Yoshio Itsumi

Electronic bonding characteristics of hydrogen in bcc iron: Part II. Grain boundaries

Electronic structure calculations were carried out for bcc iron grain boundaries (GB) with or without hydrogen, using the self-consistent Discrete Variational embedded cluster method within the first-principles local density formalism. Bonding characteristics were mainly investigated. Simple rigid body translations perpendicular to the GB plane were used for estimation of relaxed GB geometry. Analysis of bond order summation over the GB shows considerable volume expansion normal to the GB plane of a dense 23(111) twist/tilt GB and some compression for the rather open 23(110) twist configuration. These results are discussed in the context of atomistic simulations which suggest that higher energy GBs generally have larger volume expansion normal to the GB plane. H in a 23(111) GB reduces Fe-Fe bonding strength by —3% within a 0.25 nm spherical volume around the H site, associated with reduction of the 4 s and 4p occupancy of the nearest neighbor Fe. Since these orbitals contribute mainly to metallic bonding, the action of H atoms as an embrittlement inducer can be understood.

Fatigue Characteristics and Microstructures of Laser-and Non-Laser Welded Low-Cost Ti-4.5Al-2.5Cr-1.2Fe-0.1C for Use in Next Generation Aircrafts

Ti-4.5Al-2.5Cr-1.2Fe-0.1C referred to as KS Ti-531C is a newly developed low cost titanium alloy for use in next-generation air crafts by adding inexpensive elements such as C, Fe, Al and Cr. There is a possibility for the formation of TiCr and/or TiCr2 after a long heat treatment or welding. Therefore, relationships between the microstructures and fatigue properties of KS Ti-531C subjected to several heat treatments as well as laser welding were investigated systematically. The results showed that the fatigue strength of laser-welded KS Ti-531C was lower than that of non-laser welded KS Ti-531C.

Hot Deformation Behavior of Near-α Ti-Fe Alloy in (α+β) Two-Phase Region with Different Fe Content

In the present study, microstructure evolution of Ti-Fe alloys with different Fe content between 0.2-1.5mass% during hot deformation in (α+β) two-phase region is studied with focusing on effect of phase volume fraction at different deformation temperatures and strain rates. Hot deformation was conducted on the specimens quenched after β solutionizing at 1173K for 1.2ks at 1108, 1073 and 948K, by uniaxial compression by 50% at various strain rates ranged from 1 to 10-4 s-1. Initial structures are (α+β) lamellar structures of fine interlamellar spacing and colony sizes. Increase in Fe content results in increasing the fraction of the β phase at the given deformation temperature. Either colony size or interlamellar spacing is coarser at higher temperatures. At the higher deformation temperature where β phase fraction is larger, dynamic recovery of β phase is a major deformation mechanism while at a lower temperature, i.e., a higher α fraction, dynamic recrystallization of α phase occurs predominantly. It is concluded that critical strain needed for occurrence of dynamic recrystallization is decreased by increasing fraction of the α phase at the same deformation temperature, i.e., by decreasing Fe content. Furthermore, by increasing strain rate grain size of the recrystallized α is decreased.