About Us  |  Contact
Mail:sales@northalloys.com
News Center

Mechanism of strengthening and toughening of molybdenum alloys

Up to now, there are two main ways to strengthen and toughen molybdenum: pure purification and alloying.

Early plasticity of molybdenum method focuses on improving the purity of molybdenum, this is because the high pure molybdenum under the condition of static stretching, due to the rotation of grain, can make the stress concentration relax, the crack propagation become very slowly, so as to make the pure molybdenum from micro cracks produced to the material before rupture can withstand larger plastic deformation. In addition, another main reason for the high ductility of pure molybdenum is that when the stress concentration on the grain boundary reaches a certain degree, secondary cracks will appear on the interface between grains parallel to the tensile force, which causes the growth of the original main crack to be blocked or diverted, and the main crack tip to be passivated, thus achieving the role of hardening of the secondary crack.

However, due to the inherent low temperature brittleness of aluminum and the inevitable influence of carbon, nitrogen, oxygen and other impurities in the production process, especially the influence of oxygen element, it is inevitable to study other methods to optimize the plasticity of molybdenum. It is generally believed that the embrittlement of molybdenum is caused by the deposition of interstitial impurity elements on grain boundaries, and alloying is the best method to improve molybdenum embrittlement.

The purpose of molybdenum alloying and microalloying is to make molybdenum alloy obtain high strength, high plasticity, high oxidation resistance at high temperature, heat resistance, corrosion resistance and good processing properties. Through the mechanism of solution strengthening, dispersion strengthening, fine grain strengthening, fiber strengthening and second phase toughening, the high temperature properties of molybdenum alloy are not only improved, the application range of high temperature is broadened, the low temperature brittleness is greatly reduced, and the processing properties of molybdenum alloy are improved.

It has long been found that adding titanium, zirconium and hafnium to molybdenum can increase the recrystallization temperature of molybdenum. The optimum content of titanium is 0.5%. The content of zirconium is in the range of 0.1%~0.4%, and the strengthening effect of zirconium on molybdenum is much stronger than that of titanium. But in general, the strengthening effect of trace solution is weak, and the properties of molybdenum alloy are slightly higher than that of pure molybdenum. The thermal strength and hardness of molybdenum alloy can be significantly improved by a large amount of solid solution strengthening, but the machinability becomes worse.

Researchers at home and abroad have made a large number of studies on the effects of other elements on the brittle properties of molybdenum plastics. It is found that the most effective element to improve molybdenum plasticity is the VIIB group element rhenium. Adding a certain amount of rhenium can effectively reduce the yield strength of molybdenum and thus improve the plasticity of molybdenum alloy, which is usually called the "rhenium effect".

In addition, on the one hand, rhenium can greatly reduce the plastic-brittle transition temperature of molybdenum alloy, and with the increase of rhenium content, the plastic-brittle transition temperature decreases gradually, so that the molybdenum rhenium alloy has a good performance at room temperature. On the other hand, it can also improve the recrystallization temperature of molybdenum alloy, and improve the normal temperature property, welding property and radiation resistance of molybdenum alloy. The addition of rhenium can improve the comprehensive properties of molybdenum alloy, and significantly expand the application of molybdenum alloy in aerospace, electronics industry, nuclear reactor and other fields.

Since 1955, it was found that Mo-50Re alloy has better plasticity and strength than molybdenum, many scholars have made in-depth and detailed studies on the molybdenum-rhenium alloy. The results show that no obvious cracks have been found in Mo-5Re and Mo-50Re alloy ingots prepared by powder metallurgy after 25% deformation at room temperature. Rhenium is a metal with high melting point, and its crystal has a densely arranged hexagonal structure. Its elastic modulus is second only to osmium. Rhenium has a high solubility in molybdenum and does not have plastic-brittle transition at low temperature. The reasons for the excellent performance of molybdenum-rhenium alloys at room temperature can be summarized as follows: Rhenium can form MoreO4 type compounds with molybdenum, but unlike MoO2 type compounds, does not infiltrate grain boundaries; (2) Rhenium can prevent the segregation of impurity elements at grain boundaries, thus purifying grain boundaries; (3) the twin deformation of molybdenum-rhenium alloy at low temperature is different from that of pure molybdenum; (4) Rhenium changes the electronic structure of molybdenum, reduces the direction of atomic bonds, inhibits the transition of molybdenum from metal bond to covalent bond, reduces the stacking fault energy and increases the shear modulus, which can fundamentally solve the low temperature brittleness nature of molybdenum. According to the current research situation, rhenium is the only element that can fundamentally solve the intrinsic brittleness of molybdenum.

However, because of the high price of rhenium, the application of molybdenum-rhenium alloy is limited. In recent years, many researchers began to study the low rhenium molybdenum-rhenium alloys and made some progress. In addition, the α phase is very easy to appear in the molybdenum-rhenium alloy with high rhenium content, and the existence of α phase will seriously affect the properties of molybdenum-rhenium alloy, especially the processing property. Therefore, it is very necessary to develop low rhenium alloys from the aspects of alloy cost and machining property.

In addition to the Mo-Re alloy, the molybdenum doped with Si-Al-K or K-Si based on the theory of potassium bubble, and the rare earth doped molybdenum alloy developed by using the characteristics of the thermodynamic properties of rare earth oxides are very stable and the melting point is close to that of molybdenum also have excellent high temperature performance. The strengthening mechanism of molybdenum doping is that the solid particles deform together with the parent metal and the dispersion particles effectively hinder the grain boundary movement. The resistance of the recrystallized grain boundary along the axial direction is much smaller than that along the wire diameter direction. The grain boundary migrates rapidly along the axial direction but slowly along the radial direction. Finally, the lap structure with large aspect ratio is formed, which is beneficial to improve the toughness of the recrystallized material and increase the recrystallization temperature significantly.

But with the continuous development of modern science and technology, the above molybdenum alloy has been unable to meet the application needs of people. In order to pursue higher performance materials, on the basis of previous research, many domestic and foreign researchers in recent years try to combine various strengthening and toughening methods, using comprehensive strengthening method to achieve further improvement of molybdenum alloy properties. The combination of solid solution strengthening and carbide strengthening, carbide strengthening and ODS strengthening, ODS strengthening and bubble strengthening, and their many kinds of mutual combination, this kind of alloy is called multicomponent alloy. The strengthening mechanisms of molybdenum alloys are closely related. The strengthening of trace elements mainly takes place at the temperature of 1100℃~1300℃. When the temperature is higher, it will fail, and the diffusion strengthening effect of carbide is most obvious at 1400℃~1500℃. At 1500℃~1800℃, carbides soften and become unstable. At this temperature, the strengthening effect of rare earth oxides with high melting point is significant. When the temperature is higher than 2000℃, the rare earth oxides begin to soften, and the strengthening effect of bubbles of doped aluminum, potassium and silicon is significant.
Find Better Metal for Better Future
Products
Zirconium
Titanium
Tantalum
Niobium
Molybdenum
Tungsten
Nickel
Others
Tel:+86 21 56836035
Fax:+86 21 56836035
Email:sales@northalloys.com
Address:No 188,Xinfeng Road,Shanghai China 201501
sales@northalloys.com
Copyright © 2024 North Alloys All Rights Reserved Tel:+86 21 56836035 | Overview | Products&Service | Technical Assistance | Appliance