Revealing hidden supercooled liquid states in Al-based metallic glasses by ultrafast scanning calorimetry: Approaching theoretical ceiling of liquid fragility

Abstract

Metallic glasses (MGs) are amorphous alloys with a number of unique properties that are attractive for the fundamental understanding of the nature and applications of disordered systems [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Generally, MGs might be grouped into two categories based on their glass forming ability (i.e., the ease of glass formation by cooling a liquid): in one case, large or bulk volumes may be slowly cooled to the glassy state from the melt. This category is usually called as bulk metallic glass formers [18][19][20][21][22]. In contrast, certain MGs, represented by aluminum (Al)- [23,24] and some iron (Fe)- [25,26] based MGs, can be synthesized mainly by rapid solidification processes such as melt spinning or vapor deposition. These MGs are often denoted as marginal MG-formers [23][24][25].With the rapid developments of MGs, many previously identified marginal glass formers have been successfully made into bulk metallic glasses by composition strategies (such as Fe-and Ni-based MGs) [8,25,26,28]. However, one remarkable exception is the family of Al-based MGs [23,24]. Although there have been continuous efforts in optimizing compositions and improving processing techniques since its first discovery [29] several decades ago, the glass forming ability of Al-MGs is still limited nowadays [23,30,31]. Recently, Wu et al. [30] reported that the hitherto best glass forming ability for Al-MGs is from Al 86 Ni 6.75 Co 2.25 Y 3.25 La 1.75 with a fully glassy rod of 1.5 mm in diameter. Although the glassy rod can reach 2.5 mm for the same composition after a refined fluxtreatment of the liquid [31], this critical size is still much smaller than those of typical Zr-based MGs (~75 mm) [32], Mg-MGs (25 mm) [18] and Pd-MGs (80 mm) [33]. Moreover, the glass forming ability of Al-MGs is very sensitive to compositions. Only a few percent change of the constituting elements would dramatically reduce this critical size [21,27,30,34,35]. Consequently, it is more difficult to develop Al-MGs than other MGs. Fundamentally, it remains to be a puzzling issue why the Al-MGs are so unique [27,30,36].Among several factors, one clue for the distinction between bulk glasses and marginal glass formers is based upon their supercooled liquids [19,23,26,27]. The change of the relaxation dynamics (e.g., viscosity, diffusivity or relaxation time) with the temperature is highly materialspecific. Angell introduced the concept of fragility (m) for the classification of glass-forming materials, which was defined as the apparent activation energy of relaxation time τ α normalized to glass transition temperature T g : m = dlog(τ α )/(dT g /T)| T=T g [37]. A stronger deviation from Arrhenius behavior (with a larger m value) corresponds to a more fragile system; otherwise, the system is strong [37][38][39][40]. Wang et al. [41] theoretically pointed out that the fragility index for non-polymeric glasses should have a lower limit m = 16.5, and an upper limit m~175.Recently, Johnson et al.[42] conducted a critical asses...

Fragility crossover mediated by covalent-like electronic interactions in metallic liquids

“…Interestingly, although the fragility concept is introduced for describing the properties of liquids [9][10][11][12], it has been reported to correlate with a range of properties of solid glasses. These include the glass-forming ability (GFA) [4,[13][14][15] and mechanical deformability [16,17] of metallic glasses (MGs), elastic properties [18][19][20], heat capacity associated with the glass transition [21,22], the Boson peak that is associated with vibrational properties and low-temperature excess heat capacity [23,24], the surface diffusion coefficient and the formation of ultra-stable glasses [25,26]. Fragility is also an important indicator in the casting, annealing, and aging processes of glasses [27][28][29].…”

“…(i) The interference of crystallization. For example, in some Al-based and La-based MGs, the glass transitions were seldomly probed due to the early crystallization as indicated by DSC or viscosity measurements [22,[64][65][66][67]. This crystallization is also a major concern in studies of many other properties of metallic liquids.…”

“…For these major challenges, fragility data is only available for a small number of MGs or metallic liquids [4,22]. Usually, fragility is extracted only for one composition from a large alloy family, because only that specific composition exhibits good thermal stability against crystallization.…”

“…With the so-obtained wide supercooled liquid region, one could apply the DSC protocol that yields reliable fragility data. For example, the glass transition and fragility of some Al-based MGs [22] and a Ni 80 P 20 MG [70] have been recently probed with heating rates higher than 100 K s −1 .…”

Improve the Forming Ability of Al-Based Metallic Glass Under Ultrasonic Vibration at Room Temperature

“…The tensile ductility is only 5% at high temperature (He et al, 2020). Furthermore, at high temperatures, Al-based MGs have a very narrow supercooled liquid region (SLR) (Yang et al, 2019), resulting in the lack of thermoplastic forming ability compared to Zr and novel metal based MGs (Inoue and Takeuchi, 2011;Lucena et al, 2020). This induces a great difficulty in processing and forming of Al-based MGs, even after their successful preparation, which limits the development of Al-based MGs.…”