Various stages in stress strain curve of Ti Al Nb alloys undergoing SIMT

Transcription

1 Various stages in stress strain curve of Ti Al Nb alloys undergoing SIMT Archana G. Paradkar a,, S.V. Kamat a, A.K. Gogia b, B.P. Kashyap c a Defence Metallurgical Research Laboratory, Hyderabad , AP, India b Project Office (Materials), Kaveri Engine Programme, Hyderabad , AP, India c Indian Institute of Technology, Department of Metallurgical Engineering and Material Science, Mumbai , Maharashtra State, India Abstract Ti Al Nb alloys in the present range of composition were found to exhibit a typical four-stage behaviour observed in alloys undergoing stressinduced martensitic transformation (SIMT) in as well as 2 heat-treated condition. Intermittent unloading reloading during tensile test was used to measure the apparent modulus at regular strain intervals. This coupled with the observation of microstructure of the samples from tensile tests interrupted at each of the four stages was used to identify the operative mechanism of each stage. Keywords: Ti Al Nb alloys; Apparent modulus; Stress-induced martensitic transformation (SIMT) 1. Introduction Stress-induced martensitic (SIM) transformations in steels and shape memory alloys are studied extensively [1 4]. Alloy undergoing SIM transformation exhibits a typical stress-plateau in the tensile stress strain curve (Fig. 1). The curve delineates four distinct stages and the numbers in Fig. 1 denote these stages during deformation. Several investigations [1,5 14] have been carried out to clarify the nature of each stage in Ni Ti and the results of these studies are summarized in the following. Stage 1 is the initial linear elastic region. In this stage, the parent phase undergoes an elastic deformation [5,6]. Stage 2 or stress plateau region corresponds to stress-induced transformation of metastable parent phase to martensite or reorientation of martensite present in initial microstructure [1,5,7]. In case the reorientation of martensite is responsible for deformation strain, the stress plateau is flat, but, when stress-induced martensite contributes to deformation strain, this stage is reported to exhibit gradual increase in stress with increase in strain [8] resulting in a rising stress plateau. The mechanism of the deformation in stage 3 is not well established. Some authors [1,9] have suggested that the deformation in stage 3 is an elastic deformation of the martensite phase formed in stage 2. Similar observations are also reported by Vaidyanathan et al. [6]. However, transmission electron microscopy observations by Melton and Mercier [10] revealed an intersecting array of martensite laths in some part and dislocations in another part in a specimen deformed into stage 3. However, their observation is limited to a small region of stage 3. Miyazaki et al. [11], on the other hand, reported that in Ni Ti alloy, this stage corresponds to the mixed processes of elastic deformation of stress-induced martensite formed in stage 2 and reorientation of martensite in combination with the further stress-induced transformation of the parent phase. Mohamed and Washburn [1] made an electron microscopy observation of specimens elongated by 8% and found heavy irregularity of martensite boundaries. Thus, they suggested that slip occurred at the stage 4 in Ni Ti alloy. Michel [12] and Tadaki and Wayman [13] also made the electron microscopy observation of heavily cold-rolled ( 30%) specimens, which roughly corresponded to stage 4 in tensile tests. They both found high density of dislocations and the segmentation of martensites. These results are clear evidence to show that slip occurs in stage 4. Stage 4 defines the plastic deformation of oriented martensite or martensite and retained parent phase, if any, depending upon the initial microstructure. Similar four-stage stress strain curves are also reported in Cu AI Ni single crystals in specific orientations [14]. While the first two stages are similar to those seen

