Masaharu Yamaguchi

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.

Deformation and improvement of ductility of the intermetallic compounds Al3 V and Al3(V, Ti)

Abstract Polycrystalline specimens of the intermetallic compounds Al3V and Al3(V, Ti) containing 1 - 25 at.% Ti with the D022 structure have been deformed under compression temperatures between 20 and 900°C. Al3V shows only a small fracture strain at low temperatures. The deformation is mainly controlled by the motion of 〈110〉-type dislocations without twinning. The activation of 〈110〉 and 〈010〉 slip in addition to 〈110〉 slip contributes to the ductility improvement at high temperatures. Ductility of Al3V has been found to be induced by the replacement of vanadium by titanium. For example, Al3(V0.95Ti0.05) shows 7% compressive fracture strain at room temperature. In Al3(V0.95Ti0.05) deformed at room temperature, not only dislocations but also numerous deformation twins and stacking faults are observed. Therefore, the improvement of ductility of Al3V by the partial replacement of vanadium by titanium arises from a substantial increase in the activity of the ordered twinning which is not operative in the bi...

The core structures of a ⅓〈1 123〉{1122} edge dislocation under applied shear stresses in an h.c.p. model crystal

Abstract The behaviour of a ⅓〈1 123〉{1122} edge dislocation under the effect of an external shear stress applied on {1122} planes has been studied by computer calculation. Before the application of the external stress, the dislocation has either of two typos of core configuration; one spreads as two twin faults on {1121} and {1122} planes (type I) and the other as a stacking fault with a fault vector of b/2, hounding partial cores still having the same twin faults as in the type-I core (type II). When the shear stress is applied in a direction that corresponds to tension along the c axis, the {1121) twin develops from both types of core under suitable conditions. When the stress is applied in the opposite direction, the type-I core changes into type-II; the dislocation moves as an extended dislocation without development of the {1122} twin. The results are interpreted and discussed in the light of experimental observations on deformation in h.c.p. metals.

Plastic deformation of Ni2AlTi

Abstract Single-crystal and polycrystalline specimens of the intermetallic compound Ni2AlTi with the L21 structure have been deformed in compression at 923–1283 K. At temperatures higher than ∼965 K, specimens deform by slip along ⟨110⟨ directions. The observed slip planes vary with crystal orientation and, in general, they are indexed to be the high-index {hℏk} planes. However, the fundamental slip planes on which dislocations move are of the {110}, {001} and possibly {112} types. The critical resolved shear stress (CRSS) is lowest for the {110} ⟨110⟩ system and highest for the {001}⟨110⟩ system. The CRSS for these slip systems exhibits a strong temperature-dependence. Dislocation structures on the slip plane of the most predominant slip system have been studied as a function of the deformation temperature and crystal orientation. The most characteristic feature of the dislocation structure in Ni2AlTi is the occurrence of numerous dislocation nodes and networks. This has been discussed in term of the ela...

Effects of non-stoichiometry on the high-temperature deformation of Ni2AITi

Abstract High-temperature deformation experiments have been performed on two non-stoichiometric Ni2AlTi compounds lying roughly on the NiAl-NiTi quasi-binary line in the Ni-Al-Ti ternary system, and the results have been compared with those for the stoichiometric compound. The marked effects of non-stoichiometry on the deformation behaviour are observed only in the Al-rich compound; the deviation from stoichiometry on the Al-rich side produces considerable strengthening of Ni2AlTi and causes the 〈110→ 〈001〉 slip mode transition in crystals with orientations near [111]. In crystals of the Ti-rich compound and in those of the Al-rich compound with orientations away from [111], slip occurs along 〈110〉 directions, as in the stoichiometric compound. The 〈001〉 superlattice dislocations in the Al-rich compound are found to be dissociated into two 1/2〈001〉 superlattice partial dislocations coupled by a ribbon of antiphase boundaries (APB). From the separation of the superlattice partial dislocations, the energy o...

Deformation of single crystals of the L21 ordered Ag2MgZn

The orientation and temperature dependence of slip geometry and yield stress in single crystals of the L21 ordered Ag2MgZn has been studied in compression in the temperature range 290 to 580 K. The slip direction in Ag2MgZn is exclusively 〈1 1 1〉 in this temperature range, but the slip plane varies with crystal orientations; slip occurs on (¯2 1 1) for orientations near the [0 1 1]-[¯1 1 1] boundary, while for the other orientations in the [0 0 1]-[0 1 1]-[¯1 1 1] unit triangle it occurs on the (¯1 0 1). The critical resolved shear stress (CRSS) for slip on both the (¯1 0 1) [1 1 1] and (¯2 1 1) [1 1 1] systems increases abnormally with increasing temperature and reaches a maximum peak at about 0.92 of the critical temperature Tc, for the L21-type order. The peak temperature and the shape of the CRSS versus temperature curve are independent not only of crystal orientation but also of the operative slip system. It is suggested that the strength anomaly in Ag2MgZn be interpreted in terms of the mechanism based on the transition from unit dislocations to superdislocations.

Deformation and fracture behaviour of the heusler alloy Cu2MnAl single crystals

Abstract Various orientations of single crystals of the Heusler alloy Cu 2 MnAl have been tested in compression at 77–367 K. Slip occurs along 〈111〉 and the observed slip phane varies with crystal orientation. For orientations where slip of the {110}-type is activated, a sharp decrease in ductility occurs at around 240 K and below this temperature, fracture occurs explosively with practically no observable plastic deformation. For orientations exhibiting slip on {112} planes, on the other hand, crystals can be deformed plastically even at 77 K. The brittle fracture for the former orientations is probably triggered by the burst of coarse {110} slip bands. The occurrence of the coarse {110} slip appears to be explained in terms of the dissociation of superlattice dislocations which is characteristic of DO 3 and L2 1 structures.

Deformation of the Intermetallic Compound Al 3 Ti and Some Alloys with An Al 3 Ti Base

In this paper, firstly, the results of deformation experiments which have been made on Al 3 Ti and Al 3 Ti-base alloys are reviewed. Secondly, the possibility of the improvement of ductility of Al 3 Ti by alloying processes is discussed, and then possible alloying elements for this purpose are suggested. Thirdly, the oxidation resistance of Al 3 Ti is described and then possible future technological applications of Al 3 Ti are briefly discussed.

Plastic deformation of Co2AlTi

Abstract Single crystals of Co2·01Al0·95Ti1·04 and Co2·13Al0·76Ti1·11 with the L21 structure have been deformed in compression at temperatures up to 1355 K. The brittle-to-ductile transition temperature is about 1170 K. At temperatures above 1170 K, slip on both {110}〈110〉 and {110}〈001〉 systems occurs depending on crystal orientation. The critical resolved shear stress (CRSS) for the two slip systems decreases gradually with increasing temperature. The difference in the CRSS between the two slip systems is small. In crystals deformed by slip on the {110}〈110〉 system, numerous dislocation nodes and prominent strings of small loops are observed. In crystals where slip occurs mainly on the {110}〈001〉 system, dislocation nodes resulting from the interaction of 1/2〈110〉 and 〈001〉 dislocation′s exist in abundance, but prominent strings of loops are not present. A brief comparison of the plastic properties of Co2AlTi and Ni2AlTi, both of which are L21 compounds, is made, and the differences between the two comp...