Au Cd Research Articles

The thermoelastic phase transition in Au—Cd alloys studied by acoustic emission

Abstract The acoustic emission generated during the thermoelastic phase transitions in polycrystalline Au–47·5 at.% Cd and in Au–49 at.% Cd alloys has been recorded and analysed. It was found that the amount of acoustic activity depends upon the direction of the transformation, the heating or cooling rates, and the specific crystallographic features of the martensitic phases. Precursor acoustic activity was detected in both alloys at temperature differences of about 2 5°C before the M 8 point. The dynamics and kinetics of martensitic thermoelastic phase transformations are discussed in terms of the accompanying generation of acoustic emission.

High-voltage, high-resolution electron microscopy of Au–Cd alloys. I. Hexagonal long-period superstructures near 30 at. % Cd

The crystal structure of Au–Cd (30–32%) alloys has been studied at atomic level with a 1 MV electron microscope. The experimental images can be fitted well with many-beam dynamical calculations for the long-period superstructure (LPS) designated as 7a0−2H type. The structures with incommensurate periods consist of a number of domains of the 7a0−2H structure separated by antiphase boundaries which are composed of orthorhombic cells. The result demonstrates the potentiality of high-voltage, high-resolution electron microscopy for direct determination of the atomic arrangements in modulated structures of alloys.

High-voltage, high-resolution electron microscopy of Au–Cd alloys. II. Double hexagonal long-period superstructures

Ordered structures of Au3Cd with a double hexagonal stacking sequence (4H) have been investigated by 1 MV electron microscopy in combination with many-beam dynamical calculations. One- and two-dimensional long-period superstructures (LPS) were observed at atomic level by high-resolution images with [001] incidence. The two-dimensional LPS was determined to be isomorphic with the Au77Mg23 structure. Two-dimensional LPS with stacking sequences 6Hand 9R were also found in the composition range 24–31 at.%Cd.

Effect of cooling rate on the microstructure and acoustic emission energy during the thermoelastic transformation in AuCd single crystals

Effect of cooling rate on the microstructure and acoustic emission energy during the thermoelastic transformation in AuCd single crystals

Study of superelasticity associated with the thermoelastic martensitic transformation in Au—Cd alloys

Abstract Superelastic behaviour in Au-47·5 at.% Cd and Au-49·5 at.% Cd was studied using microscopic surface observation, an analysis of Laue photographs and measurement of the strain distribution. The superelasticity above the transformation temperature, T0, in Au—47·5 at. % Cd alloy was associated with the transformation, β(b.c.c.) β'(orthorhombic) upon loading and β'-β upon unloading. Below T0, superelasticity in a quenched and aged martensite was due to migration of twin boundaries in the martensite crystal. In Au—49-5 at.% Cd alloy, below T0, a significant superelasticity was observed when the specimen axis was near the [100]β pole. This superelasticity was accompanied by the transformation β“(hexagonal) β’ upon loading and β→β” upon unloading. Above T0, the superelasticity observed was identical to that found in Au-47-5 at.% Cd. In the stress-induced transformation, upon loading, a band consisting of alternate layers of the b.c.e. phase and the martensite phase was formed at one end of the specimen and advanced towards the other end. The superelastic strain δ∊τ was measured with a strain gauge.

On two stage yielding in AuCdCu thermoelastic martensite

On two stage yielding in AuCdCu thermoelastic martensite

Channeling and electrical investigations of Au doped CdTe

Channeling measurements (2MeV He +) of Au doped CdTe annealed at 800–900°C indicate Au concentrations between 10 18 and 10 20/cm 3 with a substitutional Au solubility that increases with both increasing temperature and decreasing P Cd. The partial pressure dependence is consistent with the model of doubly ionized Frenkel defects on the Cd lattice and ionized Au Cd. Optical absorption from ∼4μ to the band edge is seen to increase with both increasing anneal temperature and decreasing P Cd. Hall measurements show substantial charge compensation in all the annealed samples.

X-ray study of stacking faults in a double hexagonal close-packed Au–Cd–In alloy

A study of the x-ray powder pattern resulting from a cold-worked hexagonal close-packed Au–Cd–In alloy containing 17.5 at.% Cd and 5.0 at.% In has been made. Quantitative analysis of observed anomalous peak broadening, peak shifts, and peak asymmetry in the powder pattern shows evidence for only three of the nine faults, which have earlier been postulated for this structure, namely intrinsic-ch, intrinsic-h, and intrinsic-2h with respective fault probabilities of αch=0.045, αh=0.036, and α2h=0.002.

Martensitic transformation in near-equiatomic TiNi alloys

The crystal structure and morphology of the composition have been studied by utilizing electron martensitic phases in TiNi alloys of near-equiatomic diffraction and transmission electron microscopy. Two monoclinic martensites, β′ and β″, are observed to form as a result of the low temperature instability of the parent β phase. These transformations are similar to the 2H modulation in the AuCd system. The 2H modulation in the AuCd system has an orthorhombic symmetry whereas in the present case the symmetry is monoclinic. The β′ and β″ phases form as alternating platelets, occasionally showing dislocations at the common interface.

