Quasimartensitic Modulations: Session 3
Program Organizers: Avadh Saxena, Los Alamos National Laboratory

Tuesday 4:00 PM
July 11, 2017
Room: Gold Coast
Location: Hyatt Regency Chicago

Session Chair: Antoni Planes, Universitat de Barcelona

4:00 PM  Invited
Transmission Microscopy Studies of Premartensitic Phenomena in Ti- and Ni-based Alloys: Yasukazu Murakami1; Kodai Niitsu2; Yuta Kimura3; Ryosuke Kainuma3; Daisuke Shindo3; 1Kyushu Univeristy; 2RIKEN; 3Tohoku University
    This paper reports on transmission electron microscopy studies about the premartensitic phenomena that can be observed in both magnetic and non-magnetic shape memory alloys. We particularly focus on the issues about (1) lattice modulation observed before the onset of martensitic transformations in Ti-based alloys, and (2) change in magnetic domain structures by cooling the parent phase in Ni-based alloys; i.e., phenomenon due to magnetoelastic interplay. Principles of advanced electron microscopy methods (e.g., magnetic imaging by electron holography) will be also mentioned in this presentation.

4:40 PM  
A Design Strategy to Developing Shape Memory Alloys with Low Thermal Hysteresis: Deqing Xue1; Yumei Zhou1; Xiangdong Ding1; Dezhen Xue1; 1State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China
    The large thermal hysteresis during the temperature induced martensitic transformation is a major obstacle to the functional stability of shape memory alloys (SMAs). We proposed that SMA with the triple point composition in the composition-temperature phase diagram could possess a very low thermal hysteresis. We demonstrated such a recipe by establishing two different phase diagrams in the Ti-Ni-Pd-Cu system. In both phase diagrams, the triple point composition has a very low thermal hysteresis. We expect that this design strategy can guide the search for shape memory alloys with low thermal hysteresis.

4:55 PM  
A New Framework for the Interpretation of Modulated Martensites in Shape Memory Alloys (with OpenKIM): Ryan Elliott1; Vincent Jusuf2; 1University of Minnesota; 23M
     Modulated Martensites (MMs) are long-period stacking-order structures consisting of [110]_{B2} basal planes. First-principles computational results indicate the minimum energy phase is not a MM, but a short-period Ground State Martensite (GSM). Energy contributions from austenite-martensite kinematic compatibility are often believed to explain the observation of meta-stable MMs, as opposed to the GSM. However, a general approach for predicting MM structures and their properties has not previously been available.We develop a new framework for the interpretation of MMs as natural features of a material's energy landscape. A new understanding of MMs as a mixture of two short-period Base Martensite phases will be described. This MM Mixture Model (M^4) is capable of accurately predicting the energy, lattice constants, and structural details of an arbitrary MM. This is demonstrated by comparing the M^4 predictions to computational results from a particular empirical atomistic model, available at https://openkim.org

5:15 PM  
Glassy Behavior of Tweed in Defect-free Ferroelastics: Xiaofei Wang1; Xiangdong Ding1; Ekhard Salje1; 1State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University
    Molecular dynamics simulations were used to study the dynamic properties of tweed before ferroelastic phase transition. The system exhibits frequency dispersion and the shift of frequency fits the Vogel-Fulcher law very well. However, the Vogel-Fulcher temperature TVF, is lower than the phase transition temperature Tc, therefore we cannot see the ultimate freezing of the dynamics. In addition, Cole-Cole plots of the response show a depressed semicircle at temperatures near Tc, a typical characteristic of glass state. Spatial correlation of the lattice strain shows that the size of tweed patches reaches about 8 lattice spacing, and similar to relaxors, butterfly- and rod-shaped diffuse scattering is observed around {hh0} and {h00} Bragg spots near Tc, respectively. We believe the glass behavior is due to the growth and the inhomogenous distribution of the size of the cross-hatched tweed patches.