Characteristics and applications of molybdenum-based materials

There is an inextricable link between the properties of materials and their applications. For some applications, a certain property, such as electrical conductivity, is critical. For other applications, a certain material is the best choice because of its good overall properties. The word "optimal" is important in this context because it means that while other materials may have advantages in one or two properties, the selected material provides the best for the various engineering design problems faced by the specific part material. comprehensive solutions. Sometimes, the best solution is not a single material, but a combination or composite of multiple materials, allowing designers to tailor the desired properties to meet the challenges presented by a specific application. Regardless, the solutions that ultimately stand out from the competition are the ones that are cost-effective. This means that materials such as molybdenum metal are very expensive according to common engineering material standards, but they must demonstrate significant advantages compared to competing materials.

Table 1 summarizes the nominal chemical compositions of molybdenum-based materials sold around 2012. The table includes molybdenum-based alloys and composites of molybdenum with other materials. Molybdenum alloys have higher strength than pure molybdenum and can maintain this strength at higher temperatures than pure molybdenum. The alloy parts in the table are further subdivided into "replacement", "carbide stable" and "dispersion strengthened" sub-tables .

Table 1

Substitute alloys are the simplest class of alloys. Among them, alloy atoms replace molybdenum atoms on the alloy's body-centered cubic (BCC) crystal structure (Figure 1). When alloy atoms replace molybdenum atoms, it causes strain in the crystal lattice, which increases the material's strength.

Figure 1 shows the arrangement of molybdenum atoms on a crystal lattice represented by a "body-centered" unit cell, with atoms located at the four corners and the center.

Figure 1

Replicating this unit cell face-to-face in three-dimensional space will form a complete crystal. While alloying can increase strength, the primary method of strengthening molybdenum is by mechanical deformation anyway, typically through standard rolling, rotational forging, or deformation that can increase molybdenum's strength by up to four times, depending on the amount of deformation applied. For deep processing materials such as wire rods, even higher increase factors are possible. Annealing removes the effects of machining and restores its strength. The maximum service temperature of alternative alloys may be slightly higher than that of pure molybdenum. However, to significantly increase high-temperature strength, metallurgists have been pursuing other alloying methods.

Carbide-stabilized alloys contain small reactive metal carbide particles in a molybdenum matrix. They also benefit from a small amount of alternative alloying provided by reactive metals that are not present in the form of carbides, as well as additional interstitial hardening from carbon and elements. Oxygen atoms in non-carbide particles. This combination maintains the strength of molybdenum at higher temperatures than pure molybdenum or simple replacement alloys because the fine particles force the recovery process to occur at higher temperatures. The production process is a key factor in the success of these alloys. The process must ensure that the active metal and carbon are first dissolved in the molybdenum matrix and then precipitated in the required fine dispersed phase in subsequent processes.

Dispersion-strengthened alloys use oxides as the second phase, or in the case of doped Al/K/Si materials, use dispersed element phases that are insoluble in the molybdenum matrix. In this case, very small and stable second phase particles must be present in the material at the beginning of the deformation process. The purpose of processing is to create a special arrangement of these particles that results in extraordinary high-temperature strength and stability.

Composite materials can be divided into two categories: laminated composite materials and powder composite materials. Laminates are made by calendering composites that combine copper or nickel with molybdenum in a core. Powder composites are produced by mixing/pressing/sintering (sometimes by hot isostatic pressing). pressing (HIP, densification) or liquid phase infiltration.

Table 2 cross-references some functions and applications, pointing out the important functions of each application. Only some of the features or apps are listed here. Manufacturing characteristics such as processability and formability play an important role in the economic decision to manufacture a specific part, but the choice of basic materials is driven by the requirements of the application. It is obvious from the table that no application is built around just one component. For example, heat sinks for power semiconductors must have a certain coefficient of thermal expansion to minimize thermal stress during operation, but they must also conduct heat and electricity effectively, because their work also requires that they not only pass current, but also carry away electricity. Semiconductor heat. If the power device is used in an aircraft or spacecraft, density will become a more important factor than if it were part of a large fixed motor power control device.

 Table 2

Therefore, one must consider the "package" of properties when matching materials to applications. Once the set of properties required for a particular application is understood, the appropriate alloy or composite can be selected for that application. When none are readily available - When manufactured materials are available, consideration can be given to developing a new material with a range of customized properties. When making this decision, it is necessary to understand competing materials and their cost, availability, and reliability compared to molybdenum-based materials.

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