Search

The (100) surface of Ni2MnGa ferromagnetic shape memory alloy exhibits intrinsic surface property dissimilar to the bulk as well as influence of compositional variation at the surface. It is shown that by sputtering at room temperature and annealing at high temperature, it is possible to obtain a clean, ordered and stoichiometric surface. However, for even higher annealing temperatures, the surface becomes Mn rich. The (100) surface of Ni2MnGa is found to have Mn–Ga termination. A surface reconstruction to p4gm symmetry is observed in the austenite phase, while the expected bulk truncated symmetry at surface is p4mm. For the stoichiometric surface, the XPS valence band is compared with our calculations based on first principles density functional theory and good agreement is obtained. The ultraviolet photoelectron spectroscopy (UPS) valence band spectra depend sensitively on composition varying from Ni rich to Mn rich surfaces. A satellite feature observed in both Ni 2p core-level and valence band spectra is related to the narrow 3d valence band in Ni2MnGa.

The spontaneous generation of voltage (SGV) in single crystalline Tb 5 Si 2.2 Ge 1.8 and Gd has been studied. Temperature-induced SGVs were observed along the three principal crystallographic axes of Tb 5 Si 2.2 Ge 1.8 , but not in Gd. Field-induced SGVs were observed with magnetic fields less than 40 kOe applied along the a -axis of Tb 5 Si 2.2 Ge 1.8 , and the c -axis of Gd. The absence of the temperature-induced SGV in Gd indicates the key role first-order phase transformations play in the appearance of the effect when temperature varies. The anisotropy of the magnetic field-induced SGV in Tb 5 Si 2.2 Ge 1.8 and the existence of the field-induced SGV in Gd, highlight the importance of the magnetocaloric effect in bringing about the SGV.

There have been many investigations to identify new alloys exhibiting large magnetic field-induced strain and more recently, the giant magnetocaloric effect. For any particular alloy system, it is necessary to adjust composition to alter the magnetic ordering and structural transformation temperatures to achieve the desired field-induced response. For the Ni–Mn–Sn alloy system, this compositional tuning has dramatic effects on microstructural development significantly affecting both transitions. Solidification induced chemical variations, when not fully equilibrated, have been found to mask the intrinsic magnetostructural behavior in Ni50Mn37Sn13. The use of a phase purity criterion was found to be an insufficient descriptor for determining homogeneity. When both structural and chemical homogenization are achieved, the nature and sequence of the transitions were found to be: (1) austenite paramagnetic to ferromagnetic transition, (2) coupled ferromagnetic austenite to paramagnetic martensite magnetostructural transition, and (3) paramagnetic to ferromagnetic martensite transition with Curie–Weiss behavior on heating above its Curie temperature.

We have studied the lattice dynamics of a Ni 54 Mn 23 Al 23 (at.%) Heusler single-crystalline alloy by means of neutron scattering and ultrasonic techniques. Results show the existence of a number of precursor phenomena. We have found an anomaly (dip) in the low TA 2 phonon branch at the wave number ξ 0 ≈ 0.33 (in reciprocal lattice units) that becomes more pronounced (phonon softening) with decreasing temperature. We have also observed softening of the associated shear elastic constant ( C ′ ) with decreasing temperature. Ultrasonic measurements under applied magnetic field, both isothermally and varying the temperature show that the values of elastic constants depend on magnetic order thus evidencing magnetoelastic coupling.

A combination of characterization methods (SEM, X-ray diffraction, DTA, neutron diffraction and phase diagram analyses) has been used to determine the actual composition of a complex intermetallic compound of nominal composition Pr 6 Ni 2 Si 3 where neither X-ray nor neutron diffraction alone are capable of differentiating between vacancies or Si substitution on one of the Ni sites due to the scattering factor of two Si substitutions being equivalent to one Ni atom plus a vacancy. Using a combination of quantitative phase analysis of the diffraction data based on Reitveld refinement, metallography and mass balance calculations, the correct stoichiometry was found to necessarily contain excess Si, and is close to the stoichiometry, Pr 6 Ni 1.6 Si 3.2. Nonetheless this composition still contains around 1% second phase following long term anneal. Due to the close proximity of neighboring phases, this corresponds to an error in stoichiometry of approximately 0.02%. Assuming similar occupancies of Si and Ni on the Ni sites for other members of the homologous series R ( n +2)( n +1) Ni n ( n −1)+2 Si n ( n +1) , compositions of Pr 5 Ni 1.85 Si 3.1 and Pr 15 Ni 6.7 Si 10.2 were predicted for n = 3 and n = 4 alloys, respectively, and verified experimentally. The approach present in this paper is applicable to many other systems.

