PDF《Amorphous martensite in β-Ti alloys》

ARTICLE Amorphous martensite in β-Ti alloys Long Zhang1,2, Haifeng Zhang1, Xiaobing Ren3,4, Jürgen Eckert5,6 Yandong Wang7, Zhengwang Zhu1, Thomas Gemming

2 &

Simon Pauly2 Martensitic transformations originate from a rigidity instability, which causes a crystal to change its lattice in a displacive manner.

Here, we report that the martensitic transformation on cooling in TiCZrCCuCFe alloys yields an amorphous phase instead. Metastable β-Ti partially transforms into an intragranular amorphous phase due to local lattice shear and distortion. The lenticular amorphous plates, which very much resemble α′/α″ martensite in conventional Ti alloys, have a well-de?ned orientation relationship with the surrounding β-Ti crystal. The present solid-state amorphization process is reversible, largely cooling rate independent and constitutes a rare case of congruent inverse melting. The observed combination of elastic softening and local lattice shear, thus, is the unifying mechanism underlying both martensitic transformations and catastrophic (inverse) melting. Not only do we reveal an alternative mechanism for solid-state amorphization but also establish an explicit experimental link between martensitic transformations and catastrophic melting. DOI: 10.1038/s41467-018-02961-2 OPEN

1 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences,

72 Wenhua Road, Shenyang 110016, China.

2 IFW Dresden, Institute for Complex Materials, Helmholtzstra?e 20,

01069 Dresden, Germany.

3 Multi-disciplinary Materials Research Centre, Frontier Institute of Science and Technology, Xi'

an Jiaotong University, Xi'

an 710049, China.

4 Ferroic Physics Group, National Institute for Materials Science, Tsukuba 305-0047, Japan.

5 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstra?e 12,

8700 Leoben, Austria.

6 Department Materials Physics, Montanuniversit?t Leoben, Jahnstra?e 12,

8700 Leoben, Austria.

7 State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China. Correspondence and requests for materials should be addressed to H.Z. (email: [email protected]) or to X.R. (email: [email protected]) NATURE COMMUNICATIONS | (2018)9:506 |DOI: 10.1038/s41467-018-02961-2 |www.nature.com/naturecommunications

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A morphous materials are generally obtained by quenching liquids in order to avoid crystallization1,2 and multi- component or even monatomic metallic glasses can be prepared in this way3. Alternatively, amorphous materials can also be produced through a transformation from crystalline solids, known as solid-state amorphization (SSA)4. A variety of processes can fully or partially transform crystals into an amor- phous phase, among them high-pressure treatments5C8, irradia- tion with ions or electrons9, severe plastic deformation10, inter- diffusion in multilayers11,12, mechanical alloying13, hydrogen absorption14, or decompression of solids15C17. It has even been reported that a metastable liquid is the intermediate state in certain solidCsolid phase transformations18,19. From a thermodynamic viewpoint, SSA can be considered a melting process far below the equilibrium melting tempera- ture20,21. One requirement for solid-state amorphization4 is the generation of a metastable crystalline state containing a high density of lattice defects11,22,23. The continuous increase of lattice defects augments the static disorder up to a critical point at which the lattice becomes unstable and a glass forms9,23. Analogies between SSA and melting have been widely recognized20,21,24 and SSA without compositional change (polymorphic SSA) on cool- ing of a crystalline solid is known as inverse melting17,24,25. Such a polymorphic or (more appropriately) congruent inverse melting has been observed in various materials, such as polymers26, sili- con10,27, and TiCCr supersaturated alloys22. Polymorphic SSA generally proceeds via nucleation and growth, i.e., the amorphous phase nucleates at lattice defects followed by its growth resulting from a gradual collapse of the crystal lattice4,12,19. Similarly, the process of nucleation and growth also holds for conventional melting of crystals and the liquid begins to form at defects19,28,29, such as surfaces or internal lattice defects once the equilibrium melting temperature is reached. However, a different, rarely encountered mode of melting has been long been predicted to occur in defect-free crystals undergoing substantial super- heating via a rigidity catastrophe29C31. Several upper limits for predicting the onset of catastrophic melting have been postulated in the past decades, including an isochoric limit (i.e., the super- heated crystal and liquid have the equal volume)31, an isenthalpic limit (equal enthalpy)32 and an isentropic limit (equal entropy)31,32. Yet, since real solids generally cannot be superheated to tem- peratures at which catastrophic melting sets in, these limits for catastrophic melting have not been reached in experiments33. Consequently, it remains an open question, which limit provokes catastrophic melting29,34. In this context, congruent inverse melting can provide helpful insights because no superheating of the solid is required, which greatly facilitates experimental access. Martensitic transformations are another very common phe- nomenon and occur in a variety of materials35,36, including steels37, polymers38, ceramics39, shape memory alloys35 as well as Ti, and Zr alloys36. During cooling or deformation, phonon anomalies accompanied by elastic softening and lattice shear