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Abnormal martensitic transformation of high Zr-containing Ti alloys
Journal of Alloys and Compounds 615 (2014) 804–808
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Abnormal martensitic transformation of high Zr-containing Ti alloys S.X. Liang a,⇑, L.X. Yin a, Y.K. Zhou b, X.J. Feng a, M.Z. Ma b,⇑, R.P. Liu b, C.L. Tan c a
College of Equipment Manufacture, Hebei University of Engineering, Handan 056038, Hebei, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China c Beijing Institute of Spacecraft System Engineering, Beijing 100094, China b
a r t i c l e
i n f o
Article history: Received 8 May 2014 Received in revised form 24 June 2014 Accepted 5 July 2014 Available online 14 July 2014 Keywords: Phase transitions Microstructure Titanium alloy
a b s t r a c t Martensitic phases are intermediate phases in Ti alloys and have obvious effects on microstructure and properties of heat treated specimens. The martensitic transformation and phase stability of Ti alloys with high content Zr were investigated. The addition of high content Zr was found to greatly modify the b phase stability and martensitic transformation of Ti alloys. The b transus temperature of Ti–50Zr binary alloy is about 616 °C. Simultaneously, the phase composition of the b quenched Ti–50Zr binary alloy is composed of whole a0 martensitic phase. Reversely, the Ti–30Zr–10Al–3.5V and Ti–50Zr–10Al–3.5V quaternary alloys have the higher b transus temperatures 779 and 636 °C, respectively. However, phase composition is major a00 martensitic phase in b quenched Ti–30Zr–10Al–3.5V alloy and is major b phase in b quenched Ti–50Zr–10Al–3.5V alloy, respectively. This interesting martensitic transformation phenomenon does not conform to previous wide accepted martensitic transformation theories and laws of Ti alloys. The delayed martensitic transformation of aforementioned two quaternary alloys was considered to be caused by the local stresses resulting from the extra additions of Al and V alloying elements. Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction For most of metal and alloys, the solution plus aging heat treatment is one of the most effective strengthening methods. So, martensitic transformation is important for developing high performance metal and alloys. Researches on phase transition of Ti and Ti alloys are very important to develop new Ti alloys with excellent properties and to expand their application. Resulting from the low shear modulus and small energy associated with the parent–martensite interface, a small driving force is adequate for martensite formation in the Ti alloys [1]. Moreover, martensitic transformation in Ti alloy is dependent on the electron structure parameters bond energy and shared electron pair number on the covalent bond. The larger bond energy and shared electron pair number on the covalent bond of b phase can increase probability of occurrence for orthorhombic martensite a00 phase [2,3]. All aforementioned parameters affecting martensitic transformation are heavily composition dependent. Previous researches showed some effective laws about effects of alloying elements on phase stability and martensitic transformation of Ti and Ti alloys. The most widely accepted law shows that the martensitic structure ⇑ Corresponding authors. Tel.: +86 310 8577971. E-mail addresses: [email protected] (S.X. Liang), [email protected] (M.Z. Ma). http://dx.doi.org/10.1016/j.jallcom.2014.07.041 0925-8388/Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
of water quenched Ti and Ti alloys from b temperature zone changes as the order, a0 ? a00 ? x ? b, as the increasing of b stabilizer content [4,5]. Hereinafter, it is called b stabilizer content law. Apart from b stabilizer content law, other effective laws also have been reported, such as valence electron theory and B0 M d law. Valence electron theory indicates that the final phase compositions and crystal structure of quenched Ti alloys are greatly dependent on average number of valence electron e/a [2]. The B0 M d law shows that effects of alloying elements on phase stability of Ti alloys can be indicated by the B0 M d diagram. Here, B0 is the bond order which is a measure of the covalent bond strength between titanium and alloying elements and M d is the average d orbital energy level of formulated titanium alloys [6]. Elements Zirconium (Zr) and Ti belong to the same group in the element periodic table and show similar physicochemical properties. The interaction and influence of Zr on Ti alloys or of Ti on Zr alloys have been investigating [7–10] and some high performance TiZr-based or ZrTi-based alloys were developed [11–14]. However, effects of Zr on phase structure and stability of Ti alloys are still unclear and controversial. Even, some very different results about effects of Zr on phase stability of Ti alloys were reported [15,16]. Here, an abnormal martensitic transformation phenomenon of Ti alloys with high content Zr (>20 at.%) is reported and the transformation mechanism is also discussed.
