Georg Kaltenboeck

Beating Crystallization in Glass-Forming Metals by Millisecond Heating and Processing

Resistive heating can be used to rapidly heat a bulk metallic glass without inducing crystallization. The development of metal alloys that form glasses at modest cooling rates has stimulated broad scientific and technological interest. However, intervening crystallization of the liquid in even the most robust bulk metallic glass-formers is orders of magnitude faster than in many common polymers and silicate glass-forming liquids. Crystallization limits experimental studies of the undercooled liquid and hampers efforts to plastically process metallic glasses. We have developed a method to rapidly and uniformly heat a metallic glass at rates of 106 kelvin per second to temperatures spanning the undercooled liquid region. Liquid properties are subsequently measured on millisecond time scales at previously inaccessible temperatures under near-adiabatic conditions. Rapid thermoplastic forming of the undercooled liquid into complex net shapes is implemented under rheological conditions typically used in molding of plastics. By operating in the millisecond regime, we are able to “beat” the intervening crystallization and successfully process even marginal glass-forming alloys with very limited stability against crystallization that are not processable by conventional heating.

Glassy steel optimized for glass-forming ability and toughness

An alloy development strategy coupled with toughness assessments and ultrasonic measurements is implemented to design a series of iron-based glass-forming alloys that demonstrate improved glass-forming ability and toughness. The combination of good glass-forming ability and high toughness demonstrated by the present alloys is uncommon in Fe-based systems, and is attributed to the ability of these compositions to form stable glass configurations associated with low activation barriers for shear flow, which tend to promote plastic flow and give rise to a toughness higher than other known Fe-based bulk-glass-forming systems.

Accessing thermoplastic processing windows in metallic glasses using rapid capacitive discharge

The ability of the rapid-capacitive discharge approach to access optimal viscosity ranges in metallic glasses for thermoplastic processing is explored. Using high-speed thermal imaging, the heating uniformity and stability against crystallization of Zr35Ti30Cu7.5Be27.5 metallic glass heated deeply into the supercooled region is investigated. The method enables homogeneous volumetric heating of bulk samples throughout the entire supercooled liquid region at high rates (~105 K/s) sufficient to bypass crystallization throughout. The crystallization onsets at temperatures in the vicinity of the “crystallization nose” were identified and a Time-Temperature-Transformation diagram is constructed, revealing a “critical heating rate” for the metallic glass of ~1000 K/s. Thermoplastic process windows in the optimal viscosity range of 100–104 Pa·s are identified, being confined between the glass relaxation and the eutectic crystallization transition. Within this process window, near-net forging of a fine precision metallic glass part is demonstrated.

Shaping metallic glasses by electromagnetic pulsing

With damage tolerance rivalling advanced engineering alloys and thermoplastic forming capabilities analogous to conventional plastics, metallic glasses are emerging as a modern engineering material. Here, we take advantage of their unique electrical and rheological properties along with the classic Lorentz force concept to demonstrate that electromagnetic coupling of electric current and a magnetic field can thermoplastically shape a metallic glass without conventional heating sources or applied mechanical forces. Specifically, we identify a process window where application of an electric current pulse in the presence of a normally directed magnetic field can ohmically heat a metallic glass to a softened state, while simultaneously inducing a large enough magnetic body force to plastically shape it. The heating and shaping is performed on millisecond timescales, effectively bypassing crystallization producing fully amorphous-shaped parts. This electromagnetic forming approach lays the groundwork for a versatile, time- and energy-efficient manufacturing platform for ultrastrong metals.