Dihedral angles, martensitic transformation in thin films, and role of twin boundaries in domain evolution

March 22, 2009

Did you know that Acta these days uploads supplementary material — like videos? I didn’t till I saw the paper on martensitic transformation in thin films by Buschbeck etal — wherein, a video of AFM surface topology as a function of temperature is also uploaded — which I think is a great move.

[1] Dihedral angles in Cu–1 wt.% Pb: Grain boundary energy and grain boundary triple line effects

D Empl et al

The dihedral angle shown by intergranular lead inclusions in Cu–1 wt.% Pb alloys is measured varying the purity of the metal and the temperature. Several measurement methods are used and compared, namely classical two-dimensional (2D) methods based on metallurgical cross-section analysis and a recently developed 3D stereoscopic method that yields the true three-dimensional angle value for individual inclusions straddling a flat grain boundary. We confirm and extend earlier measurements using the new method. We show that a discrepancy found between the literature data and the stereoscopic 3D dihedral angle measurements is not caused by impurity effects. Rather, the data indicate that the discrepancy has its origin in a difference in average dihedral angle values measured between inclusions straddling two grains and values found at inclusions located where three or more grains meet.

[2] In situ studies of the martensitic transformation in epitaxial Ni–Mn–Ga films

J Buschbeck et al

The martensitic transformation of epitaxial Ni–Mn–Ga films is investigated with respect to changes of structure, microstructure, magnetic and electronic properties. For this, temperature dependent atomic force microscopy (AFM), X-ray, magnetization and resistivity measurements are performed in situ, during martensitic transformation of a 500 nm thick film. The combination of these methods gives a comprehensive understanding of the martensitic transformation and allows to identify differences of constrained epitaxial films compared to bulk. Experiments show the formation of a twinned, orthorhombic martensite with high uniaxial magnetocrystalline anisotropy from the austenite around room temperature. High resolution AFM micrographs directly reveal how martensite variants grow and show the converging of variants nucleated at different nucleation sites. While most features are in agreement with a first-order transformation, the transformation proceeds continuously to lower temperatures, an effect which can be explained by the constraint from the substrate.

[3] Domain microstructure evolution in magnetic shape memory alloys: Phase-field model and simulation

Y M Jin

A phase-field micromagnetic microelastic model is employed to simulate domain microstructure evolution in magnetic shape memory alloys. The simulations reveal that coupled motions of martensite twin boundaries and magnetic domain walls depend not only on the external magnetic field but also on internal domain configurations. It is shown that a twin boundary can continue its motion under a decreasing magnetic field or even reverse motion direction without changing magnetic field. The domain microstructure-dependent driving forces for the coupled motions of martensite twin boundaries and magnetic domain walls are analyzed; these explain the complex domain processes and resultant peculiar magnetomechanical behavior of magnetic shape memory alloys.

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