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2. Material and methods Sponge Zr (Zr + Hf P 99.5 wt%), sponge Ti (99.7 wt%), industrially pure Al (99.5 wt%), and V (99.9 wt%) were used to prepare Ti–50Zr and Ti–50Zr–10Al– 3.5V (at.%) alloys. Hereinafter, alloys Ti–50Zr, Ti–30Zr–10Al–3.5V and Ti–50Zr– 10Al–3.5V are abbreviated to TZ, TC30Z and TC50Z, respectively. The ingot was melted repeatedly at least five times by vacuum nonconsumable electro-arc furnace to ensure uniform chemical composition. Ingots were sectioned into segments with size of 10 10 5 mm as heat treatment specimens. The heat treatments were performed in a SGL-1700 type tubular vacuum heat treatment furnace with protective argon atmosphere. The specimens were firstly annealed at 800 °C for 1 h and cooled in furnace to obtain the near equilibrium structure and to prepare specimens for phase transus temperature testing. Each annealed specimen was held at temperature Tb + 50 °C (Tb is the b transus temperature) for 30 min followed by rapid quenching into water. Each quenched specimen was split in half for crystal structural and microstructural analysis. Phase transus temperatures were tested by Differential Scanning Calorimetry (DSC) at a constant heating rate of 5 °C/min in an argon atmosphere. Crystal structures were examined by X-ray diffraction (XRD) using Cu Ka radiation. Optical microscope (OM) and transmission electron microscopy (TEM) were used for microstructural analysis.
Fig. 1. DSC curves of examined Ti alloys with high Zr contents.
3. Results Fig. 1 shows DSC curves of examined Ti alloys. The Tb of TZ alloy is about 616 °C. Comparing with pure Ti, the Tb is greatly decreased by 50 at.% Zr addition. The Tb of TC30Z and TC50Z alloys are 779 and 636 °C, respectively. Likewise, the DSC results of those two quaternary Ti alloys show that the addition of high content Zr can obviously decrease the transus temperature. Fig. 2 shows the XRD patterns of water quenched Ti alloys. The TZ alloy is composed of whole a0 martensitic phase. It is similar with results of Zhou et al. [17] who showed that the Zr–Ti binary cast alloys with 10 to 90 at.% Ti are composed of a phase. Li et al. [18] also showed that the structure of quenched Ti50Zr50 alloy is hexagonal structure, namely, a0 martensite. However, the phase compositions of quenched TC30Z alloy are major a00 martensitic phase and little b phase. And the quenched TC50Z alloy is composed of major b phase and little a00 martensitic phase. Fig. 3 shows micrographs of examined Ti alloys. The microstructure of TZ alloy in Fig. 3(a) shows coarse plate a0 martensitic phase inside original b grains. However, original b grains in the TC30Z alloy are filled with major acicular fine a00 martensitic phase in Fig. 3(b). As the Zr content increases to 50 at.%, barely fine a00 martensitic phase can be found in the TC50Z alloy in micrograph Fig. 3(c). The TEM micrographs and the corresponding selected area electron diffraction patterns (SADPs) of examined Ti alloys TZ, TC30Z and TC50Z are showed in Fig. 4(a)–(c), respectively. The similar laths with a certain angle are showed in Fig. 4(a) and (b). However, the thickness of laths in Fig. 4(a) is obviously higher than that in Fig. 4(b). The corresponding SADPs confirm that the laths in TZ specimen in Fig. 4(a) are a0 martensite and the laths in TC30Z specimen in Fig. 4(b) are a00 martensite. Fig. 4(c) shows little a00 laths and major retained b phase which is attested by the corresponding SADP. The results of microstructure are coincident with XRD results. 4. Discussions Aforementioned b stabilizer content law showed that the quenched phase of Ti alloys changes orderly as a0 , a00 , x, and b phase when the Tb decreases. Many other literatures also showed that the similar trends for Ms. and Tb varied with the content of alloying elements [19–22]. According to DSC curves, the Tb decreases orderly as TC30Z, TC50Z and TZ alloys. However, the water quenched phase of TZ alloy with the lowest Tb is whole a0 martensitic phase. But water quenched TC30Z and TC50Z alloys are composed of major a00 martensitic phase and major b phase, respectively. Thus, water quenched phase in TZ and TCZ (Ti–xZr–
Fig. 2. XRD patterns of Ti alloys with high Zr contents.
10Al–3.5V) alloys disagrees with the b stabilizer content law. According to the valence electron theory, the quenched phase of Ti alloys is greatly dependent on the value of e/a. Results of Laheurte et al. [23] showed that the stability of b phase increases when the e/a ratio rises towards high value. The martensitic phase of Ti alloys with the value of e/a below 4.07 should be a0 phase. The e/a ratios of TZ, TC30Z and TC50Z alloys can be calculated as 4, 3.935 and 3.935, respectively. However, water quenched phases of TC30Z and TC50Z with e/a ratios below 4 are major a00 martensitic phase and major b phase, respectively. Thus, water quenched phases in TZ, TC30Z and TC50Z alloys also do not comply with the valence electron theory. The B0 M d law indicates that b phase stability of Ti alloys increases with the B0 value and decreases as the increasing of M d value. The B0 M d diagram of Ti alloys can be divided into a0 , a00 , x and b phase four regions [2,24]. According to the results of Morinaga et al. [6], the B0 values of TZ, TC30Z and TC50Z alloys can be calculated as 2.938, 2.843 and 2.902, respectively. Similarly, the M d value also can be calculated as 2.691, 2.548 and 2.646, respectively. Although the B0 value of Ti alloys increases with the Zr addition, the M d value also increases with Zr addition. The similar result war also reported by Abdel-Hady et al. [24]. According to above analyses and the extended B0 M d diagram of Ti alloys showed in Fig. 5 [25], it is easy to deduce that all alloys of TZ, TC30Z and TC50Z are located in the same region (such as in the a0 region) in the B0 M d diagram. Here again, the B0 M d law also cannot clearly explain the abnormal martensitic transformation phenomenon in those examined Ti alloys with high
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Fig. 3. OM micrographs of (a) TZ, (b) TC30Z, and (c) TC50Z.