2 293 Table 2 Solutionising temperatures for various alloys S/N % 2 Solution treatment temperature ( C) Ti 15Al 12Nb Ti 15Al 8Nb Ti 18Al 8Nb Fig. 1. Typical stress strain curve for an alloy undergoing SIM (numbers denote various stages during tensile testing). in Ni Ti alloy, the deformation modes in stages 3 and 4, in this case, are proved unambiguously to be due to the elastic deformation of a martensite and martensite-to-martensite transformation, i.e. successive stress-induced transformation, respectively [14]. Thus, it is seen that the operative mechanisms during the various stages of stress strain curve are dependent on the alloy system. SIM in Ti alloys is well reported in literature [15 24]. Ti Al Nb alloys containing at.% Al and 8 12 at.% Nb are also reported to undergo SIM in WQ condition [25]. The present investigation is aimed at studying the operative mechanisms during the tensile deformation of Ti Al Nb alloys undergoing stress-induced transformation for various heat treatments resulting in either fully single-phase microstructure or ( + 2 ) microstructure with different volume fraction of 2. Three alloys, viz. Ti 15Al 8Nb, Ti 15Al 12Nb and Ti 18Al 8Nb were selected for this purpose. 2. Experimental work Ingots of Ti 15Al 8Nb, Ti 15Al 12Nb and Ti 18Al 8Nb alloys were melted by consumable vacuum arc melting process. The nominal composition of the alloys is listed in Table 1. The transus temperatures of the alloys were found to be 1000, 950 and 1060 C for Ti 15Al 8Nb, Ti 15Al 12Nb and Ti 18Al 8Nb, respectively. The ingots were first forged in single-phase region and hot rolled at a temperature 100 C below the transus to 14 mm thick plates in several passes. Few samples were solution treated and water-quenched to get single-phase structure. The volume fraction of 2 is varied by heat-treating in 2 region at different temperatures so as to obtain different volume fractions of 2, viz. 10, 20 and 40% (Table 2) and then water quenched. The tensile samples of 4 mm diameter and 10 mm gage length (parallel to the rolling direction) were machined and stress relieved after machining and pickled so as to avoid masking of the true flow behaviour of the alloy [25]. The tensile tests were carried out using strain gauges on a servo-hydraulic Instron Universal Testing Machine at a crosshead speed of 1 mm/min. A set of specimens in 2 and water-quenched condition for all the three alloys were loaded, unloaded and reloaded several times at regular strain intervals during the tensile deformation with holding time of 2 min after each loading and unloading. The apparent modulus was measured by taking the average of the unloading and reloading stage. Few samples of Ti 15Al 8Nb alloy, as a representative case, were electropolished and the tensile test was interrupted at each stage for SEM observations. 3. Results and discussions Microstructures of all the alloys in 2 and waterquenched condition are similar and representative micrographs for Ti 15Al 8Nb in and 2 heat-treated conditions are shown in Figs. 2 and 3, respectively. The microstructure shows single-phase structure (Fig. 2), in water-quenched condition and a two-phase structure in 2 (Fig. 3a c) water-quenched condition, which is confirmed by XRD (Figs. 2b and 3d) to be and + 2, respectively. Ti Al Nb alloys in the present range of composition, show typical four-stage behaviour in both and ( 2 ) water-quenched condition during the tensile test, similar to observations in Ni Ti alloy [5], and representative σ ε curves for Ti 15Al 8Nb are shown in Fig. 4. In the present case, however, the stress-plateau is slightly rising. This indicates that the transformation strain in the present case is due to the stress-induced transformation of to martensite [8]. In Ti alloys, the elastic modulus of orthorhombic martensite ( ) is significantly different than that of phase. The phase Table 1 Nominal composition of the alloys Alloy Al, wt.% (at.%) Nb, wt.% (at.%) O (wt.%) N (wt.%) Ti Ti 15Al 8Nb 8 (14.4) (8.23) Balance Ti 15Al 12Nb 7.8 (14.5) (11.82) Balance Ti 18Al 8Nb (17.9) 15.9 (8.18) Balance

3 294 Fig. 2. (a) Optical micrograph and (b) XRD of the Ti 15Al 8Nb alloy in water-quenched condition showing phase. change from bcc to orthorhombic martensite during the deformation would then be reflected by a change in the elastic modulus. Thus, the measurement of the apparent modulus can be used as a tool for tracking the SIM transformation in these alloys and hence identifying the various stages in the tensile stress strain curve. The term apparent modulus is used here as the modulus obtained on unloading and reloading is not the true modulus of mixture since secondary deformation mechanisms such as stress-induced transformation and martensite reorientation are also operative [5]. Representative stress strain curves for Ti 15Al 8Nb alloy, tested with intermittent unloading reloading condition, in and 2 (10% 2 ) heat-treated conditions are shown in Fig. 5. Similar curves are obtained for other alloys in various microstructural conditions. Hysteresis is absent in stage 1. However, beyond this stage, hysteresis is seen and it decreases with increase in strain. The hysteresis seen during unloading and reloading (Fig. 5) can be attributed to mechanical reversibility of stress-induced martensite or partial reversibility of martensite reorientation [26]. Figs. 6 8 depict the measured values of the apparent elastic modulus plotted as a function of strain during different stages of tensile deformation for the three alloys. It is seen from these figures that the apparent modulus is high initially (see Table 3) and does not change in stage 1. The modulus starts to decrease as stage 2 commences and attains a stable value (Figs. 6 8) at the end of stage 2, which is significantly lower than the corresponding initial modulus values (Table 3). The value does not change subsequently in stages 3 and 4. The microstructures of the specimens during each of the four stages are also examined. Microstructure of the tensile specimen of Ti 15Al 8Nb alloy interrupted at stage 1 (Fig. 9a) shows that there is no change in the microstructure as a result of loading to stage 1. This coupled with the observation that the elastic modulus does not change in stage 1 indicates that Fig. 3. SEM micrographs of alloy Ti 15Al 8Nb in: (a) ( + 10% 2 ) WQ, (b) ( + 20% 2 ) WQ, (c) ( + 40% 2 ) WQ condition and (d) XRD pattern showing presence of 2 and.

