Neutron-scattering and electron-microscopy studies of the premartensitic phenomena inNixAl100

Abstract

Inelastic-neutron-scattering and electron-microscopy studies were performed on single crystals of the cubic phase of Ni, A1, 00, alloys for x=50, 58, 62.5, and 63.9 at. %. The [110]-TAz phonon branch, corresponding to atomic displacements along the [110]direction, is shown to exhibit anomalous behavior in that the entire branch is very sensitive to composition and exhibits a "kink" whose position in q space is also sensitive to x. A peak in the elastic diffuse scattering is also present at the same q as the kink. Amplitude-contrast electron-microscope images display a "tweed" pattern characteristic of many martensitically transforming materials. The high-resolution (phase-contrast) electron image reveals rnicroscopic deviations of the cubic structure and can be viewed as embryos of the low-temperature (5,2) martensitic phase. The displacement fields associated with these distortions are shown to be the origin of the elastic central peak observed in the experiments. Temperature-dependence studies show a substantial softening of the phonon branch in the vicinity of the kink and the elastic diffuse scattering increases as T approaches TM, the martensitic-transformation onset temperature. The mode never becomes completely soft. Furthermore, the softening is narrowly restricted to the [110]direction, which is perpendicular to [110]. Studies of the phonon dispersion curve under uniaxial stress show a softening of the phonon mode near the q value of the kink, which can be interpreted in terms of Clapp's proposed localized-soft-mode theory of nucleation of materials undergoing martensitic transformation. I. INTRQDUCTK)NThe study of martensitic transformations (MT's) have interested metallurgists for nearly a century. ' The use of MT's in physical metallurgy has been prominent in the development of alloys with high strength (e.g. , steels) and others with exotic properties (e.g. , shape-memory alloys).Various applications of MT s have evolved, despite incomplete understanding of the mechanism. Metallurgists have a precise, albeit phenomenological, definition of MT's, whereas physicists have come to use the term loosely to define many first-order phase transitions with acoustic anomalies. It is generally agreed by both disciplines, however, that MT's are first-order, displacive (meaning a coordinated displacement of atoms over distances much less than ihe atomic spacing), and diffusionless (meaning no atom by atom jumping within the cell as in order-disorder transitions).Shear strains are known to play an important role. Furthermore, nucleation and growth is important because of the firstorder nature of the transformation; the system passes through a two-phase regime with distinct interfaces between the two phases. The role of atomic displacements in the nucleation process is still not completely understood.In this paper we describe the study of one system, NiAl, over a range of compositions with the aim at understanding the role of the atomic displacements in the MT of the ordered phase. The crystal structure of the hig...

Anomalies related to theTA2-phonon-mode condensation in the HeuslerNi2Mañosa1, Gonzàlez-Comas2, Obradó3 et al. 1997Phys. Rev. B16610930View full textAdd to dashboardCite

“…When the sample is cooled down, this dip becomes more pronounced, but the softening is always incomplete and below a given temperature still above M s the frequency of the soft modes starts to increase again. 9 The prototype alloy system where similar softening effects have been observed is Ni-Al; 13 however, the amount of softening measured in Ni 2 MnGa is larger and in Ni-Al there is not an increase of the frequency below a given temperature. Also, recent measurements by some of the present authors 14 have shown that, at the temperature of phonon condensation, the thermal expansion and elastic modulus versus temperature curves exhibit a minimum, which is accompanied by an increase in the internal friction.…”

“…Another system showing an intermediate premartensitic structure accompanied by pronounced phonon softening is Ni-Al. 13 The elastic constants of this alloy, however, do not show the aforementioned anomalous behavior. 21 The different behavior in the two alloy systems is due to the existence in Ni 2 MnGa of a phase transition towards the intermediate phase.…”

The role of phase compatibility in martensite

“…From this point of view, the "soft mode" phenomenon often referred to as pre-martensitic and cited to be a characteristic of the martensitic transition is not an intrinsic softening of the austenite phase, but rather a collective and effective response of pre-existing martensitic-like embryos stabilized by static defects, such as dislocations and/or grain boundaries or even pinned defects. 34,35,40 Inside the martensitic embryos accommodation cannot be perfect, because the boundary of any finite domain cannot consist exclusively of habit planes. Therefore, elastic screening by twinning is only partial and some unrelaxed elastic energy is stored in the resulting martensitic domain and in its surrounding.…”

“…Therefore, the first embryos are essentially modulated or simply sheared as onset for the polytwinned character of the final martensite domain and cannot nucleate homogeneously through thermal fluctuations. 34,35 Nucleation is heterogeneous on pre-existing defects that generate local favorable strains. Note that, to a certain extent, these embryos exist above the equilibrium temperature T 0 and can be responsible for the softness of the effective shear elastic constant often observed experimentally well above T 0 .…”

Modeling solid-solid phase transformations: from single crystal to polycrystal behavior

“…These materials are characterized by highly mobile twin boundaries and (often) show precursor structures (e.g., tweed [4][5][6]) above the transition. Furthermore, these materials have small Bain strain, elastic shear modulus softening, and a weakly to moderately first order transition.…”