Fig. 4. TEM micrographs and the corresponding SADPs taken from circle areas of (a) TZ, (b) TC30Z, and (c) TC50Z.
Zr content. The inconsistent martensitic transition results of examined high Zr-containing Ti alloys with previous b stabilizer content law, valence electron theory and B0 M d law may result from the contents of alloying elements. Previous b stabilizer content law, valence electron theory and B0 M d law are mainly applicable to low-alloy Ti alloys. However, the Zr contents in examined Ti alloys
are more than 30 at.%. Additions of high alloying element should alter the movement of outermost and subshell electrons. The subshell electrons in transition elements such as Ti element should jump and become outermost electrons which should enhance the amount and bond strength of valence electron. Wen et al. [26] showed that the addition of interstitial impurities can enhance
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the hybridization states of Ti and Al atoms. Finally, the valence electron structures of various phases in Ti–Al alloys became considerably anisotropic. Tian et al. [27] also showed that the addition of C element changed the covalence electron numbers and bonding energy of iron aluminides matrix. Thus, the phase transition of the examined high Zr-containing Ti alloys cannot be explained entirely by theories for dilute alloy, such as aforementioned laws and/or theories. Although the martensitic transformation in Ti alloy is dependent on the stability of b phase, it is also dependent on the formation of martensite. Previous reports [28–30] showed that the addition of Co or Mn to Ti alloy forms the formation of point defects and local stresses which suppress the formation of long range ordered martensites and even form glass state instead of martensitic phase. According to the Fleischer theory, the local stress resulted from alloying elements can be expressed as
s ¼ AGe3=2 c1=2 where s is the local stress, A is a constant, G is the shear modulus, e is alloying mismatch, and c is alloying concentration. The mismatch of Al and V element is about 10.6% and 17.5% to Zr element, respectively. Thus, the additional Al and V alloying elements in TCZ alloys should also result in extra local stresses postponing the long range ordered martensitic transformation. The distance of atom migration in transformation from b phase into a00 martensite is shorter than that into a0 martensite. Thus, the martensitic transformation in TCZ quaternary alloys is sluggish. Consistently, major b phase in TC50Z alloy and major a00 martensitic phase in TC30Z alloy is obtained. Otherwise, the onset of a to b transus temperature (Ta) and width of a + b two-phase temperature rejoin (Wa+b = Tb Ta) can also be obtained from DSC curves in Fig. 1. The Wa+b of TZ alloy is only about 5 °C (616–611 °C). However, the Wa+b of TC30Z and TC50Z alloys are 204 and 76 °C, respectively. Although the Tb of TCZ alloys are higher than that of TZ alloy, the Wa+b of the TCZ alloys are also much more higher than that of TZ alloy. The high value of Wa+b of TCZ alloys should result from the additional Al and V alloying elements. It is well known that martensites such as a0 , a00 , and x phases in Ti alloys are non-equilibrium phases. The structure of martensite is not only dependent on stability of b phase but also on stability of a phase. The stability of b phase can be indicated by the Tb. Similarly, Ta can signify the stability of a
Fig. 5. The expanded B0 M d diagram of Ti alloys.
