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Deforemation and Dislocation Tem Image Simulation in L10-Fepd

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Deforemation and Dislocation Tem Image Simulation in L10-Fepd DEFOREMATION AND DISLOCATION TEM IMAGE SIMULATION IN L10-FEPD by Amal A. Al Ghaferi BE, United Arab Emirates University, 2000 Submitted to the Graduate Faculty of School of Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy University of Pittsburgh 2006 UNIVERSITY OF PITTSBURGH SCHOOL OF ENGINEERING This dissertation was presented by Amal A. Al Ghaferi It was defended on December, 4 , 2006 and approved by Dr. John A. Barnard, Professor, Department of Mechanical Engineering and Materials Science Anthony J. DeArdo, Professor, Department of Mechanical Engineering and Materials Science Ian Nettleship, Associate Professor, Department of Mechanical Engineering and Materials Science Jean R. Blachère, Associate Professor Emeritus, Department of Mechanical Engineering and Materials Science Dr. J.M.K. Wiezorek, Associate Professor , Department of Mechanical Engineering and Materials Science, Dissertation Director ii Copyright © by Amal A. Al Ghaferi 2006 iii DEFORMATION AND DISLOCATIONS TEM IMAGES SIMULATION OF L10 FEPD Amal A. Al Ghaferi, PhD University of Pittsburgh, 2006 A yield strength anomaly has been observed in intermetallic L10-ordered FePd alloys between about 300-700K. However, previous work did not provide clear explanations regarding the microstructural or mechanistic origins for this observation. FePd and γ-TiAl have the same crystal sturtucre. YSA was studied intensively in γ-TiAl system and its origin was related for the dislocation fine structure. In TiAl, the dissociation of the superdislocation [10-1] play an important role in the YSA phenomena and the existence of Yoo torque promote dissociated shockly partial to cross slip to another slip plane. This forms KW lock which is the reason of YSA in TiAl. The Elastic Constants of the tetragonal L10 phases describe the intrinsic elastic properties of the L10-FePd .Details of the dislocations fine structure, such as the details of dissociations and decompositions of dislocations and the relevant planar fault energies, are determined by the elastic constants , including the elastic interactions between partials dislocations and the effect of the Yoo torque. The elastic constants are required for the computer image simulations of WBDF TEM images of narrowly dissociated dislocation configurations. Elastic constants were calculated for FePd and were compared to Elastic constants of γ-TiAl and FePt. Dislocations TEM Image Simulation will provide contrast guidance for experimental studies in both FePd and FePt. Experimental TEM studies of the fine structure require extensive tilting of the thin crystal section within the magnetic field of the objective lens of the TEM iv instrument and are difficult to perform for the hard ferromagnetic FePd and FePt intermetallics since magnetic effects of the sample material on the electrons used for imaging lead to complicated artifacts in images upon tilting through significant angles. Hence, image simulations may assist in identifying appropriate imaging conditions under which significant and relevant differences in the dislocation image contrast may be expected for the dislocations in the two different intermetallics. This study builds the fundamental basic in the research of the origin of the YSA in FePd. Although the origin of YSA in L10 FePd has not been identified but the basic intrinsic elastic properties, i.e. elastic constants, has been measured experimentally for the fist time and these elastic constants were used in dislocations TEM image simulation . v TABLE OF CONTENTS PREFACE ..................................................................................................................................................................xiii 1.0 INTRODUCTION ..........................................................................................................................................1 2.0 BACKGROUND ............................................................................................................................................4 2.1YIELD STRESS ANOMALIES ................................................................................................................................4 2.1.1 YSA in L12 System ........................................................................................................................6 2.1.1.1 Ni3Al .................................................................................................................................................... 6 2.1.2 YSA in L10 Systems: ......................................................................................................................10 2.1.2.2 FEPT .................................................................................................................................................. 22 2.2 FEPD BACKGROUND .......................................................................................................................................25 3.0 MOTIVATION .....................................................................................................................................................32 4.0 OBJECTIVES ...............................................................................................................................................34 5.0 THEORY AND EXPERIMENTS.........................................................................................................................36 5.1 GROWTH AND PROPERTIES OF SINGLE CRYSTALS OF FEPD .........................................................................36 5.1.1 Fabrication of L10-FePd single crystals .......................................................................................36 5.1.2 Fabrication of equiaxed L10-FePd polycrystals ...........................................................................40 5.1.3 Experimental Methods:.................................................................................................................42 5.2 ELASTIC CONSTANTS CALCULATIONS ............................................................................................................45 5.2.1 Young’s Modulus : ........................................................................................................................48 5.2.1.1 YOUNG’S MODULUS FOR A TENSILE AXIS WITHIN THE (001) PLANES............................. 48 vi 5.2.1.2 YOUNG’S MODULUS FOR A TENSILE AXIS WITHIN THE (100) PLANES............................ 48 5.2.1.3 YOUNG’S MODULUS FOR A TENSILE AXIS WITHIN THE (1-10) PLANES............................ 48 5.2.2 Shear Modulus :............................................................................................................................49 5.2.2.1 SHEAR PLANE (001) ........................................................................................................................ 49 5.2.2.2 SHEAR PLANE (100) ........................................................................................................................ 50 5.2.2.3 SHEAR PLANE (1-10) WITH SHEAR STRESS ROTATED FROM [001] TO [110]...................... 50 5.3 COMPRESSION TESTS ......................................................................................................................................52 5.4 DISLOCATION TEM IMAGE SIMULATION: .......................................................................................................55 5.4.1 Image Simulation Parameters: .....................................................................................................60 5.4.2 Diffraction Contrast from Stacking Faults............................................................................62 5.4.3 Weak Beam Dark Field:................................................................................................................65 5.4.3.1 REQUIREMENT OF WEAK BEAM MICROSCOPY ...................................................................... 67 6.0 RESULTS AND DISCUSSIONS .........................................................................................................................68 6.1 PREPARATION AND CHARACTERIZATION TO L10 FEPD SINGLE CRYSTAL ......................................................68 6.1.1 XRD..............................................................................................................................................68 6.1.2 VSM Measurements: .....................................................................................................................69 6.2 ELASTIC PROPERTIES OF L10 FEPD..................................................................................................................73 6.2.1 Comparison of Young’s modulus for FePd, FePt and TiAl...........................................................76 6.2.1.1 YOUNG’S MODULUS FOR A TENSILE AXIS WITHIN THE (001) PLANES............................. 77 6.2.1.2 YOUNG’S MODULUS FOR A TENSILE AXIS WITHIN THE (100) PLANES............................. 77 6.2.1.3 YOUNG’S MODULUS FOR A TENSILE AXIS WITHIN THE (1-10) PLANES............................ 78 6.2.2 Comparison of Shear Modulus for FePd, FePt and TiAl:.............................................................80 6.2.2.1 SHEAR PLANE (001) WITH THE SHEAR STRESS DIRECTION ROTATED ………………..….80 6.2.2.2 SHEAR PLANE (100) …………………………………………………………………………………80 6.2.2.3 Shear plane (1-10) with shear stress rotated from [001] to [110]: ...........................................81 2.2.2.4 SHEAR PLANE (111) .......................................................................................................................
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