Yasuharu Shirai

Deformation of polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition

Abstract Polysynthetically twinned crystals of TiAl with a nearly stoichiometric composition have been grown and deformed in compression at room temperature. The yield stress and deformation behaviour depend strongly on the angle between the twin boundaries and the compression axis rather than on the crystallographic orientation of the compression axis. The yield stress is high when compression is perpendicular or parallel to the twin boundaries, and is generally very low for specimens where the twin boundaries are at an intermediate angle to the compression axis. The ratio of the highest to the lowest values of yield stress is almost 8:1. This large difference in yield stress has been found to be related to the difference in the deformation mode; for compression perpendicular or parallel to the twin boundaries, shear deformation always occurs across them while, for compression at an intermediate angle, shear deformation is parallel to the boundaries. The former mode of deformation is much harder than the...

Plastic deformation of MoSi2 single crystals

Abstract The deformation behaviour of MoSi2 single crystals has been studied as a function of crystal orientation in the temperature range from - 196 to 1500°C in compression. While [001]-oriented crystals can be plastically deformed only at high temperatures above 1300°C, plastic flow is possible from temperatures as low as room temperature for single crystals with orientations other than [001]. Five slip systems { 110)(111], {011)(100], {010)(100], {023)(100] and {013)(331] are identified to be operative, depending on crystal orientation. An anomalous increase in critical resolved shear stress (CRSS) is observed in the intermediate-temperature range for the former three slip systems. The anomalous temperature range as well as the magnitude of such an anomaly varies with slip system. The Schmid law is generally valid for the {110)(111], {011)(100] and (023)(100] slip systems. In particular, this is the case even in the temperature range of the anomaly in CRSS for the {110)(111j system. In contrast, the C...

Gamma Titanium Aluminide Alloys

Extensive progress and improvements have been made in the science and technology of gamma titanium aluminide alloys within the last decade. In particular, the understanding of their microstructural characteristics and property/microstructure relationships has been substantially deepened. Based on these achievements, various engineering two-phase gamma alloys have been developed and their mechanical and chemical properties have been assessed. Aircraft and automotive industries arc pursuing their introduction for various structural components. At the same time, recent basic studies on the mechanical properties of two-phase gamma alloys, in particular with a controlled lamellar structure have provided a considerable amount of fundamental information on the deformation and fracture mechanisms of the two-phase gamma alloys. The results of such basic studies are incorporated in the recent alloy and microstructure design of two-phase gamma alloys. In this paper, such recent advances in the research and development of the two-phase gamma alloys and industrial involvement are summarized.

Temperature dependence of yield stress, tensile elongation and deformation structures in polysynthetically twinned crystals of Ti-Al

Abstract The plastic deformation behaviour of polysynthetically twinned (PST) Ti-Al with three different orientations has been studied in tension and compression as a function of temperature in a range from −196 to 1100°C. With increase in temperature, the yield stress decreases rather rapidly at low temperatures and then decreases gradually at intermediate temperatures for all orientations studied although the temperature dependence at low temperatures is less significant for an orientation where shear deformation occurs parallel to the lamellar boundaries. When the loading axis is perpendicular to the lamellar boundaries, the yield stress again rapidly decreases with increasing temperature at high temperatures. This is also the case for PST Ti-Al whose lamellar boundaries are parallel to the loading axis, although a small and broad anomalous yield stress peak is observed at 800°C for this orientation. When the lamellar boundaries are inclined at an intermediate angle to the loading axis, the yield stres...

Low-temperature deformation of single crystals of a DO19 compound with an off-stoichiometric composition (Ti-36·5 at.% Al)

Abstract Deformation behaviour of a DO19, compound with an off-stoichiometric composition of Ti-36·5 at.% Al has been studied as a function of orientation at temperatures from −196°C to room temperature in compression. The critical resolved shear stress (CRSS) for {1100} 〈11 20〉 prism slip and (0001)〈1120〉 basal slip increases with decreasing temperature. a dislocations introduced by prism slip have a tendency to align along their screw orientation at −196°C, but the tendency becomes less marked with increasing temperature so that at room temperature these dislocations do not necessarily align along any particular crystallographic orientations. The CRSS for {1121} 〈1126〉 pyramidal slip shows a minimum in the CRSS-temperature curve at a temperature T min between −196 and −100°C. Most 2c + a dislocations are in screw orientation at temperatures below T min where the CRSS increases with decreasing temperature, whereas at temperatures above T min, where the CRSS anomalously increases with increasing temperatu...

