Microstructure of directionally solidified Ti–Fe eutectic alloy with low interstitial and high mechanical strength
Highlights
► Moving electric arc setup was applied to directionally solidify Ti–Fe eutectic. ► Avoids oxygen contamination and produces aligned eutectic microstructure. ► Orientation relationship between β-Ti and TiFe phases is (1 1 3)β‖(1 1 3)TiFe. ► Microhardness values increased as the solidification rate increased. ► Samples presented compressive strength up to 3000 MPa and ductility up to 25.2 ± 2%.
Introduction
Structural materials such as commercially pure (CP) titanium and its alloys are extremely important in a number of sectors, particularly in the transport, chemical, energy generation and biomaterial industries [1], [2], [3], [4], [5]. Although CP titanium possesses interesting properties, especially a good strength-to-weight ratio, high biocompatibility and enhanced corrosion resistance, its mechanical strength is relatively limited. However, its mechanical properties can be enhanced by alloying it with other elements. The mechanical behavior of the resulting alloys depends on the amount and type of alloying elements and processing routes applied. In this context, the performance of a structural material can be improved considerably by tailoring its microstructural morphology and the nature of its stabilized phases.
An approach to optimize structural materials involves combining Ti and other elements, which leads to an eutectic transformation and results in the in situ production of composites from the liquid phase [6]. In situ composites obtained by directional solidification processing of eutectic alloys have more attractive and special characteristics than their constituent phases. In this technique the eutectic liquid phase decomposition is used to produce two or more solid phases, which results in a refined microstructure whose phases are arranged side by side [7].
Eutectic transformation in the Ti–Fe system can be employed to produce in situ composite materials whose enhanced mechanical strength renders them potentially interesting structural materials. In composite materials obtained by eutectic solidification, the matrix is reinforced by means of eutectic precipitation. In these cases, the eutectic structure is composed of regularly dispersed reinforcing phase incorporated into the matrix. According to the Ti–Fe phase diagram shown in Fig. 1 [8] an eutectic phase transformation occurs at 1085 °C of a material with a composition of 32.5 wt% Fe whose liquid solidification gives rise to βTi solid solution phase and TiFe intermetallic compound. TiFe is stable at room temperature–1317 °C and presents a Pm-3m structure similar to that of CsCl, with a lattice parameter of a=0.2975 nm. The periodicity of the eutectic array depends on aspects of the growth of its solid phases and of solute fluxes across the solid/liquid interface [7].
Experimental results of the microstructure and mechanical properties of as-cast Ti–Fe revealed a mechanical strength of 2.2 GPa and ductility of 6.7%, with a microstructure composed of TiFe intermetallic compound and βTi phase [9]. Recent attempts to improve the mechanical behavior of Ti–Fe alloy have focused on the addition of alloying elements to this binary eutectic alloy. The effect of the addition of Sn to the Ti–Fe eutectic on its mechanical strength and ductility was investigated by Das et al. [10]. It was found that adding Sn decreased the strength slightly but increased the ductility, while the changes in mechanical properties were attributed to changes in morphology and phase distribution in the eutectic microstructure [10]. In another study, the addition of Sn was found to result in ultrafine composites with fracture strength of 2350–2650 MPa and plasticity of 7.4–12.5% under compression [11]. Another study revealed that the addition of V leads to microstructural refinement and formation of metastable ω-phase. While a small amount of V added to the Ti–Fe eutectic alloy improved both its strength and plasticity a large amount of V resulted in the precipitation of ω-phase leading to the degradation of the alloy's mechanical properties, especially ductility [12]. The addition of Ga to Ti–Fe eutectic was recently evaluated by Misra et al. [13], whose experimental results suggested that the composites obtained in situ exhibit fracture strength of 2290–2766 MPa and plasticity of 4.8–6.5% under compression. The addition of In and Nb was also tested recently and the results suggested that alloys with a mechanical strength of about 2350 MPa and a ductility of 4.5% were obtained. An evaluation of the microstructure showed βTi dendrites as a primary phase and Ti (Fe, In and/or Nb)+βTi as the eutectic structure [14].
Very recently, Schlieter et al. [15] investigated the effect of solidification conditions on the microstructural and mechanical properties of Ti–Fe binary eutectic cast in different conditions. Samples were also solidified directionally by the Bridgman technique in Al2O3 crucibles coated with boron nitride to minimize oxygen contamination. X-ray diffraction results indicated precipitation of an oxygen solid solution phase (TiFeO solid solution), which may alter the properties of the resulting samples. Oxygen strongly affects the microstructure and mechanical behavior of Ti alloys; hence, its content should be strictly controlled [16]. Therefore, the accurate evaluation of the properties of a directionally solidified Ti–Fe eutectic sample is possible only in the absence of oxygen contamination.
Considering the potential use of Ti–Fe alloy as a structural material with enhanced mechanical properties, the purpose of this work was to prepare, process, and characterize directionally solidified Ti–Fe eutectics. In this work the directional solidification process was carried out in a novel setup that employs a water-cooled copper crucible combined with a moving voltaic electric arc under an inert atmosphere, thus preventing oxygen contamination.
Section snippets
Experimental procedure
The ingot samples of the Ti–Fe system studied here had 32.5 wt% of Fe content. They were prepared in an arc melting furnace using a nonconsumable tungsten electrode and water-cooled copper crucible in a high purity argon atmosphere. These ingots were remelted eight to ten times to ensure the homogeneity of their chemical composition and were produced from high purity Grade 2 Ti and high purity Fe (99.99%). The mass loss caused by melting the samples in the arc furnace was found to be less than
Results and discussion
The thermal behavior of the Ti–Fe eutectic alloys was investigated initially by the DTA technique. This technique was employed to evaluate phase transformations including eutectoid and eutectic transformation, as well as to explore possible deviations of the samples equilibrium eutectic compositions. The results presented in Fig. 3 show a broad endothermic peak at 620 °C related to eutectoid transformation, a more pronounced endothermic peak at 1080 °C due to eutectic transformation and a minor
Conclusions
A new experimental setup was applied to obtain directionally solidified Ti–Fe eutectic alloy based on a water-cooled copper crucible combined with a moving voltaic electric arc under inert atmosphere. The results indicated that the use of this type of equipment for directional solidification experiments prevented interstitial contamination by oxygen and produced a well aligned eutectic microstructure, representing an improvement over the classical Bridgman technique using ceramic crucibles.
Acknowledgments
The authors gratefully acknowledge the Brazilian research funding agencies FAPESP (State of São Paulo Research Foundation) and CNPq (National Council for Scientific and Technological Development) for their financial support of this work and the Laboratory of Electron Microscopy (LME) of the Brazilian Synchrotron Light Laboratory (LNLS) for the use of its Jeol JEM 2100 HTP microscope (TEM-MSC 200 kV).
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