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@MISC{Zhang:396847,
      author       = {Zhang, Jian and Raabe, Dierk and Dippel, Ann-Christin},
      title        = {{N}ew mechanism reveals the secrets of a {T}i alloy
                      transformation},
      reportid     = {PUBDB-2017-12927},
      year         = {2017},
      abstract     = {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.},
      cin          = {DOOR / FS-PE},
      cid          = {I:(DE-H253)HAS-User-20120731 / I:(DE-H253)FS-PE-20120731},
      pnm          = {6213 - Materials and Processes for Energy and Transport
                      Technologies (POF3-621) / 6G3 - PETRA III (POF3-622)},
      pid          = {G:(DE-HGF)POF3-6213 / G:(DE-HGF)POF3-6G3},
      experiment   = {EXP:(DE-H253)P-P02.1-20150101},
      typ          = {PUB:(DE-HGF)21},
      url          = {https://bib-pubdb1.desy.de/record/396847},
}