Phase Transformation of 00 Martensite Structure by Aging in Ti-8 mass%mo Alloy

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1 Materials Transactions, Vol. 45, No. 5 (2004) pp to 1634 Special Issue on Recent Research and Developments in Titanium and Its Alloys #2004 The Japan Institute of Metals Phase Transformation of 00 Martensite Structure by Aging in Ti-8 mass%mo Alloy Yoshikazu Mantani 1, Yoshito Takemoto 2, Moritaka Hida 2, Akira Sakakibara 2 and Mamoru Tajima 1 1 Department of Mechanical Engineering, Faculty of Engineering, Kanagawa University, Yokohama , Japan 2 Department of Mechanical Engineering, Faculty of Engineering, Okayama University, Okayama , Japan The orthorhombic 00 martensite was formed in Ti-8 mass%mo alloy by quenching from 1223 K. The purpose of this study was to investigate phase transformation of the 00 martensite structure by isothermal aging. In differential scanning calorimetry curve of the quenched specimen, an exothermic peak that indicated decomposition from the 00 martensite to and phases was observed near 780 K, so that isothermal aging was performed at 723 K and 923 K for 9.0 ks. Optical microscopy, X-ray diffraction and transmission electron microscopy were performed to these specimens. Band-like products that were composed of the single variant of! phase were observed in the quenched 00 martensite structure. On the other hand, ð111þ 00 twins were observed in the 723 K-aged 00 martensite structure. The quenched 00 martensite structure indicated low elastic incline and good ductility, whereas the 723 K-aged 00 martensite structure indicated high yield stress and brittleness. It was pointed out that the! products were formed to relax the volume expansion from the phase to the 00 martensite, and the ð111þ 00 twins were formed during the isothermal aging at 723 K with the extinction of the! products. (Received November 18, 2003; Accepted January 15, 2004) Keywords: titanium-molybdenum alloy, orthorhombic martensite, omega phase, differential scanning calorimetry, transmission electron microscopy 1. Introduction The orthorhombic 00 martensite is formed by quench in titanium alloys with suitable stabilizer elements. 1 3) In metastable alloys, it has been reported that the 00 martensite was formed by deformation. 4 7) When the 00 martensite existed or was induced by deformation, very characteristic mechanical properties were observed. Y. T. Lee and G. Welsch have reported that þ 00 phase mixture structure indicated very low Young s modulus and high specific damping capacity in quenched Ti-6Al-4V alloy. 8) T. Grosdider et al. have reported that a low level of apparent yield stress and a double yielding effect were observed in metastable -Cez alloy, 7) which is associated with the formation of stress induced 00 martensite. In addition to the characteristic mechanical properties, characteristic phase transformation by isothermal aging, i.e., the reverse transformation from the 00 martensite to phase, has been confirmed in some quenched alloys. 9 11) We have also reported that the structure almost composed of the 00 martensite in Ti-40 mass%nb alloy quenched from 1223 K was drastically changed into the þ! structure by isothermal aging at 623 K for 0.9 ks. 11) Considering from these reports, the 00 martensite is a very important phase to perform functional alloy design, so that there is necessity to investigate the 00 martensite in detail. The purpose of this study is to investigate the 00 martensite in Ti-8 mass%mo alloy, especially, phase constitutions after different heat treatments, microstructures of the 00 martensite and mechanical properties, to understand the characteristics of the 00 martensite structure compared with the þ structure. 2. Experimental Procedure Ti-8 mass%mo alloy sheet of 1 mm in thickness was used in this study. The specimens for structural observation were capsulated in evacuated quartz tube and solution-treated at 1223 K for 4.5 ks. After that, one specimen was annealed by furnace cooling from 1223 K, and the other specimens were quenched into iced water. The former was called the annealed specimen, and latter was the quenched specimen. The quenched specimens were isothermal aged at 723 K or 923 K for 9.0 ks under the vacuum. The former was called the 723 K-aged specimen, and latter was the 923 K-aged specimen. These specimens were mechanically ground and electrolytically polished by using the solution of HClO 4 (6%vol) + Butanol(35%vol) + CH 3 OH(bal.) at 223 K. Optical microstructures were revealed by using the etchant of HF (33%vol) + HNO 3 (33%vol) + water(bal.). X-ray diffraction (XRD) with CuK was operated at 40 kv-200 ma to clarify the phase constitutions after these heat treatments. Differential scanning calorimetry (DSC) was operated at a constant heating rate of 5 K/min in Ar atmosphere to investigate phase transformation temperature. Microstructures of the 00 martensite structure were observed by transmission electron microscope (TEM). Thin foils for TEM observation were prepared by electrolytic polishing. TEM (Topcon EM-002B) was operated at 200 kv, and bright field (BF) images, dark field (DF) images, selected area diffraction (SAD) and micro beam diffraction (MBD) were observed. Vickers hardness test and tensile test were performed to investigate the mechanical properties of each structure. Test load of Vickers hardness was 1 kg. Tensile specimens with a gage size of 3 mm 1 mm 10 mm were heat-treated under the same conditions mentioned above. Tensile test was carried out at an initial strain rate of 8: s Results 3.1 Microstructures after heat treatments Figure 1 shows optical microstructures and XRD profiles

2 1630 Y. Mantani, Y. Takemoto, M. Hida, A. Sakakibara and M. Tajima Fig. 1 Optical micrographs and XRD profiles of Ti-8 mass%mo alloy. (a) and (b): Annealed from 1223 K. (c) and (d): Quenched from 1223 K. of the annealed and quenched specimens. (a) and (b) are those of the annealed specimen, (c) and (d) are those of the quenched specimen. Microstructure of the annealed specimen (a) was complicated, and the XRD profile (b) showed coexistence of and phases. The lattice parameters of hexagonal close packed phase were a ¼ 0:2954, c ¼ 0:4686 nm, and that of body centered phase was a ¼ 0:3261 nm. In the micrograph of the quenched specimen (c), fine needle-like traces in each grain were observed. XRD profile (d) indicated that the quenched specimen consisted only of the 00 martensite structure. The lattice parameters of the orthorhombic 00 martensite were a ¼ 0:3012, b ¼ 0:4983 and c ¼ 0:4658 nm. To investigate the phase transformation temperature, DSC measurement was performed. Figure 2 shows DSC heating curves of these specimens after different heat treatments. There was no peak observed on the annealed specimen (a). On the other hand, exothermic peak near 780 K was observed on the quenched specimen (b). The peak is considered to indicate phase decomposition from the 00 martensite to and phases. Therefore, isothermal aging treatments for the quenched specimens were performed at 723 K or 923 K that corresponded to the state of before or after the phase decomposition respectively, for 9.0 ks. In the 723 K-aged specimen (c), exothermic peak near 575 K, endothermic peak near 600 K and exothermic peak near 800 K were observed. This result implies the existence of phase transformation in the 00 martensite structure by the isothermal aging at 723 K before the phase decomposition to and phases. On the other hand, there was no peak observed in the 923 K-aged specimen (d). It is considered that the 00 martensite has Fig. 2 DSC heating curves of Ti-8 mass%mo alloy which performed different heat treatments. (a) The annealed specimen. (b) The quenched specimen. (c) The 723 K-aged specimen. (d) The 923 K-aged specimen. already been decomposed to the and phases. Figure 3 shows optical microstructures and XRD profiles of the specimens after isothermal aging at 723 K or 923 K. (a) and (b) are those of the 723 K-aged specimen, (c) and (d) are those of the 923 K-aged specimen. Though grain boundaries of the 723 K-aged specimen (a) were preferably etched, fine needle-like traces were observed in each grain. XRD profile (b) showed that the 723 K-aged specimen still consisted only

3 Phase Transformation of 00 Martensite Structure by Aging in Ti-8 mass%mo Alloy 1631 Fig. 3 Optical micrographs and XRD profiles after isothermal aging of quenched specimen in Ti-8 mass%mo alloy. (a) and (b): Aged at 723 K for 9.0 ks. (c) and (d): Aged at 923 K for 9.0 ks. of the 00 martensite structure. In the micrograph of 923 K- aged specimen (c), coarse needle-like products were formed. XRD profile (d) indicated that the 00 martensite was completely decomposed to the and phases. 