Many displacive solid-solid transformations, despite being first order, show pronounced precursor effects, such as the mesoscopic, micron-scale, "tweed'' pattern seen in shape-memory alloys. We model this tweed theoretically using a nonlinear, nonlocal elastic free energy, and argue that quenched concentration inhomogeneities drive the local tweed modulations. We report (1) the construction of a model for {l l}/(lT) shear transformations in square systems, (2) a simulation including concentration inhomogeneities, and (3) a mapping of the disordered 2D martensite onto an infinite-range spin model, identifying tweed with the spin-glass phase.PACS numbers: 8I.30.Kf, 61.70.Wp, 75.10.Nr In textbook first-order phase transitions critical fluctuations are negligible: Water shows no hint of incipient solidity above 0°C. First-order solid-solid displacive transformations are strikingly different; precursors are common for tens through hundreds of degrees away from the transitions. We study here the tweed pattern [l] observed in electron micrographs well above the transition temperature in the so-called medium and weak martensites, including the shape-memory alloys FePd [2] and NiAl [3], the superconducting ^-15's [4], and high-T^ Y-Ba-Cu-Co-O, and Y-Ba-Cu-Al-O [5,6]. The tweed appears as a characteristic cross-hatched pattern. X-rayand electron-dilTraction measurements [2,3,7-10] indicate that tweed is a local mixture of undeformed and deformed regions, and that it arises from a local {llO}/

We study the influence of elastic anisotropy on nanoscale precursor textures that exist in some shapememory alloys and show that tweed occurs in the limit of high elastic anisotropy while a nanocluster phase-separated state occurs for values of anisotropy inhibiting the formation of martensite. These results are consistent with specific heat data, elastic constant measurements, and zero-field cooling or field cooling experiments in nonstoichiometric NiTi alloys. DOI: 10.1103/PhysRevLett.100.165707 PACS numbers: 64.70.Kÿ, 62.20.Dÿ, 81.30.Kf Systems exhibiting spatially inhomogeneous states are now a subject of increasing interest [1,2]. Prototypical examples are tweed textures observed in some martensitic materials as precursors [3] to the structural transition. Tweed [4] refers to the contrast anomaly observed in TEM consisting of striations parallel to the traces of f110g planes appearing at intervals of few nanometers [5]. It originates from a static, long length scale elastic modulation of a very small strain amplitude that arises as the natural cooperative response of anisotropic long-range (elastic) interactions to local inhomogeneities [6] coupling to strain. Nanoscale precursor modulations are not exclusive to martensitic or ferroelastic systems, but instead they occur in a broad class of ferroic materials including ferromagnetic [7,8] and ferroelectric materials [9]. The specific symmetry properties of the anisotropy determine the selected pattern.In cubic shape-memory alloys (SMA) the possibility of tweed is enhanced by the existence of easy shear directions along f110g planes-reflected in a low value of shear modulus C 0 -which define natural channels that propagate correlations. This is an essential feature since it circumvents the expected cutoff of correlations associated with disorder [10]. Thus, one expects that reducing the elastic anisotropy factor (A), defined as the ratio between the two relevant shear moduli A C 44 =C 0 , will inhibit the occurrence of tweed. In the case of SMA, tweed has been observed only in materials with a significant value of A. Examples are [11] Ni-Al, Cu-Zn-Al (and other Cu-based alloys), and Fe-Pd, among others. A completely different case is that of Ni-Ti which exhibits a very rich pretransitional behavior [12] but no signature of tweed striations. The diffuse TEM contrast reported in iron-doped Ni-Ti [Ni-Ti(Fe)] has been found to correspond to very small domains of the incoming phase of almost spherical shape [13]. Energy-filtering dark-field imaging techniques enable one to visualize tiny domains down to 5 nm which are not elongated in specific directions [13]. This morphology is definitely different from that responsible for the striations in the tweed contrast [14,15]. In this case, upon cooling, the interacting domains line up and enhance the crosshatched directionality of the tweed correlations.Murakami et al. [13] have suggested that the peculiar behavior of Ni-Ti might be due to its low value of A. Nevertheless, the role of elastic anisotropy on ferroelas...

Abstract

We report on the multicaloric response of the Fe 49 Rh 51 alloy under the combined application of hydrostatic pressure and magnetic field. Experimental data are complemented by a mean field model that takes into account the interplay between structural and magnetic degrees of freedom. A large multicaloric strength has been found for this alloy, and it is shown that a suitable combination of pressure and magnetic field enables the sign of the entropy change to be reversed and thus the multicaloric effect can be tuned from conventional to inverse. It is also shown that an extended temperature window for the multicaloric effect can be achieved by taking advantage of the coupling between structure and magnetism which enables a cross response of the alloy under the application of different external fields. Mean field calculations remarkably reproduce experimental results.