Masking Effect of LPSO Structure Phase on Wear Transition in Mg97Zn1Y2 Alloy

Room- and elevated-temperature wear tests were conducted using a pin-on-disk testing machine to study wear behavior of Mg97Zn1Y2 alloy and role of long-period-stacking-ordered (LPSO) structure phase in mild–severe wear transition (SWT). Variation of wear rate exhibited a three-stage characteristic with load at various test temperatures, i.e., a gradual increasing stage, a slightly higher plateau stage, and a rapid rising stage. The wear mechanisms in the three stages were identified using scanning electron microscope (SEM), from which the first stage was confirmed as mild wear, and the other two stages were verified as severe wear. The interdendritic LPSO structure phase was elongated into strips along the sliding direction with Mg matrix deformation in the subsurface, plate-like LPSO structure phase precipitated at elevated temperatures of 150 and 200 °C. The fiber enhancement effect and precipitation effect of LPSO structure phase resulted in a little difference in wear rate between the first and second stages, i.e., a masking effect on SWT. Microstructure and microhardness were examined in the subsurfaces, from which the mechanism for SWT was confirmed to be dynamic recrystallization (DRX) softening. There is an apparently linear correlation between the critical load for SWT and test temperature, indicating that SWT is governed by a common critical DRX temperature.

Formation Process of Long-Period Stacking-Ordered Structures in Mg97Zn1Y2 Alloy Comprising HCP and Cubic Phases Fabricated by High-Pressure High-Temperature Annealing

As-cast Mg97Zn1Y2 alloy consists of α-Mg matrix and 18R-type long-period stacking-ordered (LPSO) structures. We observed that the alloy undergoes a phase transformation to D03 superlattices and α-Mg matrix due to high-pressure high-temperature (HPHT) annealing at 3 GPa and above 773 K. Further, the alloy recovered after HPHT annealing, consisting of the α-Mg matrix and D03 superlattices, transformed into 18R-type LPSO structures during further annealing at ambient pressure. An fcc structure with a lattice parameter of 1.42 nm, which was twice that of D03, emerged in both the collapse process of the 18R-type LPSO structure under high-pressure, and the formation process of the 18R-type LPSO structure at ambient pressure. This fcc phase was an intermediate structure between 18R-type LPSO and D03. From the electron diffraction results, it is considered that 18R-type LPSO is continuously present with 2H including stacking faults, which almost corresponded with previous studies.

Preparation and Thermal Properties of BN and LPSO Hybrid Reinforced Magnesium Matrix Composite

Hexagonal boron nitride (h-BN) is a ceramic material with high thermal conductivity (TC) and low coefficient of thermal expansion (CTE), which can improve multiple thermal properties of metal-matrix composites as a reinforcing particle, but its wettability with metal melt is very poor. In order to enhance the wettability between the h-BN and magnesium alloy melt, the electroless plating was used to coat a thin layer of pure nickel on h-BN particles, which was proved to be effective and efficient in this study. The results showed that by adding Y element to magnesium alloy melt to consume Ni element melted from the nickel-plating layer, the long period stacking ordered (LPSO) structure composed of Mg-Ni-Y was successfully formed, and the magnesium matrix composite reinforced by hybrid h-BN and LPSO structure was obtained. After ultrasonic treatment (UT), the TC of the magnesium composite containing 3 vol% h-BN and 14.16 vol% LPSO is 99.92 W/(m·K), which is a 8.3% enhancement compare to the composite without UT. The average CTE (293-373 K) of the composite is 18.36×10-6 K-1, which is reduced by 29.4% compared with pure magnesium.

Influence of Volume Fraction of Long-Period Stacking Ordered Structure Phase on the Deformation Processes during Cyclic Deformation of Mg-Y-Zn Alloys

Deformation mechanisms in extruded Mg-Y-Zn alloys with different volume fractions of the long-period stacking ordered (LPSO) structure have been investigated during cyclic loading, i.e., compression followed by unloading and reverse tensile loading. Electron backscattered diffraction (EBSD) and in situ neutron diffraction (ND) techniques are used to determine strain path dependence of the deformation mechanisms. The twinning-detwinning mechanism operated in the α-Mg phase is of key importance for the subsequent hardening behavior of alloys with complex microstructures, consisting of α-Mg and LPSO phases. Besides the detailed analysis of the lattice strain development as a function of the applied stress, the dislocation density evolution in particular alloys is determined.

Production of Oxide Dispersion Strengthened Mg-Zn-Y Alloy by Equal Channel Angular Pressing of Mechanically Alloyed Powder

Mg-Zn-Y alloys with long-period stacking ordered structures (LPSO) have attracted attention due to their excellent mechanical properties. In addition to the LPSO structure, Mg alloys can also be strengthened by oxide particles. In the present study, oxide dispersion strengthened Mg97Zn1Y2 (at%) alloys were prepared by equal channel angular pressing (ECAP) of mechanical alloyed (MA) powder under an oxygen gas atmosphere. The 20-h-MA powder had a particle size of 28 μm and a crystallite size of 36 nm. During the MA process followed by ECAP, an Mg matrix with dispersed Y2O3 (and MgO) particles was formed. The alloy processed by ECAP exhibited a hardness of 110 HV and a compressive strength of 185 MPa. Compared to pure Mg, the increased hardness was due to the dispersion strengthening of Y2O3 and MgO particles and solution strengthening of Zn and Y.

Effect of Al on microstructure and mechanical properties of as-cast Mg-8Gd-4Y-1Zn alloy

Purpose This paper aims to investigate the effect of aluminum addition on the microstructure and mechanical properties of Mg-8Gd-4Y-1Zn alloy. Design/methodology/approach Mg-8Gd-4Y-1Zn-xAl (x = 0, 0.5, 1.0, 1.5, 2.0 Wt.%) alloys were prepared by the conventional gravity casting technology, and then microstructures, phase composition and mechanical properties were investigated by material characterization method, systematically. Findings Results show that the as-cast microstructure of Mg-8Gd-4Y-1Zn alloy mainly consists of a-Mg matrix as well as Mg12REZn (18 R LPSO structure), and island-like Mg3(RE, Zn) phase is distributed at the grain boundary. The addition of a small amount of Al (0.5 Wt.%) can decrease the content of island-like Mg3(RE, Zn) phase, but significantly increase the content of long-period stacking ordered (LPSO) structure, resulting in the improvement of both tensile strength and elongation of Mg-8Gd-4Y-1Zn alloy. However, the addition of excessive Al will consume Re element and decrease the amount of LPSO structure, leading to the decrease of tensile properties. When the content of Al is 0.5 Wt.%, the tensile strength and elongation are 225 MPa and 9.0% of Mg-8Gd-4Y-1Zn alloy, which are 14% and 29% higher than that of Mg-8Gd-4Y-1Zn alloy, respectively. Originality/value Adding aluminum to Mg-8Gd-4Y-1Zn alloy strengthens its mechanical properties. And the effect of Al content on the alloy strengthening. The formation mechanism of LPSO structure with different aluminum content was revealed.