The effect of microstructure on creep behavior of a powder metallurgy (PM) beta gamma alloy

Transcription

1 Journal of Mechanical Science and Technology 26 (7) (2012) 2009~ DOI /s The effect of microstructure on creep behavior of a powder metallurgy (PM) beta gamma alloy D. Y. Seo 1,*, T. Sawatzky 2, H. Saari 2, D. J. Kim 3, P. Au 1 and C. S. Seok 4 1 Aerospace, National Research Council of Canada, Ottawa, ON, Canada 2 Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, Canada 3 Department of Mechanical Engineering, Andong National University, Andong, Korea 4 School of Mechanical Engineering, Sungkyunkwan University, Suwon, Korea (Manuscript Received February 22, 2012; Revised March 16, 2012; Accepted April 10, 2012) Abstract Pre-alloyed beta gamma titanium aluminide powder with a nominal composition of TiAl- 2Nb-2Mo (G2) is consolidated by hot isostatic pressing. After consolidation, a step cooled heat treatment is performed to homogenize the material and produce a fully lamellar microstructure. Various aging heat treatments are then performed to form interfacial beta phase precipitates along lamellar interfaces. The step cooled heat treatment produces a relatively fine microstructure with an average lamellar grain size of 40 μm. The aging heat treatments generate beta phase precipitates along lamellar grain boundaries as well as along lamellar interfaces, and result in limited lamellar degradation and grain growth. However, coarse intergranular grains consisting of beta and gamma grains form during aging. Constant load tensile creep tests are performed on step cooled heat treated and aged specimens. Primary creep resistance, generally, improves with aging time, even with interfacial precipitation, and the limited lamellar degradation occurs with aging. However, total creep life of aged samples decreases with aging time. The microstructures of the tested specimens are characterized and related to the creep behaviour of the TiAl-2Nb-2Mo alloy in the un-aged and aged conditions. Keywords: Beta gamma TiAl alloy; Beta phase; Creep; Powder metallurgy Introduction Titanium aluminides (TiAl) are attractive materials for elevated temperature applications due to excellent high temperature properties and low density, but material processing and component manufacturing are difficult and costly, and have limited their widespread use [1]. Attempts to overcome these issues have led to the development of alloys with high volume fraction of the beta (β) phase [2, 3] and a new class of beta gamma TiAl alloys [4, 5]. These alloys make use of the β phase to improve hot workability and machinability. However, at temperatures greater than 700 C, the presence of the β phase in significant volume fraction is detrimental to strength and creep resistance [2, 3, 6]. Thus, efforts to remove the β phase during final processing steps have been made to improve high temperature performance [7]. Conversely, complete elimination of the β phase is not necessarily advantageous as fine β precipitate particles at the lamellar interfaces have been shown to significantly improve the primary creep * Corresponding author. Tel.: , Fax.: address: dongyi.seo@nrc-cnrc.gc.ca This paper was presented at the ICMR2011, Busan, Korea, November Recommended by Guest Editor Dong-Ho Bae KSME & Springer 2012 resistance of conventional γ-tial alloys by hindering dislocation emission and motion [8-10]. Thus, a final microstructure with limited β phase in the form of precipitate particles at the lamellar interfaces, along with a fine grained fully lamellar microstructure through solution heat treatment and aging was investigated for powder metallurgy (PM) beta gamma TiAl alloys. The microstructural features such as lamellar grains, lamellae, and presence of the β phase or β precipitates are qualitatively correlated with creep performance. 2. Materials and specimens The -35 mesh gas atomized pre-alloy powders with nominal compositions of TiAl-2Nb-2Mo (at%) (G2) alloy were encapsulated in stainless steel tubes under a vacuum of torr to minimize oxygen contamination and consolidated by a twostep hot isostatic pressing (HIP) process (1250 C/200 MPa/1 hour 1100 C/200 MPa/3 hours + slow cooling to room temperature). HIPed and de-canned samples were then given a step cooled solution heat treatment (SCHT) in air (1400 C/40 min + furnace cool 12 C/min to 1280 C + air cool to room temperature). The SCHT samples were given an aging treatment at 900 C for 6 and 24 hours.

