Yukichi Umakoshi

Unique alignment and texture of biological apatite crystallites in typical calcified tissues analyzed by microbeam X-ray diffractometer system

Preferential orientation of biological apatite (Ap) crystallites in typical calcified tissues of rabbit ulna, rabbit skull, and monkey dentulous mandible was investigated using a microbeam X-ray diffractometer, with a beam spot of 100 microm in diameter, to clarify relationship between the Ap orientation and mechanical function. Preferential alignment of the c-axis of the biological Ap was evaluated by the relative intensity between (002) and (310) diffraction peaks. Preferential alignment of biological Ap in each calcified tissue varied depending on the shape and stress condition in vivo; that is, the c-axes of biological Ap in the rabbit ulna and the rabbit skull bone were preferentially observed as a one-dimensional orientation along the longitudinal axis and a two-dimensional orientation along the surface, respectively. Precise analysis of the preferential alignment along the skull surface showed an elliptical distribution of the c-axis of biological Ap elongating along the suture inside the skull surface of both lamina exterior and interior. The c-axis of biological Ap in a monkey dentulous mandible basically aligned along the mesiodistal direction in the flat bone, but this alignment changed along the normal direction to the flat bone surface parallel to the biting direction near the tooth, due to the force of mastication. It was concluded that the microscale measurement of biological Ap texture is one of the useful new methods for evaluating mechanical function and stress distribution in vivo in calcified tissues.

The asymmetry of the flow stress in Ni3(Al,Ta) single crystals

Abstract Flow stress measurements were performed on single crystalline Ni 3 (Al, Ta) as a function of temperature, orientation, strain rate and sense of the applied uniaxial stress to check the predictions of the Paidar et al. model [ Acta metall. 32 , 435 (1984)]. It was found that the critical resolved shear stress (CRSS) for (111)[101] slip depends not only on the test temperature and orientation of the samples, as other investigators have previously observed, but also on the sense of the applied stress. The orientation dependence of the tension/compression asymmetry, including the regions where the asymmetry is a maximum (positive), a minimum, and where it disappears, is as predicted by the model. The applied stress changes the activation enthalpy of cross slip primarily through its effect on constricting the Shockley partials during cross slip and only secondarily on directly promoting (111) to (010) cross slip. A maximum attainable CRSS for (111)[101] slip, the saturation stress, is also in agreement with the model. It was also found that the CRSS for (111)[101] slip is strain rate independent, but the CRSS for (001)[101] slip shows a strong positive strain rate dependence. The temperature at which the peak in the flow stress vs temperature curve occurs increases with increasing strain rate and decreases with increasing ratio of RSS on (001)[110] divided by that on (111)[101]. When the deformation occurs by (001)[110] slip the stress-strain curve exhibits clearly defined, continuous yield points.

Plastic deformation of the intermetallic compound Al3Ti

Abstract Polycrystalline specimens of the intermetallic compound Al3Ti with the D022-type structure have been deformed under compression at 25–860°C. At temperatures below 620°C, specimens fracture with very small fracture strain since extremely localized deformation occurs and cracks are initiated in intensely deformed regions almost at yielding. At temperatures above 620°C, good compression ductility is observed. Yield stress starts decreasing rapidly with increasing temperature at around 330°C and becomes less temperature dependent above 630°C. The major mode of deformation of Al3Ti is twinning of the type (111)[112] which does not disturb the D022 symmetry of the lattice. However, at high temperatures, the four (111)[112]-type twinning systems are augmented by slip of the types [110], [100] and [010]. This would supply an explanation for the good compression ductility observed at temperatures above 620°C. Suggestions are made for further experimental work on alloying additions which may improve the du...

Oxidation resistance of intermetallic compounds Al3Ti and TiAl

The oxidation kinetics and morphological features of Al3Ti and TiAl were investigated. The oxidation resistance of Al3Ti is much better than that of TiAl, for example, by a factor of about 30 at 1000° C for 48 h. The big difference in the oxidation resistance is related to the characteristics of the external oxide scales of a protective Al2O3 or a mat of crystalline TiO2 formed on Al3Ti or TiAl, respectively. Sufficient aluminium transport from Al3Ti assists the formation of the Al2O3 scale which acts as a protective film against oxidation. The poor aluminium content of TiAl produces Ti3Al phase at the interface of TiAl and oxide scales and increases the diffusivity of titanium in the Al2O3 scale. The external crystalline TiO2 scale produced by the diffusion of titanium through the Al2O3 scale enhances oxidation of TiAl.

