of strength in Ir3Zr, Ir3Hf, and Ir3Ti with an L12 structure was investigated. In Ir3Zr, we observed anomalous strength behavior. Ir3Ti and Ir3Hf showed normal strength behavior. In terms of the phase stability of the L12 structure, the strength behavior was discussed including Ir3Nb in the previous result. The phase stability of Ir3Ti is low with respect to the D024, which includes hexagonal stacking. This suggests that SF energy on {111} in Ir3Ti is relatively low and that the favorable dissociation bounded by SISF causes negative strength behavior. The relationship between the strength behavior and phase stability of Ir3Nb, which had shown anomalous behavior, is clear. The phase stability of Ir3Nb is low with respect to the D0a, which shows that both APB energy on {100} and SF energy are relatively low. As a result, normal strength behavior is caused by SISF bounding dislocation at low temperature, while APB bounding dislocation causes anomalous strength behavior. For Ir3Zr and Ir3Hf, the strength behavior cannot be explained by simply using the concept of the phase stability. However, the phase stability shows that the magnitude of APB and SF energies in Ir3Zr and Ir3Hf may be close each other and then when APB energy is lower than SF energy, anomalous strength behavior is appeared.
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Acknowledgements The authors are grateful to Mr. S. Nishikawa and Mr. T. Maruko of Furuya Metal Co. Ltd. for preparing the ingots. The authors thank Dr. T. Ogata, of the National Research Institute for Metals, for helping with the compression test at 77 K. They also thank Professor S. Miura of Hokkaido University, Dr. H. Hosoda of Tsukuba University, Professor Y. Mishima of Tokyo Institute of Technology, and Professor D.-M. Wee of Korea Advanced Institute of Science and Technology for helpful discussions concerning the phase stability.
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