Introducing transformation twins in titanium alloys: an evolution of α-variants during additive manufacturing

Abstract

Titanium alloys can experience a cooling-induced phase transformation from a body-centred cubic phase into a hexagonal close-packed phase which occurs in 12 crystallographically equivalent variants. Among them, variant selection II, 60°/$〈1\overline{2}10〉$, is very close to the orientation of $\left\{10\overline{1}1\right\}〈1\overline{2}10〉$ twins (57.42°/$〈1\overline{2}10〉$). We propose that the cyclic thermal loading during additive manufacturing introduces large thermal stresses at high temperature, enabling grain reorientation that transforms the 60°/$〈1\overline{2}10〉$ variant boundaries into the more energetically stable 57.42°/$〈1\overline{2}10〉$ twin boundaries. This transformation twinning phenomenon follows a strain accommodation mechanism and the resulting boundary structure benefits the mechanical properties and thermal stability of titanium alloys.

GRAPHICAL ABSTRACT

IMPACT STATEMENT

A new twinning mechanism, transformation twinning, was discovered in a Ti–6Al–4V alloy fabricated by selective laser melting. The resulting high density of transformation twins impact the global mechanical properties significantly.

KEYWORDS:

• Titanium alloy
• additive manufacturing
• twinning mechanism
• variant selections

Acknowledgements

The authors acknowledge the scientific and technical input and support from the Microscopy Australia node at the University of Sydney (Sydney Microscopy & Microanalysis). This project is supported by the Australia–US Multidisciplinary University Research Initiative program. XZL is also supported by the Australian Research Council [DP190102243], WX by [DP150104719], and SP by [DE180100440].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Australian Research Council: [Grant Number DE180100440, DP150104719, DP190102243]; Australia–US Multidisciplinary University Research Initiative program: [Grant Number AUSMURI].