Advanced Materials Research Vol. 409

Abstract: HRTEM observation was performed to clarify the early stage of precipitation in Mg-Gd-Y alloy aged at 473 K. The hardness increased with increasing aging time after quenching. The hardness increased slowly at early stage of aging, increased rapidly, and then reached to peak hardness. At as-quenching and the early stage of aging, the weak spots appeared at a half 100_Mg in SAED pattern. The intensity of these weak spots changed to higher with increasing aging time. Monolayers on {100}_Mg planes and β” phase existed at this aging stage. Monolayers, β” phase, the precipitates with five layers corresponding the feature of (020)_β’ plane and β’ phase co-existed after the rapidly increasing of hardness. In the peak aged specimen at 473 K, both of β” and β’ phases co-existed and β’ phase were formed dominantly.

Abstract: The Vickers micro hardness measurements and high resolution transmission electron microscopy (HRTEM) observation were carried out to investigate the effect of Zn content on aging behavior of Mg-Zn alloys. The hardness increased drastically after aging at 473 K in the Mg-Zn alloys with higher Zn concentration. And higher maximum hardness was obtained at the earlier aging time in the alloys with higher Zn concentration. In bright field image and HRTEM, the particle shaped contrasts of the precipitates were not appeared in 3.8%Zn. The particle shaped contrasts of the precipitates were appeared as indicated by arrows in 4.7 and 6.5%Zn. The contrasts of row on {110}_Mg planes and the β₁’ phase appeared in the water quenched 4.7 and 6.5%Zn alloy age at 473 K for 3.6 ks.

Abstract: Magnesium alloys containing Al have been used for industrial materials due to their lightweight and recyclability. The Mg-Al alloys are usually used for the industrial production. The Mg-Al alloys are divided into AZ-series alloys with the addition of Zn, and AM-series alloys with the addition of Mn, respectively. The addition of Zn to the Mg-Al alloy system reduces the solid solubility of Al in Mg, increases the amount of precipitate phases after ageing and thus causes a moderate increase in strength. The Mg₁₇Al₁₂ phase (γ) is reported as the precipitate formed in the Mg-Al alloys during aging after the solution heat treatment, which is the discontinuous precipitate in the grain boundary and continuous precipitate in the matrix. However, there is few report about the effect of Zn contents on age-hardening behavior and microstructure of AZ-series alloys. The propose of this study is to investigate the difference of the age-hardening behavior and microstructures of AZ-series alloys using hardness test and scanning electron microscopy (SEM) observation.

Abstract: Although numerous solidification experiments have been conducted for Al, Cu and Si alloys to investigate microstructural features like primary and secondary dendrite arm spacing, solute distribution with in secondary arms and second phase fraction, no systematic solidification study on Mg alloys has been performed to understand the evolution of microstructural features as a function of cooling rate and solute content. The present study focuses on the experimental microstructural evolution of Mg-3, 6 and 9 wt. % Al alloys in the cooling rate range of 1 K/sec to 1000 K/sec. The results suggest that secondary dendrite arm spacing and amount of second phase formation are strongly dependent on both solute content and cooling rate.

Abstract: The corrosion behavior of Mg-6mass%Al alloy with different microstructure conditions was studied by electrochemical method in Na₂SO₄ and NaCl solutions. A measurement of polarization curves was carried out in order to investigate the fundamental electrochemical characteristics. Electrochemical impedance spectroscopy was carried out to discuss the corrosion characteristics that were obtained from polarization curves. Electrochemical measurements were carried out with as-cast, as solution-treated and two kinds of aged specimens, respectively. For measurement of polarization curves, the apparent difference was exhibited in behavior showing the pitting corrosion by difference of solutions. In all specimens, the corrosion current density which occurred in four kinds of specimens was higher in the NaCl solution than in Na₂SO₄ solution.

