Twinning-induced Plasticity in Metastable Beta Titanium Alloys
Creators: name: Evans, Peter
Publisher: name: Imperial College London
Year: 2017

description: This thesis examines whether twinning can be used to provide effective work hardening in titanium alloys, and hence enhanced ductility and energy absorption, via the twinning-induced plasticity (TWIP) mechanism. The thesis focuses on a series of Ti-Cr-Ti-64 quasi-ternary alloys and on the Ti-20V (wt.%) alloy, which were produced by a conventional arc melting and hot-rolling route, followed by annealing and water quenching. After introducing the thesis, reviewing the literature and the experimental methods used, Chapter 4 examines the Ti-Cr system. It is found that Ti-8Cr (wt.%), Ti-10Cr and Ti-10Cr-25(Ti-64) all show substantial ductility (12-35%), work hardening (&gt;1.5 GPa) and profuse twinning, resulting in substantial areas under the stress-strain curve of up to 345 MPa. This is around twice the energy absorbed by a TWIP steel, per unit mass. However, the initial yield strengths, of 610-880 MPa, are rather too low for industrial application. The twin type is examined and, in contrast to Ti-Nb alloys which can deform by &lt;112&gt;&lt;111&gt; twinning but do not work harden, {332}&lt;113&gt; type twins are observed using electron-backscatter diffraction (EBSD). In Ti-8Cr, TEM examination shows no evidence of omega formation. It was also shown that cold rolling and flash annealing could be used to refine the grain size, offering a potential route to raising the yield strength. Most of the remaining alloys in the Ti-(8-14)Cr(0-75%)Ti-64 series were found to offer unattractive properties, however. Some of the alloys deformed only by slip, whilst others were observed to only form twins in the neck. It is surmised that the twinning that did occur was not profuse enough to provide su fficient work hardening to inhibit necking. Nonetheless, there were three alloys - Ti-11Cr-75(Ti-64), Ti-12Cr-75(Ti-64) and Ti-13Cr-50(Ti-64) - which despite only exhibiting deformation slip still possessed reasonable ductility combined with yield strengths on the order of 1.0-1.1GPa. These alloys may be of further industrial interest. The Ti-20V and Ti-20V-2.5Al (wt.%) alloys were also examined both after deformation and in-situ using neutron diffraction. The Ti-20V alloy showed substantial ductility (15% uniform elongation) and a yield stress of 670 MPa, while the 2.5 wt.% Al addition resulted in an alloy that was brittle. The Ti-20V alloy showed profuse twinning, the occurrence of which was much reduced in Ti-20V-2.5Al. The twin type in Ti-20V was shown by analysis of the tensor misorientation between the parent and twin to be of the {332}&lt;113&gt; type. Secondary twins crossing primary twins were also observed. The in-situ neutron diffraction allowed the diffraction elastic constants to be measured, which show relatively little anisotropy. Substantial load shedding to grains with their {200} plane normal along the loading direction was observed, and the diffracted intensity of these grains also decreased by around 50%, indicating that many of them twinned. TEM analysis indicated that both alloys possessed omega phase, which could be imaged in dark field. It is concluded that twinning induced plasticity can occur in bcc titanium alloys, and that these are an attractive potential target for the development of low density alloys for energy absorption applications. The outstanding issues that remain are how to increase the initial yield stress towards 1GPa and how to suppress omega formation. Some possible routes to achieving those goals are suggested.