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phase. Thus, apart from Tb, the martensitic transformation should also be dependent on Ta and Wa+b. 5. Conclusions An abnormal martensitic transformation in Ti alloys with high content Zr is present. The water quenched martensite is a0 phase in Ti–50Zr alloy with lowest b transus temperature. However, the major a00 martensitic phase in quenched Ti–30Zr–10Al–3.5V with moderate b transus temperature and major b phase in quenched Ti–50Zr–10Al–3.5V alloy with highest b transus temperature are detected. The mechanism of this abnormal martensitic transformation is related to the point effect, local stress and width of two-phase temperature rejoin which suppress the martensitic transformation in Ti alloys. Acknowledgements This work was supported by the SKPBRC (Grant No. 2013CB733000), NSFC (Grant Nos. 51171163/51121061/ 51271161) and NSFH (Grant No. E2014402014). References [1] S. Banerjee, P. Mukhopadhyay, Phase Transformations Examples from Titanium and Zirconium Alloys, Elsevier, Amsterdam, 2007. p. 47. [2] C. Lin, G.L. Yin, Y.Q. Zhao, P. Ge, Z.L. Liu, Analysis of the effect of alloy elements on martensitic transformation in titanium alloy with the use of valence electron structure parameters, Mater. Chem. Phys. 125 (2011) 411–417. [3] J.H. Dai, X. Wu, Y. Song, R. Yang, Electronic structure mechanism of martensitic phase transformation in binary titanium alloys, J. Appl. Phys. 112 (2012) 123718. [4] J. Syarif, T.N. Rohmannudin, M.Z. Omar, Z. Sajuri, S. Harjanto, Stability of the beta phase in Ti–Mo–Cr alloy fabricated by powder metallurgy, J. Min. Metall. Sect. B-Metall. 49 (2013) 285–292. [5] L.W. Ma, H.S. Cheng, C.Y. Chung, B. Yuan, Super elastic behavior and microstructure of Ti19Nb9Zr1Mo (at. %) alloy, Mater. Lett. 109 (2013) 172–174. [6] M. Morinaga, N. Yukawa, T. Maya, K. Sone, H. Adachi, Theoretical design of titanium alloys, Sixth World Conference on Titanium III (1988) pp. 1601-1606. [7] S.X. Liang, M.Z. Ma, R. Jing, X.Y. Zhang, R.P. Liu, Microstructure and mechanical properties of hot-rolled ZrTiAlV alloys, Mater. Sci. Eng., A 532 (2012) 1–5. [8] X. Tang, T. Ahmed, H.J. Rack, Phase transformations in Ti–Nb–Ta and Ti–Nb– Ta–Zr alloys, J. Mater. Sci. 35 (2000) 1805–1811. [9] J.I. Kim, H.Y. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Shape memory characteristics of Ti–22Nb–(2–8)Zr (at.%) biomedical alloys, Mater. Sci. Eng., A 403 (2005) 334–339. [10] R. Jing, S.X. Liang, C.Y. Liu, M.Z. Ma, X.Y. Zhang, R.P. Liu, Structure and mechanical properties of Ti–6Al–4V alloy after zirconium addition, Mater. Sci. Eng., A 552 (2012) 295–300. [11] D.Q. Martins, W.R. Os´orio, M.E.P. Souza, R. Caram, A. Garcia, Effects of Zr content on microstructure and corrosion resistance of Ti–30Nb–Zr casting alloys for biomedical applications, Electrochim. Acta 53 (2008) 2809–2817. [12] S.X. Liang, L.X. Yin, M.Z. Ma, R. Jing, P.F. Yu, Y.F. Zhang, B.A. Wang, R.P. Liu, A multi-component Zr alloy with comparable strength and higher plasticity than Zr-based bulk metallic glasses, Mater. Sci. Eng., A 561 (2013) 13–16. [13] R. Jing, S.X. Liang, C.Y. Liu, M.Z. Ma, R.P. Liu, Aging effects on the microstructures and mechanical properties of the Ti–20Zr–6.5Al–4V alloy, Mater. Sci. Eng., A 559 (2013) 474–479. [14] S.X. Liang, M.Z. Ma, R. Jing, Y.K. Zhou, Q. Jing, R.P. Liu, Preparation of the ZrTiAlV alloy with ultra-high strength and good ductility, Mater. Sci. Eng., A 539 (2012) 42–47. [15] M. Kawakita, M. Takahashi, S. Takahashi, Y. Yamabe-Mitarai, Effect of Zr on phase transformation and high-temperature shape memory effect in TiPd alloys, Mater. Lett. 89 (2012) 336–338. [16] W.F. Ho, W.K. Chen, S.C. Wu, H.C. Hsu, Structure, mechanical properties, and grindability of dental Ti–Zr alloys, J. Mater. Sci. – Mater. Med. 19 (2008) 3179– 3186. [17] Y.K. Zhou, R. Jing, M.Z. Ma, R.P. Liu, Tensile strength of Zr–Ti binary alloy, Chin. Phys. Lett. 30 (3) (2013) 116201. [18] Y. Li, Y. Cui, F. Zhang, H. Xu, Shape memory behavior in Ti–Zr alloys, Scripta Mater. 64 (2011) 584–587. [19] Y. Mantani, K. Kudou, Effect of plastic deformation on material properties and martensite structures in Ti–Nb alloys, J. Alloy. Comp. 577S (2013) S448–S452. [20] Y. Mantani, M. Tajima, Phase transformation of quenched a martensite by aging in Ti–Nb alloys, Mater. Sci. Eng., A 438–440 (2006) 315–319. [21] S.Q. Wu, D.H. Ping, Y. Yamabe-Mitarai, T. Kitashima, G.P. Li, R. Yang, Microstructural characterization on martensitic a phase in Ti–Nb–Pd alloys, J. Alloy. Comp. 577S (2013) S423–S426.
