Size Effects in Martensitic Transformations: Session 2
Program Organizers: Peter Anderson, The Ohio State University

Friday 8:00 AM
July 14, 2017
Room: Wrigley
Location: Hyatt Regency Chicago

Session Chair: William LePage, University of Michigan

8:00 AM  
Size Effects in Shape Memory Alloys: Competition between Structural and Microstructural Features in Determining Grain Scale Performance: Partha Paul1; Harshad Paranjape2; Aaron Stebner2; Peter Anderson3; L. Catherine Brinson1; 1Northwestern University; 2Colorado School of Mines; 3Ohio State University
     Stress induced phase transformation in polycrystalline NiTi is studied as a competition between two factors: extrinsic structural factors and intrinsic microstructural factors. Structural factors arise from stress concentrations around features such as holes. Microstructural factors include grain orientations and interactions between neighbors, which constrain the martensite microstructures allowed. In this work, we systematically study the relative influence of structural and microstructural factors through a combined experimental and modeling effort. The key conclusion is when the holes are much larger than the grains, we see a predominance of structural effects with a subtle influence of the microstructure effects. However, when the holes and grains are of comparable size, we see the complete domination of microstructural effects. These findings are key to understanding the dominant mechanism in porous SMAs, with pores comparable to grain size and also in choosing a reliable modeling framework for miniature SMA components such as micron-sized sensors.

8:20 AM  
Phase Field Study of Lattice Instability and Microstructure Evolution in Silicon During Phase Transformation under Complex Loading: Hamed Babaei1; Valery I. Levitas1; 1Iowa State University
    Phase field theory is developed, which takes into account crystal lattice instability conditions for martensitic phase transformations between cubic diamond and beta tin phases of silicon under complex loading obtained using molecular dynamics simulations (Levitas, Chen, and Xiong, 2016). Theory includes fully geometrically nonlinear formulation and new interpolation functions different for different material parameters. Finite element algorithm and numerical procedure is developed and implemented in the code DealII. Various problems on lattice instabilities and following nanostructure evolution in silicon single crystal under complex three-dimensional loadings are solved and analyzed. Special emphases are on the studying the combined and nontrivial effects of sample size and stress state on the nanostructure and the entire transformation process. Results are compared with molecular dynamic simulations.

8:40 AM  
Phase Transformations in Beta-metastable Titanium Alloys: Elisabeth Aeby-Gautier1; Eiichi SUKEDAI; Moukrane DEHMAS2; Benoît APPOLAIRE3; 1Institut Jean Lamour, CNRS/Université de Lorraine UMR 7198; 2CIRIMAT; 3LEM UMR CNRS-ONERA
     For beta-metastable titanium alloys, HEXRD experiments and additional SEM/TEM studies at the lower temperatures (450°C) evidenced the formation of metastable phases, the \945;’’ phase, with an orthorhombic centred structure and the ω phase with a hexagonal structure. The orthorhombic structure is often considered as an HCP (\945;) structure, when characterized by TEM. We thus name it the \945;’’/\945; phase. At temperatures near 300°C, the ω phase forms first, and the nano-sized precipitates act as nucleation sites for the further \945;’’/\945; formation. The morphology of the \945;’’/\945; precipitates is rather ellipsoidal. At the higher temperatures (near 400°C), no or poor amount of ω phase is formed before the formation of \945’’/\945 The \945;’’/\945; precipitates present a thin plate morphology with a very low width/diameter ratio and are spatially self-organized. Our contribution will discuss the morphologic features obtained considering the \945;’’/\945; formation in both conditions.

9:00 AM  
Synergies between Omega Phase and Alpha" Martensite in Beta Ti Alloys: Benoît Appolaire1; Elisabeth Aeby-Gautier2; 1LEM, ONERA/CNRS; 2CNRS-Université de Lorraine
     In beta Ti alloys, it is often reported that omega precipitation favors the appearance of alpha hcp equilibrium phase as well as alpha" orthorhombic martensite. This has been put into evidence either post-mortem by TEM, or in-situ by high energy Xray diffraction. Despite intense efforts to clarify the mechanisms for such a synergy, there seems to be no real consensus yet. Indeed, either it is proposed that alpha" nucleates as internal plates inside the omega precipitate; or it is reported that alpha" nucleates at the omega/beta boundary.In this contribution, we will investigate the different hypotheses with full field elastic calculations relying on an efficient FFT based algorithm. In particular, the elastic interactions between omega precipitates computed with a phase field model and embryos of alpha" will be examined to determined the preferred locations from an elastic point of view.

9:20 AM  
Precipitates and Nanoscale Templating of Shape Memory Alloys: Kathryn Esham1; Harshad Paranjape1; Lee Casalena1; Mike Mills1; Peter Anderson1; 1Ohio State University
    The martensitic transformation in NiTi shape memory alloys can occur up to 100⁰C. Alloying and aging with Hf creates nanoscale precipitates and raises transformation temperature. Precipitates suppress fatigue and ratcheting, yet allow twinned martensitic lathes to grow. How precipitates accomplish this is not well understood. Phase-field finite element analysis is used in conjunction with a custom user defined material code to study the effect of precipitates on microstructure transformation and plasticity. It is hypothesized that elongated H-phase precipitates constrain the phase transformation parallel to the long axis. Distortion due to misfit strain triggers nucleation, so precipitate shape and orientation control the pattern of local martensite variant formation. This ability to control both constraint and nucleation allows precipitates to serve as templates in designing the microstructure. Understanding the templating these precipitates provide will help define material properties and lifetime, opening up new design methodologies for high temperature SMAs.

9:40 AM  
Breakdown of Shape Memory Effect in Bent CuAlNi Nanopillars: When Twin Boundaries Become Stacking Faults: Xiangdong Ding1; Ekhard Salje2; Jun Sun1; 1Xi'an Jiaotong University; 2Cambridge University
    Bent CuAlNi nanopillars (diameters 90~750 nm) show a shape memory effect, SME, for diameters D > 300 nm. The SME and the associated twinning are located in a small deformed section of the nanopillar. Thick nanopillars (D > 300 nm) transform to austenite under heating, including the deformed region. Thin nanopillars (D< 130 nm) do not twin but generate highly disordered sequences of stacking faults in the deformed region. No SME occurs and heating converts only the undeformed regions into austenite. The defect-rich, deformed region remains in the martensite phase even after prolonged heating in the stability field of austenite. By using molecular dynamic simulations, we further explored some novel mechanisms for SME and superelasticity in non-transformed nanomaterials and 2D materials. (References: 1) Nano Letters, 16, 194-198, 2016; 2) Advanced Functional Materials, 26, 760-767, 2016; 3) Journal of the American Chemical Society, 138, 4772−4778, 2016)

10:00 AM Break