Elsevier

Materials Science and Engineering: A

Volume 650, 5 January 2016, Pages 404-413
Materials Science and Engineering: A

Enhanced mechanical response of an ultrafine grained Ti–6Al–4V alloy produced through warm symmetric and asymmetric rolling

https://doi.org/10.1016/j.msea.2015.10.061Get rights and content

Abstract

An equiaxed ultrafine-grained (UFG) microstructure was successfully produced in a Ti–6Al–4V alloy with an average grain size of 110–230 nm through symmetric and asymmetric warm rolling of a martensitic starting microstructure. The UFG material displayed a combination of ultrahigh strength and ductility at room temperature. Compared with the conventional symmetric rolling, the asymmetric rolling process led to a more pronounced effect of microstructure refinement and a higher tensile ductility. The optimum mechanical response was obtained though the asymmetric rolling at 70% reduction, offering an ultimate tensile strength of 1365 MPa and a total elongation of ~23%. Apart from the magnitude of grain refinement, the inclination of basal texture component from the normal towards the rolling direction during asymmetric rolling and possible strain induced β to martensite transformation may concurrently contribute to a remarkable tensile strength-ductility balance.

Introduction

The α+β Ti–6Al–4V alloy is a workhorse of titanium alloys, accounting for more than 50% of the total titanium usage. This is mainly attributed to the good balance of mechanical properties such as moderately high specific strength, good fatigue strength and intermediate fracture toughness, which are mostly governed by the microstructure characteristics 1], [2]. Depending on the thermomechanical processing (TMP), this alloy can transform into a wide range of microstructures such as fully martensitic, lamellar α+β, equiaxed α+β and bimodal microstructures. Among the various microstructures, the equiaxed structure offers a unique combination of high ductility, fatigue strength and superplastic formability at elevated temperatures. However, the typical mean grain size of the equiaxed microstructure produced through conventional TMP is in a magnitude of micrometers (i.e. ~5–10 μm) [3], which limits the mechanical performance. Recently, many attempts have been performed to obtain an ultrafine grained (UFG) and/or nano-crystalline (NC) microstructure [4] in the Ti–6Al–4V alloy to further enhance its mechanical performances for the aerospace and medical applications. Prior research suggests that the grain size reduction from micron to submicron-scale not only improves the tensile strength [5], but also significantly enhances the fatigue strength and fatigue life [6], [7]. In addition, the temperature required for the superplastic deformation is also reduced by hundreds of degrees through the microstructural refinement [8], [9], [10].

Most of the current attempts have been focused on severe plastic deformation (SPD) methods such as equal channel angular pressing (ECAP) [7], hydrostatic extrusion (HE) [5] or multistep isothermal forging [11], by which submicron-crystalline (SMC) structure can potentially be produced. However, special equipment, complicated procedures and very high plastic working energy are normally essential for these types of SPD techniques. Moreover, due to the accumulation of large strains, the as-processed SMC/NC materials are prone to exhibit a sharp fall in the engineering stress–strain curve after reaching the peak stress during tensile deformation [5], [12]. This indeed indicates a poor strain-hardening capability after SPD.

Recently, a novel TMP approach was developed for the Ti–6Al–4V alloy through the warm deformation of a martensitic starting microstructure, which led to the formation of an UFG structure comparable with that produced by many SPD techniques [13], [14], [15]. In this approach, the initial martensitic alpha laths were progressively fragmented into equiaxed ultrafine grains with a mean size of 100–800 nm, primarily through dynamic recrystallization [13], [14], [15] or spheroidization [16]. However, the extent of grain refinement was considerably influenced by the thermomechanical processing parameters such as deformation temperature and strain. In addition, a recent study suggested that the deformation mode (i.e. symmetric vs asymmetric rolling) alters the microstructure and texture evolution remarkably [17]. This may indeed affect the final mechanical properties of material considerably depending on the level of grain refinement. The current work, therefore, aims to investigate the impact of deformation mode (i.e. symmetric and asymmetric rolling) and thermomechanical processing parameters (e.g. reduction) on the microstructure characteristics, texture development and mechanical behavior of the Ti–6Al–4V alloy with an initial martensitic structure subjected to the warm rolling.

Section snippets

Experimental procedure

The material used in the current study was a Ti–6Al–4V alloy plate with a thickness of 5.75 mm received in the hot-rolled condition. The as-received material had an initial α+β microstructure of the layer-by-layer morphology (Fig. 1a). The mean thickness of α plate was ~1.3 µm while the β film (shown by arrows in Fig. 1a) had a thickness of 0.1–0.5 μm. Rolling billets with a size of 25×75 mm were sectioned from the plate along the rolling direction. These billets were subjected to a solution

Microstructure and texture of the as-rolled plates

The as-quenched martensitic microstructure was composed of martensitic laths with different orientations and an average lath thickness of ~0.45 µm, primarily extending from the prior-β grain boundaries (Fig. 1b and c). Upon warm rolling, the initial martensitic laths were gradually fragmented to different extents depending on the thermomechanical conditions (Fig. 2, Fig. 3, Fig. 4). The microstructure fragmentation/refinement was not uniform throughout the thickness of rolled specimens due to

Discussion

The current study reveals that extensive grain refinement can be attained in the Ti–6Al–4V alloy through warm rolling of the initial fine martensitic microstructure. The conversion of the martensitic microstructure into equiaxed fine grains during warm deformation of Ti–6Al–4V alloy in a temperature range of 700–800 °C takes place through a combination of different deformation mechanisms [13]. The initial martensitic microstructure consists of relatively fine laths with high dislocation density

Conclusions

In the current work, the effect of deformation mode (symmetric and asymmetric rolling) and thermomechanical processing variables (i.e. rolling reduction and temperature) on microstructure refinement, texture characteristics and mechanical behavior was investigated in a Ti–6Al–4V alloy with an initial martensitic structure using warm rolling:

  • i.

    The present work demonstrated a novel method to produce an ultrafine grained microstructure through symmetric/asymmetric warm rolling of a martensitic

Acknowledgments

Funding support from the Australian Research Council, Australia, through an Australian Laureate Fellowship (P.D.H., Grant Number: FL0992361) is gratefully acknowledged.

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