3.2 The 00 martensite structure observed by TEM In order to investigate the difference of the 00 martensite structure between the quenched and 723 K-aged specimen, TEM observation was performed. Figure 4 shows low magnification TEM images (BF) of the quenched specimen (a) and 723 K-aged specimen (b). The size and shape of the 00 martensite in (a) and (b) were similar to each other. However, band-like products observed in the 00 martensite, indicated by arrows, seem to be different. The appearance of the products in (a) is faint, and that in (b) is clear. Figure 5 shows detailed analysis of the inner band-like products in the quenched specimen. SAD pattern (b) was taken from one 00 martensite plate in BF image (a), and (c) was the key diagram of (b). Large spots represent ð111þ 00 pattern, and small spots represent single variant of commensurate! reflections. DF image (d) was taken at the same place as (a) using an! spot indicated by an arrow in (c). The inner products of the 00 martensite can be seen as bright regions in (d). Figures (e) and (f) were MBD patterns taken from the regions marked by E and F in (d), respectively. The pattern of (e) indicates the 00 martensite, and (f) indicates the single variant of commensurate! phase. Therefore, it was shown that the band-like products of the! phase existed in the quenched 00 martensite structure. Figure 6 shows detailed analysis of the inner band-like products in the 723 K-aged specimen. The inner band-like products have bamboo-like structure as shown in BF (a). SAD pattern (b) was taken from the region of (a) that included a few band-like products, and (c) was the key diagram of (b). In key diagram (c), solid circles represent ð101þ 00 pattern, and open circles represent ð101þ 00 pattern. Therefore, it was understood that SAD pattern (b) was a superposed pattern of the ð101þ 00 and ð101þ 00. The DF image using a solid circle spot was shown in (d), and that using an open circle spot was shown in (e). The arrows in (a), (d) and (e) indicate the same position. The inner band-like products were alternately formed in 00 martensite plate, and it was revealed that there was a ð111þ 00 twin relation. 3.3 Mechanical properties Mechanical properties of each structure were investigated by Vickers hardness test and tensile test. The results of Vickers hardness test are shown in Table 1. The quenched specimen indicated slightly lower hardness compared with the annealed specimen. On the other hand, the 723 K-aged specimen indicated very high hardness and the 923 K-aged specimen indicated slightly higher hardness compared with the annealed and quenched specimens. Figure 7 shows nominal stress-strain curves of the specimens that were treated by the different heat treatments. The curve of the annealed specimen is (a), the quenched specimen is (b), the 723 K-aged specimen is (c) and the 923 K-aged specimen is (d). The curves of (a) and (d) indicated similar tendency each other, because these specimens were composed of the þ structure. The curve of the quenched specimen (b), which was composed of the 00 martensite structure, showed low elastic incline and good ductility. On

4 1632 Y. Mantani, Y. Takemoto, M. Hida, A. Sakakibara and M. Tajima the other hand, the curve of the 723 K-aged specimen (c), which was also composed of the 00 martensite structure, indicated almost the same elastic incline as that of the þ structure. However, different from the quenched specimen, the 723 K-aged specimen was characterized by its high yield stress and brittle fracture. Though the quenched specimen and 723 K-aged specimen were both composed of the 00 martensite structure, it was considered that the difference of the 00 martensite inner products observed by TEM caused these drastic changes of the mechanical properties. 4. Discussion Fig. 4 TEM micrographs of the 00 martensite in Ti-8 mass%mo alloy. (a) The quenched specimen. (b) The 723 K-aged specimen. 4.1 Volume differences among the phases Volume differences among, 00 and phases are discussed from the lattice parameter obtained by XRD analysis shown in Fig. 1 and Fig. 3. It is known that there is Burgers orientation relationship between phase and phase, 11) that is: ð0001þ kð110þ; ½1120Š k½111š It is known that orientation relationship between 00 martensite and phase 5,9,11) is as follows: ½100Š 00 k½100š; ½010Š 00 k½110š; ½001Š 00 k½110š From these orientation relationships, schematic diagram of ð0001þ, ð001þ 00 and ð110þ are illustrated in Fig. 8. Open circles represent atoms on the layer below or above the layer of solid circles. Atomic coordinates of open circles in the 00 martensite are referred to what Duerig et al. reported. 5) Each phase is regarded as orthorhombic type lattice in this figure, Fig. 5 TEM micrographs of the 00 martensite in quenched Ti-8 mass%mo alloy. (a) BF image. (b) SAD pattern taken from one 00 martensite region including a few band-like products in (a). (c) Key diagram of (b). (d) DF image using arrowed spot in (c). (e) MBD pattern taken from E region in (d). (f) MBD pattern taken from F region in (d).