Martensitic pretransformation phenomena in the Au-47.5 at.% Cd and the Fe-29.7 at.% Ni alloys

Premartensitic transformation characteristics have been investigated in the Au-47.5 at.% Cd and the Fe-29.7 at.% Ni alloys by measuring electrical resistivity and magnetoresistance. Preliminary electron diffraction studies have also been made for the Au–Cd alloy. An anomalous magnetoresistance peak was observed for both of these alloys prior to the martensitic transformation. In the Au–Cd alloy this peak was about 2–3°C above the Ms, and for the Fe–Ni alloy it was about 5–6°C above the Ms. Furthermore, when the Ms was lowered by isothermal stabilization, this peak moved in conjunction with the new Ms temperature. This indicates that the observed peak is truly associated with the premartensitic instability of the parent phase. The electron diffraction pattern of the parent phase of the Au–Cd alloy shows extensive 〈110〉 and 〈112〉 streaking in the temperature range where anomalous magnetoresistance was observed. These experimental results could tentatively be interpreted in terms of anisotropic scattering centers resulting from localized strain inhomogeneities. Presumably these inhomogeneities are associated with the atomic shuffling prior to the actual phase transformation and could be identified with the so-called ``strain embryos''.

Soft Phonon Modes and the Propagation Velocity of Martensite

THE interpretation of martensitic phase transformations remains a central problem in physical metallurgy. Studies of lattice properties such as elastic and anelastic behaviour, thermal expansion and thermal conductivity have resulted in knowledge of the way in which such transformations proceed at the atomic level1–8. In particular, the martensitic phase changes in Au–Cd (ref. 1), In–T1 (refs. 2 and 7) and TiNi (refs. 5 and 6) are preceded by “softening” of certain elastic coefficients and the transitions result from the development of lattice instability evidenced as a soft acoustic mode; as the transition temperature is approached and the incipient mechanical instability rises, certain lattice vibration modes undergo a considerable energy decrease and as their frequency falls the wavelength increases and the interatomic binding forces are decreased. Ultimately, the vibration amplitude and anharmonicity are so large that the atoms adopt new sites. On the basis of this model properties which depend on the lattice vibration spectrum are expected to exhibit marked anomalies in the vicinity of the transformation. One property directly manifesting anharmonicity of lattice vibrations is the thermal expansion; a pronounced peak in the thermal expansion coefficient occurs centred around the martensitic transformation in TiNi (ref. 6), the attendant anharmonicity being reflected in the Gruneisen parameter which rises from the normal background value of about 2 to more than 40 in the transition region.

A new type of long period superlattice with hexagonal symmetry in AuCd alloys

The existence of a new type of long period superlattice having hexagonal symmetry, whose period is determined by the critical contact of the Brillouin zone boundaries with the Fermi surface, has been established. The ordering of the atoms on the close packed hexagonal plane of these structures is derived from that of the A 3 B type (Cu 3Au type) structure, but the long period is produced in the basal plane by the existence of antiphase boundaries in three directions perpendicular to the close packed plane and making 120° with each other, thus creating the hexagonal symmetry. The stacking of the close packed layers is of either the 2 H or 4 H type. The size of this hexagonal unit cell is found to be between seven and nine times as large as that of the hexagonal unit cell of the disordered close packed lattice, these limiting cases being called the 7 a 0 structure and the 9 a 0 structure respectively. Significantly, the period of this structure is found to respond to a change of e a in a fashion similar to that found in the one dimensional long period superlattice, which has been investigated in detail. In the case of the AuCd alloys the stacking is of the 2 H type and the 7 a 0 structure is identified as Au 72Cd 26 with space group P6 3 mmc , and the 9 a 0 structure as Au 42Cd 12 with P6 3 mcm . The geometry of the Brillouin zone structure indicates that the structure is stabilized by creating superlattice Brillouin zone boundaries at the flat part of the Fermi surface (in the 〈110〉 directions in the case of the fcc structure), as in the case of the one dimensional long period superlattice. Examples of these new structures are found, other than in the AuCd alloys reported here, in AuMg alloys, CuSb alloys, etc. near the A 3B composition range. These structures are usually found among the long period stacking variants of a one dimensional long period superlattice with non variable period ( M = 1, M = 2, etc.), occurring in these alloys.

The deformation of AuCd β′ alloys

The deformation of AuCd β′ alloys

Density and Resistivity Changes in Au–Cd upon Quenching

Density and Resistivity Changes in Au–Cd upon Quenching

The crystallography of martensite transformations—IV body-centred cubic to orthorhombic transformations

The theory developed in parts I and II is applied to transformations of structure from body-centred cubic to orthorhombic. Two classes (A, B) of this general type arise depending on which of the planes {111} 0 or {110} 0 the final structure is twinned. The theoretical predictions of habit planes, orientation relationships and directions of the homogeneous strain are found to be in satisfactory agreement with all the available experimental data for the transformations in Cu 3Al, Au Cd, and Ti (class A). Thus, the hypothesis that the inhomogeneous shear is a part of a twinning shear in the final structure has been further substantiated. The reported twinning plane for the {443} B -transformation in Ti-11% Mo suggests that this transformation belongs to class B but some of the remaining available experimental data are inconsistent with this hypothesis.

Effect of Quenching on the Resistivity of Au–Cd

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