2 2010 D. Y. Seo et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2009~2013 Table 1. Summary of creep properties of G2 alloy at 760 o C/270 MPa. Aging time Instantaneous strain (%) Time to 1.0% strain Minimum creep strain rate (/sec) Total creep life Fig. 1. OM and SEM micrographs of G2 alloys in the (a-c) 0 hour, (df) 6 hour and (g-i) 24 hour aged conditions. 3. Experimental procedures Creep specimens were machined by low stress grinding to a nominal gage length and diameter of 22 mm and 4 mm respectively. Constant load creep tests were performed in air at 760 C and 276 MPa. Creep strain was measured with a linear variable displacement transducer (LVDT) equipped extensometer providing a strain resolution of +/ Strain measurements were recorded at 1 second intervals near the start of testing, later increasing to a maximum interval of 30 minutes. Heat-treated and crept samples were examined by both optical (OM) and scanning electron microscopes (SEM) equipped with back scattered detector (BSE) and energy dispersive spectroscopy (EDS). Samples for OM analysis were sectioned and polished using standard metallographic procedures followed by etching in a solution of 25 ml glucose + 12 ml H 2 O + 12 ml HNO ml HF. Samples for SEM analysis were sectioned and prepared by mechanically polishing to 600 grit and then electro-polished for 3 minute using a solution of 64% methanol + 31% butanol + 5% perchloric acid at -45 C and 30V. 4. Results and discussions Fig. 1 shows the microstructures of the G2 alloy in the SCHT (0 hour aged) and SCHT + aged conditions. The SCHT and aged conditions all have a lamellar colony size of about 40 μm. The lamellar grain boundary morphologies are a mix of planar and well interlocked boundaries. While a small amount of retained β particles is present at the lamellar grain boundaries, there are no significant interfacial β precipitates in the 0 hour aged condition, as shown in Figs. 1(b) and 1(c). The average lamellar spacing is approximately 0.04 μm. Aging the G2 alloy for 6 hours results in the formation of fine β phases at grain boundary triple points as shown in Fig. 1(e). As a result of the soft nature of the β phase at elevated temperature, the β morphology at the grain boundary may result in severe grain boundary weakness at high temperatures causing higher strain rates during creep. Interfacial β precipitates with Fig. 2. Effect of aging time on creep response of the G2 alloy at 760 o C in air with a constant stress of 276 MPa. a very fine acicular morphology are also found. After aging for 24 hours, the size of the interfacial β precipitates increased compared to the 6 hour aging condition, as shown in Figs. 1(f) and 1(i). It is noted that there is no indication of severe lamellar grain growth or lamellar degradation in the aged G2 alloy, which is different from the previously investigated TiAl-4Nb- 3Mn (G1) alloy [11]. Transmission electron microscopy (TEM) analysis of these interfacial precipitates indicated that the precipitates are rich in Ti and Mo compared to the gamma (γ) phase or rich in Mo compared to the alpha-2 (α 2 ) phase [12]. From previous studies in conventional PM TiAl alloys [9, 10], only extremely fine interfacial β precipitates retard dislocation motion effectively at the early stage of creep. The creep curves of G2 alloy are shown in Fig. 2 and the creep properties are summarized in Table 1. It was found that different aging resulted in different primary creep strain response, minimum creep strain rate, and creep life. For example, the time to reach 1% creep strain for the 0 hour aged condition is 11 hours, while for the 6 and 24 hours aged conditions the values are 38 and 56 hours, respectively. The increase of the time to reach 1% creep strain increased proportionally with aging up to 24 hours, primarily aided by the reduction of instantaneous strain for the 24 hour aged condition. However, the minimum creep strain rate of the 0 hour aged condition is approximately 9.03x10-9 /s, which is slightly lower than those of the aged conditions (1.43x10-8 /s for both 6 and 24 hours aged conditions). The creep life of the 0 hour aged condition is 512 hours and rupture strain is approximately 14.5%. Overall, the G2 alloy in the 0 hour aged condition exhibits better creep life than the aged conditions. However, better primary creep resistance was achieved in the aged conditions. This behavior can be attributed mainly to the presence, in the G2 alloy, of the β precipitates at the lamellar interfaces as well as at the grain boundaries, even though the lamellar

