Figure a: Metallic glass
Metallic Glasses
Metallic glasses (MGs), a new class of metallic alloy, are very potential for micro- and nano-manufacturing. MGs possess some unique features due to noncrystalline (amorphous) structure, for instance, high yield strength and net shape forming. The noncrystallinity was absent in metals and alloys until 1960 when Pol Duwez, at California Institute of Technology, had synthesized an Au-25 at.% Si alloy in the glassy state by solidifying very rapidly. Since this amorphous material consists of metals it is referred to glassy metals or metallic glass. During last 50 years hundreds of glassy alloys have been prepared. For example, Al, Au, Co, Cu, Fe, Pd, Pt, Ti, and Zr based composition have been vitrified into glassy state. All of them are broadly classified into metal-metalloid or metal-metal type.
Metallic glasses (MGs), a new class of metallic alloy, are very potential for micro- and nano-manufacturing. MGs possess some unique features due to noncrystalline (amorphous) structure, for instance, high yield strength and net shape forming. The noncrystallinity was absent in metals and alloys until 1960 when Pol Duwez, at California Institute of Technology, had synthesized an Au-25 at.% Si alloy in the glassy state by solidifying very rapidly. Since this amorphous material consists of metals it is referred to glassy metals or metallic glass. During last 50 years hundreds of glassy alloys have been prepared. For example, Al, Au, Co, Cu, Fe, Pd, Pt, Ti, and Zr based composition have been vitrified into glassy state. All of them are broadly classified into metal-metalloid or metal-metal type.
Figure b: Thermoplastic embossing of metallic glass
Thermoplastic Embossing
Similar to thermoplastic polymers, MGs have a super cooled liquid region which is defined by the temperature between glass transition temperature (Tg) and crystallization temperature (Tx). Above Tg, the MGs become compliant due to drop of the viscosity in several order of magnitude. For example, viscosity of Zr(46.75)Ti(8.25)Cu(7.5)Ni(10)Be(27.5) MG drops to 10^6 Pa.s from 10^12 Pa.s at 463 C. Therefore, they behave like high viscous fluid and replicate the shape of the mold after high pressure is applied. The advantages of thermoplastics forming are:
a) Fabrication is carried out well below melting temperature. For instance, melting temperature of Ni60Pd20P17B3 is 656 C whereas thermoplastic forming is conducted 20-30 C above glass transition temperature, 331 C.
b) Very precise and uniform structure fabrication is possible.
c) Multiple length scale features are feasible
d) Annealing above Tg does not cause embrittlement as long as the process is conducted within supercooled liquid region.
Typical thermoplastic embossing of metallic glasses is shown in figure b where as example of different pattern surfaces made through embossing is shown in figure 4 .
Similar to thermoplastic polymers, MGs have a super cooled liquid region which is defined by the temperature between glass transition temperature (Tg) and crystallization temperature (Tx). Above Tg, the MGs become compliant due to drop of the viscosity in several order of magnitude. For example, viscosity of Zr(46.75)Ti(8.25)Cu(7.5)Ni(10)Be(27.5) MG drops to 10^6 Pa.s from 10^12 Pa.s at 463 C. Therefore, they behave like high viscous fluid and replicate the shape of the mold after high pressure is applied. The advantages of thermoplastics forming are:
a) Fabrication is carried out well below melting temperature. For instance, melting temperature of Ni60Pd20P17B3 is 656 C whereas thermoplastic forming is conducted 20-30 C above glass transition temperature, 331 C.
b) Very precise and uniform structure fabrication is possible.
c) Multiple length scale features are feasible
d) Annealing above Tg does not cause embrittlement as long as the process is conducted within supercooled liquid region.
Typical thermoplastic embossing of metallic glasses is shown in figure b where as example of different pattern surfaces made through embossing is shown in figure 4 .