Magnesium Research: Correlation between wear transition of Mg97Zn1Y2 alloy

July 1, 2021

In recent years, people have gradually discovered some characteristics about the long-period stacking ordered structure (LPSO), such as thermal stability at 500oC, kink band deformation mode, and preventing the growth of deformation twins in the magnesium matrix. Due to its special LPSO structure, Mg97Zn1Y2 alloy has excellent performance at room temperature and high temperature. It can not only be used to manufacture structural parts used under room temperature and high temperature conditions, but also can be used to manufacture wear parts such as pistons, sliding bearings and light-duty gears. Studies have shown that at room temperature, Mg97Zn1Y2 alloy shows better wear performance than AZ91 alloy. Magnesium alloys usually exhibit two different wear behaviors, namely, light wear and severe wear. Light wear is a stable wear state, which is accepted by engineering applications. At present, the wear performance of Mg97Zn1Y2 alloy at room temperature has been deeply studied in a wide load and speed range, and the safe wear area has been determined. However, so far, there are still few studies on the high-temperature wear characteristics of magnesium alloys, and the problems related to the slight-severe wear transition, such as the mild-severe wear transition mechanism, judgment criteria, critical transition load or test temperature, have not been involved. Therefore, in order to further expand the engineering application of magnesium alloys, it is extremely necessary to carry out research on the wear performance and wear transition of Mg97Zn1Y2 alloy at high temperature.

Recently, Professor An Jian from the School of Materials Science and Engineering of Jilin University and others have systematically studied the mild-to-severe wear transition of Mg97Zn1Y2 alloy at room temperature and studied the high-temperature wear properties of Mg97Zn1Y2 alloy in the range of 20-200oC. -The changes in the structure and properties of the subsurface layer before and after the severe wear transition reveals that the mechanism of the slight-severe wear transition is the softening induced by the dynamic recrystallization transition of the subsurface layer. The wear transition obeys the critical surface dynamic recrystallization temperature criterion, and the critical transition load can be determined by this criterion. Evaluation.

By measuring the wear rate-load change curve of Mg97Zn1Y2 alloy at different experimental temperatures and a test rate of 0.5 m/s (Figure 1), the effect of load and temperature on the wear rate was systematically studied, and it was found that: (1) Wear rate It increases with the increase of load; (2) In the range of 20-100oC, the influence of temperature is not a simple positive correlation, but in the range of 150-200oC, the wear rate increases with the increase of temperature; (3) In every At each test temperature, the wear rate-load curve can be divided into two regions, and the turning point between the two regions essentially corresponds to the transition from mild to severe wear. Through SEM and EDS technical means, the morphological characteristics and chemical composition changes of the worn surface are analyzed, and the main wear mechanisms in the process of minor wear are determined as oxidation, abrasive particles, peeling, slight plastic deformation, and the main wear mechanisms in the process of severe wear. For severe plastic deformation, peeling of the oxide layer and surface melting. On this basis, the wear rate diagram and the wear mechanism transition diagram are drawn, as shown in Figure 2.

 

 

A comparative analysis of the changes in the wear sub-surface structure before and after the slight-severe wear transition is shown in Figure 3. It is found that plastic deformation occurs in the sub-surface layer in the slight abrasion stage, and the depth of the deformation zone increases as the load increases. In the severe wear stage, when the load exceeds the transformation load, the friction-affected zone includes two sub-zones, the dynamic recrystallized fine grain sub-zone located in the upper part and the plastic deformation sub-zone in the lower part. When the load is further increased and the surface melting and wear mechanism appears, the friction-affected zone consists of three sub-zones from top to bottom: solidification sub-zone, dynamic recrystallization fine-grain sub-zone and plastic deformation sub-zone. A comparative analysis of the hardness changes of the worn sub-surface layer before and after the slight-severe wear transition is shown in Figure 4. The change of the hardness gradient of the worn sub-surface layer shows that in the light wear stage, the hardness decreases monotonously with the increase of depth. At this time, the greater the load, the higher the overall level of hardness, indicating that strain strengthening has occurred. In the severe wear stage, there is a low hardness in the near-surface area, indicating that softening has occurred, which proves the dynamic recrystallization of the surface layer. The above results indicate that strain strengthening dominates the change of sub-surface properties in the lightly worn stage, while dynamic recrystallization and softening play a leading role in the severely worn stage. The tissue transition before and after the slight-severe wear transition is shown in Figure 5.