Emerging Evidence of a Second Glass Phase in Strong to Ultra-Fragile Bulk Metallic Glass-Forming Liquids

This thesis compiles three experimental works that provide evidence for distinct bulk metallic glass (BMG) phases across a range of kinetic fragilities. Motivated by An et al.’s computational discovery of a secondary heterogeneous glass phase in pure Ag and binary AgCu and CuZr, the thesis reports the distinct glass phases in the high and ultra-high fragile regime with a tunable Pt80-XCuXP20 system, and the kinetically strong Ni71.4Cr5.64Nb3.46P16.5B3 (Ni208) BMG. The high-fragility work utilizes direct measurement techniques for liquid configurational enthalpy as a function of temperature on anneal-equilibrated samples (Chapter 2). An apparent first-order glass-melting transition is revealed across kinetic fragilities ranging from m = 60 to over 90. The glass-melting temperature, Tgm, traverses up the ∆T region with increasing Cu content, X. A further experimental study of PtCuP explored the traditional and second glass phases to determine if they are in fact equivalent to the two glasses of An et al. (Chapter 3). Hardness data reveal that while the high-fragility samples grow the second glass during anneal, it forms in the ultra-fragile samples on quenching. Further, this apparent glass-melting transition is visible via traditional thermodynamic methods in ultra-fragile samples. For X = 20, where Tgm is in the inaccessible ∆T region, rapid capacitive discharge heating visualizes Tgm as well. Investigation of a kinetically stronger Ni-based BMG connects the presence of the secondary glass to the embrittlement transition in Ni208 (Chapter 4). Inclusions are only present in embrittled samples, and are suppressed to lower temperatures when the initial melt is overheated above a critical toughening temperature. The inclusions show a heterogeneous structure and 30% increased hardness, similar to the computational Ag secondary glass phase. These works provide compelling evidence for the existence of a secondary glass phase across the spectrum from strong to ultra-fragile glasses, and validates the initial computational discovery. This proves to be a significant work, as it presents direct experimental evidence of a novel phenomenon in metallic liquids, and presents a new solid-like glass phase.