Limits of Cation Solubility in AMg2Sb2 (A = Mg, Ca, Sr, Ba) Alloys

A M 2 X 2 compounds that crystallize in the CaAl 2 Si 2 structure type have emerged as a promising class of n- and p-type thermoelectric materials. Alloying on the cation (A) site is a frequently used approach to optimize the thermoelectric transport properties of A M 2 X 2 compounds, and complete solid solubility has been reported for many combinations of cations. In the present study, we investigate the phase stability of the AMg 2 Sb 2 system with mixed occupancy of Mg, Ca, Sr, or Ba on the cation (A) site. We show that the small ionic radius of Mg 2 + leads to limited solubility when alloyed with larger cations such as Sr or Ba. Phase separation observed in such cases indicates a eutectic-like phase diagram. By combining these results with prior alloying studies, we establish an upper limit for cation radius mismatch in A M 2 X 2 alloys to provide general guidance for future alloying and doping studies.

Site Occupation and Defect Structure of Fe2Nb Laves Phase in Fe-Nb-M Ternary Systems at Elevated Temperatures

For the Fe2Nb Laves phase with C14 structure in the Fe-Nb-M (M : Cr, Mn, Co, Ni) systems, the site occupation of M in Fe2Nb has been examined in terms of XRD Rietveld analysis, particularly paying attention to the two Fe sublattice sites of Fe1 (36-net in the triple layer : t) and Fe2 (kagome-net of the single layer : s) with the fraction of 0.25 and 0.75, respectively. In any these four ternary systems, the Fe2Nb Laves phase region largely extends along the equi-Nb concentration direction; for Mn complete solid solubility exists, and the solubility of Cr and Co in Fe2Nb is more than 50 at.% and that of Ni is 44 at.%. Thus, at least two thirds of all Fe sublattices in Fe2Nb are occupied by M in all cases. Rietveld analysis revealed that Cr and Mn with which have a larger atomic size than Fe prefer to occupy the Fe1 sublattice site when the amount in solution is less than 0.25 fraction of Fe in Fe2Nb and the preferred occupation site changes to the Fe2 sublattice site when the amount in solution increases beyond 0.25. In contrast, Co and Ni whose atomic size is smaller than Fe preferentially occupy the Fe2 sublattice site, regardless of the amount. The c/a ratio of stoichiometoric Fe2Nb increases and becomes closer to the ideal value (1.633) of the cubic C15 structure when the Fe1 sublattice site is occupied by Cr and Mn. However, the degree of symmetries of both tetrahedron and kagome-net formed by Fe atoms become better when Fe2 sublattice site is occupied by a certain amount of Ni.

Relating Mechanical Properties with Dislocation Cores in Ni3Ge-Fe3Ge Intermetallic Alloys

ABSTRACTThe transition from positive to negative temperature dependence of 0.2% yield stress is investigated m the model pseudo-binary Ni3Ge-Fe3Ge system. Ni3Ge and Fe3Ge, both Ll2 intermetallic alloys, show complete solid solubility as Fe is continuously substituted for Ni across the composition range. However, Ni3Ge exhibits the yield stress anomaly, whereas the yield stress of Fe3Ge shows a normal decline with temperature. Mechanical testing has verified this behavior, with the anomalous behavior gradually disappearing with increasing Fe content. It is proposed that this transition results from changes in the structure of dissociated superdislocation cores. Alloys with anomalous behavior from this system are characterized by the presence of screw superdislocations locked in the Kear-Wilsdorf (KW) configuration. Conversely, alloys with normal yield stress dependence are observed to contain curvilinear superdislocations that glide on the cube planes. Results from mechanical testing are presented and correlated with TEM observations of deformation structures. These results are discussed in light of planar fault energies determined through computer simulations of images.

