Elsevier

Acta Materialia

Volume 60, Issues 13–14, August 2012, Pages 5067-5078
Acta Materialia

On the formation of an ultrafine-duplex structure facilitated by severe shear deformation in a Ti–20Mo β-type titanium alloy

https://doi.org/10.1016/j.actamat.2012.05.042Get rights and content

Abstract

Severe plastic deformation in the form of equal channel angular pressing (ECAP) has been adopted to introduce severe shear strain into a Ti–20 wt.% Mo β-type titanium alloy to elucidate the aging response of the severely deformed β matrix. Upon isothermal aging in the (α + β) phase field, selective heterogeneous α nucleation and growth resulted in a mixed precipitation microstructure. An ultrafine-duplex (α + β) structure composed of equiaxed α precipitates formed inside the shear bands (SBs) created during ECAP, whereas acicular α precipitates were favoured outside the SBs. This distinct precipitation structure has been correlated to the structural characteristics of the SBs: high disorder with dislocation cells characteristic of low-angle boundaries and enhanced atomic diffusivity. The highly disordered structure results in a weak variant selection and thereby promotes randomly orientated α precipitation without obeying the Burgers orientation relationship. Furthermore, the enhanced atomic diffusivity facilitates rapid growth of the α nuclei to form the ultrafine-duplex (α + β) structure.

Introduction

The age-hardening characteristics of metastable body-centered cubic (bcc) β-Ti alloys originate from the precipitation of the hexagonal close-packed (hcp) α phase in the β matrix upon thermomechanical processing or aging [1], [2], [3], [4], [5]. A microstructure composed of uniformly dispersed fine α precipitates is desirable for attaining ultra-high strength. The main strategy adopted to promote uniform α precipitation is to generate a large number of heterogeneous nucleation sites by means of hot/cold deformation or forming intermediate metastable phases prior to α nucleation. The potential nucleation sites for α in β include dislocations, grain boundaries (GB), sub-grain boundaries [4], and interfaces between β and secondary phases such as ω and β′ [1], [5]. ω-Assisted α nucleation has been reported in a number of β titanium alloy systems [1], [5]. For alloys with large β/ω misfit (such as Ti-V), cuboidal ω is dominant and α tends to nucleate on ledges and misfit dislocations at the β/ω interfaces [1], [5]. On the other hand, for alloys with low β/ω misfit (such as Ti–Mo and Ti–Nb) ellipsoidal ω is often observed but seldom acts as active sites for α nucleation [1], [5]. Furthermore, for alloys with low β/ω misfit, ω-assisted α nucleation strongly depends on the heating rate during aging [5]. A higher heating rate is found to retard the formation of ω and dramatically reduce the number of potential nucleation sites, leading to a non-uniform dispersion of coarse α precipitates in the β matrix [6].

An alternative but commonly adopted approach is the introduction of hot/cold deformation prior to aging to create a high density of dislocations and a large number of grain and sub-grain boundaries to accelerate the kinetics of α precipitation and to attain a uniform dispersion of fine α precipitates in the β matrix [3], [4], [7], [8], [9]. Upon slight cold rolling, a large number of slip bands were generated in a Ti–15V–3Cr–3Sn–3Al β alloy and subsequent aging resulted in a preferential nucleation of acicular α precipitates on dislocations in the slip bands [4], [7]. On the other hand, cold rolling to a large strain (>50%) led to a mixed aging microstructure with the concurrence of acicular α precipitates in the β matrix and an ultrafine-duplex structure composed of equiaxed α precipitates in the β matrix [4], [7]. The volume fraction of the ultrafine-duplex (α + β) structure was found to increase with increasing imposed strain. A similar ultrafine-duplex (α + β) structure was also reported for a hot-rolled VT22 β-Ti alloy [3].

The formation of the ultrafine-duplex (α + β) structure was attributed by Inaba et al. [7] to the nucleation of α precipitates at β sub-grain boundaries formed through recovery of the β matrix. Later, it was argued by Makino et al. [4] that direct aging after heavy cold rolling might lead to slow recovery and the absence of β sub-grains, and instead it was proposed that α nucleated on dislocations prior to the formation of β sub-grains. Nevertheless, none of these studies have identified the exact location of the ultrafine-duplex (α + β) structure, nor have they clarified its dependence on deformation strain and the underlying causes leading to the formation of the equiaxed α precipitates. Moreover, the limited strain imposed during cold or hot rolling is not sufficient to attain homogeneous deformation across the specimen and is thus unable to generate a uniform ultrafine-duplex (α + β) structure when subjected to subsequent aging [4], [7].

