Akira Nagasawa

Anharmonicity and Martensitic Phase Transition in β-Phase Alloys

We obtain the third order elastic constants in the β 1 phases of Au–Cu–Zn and Cu–Al–Zn alloys by measuring the pressure dependence of the second order elastic constants under the uniaxial pressure. The β 1 phases are highly anharmonic. In any alloy, the free energy barrier to form the martensite is of the order of only meV/atom. We also calculate the pressure derivatives of the second order elastic constants as a function of the pressure directions. They are considerably anisotropic. Using the results, we consider the martensitic nucleation. The martensitic nucleation mechanism is originated from the anharmonic property of the β 1 phase.

The effect of quenching on the martensitic transformation in ß1 phase of Au-Cd alloys

The effect of quenching on the martensitic transformation mechanism in β1 Au-Cd alloys has been investigated by measurements of the electrical resistivity and X-ray diffraction. In the case of the Au-47.5 at%Cd alloy, the ζ′2-martensite is the characteristic product under quenching conditions, but it always exists with the equilibrium γ′2-martensite phase. Consequently, the β1 ↔γ′2 and β ↔↔′2transformations occur simultaneously during the heating and cooling cycles. The corresponding resistivity behaviour is very complicated and extremely sensitive to thermal treatments such as quenching temperature and thermal cycling. On the other hand, in the case of the Au-49.0 at%Cd alloy, only the γ1 ↔ζ′2 transformation occurs even when quenched, and the transformation is unaffected structurally by quenching. A distinct resistivity anomaly, which is considered to be due to the disappearance of quenched-in vacancies, is observed in quenched alloys. Some important characteristics of this anomaly are determined. In particular, the quenching effect disappears when the specimen is heated above the temperature at which the resisitivity anomaly begins. This result suggests that the quenched-in vacancies play an essential role in the martensitic transformation process under quenching conditions.

X-ray and neutron diffraction anomalies preceding martensitic phase transformation in AuCuZn2 alloys

The present paper gives the results obtained by the X-ray and neutron diffraction studies on the single crystals of the beta-1 AuCuZn2 alloys. As precursor phenomena, the dispersion relation of the [110] TA1 phonon exhibits significant dip near 2/3 [110]qmax position and anomalous peaks appear around 1/3 and 2/3 [110]qmax positions. Characteristics of the interplanar force constants, obtained by the analysis of the dispersion relation, and the positions of the anomalous peaks predict the martensite structures to be formed in the beta phase alloys. In the present case, both the 6R and 18R martensites will be formed by cooling and/or under the stress field.

Electric resistivity anomaly and martensitic phase transformation in AuCuZn alloys

The martensitic phase transformation in the beta-1 phase of Au-Cd alloys exhibits very complicated behaviours (1). In the case of Au-47.5 at % Cd alloy, for example, the electric resistivity of the slowly cooled beta-1 phase tends downward during the martensitic phase transformation while the reverse change occurs in the quenched one. When the quenched alloy is heated, the electric resistivity increases linearly and then its temperature gradient begins to decrease at about 385K. Above 420K, the electric resistivity rises again linearly with increasing temperature. Such a resistivity anomaly is an irreversible phenomenonon the thermal cycles. We confirm by the X-ray analysis that it is not concerned with any structural change. In the case of the slowly cooled alloy, on the other hand, the resistivity anomaly does not occur. We conclude that the resistivity anomaly is originated from the annihilation of quenched-in vacancies introduced into the beta-1 phase.

Phonon dispersion relations of premartensitic β1-phase in AuZn alloys

Abstract Phonon dispersion relations in AuZn alloys were measured at 293, 150 and 110 K. The [110] TA 1 branch had a dip and the phonon energy decreased with decreasing temperature. The energy of the [111] LA branch increased with decreasing temperature. The structure of the martensite was proved to be trigonal. The martensitic transformation in the AuZn β-phase alloy is thought to occur by a (110) 110 shear through the same mechanism as the ζ -martensite in AuCd alloys.

Lattice instability in β1-AgZn

Phase transition of β 1 -AgZn alloy was studied by means of neutron inelastic and diffuse scattering techniques. Acoustic phonon dispersion relation along the main crystal axes of the alloy was obtained. No [ζζζ]LA phonon anomalies related to the ζ-phase was observed, although previous sound experiment followed by elastic constant analysis predicts that possibility. The observed anomalies in phonon energy of both [ζζ0]TA 1 and [ζζ-2ζ]TA modes associated with the diffuse scattering peaks indicate that a strong lattice instability is involved in the phonon modes. It is pointed out that the lattice instability plays important roles in the structure transition from β 1 -phase to ζ-phase or to the martensitic phase in the AgZn alloy.

Elastic behavior and phase stability of β1-AgCd alloy

Obtention de monocristaux de haute purete; mesure de la vitesse des ultrasons; analyse thermique differentielle; variations thermiques des parametres cristallographiques et des constantes elastiques de 150 a 350 K; temperature de transition

Elastic Behavior and Phase Stability of the β1-AgZn Alloy

To elucidate the dynamical mechanism of the β 1 to ζ transition the elastic constants of metastable β 1 phase of Ag-Zn alloys containing 46.5 and 48.5 at.% Zn are measured in the range between room temperature and the β 1 to ζ transition temperature using an ultrasonic pulse-echo overlapping method. The elastic constants C L [110]=(C 11 +C 12 +2C 44 )/2 and C 44 decrease linearly with increasing temperature up to 340 K and then fall more rapidly on further heating. On the other hand, C 11 decreases linearly on heating up to 340 K and then increases at higher temperatures. Furthermore, C L [111]=(C 11 +2C 12 +4C 44 )/3 which is mainly related to the β 1 to ζ transition mechanisum decreases rapidly above 340 K. These elastic constants show a characteristic temperature hysteresis with heating and cooling. Based on the obtained results, we discuss the transition mechanism and stability of the β 1 phase.

[110]TA1 Phonon Branch and Anomalous 2/3[110] Elastic Peak in Heusler and B2 Phases of a AuCuZn2 Alloy

The present study on the [110]TA 1 phonon dispersion relation in a AuCuZn 2 alloy reveals that the lattice instability towards the 18R martensite is characteristic of only the Heusler phase existing stably below about 670 K, which is the disordering temperature to the B2-phase. Anomalous peak appearing near ζ=2/3[110] in the Heusler-phase is also examined by neutron elastic scattering experiments in a wide range of temperature. It is evident that such a peak has no relation to the 2/3-dip on the [110]TA 1 phonon branch observed in the Heusler-phase but it is originated from vacancies existing inherently and regularly in the B2-lattice.

[110]TA1 phonon dispersion relation of the β1-phases in Ni–Co–Al alloys

Abstract The present study is mainly concerned with the relation between electron concentration and anomalous dip appearing on the [110]TA1 phonon branch of the β1-phase (B2-structure) in Ni–Al alloys. By substituting Co for Ni, the dip-position shifts toward the Brillouin zone center. In addition, we prove that the dip appears also in the equilibrium β1-phase existing above Tb-temperature, below which Ni3Al precipitates. The obtained results indicate clearly that the dip is not any precursor of the martensitic transition but a reflection of the electronic property in the β1-phase.