Strength and ductility-related properties of ultrafine grained two-phase titanium alloy produced by warm multiaxial forging
Highlights
► Strength of Ti–6Al–4V increases up to 33% due to grain refinement to UFG regime. ► Ductility-related properties decrease with decreasing grain size to UFG regime. ► Increasing ductility of UFG alloy by annealing is limited by intensive softening. ► Large alpha particles in UFG matrix enhance ductility without strength loss.
Introduction
Two-phase Ti–6Al–4V alloy is one of the most widely used titanium alloys due to an excellent combination of mechanical properties, corrosion resistance and workability. Considerable enhancement in the mechanical properties of the alloy can be attained by microstructure refinement to the ultrafine grained (UFG, grain size less than 0.5–1 μm) regime [1], [2], [3], [4] that, in turn, may broaden application of the alloy. Improvement in the mechanical properties through grain refinement has contributed to the rapidly expanding field of materials engineering, in which UFG materials are produced using severe plastic deformation (SPD). Well-established SPD techniques include high-pressure torsion [5], equal channel angular extrusion [6] twist extrusion [7] and multiaxial forging (MAF) [8]. An advantage of these methods is the almost unlimited ability to accumulate deformation while maintaining the initial shape and size of the specimen.
Generally, it is well established that strength, hardness, high-cycle fatigue resistance and superplastic properties are improved by grain size reduction [1], [2]. However considerable microstructure refinement usually results in a decrease in ductility-related properties (tensile elongation, impact toughness, crack propagation resistance) [1], [2] due to low ability of such materials to strain hardening. Therefore, additional investigations are needed in order to evaluate the feasible area of application of UFG two-phase titanium alloys and to improve ductility of such materials.
The aim of the present work was to study the most important room temperature mechanical properties of two-phase Ti–6Al–4V alloy with UFG microstructure produced by warm multiaxial forging, and to develop approaches to further improvement of ductility-related properties.
Section snippets
Material and experimental procedures
The material used in this study was an alpha/beta titanium alloy Ti–6Al–4V with a measured composition (in weight pct.) of 6.3 Al, 4.1 V, 0.18 Fe, 0.03 Si, 0.02 Zr, 0.01 C, 0.18 O, 0.01 N. It was received in the form of a hot-rolled 40-mm diameter bar with a beta-transus temperature (at which alpha + beta → beta) of 980 °C.
Ultrafine grained microstructure in the alloy was produced by warm MAF consisted of sequential compression of a sample along variable directions as it is schematically shown in
Mechanical properties of the UFG Ti–6Al–4V alloy
The microstructures of the Ti–6Al–4V alloy in the UFG and MC conditions are shown in Fig. 2. The microstructure of the UFG alloy consisted of alpha and beta particles with a mean grain size of 150 nm (last deformation temperature was 475 °C) (Fig. 2a). The MC condition consisted of primary alpha globules in the beta transformed matrix (Fig. 2b). During cooling from the top of the alpha/beta field the beta phase decomposed into a mixture of very fine (less than 1 μm) alpha and beta lamellae. The
Summary
Mechanical properties of two-phase Ti–6Al–4V alloy in the ultrafine grained condition produced by multaxial forging and in the heat strengthened microcrystalline condition were studied. The strength of the UFG condition was found to be up to 33% higher than that of the MC condition (1400 and 1050 MPa, respectively). However, ductility-related properties including tensile elongation, impact toughness and fracture toughness decreased with decreasing the grain size. Any noticeable increase in
Acknowledgement
This work was supported by the Russian Foundation for Basic Research, under Grant no. 10-08-00701-а.
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