Elsevier

Materials & Design

Volume 107, 5 October 2016, Pages 406-415
Materials & Design

Characterization of microstructure evolution in β-γ TiAl alloy containing high content of Niobium using constitutive equation and power dissipation map

https://doi.org/10.1016/j.matdes.2016.06.064Get rights and content

Highlights

  • β-γ TiAl alloys containing high content of Nb have excellent hot-working characteristic.

  • The developed Arrhenius-type constitutive equation reflects the good predicted accuracy.

  • 3D power dissipation maps based on Arrhenius-type constitutive equations and dynamic material model were established.

  • The developed power dissipation maps can predict and characterize microstructure evolution during hot deformation.

Abstract

In order to study the hot deformation behavior and microstructure evolution of β-γ TiAl alloy containing high content of Nb, isothermal hot compression tests were conducted in the strain rate range of 0.001–1.0 s 1 and temperature range of 1273–1473 K. The Arrhenius-type constitutive equation was successfully established for expressing the non-linear relation among true stress, strain, strain rate and deformation temperature. The average absolute relative error and correlation coefficient are 6.009% and 0.9961, respectively, which reflects good predicted accuracy of developed constitutive equation. Conventional and 3D power dissipation maps based on the developed constitutive equations and dynamic material model were successfully established. Efficiency of power dissipation increases with higher deformation temperature, lower strain rate and higher strain, which indicates that more power is dissipated through changing microstructure. These deformed specimens with equal efficiency of power dissipation exhibit similar microstructures during hot deformation. The content of dynamic recrystallization (DRX) grains increases with the increase of efficiency of power dissipation (η). The microstructures with η  0.55 mainly consist of equiaxed DRX grains, corresponding to the temperature range of 1273–1473 K and the strain rate range of 0.001–0.01 s 1 at the strain of 0.5, which could be the optimum hot working window of alloy.

Introduction

Intermetallic TiAl based alloys have the advantages of low density, excellent high temperature strength, good creep properties and high resistance to oxidation, which has extensive applications in aerospace, automotive and energy industries [1], [2], [3]. However, the applications of TiAl based alloy are restricted because of low ductility and fracture toughness at low temperature. Optimizing microstructure and composition has made good progress in improving these mechanical behaviors [4], [5]. TiAl alloys containing high content of Nb exhibit not only excellent oxidation resistance [6], [7] and creep strength [8], but also enhancive tensile strength at room and elevated temperatures [9]. It is reported that the addition of Niobium can increase the critical resolved shear stress (CRSS) for ordinary slip and decrease the stacking fault energy (SFE) [10], which raises the stability of microstructure and enhances the difficulty of dislocation climb in the entire temperature range.

Conventional TiAl alloys containing high content of Niobium mainly consisting of α and γ phases can only be deformed under canned or isothermal conditions [11], [12]. It is found that increasing fraction of β phase can improve hot workability of β-γ TiAl alloys containing high Nb [13], [14], [15], because the disordered β phase with BCC lattice benefits to hot deformation and dynamic recrystallization due to providing more independent slip systems at elevated temperature [16], [17], [18]. The disordered β phase transforms to ordered B2 phase at low temperature [14]. B2 phase can deteriorate room-temperature ductility and creep behaviors [19], and the heat treatment after hot-working can effectively minimize or even eliminate remaining B2 phase [19], [20].

The flow stress study of alloy plays a significant role in determining hot working parameters that influence the microstructure and mechanical behaviors of workpieces [21], [22]. Generally, proper constitutive equations can accurately describe the non-linear relation among true stress, strain, strain rate and deformation temperature. Sellars and Mctegart [23], [24] proposed a hyperbolic sinusoidal Arrhenius-type equation that adapts to a wide range of true stress. The developed constitutive equation of β-γ TiAl alloy containing high content of Nb can be applied to finite element analyses of hot deformation, which improves the accuracy of simulation and shortens research cycle.

The dynamic material model (DMM), based on the principles of irreversible thermodynamics of large plastic flow, has been widely applied to examine hot deformation behavior of metals and alloys [25], [26]. Srinivasan and Prasad [27] proposed that the DMM characterizes deformation behaviors and microstructure evolution of metallic material at elevated temperature. Hot deformation of workpiece is a process of power dissipation, and the total input power is applied to increase temperature and change microstructure, corresponding to the dissipative content (G) and dissipative co-content (J). The optimum hot working condition of strain rate and temperature is maximum value of J. The efficiency of power dissipation (J/Jmax) is related to dynamic microstructural changes during hot deformation [28]. The variation of efficiency of power dissipation, as a dimensionless parameter, with strain, strain rate and deformation temperature constitutes power dissipation map [29]. Comparing with traditional power dissipation map, the 3D power dissipation map can better express the processing conditions of whole deformation process in a better way [30], [31]. The developed power dissipation map based on dynamic material model is employed to optimize hot workability and control microstructure of alloy [32], [33], [34]. Objectives of the present study are that:

  • 1.

    The Arrhenius-type constitutive equations of β-γ TiAl alloy containing high content of Nb are established by analyses of experimental true stress–true strain data;

  • 2.

    The power dissipation maps based on Arrhenius-type constitutive equations and dynamic material model are established at various strains;

  • 3.

    The prediction of microstructure evolution with the power dissipation maps is verified by isothermal hot compression tests.

Section snippets

Experimental

The chemical compositions (at.%) of β-γ TiAl alloy containing high content of Nb used in the present investigation are 45.0Al, 8.0Nb, 2.0Cr, 2.0Mn, 0.5Y and Ti (balance). The initial microstructure consists of dominant γ phase and a little B2 phase identified by SEM-EDS and EBSD techniques. The fraction of B2 phase is about 11.4%, and almost no α phase is found due to the addition of β-stabilizing elements(Nb, Cr and Mn), as shown in Fig. 1, Fig. 2.

Cylindrical specimens of 12 mm height and 8 mm

Flow behavior and constitutive analysis

The experimental flow stress curves of β-γ TiAl alloy containing high content of Nb at different temperatures and strain rates are illustrated in Fig. 3. True stress increases rapidly and reaches the peak value at an initial stage of hot deformation as a result of work hardening. And then the true stress decreases gradually attributing to dynamic recovery and dynamic recrystallization [35]. It is observed that true stress increases with the increase of strain rate and the decrease of

Conclusions

The following conclusions can be drawn based on the study.

  • (1)

    The Arrhenius-type constitutive equation of β-γ TiAl alloy containing high content of Nb was established by experimental flow stress data from isothermal hot compression tests, which conducted in the strain rate range of 0.001–1.0 s 1 and temperature range of 1273–1473 K. The average absolute relative error (AARE) and correlation coefficient (R) are 6.009% and 0.9961, respectively, which reflects the good predicted accuracy of developed

Acknowledgements

This work was supported by the Beijing Natural Science Foundation (2162024) and the National Key Basic Research Program of China (No. 2011CB605502).

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