S. De Negri

The Al–R–Mg (R=Gd, Dy, Ho) systems. Part II: Thermodynamic modelling of the binary and ternary systems

Abstract The thermodynamic modelling and optimisation of the Al–R, R–Mg and Al–R–Mg (R=Gd, Dy, Ho) systems has been carried out, the Al–Mg system being already optimised in literature. The Compound Energy Formalism (CEF) has been used to describe the thermodynamic functions of either solutions and stoichiometric phases present in the systems. In particular, the order/disorder relation between cI2-W (A2) and cP2-CsCl (B2) phases present in the systems has been taken into account and thermodynamically modelled. Moreover γ-(Mg,Al) and R5(Mg1-xAlx)24 (R=Dy, Ho) phases, related to the cI58-αMn (A12) type structure, have been modelled as different sublattice occupations of the same A12 phase. The thermodynamic modelling of the ternary systems is mainly based on the experimental investigation carried out in our laboratory and reported in a separate paper in this issue [1]. As a result, a complete description of the solid-solid and solid-liquid phase equilibria in the six binary and three ternary systems has been obtained.

Influence of the rare earth content on the electrochemical behaviour of Al-Mg-Er alloys

Abstract The electrochemical behaviour of a number of Al–Mg alloys with increasing content of erbium (rare earth metal) has been evaluated. To this end borate solutions with or without chloride have been used. The examined alloys show a complex microstructure, in which the Al solid solution coexists with such intermetallic compounds as Al3Mg2, Al66.7Mg23.3Er10 (τ) and Al3Er. In chloride-free solutions the alloys exhibit a good electrochemical behaviour independent of composition and microstructure. In chloride-containing solutions the intermetallic compounds have a strong influence due to the difference in electrochemical activity between these phases and the Al matrix; local galvanic cells form affecting the stability of the protective superficial oxide and lead to an accelerated localised corrosion attack. The presence of erbium is beneficial; the analysis of the anodic polarisation curves reveals a progressive increase in the passive range with increasing rare earth concentration.

Effect of the Al content on the optical phonon spectrum in Mg1-xAlxB2

Raman and infrared absorption spectra of Mg(1-x)Al(x)B(2) have been collected for 0<x<0.5 in the spectral range of optical phonons. The x-dependence of the peak frequency, the width and the intensity of the observed Raman lines has been carefully analized. A peculiar x-dependence of the optical modes is pointed out for two different Al doping ranges. In particular the onset of the high-doping structural phase previously observed in diffraction measurements is marked by the appearence of new spectral components at high frequencies. A connection between the whole of our results and the observed suppression of superconductivity in the high doping region is established.

Microstructure and in vitro degradation performance of Mg–Zn–Mn alloys for biomedical application

Manganese and zinc were selected as alloying elements to develop a Mg-based ternary alloy for biomedical applications, taking into account the good biocompatibility of these metals. The microstructures of Mg-Zn-Mn alloys containing 0.5 or 1.0 mass% of manganese and 1.0 or 1.5 mass% of zinc were investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. Their corrosion properties were assessed by means of potentiodynamic polarization and electrochemical impedance spectroscopy measurements performed in Ringers physiological solution that simulates bodily fluids. All tested samples are two-phase alloys formed by a Mg-based matrix, consisting of a Mg-Zn-Mn solid solution, and a Mg-Zn binary phase. The electrochemical results show an improvement of the corrosion behavior of the investigated alloys with increasing Zn and Mn content. This is attributed to the formation of a partially protective Mg(OH)(2) surface film whose protective capabilities are increased by the alloying elements. The reduced influence of the Mg-Zn intermetallic compound on the corrosion rate of Mg-Zn-Mn alloys in the presence of a partially protective surface layer can be ascribed to an increasing resistance between the Mg-Zn-Mn solid solution and the second phase, thereby decreasing the effective driving force for microgalvanic corrosion. Owing to its highest corrosion protective ability, the Mg-1.5Zn-1Mn alloy is a promising candidate for the development of degradable implants, such as screws, plates, and rods.

High Tc superconductivity in a critical range of micro-strain and charge density in diborides

Expansion of the superlattice of boron layers, with AB2 structure, due to different intercalated A atoms has been studied to understand the emergence of high Tc superconductivity in the diborides. The structure of these metal heterostructures at the atomic limit (MEHALs) (with A = Al, Mg, Ti, Hf, Zr) has been measured by synchrotron x-ray diffraction. The increasing atomic radius of the intercalated A ions induces an increase of (1) the separation between the boron layers and (2) the tensile micro-strain e of the B–B distance within the boron layers. The results show that the superconductivity in these MEHALs appears in a critical region in a phase diagram controlled by two variables, the micro-strain and the charge density (e, ρ).

ANISOTROPIC THERMAL EXPANSION IN DIBORIDES AS A FUNCTION OF MICRO-STRAIN

Thermal expansion of diborides (AB2), with different intercalated atoms (A) is studied as a function of temperature in the range of 100-370 K by high-resolution x-ray powder diffraction. The results indicate a well defined relationship between the anisotropy of the thermal expansion and the micro-strain of the boron layers, ea = (a-a0)/a0 (where a0 = c/1.08 is the equilibrium a-axis for an unstrained AB2 system), determined by the atomic radius of the intercalated atoms. The thermal expansion is isotropic at ea = 0 (i.e., near c/a = 1.08) while the AB2 system is unstrained, and it gets anisotropic away from the equilibrium. The anisotropy increases with increasing micro-strain in both directions (positive or negative, i.e. tensile or compressive) suggesting that the micro-strain is a key variable to define the state of diborides. As a matter of fact, the MgB2 with the highest Tc shows a tensile micro-strain, ea = 6% and a large thermal expansion anisotropy.

The role of boron lattice expansion in superconducting diborides

Abstract Lattice structure of the diborides has been studied to explore a possible relation between high T c superconductivity and expansion of the boron plane. High purity Mg 1− x A x B 2 diborides have been prepared by direct reaction of the elements and the lattice parameters are determined by X-ray diffraction, providing a measure to the strain ϵ of the B–B distance. The results show that the superconductivity in these intermetallics appears in a critical region of tensile strain and charge density in the boron plane.

EFFECTS OF THE Al CONTENT IN MgB2: A RAMAN STUDY

The partial substitution of Al for Mg in the MgB2 superconductor produces both a progressive reduction of its superconducting transition temperature Tc and the occurrence of two distinct structural phases at low and high Al content, as shown by X-ray diffraction data. To achieve a deeper understanding of the effect of the Al content, we investigated the x dependence of the Raman phonon spectrum in Mg1-xAlxB2 compounds. We analyze the evolution of the E2g Raman-active phonon mode observed on increasing x both in our measurements (0 ≤ x ≤ 0.5) and in recent experiments (0 ≤ x ≤ 1). Our analysis shows the connection between the evolution with x of the E2g phonon and the Tc suppression, thus supporting the hypothesis that superconductivity in MgB2 is mediated by a strong e-ph coupling with the E2g mode. We then derive the electron-phonon coupling constant λ in the 0-0.5 x range.