5 Phase Transformation of 00 Martensite Structure by Aging in Ti-8 mass%mo Alloy 1633 Fig. 6 TEM micrographs of the 00 martensite in 723 K-aged Ti-8 mass%mo alloy. (a) BF image of inner one 00 martensite. (b) SAD pattern taken from the region including a few band-like products in (a). (c) Key diagram of (b). (d) DF image using a reflection of solid circle in (c). (e) DF image using a reflection of open circle in (c). Table 1 Vickers Hardness (Hv) Vickers hardness number of each specimen. Annealed Quenched 723 K-aged 923 K-aged b= 3 a a 1400 (0001) α Nominal Stress, σ /MPa Nominal Strain, ε (%) a a b b= 2 a (001) α" (110) β Fig. 8 Schematic illustration of structures arranged to the orthorhombic type lattice. (a) ð0001þ. (b) ð001þ 00. (c) ð110þ. Fig. 7 Nominal stress-strain curves of Ti-8 mass%mo alloy which performed different heat treatments. (a) The annealed specimen. (b) The quenched specimen. (c) The 723 K-aged specimen. (d) The 923 K-aged specimen. and the lattice parameters, axis ratios and volume expansions compared with the phase are shown in Table 2. The phase is expanded by 2.10% and the 00 martensite is expanded by

6 1634 Y. Mantani, Y. Takemoto, M. Hida, A. Sakakibara and M. Tajima Table 2 Lattice parameters, axis ratios and volume expansions of each phase regarded as orthorhombic type lattice. Phase a b c Volume b=a c=a (nm) (nm) (nm) expansion þ2:10% þ0:79% % compared with the phase. Therefore, The 00 martensite has a nearer structure to the phase compared with the phase. In the quenched specimen, band-like products of the! phase were observed in the 00 martensite structure by TEM, as shown in Fig. 5, though it was not recognized by XRD. It is considered that these! products were formed to relax the strain of volume expansion caused by the 00 formation with quench. These! products are considered to be effective to relax the volume expansion, because those were able to form with smaller displacement compared with forming the phase. 4.2 Phase transformation of 00 martensite structure In the 723 K-aged specimen, not band-like products of! phase but ð111þ 00 twins were observed in the 00 martensite structure by TEM, as shown in Fig. 6, though XRD profiles were the same as the quenched specimen. It is considered that band-like products of the! phase were changed into the 00 martensite by aging at 723 K, and the ð111þ 00 twins were formed through this process. Only the exothermic peak near 780 K, which was considered to indicate phase decomposition from the 00 martensite to the and phases, was observed in DSC curve Fig. 2(b), and it was hard to detect changes with slow reaction rate such as those realized by isothermal aging. On the other hand, exothermic peak near 575 K and endothermic peak near 600 K were observed in DSC curve, Fig. 2(c), before exothermic peak near 800 K. It is considered that these peaks were not concerned with the precipitation of! phase in the 723 K-aged specimen, though DSC peaks near 600 K were concerned with that in metastable -type titanium alloys. Age precipitation on the prior grain boundary (Fig. 3(b)) and distortion of the diffraction spots (Fig. 6(b)) were observed in the 723 K-aged specimen, so that it was considered that - like or -like structures with age diffusion were included in the specimen, though these were recognized by XRD, and the changes of these structures were observed as DSC peaks. Therefore, tensile properties of the 723 K-aged specimen are regarded as not! brittleness. The phase transformation of the 00 martensite structure involved slight change, which cannot be recognized by XRD analysis, but mechanical properties shown in Table 1 and Fig. 7 were drastically changed. The quenched specimen indicated low hardness, low elastic incline and good ductility. On the other hand, the 723 K-aged specimen indicated high hardness, high yield stress and brittleness. The 00 martensite in the 723 K-aged specimen indicates the same tendency of the mechanical properties as the martensite of steel that has high hardness and brittleness. Therefore, tensile property of the quenched 00 martensite is very characteristic, and it is considered the band-like products of! phase greatly affect to the mechanical properties. Using the 00 martensite structure, functionally alloy design will be expected. However, the details such as phase transformation behavior and mechanical properties of the 00 martensite structure should be studied further. 5. Conclusion The phase constitutions obtained by different heat treatments and the phase transformation of the 00 martensite structure has been investigated by means of optical microscopy, X-ray diffraction, differential scanning calorimetry and transmission electron microscopy in Ti-8 mass%mo alloy. The conclusions obtained are as follows: (1) The annealed specimen was composed of the and phases, and the quenched specimen was composed of the 00 martensite. Though the 00 martensite still remained after the isothermal aging at 723 K for 9.0 ks, it was decomposed to the and phases by the aging at 923 K for 9.0 ks. (2) The band-like products of! phase were formed in the quenched 00 martensite structure. On the other hand, not the! phase products but the ð111þ 00 twins were observed in the 723 K-aged 00 martensite structure. The quenched 00 martensite indicated slightly lower hardness, low elastic incline and good ductility. 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