Elsevier

Materials Characterization

Volume 151, May 2019, Pages 129-136
Materials Characterization

Microstructural behavior and mechanical properties of nanocrystalline Ti-22Al-25Nb alloy processed by high-pressure torsion

https://doi.org/10.1016/j.matchar.2019.02.029Get rights and content

Highlights

  • The nanocrystalline Ti-22Al-25Nb alloy was first prepared by high-pressure torsion.

  • The phase transformation of B2→O at ambient temperature was observed during high-pressure torsion.

  • The hardening mechanism is attributed to grain refinement and the high density of dislocations induced by HPT process.

Abstract

In the work reported in this paper, high-pressure torsion (HPT) was conducted on Ti-22Al-25Nb alloy under the applied pressure of 6 GPa at ambient temperature in 1, 5, and 10 turns. Investigation of the microstructural characteristics and microhardness evolution of the HPT-processed Ti-22Al-25Nb alloy showed that the lamella B2-phase could be transformed to O-phase at ambient temperature. This was due to severe shear transformation of the B2 lattice caused by the high density of dislocations generated during HPT. Significant grain refinement was achieved, from grain size of ~67 μm in the initial annealed specimen to ~53 nm after 10 turns in the HPT process. In addition to dislocations, nanotwins played an important role in the grain refining process. With increasing accumulation of equivalent strain, the microhardness value of this alloy increased and reached a saturated value of about ~600 HV when the equivalent strain reached ~20. This is higher than that obtained by conventional processes. The hardening mechanisms were primarily attributed to pronounced grain refinement and the high density of dislocations induced by the HPT process.

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 Mgsingle bondRe 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.

References (50)

  • X.C. Zhao et al.

    The processing of pure titanium through multiple passes of ECAP at room temperature

    Mater. Sci. Eng. A

    (2010)
  • M.A. Afifi et al.

    Effect of heat treatments on the microstructures and tensile properties of an ultrafine-grained Al-Zn-Mg alloy processed by ECAP

    J. Alloys Compd.

    (2018)
  • M. Karimi et al.

    Nanostructure formation during accumulative roll bonding of commercial purity titanium

    Mater. Charact.

    (2016)
  • A.P. Zhilyaev et al.

    Using high-pressure torsion for metal processing: fundamentals and applications

    Prog. Mater. Sci.

    (2008)
  • W.T. Sun et al.

    Evolution of microstructure and mechanical properties of an as-cast Mg-8.2Gd-3.8Y-1.0Zn-0.4Zr alloy processed by high pressure torsion

    Mater. Sci. Eng. A

    (2017)
  • A.R. Kilmametov et al.

    The α-w and β-w phase transformations in Ti-Fe alloys under high-pressure torsion

    Acta Mater.

    (2018)
  • G.H. Cao et al.

    Microstructural evolution of TiAl-based alloys deformed by high-pressure torsion

    Acta Mater.

    (2015)
  • A.P. Zhilyaev et al.

    Experimental parameters influencing grain refinement and microstructural evolution during high-pressure torsion

    Acta Mater.

    (2003)
  • H. Shahmir et al.

    Using heat treatments, high-pressure torsion and post-deformation annealing to optimize the properties of Ti-6Al-4V alloys

    Acta Mater.

    (2017)
  • R.B. Figueiredo et al.

    Using finite element modelling to examine the temperature distribution in quasi-constrained high-pressure torsion

    Acta Mater.

    (2012)
  • J. Tiley et al.

    Quantification of microstructural features in α/β titanium alloys

    Mater. Sci. Eng. A

    (2004)
  • J.L. Yang et al.

    High-temperature deformation behavior of the extruded Ti-22Al-25Nb alloy fabricated by powder metallurgy

    Mater. Charact.

    (2018)
  • Z.Q. Zhang et al.

    Microstructure refinement of a dual phase titanium alloy by severe room temperature compression

    Trans. Nonferrous Metals Soc. China

    (2012)
  • D.X. Wei et al.

    Refinement of lamellar structures in Ti-Al alloy

    Acta Mater.

    (2017)
  • D.X. Wei et al.

    Control of γ lamella precipitation in Ti-39 at.% Al single crystals by nanogroove-induced dislocation bands

    Acta Mater.

    (2015)
  • Cited by (0)

    View full text