Influence of cooling rate on microstructure formation during rapid solidification of binary TiAl alloys

doi:10.1016/j.jallcom.2015.03.016

Journal of Alloys and Compounds

Volume 637, 15 July 2015, Pages 242-247

https://doi.org/10.1016/j.jallcom.2015.03.016 Get rights and content

Highlights

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Rapid solidification studies with varying cooling rates were realized for Ti–Al.
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Experiments were combined with finite element simulations of heat transfer.
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The resulting microstructure of Ti–Al alloys is strongly dependent on the Al content.
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The microstructure and phase transformation behavior can be predicted.
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The method allows alloy development for processes involving rapid solidification.

Abstract

Titanium aluminides as structural intermetallics are possible candidates for a potential weight reduction and increased performance of high temperature components. A method for the characterization of the microstructure formation in rapidly solidified alloys was developed and applied for binary Ti–(44–48)Al (at.%). The results show a strong dependency of the microstructure on the Al content at cooling rates between 6 ⋅ 10² and 1.5 ⋅ 10⁴ K s⁻¹. The formation of α → α₂ ordering, lamellar α₂ + γ colonies and interdendritic TiAl γ-phase were observed, depending on the Al amount. Based on thermodynamic calculations the observed microstructure can be explained using the CALPHAD approach taking into account the non-equilibrium conditions. The presented method provides a useful tool for alloy development for processing techniques involving rapid solidification with varying cooling rates.

Introduction

Titanium aluminides are structural intermetallics that are promising candidates for high temperature applications [1], [2]. The binary Ti–Al phase diagram shows several intermetallic phases [3] (cf. Fig. 1). However, only the Ti₃Al α₂- and the TiAl γ-phases have been found to be of engineering importance. Furthermore, these two phases show the largest solubility range of intermetallic phases, whereas all other phases have rather limited solubility or are even line compounds. The range of technical alloys is restricted to the α₂ + γ phase field due to the formation of a lamellar structure leading to enhanced toughness. The alloy properties strongly depend on the phase morphology [2].

The emerging field of additive manufacturing (AM) requires alloys that are suitable for rapid solidification and subsequent cooling at small melt pool dimensions as well as multiple reheating and cooling cycles. Some attempts on the AM of titanium aluminide alloys have been reported recently [4], [5], [6]. While many experimental data are available regarding the phase evolution during solid state heat treatments and subsequent quenching [1], [7], [8], [9], [10], comparably little is known about microstructure formation during rapid solidification [11], [12], [13], [14]. The Ti–Al system is well studied in the solid state up to cooling rates of 400 K s⁻¹ [7], [8]. However, the developed continuous cooling diagrams do not take into account the solidification step and established knowledge of rapid solidification is typically restricted to cooling rates above 10⁵ K s⁻¹. Several studies using undercooled levitated alloys established the phase selection principles at high solidification speed and moderate cooling rates [15], [16], [17], [18]. These studies rely on the absence of nucleation sites to achieve high undercooling, which is not realistic for AM. The AM of titanium aluminide alloys is still at its infancy for several reasons. First, titanium aluminides are inherently brittle at room temperature rendering them susceptible for severe cracking during processing due to thermal stresses. Second, many alloys have pronounced element segregation during casting [1], an effect that usually increases with a higher cooling rate. Furthermore, titanium aluminides undergo several solid state phase transformations. These transformations may be altered or completely suppressed at the typical cooling rates of 10³–10⁴ K s⁻¹ of laser-based AM techniques and metastable phase relations can occur [11]. In this work, we systematically studied the microstructure and the phase evolution during rapid solidification of binary Ti–(44–48)Al (at.%) alloys at defined cooling rates between 6 ⋅ 10² and 1.5 ⋅ 10⁴ K s⁻¹. For this, we developed a method which combines rapid solidification experiments of small alloy samples with finite element simulations of the temperature evolution in the samples and thermodynamic simulations using the CALPHAD method.

Section snippets

Materials and Methods

The Ti–(44–48)Al (at.%) master alloys for the rapid solidification experiments were produced by non-consumable tungsten electrode arc melting in water cooled Cu crucibles in 500 mbar Ar atmosphere, purified by an OXISORB cartridge (Messer). As starting materials Ti (99.98%) and Al (99.999%) from Alfa Aesar were used. Every alloy was re-melted eight times to ensure good sample homogeneity. No chemical analysis was performed because the material losses after arc melting were <0.3%. The rapid

Results and discussion

The resulting microstructures of the different alloys show a strong dependency on the Al content. According to the equilibrium phase diagram, all alloys have an α₂ + γ microstructure with varying amounts of α₂ and γ. Thermodynamic calculations and previous studies confirm that the Al-lean alloys (<44.5 at.% Al) show full solidification via the bcc β-Ti phase while high Al containing alloys undergo a high-temperature peritectic reaction involving the bcc β-Ti, the hcp α-Ti and the liquid phase [1].

Conclusion

In summary, it was shown that:

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Systematic rapid solidification studies with varying cooling rates can be realized by a combination of experiments with heat transfer analysis based on finite element simulations.
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At the studied cooling rates, the resulting microstructure of binary Ti–Al alloys is strongly dependent on the Al content. At Al $⩽$ 45 at.% the alloys undergo α → α₂ ordering, while intermediate alloys with 45 < Al < 48 at.% show a mixed microstructure consisting of coarse α/α₂ grains, local γ-phase

Acknowledgement

The authors thank Dr. M. Koster for the high-speed camera measurements and Dr. T. Ivas and A. Lis for assistance with the finite element modeling.

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Influence of cooling rate on microstructure formation during rapid solidification of binary TiAl alloys

Highlights

Abstract

Introduction

Section snippets

Materials and Methods

Results and discussion

Conclusion

Acknowledgement

J. Alloys Comp.

Intermetallics

J. Mater. Process. Technol.

Mater. Lett.

Mater. Sci. Eng. A

Scr. Metall. Mater.

Acta Met. Mater.

J. Alloys Comp.

Scr. Metall.

Scr. Metall.

Acta Metall.