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[22] S. Banerjee, P. Mukhopadhyay, Phase Transformations Examples from Titanium and Zirconium Alloys, Elsevier, Amsterdam, 2007. p. 282. [23] P. Laheurte, F. Prima, A. Eberhardt, T. Gloriant, M. Wary, E. Patoor, Mechanical properties of low modulus b titanium alloys designed from the electronic approach, J. Mech. Behav. Biomed. 3 (2010) 565–573. [24] M. Abdel-Hady, H. Fuwa, K. Hinoshita, H. Kimura, Y. Shinzato, M. Morinaga, Phase stability change with Zr content in b-type Ti–Nb alloys, Scripta Mater. 57 (2007) 1000–1003. [25] M. Abdel-Hady, K. Hinoshita, M. Morinaga, General approach to phase stability and elastic properties of b-type Ti-alloys using electronic parameters, Scripta Mater. 55 (2006) 477–480. [26] Z. Wen, L. Wen, G. Hani, Influence of interstitial impurities on the valence electron structures and phase transformation behavior inintermediate Ti–Al alloys, J. Mater. Sci. 42 (2007) 8139–8143.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Abnormal martensitic transformation of high Zr-containing Ti alloys S.X. Liang a,⇑, L.X. Yin a, Y.K. Zhou b, X.J. Feng a, M.Z. Ma b,⇑, R.P. Liu b, C.L. Tan c a
College of Equipment Manufacture, Hebei University of Engineering, Handan 056038, Hebei, China State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao 066004, China c Beijing Institute of Spacecraft System Engineering, Beijing 100094, China b
a r t i c l e
i n f o
Article history: Received 8 May 2014 Received in revised form 24 June 2014 Accepted 5 July 2014 Available online 14 July 2014 Keywords: Phase transitions Microstructure Titanium alloy
a b s t r a c t Martensitic phases are intermediate phases in Ti alloys and have obvious effects on microstructure and properties of heat treated specimens. The martensitic transformation and phase stability of Ti alloys with high content Zr were investigated. The addition of high content Zr was found to greatly modify the b phase stability and martensitic transformation of Ti alloys. The b transus temperature of Ti–50Zr binary alloy is about 616 °C. Simultaneously, the phase composition of the b quenched Ti–50Zr binary alloy is composed of whole a0 martensitic phase. Reversely, the Ti–30Zr–10Al–3.5V and Ti–50Zr–10Al–3.5V quaternary alloys have the higher b transus temperatures 779 and 636 °C, respectively. However, phase composition is major a00 martensitic phase in b quenched Ti–30Zr–10Al–3.5V alloy and is major b phase in b quenched Ti–50Zr–10Al–3.5V alloy, respectively. This interesting martensitic transformation phenomenon does not conform to previous wide accepted martensitic transformation theories and laws of Ti alloys. The delayed martensitic transformation of aforementioned two quaternary alloys was considered to be caused by the local stresses resulting from the extra additions of Al and V alloying elements. Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction For most of metal and alloys, the solution plus aging heat treatment is one of the most effective strengthening methods. So, martensitic transformation is important for developing high performance metal and alloys. Researches on phase transition of Ti and Ti alloys are very important to develop new Ti alloys with excellent properties and to expand their application. Resulting from the low shear modulus and small energy associated with the parent–martensite interface, a small driving force is adequate for martensite formation in the Ti alloys [1]. Moreover, martensitic transformation in Ti alloy is dependent on the electron structure parameters bond energy and shared electron pair number on the covalent bond. The larger bond energy and shared electron pair number on the covalent bond of b phase can increase probability of occurrence for orthorhombic martensite a00 phase [2,3]. All aforementioned parameters affecting martensitic transformation are heavily composition dependent. Previous researches showed some effective laws about effects of alloying elements on phase stability and martensitic transformation of Ti and Ti alloys. The most widely accepted law shows that the martensitic structure ⇑ Corresponding authors. Tel.: +86 310 8577971. E-mail addresses: [email protected] (S.X. Liang), [email protected] (M.Z. Ma). http://dx.doi.org/10.1016/j.jallcom.2014.07.041 0925-8388/Crown Copyright Ó 2014 Published by Elsevier B.V. All rights reserved.
of water quenched Ti and Ti alloys from b temperature zone changes as the order, a0 ? a00 ? x ? b, as the increasing of b stabilizer content [4,5]. Hereinafter, it is called b stabilizer content law. Apart from b stabilizer content law, other effective laws also have been reported, such as valence electron theory and B0 M d law. Valence electron theory indicates that the final phase compositions and crystal structure of quenched Ti alloys are greatly dependent on average number of valence electron e/a [2]. The B0 M d law shows that effects of alloying elements on phase stability of Ti alloys can be indicated by the B0 M d diagram. Here, B0 is the bond order which is a measure of the covalent bond strength between titanium and alloying elements and M d is the average d orbital energy level of formulated titanium alloys [6]. Elements Zirconium (Zr) and Ti belong to the same group in the element periodic table and show similar physicochemical properties. The interaction and influence of Zr on Ti alloys or of Ti on Zr alloys have been investigating [7–10] and some high performance TiZr-based or ZrTi-based alloys were developed [11–14]. However, effects of Zr on phase structure and stability of Ti alloys are still unclear and controversial. Even, some very different results about effects of Zr on phase stability of Ti alloys were reported [15,16]. Here, an abnormal martensitic transformation phenomenon of Ti alloys with high content Zr (>20 at.%) is reported and the transformation mechanism is also discussed.