3 D. Y. Seo et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2009~ Fig. 4. Deformed microstructures of the (a-c) 0 hour, (d-f) 6 hours and (g-i) 24 hour aged conditions. Fig. 3. Comparison of time to (a) 1% creep strain; (b) rupture for G2 alloy with conventional and non-conventional γ-tial and β solidifying γ-tial alloys. grain size is relatively fine. In general, fully lamellar TiAl alloys at elevated temperatures deform primarily through dislocation emission and glide along lamellar interfaces, and the β precipitates limit this dislocation emission and motion during creep [13]. The creep performance of the G2 alloy in this study was compared with those of γ-ti-48al-2cr-2nb alloys conducted by Saari et al. [10, 14], Ti-48Al-2W alloys conducted by Zhu and Seo et al. [9, 15] and β solidifying γ-tial alloys conducted by Lapin et al. [16] as shown in Fig. 3. Test results at 276 MPa reveal that the time to 1% creep strain for G2 alloy is in the same performance bracket as the Ti-48Al2Cr-2Nb alloy and β solidifying γ -TiAl alloys. A comparison between the Larson-Miller Parameter (LMP) with aging shows that the 24 hour aged condition performs better than the Ti-48Al-2Cr-2Nb+W alloy with the exception of the Ti-48Al2W alloy (SCHT+ 5 hour aging) [9]. When comparing the LMP for overall creep life as shown in Fig. 3(b), a reversal in performance is observed with the SCHT heat treatment providing the longest time to rupture in the G2 alloy. At the same stress level, although the Ti-48Al-2W alloys [15] exhibits higher performance of rupture life, the G2 alloys perform adequately compared with conventional γ-tial. Fig. 4 shows the deformation microstructures of crept samples in the 0 hour, 6 hour, and 24 hour aged conditions with the applied load in the vertical direction. It shows substantial voids formed and coalesced at grain boundaries where coarse β and γ grains were observed for all aging conditions. It is noted that in the 24 hour aged condition a number of coarse grains at the grain boundaries were formed before creep. During creep, grain growth did not occur for the fully lamellar structures, however, equiaxed β and γ growth is evident along lamellar grain boundaries, especially in the aged conditions as shown in Figs. 4(e) and 4(h). In addition, lateral dissolution of the lamellar structure was limited during creep for the unaged condition, which is different from TiAl-4Nb-3Mn (G1) alloy in the same heat treatment condition [11]. It is noted that in the 0 hour aged condition fine interfacial β precipitates were formed during creep due to dynamic precipitation, which is similar microstructrural evolution during creep reported previously [17, 18]. Previous studies have found that α2 dissolution during the α2 to γ transformation may lead to dislocation generation that accelerates primary creep in unaged fully lamellar TiAl alloys [19]. Also, lateral dissolution of the α2 lath may occur during high temperature exposure as a result of a reduction in interfacial energy [20]. Therefore, it can be speculated that in the 0 hour condition, primary creep strain rate is relatively high compared to the aged condition due to the lack of the interfacial precipitates and α2 dissolution, but the strain rate is decreasing gradually with increasing time because of dynamic interfacial precipitation, eventually exhibiting lower minimum creep strain rate than that of the aged conditions as shown in Fig. 2 and Table 1. Also, less growth of equiaxed β and γ grains at grain boundaries in the 0 hour aged condition are expected, which will lead to limited nucleation sites for the void formation at tertiary creep stage. However, in the aged condition, these interfacial precipitates and equiaxed β and γ grains coarsened during creep as shown in Figs. 4(e), 4(f), 4(h) and 4(i). Previous studies indicated that tertiary creep strain in the aged Ti-48Al-2W alloy predominantly occurred in lamellar grain boundary regions, producing extensive dislocation and twinning activity, and causing local hardening and strain discontinuities across grain boundaries [13, 17]. Eventually, the presence of the relatively coarse intergranular grains acted as an effective void initiation sites leading to the formation of intergranular voids during tertiary creep. Similarly, in the aged G2 alloys the propensity for intergranular void formation increased during tertiary creep due to coarsening of β and γ grains at lamellar grain boundaries in Fig. 1(e) and 1(h). Therefore, the more extensive tertiary creep and longer creep life of the unaged sample is a result of more balanced accommodation of creep strain between lamellar and

