Determination of liquid diffusion coefficients along a liquidus phase boundary

A method for determination of liquid phase diffusion coefficients along the liquidus curve in a binary alloy is presented. The technique is similar to that proposed by Watson and Hunt [1] but is more applicable to a larger selection of alloy systems. The technique utilizes temperature gradient zone melting (TGZM) to produce compositional adjustments along the length of a stationary sample placed in a temperature gradient. Diffusion coefficients as a function of position and, hence, temperature in the sample are calculated from measurement of the compositional changes occurring during TGZM and the temperature profile in the alloy. Since coefficients can be measured over a wide range of temperatures, the activation energy for diffusion can also be determined. The technique is demonstrated for a Cu-85 wt pct Sn alloy

Tiling of the five-fold surface of Al(70)Pd(21)Mn(9)

The nature of the five-fold surface of Al(70)Pd(21)Mn(9) has been investigated using scanning tunneling microscopy. From high resolution images of the terraces, a tiling of the surface has been constructed using pentagonal prototiles. This tiling matches the bulk model of Boudard et. al. (J. Phys.: Cond. Matter 4, 10149, (1992)), which allows us to elucidate the atomic nature of the surface. Furthermore, it is consistent with a Penrose tiling T^*((P1)r) obtained from the geometric model based on the three-dimensional tiling T^*(2F). The results provide direct confirmation that the five-fold surface of i-Al-Pd-Mn is a termination of the bulk structure.Comment: 4 pages, 4 figure

Experimental investigation of the electronic structure of Gd5Si2Ge2 by photoemission and x-ray absorption spectroscopy

The electronic structure of the magnetic refrigerant Gd5Ge2Si2 has been experimentally investigated by photoemission and x-ray absorption spectroscopy. The resonant photoemission and x-ray absorption measurements performed across the Gd N4,5 and Gd M4,5 edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy (4f7) and the character of the states close to the Fermi edge. The presence of Gd 5d states in the valence band suggests that an indirect 5d exchange mechanism underlies the magnetic interactions between Gd 4f moments in Gd5Ge2Si2. From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of states do not display clear variations. A significant change is instead detected in the photoemission spectra at higher binding energy, around 5.5 eV, likely associated to the variation of the bonding and antibonding Ge(Si) s bands across the phase transition

Phonon precursors to the high temperature martensitic transformation in Ti50_{50}Pd42_{42}Cr8_8

Inelastic neutron scattering measurements were carried out on the Ti$_{20}$Pd$_{50-\rm x}$Cr$_{\rm x}$ alloy, which is a high temperature shape memory material. For $\rm x=0$, the transformation temperature is ~800K and for the composition studied ($\rm x=8$ at %) M$_{\rm s}\sim 400$K. The majority of the measurements were performed in the parent, B2-phase, up to 873K. Most of the phonons propagating along the three symmetry directions [ $\zeta$00] , [ $\zeta \zeta \zeta$] , and [ $\zeta\zeta$0] were well defined with the exception of the [ $\zeta\zeta$0] transverse acoustic mode with displacements along the [ -$\zeta\zeta$0] corresponding to the $\rm C'=l/2 (C_{11}-C_{12}$) etastic constant in the $\zeta \to 0$ limit. These phonons are well defined for small $\zeta$, but for $\zeta > 0.10$ they are strongly overdamped near the transition temperature and become better defined at higher temperatures. An elastic peak develops in the cubic phase at $\zeta =0.22$ and increases in intensity as M$_{\rm s}$ is approached. However, the dispersion curves show no anomaly at this particular wavevector, in marked contrast to the lattice dynamic studies of other systems exhibiting Martensitic transformations

Iron deposition on the five-fold surface of the icosahedral Al-Pd-Mn quasicrystal

The room temperature adsorption behaviour of Fe on the five-fold surface of i-Al-Pd-Mn has been studied using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). A complex growth scenario for Fe adsorption on this substrate is observed with STM. At coverages up to about 3 MLE (monolayer equivalent), layer-by-layer growth is observed whereby small clusters and islands are formed which eventually coalesce into almost complete monolayers. No LEED pattern is observed, indicating that the layers are disordered. The AES results rule out intermixing. Above this coverage, there is a transition to a multilayer island growth mode. The islands are rotated by 72° and have the bcc(1 1 0) Fe structure. The results are compared with previous work on Fe adsorption on this substrate and on Al and Fe single crystal substrates