S.X. Liang et al. / Journal of Alloys and Compounds 615 (2014) 804–808
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2. Material and methods Sponge Zr (Zr + Hf P 99.5 wt%), sponge Ti (99.7 wt%), industrially pure Al (99.5 wt%), and V (99.9 wt%) were used to prepare Ti–50Zr and Ti–50Zr–10Al– 3.5V (at.%) alloys. Hereinafter, alloys Ti–50Zr, Ti–30Zr–10Al–3.5V and Ti–50Zr– 10Al–3.5V are abbreviated to TZ, TC30Z and TC50Z, respectively. The ingot was melted repeatedly at least five times by vacuum nonconsumable electro-arc furnace to ensure uniform chemical composition. Ingots were sectioned into segments with size of 10 10 5 mm as heat treatment specimens. The heat treatments were performed in a SGL-1700 type tubular vacuum heat treatment furnace with protective argon atmosphere. The specimens were firstly annealed at 800 °C for 1 h and cooled in furnace to obtain the near equilibrium structure and to prepare specimens for phase transus temperature testing. Each annealed specimen was held at temperature Tb + 50 °C (Tb is the b transus temperature) for 30 min followed by rapid quenching into water. Each quenched specimen was split in half for crystal structural and microstructural analysis. Phase transus temperatures were tested by Differential Scanning Calorimetry (DSC) at a constant heating rate of 5 °C/min in an argon atmosphere. Crystal structures were examined by X-ray diffraction (XRD) using Cu Ka radiation. Optical microscope (OM) and transmission electron microscopy (TEM) were used for microstructural analysis.
Fig. 1. DSC curves of examined Ti alloys with high Zr contents.
3. Results Fig. 1 shows DSC curves of examined Ti alloys. The Tb of TZ alloy is about 616 °C. Comparing with pure Ti, the Tb is greatly decreased by 50 at.% Zr addition. The Tb of TC30Z and TC50Z alloys are 779 and 636 °C, respectively. Likewise, the DSC results of those two quaternary Ti alloys show that the addition of high content Zr can obviously decrease the transus temperature. Fig. 2 shows the XRD patterns of water quenched Ti alloys. The TZ alloy is composed of whole a0 martensitic phase. It is similar with results of Zhou et al. [17] who showed that the Zr–Ti binary cast alloys with 10 to 90 at.% Ti are composed of a phase. Li et al. [18] also showed that the structure of quenched Ti50Zr50 alloy is hexagonal structure, namely, a0 martensite. However, the phase compositions of quenched TC30Z alloy are major a00 martensitic phase and little b phase. And the quenched TC50Z alloy is composed of major b phase and little a00 martensitic phase. Fig. 3 shows micrographs of examined Ti alloys. The microstructure of TZ alloy in Fig. 3(a) shows coarse plate a0 martensitic phase inside original b grains. However, original b grains in the TC30Z alloy are filled with major acicular fine a00 martensitic phase in Fig. 3(b). As the Zr content increases to 50 at.%, barely fine a00 martensitic phase can be found in the TC50Z alloy in micrograph Fig. 3(c). The TEM micrographs and the corresponding selected area electron diffraction patterns (SADPs) of examined Ti alloys TZ, TC30Z and TC50Z are showed in Fig. 4(a)–(c), respectively. The similar laths with a certain angle are showed in Fig. 4(a) and (b). However, the thickness of laths in Fig. 4(a) is obviously higher than that in Fig. 4(b). The corresponding SADPs confirm that the laths in TZ specimen in Fig. 4(a) are a0 martensite and the laths in TC30Z specimen in Fig. 4(b) are a00 martensite. Fig. 4(c) shows little a00 laths and major retained b phase which is attested by the corresponding SADP. The results of microstructure are coincident with XRD results. 4. Discussions Aforementioned b stabilizer content law showed that the quenched phase of Ti alloys changes orderly as a0 , a00 , x, and b phase when the Tb decreases. Many other literatures also showed that the similar trends for Ms. and Tb varied with the content of alloying elements [19–22]. According to DSC curves, the Tb decreases orderly as TC30Z, TC50Z and TZ alloys. However, the water quenched phase of TZ alloy with the lowest Tb is whole a0 martensitic phase. But water quenched TC30Z and TC50Z alloys are composed of major a00 martensitic phase and major b phase, respectively. Thus, water quenched phase in TZ and TCZ (Ti–xZr–
Fig. 2. XRD patterns of Ti alloys with high Zr contents.