4 295 Fig. 4. Tensile stress strain curves in and 2 water-quenched condition: (a) 1040 C/1 h/wq, (b) 965 C/1 h/wq, (c) 926 C/1 h/wq and (d) 886 C/1 h/wq for Ti 15Al 8Nb alloy. Table 3 Initial modulus of the alloys from σ ε curves Alloy 10% 2 20% 2 40% 2 Ti 15Al 12Nb Ti 15Al 8Nb Ti 18Al 8Nb Fig. 5. Intermittent loading unloading curves in: (a) WQ condition and (b) 2 heat-treated condition (10% 2 ) for Ti 15Al 8Nb alloy. Fig. 6. Variation of apparent modulus of elasticity vs. engineering strain for Ti 15Al 12Nb alloy in various heat-treated conditions.

5 296 Fig. 7. Variation of apparent modulus of elasticity with engineering strain for Ti 15Al 8Nb alloy in various heat-treated conditions. this stage corresponds to elastic deformation of the starting microstructure. The stress-induced martensite in Ti alloys is known to have orthorhombic structure [27]. A representative micrograph (Fig. 9b) of the Ti 15Al 8Nb alloy in heat-treated condition, in the beginning of stage 2, shows a small volume fraction of orthorhombic martensite ( ) needles along with retained. The microstructure at the end of stage 2 shows a considerably higher volume fraction of needles along with some retained. There is no significant change in the microstructure in stage 3(Fig. 9c). The point to note is that no slip lines are observed in the micrograph either in stage 2 or 3. However, the micrograph Fig. 8. Variation of apparent modulus of elasticity with engineering strain for Ti 18Al 8Nb alloy in various heat-treated conditions.

6 297 Fig. 9. SEM micrographs of electro-polished tensile sample interrupted at various stages: (a) elastic region in stage 1, (b) at the beginning of stage 2 showing SIMT of to, (c) in stage 3 showing absence of slip lines and (d) beginning of stage 4 showing slip lines for Ti 15Al 8Nb alloy in treated condition. (Fig. 9d) in stage 4 clearly indicates the presence of slip lines. The observation of the apparent modulus variation with strain as well as the microstructure in the different stages confirmed that the behaviour is similar in 2 heat-treated specimens. The only difference is the presence of 2 in addition to. Stress-induced produced at the beginning of stage 2 in 2 Fig. 10. Similar stages are observed for alloy in 2 heat-treated condition. Microstructure shows the presence of an additional phase primary 2. SEM micrographs of electro-polished tensile sample interrupted at the beginning of stage 2 showing SIMT of to are seen for (20% 2 + ). Fig. 11. XRD of Ti 15Al 8Nb alloy after deformation in: (a) WQ and (b) 2 WQ condition.

