Alfons Gonzàlez-Comas

PREMARTENSITIC TRANSITION DRIVEN BY MAGNETOELASTIC INTERACTION IN BCC FERROMAGNETIC NI2MNGA

We show that the magnetoelastic coupling between the magnetization and the amplitude of a short wavelength phonon enables the existence of a first order premartensitic transition from a bcc to a micromodulated phase in Ni2MnGa. Such a magnetoelastic coupling has been experimentally evidenced by ac susceptibility and ultrasonic measurements under an applied magnetic field. A latent heat around 9J ymol has been measured using a highly sensitive calorimeter. This value is in very good agreement with the value predicted by a proposed model. [S0031-9007(97)04508-0] Martensitic transitions (MT) are first order displacive structural phase transitions accompanied by a significant strain of the unit cell. In general, homogeneous and intracell (short-wavelength phonons) strains are necessary in order to describe the path followed by the atoms at the transformation. An interesting feature displayed by the systems undergoing this kind of first order transitions is the existence of precursor effects [1,2]. They reflect that, in a sense, the system prepares for the phase transition before it actually takes place. For instance, in bcc materials the TA2f110g phonon branch has low energy, the corresponding elastic constant (C 0 ) is very low and softening of all these phonons and C 0 occurs on cooling [2]. The Ni2MnGa Heusler alloy is investigated in the present Letter. At high temperature it is ferromagnetic (the Curie temperature is Tc › 381 K), displays a bcc structure with an L21 atomic order (space group Fm3m), and transforms martensitically at TM › 175 K. This alloy is unique in the sense that (i) it is the only known bcc ferromagnetic material undergoing a MT and (ii) the MT is preceded by a structural phase transition (intermediate transition) to a micromodulated phase (the cubic symmetry is preserved) resulting from the freezing of a q › 0.33TA2 phonon which becomes the intracell strain characterizing the new phase. Such a phase transition has been evidenced by neutron scattering [3], x-ray [4], electron microscopy [5], and ultrasonic measurements [6,7]. We have recently suggested that this transition to the premonitory (intermediate) phase is a consequence of the magnetoelastic interplay between the phonon and the magnetization [7]. At the MT the system transforms to a modulated structure with tetragonal symmetry (homogeneous strain) [8]. The modulation of the martensitic structure is different from that of the premartensitic phase. It has been argued [7] that the intermediate transition has to be first order because there is no complete softening of the frequency of the soft phonon; nevertheless, attempts in measuring a latent heat have not been successful [9] and a small thermal hysteresis has been detected [10] only in samples subjected to external stresses (which can result in a modification of the characteristics of the transition). In this Letter we present a phenomenological model for the intermediate transition based on a Landau expansion, which includes a magnetoelastic coupling. The primary order parameter is the amplitude h of a TA2 f110g phonon, and secondary order parameters are « ,a s110 df 110g homogeneous shear suitable to describe a cubic to tetragonal change of symmetry, and M, the magnetization (considered here to be a scalar). In terms of these three order parameters we assume the free energy function to have the following general form: F sh, «, Md › Fstr sh, «d 1 FmagsM d 1 Fmesh, «, Md ,

A comparative study of the post-quench behaviour of Cu–Al–Be and Cu–Zn–Al shape memory alloys

Abstract This paper reports a comparative investigation of the effect of quenching on the Cu–Al–Be and Cu–Zn–Al shape memory alloys by the use of several experimental techniques. In a first stage, the order–disorder transitions in these alloys have been characterized by means of modulated calorimetry. Results have proved that the A2⇋DO3 transition in Cu–Al–Be is first order with a latent heat of 1160 J/mol; the B2⇋L21 transition in Cu–Zn–Al is second order, and a peak in the specific-heat vs temperature curve has been observed. Secondly, the post-quench behaviour of these alloys, when subjected to some of the typical heat treatments used to stabilize the β phase, has also been studied by means of neutron diffraction, positron annihilation and highly sensitive calorimetry. A different post-quench time evolution of the martensitic transition temperatures has been found for the two alloys. For Cu–Al–Be, this evolution has been shown to be correlated with positron annihilation data, while, for Cu–Zn–Al, a correlation with neutron diffraction data has been established. These results show that the measured shifts in the transition temperatures induced by a quench are mostly due to an excess of vacancies in the case of Cu–Al–Be, and to an incomplete degree of L21 atomic order in Cu–Zn–Al.

Magnetoelastic behavior of the Heusler Ni2MnGa alloy

In this work the effect of the interplay between magnetic and structural degrees of freedom in the structural transitions undergone by Ni2MnGa alloy is investigated. Elastic constant and magnetic susceptibility measurements in a magnetic field are presented. A simple phenomenological model is proposed to account for the experimental observations.

Premartensitic phase transformation in the Ni2MnGa shape memory alloy

Abstract We present a phenomenological model for the premartensitic phase transition in ferromagnetic NiMnGa alloys. The model contemplates the effects of an applied mechanical stress on the premartensitic transition. It is shown that the premartensitic transition is driven by a magnetoelastic coupling. The first order character of this transition is enhanced by the application of a mechanical stress. Experimental data are also presented which support the predictions of the proposed model.

Lattice stability and martensitic transition in β-phase Cu-based shape memory alloys : long-wavelength acoustic mode anharmonicity

Abstract We report measurements of the complete set of third-order elastic constants of β-phase Cu—Al—Ni and Cu—Al—Be alloys as a function of temperature. These data provide the experimental information needed for the study of the vibrational anharmonicity of long-wavelength acoustic modes in Cu-based alloys. The long-wavelength acoustic mode Grüneisen parameters have been computed for different families of Cu-based alloys. The modes associated with the two shears necessary to accomplish the lattice distortion from the β phase to the close-packed martensite structure have been found to exhibit the largest anharmonicity. It has also been found that the Grüneisen parameters of these two relevant modes take fixed values (independent of alloy composition and structure of the martensitic phase) at the transition temperature. Based on the results, the mechanical stability of the β phase and the nucleation mechanism is discussed in terms of the vibrational anharmonicity.

Non-linear acoustic properties and acoustic-mode vibrational anharmonicity of 18R martensite Cu-Zn-Al shape-memory alloy

To shed further light on the dynamics of the martensitic transformation in shape-memory alloys, measurements have been made of the effects of hydrostatic pressure on the velocities of 15 different pure ultrasonic modes propagated at room temperature in a single-crystal alloy in its 18R martensitic phase. Although the point symmetry group of the 18R martensite is monoclinic, the elastic behaviour can be reasonably described in terms of an orthorhombic structure; the further monoclinicity beyond such an orthorhombic representation is very small and can be neglected to a first approximation. The results obtained for the nine independent second-order elastic stiffness tensor components in the 18R structure and their hydrostatic-pressure derivatives have been used to calculate the mode Gr?neisen parameters , which quantify the vibrational anharmonicity of long-wavelength acoustic phonons. Results are compared with those computed from TOEC data measured on a -phase Cu-Zn-Al single crystal with a similar composition. At room temperature, about 28 K below the martensitic transition temperature (= 323 K), the longitudinal mode have normal, positive values; however, those associated with some shear modes have negative values. A pronounced minimum has been found in the Gr?neisen parameter of the shear mode propagated along the [111] direction with polarization vector. It is concluded that the lattice stability against this particular shear mode decreases as the sample approaches the martensitic transition.