Abstract: Our recent studies showed that continuous and cellular precipitates are covered with the whole of crystal grain in age hardable AM60 magnesium alloy cast into permanent molds, which have the average grain size of 75-85μm. Also, continuous precipitation is generated nearby grain boundary in the same alloys cast into sand molds, which have the average grain size of 138-147μm. It’s thought that permanent mold castings have the age hardening behavior of intragranular precipitation participation type that is influenced by continuous precipitates. It’s also thought that sand mold castings have the age hardening behavior of grain boundary participation type that is influenced by cellular precipitates. In this study, AM60 magnesium alloy with larger grain size was used to detect the grain size dependence of microstructure and aging behavior. In the microstructure of as-cast condition, the larger the grain size, it was shown that the none-equilibrium crystallized β phase with eutectic reaction during the solidification between liquidus and solidus temperatures becomes large-size. In the age hardening curves, the peak hardness values become higher with decreasing of grain size.

Abstract: Magnesium alloys containing aluminum have been used for industrial materials due to their lightweight and recyclability. And the Mg-Al based alloys are usually used for the industrial production. The Mg₁₇Al₁₂ phase is reported as the precipitate formed in the Mg-Al alloys during aging after the solution heat treatment, which is the discontinuous precipitate in the grain boundary and continuous precipitate in the matrix. The Mg-Al alloys are divided into AZ-series alloys with the addition of Zn, and AM-series alloys with the addition of Mn, respectively. AM-series and AZ-series Mg-Al alloys have been used for industrial products widely, particularly for AM-series alloys because of better toughness and impact absorption properties than AZ-series alloys. However, there is few report about the effect of Al and Mn contents on age-hardening behavior and microstructure of AM-series alloys. The propose of this study is to investigate the difference of the age-hardening behavior and microstructures of AM-series alloys using hardness test and scanning electron microscopy (SEM) observation.

Abstract: Magnesium alloys have received considerable attention because of their lightweight and recyclability. AM-series and AZ-series Mg-Al alloys have been used for industrial products widely, particularly for AM-series alloys because of better toughness and impact absorption properties than AZ-series alloys. The Mg₁₇Al₁₂ intermetallic compound is the only precipitate formed during ageing after the solution heart treatment. Discontinuous precipitates exist in grain boundary randomly, and continuous precipitates exist in the matrix. However, there is few report about the orientation relationship between the discontinuous precipitates and the matrix. The purpose of this study is to investigate the orientation relationship between the discontinuous precipitates and the matrix of AM-series magnesium alloys. Hardness measurement, SEM observation, the electron backscattered diffraction (EBSD) techniques were preformed in order to understand the relationship between the discontinuous precipitate and crystallographic orientation of grains in AM-series magnesium alloy. TEM samples with discontinuous precipitates were prepared using the focused ion beam (FIB). And TEM observation was performed to investigate the discontinuous precipitates and crystallographic orientation in the matrix.

Abstract: In the present research, seven multi-phase diffusion couples, with terminal alloys having different microstructural features, were prepared and annealed for 4 weeks at 335°C. The phase relations and change of morphological characteristics of each phase were studied along the diffusion zone by means of scanning electron microscopy/energy dispersive X-ray spectroscopy and quantitative electron probe microanalysis. Depending on the different terminal compositions of the diffusion couples, the morphological evolution in the diffusion zone can be: tooth-like, matrix phase with isolated and/or dendritic precipitates. Electron back-scattered diffraction analysis was carried out to investigate the crystal orientation of the ternary compounds and the crystal orientation relations at the interface of the diffusion zones.

Abstract: The induction heating is a surface heat treatment that exhibits some relevant industrial advantages. In fact, the process is not energy-consuming compared to thermo-chemical processes such as carbonizing and nitriding because it allows generating high power and focusing it locally and during a short time to achieve hardness at the surface area without affecting the part core. Using no plating phase, the induction heating process is qualified as green and sustainable manufacturing process but should be better understood to help developers to reach optimized recipes in a small number of process iterations. Globally, for a given range of parts to be manufactured, one has to proper select the frequency and power of the equipment to be. This work will show how part geometry, generator frequency and power are closely linked. This work is carried principally by simulation efforts using computer-modeling software (COMSOL). A developed 2D model includes the coupling between electro-magnetic and thermal fields, and takes account of the non-linear behavior of material properties versus temperature. The simulation allows optimizing the machine according to the dimensions of gear. This paper also proposes a method to approximate the power amount required to achieve a desired hardness profile.