What are Bulk Metallic Glasses?

An alloy is a solid-state metallic compound prepared by mixing two or more pure metals (in some cases non-metals) together. Alloys are preferred over pure metals because of improvement in material properties such as strength, corrosion resistance, high-temperature applicability, etc. For example, a commonly known alloy is steel where Iron (Fe) is mixed with Carbon (C) to improve its fracture toughness and strength. These alloys generally have a crystalline structure which means their constituents are arranged in a highly ordered microscopic structure that defines their properties. Amorphous Alloys are a class of alloys that are devoid of any atomic ordering and as a result, have properties that are unique to their class of materials.

The process of crystallization takes place in the alloy melt when it is allowed to cool down over a period of time. As the atoms in the melt lose thermal energy, they arrange themselves in some commonly observed patterns. Theoretically, any melt can be cooled down at a characteristic Critical Cooling Rate (CCR) to deny the atoms an opportunity to crystallize and kinetically arrest the structure in a disordered state. However, the CCR for most alloy compositions tends to be extremely high which makes it impractical to produce at scale. Bulk Metallic Glasses (BMGs) are a special subset of Amorphous Alloys that do not require a high CCR and can be produced through conventional casting methods like Arc-melting and Copper mold casting in bulk quantities.

Due to their amorphous nature, BMGs tend to have some unique and interesting properties. Some of which are described below:

  1. Thermoplastic Formability: Unlike traditional alloys, BMGs do not have a well-defined melting point but instead exhibit a wide supercooled region. When heated in this temperature range, BMGs can undergo continuous softening (much like plastics) without any events of crystallization upon cooling. This property makes BMGs especially useful for net-shape processing of articles made from BMGs through Metal Injection Molding (MIM). BMGs undergo minimal expansion or shrinkage with temperature changes and therefore are also excellent in retaining the shape of the mold.

  2. High Yield Strength and Higher Elastic Strain Limits: Crystalline alloys exhibit defects (such as dislocations) in their crystalline lattice, which are mechanically considered as weak spots that limit their strength. BMGs, due to their amorphous nature, do not have any such defects and can undergo a large range of reversible (elastic) deformation without failing. Some Zirconium-based alloys have a tensile strength that is almost twice that of high-grade Titanium.

  3. Wear and Corrosion Resistance: A common surface feature of crystalline alloys is grain boundaries that arise due to imperfections or defects in the lattice. BMGs offer a high surface smoothness due to the absence of any grain structure which leads to significantly less wear and improved surface corrosion resistance.

  4. Soft-Magnetic Properties: It has been observed that some magnetic metals-based BMGs have high magnetic susceptibility, with low coercivity and high electrical resistance. This leads to significantly lower losses by eddy currents, making them useful in transformer cores and electrical motors.

In light of these properties, BMGs can be used A) to produce components that undergo high mechanical stress like gears, sensor mounts, or structural components, B) in medical instruments, implants, and other components that operate in corrosive environments, and C) in electric motors, transformer cores, etc.

BMGs are a unique class of materials with a highly specialized set of applications. Further efforts need to be made in this field to reduce both the cost of development and production of these materials. New production processes like Metal Injection Molding and Metal 3D Printing show promise in achieving these goals and unlocking new applications of BMGs.

Finding the right alloy composition is crucial to successfully develop a BMG. This requires a deep understanding of the structure-property relationship as well as the effects of individual elements on the resulting alloy. Phaseshift Technologies uses a high-throughput computational approach to explore a vast compositional space to identify novel BMGs with improved material properties in reasonable time-spans. This involves exploration of a large and complex design space with the use of Machine Learning, as well as modeling of material properties through state-of-the-art simulation methods.