One-dimensional Antiphase Domain Structure Research Articles

Long-period ordering in a TiAl single crystal with a gradient composition

A series of long-period superstructures (LPSs) formed in an Al-rich TiAl single crystal with a gradient composition has been observed by transmission electron microscopy. A Ti–54.7 at.% Al single crystal was heat treated within molten Al at 1234°C, and a composition-gradient layer with an Al concentration from 55 to 75 at.% was formed at the surface of the single crystal. The composition-gradient layer exhibits ordering into different LPSs depending on the Al concentration as follows: short-range order, h-Al2Ti and one-dimensional antiphase domain structure. These LPSs have an alternate stacking of Al(002) and Ti–Al(002) layers and the ordering of the LPSs proceeds in the TiAl matrix, maintaining an orientation relationship in which Al(002) layers of the LPSs and the TiAl matrix are parallel to each other. The ordering of this type of LPS and the phase transition between these LPSs are thus interpreted as structural changes in Ti(002) layers of the Al-rich TiAl crystal. It was also observed that ordering of the orthorhombic h-Al2Ti exhibits anisotropic features depending on the direction of the composition gradient. This suggests that the long-period ordering in the TiAl crystal depends on diffusion processes as well as the local concentrations of Ti and Al atoms.

TEM Analysis of Long-Period Superstructures in TiAl Single Crystal with Composition Gradient

ABSTRACTThe ordering mechanism of long-period superstructures (LPSs) in Al-rich Ti-Al alloys was studied using a TiAl single crystal with a composition gradient. A TiAl single crystal with gradient compositions from 55 to 75 at.% Al was prepared by annealing in a molten Al at 1234°C. The single crystal exhibits long-period ordering into different LPSs depending on the Al concentration as follows: an Al5Ti3 type short-range order, h-Al2Ti and one-dimensional antiphase domain structures. These LPSs show an orientation relationship in which Al (002) layers of the LPSs are parallel to those of the TiAl matrix. The atomic arrangements of the LPSs are characterized in common as the alternate stacking of the Al (002) layers and Ti-Al (002) layers. It is thus concluded that the ordering of this type of LPSs and the phase transition between these LPSs are explained as structural changes in Ti-Al (002) layers of the Al-rich L10-TiAl crystal.

Phases and phase equilibria in the Al-rich part of the Al–Ti system above 900 °C

The Al-rich part of the Al–Ti system in the composition range 55–75 at.% Al has been investigated in the temperature range 900–1300 °C. By combining results obtained by metallography, X-ray diffraction (XRD), electron probe microanalysis (EPMA), differential thermo analysis (DTA) and transmission electron microscopy (TEM), the phase equilibria in the respective part of the Al–Ti system have been established. According to these results the existence of the phase Ti 1− x Al 1+ x is ruled out, at least in the investigated temperature range. The phase h-Al 2Ti is a metastable phase which forms instantly from TiAl supersaturated with aluminium during cooling but transforms only very slowly into the equilibrium phase r-Al 2Ti. h-Al 2Ti is frequently observed because the solubility of aluminium in TiAl increases markedly with increasing temperature. Above 1215 °C a continuous homogeneity range exists which extends up to about 71 at.% Al. It is not yet clear whether TiAl is stable up to this Al-content or if there is a second-order transition to the so-called one-dimensional antiphase domain structures (1d-APSs) which are observed in all alloys with at least 68 at.% Al by TEM after cooling. The XRD patterns of the 1d-APSs could be completely indexed by the Al 5Ti 2-structure. The 1d-APSs/Al 5Ti 2 are stable down to a temperature of about 980 °C where they disintegrate at a final composition between 70 and 71 at.% Al by eutectoid decomposition: 1d-APSs/Al 5Ti 2 ↔ r-Al 2Ti + Al 3Ti.

Phase stability and microstructure of Al-Ti-Fe near Al3Ti

The phase stability and microstructure of Al-Ti-Fe near Al3Ti at 800 °C are studied by using high resolution electron microscopy and backscattered electron imaging. One of the most significant changes resulting from the addition of small amounts of iron in this system is to convert tetragonal Al3Ti or Al2Ti into cubic Ll2 structures. In addition, the added iron can change the stacking structure of Al2Ti and the domain size of the long-period one-dimensional antiphase domain structures. A partial ternary phase diagram in this region is also determined.

