47 research outputs found
Molecular dynamic simulation of a homogeneous bcc -> hcp transition
We have performed molecular dynamic simulations of a Martensitic bcc->hcp
transformation in a homogeneous system. The system evolves into three
Martensitic variants, sharing a common nearest neighbor vector along a bcc
direction, plus an fcc region. Nucleation occurs locally, followed by
subsequent growth. We monitor the time-dependent scattering S(q,t) during the
transformation, and find anomalous, Brillouin zone-dependent scattering similar
to that observed experimentally in a number of systems above the transformation
temperature. This scattering is shown to be related to the elastic strain
associated with the transformation, and is not directly related to the phonon
response.Comment: 11 pages plus 8 figures (GIF format); to appear in Phys. Rev.
Development of a tight-binding potential for bcc-Zr. Application to the study of vibrational properties
We present a tight-binding potential based on the moment expansion of the
density of states, which includes up to the fifth moment. The potential is
fitted to bcc and hcp Zr and it is applied to the computation of vibrational
properties of bcc-Zr. In particular, we compute the isothermal elastic
constants in the temperature range 1200K < T < 2000K by means of standard Monte
Carlo simulation techniques. The agreement with experimental results is
satisfactory, especially in the case of the stability of the lattice with
respect to the shear associated with C'. However, the temperature decrease of
the Cauchy pressure is not reproduced. The T=0K phonon frequencies of bcc-Zr
are also computed. The potential predicts several instabilities of the bcc
structure, and a crossing of the longitudinal and transverse modes in the (001)
direction. This is in agreement with recent ab initio calculations in Sc, Ti,
Hf, and La.Comment: 14 pages, 6 tables, 4 figures, revtex; the kinetic term of the
isothermal elastic constants has been corrected (Eq. (4.1), Table VI and
Figure 4
Disorder-Driven Pretransitional Tweed in Martensitic Transformations
Defying the conventional wisdom regarding first--order transitions, {\it
solid--solid displacive transformations} are often accompanied by pronounced
pretransitional phenomena. Generally, these phenomena are indicative of some
mesoscopic lattice deformation that ``anticipates'' the upcoming phase
transition. Among these precursive effects is the observation of the so-called
``tweed'' pattern in transmission electron microscopy in a wide variety of
materials. We have investigated the tweed deformation in a two dimensional
model system, and found that it arises because the compositional disorder
intrinsic to any alloy conspires with the natural geometric constraints of the
lattice to produce a frustrated, glassy phase. The predicted phase diagram and
glassy behavior have been verified by numerical simulations, and diffraction
patterns of simulated systems are found to compare well with experimental data.
Analytically comparing to alternative models of strain-disorder coupling, we
show that the present model best accounts for experimental observations.Comment: 43 pages in TeX, plus figures. Most figures supplied separately in
uuencoded format. Three other figures available via anonymous ftp
Vibrational properties of the one-component phase
A structural model of a one-component -phase crystal has been
constructed by means of molecular dynamics simulation. The phonon dispersion
curves and the vibrational density of states were computed for this model. The
dependence of the vibrational properties on the thermodynamical parameters was
investigated. The vibrational density of states of the -phase structure
is found to be similar to that of a one-component glass with icosahedral local
order. On the basis of this comparison it is concluded that the phase
can be considered to be a good crystalline reference structure for this glass
ARE MARTENSITIC PHASE TRANSITIONS IN PURE GROUP 3 AND 4 METALS DRIVEN BY LATTICE VIBRATIONS ?
The phonon dispersion in the high temperature bcc phase of the group 3 and 4 metals Sc, La, Ti, Zr and Hf have been determined by inelastic neutron scattering. In these pure elements the lattice vibrations are characterized by a valley of low energy modes, which are extremely damped, i.e. lifetimes as short as one vibrational period are measured. It is shown that these low energy vibrations exhibit a particular temperature behavior and are directly related to the transformation into the low temperature hcp or fcc phase or into the hexagonal [MATH]-phase under increasing pressure
Entropy change of martensitic transformations in Cu-based shape-memory alloys
We have investigated the different contributions to the entropy change at the martensitic transition of different families of Cu-based shape-memory alloys. The total entropy change has been obtained through calorimetric measurements. By measuring the evolution of the magnetic susceptibility with temperature, the entropy change associated with conduction electrons has been evaluated. The contribution of the anharmonic vibrations of the lattice has also been estimated using various parameters associated with the anharmonic behavior of these alloys, collected from the literature. The results found in the present work have been compared to values published for the martensitic transition of group-IV metals. For Cu-based alloys, both electron and anharmonic contributions have been shown to be much smaller than the overall entropy change. This finding demonstrates that the harmonic vibrations of the lattice are the most relevant contribution to the stability of the bcc phase in Cu-based alloys