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New mechanism reveals the secrets of a Ti alloy transformation

By Eva Karatairi March 15, 2017
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The final nanolaminate composite microstructure throughout the bulk Ti gum metal. The image is acquired with a scanning electron microscope with an integrated back-scattered electron detector. Credit: Jian Zhang, MPIE

A new phase transformation mechanism has been described for a variation of the gum metal, an oxygen-free, β-titanium (Ti) alloy, with niobium (Nb), tantalum (Ta), and zirconium (Zr) as alloying elements. The scientists behind the discovery believe that their findings will serve as a guide for future developments of new and improved varieties of Ti-alloys. Their work is published in Nature Communications.

Pure titanium undergoes a crystallographic transformation at 882°C, from the alpha (α) phase, where the atoms are in a hexagonal lattice structure (hcp), to the beta (β) phase, which exhibits a body centered cubic crystalline structure (bcc). Depending on the mixture of chemical elements that are used for the alloying, this transformation temperature can be altered significantly. 

Gum metals, in which the β-phase is stabilized at room temperature, are a very special class of β-titanium alloys, with very low elastic stiffness and nearly hardening-free plasticity. Contrary to other metals, gum metals do not become harder or brittle when deformed, but easily change their shape and “bend almost like honey” as Dirk Raabe, director at the Max-Planck-Institut for Iron Research (MPIE) and co-author of the article, describes. This exceptional mechanical behavior makes them very important for biomedical and aerospace applications.

Understanding the underlying transformation mechanisms in Ti-alloys is considered crucial for designing gum metals or gum-metal-like materials for desired microstructures and mechanical properties.

Experts from MPIE and DESY Research Center in Germany, the State Key Laboratory for Mechanical Behavior of Materials in China, and the Massachusetts Institute of Technology, led by Jian Zhang of MPIE (currently at the State Key Laboratory), have unravelled the mechanism behind a new transformation phenomenon that may explain why and how the gum metal can be deformed to such a high degree. The phase transformation appears upon fast cooling (quenching) the Ti-23Nb-0.7Ta-2Zr at.% alloy from the β-phase region.

The transformation unfolds in four structural steps and is characterized as martensitic since an αꞌꞌ martensite phase with orthorhombic crystal structure is involved. The detailed study of the individual steps also uncovered a new structure confined and stabilized in the interface of the adjacent αꞌꞌ and β phases, the omega (ω) planar complexion. A planar complexion refers to a metastable phase confined and stabilized in the interfaces of the adjacent phases. It was shown that the new ω-complexion plays a significant role in the transformation, as it mediates the β-to-αꞌꞌ transition The formation of the ω-complexion is induced by a diffusionless transformation in which the structural changes occur by coordinated movement of atoms toward the energetically more favorable arrangement of the αꞌꞌ phase. The mechanical stresses induced by the atomic movements along the phase boundaries have a significant influence on the morphology of the resulting phase: thus when the stress rises above a critical value, ω-complexion emerges before the decrease of the stress value causes the formation of a new αꞌꞌ layer.

This mechanism leads to a final nanostructure of many layers of αꞌꞌ martensite, alternating with ω-planar complexions. Confirming the microstructure in the bulk sample was one of the most challenging parts, according to Zhang. The in situ synchrotron x-ray diffraction (SXRD) heating/cooling measurements and the decoding of the complex SXRD pattern were performed with the assistance of A.-C. Dippel’s research group at the PETRA III ring accelerator in the DESY facilities in Hamburg, Germany. The results demonstrate the co-existence of αꞌꞌ and ω phases, and a small volume of remaining β-phase.

A great finding for the team was the way the nanostructure of the bulk sample fully mirrors its microscopic structure. To Zhang’s astonishment the micrographs from the scanning electron microscope with an integrated back-scattered electron detector showed the nanolaminate composite microstructure expanding to the macroscopic scale.  

David Dye, a professor at Imperial College London, an expert on design of titanium and nickel/cobalt superalloys who was not involved in the study, describes the idea that the high-pressure ω phase of Ti is formed at the interfaces of the orthorhombic stress-induced martensite αꞌꞌ by the interface strain and confinement as “very beautiful.”

“Showing it so clearly in the TEM [transmission electron microscope] is also very nice work,” Dye says, adding that “previously, the variety of shear-related products—superelastic martensite, omega, twins—in these alloys has been very mysterious, and this paper helps explain the picture.”

The researchers are now interested in finding out how the microstructure of the alloy will evolve during a cold deformation procedure. “It is not yet clear to us how this phase transformation will contribute to the final properties of the material; this is something we are trying to figure out,” Zhang says. The team also wants to understand why the β-phase in the gum metal system is unstable toward both the αꞌꞌ and ω phases. According to Zhang, this is the reason why the transformation from β to αꞌꞌ phase can induce the transformation from β to ω phase at the interphase. The researchers believe that this may be the key for designing new materials that could be even better than the gum-like titanium alloys.

Read the article in Nature Communications.