Study of the Effects of Al, Ta, W Additions on the Microstructure and Properties of Mosi2 Base Alloys

Solidification microstructures and phase selection in ingots and melt-spun ribbons of MoSi2, MoSi2-W-Ta and MoSi2-Al alloys were studied. Rapid quenching from the melt refined the grain size of all alloys by two orders of magnitude compared to the ingots. Zone A microstructures were formed in the wheel side of all alloy ribbons. Addition of W and Ta and of Al suppressed the formation of Mo5Si3 in the ingots and ribbons. Tungsten exhibited complete solid solubility in MoSi2. In the ribbons the solubility of Ta in MoSi2 was extended to 2 at% in Zone A, and in the ingots and zone B of the ribbons hexagonal (C40) TaSi2 and tetragonal (C11b) MoSi2 structures were formed. A fine lamellar microstructure between the Cllb and C40 phases was formed at 1350°C after 48h. Addition of Al changed dramatically the solidification microstructure which consisted of two phases C40 Mo(Al0.5Si0.5)2 and C54 MoAl1.3Si0.7. In the ingots the room temperature hardness decreased with addition of W and Ta and of Al. Addition of Al increased the thermal stability of the MoSi2-Al alloy.

The Microstructure of Nb3Al-Mo Intermetallics

The microstructure of arc melted ingots and melt-spun ribbons of Nb-18A1, Nb-18Al-(20–40) Mo (at%) were studied. Rapid quenching from the melt refined the grain size by at least two orders of magnitude. In the binary alloy ingots and ribbons the Nb3Al (A15) and B2 phases were present. Molybdenum exhibited complete solid solubility in the bcc Nb-Al solid solution as well as in the Nb3Al (A15 phase). The former was the only phase present in the ribbons of the ternary alloys, while Nb3Al was detected only in the ingots. B2 phase was formed in the alloy with 20 at% Mo and the A2 phase was present in the alloy with 40 at% Mo. Furthermore, Mo addition increased the room temperature microhardness of the Nb-Al solid solution by ≈7 Kgmm−2/ at%Mo.

Composition Dependence of Hardness and Moduli in GeSi/Si-Heterostructures Measured by Nanoindentation

GexSi1-x layers (0 ≤ × ≤ 1), with thicknesses ranging from 0.1 to 1.2 µm, were grown on Si(100) and Si(lll) by chemical vapor deposition (APCVD, LP/RTCVD) and liquid phase epitaxy (LPE), respectively. A NanoTest 500 machine served for nanoindentation measurements to evaluate the hardness and elastic moduli. The GeSi layers show strong alloy hardening with an increase varying proportional to x(l-x) as reported for III-V and H-VI-semiconductors. Maximum hardness is close to x = 0.45 at one and a half of the silicon hardness. For binary alloys such as Ge-Si, which show complete solid solubility, the elastic moduli are generally assumed to vary linearly with composition. In contrast to that we found for the indentation modulus E/(l-v2) a positive deviation of 30 % from linear interpolation (Vegard’s law), proportional to x(l-x). The increase in the elastic constants is explained by the structural properties of the Ge-Si alloy.

Das System P4S3–P4Se3 / The P4S3-P4Se3 System

The system P4S3-P4Se3 was investigated by room temperature and high temperature X-ray techniques, MS- and 31P NMR spectroscopy and differential scanning calorimetry. At room temperature the phase diagram contains three regions of solid solubility, one from P4S3 to ca. 45 mole% P4Se3 with the a-P4S3 structure, the second from ca. 48 to 60 mole% P4Se3 of unknown structure, and the third from ca. 65 mole% P4Se3 to P4Se3 with the β-P4Se3 structure. On heating all these phases transform into a plastic-crystalline modification (ß) with complete solid solubility between P4S3 and P4Se3. For β-P4S3 d-values are determined with Cr-Kɑ1-radiation. At higher temperatures the ß-phase transforms into a γ-phase, which was characterized by high-temperature X-ray methods.Mass and 31P NMR spectra reveal a multicomponent system consisting of all possible molecules of the type P4SχSe3-χ (χ = 0-3). All samples prepared from the melt are composed of a mixture of all these cage molecules.For P4S3 a new modification is found by the DSC measurements. It can be shown that the enthalpy of melting consists of the ß-γ transition enthalpy and the real enthalpy of fusion. Metastable γ-P4S3 can be obtained over a broad temperature region (ca. 440-405 K) on cooling of the melt; it is probably isostructural with γ-P4Se3. For the a-ß and ß-γ transitions several kinds of hysteresis effects are described