Key Takeaways & Executive Findings
- •• Directional annealing of Ti43Al alloy in the α single-phase region successfully produces columnar grains up to 22.29 mm in length at 1,350 °C and 8 K·mm-1 temperature gradient. • The process yields fully lamellar microstructures, with grain growth appearing stochastic based on orientation distribution analysis. • Optimal mechanical properties (tensile strength 411.23 MPa, elongation 2.29%) are achieved at 1,400 °C and 8 K·mm-1, while other conditions result in near-zero plasticity. • The study provides a crucible-free alternative to conventional directional solidification, avoiding melt contamination and simplifying processing.
Abstract
Abstract: The directional annealing technique is widely used to prepare columnar grains or single crystals. To investigate the effect of hot zone temperature and temperature gradient on the growth of columnar crystals, Ti43Al alloys were heat treated by the directional annealing technique and their mechanical properties were tested. The results show that columnar grains with a maximum size of 22.29 mm can be obtained at a hot zone temperature of 1,350 °C and a temperature gradient of 8 K·mm-1. During the directional annealing process, Ti43Al alloys are heated to α single-phase domain to start the phase transformation. Columnar grains with a microstructure of fully lamellar colonies are obtained at different hot zone temperatures and temperature gradients. The distribution of the orientation difference for the α2 phase was found to be more random, suggesting that the growth of the columnar crystals may be stochastic in nature. Tensile testing results show that the strength and elongation of directional annealed Ti43Al alloy at 1,400 °C-8 K·mm-1 are 411.23 MPa and 2.29%, and the remaining directional annealed alloys show almost plasticity.
1. Introduction
TiAl-based alloys have a broad prospect and become the optimal candidate for superalloys in the fields of aerospace and gas turbines, due to their attractive properties such as low density, high specific strength, and good creep resistance [1-5]. Especially, the directionally solidified TiAl alloys with columnar grains or single crystals, exhibit excellent room-temperature ductility and enhanced high-temperature mechanical properties [6-7].
Up to the present, mainly four pathways can prepare directionally solidified TiAl alloys: (1) traditional Bridgman solidification [8]; (2) optical floating zone melting (OFZM) [9-10]; (3) electromagnetic constraining shaping [11]; (4) cold crucible directional solidification technique (CCDS) [6, 12]. However, the TiAl alloy melt has great chemical activity, which is easy to react with the crucible materials and introduce impurities [8, 13, 14]. In the OFZM and CCDS, melt contamination can be avoided, but the operation process is complex and the directional microstructure is difficult to control [6, 8, 15].
In recent years, some researchers have developed directional annealing to prepare columnar grains in metals [16-18]. The TiAl specimen is fixed on a lifting platform and induction-heated to a higher temperature below the melting point. As the effective heat treatment area moves, non-preferred grains are eliminated and these grains with specific orientations continuously grow. For instance, Ukai et al. [19] prepared zone annealed in a Ni-based oxide dispersion-strengthened (ODS) superalloy, they found that grains with the texture of cube orientation (001)[0 0] as well as Goss orientation (110)[001] preferentially grew into columnar grains. By controlling the hot zone temperature in β single-phase region and withdrawing rate, columnar grains could be obtained [17]. The plasticity of Ti44Al6Nb1Cr alloy prepared by Liu [20] was improved after directional annealing, which show an improved elongation of 2.3% at room temperature.
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Li Zhou, Jie-ren Yang, Yun-lu Ma, Ze-dong Liu, Rui-run Chen (2025). Microstructural evolution and mechanical properties of Ti43Al alloy by directional annealing. China Foundry. https://doi.org/10.1007/s41230-024-4050-5
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Frequently Asked Questions
What is the maximum columnar grain size achieved in this study?
The maximum columnar grain size achieved was 22.29 mm, obtained at a hot zone temperature of 1,350 °C and a temperature gradient of 8 K·mm-1.
What are the optimal mechanical properties of the directionally annealed Ti43Al alloy?
The optimal mechanical properties were achieved at 1,400 °C and 8 K·mm-1, with a tensile strength of 411.23 MPa and elongation of 2.29%.
Why is directional annealing preferred over traditional directional solidification for TiAl alloys?
Directional annealing avoids melt contamination by eliminating the need for a crucible, simplifying the process and offering better control over the directional microstructure.
What is the significance of the α single-phase region in the directional annealing process?
Heating the alloy to the α single-phase region initiates phase transformation and promotes grain growth, leading to the formation of columnar grains with fully lamellar colonies.
What does the EBSD analysis reveal about the growth of columnar crystals?
The EBSD analysis shows a random distribution of orientation differences for the α2 phase, suggesting that the growth of columnar crystals may be stochastic in nature.
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