Rolling-induced Face Centered Cubic Titanium in Hexagonal Close Packed Titanium at Room Temperature.

H C Wu,C N Tomé,A Kumar,J Wang,X F Bi,Z Zhang,S X Mao

doi:10.1038/srep24370

Abstract

Combining transmission electron microscopes and density functional theory calculations, we report the nucleation and growth mechanisms of room temperature rolling induced face-centered cubic titanium (fcc-Ti) in polycrystalline hexagonal close packed titanium (hcp-Ti). Fcc-Ti and hcp-Ti take the orientation relation: 〈0001〉hcp||〈001〉fcc and , different from the conventional one. The nucleation of fcc-Ti is accomplished via pure-shuffle mechanism with a minimum stable thickness of three atomic layers, and the growth via shear-shuffle mechanisms through gliding two-layer disconnections or pure-shuffle mechanisms through gliding four-layer disconnections. Such phase transformation offers an additional plastic deformation mode comparable to twinning.

Highlights

Using TEM, we observed rolling-induced face-centered cubic titanium (fcc-Ti) bands at room temperature in hexagonal close packed titanium (hcp-Ti) polycrystalline aggregates
Platelet bands with 5 nm to 30 nm in thickness were observed in the hcp-Ti matrix after the sample underwent a total rolling reduction of 50% in thickness, as indicated in Fig. 1 by red arrows
This orientation relation is different from the conventional one (〈 1120〉 hcp||〈 011〉 fcc and {0001}hcp||{111}fcc), indicating that the corresponding phase transformation can’t be accomplished via successive glide of Shockley partial dislocations on hexagonal close packed planes

Summary

Introduction

Using TEM, we observed rolling-induced fcc-Ti bands at room temperature in hcp-Ti polycrystalline aggregates. Such phase transformation offers an additional plastic deformation mode comparable to twinning. It is known that pure Ti has the hexagonal closed packed (hcp) structure at ambient temperatures and body centered cubic (bcc) structure at elevated temperatures, while fcc phase does not exist in the equilibrium phase diagram[15,16]. Density functional theory (DFT) calculations, and microscopes, we identified the nucleation and growth mechanisms of fcc-Ti in hcp-Ti. We found the nucleation via pure-shuffle and the growth via shear-shuffle mechanisms, different from the established phase transition path via the collective gliding of Shockley partial dislocations on hexagonal close packed planes

Methods

Results

Conclusion