Mg-Sn Alloys Research Articles

Microstructure and Impression Creep Characteristics of Cast Mg-5Sn-xBi Magnesium Alloys

The microstructure and creep behavior of a cast Mg-5Sn alloy with 1, 2, and 3 wt pct Bi additions were studied by impression tests in the temperature range 423 K to 523 K (150 °C to 250 °C) under punching stresses in the range 125 to 475 MPa for dwell times up to 3600 seconds. The alloy containing 3 wt pct Bi showed the lowest creep rates and, thus, the highest creep resistance among all materials tested. This is attributed to the favorable formation of the more thermally stable Mg3Bi2 intermetallic compound, the reduction in the volume fraction of the less stable Mg2Sn phase, and the dissolution of Bi in the remaining Mg2Sn particles. These particles strengthen both the matrix and grain boundaries during creep deformation of the investigated system. The creep behavior of the Mg-5Sn alloy can be divided into the low- and high-stress regimes, with the respective average stress exponents of 5.5 and 10.5 and activation energies of 98.3 and 163.5 kJ mol−1. This is in contrast to the creep behavior of the Bi-containing alloys, which can be expressed by a single linear relationship over the whole stress and temperature ranges studied, yielding stress exponents in the range 7 to 8 and activation energies of 101.0 to 107.0 kJ mol−1. Based on the obtained stress exponents and activation energies, it is proposed that the dominant creep mechanism in Mg-5Sn is pipe-diffusion controlled dislocation viscous glide the low-stress regime and dislocation climb creep with back stress in the high-stress regime. For the Mg-5Sn-xBi alloys, however, the controlling creep mechanism is dislocation climb with an additional particle strengthening effect, which is characterized by the higher stress exponent of 7 to 8.

The microstructure, creep resistance, and high-temperature mechanical properties of Mg-5Sn alloy with Ca and Sb additions, and aging treatment

The microstructure, creep resistance, and high-temperature shear strength of the Mg-5Sn, Mg-5Sn-1.4Ca, and Mg-5Sn-0.4Sb alloys were studied in the as-cast and aged conditions. Impression creep tests were performed at 150 and 200 oC and under punching stresses in the range 250-500 MPa. The shear yield stress (SYS) and ultimate shear strength (USS) were measured by the shear punch test (SPT) in the temperature range 25-250 oC. The results showed that creep resistance and shear strength of the base alloy were significantly enhanced with the addition of Sb and Ca. This was attributed to the concurrent formation of Mg 2Sn particles with the more thermally stable CaMgSn or Mg 3Sb 2 phases which strengthen both matrix and grain boundaries during deformation in the investigated system. Furthermore, mechanical properties and creep resistance of the aged alloys were much better than those of the as-cast materials, due to the higher volume fraction of Mg 2Sn, Mg 3Sb 2, and CaMgSn precipitates distributed homogeneously in the microstructure of the aged materials.

Effect of Microstructural Inhomogeneity on Creep Response of Mg-Sn Alloys

The development of new creep resistant magnesium alloys has become a major issue in recent years. The alloys investigated in the present work are based on the binary system Mg-Sn. Sn as major alloying element was chosen due to its high solid solubility over a wide temperature range and due to the possible formation of Mg2Sn intermetallic precipitates with a high melting temperature of about 770°C. These characteristics suggest that a fairly large volume fraction of thermally stable Mg2Sn particles can be formed during solidification. This makes it possible that the Mg-Sn alloys can be developed as creep resistant magnesium alloys. In fact, previous investigations indicate that the Mg-Sn alloys have a comparable or even better creep property than AE42 alloy. The present work investigates the microstructure of Mg-Sn alloys with and without creep deformation using SEM and TEM technique. The effects of microstructural inhomogeneity on the creep response are presented. Based on the microstructural analysis, the mechanism responsible for improving the creep resistance will be discussed. It is shown that the grain boundary sliding is a dominant creep mechanism for the Mg-Sn binary alloy.