High temperature deformation of MoSi2 single crystals with the C11b structure

Abstract Single crystals of MoSi2 with the C11b structure have been deformed in compression at 900–1500°C. Slip of the {013}〈3 3 1〉- and/or the {013}〈3 3 1〉- type is activated depending on crystal orientation. The critical resolved shear stress (CRSS) for {013}〈3 3 1〉- slip is much higher than that for {013}〈3 3 1〉- slip . At about 1300°C where ductility is remarkably improved, 〈100〉- and 〈110〉-type ordinary dislocations are observed and the climb mobility of these dislocations is very high. The CRSS for {013}〈3 3 1〉- slip , which depends on orientation exhibits a slight anomalous peak at 1000–1100°C. The anomalous strengthening is interpreted by the cross-slip mechanism as similar to that in L12 ordered alloys. Ductility improvement of MoSi2 above about 1200°C is assisted by the formation of stacking fault which is closely related to the instability of the C11b against to the C40 structure.

Deformation of FeAl single crystals at high temperatures

Abstract Single crystals of the intermetallic compound FeAl with the CsCl structure have been deformed in compression at temperatures between 470 and 1000 K to study the slip geometry and the dislocation arrangements in this material at high temperatures. At temperatures below 0·44 of the melting temperature, T m, slip occurs on the {110}〈111〉 systems, while at 0·44T m, the transitions in the slip directions from 〈111〉 to 〈001〉 takes place for orientations in the right-hand side of the unit stereographic triangle, i.e. near the [011]-[111] boundary and, as the deformation temperature is increased, the region showing the 〈001〉-type slip becomes wider. The critical resolved shear stress for both types of slip exhibits a strong temperature dependence above 0·44T m. The dislocation arrangements in crystals deformed by the slip of 〈001〉 type is found to consist mostly of edge dislocations. The occurrence of the 〈001〉-type slip at such a high temperature is suggested to be associated with compositional deviatio...

The effect of compositional deviations from stoichiometry on the transition in slip directions from (111) to (001) in FeA1 at high temperatures

Abstract Single crystals of iron-rich FeA1 with three different compositions have been deformed in compression in the range 573–973 K. The effect of the deviation from stoichiometry on the (111)→〈001〉 slip-mode transition which occurred at around 0.5 Tm was studied.

The role of ordered domains and slip mode of α2 phase in the plastic behaviour of TiAl crystals containing oriented lamellae

Abstract TiAl crystals containing a single set of lamellae composed of the α 2 and γ phases were grown using the floating zone method. The lamellar spacing was controlled in the range between 0.37 and 1.8 μm. The plastic behaviour of these crystals depends strongly on the deformation mode of α 2 phase, the domain size and the lamellar spacing. When the stress is applied parallel to the lamellar planes, shear deformation occurs across the lamellae and the {10 1 0} prismatic slip is activated in the α 2 phase. For specimens with the loading axis perpendicular to the lamellar planes, the {11 2 1} pyramidal slip occurs with application of high stress. The mode of deformation in the α 2 phase is responsible for a large anisotropy of plastic behaviour of TiAl containing lamellae. The lamellar spacing and the domain size affect the yield stress similar to the effect caused by grain size. The lamellar boundaries between thin α 2 plates and γ phase provided more effective strengthening than the domain boundaries.

The effect of orientation and lamellar structure on the plastic behavior of TiAl crystals

Abstract Crystals of binary TiAl (48.1–51.6 at.% Al) containing a single set of lamellae were grown using the floating zone method, and the spacings between the lamellae were changed by altering the crystal growth rate and the alloy composition. The plastic behavior of TiAl was found to depend strongly on the lamellar spacing and the angle between the loading axis and the lamellar planes. When the stress is applied parallel or perpendicular to the lamellar planes, shear deformation proceeds on {111} planes which cross the lamellae, the specimens are very strong and they exhibit low ductility. However, when the stress is applied at intermediate angles, slip occurs on {111} planes parallel to the lamellae at a low applied stress and good ductility is observed. The mode of deformation in the α2 phase, i.e. {11 2 1}〈 11 26〉 or 〈10 1 0〉〈 2 10〉 slip, strongly affects the plastic behavior of lamellar TiAl compounds. A fine and uniform distribution of lamellae is necessary to obtain high strength and good ductility.

Orientation and temperature dependence of yield stress and slip geometry of Ti3Al and Ti3Al-V single crystals

Single crystals of binary Ti3Al and ternary Ti3Al-V alloys with the D019 structure were deformed in compression at 20–900°C. Slip systems of the {1010}〈1210〉-type and the {1121}〈1126〉-type were observed in these alloys throughout the entire temperature range depending on orientation, but the {1121}〈1126〉-slip was limited to orientations near [0001]. the basal (0001)〈1210〉-slip activated in quenched Ti3Al. The CRSS for the {1010}〈1210〉-slip in the binary and ternary alloys decreases monotonically with increasing temperature. In the ternary alloy the CRSS for the {1010}〈1210〉-slip shows a violation of Schmids law, while the binary alloy obeys the CRSS law. When Ti3Al is deformed by {1121}〈1126〉-slip the CRSS for the slip exhibits an anomalous peak in the temperature-CRSS curve but the addition of vanadium suppresses the extent of the anomalous strengthening.