The kinetics of hydrogen-induced ordering in Pd 3Mn to the L1 2-type structure

A study of the kinetics of hydrogen-induced ordering from “as-quenched” and initially ordered (L1 2-s -type) states in Pd 3Mn to the L1 2-type structure has been carried out to examine the conditions necessary for the ordering, i.e. the hydrogen pressures, temperatures and times and to consider the ordering mechanism, using electrical resistance measurements. The ordering rate for the initially ordered alloy of the L1 2-s -type structure is slower than that for the initially “as-quenched” state. The hydrogen-induced ordering to the L1 2-type structure may occur from the periodic one-dimensional antiphase domain structure of the L1 2-s type with a cubic fundamental cell, in which a step shift, ( a 2+ a 3) /2, is introduced at every two cells along the a 1 axis, by attaining a particular stress state due to the lattice expansion caused by the preferential occupation of hydrogen in the octahedral sites of six palladium nearest-neighbor atoms.

Hydrogen-induced ordering in palladium-rich PdMn alloys

Evidence for the hydrogen-induced formation of an L1 2 ordered structure from an α-L1 2 − s(Ag 3Mg-type) structure was observed in the presence of hydrogen at a pressure p H 2 = 5.0 MPa at 726 K in the α-phase of rapidly quenched alloys of Pd-12.5–31.0 at.% Mn. In Pd-29.0 at.% Mn and Pd-31.0 at.% Mn alloys, another periodic one-dimensional anti-phase domain structure was observed having a domain size M = 2 and f.c.t. fundamental cells. This arises from the β 1-phase (CuAu(I)-type) which coexists with the α-phase. The superlattice structure will be referred to here as a “ β 1- L1 2 − s ( M = 2)” structure. It was observed that in the presence of hydrogen the β 1- L1 2 − s ( M = 2) transforms into β 1- L1 2 − s ( M = 4), i.e. the periodic anti-phase domain size increases from M = 2 to 4.

Hydrogen-induced ordering in palladium-rich Pd-Mn alloys

Evidence for the hydrogen-induced formation of an L1 2 ordered structure from an α-L1 2 − s (Ag 3 Mg-type) structure was observed in the presence of hydrogen at a pressure p H 2 = 5.0 MPa at 726 K in the α-phase of rapidly quenched alloys of Pd-12.5–31.0 at.% Mn. In Pd-29.0 at.% Mn and Pd-31.0 at.% Mn alloys, another periodic one-dimensional anti-phase domain structure was observed having a domain size M = 2 and f.c.t. fundamental cells. This arises from the β 1 -phase (CuAu(I)-type) which coexists with the α-phase. The superlattice structure will be referred to here as a “ β 1 - L 1 2 − s ( M = 2)” structure. It was observed that in the presence of hydrogen the β 1 - L 1 2 − s ( M = 2) transforms into β 1 - L 1 2 − s ( M = 4), i.e. the periodic anti-phase domain size increases from M = 2 to 4.

The one-dimensional anti-phase domain structures. III. An alternative interpretation of the structure with a non-integral value of the half period,šub>M

The non-integral value for the half period, \~ M (M < \~ M < M + 1, M:integer), as experimentally found in one-dimensional anti-phase domain structures, is explained by a disordered structure consisting of a mixture of two kinds of structure units with thicknesses of M and M + 1 layers. The diffraction intensity for the disordered structure is calculated by the use of the general diffraction theory for a one-dimensionally disordered crystal given by Kakinoki & Komura [Acta Cryst. (1965), 19, 137]. With this model the non-integral value of the half period, \~ M, can be explained as due to the shift between two peaks, one due to a simple APD (anti-phase domain) structure with half period, M, and the other to another simple APD structure with half period, M + 1. It is not necessary to consider a very large value of the period as was required in Fujiwara's model [Fujiwara, J. Phys. Soc. Japan (1957), 12, 7], which was proposed assuming a disordered structure, deviating from the standard structure defined by a step function.

