Mechanical behaviour and microstructural evolution in fine grain Ti-6Al-4V alloy under superplastic conditions

Laurie Despax,Hiroaki Matsumoto,Vincent Velay,Denis Delagnes,Vanessa Vidal,Moukrane Dehmas

doi:10.1051/matecconf/202032111011

Abstract

Ti-6Al-4V is able to support high level of deformations like superplastic deformation for aeronautical structural applications. However, the applied temperature during forming induces changes in phase fraction, which may have an impact on the mechanisms of deformation involved and the final part. Mechanisms described in the literature, like dislocation glide, diffusional creep, Grain Boundary Sliding (GBS) accommodated by dislocation or diffusion, are still controversial as there are mainly based on post mortem analysis or on stress-strain data. The purpose of this work was to combine interrupted tensile tests and heat treatments to improve the understanding of the mechanisms of deformation on each stage of deformation. The chosen test temperatures were 750°C and 920°C which correspond to different β phase fractions. The microstructural features like grain size and phase fraction were studied by Scanning Electron Microscope (SEM) combined with image analysis. Moreover, EBSD was used to follow the change of crystalline orientation of α grains to distinguish the involved mechanisms as a function of the deformation. Indeed, it would appears that several mechanisms could be activated depending on the deformation stage and on the temperature.

Highlights

Superplastic forming process on titanium alloys is expensive by requiring high temperatures temperature (T ≥ 900°C)and low strain rates ()
The α phase amount was determined on the sample surface along three directions (rolling direction (RD), normal direction (ND), transversal direction (TD)) by backscattered electron (BSE)-Scanning Electron Microscope (SEM) and image analysis
The {0001} pole figure of the α phase determined by X-Ray Diffraction (XRD) indicates a rolling texture with basal plan {0001} tilted by +/-25° from the ND to the RD (Fig.1c)

Summary

Introduction

Following the initial microstructure (grain size and grain shape - equiaxial, lamellar, martensitic) and the mechanical conditions (temperature, strain rate), the proposed deformation mechanisms are different and have an impact on the final response of the material (softening, hardening). Otherwise Kim et al proposed the sliding as a phase boundary sliding rather than GBS due to the presence of dislocations only at α/β interfaces [6] These studies are mainly based on mechanism according the α phase. Interrupted tensile tests and SEM-EBSD measurements are used to analyse the mechanical behaviour and the microstructural evolution (phase fraction, grain size and α grain orientation) on each step of the deformation

Objectives

Results

Discussion

Conclusion