7 298 of the initial microstructural constituents. The stress-induced martensitic transformation commences in the beginning of stage 2 and whatever transformation has to occur is completed by the end of stage 2. Stage 3 corresponds to the elastic deformation of retained + structure prevailing at the end of stages 2 and 4 corresponds to plastic deformation of this mixture. Thus, the change in apparent modulus with strain can be used as an excellent tool to track the different stages of tensile deformation. The change in apparent modulus with strain can also be used to estimate the volume fraction of martensite ( ) if one knows the modulii of and 2 phases. This method could overcome the limitation of measurement of volume fraction of martensite by optical method which is not only time consuming and tedious but also may not be very accurate due to the uncertainties associated in resolving smaller martensitic laths. 4. Summary The tensile curves for all the three alloys in both, and 2, solution-treated and water-quenched conditions depict four-stage behaviour. It is established that stage 1 represents the elastic deformation of the phases present in initial structure, stage 2 corresponds to stress-induced transformation of to martensite ( ), stage 3 represents the elastic deformation of all the constituent phases and stage 4 corresponds to the plastic deformation of the constituent phases. Acknowledgements Fig. 12. TEM micrographs of Ti 15Al 8Nb alloy after deformation in: (a) WQ (1040 C/1 h/wq) and (b) 2 WQ (925 C/1 h/wq) conditions, showing presence of. heat-treated condition for Ti 15Al 8Nb alloy for ( + 20% 2 ) is shown in Fig. 10. The transformation of retained to orthorhombic martensite ( ) during the tensile deformation is also clearly evident from a representative XRD patterns of the Ti 15Al 8Nb alloy after deformation as depicted in Fig. 11. All the alloys exhibit presence of an additional phase, i.e. orthorhombic martensite ( ) after deformation. TEM micrographs of all the alloys after deformation, both in and 2 solution treatment and water-quenched conditions, also indicate the presence of an additional phase, orthorhombic martensite ( ) as shown in representative micrograph of the Ti 15Al 8Nb alloy after deformation (Fig. 12). This further corroborates the results of XRD studies (Fig. 11). The observation of the change in apparent modulus with the strain as well as microstructure at each stage clearly indicates that in the present alloys stage 1 represents elastic deformation The authors like to thank all the members of Titanium group for melting, members of rolling and forging group for processing and members of SFSG group for characterization of the alloys. Authors thank Dr. Vikas Kumar for his help during the experimentation and for helpful discussions. The authors would like to thank DRDO, India, for providing funding and facilities for carrying out this work. The authors would also like to thank Director DMRL, Hyderabad, for permission to publish this work. References [1] A. Mohamed, J. Washburn, J. Mater. Sci. 12 (1977) 469. [2] W.C. Leslie, Physical Metallurgy of Steels, Hemisphere Press, McGraw- Hill, New York, 1981, p [3] R.C. Garvie, R.H. Hannink, R.T. Pascoe, Nature 258 (1975) 703. [4] M. Young, E. Levine, H. Margolin, Met. Trans. 5A (1974) [5] Y. Liu, H. Xiang, J. Alloys Compd. 270 (1998) 154. [6] R. Vaidyanathan, M.A.M. Bourke, D.C. Dunand, J. Appl. Phys. 86 (1999) [7] M. Young, E. Levine, H. Margolin, 5 (1974) [8] Y. Liu, P.G. McCormick, ISIJ Int. 29 (1989) 417. [9] J. Perkins, Scr. Met. 8 (1974) [10] K.N. Melton, O. Mercier, Metall. Trans. A 9A (1978) [11] S. Miyazaki, K. Otsuka, Y. Suzuki, Scr. Metall. 15 (1981) 287. [12] M. Michel, Ph.D. Thesis, Stanford University, [13] T. Tadaki, C.M. Wayman, Scr. Met. 14 (1980) 911. [14] K. Otsuka, H. Sakamoto, K. Shimizu, Acta Metall. 27 (1979) 585. [15] Y.T. Lee, L. Welsch, Mater. Sci. Eng. A 128A (1990) 77. [16] T.W. Duerig, G.T. Terelinde, J.C. Williams, Metall. Trans. 11A (1980) 1987.

8 299 [17] T.W. Duerig, J. Albrecht, D. Richter, P. Fischer, Acta Metall. 30 (1982) [18] T. Grosdidier, C. Roubaud, M.J. Philippe, Y. Combres, Scr. Met. 36 (1997) 21. [19] T. Grosdidier, Y. Combres, E. Gautier, M.J. Philippe, Metall. Trans. A 31A (2000) [20] H. Sasano, T. Suzuki, Titanium: Sci. and Technology Proc. Fifth Int. Conf. on Titanium, Munich, Germany, 1984, p [21] Y.T. Lee, M. Peters, G. Welsch, Metall. Trans. 22A (1991) 709. [22] C. Lei, M.H. Wu, L.Mc.D. Schetky, C. Burstone, in: Pelton, et al. (Eds.), SMST-97, Proc. Second Int. Conf. on Shape Memory and Super Elastic Technologies, Pacific Grove, CA, USA, 1997, p [23] M.H. Wu, P.A. Russo, J.G. Ferrero, Proc. Int. Conf. Shape Memory Super Elastic Technol., Pacific Grove, CA, 2003, p [24] R.W. Margevicius, J.D. Cotton, Metall. Trans. 29A (1998) 139. [25] A.G. Paradkar, Ph.D. Thesis, IIT Bombay, India, [26] T.W. Duerig, R. Zando, in: T.W. Duerig, K.N. Melton, D. Stockel, C. M (Eds.), Engineering Aspects of Shape Memory Alloys, Butterworth Heinemann, London, 1990, p [27] J.C. Williams, in: R.I. Jaffee, H.M. Burte (Eds.), Titanium: Sci. and Technology Proc. Second Int. Conf. on Titanium, vol. 3, Planum Press, Boston, 1972, p