A theoretical evaluation of chemical ordering and glass transition in liquid Mg-Sn alloys

The thermochemical properties and experimental phase diagram of the Mg-Sn system are assessed by means of a computer operated method in order to obtain thermodynamic functions for the individual phases. The extrapolation of the properties of the liquid to low temperatures is discussed and a method based on an isentropic temperature (where the liquid and the crystalline phases have the same entropy) at a third of the melting point of the crystalline phase is applied. A new model, which accounts for the tendency for chemical order in the liquid, is applied and excellent agreement with the various kinds of experimental information is obtained. The new thermodynamic description is compared with previous ones based on the associate solution model, and it is concluded that the present analysis is superior because more information has been taken into account. T0, isentropic, and nucleation temperatures are calculated in order to investigate the glass forming ability. For alloys the isentropic temperature may be defined with or without the positional entropy due to chemical disorder. The results are different. An alternative assessment based on a different way of extrapolating the properties of the liquid to low temperatures is found to yield an almost identical result for stable phases and phase equilibria but a very different result for the glass transition. It is thus concluded that it is not possible to extract information for the glass transition from the high-temperature data.

Charge transfer in liquid alloys

The charge transfer in liquid alloys is discussed in terms of the partial structure factors in the long-wavelength limit and its validity is also examined. Numerical calculations are carried out for liquid Mg-Sn alloys.

Compressibility and concentration fluctuations in liquid magnesium alloys

The adiabatic compressibility of liquid Mg-Sn alloys has been determined to an accuracy of 0.5% through measurements of the sound velocity (up to 920 degrees C) and knowledge of the density. Comparison is made with the compressibility of liquid Mg-Bi. Whereas the compressibility of liquid Mg-Bi displays a distinct maximum at the composition Mg3Bi2, only a slight anomaly is observed in the Mg-Sn system, at the Mg2Sn composition. These anomalies are found to correlate with a minimum in the concentration fluctuations which have been evaluated from available thermodynamic data. In addition, the zero wavevector partial structure factors have been evaluated for the liquid Mg-Sn system.

Partielle Interferenz-und partielle Atomverteilungsfunktionen sowie elektrischer Widerstand von geschmolzenen Magnesium—Zinn-Legierungen

Abstract From measured intensity functions, published in a previous paper, partial interference functions, partial pair correlation functions, partial atomic distribution functions, and partial coordination numbers were calculated and discussed. According to the theory of Faber and Ziman, the electrical resistivity of molten Mg-Sn alloys was calculated from the partial interference functions and from pseudopotentials reported by Animalu and Heine. The calculated values, being normalized to the resistivity of the molten components, are in good agreement with values measured by a rotating field method. The partial interference functions are derived by the assumption of independency of concentration. This assumption is proved by discussion of the partial coordination numbers and the results of electrical resistivity determinations.

Anomaly in energetic and structural properties of MgSn liquid alloys

The observed asymmetry in the properties of mixing of MgSn alloy in molten state is successfully explained on basis of the quasi-lattice model. It has been utilized to determine the free energy of mixing, entropy of mixing, heat of mixing, the concentration- concentration structure factor in long wave-length limit [Scc(0)] and the Warren Cowley short range order parameter (α1) of MgSn alloy in molten state at 1073 K. The analysis suggests that heterocordiantion leading to the formation of chemical complex Mg2Sn is likely to exist in the melt but is of a weakly interacting in nature. The interaction energies between the species of the melt are found to depend considerably on temperature and the alloy is more ordered towards Mg rich region Keywords: Quasi-lattice model; Binary alloys; Short range order parameter; Asymmetry DOI: 10.3126/bibechana.v7i0.4042BIBECHANA 7 (2011) 33-38

Solution hardening of magnesium single crystals by tin at room temperature

Tensile tests on homogenized single crystals of Mg-Sn alloys were performed at room temperature. The hardening, i.e. the increase in critical resolved shear stress (crss) per at. % solute, can be partially explained by short range order (sro). If the part due to sro is subtracted from the overall hardening, the remainder fits in the scheme of solution hardening proposed by Levine et al. (8)