The one-dimensional anti-phase domain structures. II. Refinement of Fujiwara's method of the analysis of the structure with a non-integral value for the half period,šub&gt;M

One-dimensional anti-phase domain structures with an out-of-step vector u = (a + b)/2 sometimes have half periods of non-integral value. Fujiwara interpreted this structure as a disordered structure (a statistical assembly) deviating from a standard structure which he defined by a step function. In the present paper, it is pointed out that the unitary intensity of a superlattice reflexion for the standard structure is obtained in a much simpler form than that given by Fujiwara. By the use of this intensity formula we verify the fact that a pair of intensities with special l-indices, v and −v, is very strong while the others are extremely weak, so that we apparently obtain a non-integral value for the half period from the strongest pair. The statistical assembly, i.e. the disordered structure presented by Fujiwara, is interpreted using an easily understandable model, and a simple form of the corresponding intensity formula is obtained, which indicates that the intensities other than the pair, {\bar I}v and {\bar I_{v}}, practically vanish. This fact implies that the non-integral half period, \~M, may be obtained experimentally from a pair of satellites corresponding to {\bar I}v and {\bar I_{v}}. Some important remarks are made in the Appendix concerning the Fourier expansion of the step function.

The one-dimensional anti-phase domain structures. I. Classification of structure and the Patterson method applied to the layer sequence determination

The one-dimensional anti-phase domain structures with an out-of-step vector u = (a + b)/2 are classified into the following three kinds: (1) The complex out-of-step structure, (2) the complex APD (antiphase domain) structure, (3) the simple APD structure. These structures are characterized by the use of the similar symbols to the Zhdanov symbol. Intensity formulae are derived for some typical cases. The application of the Patterson method gives some useful relations between the symbol adopted and a quantity which is obtained by Fourier cosine transformation of the unitary intensities. Since this quantity is any one of a set of integers of the form (P2 − 4qP) (P: period, q: integer), the correct layer sequence may be obtained even if the observed intensities are not so accurate. Applications for some ideal and real cases are shown.

Crystal structures and phase transitions of the gold-rich gold-cadmium alloy

The gold-cadmium alloys in the composition range between 23 and 29 at. % cadmium were studied by X-ray diffraction and by measurements of specific heat and electrical resistance. By means of an X-ray single crystal diffraction method, it was confirmed that the ordered alloy Au 3Cd (α 1 phase) has a one-dimensional anti-phase domain structure of the Ag 3Mg type exhibiting a face-centered tetragonal fundamental cell a = 4.116 A ̊ , c a = 1.005 (24.6 per cent Cd). It was also established that a regular arrangement of stacking faults exists in the hexagonal α 2 phase of the 25.5% Cd alloy. Changes of specific heat and electrical resistance due to the order-disorder transition of the alloy Au 3Cd as well as those due to the lattice transition from hexagonal ( α 2) to cubic (α) were measured. The results are qualitatively discussed in connection with the structures of these phases.

The direct observation of the long period of the ordered alloy CuAu(II) by means of electron microscope

According to the electron diffraction studies on ordered alloys such as CuAu(II) (1), Cu3Pt (2), Ag3Mg (3), Cu3Pd (4), Au3Mn (5), Au4Zn (6) and Au3Zn (6), it was revealed that they possess superlattices consisting of anti-phase domains with definite size in a stable state. For example, CuAu(II) has a one-dimensional anti-phase domain structure characterized by the first kind of out-of-step, as shown in Fig. 1, while Cu3Pd has a two-dimensional anti-phase domain structure characterized by combination of the first kind and the second kind of out-of-step.

Study on the Ordered Alloy Ag3Mg

One-dimensional anti-phase domain structure of the ordered alloy Ag 3 Mg has been confirmed by X-ray diffraction study using a single crystal as well as polycrystals and by electron diffraction study with evaporated thin films. The measurements of specific heat and electrical resistivity have also been done, and these results have been qualitatively discussed in connection with the structure of this alloy.