B2 grain growth and particle pinning effect of Ti-22Al-25Nb orthorhombic intermetallic alloy during heating process
Highlights
► The B2 grain growth kinetic of Ti-22Al-25Nb alloys has been studied. ► The growth velocity is confirmed not the same for the heating temperature below and above B2-transus. ► The activation energy was increased with extending the holding time in B2 single phase region. ► Zener model is a reliable model to predict the experimental data after taking m = 1, β = 4/9 into account.
Introduction
In recent years, Ti2AlNb based alloys have been of great interest for most potential high-temperature structural materials used in 600 °C–800 °C, which were widely applied in the aerospace field due to attractive properties such as low density, high strength-to-density, good oxidation resistance and nonmagnetic property [1], [2]. Although it still remains some imperfections in terms of ductility, toughness and workability, Ti-22Al-25Nb alloy has excellent potential to be applied in the engineering practice [3], [4]. Thus, much work has been done in order to obtain alloy with the best properties. The study for evolution of crystallographic texture in the orthorhombic phase of Ti-22Al-25Nb alloy [5], the oxidation tests on cast Ti-22Al-25Nb matrix material [6], and the annealing response of the intermetallic Ti-22Al-25Nb alloy [7] are all quite comprehensive and deep.
As is well known, the mechanical properties of the Ti2AlNb alloys are strongly affected by the microstructures [8]. α2, B2 and O phases are the common phases that present in the Ti2AlNb alloys. The O phase exhibits higher tensile strength and better creep properties than the α2 phase [9], [10]. The B2 phase is beneficial to improving its room temperature ductility [4]. The grain size of the B2 phase is one of the most important factors to determine the tensile ductility for Ti-22Al-25Nb orthorhombic intermetallic alloy and the hot workability which is usually carried out at the temperature beyond 1000 °C. In the past few decades, two types of methods are generally used to analyze the grain growth behavior. One type emphasizes the research of grain growth kinetics via measuring average grain size under different temperatures and holding times. Then average grain growth rates or activation energies are calculated [11], [12], [13], [14]. While another type is favor to study the features of microstructural morphology, like crystallographic texture, grain shapes and boundary angles [15], [16].
In this article, the B2 grain normal growth model is investigated in the B2 single phase via measuring average grain size under different temperatures and holding times. However, according to the Ti-22AlxNb phase diagram, there exist an inclusion phase which plays an important role for the B2 grain growth at the temperature below 1060 °C for Ti-22Al-25Nb alloy [17]. Thus it is rather necessary to study the B2 grain growth behavior in this case.
Section snippets
Experiment
The alloy used in the present work with the nominal composition of Ti-22Al-25Nb was provided by Central Iron and Steel Research Institute (CISRI) in a bar form with a diameter of 85 mm. The chemical analysis showed that the cast ingot composition was in good agreement with nominal composition (see Table 1). In order to improve the property of the alloy, the contents of H, O and N were strictly controlled which was confirmed the optimal proportion via experimental and theoretical investigation.
Effect of temperature and holding time on morphology and grain size of B2 phase
Generally, the microstructures of the materials are significantly influenced by the heating temperature and holding time. The investigation of microstructural evolution of materials during different heating processes is quite helpful to validate the mechanisms of the B2 grain growth. Fig. 2 shows the morphology of the alloys at different heating temperatures for a holding time of 30 min. As is seen from Fig. 2(a) and (b), a large amount of fine α2 phase distribute in B2 phase interior and
Conclusion
The behavior of B2 grain growth of Ti-22Al-25Nb alloy was studied and the following conclusions are drawn from this work:
- 1.
The grain size increases with the increasing heating temperature. However, the growth velocity is confirmed not the same for the heating temperature below and above B2-transus. The pinning effect of the α2 particles significantly plays an important role for controlling the grain growth.
- 2.
The B2 grains grow up 23, 34, 53, 61, 85 and 108 μm/h in average as the temperature
Acknowledgments
This work was financially supported by Research Fund for the Docoral Program of Higher Education of China with No. 20116102110015, the New Century Excellent Talents in University with No. NCET-07-0696, and the National 973 Project of China with No. 2007CB613807.
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