Thermo-mechanical and magneto-mechanical properties were studied for iron substituted with 17.5, 19.1 and 20.5 at.% gallium quenched from 1000 °C and subsequently heated to 400 °C and slow-cooled. A phase change was observed in the 19.1 and 20.5 at.% Ga quenched samples upon heating. X-ray diffraction studies on the single crystal samples showed that the phase change observed was due to the precipitation of D0 3 upon heating from the quenched state. Magneto-mechanical characterization showed that the presence of ordered D0 3 phase in the slow-cooled 19.1 at.% Ga sample reduced the saturation magnetostriction in comparison to the quenched sample of same composition. The magneto-mechanical properties of both the quenched and slow-cooled 17.5 at.% Ga samples were similar owing to the presence of only the disordered A2 phase in both these samples.

Fe–Ga alloys have exhibited significant increases in magnetostriction as a function of Ga content, reaching a maximum near the solubility limit of Ga in BCC α-Fe (A2). At this limit, the magnetostriction is dependent on the thermal history of the sample and the formation of long-range chemically ordered D0 3 phase. However, due to nearly identical unit cells of A2 and D0 3 phases and similar atomic scattering factors of Fe and Ga, detection and quantification of phase mixtures in Fe–Ga alloys is problematic. This work demonstrates the use of high resolution powder X-ray diffraction in detecting the presence of two-phase mixtures in Fe–Ga alloys. Peak splitting of the primary reflections is resolved, irrespective of whether or not superlattice reflections from the ordered structure are detectable. This peak splitting is clearly discernible from Kα 1 –Kα 2 splitting at moderate and high 2 θ values and texturing of the samples does not interfere or mask the peak splitting. Quantitative analysis showed Fe-19.5 at.% Ga was 100% A2 when quenched from high temperatures, but was a two-phase mixture, 67% A2 and 33% D0 3 by volume, when slow cooled. Conversely, Fe-22 at.% Ga alloy was a two-phase mix, 40% A2 and 60% D0 3 when quenched and 100% D0 3 when slow cooled.

R5(SixGe1−x)4 alloys, where R is Tb, Dy, and Er, have been examined. Their microstructures consisted of large grains with thin plates of a second phase. Energy dispersive spectrometry of the plates revealed a rare earth rich composition. It is proposed that the plates are R5(SixGe1−x)3 compounds, similar to results seen in Gd5(SixGe1−x)4.

Single crystals of Gd 5 (Si x Ge 1− x ) 4 have been prepared by the Bridgman method using tungsten crucibles. The tungsten crucible was found to be chemically inert with respect to Gd, Si, Ge but did have limited solubility in liquid Gd 5 Si 2 Ge 2 , resulting in the precipitation of pure tungsten throughout the bulk crystal. Overall, the bulk crystal solidified in the monoclinic phase and a slight increase in Si content and decrease in Ge content at the growth proceeded was found. This increase in Si:Ge ratio resulted in a slight increase in lattice parameter of the monoclinic phase and an increase in the magnetostructural transformation temperature of Δ T = 10 K. AC susceptibility measurements indicated a small fraction of orthorhombic phase was present throughout the ingot which accounts for the decrease in MCE values along the length of the ingot.

We report on investigations of the potential for using quasicrystal surfaces as templates for molecular adsorption. The quasicrystalline surfaces of the i -Al–Pd–Mn and the d -Al–Ni–Co quasicrystals have been dosed with various molecules and the surface reactivity characterised by Fourier transform infrared spectroscopy (FTIR). The reflected signal intensities obtained are much weaker than those obtained from other crystals such as metals. Although both quasicrystals are Al-based their surface reactivities are seen to differ. For the Al–Pd–Mn surface, NO and HCOOH both dissociate at the surface and CO does not adsorb. CD 4 O adsorption leads to multilayers as in other crystals. C 6 H 6 molecules stick at low coverages and at low temperatures whilst not affecting the LEED pattern. For the Al–Ni–Co surface, CO molecules stick via activated adsorption to atop sites with vibrational frequencies very similar to CO adsorption on Ni. HCOOH forms multilayers at low temperatures which is reduced to a monolayer upon heating to 173 K; changes in the symmetry of the bonding are observed, which is also seen for HCOOH on Ni surfaces. NO and CD 4 O showed no characteristic spectra, which may be due to dissociation or a very low sticking probability. We summarise these observations and draw some conclusions as to the most favourable route for molecular overlayer formation.

An X-ray standing waves experiment was performed at the ID32 beam line of the ESRF on an Al–Pd–Mn quasicrystal with the X-ray beam at normal incidence. The X-ray photoemission core levels for each element were recorded to probe the surface. The drain current, i.e. the electron current from ground needed to neutralize the irradiated sample and to compensate the photoemitted electrons was also recorded as a bulk probe that integrates over all atoms of the sample. Within the two-beam approximation applied to quasicrystals, a simple analysis of this effect is proposed. This provides a direct-space illustration of the diffraction process in a quasiperiodic structure.

