On the work hardening of titanium: new insights from nanoindentation

Arnas Fitzner,Matthew Thomas,Michael Preuss,Jack Palmer,Ben Gardner,Joao Quinta Da Fonseca

doi:10.1007/s10853-019-03431-w

Arnas Fitzner, Matthew Thomas + Show 4 more

Open Access

https://doi.org/10.1007/s10853-019-03431-w

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Abstract

Nanoindentation was used to probe the local slip resistance in CP-Ti deformed in compression to different extents. Changes in hardness in the deformed grains and twins were compared with the change in flow stress measured during deformation, with the aim to elucidate the relative contribution of slip and twinning to the work hardening of Ti alloys. The hardness values were calibrated with measurements on binary Ti–Al alloys. The hardness increased only slightly with deformation and cannot explain the observed work hardening. Although twinned regions were found to be harder than the parent grains, this increase was found to be small once the effect of crystal orientation was accounted for. The increase in hardness in the twins was slightly higher for compressive twins than for tensile twins. It is proposed that this modest hardness increase in the twins is more consistent with the presence of twinning stresses than with a change in the local flow stress caused by dislocation interactions. The implications of these findings to the work hardening of CP titanium are discussed.

Highlights

The plastic deformation of commercial pure a-titanium at room temperature is strongly anisotropic at the single crystal level [1] because slip parallel to the closely packed planes of its HCP lattice is much easier than along other directions. a-titanium twins relatively in a number of different ways
It can be seen that the compressive proof stress is about 200 MPa when loaded along rolling direction (RD) and at 45°, while loading along normal direction (ND) gave a slightly higher proof stress of about 230 MPa
As for the commercial purity (CP)-Ti sample, the proof stress is highest along ND and the lowest values along RD

Summary

Introduction

The plastic deformation of commercial pure a-titanium at room temperature is strongly anisotropic at the single crystal level [1] because slip parallel to the closely packed planes of its HCP lattice is much easier than along other directions. a-titanium twins relatively in a number of different ways. The plastic deformation of commercial pure a-titanium at room temperature is strongly anisotropic at the single crystal level [1] because slip parallel to the closely packed planes of its HCP lattice is much easier than along other directions. The deformation behaviour is controlled by the relative activation of the different deformation mechanisms and their interaction, which will depend strongly on the crystallographic texture and deformation mode and is difficult to predict. As a consequence of the activity of multiple, interacting deformation mechanisms, the hardening behaviour of a-titanium is anisotropic, path dependent, asymmetric and shows pronounced reverseloading effects [2,3,4,5]. During plane strain compression (PSC), the hardening behaviour depends on the loading direction with respect of the starting texture [5]. The work hardening rate increases with strain rate and decreases with temperature [6]

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