SPS preparation of ultra-fine crystal molybdenum alloy

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Molybdenum and its alloys have excellent mechanical properties at high temperature, low coefficient of expansion and high conductivity and thermal conductivity. They are very promising refractory metal materials and are widely used in aerospace, electronics, nuclear energy and many other industrial fields. Some scholars have done a lot of research work to improve the mechanical properties of molybdenum alloy. Although the strength and hardness of the alloy have been greatly improved, the toughness of the alloy tends to decline at this time. In order to take both into account, grain refinement is one of the best solutions.

SPS sintering method developed in recent years uses the instantaneous high temperature field generated by pulse energy, discharge pulse pressure and Joule heat to achieve uniform sintering process. It has the distinct characteristics of fast heating rate, short sintering time, uniform structure, high density and controllable. It is considered to be an effective way to fine grain and improve the performance of traditional materials. SPS has the characteristics of activation and rapid consolidation on the surface of powder particles, and has obvious advantages in the preparation of fine-grained, ultra-fine grained and even nanocrystalline materials.

1. The effect of sintering temperature on the density of sintered billet

With the increase of sintering temperature and holding time, the density of sintered billets will increase rapidly, but it will not increase after reaching the maximum value, and even the density will decrease due to the thermal expansion of gases and impurities. The SEM images of Mo La alloys prepared at different sintering temperatures are shown in the figure.

It can be seen from the figure that when the sintering temperature is 1500 ℃, the average grain size is 3.34um, at this time, the density is low, there are obvious voids between the grains, and the sintering effect is not ideal; when the sintering temperature is 1600 ℃, the grain size is 3.74um, it can be seen that the grain size of the alloy does not grow up obviously, but the compactness is obviously improved; when the sintering temperature is 1700 ℃, the grain size is 3.74um, it can be seen that the grain size of the alloy has no obvious growth, but the compactness is obviously improved The grain size is about 9.5um.

2. Effect of holding time on grain size of Mo La alloy sintered billet

At 1600 ℃ for different holding time, it is found that with the increase of holding time, the grain size of the sintered billet changes little at first, and it will grow up obviously after conversion to metallurgical bonding. However, the microhardness of the sintered billets began to increase significantly with the change from mechanical bonding to metallurgical bonding, and then decreased due to the growth of grains. The SEM images of Mo La alloy at the same sintering temperature for different holding time are shown in the figure.

It can be seen from the above figure (a) that there is obvious sintering neck in the powder blank when the holding time is short. With the increase of holding time, the sintering neck disappears, the pores close, and the density increases significantly. At this time, the grain size does not grow significantly, as shown in figure (b). After holding time, the grain growth is obvious, as shown in figure (c).

3. Mechanical properties of Mo La alloy sintered by SPS and hydrogen

The tensile strength and compressive strength of molybdenum lanthanum alloy sintered by SPS (1600 ℃ for 7 min) and hydrogen protection sintering (1800 ℃ for 6 h) were tested. It was found that the tensile strength and compressive strength of molybdenum lanthanum alloy sintered by SPs were increased by 765 MPa and 2307 MPa respectively. The fracture analysis of the two samples shows that the grain size of SPS sintered alloy is more uniform and fine.

From the above experiments, it can be seen that compared with the traditional atmosphere sintering, SPS can well sinter fine grain alloy, and avoid grain growth in the process of sintering, which is not possessed by other processes. Moreover, the compactness and mechanical properties of SPS sintered billets are better, which can provide a broader application prospect for the preparation of new materials.