Microstructural behavior and mechanical properties of nanocrystalline Ti-22Al-25Nb alloy processed by high-pressure torsion
Introduction
Ti2AlNb-based alloys are high-temperature structural materials considered potentially useful due to their attractive properties, such as high strength-to-weight ratio, great fracture toughness, and excellent workability [[1], [2], [3]]. As a second generation Ti2AlNb based alloy, the Ti-22Al-25Nb alloy exhibits orthorhombic O (Cmcm symmetry based on Ti2AlNb), hcp α2 (Ti3Al, DO19 structure), and β/B2 (disordered/ordered structure) phases [4,5]. In previous studies, it was reported that the mechanical properties of the Ti-22Al-25Nb alloy are affected extensively by their microstructural factors such as phase content, morphology, texture, etc. [[6], [7], [8], [9], [10]]. Depending on the microstructure, tensile elongation of Ti-22Al-25Nb alloy at room temperature can vary from 0 to 16%, while the yield strength can be tailored from 650 to 1600 MPa [11]. Several metal-working techniques have been used in attempts to refine the grain structure and thereby improve the mechanical properties. Unfortunately, it is generally impossible to achieve a nanostructured alloy by conventional thermomechanical processing due to the limited accumulation of strain and the non-equilibrium feature of the grain boundaries [12,13].
Recently, a number of severe plastic deformation (SPD) processes, such as equal-channel angular pressing (ECAP) [14,15], accumulative roll bonding (ARB) [16], and high-pressure torsion (HPT) [[17], [18], [19], [20]], have been developed to achieve exceptional grain refinement in many metallic materials. Compared to the other SPD processes, the HPT process is advantageous because of its simpler operating mechanism, finer grain structures [21], and grain boundaries with larger fractions of high misorientation angles [22]. For example, Sun et al. [18] obtained nanostructured MgRe alloy with grain size of ~55 nm after 16 turns of HPT. Shahmir et al. [23] studied the microstructural behavior of Ti-6Al-4V alloy during HPT and found that the grain size could be refined to 30–40 nm after 10 turns. However, little attention has been paid to the HPT process with Ti-22Al-25Nb alloys. Therefore, it is certainly important and worthwhile to investigate systematically the evolution of microstructure and resultant mechanical properties of nanocrystalline Ti-22Al-25Nb alloys.
In the work reported in this paper, Ti-22Al-25Nb alloys, prepared by elemental powder metallurgy and hot extrusion, were subjected to the HPT process. The influence of the HPT parameters on the phase transformation, evolution of microstructure, and mechanical properties were investigated. In particular, the refining mechanism and the relationship between equivalent strain and micro-hardness were analyzed in detail.
Section snippets
Experimental procedure
Considering the large difference of the main constituent elements in the aspects of melting point, density and diffusion coefficient, elemental powder metallurgy and subsequent hot extrusion was selected as the starting materials used in the present Ti-22Al-25Nb (at.%) alloy [12]. First, Ti, Al, and Nb powders were low-energy planetary milled (QM-3SP4) at a speed of 190 rpm for 4.0 h under argon protection with an atomic ratio of 53:22:25. The blended powders were then step sintered at
Initial microstructure
Fig. 2 shows the initial microstructure of the Ti-22Al-25Nb alloy before HPT process. Due to annealing at 850 °C for 12 h releasing the residual stress, the microstructure is composed of B2-phase and O-phase. A large number of lamellar gray O-phase particles are precipitated in white B2 grains. The O-phase and B2-phase present alternate distribution as shown in Fig. 2b. The extruded alloy has an average B2 grain size of ~67 μm according to a previous study [26].
XRD analysis of the HPT-processed samples
Fig. 3a depicts XRD patterns of
Conclusions
In this study, the Ti-22Al-25Nb alloy prepared using elemental powder metallurgy and subsequent hot extrusion, was subjected to HPT processing at ambient temperature under 6 GPa of pressure with 1 to 10 turns. The influences of the strain imposed on both microstructure refinement and mechanical properties were investigated. The conclusions are summarized as follows:
- (1)
The phase transformation of B2 → O at ambient temperature was observed for the first time. This could be due to shear
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. 51875122) and the National Research Foundation of Korea (No. 2017R1A2A1A17069427). This work also thanks the China Scholarship Council.
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