10Al–3.5V) alloys disagrees with the b stabilizer content law. According to the valence electron theory, the quenched phase of Ti alloys is greatly dependent on the value of e/a. Results of Laheurte et al. [23] showed that the stability of b phase increases when the e/a ratio rises towards high value. The martensitic phase of Ti alloys with the value of e/a below 4.07 should be a0 phase. The e/a ratios of TZ, TC30Z and TC50Z alloys can be calculated as 4, 3.935 and 3.935, respectively. However, water quenched phases of TC30Z and TC50Z with e/a ratios below 4 are major a00 martensitic phase and major b phase, respectively. Thus, water quenched phases in TZ, TC30Z and TC50Z alloys also do not comply with the valence electron theory. The B0 M d law indicates that b phase stability of Ti alloys increases with the B0 value and decreases as the increasing of M d value. The B0 M d diagram of Ti alloys can be divided into a0 , a00 , x and b phase four regions [2,24]. According to the results of Morinaga et al. [6], the B0 values of TZ, TC30Z and TC50Z alloys can be calculated as 2.938, 2.843 and 2.902, respectively. Similarly, the M d value also can be calculated as 2.691, 2.548 and 2.646, respectively. Although the B0 value of Ti alloys increases with the Zr addition, the M d value also increases with Zr addition. The similar result war also reported by Abdel-Hady et al. [24]. According to above analyses and the extended B0 M d diagram of Ti alloys showed in Fig. 5 [25], it is easy to deduce that all alloys of TZ, TC30Z and TC50Z are located in the same region (such as in the a0 region) in the B0 M d diagram. Here again, the B0 M d law also cannot clearly explain the abnormal martensitic transformation phenomenon in those examined Ti alloys with high
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Fig. 3. OM micrographs of (a) TZ, (b) TC30Z, and (c) TC50Z.
Fig. 4. TEM micrographs and the corresponding SADPs taken from circle areas of (a) TZ, (b) TC30Z, and (c) TC50Z.
Zr content. The inconsistent martensitic transition results of examined high Zr-containing Ti alloys with previous b stabilizer content law, valence electron theory and B0 M d law may result from the contents of alloying elements. Previous b stabilizer content law, valence electron theory and B0 M d law are mainly applicable to low-alloy Ti alloys. However, the Zr contents in examined Ti alloys
are more than 30 at.%. Additions of high alloying element should alter the movement of outermost and subshell electrons. The subshell electrons in transition elements such as Ti element should jump and become outermost electrons which should enhance the amount and bond strength of valence electron. Wen et al. [26] showed that the addition of interstitial impurities can enhance
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the hybridization states of Ti and Al atoms. Finally, the valence electron structures of various phases in Ti–Al alloys became considerably anisotropic. Tian et al. [27] also showed that the addition of C element changed the covalence electron numbers and bonding energy of iron aluminides matrix. Thus, the phase transition of the examined high Zr-containing Ti alloys cannot be explained entirely by theories for dilute alloy, such as aforementioned laws and/or theories. Although the martensitic transformation in Ti alloy is dependent on the stability of b phase, it is also dependent on the formation of martensite. Previous reports [28–30] showed that the addition of Co or Mn to Ti alloy forms the formation of point defects and local stresses which suppress the formation of long range ordered martensites and even form glass state instead of martensitic phase. According to the Fleischer theory, the local stress resulted from alloying elements can be expressed as
s ¼ AGe3=2 c1=2 where s is the local stress, A is a constant, G is the shear modulus, e is alloying mismatch, and c is alloying concentration. The mismatch of Al and V element is about 10.6% and 17.5% to Zr element, respectively. Thus, the additional Al and V alloying elements in TCZ alloys should also result in extra local stresses postponing the long range ordered martensitic transformation. The distance of atom migration in transformation from b phase into a00 martensite is shorter than that into a0 martensite. Thus, the martensitic transformation in TCZ quaternary alloys is sluggish. Consistently, major b phase in TC50Z alloy and major a00 martensitic phase in TC30Z alloy is obtained. Otherwise, the onset of a to b transus temperature (Ta) and width of a + b two-phase temperature rejoin (Wa+b = Tb Ta) can also be obtained from DSC curves in Fig. 1. The Wa+b of TZ alloy is only about 5 °C (616–611 °C). However, the Wa+b of TC30Z and TC50Z alloys are 204 and 76 °C, respectively. Although the Tb of TCZ alloys are higher than that of TZ alloy, the Wa+b of the TCZ alloys are also much more higher than that of TZ alloy. The high value of Wa+b of TCZ alloys should result from the additional Al and V alloying elements. It is well known that martensites such as a0 , a00 , and x phases in Ti alloys are non-equilibrium phases. The structure of martensite is not only dependent on stability of b phase but also on stability of a phase. The stability of b phase can be indicated by the Tb. Similarly, Ta can signify the stability of a
Fig. 5. The expanded B0 M d diagram of Ti alloys.
