Elsevier

Intermetallics

Volume 133, June 2021, 107169
Intermetallics

Short communication
γ→β phase transformation in Ti-42.9Al-4.6Nb–2Cr

https://doi.org/10.1016/j.intermet.2021.107169Get rights and content

Highlights

  • γ→β phase transformation occurred during isothermal compression of β-solidifying γ-TiAl alloys.

  • Stacking faults pairs sometimes acted as β/γ phase interface in isothermally compressed samples.

  • Deformation-induced stacking faults pairs were occasionally retained for lower SISF energy and insufficient Nb diffusion.

Abstract

This paper investigates the γ→β phase transformation in a β-solidifying γ-TiAl alloy (Ti-42.9Al-4.6Nb–2Cr, at. %), by means of SEM and TEM. The results show that part of β/γ phase interface is flat, and TEM observations reveal that stacking faults pairs sometimes act as the β/γ phase interface in isothermally compressed samples. The γ→β phase transformation was a diffusional process, and was mainly governed by diffusion of Ti, Al and Cr. The occurrence of stacking faults pairs during γ→β phase transformation at the β/γ phase interface was related to low SISF energy and sluggish Nb diffusion.

Introduction

With the development of aviation industry, lighter and stiffer materials are required for weight-saving and performance improvement of aero engines. γ-TiAl alloys with low density, high specific yield strength and oxidation resistance, are promising materials meeting these requirements [[1], [2], [3]]. Thermomechanical processing is necessary for fabricating γ-TiAl alloys components like low-pressure turbine blades with improved performance [4]. However, the industrial cost of thermomechanical processing is still high due to poor ductility and high-temperature sensitivity.

For this reason, β-solidifying γ-TiAl alloys for improved hot workability were designed [5,6]. Although the improvement of hot workability is mainly ascribed to soft β phase introduced by the addition of Nb or other elements, β phase would decompose to brittle ω phase (Ti4Al3Nb, B82 structure) at a temperature of ~850 °C in most cases, which is detrimental to service performance [7,8]. Xiang et al. [9] reported that α2→β phase transformation occurred when TNM alloy was isothermally compressed at a temperature of 1200 °C, strain rate of 0.0001 s−1 to a true strain of 0.9. Clemens et al. [5] found that the volume fraction of β phase firstly decreased and then increased as the temperature increased from 1220 °C to 1360 °C for TNM alloys (Ti-43.5Al–4Nb–1Mo-0.1B, at%). Phase transformation in β-solidifying γ-TiAl alloys is complicated, depending on chemical compositions, processing conditions and so on.

Thermomechanical processing is frequently conducted in the (α+β/β0+γ) phase field where α→γ phase transformation occurs. Considering the decomposition of β phase at service temperature, it is quite necessary to investigate the possible γ→β phase transformation for precise microstructure control during thermomechanical processing of β-solidifying γ-TiAl alloy. Thus, a commercial β-solidifying γ-TiAl alloy was chosen. Scanning electron microscopy (SEM) in backscattered electron mode was used to investigate the initial and as-compressed microstructure. Moreover, transmission electron microscopy (TEM) was used to investigate the γ→β phase transformation during isothermal compression.

Section snippets

Material and methods

The commercial β-solidifying γ-TiAl alloy was supplied as Ti-42.9Al-4.6Nb–2Cr (atomic percentage), which was obtained by hot-canned extrusion and subsequent heat treatment. The initial microstructure is shown in Fig. 1 a and b. Before optical microscopy (OM) using a Leica DMI3000M OM, the initial Ti-42.9Al-4.6Nb–2Cr was mechanically polished and then quickly etched in a solution of 1 vol% HF+10 vol% HNO3+89 vol% H2O at ambient temperature. As seen from Fig. 1 a and b, the initial microstructure

Results and discussion

Fig. 1 c-d display the microstructure of Ti-42.9Al-4.6Nb–2Cr isothermally compressed at a deformation temperature of 1200 °C, strain rate of 0.01 s−1 to height reductions of 20% and 50%. Compared with Fig. 1 a-b, the as-compressed microstructure presents a near gamma structure, which consists of some remnant α2/γ lamellar colonies (<9.17 vol%) with various orientations against to the compression axis. Although Wei et al. [[12], [13], [14]] reported that both γ lamellae and α nano-twins can be

Conclusions

In conclusion, in specimens isothermally compressed at a deformation temperature of 1200 °C, strain rate of 0.01 s−1 to height reductions of 20%–50%, it is observed that γ grains or γ lamellae encompass with β phase sometimes with a flat β/γ phase interface. Such phenomena are related to the γ→β phase transformation that occurs during isothermal compression of Ti-42.9Al-4.6Nb–2Cr with a duplex structure. The γ→β phase transformation is achieved by the rearrangement of {111}γ planes, and the

Credit author statement

Runrun Xu: Investigation, Conceptualization, Writing. Miaoquan Li: Writing, Validation, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51975478) and the Fundamental Research Funds for the Central Universities (Grant No. 3102019MS0403).

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