טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBarzilay Maya
SubjectDeformation Mechanisms in B2 Structured Intermetalic
Compounds
DepartmentDepartment of Materials Science and Engineering
Supervisor Research Professor E Dan Shechtman
Full Thesis textFull thesis text - English Version


Abstract

Superior physical, chemical and mechanical properties of intermetallic compounds, compared to properties of commonly used metals, should have resulted in their extensive use for various applications. However, low room temperature ductility renders these intermetallics useless for structural applications.

A new class of ductile intermetallic compounds was recently discovered made of equal atomic ratios of a rare-earth element and late transition metal or early p-element, and have cubic CsCl-type (B2) structure.

These intermetallics have high ductility and high fracture toughness at room temperature, with several of them exceeding a remarkable value of 21% tensile strain. The reason for the high ductility and strength is not entirely clear.

In an effort to understand the role of structural defects of the B2 structure in determining the mechanical properties, scandium-base B2 intermetallics were studied. We report on the nature of defects found in ScNi, ScRu and ScAl.

The intermetallics were cast in an arc furnace, annealed under protective atmosphere and then cut and thinned for transmission electron microscopy studies. We have performed the structural studies by transmission electron microscope (TEM) diffraction contrast using g·b analysis and have also used phase contrast in a high resolution transmission electron microscope (HRTEM) for lattice imaging and analysis. Dislocation slip vectors and slip planes were determined and twin formations studied.

The ScNi and ScRu samples contain high density of entangled dislocations. Two types of dislocations were found to form in these intermetallics with slip vectors b=<100> and b=<111>. A B2 material that deforms only by > 100 < dislocation has only three of the five independent slip modes required by the von Mises criterion for polycrystalline ductility. However, the activity of > 111 < dislocations satisfies the criterion and may explain the high ductility of this material.

Slip in B2 structures may occur on the {100} and {110} planes. In this study it was found that ScNi and ScRu slip only on the {110} planes and no {100} slip was observed.

The ScAl structure is different, as TEM diffraction patterns indicate some deviation from the cubic structure. Contradictory data appear in earlier studies concerning the crystal structure of ScAl, described either as cubic cP2-CsCl type or orthorhombic oP8-CrB. Stacking faults on {110} planes, associated with a slip by ½ <001> may transform the B2 to B33. In our study it was found that the transition occurred under the electron beam in the HRTEM (300KeV) and we were able to record it.