Elsevier

Materials Science and Engineering: A

Volume 716, 14 February 2018, Pages 260-267
Materials Science and Engineering: A

Fracture toughness at cryogenic temperatures of ultrafine-grained Ti-6Al-4V alloy processed by ECAP

https://doi.org/10.1016/j.msea.2017.12.106Get rights and content

Abstract

This research is focused on a study of the relationship between mechanical behavior, microstructure and fracture toughness of a Ti-6Al-4V alloy in both coarse-grained (CG) and ultrafine-grained (UFG) conditions. The UFG state with a primary alpha-phase grain size, but with different orientations with respect to the testing direction, was produced by equal-channel angular pressing (ECAP) after thermo-mechanical treatment. Fracture toughness and mechanical testing were conducted at a temperature of − 196 °C. A duplex UFG structure formation in the Ti-6Al-4V alloy led to an enhancement of yield stress and a decrease in the fracture toughness at − 196 °С by comparison with the CG alloy. The lowest values of fracture toughness were observed in a sample in which there were elongated grains lying parallel to the loading direction during testing compared to the situation where the grains were perpendicular to this direction. The reasons for the reduction in fracture toughness in the UFG Ti alloy are discussed.

Introduction

Two-phase Ti alloys are widely used as structural materials in aircraft and engine construction due to their high specific strength and corrosion resistance. One of the most utilized alloys in industry is Ti-6Al-4V owing to an effective combination of alloying elements and good technological properties [1]. It is well known that the strength, ductility, fatigue endurance limit, creep strength and fracture toughness determine the efficiency of structural materials. The brittle fracture strength or crack resistance (fracture toughness), which is determined during static single loading and characterized by a КIC value, is an especially important characteristic of materials which becomes a critical factor in the design of parts for technological applications [2]. In particular, the fracture toughness of Ti-6Al-4V (~ 60 MPa m−2 on average) is higher than for Al alloys (~ 35 MPa m−2) but lower than in steels (~ 115 MPa m−2) [2]. This parameter of the Ti-6Al-4V alloy depends strongly on the oxygen content and the thermomechanical treatment (TMT) regime when the КIC value can change in the range from ~ 30 to ~ 100 MPa m−2 [2]. This is because the crack propagation resistance is a structure-sensitive parameter and, depending primarily on the TMT regime used for the Ti-6Al-4V alloy, the structure parameters such as the size of the β-phase grains and the shape and size of the α-phase may change significantly [1], [2].

The formation of ultrafine-grained (UFG) states in metals and alloys by different severe plastic deformation (SPD) techniques, including the development of true nanostructured materials with average grain sizes of < 100 nm, is one of the most promising approaches for improving the strength and ductility characteristics and significantly increasing the fatigue endurance limit and other performance properties [3], [4], [5], [6], [7]. For example, it is established that UFG structure formation in titanium and the Ti-6Al-4V alloy may lead to a high level of strength, improved fatigue endurance limit and even to a superplastic forming capability [8], [9], [10], [11], [12], [13] and this creates conditions for use of this alloy under extreme conditions. Accordingly, a study of crack resistance in this alloy in the UFG state is required urgently in order to achieve greater efficiency in the design of real structures. It is important to note also that processing by SPD techniques may induce phase transformations in the Ti-6Al-4V alloy [14].

Recent studies have shown that the formation of a nanostructured state in metals leads to a decrease in the fracture toughness as shown, for example, in nickel, iron and titanium [15], [16], [17] and this effect is frequently associated with the decrease in the strain hardening capacity of nanostructured materials [17], [18]. It is well known that the relationship between KIC and the strain-hardening coefficient is described by the relationship [19]:KIC=ncEσysεf,

where n is the strain hardening coefficient, c is a constant, σys is the yield strength, E is the elastic modulus and εf is the fracture strain.

These UFG materials often demonstrate an early localization and neck formation because of complex interactions between dislocation nucleation and accumulation within the UFG structure [20]. However, to date there are only a limited number of reports describing investigations of the fracture toughness of nanostructured materials and this is associated with experimental problems such as the small sample size after SPD processing. As is well known [21], the main fracture mechanics criterion in the evaluation of KIC is a fulfillment of the relationship t/(KIC/YS2)2 ≥ 2.5, where t is the sample thickness, and this must ensure fracture under conditions of plane strain. In addition, at higher temperatures a ductile cleavage zone may develop so the values of KIC may be overestimated.

In the present experiments, an UFG structure formation was achieved in a Ti-6Al-4V alloy by equal-channel angular pressing (ECAP) and this led to the production of bulk billets which permitted the machining of standard samples for evaluating the fracture toughness in the UFG alloy. In order to create the ultimate state of the material under conditions of maximum constraint of plastic deformation, tests were conducted in liquid nitrogen at a temperature of − 196 °C. Specimens were produced by ECAP with the same primary alpha-phase grain size but with different grain orientations relative to the testing direction. Thus, the aim of this investigation was to study the relationship between the mechanical behavior, microstructure and fracture toughness of the Ti-6Al-4V alloy in duplex coarse grained (CG) and UFG conditions.

Section snippets

Experimental materials and procedures

The experiments were conducted with a Ti-6Al-4V alloy produced in the form of hot-rolled rods with diameters of 40 mm and containing the following chemical composition (in wt%): 6.2% Al, 4.3% V, 0.02% Zr, 0.039% Si, 0.16% Fe, 0.06% C, 0.168% O, 0.015% N, 0.003% H, Ti-base. The results of differential scanning calorimetry (DSC) showed the temperature of the polymorphous transformation (ТPT) as 975 ± 5 °С. In order to produce the duplex structure in a Ti-6Al-4V rod, the billets were subjected to a

Microstructure of Ti-6Al-4V sample in the conditions HT, UFG1 and UFG2

Fig. 2 displays the microstructure of the alloy after HT, which is a mixed globular-platelet structure consisting of grains of primary αp-phase with an average size of ~ 8 µm, a volume fraction of ~ 30% (Fig. 2a) and areas (α + β) with thin plate-like structures (Fig. 2b). An X-ray phase analysis showed the ratios of the α- and β-phases were about 85% and 15%, respectively.

Fig. 3, Fig. 4 display the UFG1 and UFG2 structures. The overall structure is shown in Fig. 3a and the interiors of the

Discussion

The results obtained within this research show that the formation of a UFG structure in Ti-6Al-4V leads to an increase in the yield stress in accordance with the Hall-Petch relationship both at room temperature and at the cryogenic temperature (Table 1). During the formation of the UFG structure the alloy ductility is reduced, where this generally characterizes materials processed by SPD [17], [18], [24]. It is also apparent that the total elongations of samples with UFG1 and UFG2 structures is

Summary and conclusions

  • 1.

    ECAP was combined with TMT to form a duplex UFG structure wherein ~ 70% consisted of areas with equiaxed ultrafine α-phase grains with an average size of ~ 350 nm and the remaining volume was occupied by elongated poorly-deformed grains of the primary α-phase with lengths up to ~ 10 µm.

  • 2.

    The duplex UFG structure formation in the Ti-6Al-4V alloy leads to an increase in the yield stress and a concomitant decrease in the fracture toughness at the cryogenic temperature of − 196 °С by comparison with the

Acknowledgements

This work was supported by the Ministry of Education and Science of the Russian Federation within the scope of the basic part of the State Assignment (Grant no. 11.1235.2017), in Sections 2, 3.1, 3.2, 3,3. Sections 3.4 and 4.0 was sponsored by the RFBR Grant no. 16-58-1006116. One of the authors was supported by the European Research Council under ERC Grant agreement no. 267464-SPDMETALS (TGL).

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