Recently, severe plastic deformation (SPD) techniques in the form of equal channel angular pressing (ECAP) and high-pressure torsion (HPT) have demonstrated a high capability of introducing large accumulated strain through intensive and uniform shear deformation to dramatically refine coarse grains into ultrafine or nanoscale structures [10], [11]. The present study is thereby inspired to utilize ECAP as a means of imposing large strain on a Ti–20 wt.% Mo β alloy to explore the possible origins of the ultrafine-duplex (α + β) structure. As the underlying nucleation mechanism of the equiaxed α precipitates remains unclear, efforts have been made to gain better insights into the nucleation of α precipitates promoted by severe shear strain which ultimately leads to the formation of the ultrafine-duplex structure.

Section snippets

Experimental materials and procedures

The Ti–20 wt.% Mo alloy was prepared by arc-melting a mixture of high-purity elements (99.99%) under an argon atmosphere with a Ti getter. Cylindrical rods 10 mm in diameter were fabricated by cold crucible casting followed by solution treatment (ST) and homogenization at 1273 K for 48 h and subsequent water quenching. Samples of the rods of 30 mm in length were inserted into a preheated ECAP die (with a channel intersection angle of 90°) at 473 and 873 K, respectively, held for 20 min and pressed at

Solution treated and aged Ti–20 wt.% Mo

Metastable β with an average grain size of about 400 μm was obtained for Ti–20 wt.% Mo solution treated at 1273 K followed by water quenching (Fig. 1a). From the XRD analysis (Fig. 1a), the isothermal ω phase was revealed to form in samples aged at 523 and 673 K for 1 h, whereas it was not detectable either below 473 K, where only the β phase was present, or above 823 K, where α formed in the β matrix. Fig. 1b depicts the dependence of the Vickers microhardness on aging temperature for a fixed aging

Discussion

It has been demonstrated that α precipitation in the severely shear deformed β matrix strongly depends on the local features of the deformed structure. In particular, it is evident that the formation of the ultrafine duplex (α + β) structure is closely correlated with pre-existing SBs. As described earlier, an ultrafine-duplex (α + β) structure was also revealed in cold-rolled Ti-15V-3Cr-3Sn-3Al [4], [7] and hot-rolled VT22 [3] β alloys. The formation mechanism of such an ultrafine-duplex structure

Conclusions

A solution-treated Ti–20 wt.% Mo β alloy has been subjected to SPD by using ECAP and subsequent isothermal aging to elucidate the effect of imposed severe shear strain on the nucleation of α precipitates in the β matrix. The following conclusions can be drawn.

  • 1.

    Selective heterogeneous α nucleation and growth have been identified in the severely deformed β matrix, resulting in mixed precipitate morphologies: an ultrafine-duplex (α + β) structure inside the SBs and acicular α precipitates dispersed

Acknowledgements

The authors greatly appreciate the financial support from the Australian Research Council with the Centre of Excellence for Design in Light Metals and the Alexander-von-Humboldt Foundation. The work has further benefited from support through the EU-ITN BioTiNet (Grant No. 264635), support provided by the Free State of Saxonia in the framework of the European Centre for Emerging Materials and Processes (ECEMP), and through the German Science Foundation under the scheme of the SFB-TR79. W.X.

References (40)

  • S. Ankem et al.

    Mater Sci Eng A

    (1999)
  • T. Makino et al.

    Mater Sci Eng A

    (1996)
  • S. Nag et al.

    Acta Mater

    (2009)
  • O.M. Ivasishin et al.

    Mater Sci Eng A

    (2005)
  • F. Sun et al.

    Mater Sci Eng A

    (2010)
  • T. Furuhara et al.

    J Mater Process Technol

    (2001)
  • R.Z. Valiev et al.

    Prog Mater Sci

    (2006)
  • A.P. Zhilyaev et al.

    Prog Mater Sci

    (2008)
  • D.H. Shin et al.

    Acta Mater

    (2000)
  • I. Kim et al.

    Mater Sci Eng A

    (2003)
  • D.H. Shin et al.

    Acta Mater

    (2001)
  • S.P. Timothy

    Acta Metall

    (1987)
  • Q. Xue et al.

    Mater Sci Eng A

    (2008)
  • W.Z. Han et al.

    Acta Mater

    (2009)
  • H.J. Roven et al.

    Mater Sci Eng A

    (2008)
  • J.S. Lian et al.

    Acta Metall Mater

    (1995)
  • Y. Amouyal et al.

    Acta Mater

    (2007)
  • S.V. Divinski et al.

    Acta Mater

    (2011)
  • A.A. Nazarov et al.

    Acta Metall Mater

    (1993)
  • S.V. Divinski et al.

    Acta Mater

    (2009)
  • Cited by (0)

    View full text