Abstract
We have performed quantitative analysis of {332}〈113〉 twinning in a β-Ti-15Mo (wt.%) alloy by in situ scanning electron microscopy and electron backscattering diffraction (EBSD). Microstructure-twinning relations were evaluated by statistical analysis of the evolving twin structure upon deformation at room temperature. Our analysis reveals that at the early stages of deformation (ε < 1.5 to 2.0%), primary twinning is mainly determined by the applied macroscopic stress resolved on the twin system. Most of the primary twins (~70–80% of the analyzed twins) follow Schmid’s law with respect to the macroscopic stress, and most of the growth twins (~ 85% of the analyzed twins) correspond to the higher stressed variant. In the grain size range studied here (40–120 μm), we find that several twin parameters such as number of twins per grain and number of twins per grain boundary area exhibit grain size dependence. We ascribe these effects to the grain size dependence of twin nucleation stress and apparent critical resolved shear stress for twinning, respectively.
Highlights
Deformation twinning involves the cooperative motion of a large number of partial dislocations on periodic and adjacent spaced twinning planes
scanning electron microscopy (SEM) images were taken in a Sigma Zeiss field emission gun scanning electron microscope (FEG-SEM) equipped with a TSL orientation imaging microscopy (OIM) electron backscattering diffraction (EBSD) system
The yield strength (YS) of the SG sample was 425 MPa, which is 1.12 times greater than that of the LG sample (370 MPa). These YS values are typical of Ti-15Mo alloys with micrograin size [26,27]
Summary
Deformation twinning involves the cooperative motion of a large number of partial dislocations on periodic and adjacent spaced twinning planes. In bcc metals, {112}〈111〉 twinning is the common twinning system, which involves the cooperative gliding of 1/6 〈111〉 twin dislocations [1,2]. In bcc β-Ti alloys the operative twinning system is {332}〈113〉[3,4,5,6,7,8,9]. According to the crystallographic theory of twinning by Bilby and Crocker [10], {332}〈113〉 twinning involves homogeneous shear on {332} twinning planes with shear magnitude of 0.3536 coupled with atomic shuffling on {011} planes along 〈011〉 directions. Experimental data in cubic metals indicate that twinning occurs on the most highly stressed twin planes and a critical resolved shear stress for twinning has been commonly defined [1,11]. Several works in β-Ti alloys have reported that {332}〈113〉 twinning follow Schmid’s law
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