ABSTRACT
Calcium aluminosilicate CaO-Al2O3-SiO2 (CAS) melts with compositions (CaO-SiO2)(x)(Al2O3)(1-x) for x < 0.5 and (Al2O3)(x)(SiO2)(1-x) for x ≥ 0.5 are studied using neutron diffraction with aerodynamic levitation and density functional theory molecular dynamics modelling. Simulated structure factors are found to be in good agreement with experimental structure factors. Local atomic structures from simulations reveal the role of calcium cations as a network modifier, and aluminium cations as a non-tetrahedral network former. Distributions of tetrahedral order show that an increasing concentration of the network former Al increases entropy, while an increasing concentration of the network modifier Ca decreases entropy. This trend is opposite to the conventional understanding that increasing amounts of network former should increase order in the network liquid, and so decrease entropy. The two-body correlation entropy S2 is found to not correlate with the excess entropy values obtained from thermochemical databases, while entropies including higher-order correlations such as tetrahedral order, O-M-O or M-O-M bond angles and Q(N) environments show a clear linear correlation between computed entropy and database excess entropy. The possible relationship between atomic structures and excess entropy is discussed.
ABSTRACT
The noncentrosymmetric space group P43m but a pseudocentric cubic crystal structure were reported for the compound Ni5.20Sn8.7Zn4.16Cu1.04 [Larsson et al. (1994). Acta Cryst. C50, 9-12]. The recently described Ni2Sn2Zn shows a closely related structure, although it is centrosymmetric and contains additional voids which are partially occupied. Therefore, a new refinement of Ni5.20Sn8.7Zn4.16Cu1.04 based on the originally published structure factors was performed. The results indicate that the structure can indeed be described in the centrosymmetric space group Pm3m; no justification for the absence of the inversion centre could be found within the accuracy of the available data. In comparison with Ni2Sn2Zn, slight but significant differences were confirmed; consequently, the two structures are topologically related but not isotypic.
ABSTRACT
Dinickel ditin zinc, Ni(2)Sn(2)Zn, crystallizes in the cubic space group Pm3m, with a lattice parameter of a = 8.845 (1) Å and with all atoms occupying special positions. The crystal structure exhibits pronounced similarities with that of the quaternary compound Ni(5.20)Sn(8.7)Zn(4.16)Cu(1.04). It shares structural features with other compounds in the Ni-Sn-Zn system, such as Ni(5)Sn(4)Zn and Ni(3)Sn(2).
ABSTRACT
Work on the ternary Ni-Sn-Zn phase diagram revealed the existence of the title compound pentanickel tetratin zinc, Ni(3.17)Sn(2.67)Zn(0.67) [Schmetterer et al. (2012). Intermetallics, doi:10.1016/j.intermet.2011.05.025]. It crystallizes in the Ni(5)Ga(3)Ge(2) structure type (orthorhombic, Cmcm) and is related to the InNi(2) type (hexagonal, P6(3)/mmc) of the neighbouring Ni(3)Sn(2) high-temperature (HT) phase, but is not a superstructure. The crystal structure was determined using single-crystal X-ray diffraction. Its homogeneity range was characterized using electron microprobe analysis. Phase analysis at various temperatures indicated that the phase decomposes between 1073 and 1173 K, where a more extended ternary solid solution of the Ni(3)Sn(2) HT phase was found instead.
ABSTRACT
In this work three complete isothermal sections of the Ni-Sn-Zn system at 700, 800 and 900 °C are presented. They were constructed based on experimental investigation of more than 60 alloy samples. Powder XRD, single crystal XRD, EPMA, and DTA measurements on selected samples were carried out. Two new ternary compounds, designated as τ2 (Ni5Sn4Zn) and τ3 (Ni7Sn9Zn5), were identified and their homogeneity ranges and crystal structures could be described. Whereas τ3 is only present at 700 °C, the τ2-phase was found at both 700 and 800 °C. No truly ternary compound could be found in the isothermal section at 900 °C. A seemingly ternary compound at 20 at% Sn in the Ni-rich part of Ni-Sn-Zn was found at 800 and 900 °C. Our XRD results, however, indicate that this phase is a ternary solid solution of Ni3Sn-HT from constituent binary Ni-Sn. It is stabilized to lower temperatures by additions of Zn. These new experimental results will provide valuable information to the thermodynamic description of alloy systems relevant for high-temperature lead-free soldering.
ABSTRACT
The Al-Fe-Si system was studied for an isothermal section at 800 °C in the Al-rich part and at 900 °C in the Fe-rich part, and for half a dozen vertical sections at 27, 35, 40, 50 and 60 at.% Fe and 5 at.% Al. Optical microscopy and powder X-ray diffraction (XRD) was used for initial sample characterization, and Electron Probe Microanalysis (EPMA) and Scanning Electron Microscopy (SEM) of the annealed samples was used to determine the exact phase compositions. Thermal reactions were studied by Differential Thermal Analysis (DTA). Our experimental results are generally in good agreement with the most recent phase diagram versions of the system Al-Fe-Si. A new ternary high-temperature phase τ12 (cF96, NiTi2-type) with the composition Al48Fe36Si16 was discovered and was structurally characterized by means of single-crystal and powder XRD. The variation of the lattice parameters of the triclinic phase τ1 with the composition Al2+x Fe3Si3-x (-0.3 < x < 1.3) was studied in detail. For the binary phase FeSi2 only small solubility of Al was found in the low-temperature modification LT-FeSi2 (릧 ) but significant solubility in the high-temperature modification HT-FeSi2 (ζα ) (8.5 at.% Al). It was found that the high-temperature modification of FeSi2 is stabilized down to much lower temperature in the ternary, confirming earlier literature suggestions on this issue. DTA results in four selected vertical sections were compared with calculated sections based on a recent CALPHAD assessment. The deviations of liquidus values are significant suggesting the need for improvement of the thermodynamic models.
