Observation of Deformation Substructure in Fatigued Intermetallic Alloys Using Magnetic Properties
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Observation of Deformation Substructure in Fatigued Intermetallic Alloys Using Magnetic Properties Yukichi Umakoshi and Hiroyuki Y. Yasuda* Department of Materials Science and Engineering, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871 Japan *Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1, Mihogaoka, Ibaraki, Osaka 567-0047 Japan ABSTRACT Magnetic properties in ferromagnetic materials are very sensitive to lattice defects such as dislocations and planar faults. Dislocation substructures and planar faults in fatigued intermetallic compounds of Ni3Fe, Fe3Al and FeAl were observed using change in magnetic properties of spontaneous magnetization and high-field susceptibility. This magnetic technique was applied for the nondestructive evaluation of fatigue life. INTRODUCTION Cyclic deformation behavior and fatigue life of intermetallic compounds differ from those of disordered alloys due to the low frequency of cross slip events of superlattice dislocations and the formation of planar faults in the ordered lattice [1]. Spatial distribution and morphology of lattice defects such as dislocations and planar faults produced during the to-and-fro motion of mobile dislocations are very important to understand the fatigue behavior of compounds. However, exact observation of the type of dissociation of the dislocations and the planar faults in the fatigued compounds is very difficult because of high density of the lattice defects. Development of a new method to observe highly dense lattice defects is required for understanding the fatigue behavior of intermetallic compounds. Magnetic properties are known to be influenced by plastic deformation. For example, Pt3Fe is anti-ferromagnetic in the fully ordered L12 structure while it shows strong ferromagnetism after cold rolling because the atomic environment and magnetic couplings are changed by cold rolling. Since Ni3Mn shows ferromagnetism in the ordered state and antiferromagnetism in the disordered state, atomic rearrangement by plastic deformation induces ferromagnetism [2]. In ferromagnetic materials, magnetic properties are very sensitive to lattice defects such as dislocations and planar faults due to change of magneto-elastic coupling energy on the basis of the inhomogeneous residual stresses around dislocations and the atomic rearrangement in the vicinity of planar faults [3, 4]. In this study dislocations and anti-phase boundary (APB) tubes in fatigued single crystals of the ordered Ni3Fe and Fe3Al are observed by means of magnetic technique and cyclic hardening/softening behavior is discussed. The magnetic technique is applied for the nondestructive evaluation of fatigue life of FeAl single crystals. RESULTS AND DISCUSSION Observation of dislocations and APB in fatigued crystals through magnetic properties
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In ferromagnetic crystal containing dislocations, the magnetic free energy (Φ) can be expressed by the sum of the magnetocrystalline energy (Φk), exchange energy (Φex), mag
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