Interlaboratory Comparison of Positron Annihilation Lifetime Measurements

An interlaboratory comparison for positron annihilation lifetime (PAL) measurements for pure nickel and fused silica was performed. Based on the reported positron (for nickel) and positronium (for fused silica) lifetimes, the uncertainties in the PAL measurements were estimated.

Electrical resistivity of Ll2 trialuminides containing 3d transition element

Abstract Electrical resistivity and its temperature dependence have been measured in a wide temperature range of 4.2–1500 K for a number of Ll 2 ordered (AlX) 3 Ti and (AlX) 3 Zr compounds, where X = Cr, Mn, Fe, Co, Ni, Cu, Pd, or Ag, and quite similar results are obtained in both compound systems. The electrical resistivity of the ternary compounds investigated is found to vary systematically as a function of the atomic number of the transition alloying element X. A maximum residual resistivity (30 μΩ cm at % ) is observed when X = Fe for both compound systems. Observed resistivity change with the transition elements can be qualitatively explained by the Mott-Friedel-Anderson theory. Compounds containing Mn exhibit negative temperature dependence over the whole temperature range, while those with Cr or Fe show a convex change with a resistivity maximum. Such characteristic temperature dependencies are possibly associated with the structure of the virtual bound state of each transition element.

Radiation damage and the recovery of neutron-irradiated TiAl studied by positron lifetime spectrometry

Abstract Radiation damage and the recovery of neutron-irradiated γ-TiAl (Ti 44 Al 56 ) have been studied using positron annihilation lifetime spectrometry. The recovering process of the neutron-irradiated TiAl can be divided into two stages. The first stage, below 660 K, is ascribed to the migration of point defects and the second stage, above 660 K, is due to the dissolution of secondary defects formed at the former stage. The major defect induced by reactor neutron-irradiation is collapsed planar aggregates of point defects. Although some non-collapsed vacancy clusters are also induced, they consist of only a few vacancies each, seldom grow into larger voids during the subsequent annealing, and almost disappear below 700 K. Neutron-irradiated Ti 44 Al 56 recovers to the fully annealed state below 960 K. γ-TiAl has good tolerance for radiation.

Vacancies and their clusters in MoSi2 studied by positron lifetime spectrometry

Abstract Vacancies and their clusters in MoSi2 single crystals irradiated with 1MeV electrons, irradiated with 13 MeV protons, water-quenched from 1473 K and aircooled from temperatures in the range of 973–1473 K were studied by positron lifetime spectrometry. The results of isochronal annealing of these specimens reveal that (1) most of defects retained in electron-irradiated specimens are single vacancies at Si sites, (2) such single vacancies begin to migrate at around 240 K and form vacancy clusters, (3) such vacancy clusters begin to be annealed out through the evaporation of vacancies at around 500 K and the recovery of electron-irradiated specimens is completed at around 800 K, (4) the recovery of proton-irradiated specimens begins just after irradiation at 100 K and is completed at around 800 K, similarly to that of electron-irradiated specimens, (5) a much longer positron lifetime is observed in proton-irradiated specimens than in electron-irradiated specimens, indicating that large-scale vacancy clusters are formed in the former specimens, (6) vacancy-type defects can be retained not only in water-quenched but also in air-cooled specimens.

Vacancies and their properties in L12 Al67Mn8Ti25 studied by positron lifetime spectroscopy

Abstract Vacancies and their properties have been studied on electron-irradiated L1 2 Al 67 Mn 8 Ti 25 by using positron annihilation lifetime spectroscopy. No constitutional vacancies have been detected in the material; all constitutional point defects are substitutional anti-site atoms. The irradiated material recovers in two major stages. The first stage, around 350 K, is ascribed to the migration of vacancies. The final stage, around 500 K, is due to the dissolution of secondary defects formed at the first stage. These secondary defects are not voids but collapsed vacancy-clusters that consist of dislocations. Electron-irradiated L1 2 Al 67 Mn 8 Ti 25 recovers almost to the fully annealed state below 550K.