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Bulk Amorphous Metals for Advanced Applications
Usually metals and alloys are in a crystalline state. However, metallic glasses ( amorphous metals ) can be obtained from certain alloys with the right composition, when they are quenched from the liquid state at a high enough cooling rate. Since their discovery in 1960, amorphous metals or metallic glasses have been developed from binary systems of micron size (ribbons and powders) to multi-component systems of up to several centimeters even under conventional casting conditions. This arises from the discovery and use of unique alloy compositions. The ability to obtain amorphous metals in bulk opened up their potential for structural applications. Their elastic strain limit of 2% is much higher than that of crystalline metallic alloys ( less than 1%) and they have extremely high tensile yield strength of 2 GPa which is twice that of stainless steel and titanium.
Combined with low density (3 - 6 g/cm³) these amorphous alloys have high strength-to-weight ratio. Because of these unique properties, bulk amorphous metals or bulk metallic glasses (BMGs) have huge potential to replace some some conventional crystalline materials.
These types of new materials found their first application in making golf club heads. The amorphous drivers are twice as hard and four times as elastic as Ti drivers; about 99% of the impact energy from a BMG head is transferred to the ball compared to 70% for Ti. Amorphous metals which can be easily molded and cast into near net-shaped parts with a smooth, shining finish are ideal for use as casings for personal electronic devices such as cell phones, laptops and cameras. They are also used for leisure equipment such as baseball bats (which require good rebound), fishing equipment, scuba gear, marine applications and bicycle frames. In the medical field, BMG is ideal for corrosion- and wear-resistant applications due to its highly biocompatible and nonallergic properties. In the munitions industry, recent development of BMG composites have allowed making tank-armor penetratrating rounds from metallic glasses to replace the current depleted uranium penetrator, which is suspected of biological toxicity.
One of the key issues in the development of these amorphous alloys is to identify the right composition to form large-sized amorphous components. Over the years, we have focused on the factors that influence the glass forming ability of BMGs, i.e. to devise a method that can be used to find the best glass forming alloys. Recently, we derived a new criterion for glass forming ability (GFA). Treating glass as one of the competing phases, we expect glass formation upon complete suppression of crystal growth, when the glass transition temperature Tg is higher than the growth temperature of all the crystalline phases, including eutectic and primary phases. Our analysis shows that the best glass-forming zone can be either symmetric, or asymmetric, about the eutectic composition.
One of the conclusions derived from this criterion is that the best glass former is enclosed by the alloys which have the ability to form similar-sized in situ crystalline phase reinforced composite. When this happens, a switch of the primary phase in the composite which encompasses the best glass-forming zone/composition is expected. Furthermore, the amount of crystalline phase in the matrix will decrease when the alloy' s composition is near that of the best glass former. Thus, as a function of alloy composition, the microstructure in a cast sample would change from a composite (a primary phase in the amorphous matrix), to fully amorphous, and then to another composite (a different primary phase in the amorphous matrix). In essence, our analysis establishes a microstructure-based method for a practical strategy to experimentally pinpoint the alloy composition with the optimum GFA by monitoring the microstructure evolution with composition. Applying this method of observing phase switch in the amorphous matrix recently lead us to pinpoint many best glass forming alloys. Figure 1 shows amorphous ingots of Zr-(Cu,Ni)-Al alloy having diameter of more than 20 mm. Such results prove that our method is more efficient than any other method currently employed by others.
Obviously the challenge is to develop bulk metallic glasses with critical size larger than 10mm in Engineering Fe and Al based alloys. Our aim in the near future is to employ our newly developed method to discover alloy systems that have large glass forming ability for potential engineering applications.
Contact Person : Assoc Prof Y Li
E-mail: mseliy@nus.edu.sg
Tel : 6516 3348
Fax: 6776 3604