posted on 2017-01-10, 04:13authored byBattaini, Michael
The deformation behaviour and twinning mechanisms of commercially pure titanium
alloys were investigated using complementary diffraction techniques and crystal
plasticity modelling. The main motivation for conducting this investigation was
to improve understanding of the deformation of titanium to help achieve the long
term aim of reducing manufacturing and design costs. The deformation behaviour
was characterised with tension, compression and channel die compression tests for
three important variables: orientation; temperature from 25 C to 600 C; and composition for two contrasting alloys, CP-G1 and CP-G4. The experimental data
used to characterise the behaviour and determine the mechanisms causing it were:
textures determined by X-ray diffraction; twin area fractions for individual modes
determined using electron back-scatter diffraction; and lattice strains measured by
neutron diffraction.
A strong effect of the orientation–stress state conditions on the flow curves (flow
stress anisotropy) was found. The propensity for prism hai slip was the dominant
cause of the behaviour – samples that were more favourably oriented for prism hai slip
had lower flow stresses. Twinning was the most significant secondary deformation
mode in the CP-G1 alloy but only had a minor effect on flow stress anisotropy in
most cases. In the CP-G4 alloy twinning generally did not play a significant role
indicating that hc + ai slip modes were significant in this alloy. Differences in the
flow stress anisotropy between the two alloys were found to occur largely in the
elasto-plastic transition and initial period of hardening. Modelling results indicated
that larger relative resolved shear stress values for secondary deformation modes in
the higher purity alloy increased the initial anisotropy.
Decreasing flow stresses with increasing temperature were largely caused by a
decrease in the critical resolved shear stress (CRSS) values for slip, but also by a
decrease in the Hall-Petch parameter for slip.
The propagation of twinning was found to be orientation dependent through a
Schmid law in a similar way to slip – it was activated at a CRSS and hardened so
that an increasing resolved shear stress was required for it to continue operating.
The CRSS values determined for the individual twin modes were – 65MPa, 180MPa,
83MPa for {1012}, {1122} and {1011} twinning, respectively. Further, twinning was
found to be temperature insensitive except when the ability to nucleate twins posed
a significant barrier (for {1011} twinning). Also, the CRSS for {1012} twinning was
clearly shown to increase with decreasing alloy purity.
A thorough method for determining crystal plasticity modelling parameters based
on experimental data was formulated. Additionally, twinning was modelled in a
physically realistic manner influenced by the present findings using the visco-plastic
self-consistent (VPSC) model. In particular: the activity of twinning decreased in a
natural way due to greater difficulty in its operation rather than through an enforced
saturation; and hardening or softening due to changes in orientation and dynamic
Hall-Petch hardening were important. The rigorous modelling procedure gave great
confidence in the key experimental findings.