Date of Final Oral Examination (Defense)

8-2011

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science & Engineering

Department

Materials Science and Engineering

Major Advisor

Peter Müllner, Ph.D.

Comments

The reversible magnetic-field-induced plastic deformation that occurs in Ni-Mn-Ga alloys proceeds through the field-induced displacement of twin boundaries in the martensite phase. On the microscopic scale, the twin boundaries move through the motion of twinning dislocations. A fundamental understanding of the motion of twin boundaries requires detailed characterization of the twinning systems and the associated twinning dislocations. Twinning dislocations and twin microstructure of non-modulated Ni-Mn-Ga martensite were characterized with transmission electron microscopy. For ease of interpreting results, two different axis systems were used to represent the non-modulated structure: a face-centered tetragonal (T) lattice and a body-centered monoclinic (2M) lattice.

Two types of martensite variant interfaces were studied. The habit plane of the twins within each variant is (001)2M/(101)T. One type of interface appears smooth, where the martensite variants themselves are related with the habit plane
.
A second type of interfaces appears stepped, with a microscopic habit plane

Close to an inter-variant martensite interface, the thickness of twins varies. This is explained in terms of twin branching. It is shown that the modulated martensite structures can be constructed from branching of the non-modulated structure into nano-twinned variants.

Off-axis imaging of twins produced a distinct fringe pattern. Three fringe contrast levels appeared in a repeating sequence. The repeating contrast is consistent with a twin produced by an array of twinning dislocations with Burgers vector

and a step height equal to the d spacing of the

planes.

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