Flux-growth techniques have been successfully applied to the preparation of several families of quasicrystals and related approximant phases. Large (up to 1 cm 3 ) single-grain samples have been obtained for icosahedral R–Mg–Zn (R=Y, Gd, Tb, Dy, Ho and Er), decagonal Al–Ni–Co, icosahedral Al–Pd–Mn, icosahedral Al–Ga–Pd–Mn, and the ξ′ Al–Pd–Mn approximant phase. The samples are obtained by slowly cooling ternary (or quaternary) melts of a composition that intersects the primary solidification surface of the desired phase in the equilibrium ternary alloy phase diagram (i.e. this is a self-flux technique). The technique clearly reveals the growth habit, and results in remarkably strain-free quasicrystals.

Directional solidification and quenching experiments have been conducted to examine the formation and origin of the faceted porosity commonly observed in icosahedral Al–Pd–Mn alloys. Faceted porosity has been previously observed in both Czochralski grown and Bridgman grown single crystals during single-phase solidification of the icosahedral phase. Detailed observations in Bridgman grown single crystals indicate that the distribution and quantity of pores are non-uniformly distributed along the length of the as-grown ingot. This non-uniform distribution suggests that their formation is influenced by both chemical segregation during solidification as well as convection in the liquid. Controlled solidification of this alloy followed by quenching of the solid–liquid interface provides for a time–temperature history of the porosity formation and morphological development as a function of processing conditions.

Fivefold surfaces of Al–Pd–Mn quasicrystal have been prepared under ultra-high vacuum (UHV) by sputtering and annealing above 700 K and characterized by electron diffraction, Auger spectroscopy and X-ray photoemission spectroscopy (LEED, AES, XPS). X-ray reflectivity at grazing incidence reveals that an in situ temperature annealing at 900 K is necessary to irreversibly produce the signature of a flat and bulk terminated surface. Air-AFM investigations of surfaces prepared below or in the vicinity of this transformation temperature show a large scale structure in the micrometer range with high steps and parallel terraces. Terrace morphology is dominated by clusters with a typical size of about 10 nm. Nanometer steps can coexist with this cluster structure. With UHV-STM, terrace roughness based on cluster assembly is found to extend down to the nanometer scale.

The mechanical properties and the dislocation microstructure of single crystals with a range of compositions within the (Fe,Ni)–Al pseudobinary system have been investigated, with the purpose of bridging the behavior from FeAl to NiAl. Initial experiments are focused on compression testing of (001) oriented single crystals with a family of compositions of the form Fe x -Ni 60 _ x –Al 40 (where x = 60, 50 and 10 at.%). Measurements of yield strength and work hardening rates have been correlated with overall dislocation morphology, dislocation dissociation width and the presence of APB tubes.

A survey of a number of different techniques including electron beam float zone, inductively coupled plasma pedestal growth, and induction float zone in atmospheric and pressurized chambers was conducted for the synthesis of MoSi 2 single crystals. These techniques were evaluated as to their suitability for growth of MoSi 2 . Of concern is the high vapor pressure of silicon at the melting point which can result in silicon loss and the formation of Mo 5 Si 3 . Of the techniques surveyed, pressurized induction float zone offers the best potential for compositional control during crystal growth. Some success is demonstrated using induction float zone techniques but, unfortunately, in order to maintain a stable liquid zone, crystal growth rates 3–5 times faster than typical float zone growth rates are necessary. As a result, the size of crystals prepared thus far is limited to approximately 3 mm in diameter.

Several surface phases have previously been identified on the five-fold surface of the Al70Pd21Mn9 quasicrystal. Two of these, the clustered and terraced phases, have been discovered by scanning tunnelling microscopy (STM) studies. In this paper, we report STM images showing the existence of a new phase on this surface. This new topography was obtained by annealing the sample close to its melting point for several minutes. A model is presented which exhibits the main features of the structure. The unit-cell dimensions match those of the Al3Mn orthorhombic phase. Mechanical polishing followed by sputtering and annealing restored a pentagonal quasicrystalline surface indicating that the decomposition is a near-surface phenomenon.

The elastic constants C′, C44 and CL of a single crystal of the Ni40Ti50Cu10 (numbers indicate at.%) alloy have been measured in the premartensitic region (320–350 K) and have been found to exhibit softening on approaching the B2→B19 transition from high temperature. This means that these elastic constants decrease with decreasing T. The anisotropy factor A=C44/C′ is relatively small (≅2.2) over all the temperature range and slightly increases with decreasing temperature down to the transition start-temperature Ms. Thus, its temperature coefficient ∂A/∂T has opposite sign of that exhibited by the binary Ni50.8Ti49.2 alloy prior to the B2→B19′ transition. The present results support the view recently expressed by some authors that the B2→B19′ transition occurring in binary Ni–Ti alloys is a consequence of concomitant lattice instabilities against {1 1 0}〈1 1 0〉 B 2 and {0 0 1}〈 1 1 0〉 B 2 shear strains respectively corresponding to the C′ and C44 ultrasonic propagation modes.