4 2012 D. Y. Seo et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2009~2013 intergranular regions. In constrast, for the aged samples, larger intergranular β and γ grains and a more resistant lamellar structure with interfacial precipitates resulted in reduced tertiary creep life even though there was an improvement of primary creep resistance. Given these results, excellent primary creep performance was achieved for the 24 hour aged condition, while superior rupture life was achieved for the 0 hour aged condition. An intermediate aging time of 6 hour reveals a trade-off between rupture life and primary creep, indicating a heat treatment cycle has not been fully optimized yet for the G2 beta gamma TiAl alloy. Therefore, the controlling initial microstructure of beta gamma G2 alloy will be a key factor if both primary creep and creep life is concerned. In addition, an optimal microstructure for a good balance between primary creep strain and rupture life would contain a lamellar structure with continuous α 2 lamellae, fine interfacial precipitates, and limited intergranular grains probably achieved by an optimized aging time between 0 hours and 6 hours or potentially lower aging temperature. 5. Summary and conclusion (1) The PM beta gamma TiAl-2Nb-2Mo (G2) alloy in the unaged condition exhibited a fully lamellar microstructure with grain size and lamellar spacing averaging 40 μm and 0.04 μm, respectively, which is substantially fine fully lamellar structure compared to conventional PM TiAl alloys as well as investment-cast TiAl alloys. (2) With aging, no lamellar grain coarsening, β and γ grains growth at the grain boundaries, interfacial β precipitates occurred, and limited lateral dissolution of the α 2 lamellae were observed in the G2 alloy. (3) During creep, grain growth did not occur for the fully lamellar structures in the unaged and aged conditions. The primary creep resistance of the aged conditions was improved by interfacial precipitates acting as a barrier to dislocation motion during creep, while creep lives of the aged condition were reduced possibly because of coarsening of β and γ grains at lamellar grain boundaries, leading to the formation of intergranular voids during tertiary creep. (4) In the unaged condition, fine interfacial β precipitates were formed during creep due to dynamic precipitation resulting in lower minimum creep strain rate than that of the aged conditions. Also, limited growth of equiaxed β and γ grains at grain boundaries leads to less nucleation of void at tertiary creep stage resulting in longer creep life compared to the aged condition. (5) Given these results, it can be suggested that controlling the initial microstructure of beta gamma G2 alloy is a key factor if both primary creep and creep life are a concern. Also, it can be suggested that a lamellar structure with continuous α 2 lamellae, and fine interfacial precipitates would be an optimal microstructure for a good balance between primary creep strain and rupture life. Acknowledgment This research has been supported by NSERC through the Discovery Grants and Post Graduate Scholarship Programs. The authors would like to express their appreciation to the Institute for Aerospace Research, National Research Council Canada, for making available the facility for microstructural characterization. References [1] A. Lasalmonie, Intermetallics, 14 (2006) [2] T. Tetsui, K. Shindo, S. Kobayashi and M. Takeyama, Scripta mater., 47 (2002) 399. [3] T. Tetsui, K. Shindo, S. Kaji, S. Kobayashi and M. Takeyama, Intermetallics, 13 (2005) 971. [4] J. S. Kim, Y. H. Lee, Y. -W. 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5 D. Y. Seo et al. / Journal of Mechanical Science and Technology 26 (7) (2012) 2009~ Dongyi Seo received his Ph.D in Materials Science from Michigan State University in During his Ph.D program, he was honoured at the 2nd International Symposium on Structural Intermetallics for the Best Paper Award in Dr. Seo is currently an associate research officer in the Aerospace, National Research Council Canada and managing several major projects on characterization and evaluation of high temperature materials and coatings, and repair technology. Also, he is an adjunct professor at Department of Mechanical and Aerospace Engineering at Carleton University. He was a chair of Ottawa Chapter of ASM international in 2004/2005 and is acting as a treasure since 2006.