Pressure induced amorphization of materials

In ferromagnetic alloys with shape memory large reversible strains can be obtained by rearranging the martensitic domain structure by a magnetic field. Magnetization through displacement of domain walls is possible in the presence of high magnetocrystalline anisotropy, when martensitic structure rearrangement is energetically favorable compared to the reorientation of magnetic moments. In ferromagnetic Heusler alloys Ni2+xMn1–xGa the Curie temperature exceeds the martensitic transformation temperature. The fact that these two temperatures are close to room temperature offers the possibility of magnetically controlling the shape and size of ferromagnets in the martensitic state. In Ni2+xMn1–xGa single crystals, a reversible strain of ~6% is obtained in fields of ~1 T.

Shape memory ferromagnets

This paper gives a brief review of the effect of temperature and pressure on Raman spectra. Anharmonicity, defined by the cubic, quartic and higher terms in the potential expansion, is shown to be responsible for various properties such as dilatation or for variations of wavenumber and half-width of Raman bands with temperature and pressure, or may be strongly involved in phase transitions. This contribution does not pretend to be an exhaustive review on the subject but aims to introduce some basic concepts in a tutorial way before showing how intricated are these manifestations of anharmonicity. Copyright © 2003 John Wiley & Sons, Ltd.

Effect of pressure and temperature on Raman spectra of solids: anharmonicity

The elastic constants measurements have been made for the first time on a single crystal of the near‐equiatomic NiTi alloy. The results show that the elastic constants C′ and C44 both have a low value and a positive slope with temperature; this behavior makes NiTi unique in the class of alloys showing a martensitic transition.

Single‐crystal elastic constants of the equiatomic NiTi alloy near the martensitic transformation

Continuous-wave measurements of the shear elastic constants of bcc Fe–Ga solid solutions show that the constant C44 is independent of the gallium composition while the constant 12(C11–C12) decreases linearly and extrapolates to zero at 26 at. % Ga. This result indicates that the Fe–Ga solid solution is inhomogeneous and contains clusters consisting of embryos of a martensitic phase. The increasing softness of 12(C11–C12) then reflects an increase of the embryo density. A model assuming that the clusters are centered around next-nearest Ga pairs reproduces the known composition dependence of the magnetostrictive constant λ100 in the composition range 0<XGa<0.23 with only one adjustable parameter. Its value suggests a cluster size of 12 atoms.

Elasticity and magnetoelasticity of Fe–Ga solid solutions

The Landau theory of phase transitions has been applied to elastic phase transitions pertaining to crystals belonging to each of the 32 crystallographic point groups. Investigation of the Landau free-energy expansions provides information about: (i) the order of the possible transitions, (ii) the associated acoustic mode (which for a second-order transition should soften completely at the transition temperature) and (iii) the higher-order elastic constant contributions to thermal expansion only. Emphasis has been placed on finding the form of third-order invariants (the fourth-order terms also have been obtained for cubic crystals), which are needed in any quantitative assessment of the applicability of the Landau theory to elastic phase transitions in which the order parameter is just a strain tensor component. Application of the theory to the f.c.c. to f.c.t., transition in In-TI alloys is discussed.

Application of the Landau theory to elastic phase transitions

Indium rich, In-Cd alloys undergo a f.c.c. to f.c.t. structural phase transition which the presence of a cubic invariant in the Hamiltonian renders first order. Ultrasonic wave velocity measurements have been made in single crystals of both the f.c.c. and f.c.t. phases, emphasis being placed on behaviour near the phase transition. The shear modulus ½(C 11-C 12) tends to very small values in both phases as the transition temperature T c is approached: the onset of instability is manifested as softening of the acoustic phonon branch with wave vector (110), particle displacement (110) near the Brillouin zone centre. However, in contrast to its behaviour in the In-Tl alloys, ½(C 11-C 12) does not go to zero in the In-Cd alloys. This finding corresponds to the more recognizable first-order character of the transition in the In-Cd compared to that in In-Tl alloys. The effect of the enhanced acoustic mode softening at T c on the wave velocity and Young's modulus surfaces is discussed. In addition to the...

An ultrasonic study of the elastic phase transition in In-Cd Alloys

The third-order elastic constants of cobalt have been determined from measurements of the hydrostatic and uniaxial pressure dependences of the velocities of ultrasonic waves propagated through single crystals. The hydrostatic pressure dependences of the second-order elastic constants have also been obtained. Comparison between these experimental results and predicted values show that, not unexpectedly, a simple Lennard-Jones model employed by previous workers for the lattice potential does not give a realistic assessment of the elastic behaviour of cobalt under pressure. The volume dependence of cobalt upon pressure, i.e. the compression, is estimated by extrapolation up to high pressure of the ultrasonic wave velocity data. The lack of acoustic-mode softening establishes that the h.c.p. to f.c.c. transition in cobalt is not associated with ferroelastic behaviour. The anharmonicity of the long-wavelength acoustic modes is discussed in terms of mode Gruneisen parameters obtained in the elastic con...

G. A. Saunders

Papers

Application of the Landau theory to elastic phase transitions

An ultrasonic study of the elastic phase transition in In-Cd Alloys

Third-order elastic constants and acoustic-mode vibrational anharmonicity of cobalt