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phase. Thus, apart from Tb, the martensitic transformation should also be dependent on Ta and Wa+b. 5. Conclusions An abnormal martensitic transformation in Ti alloys with high content Zr is present. The water quenched martensite is a0 phase in Ti–50Zr alloy with lowest b transus temperature. However, the major a00 martensitic phase in quenched Ti–30Zr–10Al–3.5V with moderate b transus temperature and major b phase in quenched Ti–50Zr–10Al–3.5V alloy with highest b transus temperature are detected. The mechanism of this abnormal martensitic transformation is related to the point effect, local stress and width of two-phase temperature rejoin which suppress the martensitic transformation in Ti alloys. Acknowledgements This work was supported by the SKPBRC (Grant No. 2013CB733000), NSFC (Grant Nos. 51171163/51121061/ 51271161) and NSFH (Grant No. E2014402014). References [1] S. Banerjee, P. Mukhopadhyay, Phase Transformations Examples from Titanium and Zirconium Alloys, Elsevier, Amsterdam, 2007. p. 47. [2] C. Lin, G.L. Yin, Y.Q. Zhao, P. Ge, Z.L. Liu, Analysis of the effect of alloy elements on martensitic transformation in titanium alloy with the use of valence electron structure parameters, Mater. Chem. Phys. 125 (2011) 411–417. [3] J.H. Dai, X. Wu, Y. Song, R. Yang, Electronic structure mechanism of martensitic phase transformation in binary titanium alloys, J. Appl. Phys. 112 (2012) 123718. [4] J. Syarif, T.N. Rohmannudin, M.Z. Omar, Z. Sajuri, S. Harjanto, Stability of the beta phase in Ti–Mo–Cr alloy fabricated by powder metallurgy, J. Min. Metall. Sect. B-Metall. 49 (2013) 285–292. [5] L.W. Ma, H.S. Cheng, C.Y. Chung, B. Yuan, Super elastic behavior and microstructure of Ti19Nb9Zr1Mo (at. %) alloy, Mater. Lett. 109 (2013) 172–174. [6] M. Morinaga, N. Yukawa, T. Maya, K. Sone, H. Adachi, Theoretical design of titanium alloys, Sixth World Conference on Titanium III (1988) pp. 1601-1606. [7] S.X. Liang, M.Z. Ma, R. Jing, X.Y. Zhang, R.P. Liu, Microstructure and mechanical properties of hot-rolled ZrTiAlV alloys, Mater. Sci. Eng., A 532 (2012) 1–5. [8] X. Tang, T. Ahmed, H.J. Rack, Phase transformations in Ti–Nb–Ta and Ti–Nb– Ta–Zr alloys, J. Mater. Sci. 35 (2000) 1805–1811. [9] J.I. Kim, H.Y. Kim, T. Inamura, H. Hosoda, S. Miyazaki, Shape memory characteristics of Ti–22Nb–(2–8)Zr (at.%) biomedical alloys, Mater. Sci. Eng., A 403 (2005) 334–339. [10] R. Jing, S.X. Liang, C.Y. Liu, M.Z. Ma, X.Y. Zhang, R.P. Liu, Structure and mechanical properties of Ti–6Al–4V alloy after zirconium addition, Mater. Sci. Eng., A 552 (2012) 295–300. [11] D.Q. Martins, W.R. Os´orio, M.E.P. Souza, R. Caram, A. Garcia, Effects of Zr content on microstructure and corrosion resistance of Ti–30Nb–Zr casting alloys for biomedical applications, Electrochim. Acta 53 (2008) 2809–2817. [12] S.X. Liang, L.X. Yin, M.Z. Ma, R. Jing, P.F. Yu, Y.F. Zhang, B.A. Wang, R.P. Liu, A multi-component Zr alloy with comparable strength and higher plasticity than Zr-based bulk metallic glasses, Mater. Sci. Eng., A 561 (2013) 13–16. [13] R. Jing, S.X. Liang, C.Y. Liu, M.Z. Ma, R.P. Liu, Aging effects on the microstructures and mechanical properties of the Ti–20Zr–6.5Al–4V alloy, Mater. Sci. Eng., A 559 (2013) 474–479. [14] S.X. Liang, M.Z. Ma, R. Jing, Y.K. Zhou, Q. Jing, R.P. Liu, Preparation of the ZrTiAlV alloy with ultra-high strength and good ductility, Mater. Sci. Eng., A 539 (2012) 42–47. [15] M. Kawakita, M. Takahashi, S. Takahashi, Y. Yamabe-Mitarai, Effect of Zr on phase transformation and high-temperature shape memory effect in TiPd alloys, Mater. Lett. 89 (2012) 336–338. [16] W.F. Ho, W.K. Chen, S.C. Wu, H.C. Hsu, Structure, mechanical properties, and grindability of dental Ti–Zr alloys, J. Mater. Sci. – Mater. Med. 19 (2008) 3179– 3186. [17] Y.K. Zhou, R. Jing, M.Z. Ma, R.P. Liu, Tensile strength of Zr–Ti binary alloy, Chin. Phys. Lett. 30 (3) (2013) 116201. [18] Y. Li, Y. Cui, F. Zhang, H. Xu, Shape memory behavior in Ti–Zr alloys, Scripta Mater. 64 (2011) 584–587. [19] Y. Mantani, K. Kudou, Effect of plastic deformation on material properties and martensite structures in Ti–Nb alloys, J. Alloy. Comp. 577S (2013) S448–S452. [20] Y. Mantani, M. Tajima, Phase transformation of quenched a martensite by aging in Ti–Nb alloys, Mater. Sci. Eng., A 438–440 (2006) 315–319. [21] S.Q. Wu, D.H. Ping, Y. Yamabe-Mitarai, T. Kitashima, G.P. Li, R. Yang, Microstructural characterization on martensitic a phase in Ti–Nb–Pd alloys, J. Alloy. Comp. 577S (2013) S423–S426.
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