Metastable phase and microstructural degradation of a TiAl alloy produced via selective electron beam melting

Highlights

• Metastable Ti₂Al phase was observed in the SEBM-produced TiAl alloy.

• The mechanism of microstructure degradation was illustrated.

• Disordered α phase was observed in the top surface.

Abstract

Selective electron beam melting (SEBM) is an attractive rapid prototyping technology to fabricate TiAl alloys with a complex structure. A near fully dense Ti–47Al–2Cr–2Nb alloy was fabricated by SEBM by using a low-energy density of 20.04 J/mm³. The phase transformation, microstructural degradation and microhardness of the SEBM-produced TiAl alloy were investigated. The results showed that the microstructure was inhomogeneous along the building direction. Multiple phase transformations were observed during the in-situ thermal cycle. Disordered α phase was observed in the last solidifying tracks in the top surface due to the rapid cooling rate. A metastable Ti₂Al phase was observed due to the relatively high residual stress that was caused by a quick cooling rate when fabrication occurred under a low energy density of 20.04 J/mm³. The decomposition of α₂ lamellae, continuous coarsening and discontinuous coarsening of lamellar structure were the three main routes for microstructural degradation for SEBM-produced TiAl alloys. The microhardness exhibited an increasing tendency from a height of 18 mm to the top surface due to the increasing content of the lamellar colony. Finally, the degradation mechanism of the lamellar colony was discussed in detail.

Introduction

TiAl alloys are promising high-temperature structural material in the aerospace field due to their low density, excellent specific strength, good oxidation resistance and excellent creep properties at elevated temperature [[1], [2], [3]]. However, their low room-temperature ductility and poor deformability make it difficult to form complex shapes and limit their extensive applications [4,5].

Many solutions have been suggested to overcome these limitations, including additive manufacturing, or three-dimensional printing technology, which has gained increased attention [[6], [7], [8], [9]]. Selective electron beam melting (SEBM), as an additive manufacturing technique, has a relatively high preheating temperature, which can reduce the residual stress and avoid crack generation, and is considered a suitable method for manufacturing TiAl alloy components [10,11].

The lamellar structure of the TiAl alloy is unstable and degrades above 800 °C [[12], [13], [14]]. The SEBM technique was characterized by layer upon layer to make parts from 3D model data. Phase transformations and the in-situ decomposition of lamellar colonies has resulted in as-built samples based on the SEBM characteristics because of the high preheating temperature (>1000 °C) and the cycle heating and cooling during SEBM. Some studies reported that lamellar structure degradation deteriorated the TiAl alloy mechanical properties [15,16]. Yang et al. [16] investigated the SEBM-produced titanium alloy microstructure and found that an inhomogeneous microstructure resulted along the building direction because of the difference in thermal history for each power layer. Various degradation methods of lamellar structure exist at elevated temperature, which play an important role on microstructural evolution and mechanical properties.

Previous studies indicated that the SEBM process parameters had an important effect on the microstructure and mechanical properties of the SEBM-produced TiAl alloy, and the microstructure transformed from a fine duplex structure to a γ/B2 structure when the energy density increased [17]. Therefore, the phase transformation and microstructural evolution during SEBM were related closely to the process parameters because the process parameters affected the thermal history of the SEBM-produced TiAl alloy samples. Different SEBM process parameters resulted in different heat inputs and thermal histories, which led to different phase transformations and microstructural evolutions. SEBM-produced TiAl alloys with a fine duplex structure exhibited excellent mechanical properties when the energy density input was 20.04 J/mm³ [17]. Chen et al. [18] investigated the phase transformation and microstructural evolution of TiAl alloys during SEBM when the energy density was 24.07 J/mm³.

To the authors’ best knowledge, the intrinsic heat treatment in the SEBM process is critical in the final phase composition. There is a lack of systematic research on the phase transformation at a low energy density during SEBM. Limited information exists on the microstructural degradation mechanism of a lamellar colony of the SEBM-produced TiAl alloy, which made it challenging to achieve fine control of the target performance. Therefore, the phase transformation and degradation mechanism of a lamellar colony of the SEBM-produced TiAl alloy sample that was fabricated at a low energy density should be elucidated.

In this work, a Ti–47Al–2Cr–2Nb alloy was fabricated via SEBM at a low energy density and the SEBM-produced sample microstructure at different heights was characterized. The phase transformation and microstructural degradation mechanism were investigated and the formation mechanism of a metastable Ti₂Al phase was discussed. Finally, the degradation mechanism of a lamellar colony was discussed in detail.