BHM, Jvn. Jg. (2014), Heft XX © Springer-Verlag Wien Klein et al.

Originalarbeit

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BHMDOI 10.1007/s00501-014-0266-9© Springer-Verlag Wien 2014

Phase Transformations and Grain Growth in β-stabilized TiAl Alloys

Thomas Klein1, Salar Niknafs2, Rian Dippenaar2, Helmut Clemens1 and Svea Mayer1

1Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben, Leoben, Austria2Faculty of Engineering, University of Wollongong, Wollongong, Australia

1. Introduction

Advanced intermetallic γ-TiAl based alloys, such as the TNM alloy with a compositional range of Ti-(42–44)Al-(3–5)Nb-(0.1–2)Mo-(0.1–1)B (in at%), are considered as seminal materials for high-temperature lightweight applications [1, 2]. The development of these materials is driven by an increasing ecological awareness, in particular the demand for a significant reduction of pollutant generation by auto-mobile combustion engines and aircraft engines [3].

Advantages of γ-TiAl based alloys are, next to a low density of about 4.1gcm−3, a high oxidation resistance, a high creep resistance, high specific strength and modu-lus retention, and resistance against “Titanium Fire” [1]. However, some inherent drawbacks that have to be over-come, caused by the intermetallic character of all cons-tituent phases, are their low ductility and brittleness [1, 2]. Thus, enhanced microstructural control is required to achieve a fine grained, isotropic, and homogeneous microstructure. Foremost impacts on the final micros-tructure are solid-state phase transformations and grain growth processes elapsing during heat treatments. This study aims to illustrate the kinetics of grain growth during isothermal annealing in the single β-phase region and to contribute to the understanding of the β to α phase trans-formation in dependence of the cooling rate in the TNM alloy system by the use of HTLSCM.

2. Experimental

The experiments were carried out on material supplied by GfE Metalle und Materialien GmbH, Nuremberg, Ger-many. For details on the manufacturing technique see [1].

For the HTLSCM investigations bars of 8 mm in diam-eter were eroded out of the ingots. These bars were cut to disks of 1.4  mm thickness, which were subsequently ground and polished. All experiments were carried out on

Abstract: Solid-state phase transformations and grain growth of an intermetallic γ-TiAl alloy were investigated in-situ using high-temperature laser scanning confocal microscopy (HTLSCM). During isothermal annealing in the single β-phase region, significant grain coarsening was observed. On cooling beneath the β-transus tem-perature with different rates, a CCT diagram was evalu-ated for the initiation of β to α phase transformation and changes in the morphology were observed.

Keywords: Laser scanning confocal microscopy, Phase transformations, Titanium aluminides

Phasenumwandlungen und Kornwachstum in β-stabilisierten TiAl Legierungen

Zusammenfassung: Phasenumwandlungen und Korn-wachstum wurden in einer intermetallischen γ-TiAl Legie-rung in-situ unter Verwendung eines Hochtemperatur Kon-fokalen Laser-Scanning-Mikroskops untersucht. Während isothermer Glühungen im β-Einphasenfeld trat erhebli-ches Kornwachstum auf. Bei Kühlung unter die β-transus Temperatur mit unterschiedlichen Abkühlraten wurde ein ZTU-Diagramm für die β/α-Phasenumwandlung aufge-nommen und die Morphologie der gebildeten α-Phase wurde untersucht.

Schlüsselwörter: Konfokale Laser-Scanning Mikroskopie, Phasenumwandlungen, Titanaluminide

Dipl.-Ing. T. Klein ()Department of Physical Metallurgy and Materials Testing, Montanuniversität Leoben,Franz-Josef Straße 18,8700 Leoben, Austriae-mail: thomas.klein@unileoben.ac.at

Received 18 April 2014; accepted 28 April 2014

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Originalarbeit

Klein et al. © Springer-Verlag Wien BHM, Jvn. Jg. (2014), Heft XX

a LSCM type 1LM21 H supplied by Lasertec Cooporation, Japan. For supplementary information on the LSCM tech-nique see [4].

3. Results and Discussion

Specimens were annealed in the infrared furnace of the HTLSCM at 1405 °C. At this temperature a single β-phase field region is present. A region in the vicinity of the sam-ple center (at a constant temperature) was mapped and the grain size evolution was determined using mean linear intercept method. Grain growth occurs rapidly after a period of initiation, due to the high state of thermal activa-tion. Grain boundaries were often observed to move in a staggered motion, which might be due to pinning effects by borides or thermal grooves.

When cooling the sample from 1405 °C beneath the β-transus temperature, the disordered hexagonal α-phase is formed. The β to α phase transformation was observed at different cooling rates, which yielded different mor-phologies. At slow cooling rates, close to equilibrium the α-phase grew in the form of equiaxed grains (depicted in Fig. 1, growth fronts are marked by arrows), whereas at high cooling rates lath-like structures developed. Fig-ure 1a was taken at 1375 °C and Fig. 1b at 1368 °C. Nucle-ation mainly took place at grain boundaries, triple points and borides. A temperature calibration enabled the quan-titative analysis of transformation temperatures. Thus, a CCT diagram could be derived for the initiation of β to α phase transformation. Within the cooling rates applied, the transformation mechanism remained diffusive.

4. Conclusion

HTLSCM was successfully used for the study of solid-state phase transformations and grain growth in complex multi-phase γ-TiAl alloys. Results demonstrate that long annea-ling periods within the single β-phase field region have to be avoided due to significant grain coarsening. Further-more, cooling rate influence on the morphology of the β to α phase transformation was studied. The transformation temperatures were additionally analyzed, and a CCT dia-gram was derived. Results obtained can be employed for the improvement of industrial heat treatments.

Literatur

1. Clemens, H.; Mayer, S.: Design, processing, microstructure, prop-erties, and applications of advanced intermetallic TiAl alloys, Advanced Engineering Materials, 15 (2013), no. 4, pp 191–215

2. Kim, Y.-W.: Microstructural evolution and mechanical properties of a forged gamma titanium aluminide alloy, Acta metallurgica et materialia, 40 (1992), no. 6, pp 1121–1134

3. Lavery, N.P.; Jarvis, D.J.; Voss, D.: Emission mitigation potential of lightweight intermetallic TiAl components, Intermetallics, 19 (2011), no. 6, pp 787–792

4. Phelan, D.: In-situ studies of phase transformations in iron alloys, PhD Thesis, University of Wollongong, Australia, Department of Materials Engineering, 2002

Fig. 1: LSCM micrograph of the evolving α-phase (arrows) during con-tinuous cooling with a rate of 0.03°Cs−1. Image a was taken at 1375 °C and b was taken at 1368 °C